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Marketing information

Load cell application manual

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LOAD CELL APPLICATION MANUAL

1 GENERAL ASPECTS OF WEIGHING

1.0 Principle of electronic weighing 5

1.1 Mounting load cells 61.1.1 Mounting compression load cells1.1.2 Mounting tension cells in principle1.1.3 Mounting beams in principle

1.2 Stability and statically (un)defined systems 151.2.1 Statically undefined support1.2.2 Stability of an object on load cells1.2.3 Horizontal natural frequency and restoring force

1.3 General recommendations on the design of an electronicweighing installation

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1.3.1 Design criteria1.3.2 Stiff and rigid foundation1.3.3 Traffic1.3.4 Installation of vessels1.3.5 Load cell selection1.3.5.2 Installation of mounting kits1.3.5.3 Protection of load cells against high temperature1.3.5.4 Protection of load cells against overload1.3.5.5 Protection against dynamic overload

1.4 Constraining 391.4.1 Using constrainers1.4.2 Constraining of a suspended object1.4.3 Orientation of the constrainers1.4.4 Types of constrainers1.4.4.1 MiniFLEXLOCK (general)1.4.4.2 Rigidly clamped struts1.4.4.3 Flexbeams1.4.4.4 Pivoting rod1.4.4.5 Rocking pin1.4.5 Types of stops1.4.5.1 Horizontal stops1.4.5.2 Vertical stops (lift- off- protections)

1.5 Disturbing influences 581.5.1 Environmental influences1.5.1.1 Wind forces1.5.1.2 Heat and heat transfer1.5.1.3 Freezing environmental conditions (ice, snow)1.5.1.4 Dust, rain1.5.2 Friction1.5.3 Vibration, shock loading

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1.7 The weighing result 711.7.1 Terminology for load cells1.7.1.2 Load cell and weighed installation1.7.2 Influences from the construction1.7.3 Approved installations1.7.3.1 W&M regulations1.7.3.2 Weighbridges1.7.4 Standard accuracy: non- W&M installations1.7.4.1 Installation specification1.7.4.2 Use of the weighing installation

1.8 Installation and commissioning 801.8.1 Mechanical installation1.8.2 Electrical installation1.8.3 Calibration1.8.3.1 Calibration of vessels of more than 5 tons1.8.4 Corner point adjustment1.8.5 Load cell check

2 MOUNTING THE LOAD CELLS 88

2.1 Mounting the compression load cell PR 6201 892.1.1 Mounting kit PR 61452.1.2 MiniFLEXLOCK PR 6143

2.3 Mounting the ultra flat PanCake load cell PR 6251 95

2.4 Mounting load beams 96

2.5 Mounting the S- type load cell 982.5.1 Mounting kits PR 6041/30, .../402.5.2 MiniFLEXLOCK PR 6043/30, .../402.5.3 Standard mounting kits for the tension load cell PR 6246

2.7 Mounting the compact load cell PR 6211 1042.7.1 Mounting kits for the small type (30kg...300kg)2.7.1.1 Mounting kit PR 6011/002.7.1.2 Rubber mounting kit PR 6011/032.7.1.3 MiniFLEXLOCK PR 6011/202.7.2 Mounting kits for the big type (500kg...10t)2.7.2.1 Mounting kit PR 6011/102.7.2.2 MiniFLEXLOCK PR 6011/302.7.2.3 SeismoFLEX PR 6011/40

2.8 Accessories for the installation 1122.8.1 Cable junction boxes2.8.1.1 Plastic cable junction box PR 6130/082.8.1.2 Universal cable junction box PR 6130/60S, .../68S2.8.2 Installation cable PR 6135 (PR 6136)

2.9 Constraining devices 1152.9.1 Horizontal constrainers PR 6152/022.9.2 Constrainer PR 6143/80, .../83

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3. TANK WEIGHING 118

3.1 Overview 118

3.2 Application examples 1193.2.1 Some hints for installations with pivots3.2.2 Installations with load cells only

3.4 Pipes, bellows ... 1203.4.1 Pipes3.4.1.1 Influences of stiff pipe connections3.4.1.2 Calculation of the pipe stiffness3.4.1.3 The constraining effect of pipes3.4.2 Bellows3.4.2.1 Influences of gas pressure3.4.2.2 Influence of vertical bellows

3.5 Level control using pivots 1283.5.13.5.1.1 Standardized pivots3.5.1.2 Mounting hints3.5.2 Calculation of an I beam pivot

APPENDICES 139A Alphabetic index 139

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1.0 Principle of electronic weighing

Definition.Weighing is the determination of mass.

To weigh electronically an industrial object, this object is put on load cells. The load cells transform the weight ofthe object into an electric signal, which is led to an electronic measuring apparatus by means of an electriccable. Here the weight can be indicated, printed, and used for automatic control of an industrial process.

Fig. 1.0-1 Main parts of a weighing system

General remarks to the design of the weighing installation · Also after placing the load cells the object must remain a stable and reliable part of the industrial installation.

· Only the weight of the object and no other vertical force shall flow through the load cells.

· Vertical load cell position and simple mounting parts assure that this force flows through the primary axis ofthe load cell.

· Parasitic vertical forces must be avoided or made very small. Examples are· friction of the object to the surroundings· forces caused by gas pressure· elastic forces of e.g. pipe connections· wind

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1.1 Mounting load cells

Load cell mounting is the base for accurate weighing in the industrial surroundings, particularly formeasurements with W&M installations. This chapter describes in the first place various types of bearings andtheir properties from the mechanical point of view. Afterwards the mounting principles for compression loadcells, tension load cells and beams are shown in general.

Load cells can be regarded as the bearings of a vessel. Usually the different types of bearings are classifiedaccording to their degrees of freedom. There is a number of possible arrangements, but only 3 methods areapplied for load cell mounting. Each method is described with its advantages and disadvantages. Thedescription of these methods shall enable you to criticize an installation and to choose an appropriate load cellfor every purpose.

Fig. 1.1-1 Mounting method 1: articulated column

- examples: PR 6201PR 6241 with mounting parts PR 6041/31S

characteristics- spherical top and spherical bottom- load cell transfers only the weight, neither side loads nor momenta

⇒ measurement is unaffected by disturbing side forces and momenta- proper constraining is absolutely necessary to keep the system in a stable position especially if the centre

of gravity is above the plane of the load cells- depending on the load cell construction restoring forces can be expected- highest measuring accuracy- principle usually applied for load cells in W&M installations or where high accuracy is required

Fig. 1.1-2 Mounting method 2: articulated bearing

- example: PR 6211characteristics- spherical top or spherical bottom- load cell transfers side forces and weight, but no momentum

⇒ influences on the measurement can be expected- constraining necessary to eliminate the influence of the side forces on the result- medium accuracy

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Fig. 1.1.-3 Mounting method 3: load cell clamped- examples: MP 49- load cell fixed with bolts to both the foundation and the object- high momenta caused by this mounting method- load cell transfers weight, side forces, and momenta

measurement is influenced by all disturbing forces and momenta- no constraining necessary- lowest measuring accuracy

1.1.1 Mounting compression load cells in principle

Fig. 1.1-4 Main installation parts for a compression type load cell

1. Standard mounting parts are advised: a special ‘load button’ and a special ‘bottom plate’. They allow- to have the most optimal material for the contact with the load cell top and the load cell bottom

(standardized conditions)- easy exchange in case of wear- a standardized height between object and foundation

2. The ‘electric shunt’ as a protection against possible heavy stray currents in the structure has to beconnected (further details are described in chapter 1.3)

3. ‘Shims’ are used in case of more than three load cells under the object; their purpose is to distribute theload evenly over all load cells (refer to ‘statically undefined systems’)

4. Make sure that both, the weighed object and the foundation, are rigid and stiff as to take the actual loadsduring the operation

5. Avoid vertical force shunts: pipes, heavy cables, contact to the surroundings etc.6. Observe the external influences (wind, temperature) on the weighed object7. Every object has to be properly constrained. Remember to install the lift-off-protection.

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8. Vertical position of the load cellThe vertical position of the load cell can be adjusted by a horizontal movement of the foundation plate.The vertical position can be measured directly with a spirit level. Indirectly it can be checked by checkingthe square angle between foundation plate and load cell

As to reach the best measuring results the following angles should preferably not be surpassed during theinstallation procedure:

upper loading plate α < ±2°foundation plate β < ± 0.5°load cell tilt γ < ± 1°

Those angles are also limits for the installation in use.

Fig. 1.1-5 Permissible inclination

1.1.2 Mounting tension load cells in principle

A safety hint

If a break in suspension, support, load cell, or mounting part etc. represents ahazard to the life and health of men and animals, or if goods may be damaged,additional safety devices have to be provided.

When taking the appropriate standards into consideration, the dimensions of all mounting and structuralelements have to be calculated so that sufficient overload capacity is ensured for the design load. In particular,upright weighing objects have to be safeguarded against the weighing installation turning over or being shifted.

Measuring principleOnly the weight of the object should flow through the tension cells. For this reason, make sure that disturbingvertical forces caused by e.g. stiff pipe connections, wind, gas pressure, dirt are avoided.

- the tension cells should be mounted in the upright position- (the name stands upright)

- the cell should be mounted vertically within ±2°- the cell has to be mounted between two cardanic pivoting points- a torque around the axis should be avoided

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The tension load cells must be mounted in such a way that the movement and bending of the cable do notinfluence the weighing result. The correct positions are shown in the sketches above.

Fig. 1.1-6 Mounting PR 6206 Fig. 1.1-7 Mounting the S type

Remarks1. in the wrong position corrosion could be feared by collected water, e.g. from rain, if the load cell is not

made of stainless steel2. especially in the case of a light object, the cable weight or the cable rigidity can influence the vertical

position of tension cell. So, provide a good support of the cable.

A tension cell is designed to measure only the force flowing through its ‘primary axis’. The weight G acts invertical direction, therefore the tension cell has to be mounted vertically (if possible within ±2). This is notcritical. A deviation of 2 changes the ‘span’ of the measure ment only 0.06%! (Explanation: with no friction inthe pivots, the force through the primary axis is G/cos )

Different mounting methodsIn general two different ways to mount tension cells can be advised:

- mounting with pivoting points (use of standard mounting kits)- mounting with long rods (this system is a very cheap alternative but needs a lot of free space)

Method 1: tension load cells mounted with pivoting pointsThe mounting should be done in such a way that one cardanic pivoting point is provided at the top end andanother one at the bottom end of the tension cell (cardanic means that the tension cell can swing in alldirections). This is to prevent that bending momenta or side forces act on the tension cell.

Fig. 1.1-8 Installation of tension load cells

⇒ the danger of side forces on the load cell caused by thermal expansion of the object is eliminated⇒ external horizontal forces are kept away from the tension cells. If necessary, those forces can be taken up

by horizontal constrainers.

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Examples for such pivoting points are swivel bearings (fig. 1.1-9) and to a certain degree pairs of sphericalwashers (fig. 1.1-10).

Fig. 1.1-9 Swivel eye Fig. 1.1-10 Mounting with spherical washers

If the horizontal movement is not constrained, make sure that the part between the two pivots never touches afixed point. This can cause extreme side forces on the load cell and result in its damage.

Fig. 1.1-11 The load cell must not touch its surroundings

Beside external forces on the load cell torsional momenta must be avoided. By external influences e.g. a stirringdevice a torsional momentum could occur on the tension cell. Mounting parts and constrainers can take care ofthis.

Method 2: Long rod mountingVery long rods are flexible to transversal and rotational movements. This property can sometimes be used for acheap mounting solution. The vessel is hung up with the help of long rods. The length between fixation pointsat the ceiling and the object respectively has to be more than 1.600mm.

Two threaded rods of at least 900mm length are screwed into the load cell and locked with a spring washer. Thematerial of these threaded rods has to have a tensile strength of at least 450 N/mm2 (that is e.g. class 6.8). Thefixation at the foundation and at the object has to be done with two pairs of spherical washers (DIN 6319) toavoid undesired pre-stresses. Also therefore, the nuts have to be tightened strongly only after the vessel ishanging in its final position. For security one can provide splitpens at the two ends.

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REMARKHalf way in between the foundation and the object fixation points the bending momentum in the rod willalways remain zero. Therefore this is the best position for the tension load cell to avoid bending influences onthat cell.

Fig. 1.1-12 Calculation for long rod mounting

This mounting principle can easily be described, together with its reaction forces and moments.

L 64D E 12

= xF

= k3

4

••••π

The rod stiffnesses cause here some disturbing effects on the tension cell.The bending stiffness of a steel rod with a length L and a circular diameter D can be calculated fromThis is the force (in newton), necessary for a lateral displacement of x mm.

LD G

= M = k4

tt

••πφ

The torsional stiffness of a steel rod with a length L and a circular diameter D isThis is the torque around the vertical axis, necessary for a torsion angle of 1 radian. The following table showsboth stiffnesses for several rod lengths at the standardised rod diameters for the different tension cell types.

L = 1500 mm L = 2000 mm

k kt k kt

M12 0.7 N/mm 117.5 Nm/rad 0.3 N/mm 88.1 Nm/rad

M16 2.3 N/mm 371.4 Nm/rad 1.0 N/mm 278.5 Nm/rad

M20 x 1.5 5.7 N/mm 906.7 Nm/rad 2.4 N/mm 680.0 Nm/rad

M30 x 2 28.8 N/mm 4,590 Nm/rad 12.2 N/mm 3,443 Nm/rad

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Examples for long rod mounting

A vessel is normally suspended by three tension cells. Theexamples below show the effects for two situations ofvessels moving.Case a: the vessel moves only sideways over 10 mm

distanceCase b: the vessel rotates around its axis with an amplitude

of 10 mm at the radius R.

Fig. 1.1-13 Vessel with long rod mounting

Example 1Vessel 1 gross weight 400 kg3 pcs PR 6206/22 placed at a radius of R = 400 mmdiameter of the rod D = 12 mmlength of the rod L = 1500mm

case a side force F = 7.3 N no torque

case b side force F = 7.3 N torque M = 3 Nm

Example 2Vessel 2 gross weight 3000 kg3 pcs PR 6206/13 placed at a radius of R = 600 mmdiameter of the rod D = 16 mmlength of the rod L = 2000mm

case a side force F = 10 N no torque

case b side force F = 10 N torque M = 5 Nm

Example 3vessel 3 gross weight 15000 kg3 pcs PR 6206/53 placed at a radius of R = 1100 mmdiameter of the rod D = 30 mmlength of the rod L = 2000 mm

case a side force F = 120 N no torque

case b side force F = 120 N torque M = 32 Nm

Conclusion In all examples the long rods are good enough and do not disturb the measurement.

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1.1.3 Mounting beams in principleAvoid the application of more than three beams to suspend a stiff object. In such a case the installationbecomes statically undefined: it not certain that the load is equally distributed. Maybe even not all the beamscarry the object. Overloading and subsequent damage to some of the beams can be the result. In case of abeam with a built-in-overload-protection a measuring error occurs.

Fig. 1.1-14 Load beam (principle diagram)

Further mounting directives1. The foundation plate for fixing the beam must have a smooth, clean, flat surface and must be horizontal

within ±0.5°. A deviation causes a sensitivity error, which, however, can be compensated by themeasuring instrument.

2. The foundation must be rigid enough to avoid yielding under load. Change of the horizontallity by yieldingof more than 1° between zero load and full load causes an additional non-linearity error of more than 10-4.

3. Use the prescribed mounting parts to suspend the object. However, the suspension rod might be longer ifthere is enough room for it.

4. Provisions must be present to adjust the vertical position of the suspension rods within β < ±2° to avoidbig side forces on the beam. (FH = FG ⋅ tanβ, or FH = 0.035 ⋅ FG, β=2°; REMARK: This is easier for longersuspension rods.)

5. Grease all pivoting points to avoid too much friction (or even sticking by corrosion).6. If the object is suspended with only one beam, adequate measures must be taken to avoid torsion of the

object.7. Provisions for lifting the object during installation must be present. Also provisions for suspending

calibration masses.8. Protect beams and especially the bellows against mechanical damage (falling tools etc.).9. Avoid parasitic loads on the beam (e.g. on the bellows).

Fig. 1.1-15 Load distribution (theory) Fig. 1.1-16 Choice of the mounting bolts

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The sketches on the previous page illustrate how to mount shear beams and advise the provisions to be taken.Some interesting details should be observed:

- the mounting bolts S1 and S2 are not equally loaded- standard screws (4.6 and 5.6), which are widely used for constructions, are insufficient for mounting this

shear beamproperty class 8.8 means

conventional limit of elasticity ≥640 N/mm2 (no plastic deformation of the bolt up to this limit)tensile strength ≥800 N/mm2 (no destruction of the bolt up to this limit)

- the nut (or the material) that acts together with the screw has to have the same propertiesnut: property class 8material: low alloy steel (conventional limit of elasticity ≥640 N/mm2)

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1.2 Stability and statically (un)defined systems

Combining load cells to weighing systems („scales“) is usually quite easy if a few points are observed:the stability of the installation, (→ this chapter)the fixed position of the installation. (→ chapter 1.4)

A system with three load cells is always as stable as a three-leg-chair. A system with more than 3 load cells,e.g. 4 or even more load cells, must be shimmed to ensure an even load distribution.

The term stability depends on the equilibrium of the installation.Definition EQUILIBRIUM - stable equilibrium system returns to its centre position after slight deviations

Fig. 1.2-1 Stable equilibrium - neutral equilibrium system is in equilibrium in every position

Fig. 1.2-2 Neutral equilibrium

- unstable equilibrium the system does not return to its centre position after a slight excitation

Fig. 1.2-3 Unstable equilibrium

How can the state of the equilibrium of a weighing installation be detected? At first the position of the centre ofgravity must be known: under or over the plane of the load cells. - definition centre of gravity

In this point you can imagine the whole load of the weighing object concentrated. - definition plane of the load cells

This is the plane directly above the load cell tops.

Fig. 1.2-4 Some definitions

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1.2.1 statically undefined supportConsider the two situations described below:A. Imagine you sit on a four-leg chair on an uneven floor. Probably only three legs carry your weight. (Or even

sometimes only two!). The result is a rocking chair. Such a system is called a statically underdefinedinstallation: the system has freedom to move (it can waggle).Problem: Only some of the load cells carry the weight. This can cause overloading.

B. Imagine a vessel supported by three load cells and constrained with more than three constrainers in oneplane. This system is completely fixed and has no freedom to move. Furthermore, the system can getclamped causing the constrainers to transfer also vertical forces. Such a system is called staticallyoverdefined.Problem: Vertical force shunts can result in measuring errors.

Both cases A and B can be collected under the term „statically undefined„ meaning the forces at the supportingpoints are not predictable with the help of the standard equations.

Since both situations A and B can cause problems, every effort should be taken to minimize their negativeresults.

Waggling should be avoided since it can damage load cells and its mounting parts by hammering. Additionally,it causes unnecessary movement of e.g. pipe connections with subsequent possible zero error. The abovedescribed situation could be improved

1. by increasing the vertical flexibility at the supporting points. More flexibility makes the necessity of thealignment (adjustment of the height) less, or the alignment less critical. It does not matter if the flexibility iscaused by a non stiff object or by the flexibility of the foundation.Examples: Open trays or platforms are flexible objects.

Softness of the subsoil can cause a flexible foundation, however, this is also the case ifthe load cells are placed on steel beams which bend under load.

Such an installation with a flexible support, however, can cause other problems, e.g. in stability andaccuracy. This area is covered in chapter 1.3.

2. by shimming, i.e. adjusting the height by fitting thin metal sheets between load cells and weighed object.

1.2.2 Stability of an object on load cellsIn principle an object on three or more load cells and with three constrainers, as described in chapter 1.4, givesa reliable, stable construction.

The object could only collaps under the following improbable circumstances: • A foundation point cannot bear the vertical load.

This is of course a very evident case which has not to be explained further. There is only one reason for it:a design error

• Constrainers or their fixation points cannot take the actual horizontal loads.By breakdown, the plane of the load cell supporting points could make a transversal movement or arotation. In an extreme case this can cause capsizing of the load cell. However, correct dimensioning ofthe constrainers overcomes this problem.

Fig. 1.2-5 Possible object movement in case of wrong constraining

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• The weighing construction itself is not rigid enough to take the weight, the internal andexternal forces without a big deformation. Then capsizing of the load cells could occur by two effects:1) if the plane of the constrainers is not put in the plane of the load cells and there is a big movement

between these two planes

Fig. 1.2-6 Constrainer installed above the plane of the load cells

2) if the plane of the constrainers is deformed.

Fig. 1.2-7 Plane of the constrainers is deformed

This can happen only if the object is deformed under the horizontal forces.⇒ design error

Conclusion:All the effects, described above, normally must not occur. It is the task of the mechanical designer to assure thathis construction is strong and rigid enough to take vertical loads and horizontal forces. (If he fails, it is not theresponsibility of GLOBAL Weighing who can only be made responsible for the measuring properties of theweighing installation.)

1.2.3 Horizontal natural frequency and restoring forceChapter 1.1 explained that the object should be free to move in the horizontal direction if no constrainers areinstalled. This is realized by pivoting or rolling constructional elements, which make the weighing installation ahorizontally swinging system.

Fig. 1.2-8 Stable equilibrium of suspended objects

Suspended objects are in a stable equilibrium if their centre of gravity is below their suspension plane.In all stable cases the object can swing: the object is moving as indicated in fig. 1.2-9 and tries to return to itslowest position. The swinging object has a natural frequency of f0. If the object is moved out of its centre positionover a distance of x, a restoring force FR tries to move it back to that centre position. These properties dependon the dimensions of the construction.

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Fig. 1.2-9 Restoring force of a suspended object- a: pendulum length- x: deflection

The horizontal deflection x is counteracted by a horizontal force FR which can be calculated with the equation

ax

g m g m = F R ••≈•• αsin

The object swings with a natural frequency f0

ag

2

1 = f 0 •

•π

The shorter the pendulum length a, the higher the natural frequency, i.e. the better the stability of theinstallation. The 'pendulum length' is the distance between the upper and the lower pivoting point in case of twoor more tension cells.

pendulum length [mm] natural frequency [1/s]

150 1.30

200 1.10

500 0.70

1000 0.50

2000 0.35

5000 0.23

Example:With a pendulum length of a = 500 mm a force of FR is necessary to bring an object of 3t 12 mm out of itscentre position.

N 720 = s

m 9.81 kg 3000

mm 500mm 12

= F 2R ••

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Supported objects (like compression load cells) can also work in a stable or unstable region.

Fig. 1.2-10 Different types of equilibrium with supported load cells

Fig. 1.2-11 Restoring force for a compression load cell

A horizontal deflection is counteracted by a restoring force FR

aa - R

ax

g m = Fb

R •••

Example: PR 6201Datasheet states FR = 0.5% • FL per mm deflectionThis means for an installation with three load cells FR = 250 N/mm

The object swings with a natural frequency f0 in the horizontal direction

ag

a

a - R 2

1 = f b

0 •••π

REMARKS1. There is a general relationship between natural frequency and restoring force FR

F g

x f F G

2 0

R ••

≈

So the conclusion is that this force is smaller if the natural frequency is lower.

2. The object only returns to the centre position if the restoring force is bigger than the friction forces.

3. Constraining could be necessary if the disturbing horizontal forces are bigger than the restoring force.

4. With compression load cells the restoring force is bigger for a larger radius of the spherical segment. Thishad to be limited because a too big restoring force could lead to a not- allowed side force on the load cell.

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5. Under load the radius of the spherical segment is increased by deformation. This could lead to a biggerrestoring force than specified in the datasheets.

6. In cases where the object can freely swing this moving can be used to check:- are there force shunts present?- is the friction small enough in the pivoting points

7. Swinging could be limited by ‘stops’ provided there is a stable equilibrium for the weighing system, andlow friction in the pivoting points, and no contact between stops and weighing system during operation.

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1.3 General recommendations on the design of an electronic weighing installation

The design of an electronic weighing installation covers two aspects:1. the mechanical design influences the accuracy of the weighing2. the cabling influences the weighing

This chapters deals with both aspects.

Using computers for the calculation of the stress applied to the constructions results in smaller machinery andlighter buildings if you compare them to constructions which were designed without the help of computers. Despitethis fact the power of the machinery and the safety of the buildings remain unchanged or are even increased. As ageneral rule you could say that the strength of the materials is used in a higher degree. This means:

- the cost for the construction is lower- in case of an emergency, the constructions have a lower safety factor

1.3.1 Design criteriaThe following statement is taken from one load cell operating manual:

‘The foundation and the steelwork for a weighing installation must be stable against the maximal expected load.It must be rigid enough to allow for an accurate measurement. Furthermore the foundation must be horizontaland plain below the load cells.The weight direction has to be vertical through the load cell as accurate as possible.’

These sentences and the like can be read throughout all our documentations for load cells and accessories. Thischapter is to explain the meaning of these words.

The design of machinery and buildings is done due to specific design concepts based on national and internationalregulations. All national standards for civil engineering require a construction to be resistant against the ultimateload.

Definition ULTIMATE LOADThe design due to the ultimate load ensures that a machine or building does not break because of• fast burst• fatigue fracture• non permissible deflections• instability

Most (inter)national standards refer to this design concept, e.g. ISO 18800. From the weighing point of view thisconcept is only a basic demand for each weighing system. The usual demand for a weighing system consists inanother concept, called the ‘security from malfunction’ concept. To put it in other words, the machinery or a wholefactory has to work all day long and to ensure the quality of their products. Furthermore, all dangerous andundesirable states of the production process have to be avoided such as standstill because of broken parts.During the last years a lot of different concepts has been developed:

• design to ensure the safety of the workers• design so that the environment is affected as less as possible

These considerations are quite general and abstract. They only inform you about the basic design concepts. Themost important concept from the weighing point of view is the concept of ‘security from malfunction’: always ensureaccurate weighing by proper design. The design rules given in this chapter are based on that concept.

1.3.2 Stiff and rigid foundationDefinition RIGID foundation

A rigid foundation does not deflect under load.

The word RIGID describes a mechanical model since every real object loaded by a force deflects under it. Thereare differences in the degree of deflection. The recommendations are meant to assist you to ensure a design withthe least deflection .

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The first important facility for a weighing system is a weighing frame (cp. fig. 1.3-1). It mechanically connects allload cells under a vessel etc. and ensures that they cannot move into different directions independently from eachother. Since the load cells only transfer vertical compression forces the design has to ensure that in case of externalforces the load cells cannot tilt. The constrainers take this task although they allow the load cell to move in case ofthermal expansion.

Fig. 1.3-1 Standard installation with two frames

Where should the load cells be placed? Fig. 1.3-2 shows a first solution: the load cells are placed in the middle ofthe beams in the steelwork.

Fig. 1.3-2 Solution 1: Load cells in the middle of long beams (not the best solution)

Solution 1 (see fig. 1.3-2 above)As long beams are elastic they deflect under the load. Perhaps the load distribution is not even or the beams

have different stiffnesses or ... This could result in clamped constrainers or tilted load cells or in a wagglinginstallation with two load cells taking 90% of the whole load and the other two only 10%... The chance to run intoproblems with such a design is quite high.

Fig. 1.3-3 Solution 2: Load cells placed on short beams (preferred installation)

Solution 2 (see fig. 1.3-3 above)The load cells are placed on additional short beams. As the deflection rises with the third power of the beam

length they do not deflection so much in this solution. An uneven load distribution is easier to compensate byshimming.

For this reason solution 2 is to be preferred.

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But not only long beams can disturb your measurements because of their large deflections. Another source fortroubles are weak floors. Avoid to place a weighing system on a floor which is quite weak. Try to find anothersolution by supporting the system from another floor below. (see fig. 1.3-4)

Fig. 1.3-4 How to support a weighing system on the first floor

A weighing system installed in an upper story of a building is influenced by a lot of different factors:1. wind, wind induced movements, and vibrations of the building2. machinery in a lower story can cause the building to vibrate at low frequencies3. stiffness of the floor and the walls

If possible such installations should be avoided.

Fig. 1.3-5 Avoid weighing systems in upper stories

Vessels supported by beams, which are not of equal stiffness, can tilt under the load. Therefore such an installationshould be avoided.

Fig. 1.3-6 Vessel supported by beams of different stiffness

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Another mistake in the design that is quite common is to interconnect several installations by putting them onto thesame steelwork. Then the measuring result of one installation depends on the level inside the other vessel. For thisreason, every vessel is recommended to be placed on a separate foundation without any interconnection to others.Fig. 1.3-7 suggests a design.

Fig. 1.3-7 Every vessel stands independently

There are a lot of ways to interconnect several vessels so that only a small number of wrong installations can beshown here. A common mistake is to place two vessels, which are loaded and unloaded separately, on only onesupporting beam without additional columns in between.

Fig. 1.3-8 Vessels interconnected through one soft beam (WRONG)

Fig. 1.3-9 Avoid installation with a common supporting beam (WRONG)

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Fig. 1.3-10 Insufficient supporting columns (WRONG)

Stiffening the steelworkSometimes the systems is installed and does not work properly. The user asks our help and assistance to improvehis installation. Usually, the steelworks can be improved by stiffening. To choose the right way of stiffening somedetails must be thought about:

- how to stiffen the construction- where to place the stiffening beams

In general there is a huge variety of methods to stiffen a construction: different shapes of beams, many possibilitieshow to place them ... This paragraph suggests two methods which have proven useful.

Method 1: K-shape framework (fig. 1.3-11)

Fig. 1.3-11 K shape framework (preferred)

The K-shape framework has several advantages:- you can use rather small beams and achieve a high stiffness- the design allows traffic to pass under it- the design can be done in such a way that the beam supports directly the points where the load is induced

Method 2: one single beam (fig. 1.3.12)

Fig. 1.3-12 Single beam stiffening (not recommended)

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This method is quite easy because only one beam must be welded to the construction. However, there are certaindisadvantages:

- rather strong and heavy beams are needed- usually no traffic can pass below the construction- the buckling effect in the stiffening beam has to be observed

Summary:The platform, the floor and the construction below the weighing object must be stiff and rigid to take all loads• compression or tension load (weight to be measured)• horizontal load (thermal expansion, displacement etc.)

The next paragraphs describe some common error sources which are related to the design.Possible influences on the installation come from• moving vehicles and people on platforms which cause deflections and vibrations• motors on a weak platform or floor which transmits the induced vibrations• heavy machinery on the same platform which causes static deflections

1.3.3 Traffic

Fig. 1.3-13 Traffic beside a vessel Fig. 1.3-14 Traffic above a suspended vessel

You may say that this point is included in the chapter on the stiff floor. You are right. However, sometimes it is easierto identify an error when you have an idea what to look for. If there are failures in a weighing system, which onlycome up from time to time, the reason 'traffic' must be considered.

1.3.4 Installation of vesselsSome general recommendations · Normally the load cell may be operated only up to temperatures up to 95°C (for exceptions see the relevant

data sheets). Higher temperatures can destroy the cell. If the temperature in the weighing object or in theenvironments exceeds the a.m. value, the load cell should be protected:against conduction by heat protection plates and boxesagainst radiation by shields, screens

· To achieve the highest measuring accuracy the load cell must be protected from high temperature changes.The allowed rate is 5K/h for W&M load cells and15K/h for industrial types.Such effects may arise if you find wandering shadows etc.

· The weighing object must be designed that wind is of no significance. Use lift-off-protections

· The load cells must be protected from impact loads which are exerted by falling material or forklifters drivingagainst the weighing object. The load cells can be protected by overload protections, like rubber springs

· The weighing object must be free from vertical force shunts. The whole installation, like pipes and electricalcables, must be connected so flexible, that they do not exert a force on the weighing object.

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· No friction between weighing object and wall should disturb the weighing. Therefore the weighing object muststand free.

· load distributionthe weighing installation is equipped normally with three or four load cells

3 load cells- all cells bear the load (if possible: even load distribution)

4 load cells- higher stability than an installation with 3 load cells- uneven load distribution (needs shimming in almost every case)

· use the adequate mounting kitsthe load cells must stay in vertical position (use spirit level)

Installation drawing

A drawing of the mechanical mechanical part of the installation with a clear view of:- load cell mounting- constraining- piping

shall be made to be able to judge the measuring properties.

The wall of the vessel must be so stiff that there is no bending under load.

Fig. 1.3-15 Vessel with a stiff wall

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The construction of the weighing object with its surroundings must allow easy access to the process without in-fluencing it. Imagine a viewport in a vessel which can only be reached by ascending a ladder. The construction mustensure that this ladder is supported only by the surroundings of the weighing installation but not by the weighingobject itself.

Fig. 1.3-16 Observing the process

In principle a weighed object has to be free from its surroundings to achieve an accurate measurement. However, inindustrial applications there are often links between the object and the “outer world”. Examples are

- pipes and tubes- pneumatic and hydraulic hoses- electric cables- bellows and slabs- dirt and stones in a gap between object and its surroundings

In general these “links” have the following three effects:1. An undefined part of the weight of the link acts as a disturbing force on the object. Mostly this is a constant

force, which can be treated as a part of the dead weight. However, sometimes it can be variable, e.g. in thecase of an electric cable with a changing position or in the case of a pipe with changing contents. If this forceis not constant, it can introduce a non-reproducibility error.

2. The stiffness effect of the link.In principle this can give a span error and a zero error. The effect is described and calculated in detail inchapter 3.4

3. The friction effect of the linkThis leads to an undefined zero-error (non-reproducibility and hysteresis effect). The friction effect can becaused by friction in pivoting points, by friction of parts pressed against the object, or by internal friction in thematerial of hoses and the like.

Fig. 1.3-17 Principle of installation for pipes

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Fig. 1.3-18 Avoid the influence of storage silos

1.3.5 Load cell selectionAn industrial vessel can be put on load cells or be suspended by tension cells or load beams. The sketch belowgives a rough idea of the application areas of important GLOBAL Weighing load cell types. The load cells havenominal capacities between 10 kg and 300 t.

Fig. 1.3-19 Programme overview

The first step during the design phase is to decide if the weighing object should be suspended or stand upright. Asuspended design usually offers a higher stability for the installation whereas an upright standing vessel can moreeasily be added to an existing concept.

The second step is to decide how many load cells should be used. When choosing the number of load cells for yourweighing installation keep the following arguments in mind: - Less supporting points require that strength and stiffness of both the construction and the foundation must be

increased - If more than 3 load cells are used, the support is statically undefined. This fact makes it necessary to shim the

load cells in such a way that the load distribution becomes even (cp. chapter 1.2). - If an object is suspended by tension cells or load beams and the centre of gravity is below the supporting points,

it is sometimes possible to use less than 3 constrainers.

The third step is to choose the nominal load Ln of the applied load cell. The following aspects should be considered: - All load cells supporting the object have to be of the same nominal load. This is necessary for proper summing

up all loads by the parallel circuiting of the load cells. - The maximum actual load at the supporting points is calculated under

1) extreme conditions (e.g. storm, overloading)2) normal measuring conditions

These load values are compared with the 'load limits' in the load cell specification.

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→ the extreme load may never exceed the limiting load Ll

→ if possible choose the nominal load Ln higher than the maximum load under measuring conditions in order tobe sure of all guaranteed load cell properties.

The fourth step is the choice of the accuracy class. The purpose of the weighing installation defines the accuracywith which the measurements must be done. If the installation has to be approved by the local W&M authorities, youhave to choose a load cell type which already got its test certificate. In other cases, when you have to fulfil a more or

less clear accuracy desire of the customer, you must choose based on your experience. You could try to improveyour own intuition on this subject by studying this load cell application manual. (refer to chapter 1.7)

Selection in short

- choose Ln bigger than the maximum load during measuring- check, if disturbing forces never give a bigger load than Lu

- choose the load cell type for the desired accuracy- check, if the load cell output voltage is sufficient for the desired net scale

Selection example Vessel on three load cells - technical data

dead load 2.8tnet weight 7.0tgross weight 9.8t

- with even load distribution maximum load per load cell is 3.3t- extreme wind causes an extra load of 0.8t. So extreme maximum load is 4.1t.

([ lower than Lu)- total nominal load is 3 • 5t = 15t

With the used PR 1592 indicator the minimum scale span is 20% so 0.2 • 15t = 3t.([ this is lower than the net weight)

- for batching without W&M requirements we choose PR 6201/53D1

As you see, for normal cases the selection is very easy...

Fig. 1.3-20 load limits (according to VDE/VDI 2637)

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1.3.5.2 Installation of the mounting kitsAll load cells can easily be installed by using the specially designed mounting kits. The correct adaptation to aconstruction is shown in fig. 1.3-21. Especially supporting steel beams must be stiffened as shown. Furthermore theconstrainer must be mounted in longitudinal direction of the steel beam: the side forces are only transferred in thisorientation.

Fig. 1.3-21 Gussets stiffen the supporting I beam

1.3.5.3 Protection of load cells from high temperatureThe first hint concerns the environmental temperature and heat sources next to the load cell. Most load cells canonly be operated up to 95°C (exception: PR 6211LT may be operated up to 155°C). If higher temperatures areexpected and load cells like PR 6201 are used, they must be protected from being overheated. Fig. 1.3-21 givessome advice. The heat protection shield is made of sheet metal and protects the load cell from heat radiation (directsunlight, hot oven etc.). A heat protection plate is often made of ceramics and protects the load cell from a heat flowcaused by a temperature difference.

Fig. 1.3-22 Different heat protection devices

1.3.5.4 Protection from overloadAnother complication may arise from falling loads. The load cell specification contains the value Lu, which tells thenominal load. Above this limit a zero shift may occur. The value Ll is the limiting load. Above this value there couldoccur damage to all measuring properties.

When selecting a load cell care must be taken that the nominal load of the load cell is big enough to preventoverloading in the weighing installation. Sometimes, however, this is not possible. E.g. if a very small measuringrange, compared to possibly occurring big forces, has to be used. In such cases an overload protection can avoiddamage to the load cell.

Some load cells, e.g. PR 6211D1 with a capacity of 300kg or lower, have an overload protection already built-in. Ifthe load cell is loaded by a weight bigger than 1.5 Ln the measuring element touches the stop and cannot deflectfurther. This avoids the damage of the load cell. Fig. 1.3-23 shows the behaviour of this internal overload protection.

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Fig. 1.3-23 Load cell PR 6211 with built-in overload protection

In figure 1.3-24 the principle of an external protection of a load cell against overload is shown.

Fig. 1.3-24 Static overload protection with a prestressed spring

The device of figure 1.3-24 makes use of a prestressed spring, which causes that under normal conditions (withoutoverload) the platform is interconnected rigidly with the load cell. This is the big advantage of such a construction.However, the disadvantage is that it does not give a protection against dynamic overload. This is explained inchapter 1.3.5.5.

Figure 1.3-25 gives the principle of a protection device with a free spring.

Fig. 1.3-25 Principle of an overload protection with a spring

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With this construction there are some problems to take care of: - the platform is no longer a rigid element, but easily swings up and down and even sideways - if the spring is tall, this horizontal movement makes the load cell position unstable (the load cell can capsize)- with some spring materials the deflection under a constant load increases with time. In these cases it is difficult to

adjust the clearance between the stop and the platform. One should be sure that the platform touches the stopunder all conditions at say 140% of Ln and it remains free under normal weighing conditions say up to 100% of Ln.

Fig. 1.3-26 Devices for overload protection

Figure 1.3-26 suggests some spring elements for static overload protection. A rubber element is usually suitable upto capacities of 2t; the capacity of the element which consists of cup springs depends on the type of the cupsprings.

1.3.5.5 Protection from dynamic overloadIf the load on a weighing installation is slowly changing, we speak of (quasi-)static loading. In that case we can takemeasures against overload as described above (chapter 1.3.5.4). However, if the load increases suddenly (e.g. byfalling goods), the loading is called impact loading. This could damage thew load cells. The reason for this is thatthe very short impact loading causes a shock wave travelling through the platform to the interior of the load cell. Thiscan even happen if there are measures taken against static overloading, because the impact time interval is tooshort for the protection springs to come into action. Therefore we have to take care that the shock wave cannottravel through the protection springs.

Fig. 1.3-27 Falling mass

In order to get a better understanding of the dynamic forces, we take a simple model and try to calculate itsbehaviour under dynamic loading. A mass m is falling from a height h on the platform (with mass M). We assumethat the point of contact can be represented by a spring with a constant kC (see fig. 1.3-27). At the moment ofcollision the mass has the velocity v0

hgv ⋅⋅= 20

Then the mass becomes part of the vibrating system (consisting of mass m, platform mass M, and platform stiffnesskC) and makes a vibration movement of half a sine during half a period before it loses contact with the platform. Thenatural frequency of the vibrating system be ω0 and the time interval be ∆t.

00 ,

ωπω =∆

+= t

Mm

kc

To make a practical estimation of the time interval, we can bring the formula in the following form

hg

x

v

xt

⋅⋅⋅=⋅=∆

20

0

0 πω

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ExampleFor a height of h = 0.1 m and a deflection x0 = 1 mm you get ∆t = 1.6 ms.This value could be a good estimate for the impact time interval.

In that very short time the springs under the platform must work to keep the impact away from the load cells. Theprotection devices described above deflect during that impact interval a distance x. The device with pre-stressedspring (fig. 1.3-24) with an excess load of e.g. 0.3 Ln and a platform mass M = 0.4 Ln the platform is accelerated by

25.7

4.0

3.0

s

mga =⋅=

That results in a platform travel of

mmtax 015.0)(2

1 2 =∆⋅⋅=

The same platform load means a load of 1.5 Ln on the device with the free spring (fig. 1.3-25). With the sameplatform mass the acceleration is calculated

25.37

4.0

5.1

s

mga =⋅=

And the platform travel

mmtax 075.0)(2

1 2 =∆⋅⋅=

The platform travel in both cases is so small that the protection springs never come into action during the impactinterval. The energy of the impact load is transported with sound velocity through the platform, the springs, and theload cells. This wave of mechanical energy means local deformation, everywhere the wave passes, with thepossibility of local destructive effects.

As you see, the devices protecting from static overload do not protect from dynamic overloading. Some designadvice should help you to avoid such situations

· Make the falling height as small as possible. This minimizes the velocity at the collision point.

· Make the distance between the position of the impact and the load cells as big as possible. The shock waveloses energy by damping effects inside the material before it reaches the load cells.

· Put a layer of soft material, e.g. wood or rubber, on top of the platform where the impact takes place. Thisincreases the impact time interval ∆t, because of the lower kC value. The result will be a smaller value of theimpact force F0.

· Put a rubber spring between platform and load cell in the path of the shock wave. The basic idea is that awave cannot easily propagate from one medium to another if there is a big difference between the acousticalimpedances. The acoustical impedance of a material can be found from

Ez ⋅=∞ ρρ specific mass of the materialE Young's modulus

some valueszac = 3.9 • 107 Ns/m3 steelzac = 6.7 • 104 Ns/m3 rubber

On the border between two media a big part γ of the wave is reflected. The reflection coefficient can becalculated by

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2

21

21 )(zz

zz

+−

=γ

Example: reflection coefficient calculationz1 = 3.9 • 107 Ns/m3 steelz2 = 6.7 • 104 Ns/m3 rubber

9932.0,)1

21( 2

2

1

≈−

−= γγ

z

z

1.3.6 Measures against electrical damageGeneral recommendations - The cable of the load cells must be paralleled in a cable junction box. Avoid the penetration of moisture into thecable.

- Protect the complete weighing object against overvoltage; otherwise the load cells could be damaged bylightning etc.

- The complete cabling must be screened. The screen must only be connected to the ground at one single point toavoid erroneous currents.

- All electric arc welding next to the weighing object has to be done as carefully as possible so that the load cellscan not be damaged.

- Avoid the installation of power cables next to the measuring cables (min distance 1m).

If a weighing object is not protected from overvoltage, e.g. resulting from electric welding or lightning strikes in theneighbourhood, heavy currents could flow through the weighing structure and in particular through the load cells.Apart from magnetic or static induction effects in the cabling between the load cells and the measuring instrumentsuch an event can damage the load cells. Two effects can mainly cause the damage

Voltage effectThe potential difference between load cell body and ground gets high, because of the current flowing through thestructure. The strain gauge filament, however, remains on ground potential, because it is directly connected tothe measuring instrument. The high voltage difference between filament and billet material can destroy theinsulation layer. Such a load cell no longer operates.

Current effectEspecially for the possible very short pulses of flowing current, the skin effect can cause a very high currentdensity through the surface layer of the billet where the strain gauges are located.In practice we encountered completely burned and even evaporated strain gauge filaments, perhaps due to thiseffect.

If there is any chance of heavy currents in the weighing structure the following measures have to be taken: - The earthing screw of the load cell should be connected electrically to the central earthing junction, where the

measuring instrument is also earthed (≥ 6mm² Cu).

- Possible electrical resistances at the load cell contact points with object and foundation should be shunted. Usethe flexible copper strap, delivered with the load cells for interconnecting upper loading plate and foundationplate.ATTENTION: the strap should be installed as close to the load cell as possible, but the strap must not transfermechanical forces to the load cell

- Electric welding in the neighbourhood of the load cells is not allowed if the load cell measuring cable is alreadyconnected to the measuring instrument.

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1.3.6.1 EarthingEarthing of an electronic weighing installation serves three purposes: safety, prevention of interference, anddamage prevention.

1. safetyAll electrical equipment connected to the mains must not cause a danger of life if being touched. The legalregulations are to observed.

2. InterferenceCapacitive coupling of the outer world to the measuring circuit can disturb the measurement. This can be avoidedwith cable screens which have to be at the same potential as the measuring circuit. Therefore the followingmeasures are to be taken - provide armoured steel conduits for the cabling - least distance between power cables and measuring cables 1m.Mind that all screens have to be earthed at one point only, to avoid that otherwise stray currents still could changetheir potential.

3. Damage preventionIt is a very dangerous situation if the strain gauge filament has a too big voltage difference to the load cell body. Forthat reason the load cells have an earthing screw which can be interconnected with the ‘central earth rail’. Heavystray currents can be expected if e.g. the weighed object is situated outdoors or at a big distance from the electronicmeasuring equipment (weighbridge, big bunkers, etc.) In those cases you have to take the following practicalmeasures:

a) An ‘earthing tube’ or ‘earthing plate’ with an earth resistance of <5Ω should be put into the ground in theneighbourhood of the weighed object.b) This earthing electrode has to be connected with the 'central earth rail', mounted in the neighbourhood of thecable junction box.c) now make the following connections:

(the thick lines in the wiring diagram)- all load cell earthing screws with the central earth rail with conductors of a t least 6mm2 copper- the ‘measuring earth terminal’ of the electronic measuring instruments with the central earth rail

(equalizing line)NOTES:• Mostly 16mm2 is enough. You can make a rough check, if you can estimate the possible current

flowing through the equalizing line. The voltage drop may not exceed 100V, which is the maximumallowed voltage between strain gauge filament and load cell body to avoid electrical damage.

• The measuring cable PR 6135 is mounted in a steel pipe. This pipe could be used as a shuntresistance to the equalizing line to lower its resistance.Attention: Never use cable screenings for this purpose!

Fig. 1.3-28 Earthing of a weighing installation

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1.3.6.2 Measures against lightningThere are different methods to safeguard an electrical installation against lightning strikes. The most simplesolution for our weighing equipment is the installation of surge arrestors (cp. fig. 1.3-29)

Fig. 1.3-29 Cable junction box with overvoltage protection

1.3.6.3 Measures against damage caused by weldingWelding with load cells in position: During the construction phase of a plant it is recommended to install load celldummies in order not to damage the load cells by welding or by wrong handling. If after the installation of the loadcells some welding must be done please follow the instructions below:

1. disconnect the load cells from the electronics

2. position mass connection and welded joint at one side of the load cell as shown in the sketchvalues for the parameters a and b are given below under the headline ‘welding during operation’ since theyare only necessary in this case

3. after these preparations welding may be done

Fig. 1.3-30 Welding near an installed load cell

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In some plants welding takes place during operation from time to time. Therefore, it could be impossible todisconnect the load cells. Always keep in mind that welding near load cells in operation can cause damage. Thereare no guarantees that they work properly after the welding. If it seems to be absolutely necessary to weld duringoperation keep the following distances:

a > 600mm, b < 600mma > 3 • b

Fig. 1.3-31 No welding in this situation

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1.4 ConstrainingA weighing object is mounted on load cells. They transfer the vertical load but no horizontal forces at allbecause of their design principle (articulated column). In order to avoid influences from external horizontalforces on the measurement additional safety devices must be installed to fix the position of the weighing object.We find different types of horizontal forces: - constant loads, quasi constant loads (e.g. thermal expansion) - vibration (motors, stirrers, vibration feeders...) - shock loading, impactsThere exist two different types of devices for taking the disturbing horizontal forces:

ConstrainersConstrainers are used to eliminate horizontal loads constantly. They even operate during the weighing.Constrainers do not permit any movement of the weighing object in the constrained direction. Theycompletely take the load.

StopsStops only limit the possible movement of the weighing object. For this reason their adjustment is critical.They are used to prevent a weighing object from movements which can damage it. During weighing theymust not touch the weighing object to avoid force shunts.

If such a device either constrainer or stop shows internal ‘clearance’ (space for motion), horizontal forces andimpact loads can cause a horizontal object velocity. This means kinetic energy to the object. At the end of thefree object travel, this kinetic energy causes a deformation of the weakest points between the object and thefoundation. This ‘hammering effect’ can be very destructive to some parts of the construction, e.g. if it causespermanent deformations or even fracture at these points.

1.4.1 Using constrainersFundamentally an object has 6 degrees of freedom: 3 degrees of translation, 3 degrees of rotation. Theinstallation must be designed in such a way that load cells and constrainers together eliminate all the degrees offreedom.

1. load cells and 5 constrainers (see fig. 1.4-1, left sketch)This is the theoretical solution: one degree is eliminated by the load cells, the others are taken by theconstrainers. Complications can arise if thermal expansions disturb the object. The expansion causesadditional forces on the load cells. Additionally it is often not practicable to use 5 constrainers because of thelack of fixing points. Furthermore it is quite difficult to adjust the constrainers correctly without clamping.

Fig. 1.4-1 Different arrangements of load cells, constrainers and lift-off-protections

2. load cells and 3 constrainers and 2 lift-off-protections (see fig. 1.4-1, middle)This is another theoretical solution: it is based upon the fact that the load cells take only one direction of avertical load (downwards). The direction vertical upwards is taken by the lift-off-protection.

3. load cells and 3 constrainers (see fig. 1.4-1, right sketch)In quiet installations it is sometimes possible to omit the devices which take rotations around the horizontalaxis.

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Some installations do not require all suggested constrainers, e.g. - if the object is quiet (no permanent vibrations, only small horizontal forces ...) 2 constrainers can even be

omitted - constrainers can limit the resolution of the installed weighing equipment- with objects, suspended by tension cells or by load beams, the centre of gravity is lower than the point of

suspension, which causes a stable equilibrium.

Fig. 1.4-2 Standard installation diagrams

4. using more constrainers than necessaryThis can cause big forces in the constrainers because the system is statically undefined (overdetermined).More constrainers than necessary can cause undesired big axial forces in all constrainers. This happens ifnot all parts of the construction have the same temperature, or if the fixation points of the constrainers to thefoundation make a very small movement by the same reason. As an illustration of this effect we will look to aweighing platform with four constrainers instead of the necessary three ones (fig 1.4-3)

Constrainers like pivoting rods or flexbeams are in fact rods which will change in length a little bit under theinfluence of an axial force. They behave like springs with a very big axial stiffness. We assume that thelongitudinal constrainers have an axial stiffness kB and the two lateral constrainers kA. Further we assumethat the platform is completely rigid. By some effect the fixation point of the constrainer at point ‘1' movesover a distance ∆l. The problem is now to calculate the forces FA and FB which will work on the platform (andtheir reactions in the constrainers respectively). Because of the equilibrium of the object the followingconclusions are made:- sum of the forces in y- direction =0: forces FB have the same value- sum of forces in x- direction =0: forces FA have the same value- sum of momenta = 0: FA × a = FB × b

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Fig. 1.4-3 Weighing platform with 4 constrainers

The values of the displacements can easily be obtained:

kF + = x

kF - = x

A

A2

A

A3

The displacements x2 and x3 cause a rotation of the object over angle α

a

b

kF 2 =

ax - x =

2A

A32 ••α

This rotation causes a displacement in the y- direction of point ‘1' relative to point ‘2' of b•a. The objecttranslates over y2.

kF + = y

B

B2

In total the displacement of point ‘1' is

))ab

( k

2 +

k

1( F = b + y = y 2

ABB21 •••α

The spring at point ‘1' is compressed over a distance ∆l - y1. This results in a spring force)y - l( k = F 1BB ∆•

Combination of the last two equations gives

))ab

( kk + (1 2

l k = F2

A

B

BB

••

ƥ

FA can be found now with the momentum equilibrium

F ab

= F BA •

A practical exampleA weighbridge is constrained with four flexbeams. The stiffnesses were calculated

kA = 3.0 • 105 N/mmkB = 3.6 • 105 N/mm

Further isa = 6,000 mm, b = 1,500 mml = 1,100 mm (constrainer length)

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If the constrainer at point ‘1' has a temperature difference of 1°C with the other longitudinal constrainer, thenthe dilatation is about

mm10 1.3 = 1100mm 10 1.2 = l -2-5 •••∆

The force in the constrainers isFB ≈ 220 kgFA ≈ 60 kg

In most cases the influences of the lateral constrainers can be neglected as you can see from the calculationabove. Therefore you can simplify the above equation

l k 21

F BB ∆••≈

ConclusionAlready small temperature differences can cause big forces in the constrainers. Therefore with too manyconstrainers much care should be given to their horizontal position. 1° non- horizontallity already causes avertical force shunt of about 1.7 per cent of the axial force (that is in the example 4kg zero error for oneconstrainer). Only a relatively non-rigid platform construction could decrease the disturbing effect, but this is notvery open to a serious calculation and you cannot reckon on this!

“Feeling the constrainers“In the chapter 1.5 you find some possible causes for external forces, which can act on the constrainers.However it is interesting to know what axial constrainer forces can be expected if there are no external forces.This is especially important if you want to judge by „feeling the constrainers“ of the proper functioning of theweighing installation. “Feeling the constrainers“ results, e.g. with pivoting rods or rocking pins, in a roughestimate.

We consider several mounting conditions and ask for their influences:a. standard conditions without external forces

These conditions are- base plates and upper loading plates horizontally mounted- primary axis of load cell verticalIn this case, only the effect of the friction in the pivoting point at the load cell top can cause an axialconstrainer force of at maximum ±1% of the weight of the object.(N.B. only for PR 6201/54 2%)

b. effect of non-horizontal platesOnly about 0.1%/degree of the vertical load on the load cell

c. effect of non vertical load cellFor PR 6201 there is a the restoring force of nearly 1%/degree.(about 6%/degree for PR 6201/54)

d. effect of non-alignment of the load cells in a horizontal planeno effect at all

The main conclusion from that points is:If you feel that the constrainers are clamped and a possible foundation vibration does not influence thefriction effect, then there is something wrong in the installation.

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Some possible error sources- bad constrainer mounting

rocking pins mounted without enough clearancerocking pins mounted with too much distance between the two pins of a pairthe constrainers are mounted in a bad pattern (at least two constrainers in one line, three or moreconstrainers pointing to the same point)

- unsuspected external side forcea stone or other part is clamped between the object and its surroundingsa rigid pipe connection is expanded by temperaturean internal pressure inside bellows produces a side force

- dirt or wear at the load cell contact surfacesgreasing these places could prevent from increasing the frictionflattening of the spherical segment, or impression in the base plate, or dirt can be the causes for anincreasing restoring force.

Influence of constrainers on the resolutionIn order to avoid vertical force shunts the adequate constrainer must be chosen for each installation. The con-strainer reaction in vertical direction must be as small as possible. For this reason, the vertical stiffness ky of theconstrainer is limited. You can calculate it by comparing it with the deflection either of the load cell or of theconstruction. This depends on the mounting position of the constrainer.

A z

MR

h

g k

ny ••≤

explanation of the variablesky permissible vertical stiffness of the constrainerg gravitational field strength (» 9.81 m/s2)hn static deflection under load

1. for load cell » 0.5mm2. for the construction: value given by manufacturer

MR measuring range1. number of load cells x capacity of single load cell (in kg)2. real measuring range

z number of constrainersA constrainer influence on measurement

(≈ 10-4 = 0.01%)

Examples1. 4 load cells, 50t, 3 constrainers, fst = 0.5mm

ky ≤ 131 N/mm2. weighbridge 60t, 1 constrainer connected to the centre of a weighing platform, deflection 3mm

ky ≤ 20 N/mm

1.4.2 Constraining a suspended objectGenerally there are no special rules for constraining a suspended object. As however its centre of gravity isusually below the load cells such installations are more easily stable than those with compression cells.

Omitting constrainers with quiet suspended objectsWith quiet suspended objects constraining seems not always necessary. However to prevent undesired torqueon the tension cells, rotation around a vertical axis should be limited. This can be done with two constrainers(see fig. 1.4-4a). If the pendulum length is short, the fixation point at the object is a quasi fixed ‘stable point' (fig.1.4-4b). Then only one constrainer can be sufficient in case of the object suspended by one tension cell. If twoor more tension cells with small ‘pendulum length’ are used no constrainers are needed to limit the rotation (fig.1.4-4c). The larger the distance d the better the constraining against rotation around a vertical axis.

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Fig. 1.4-4 How to avoid torque on load cells

Pendulum length and restoring momentum (rotation around a vertical axis)The principle description of the restoring force is given in chapter 1.2. This paragraph concentrates on theinteraction between restoring force FH and constraining. The restoring force FH depends on the horizontalmovement x of the installation and the pendulum length l.

F lx

= F GH •

For the calculation it is necessary to know the connection between horizontal movement and rotation around thefixation point of the constrainer. This is described in fig. 1.4-5.

1.4-5 Geometrical conditionsThe object rotation is

dx

= θ

Calculating the restoring force FR and the restoring moment MR results in

F l

d = M ,F ld

= F G

2

HGH •••• θθ

The object rotation is in both cases counteracted by a restoring momentum, which is bigger for bigger values ofd2/l. It depends on the external momenta acting on the object and the possible free torsional movement(‘clearance’) which is in the tension cell suspension if a certain value of d2/l is allowed.

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1.4.3 Orientation of the constrainersThe mounting direction of constraining rods must be carefully observed. The rod takes high forces and transmitsthem into the steelwork. Fig.1.4-6 shows the right way of installation: the steelworks only take tension andcompression forces.

Fig. 1.4-6 Correct constrainer orientation (constrainer and steel beam parallel)

Fig. 1.4-7 shows a difficult way to install: the steelworks take momenta which act around the weak axes of an I-beam. For a simple silo without any devices like motors, stirrers etc. this installation does not normally causedifficulties. But if there are active devices like motors, stirrers etc. you may run into problems because the smallmovements act like forced oscillations. In this case the I-beams take side loads. They operate like torsionalsprings: they save and restore the kinetic energy via the constraining rod.

Fig. 1.4-7 Wrong constrainer installation

1.4.4 Types of constrainersConstraining can be done with several devices. They can be classified

- integrated devices for load cell mounting and constraining(e.g. MiniFLEXLOCK)

- special constraining devices(e.g. horizontal constrainers, rocking pin)

As a general rule you could say that the integrated devices are usually applied for all types of tank weighers.They have become the standard devices now. The special devices are used in truck scales or in case that theintegrated devices are not capable of taking the side forces. The special devices are described in more detail inthe following paragraphs.

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1.4.4.1 MiniFLEXLOCK (general overview)The mounting kit „MiniFLEXLOCK“ has been introduced by GWT as an integrated solution for mounting andconstraining in the beginning of the 80s. It is available for all load cell families. This mounting and constrainingdevice offers several advantages: • constraining and mounting combined in one single kit • constraining at the optimal point (the plane of the load cells) • avoiding problems which arise from bad constrainer installation • MiniFLEXLOCK is designed for every standard installation • easy exchange of load cells • available for every compression load cell family (Þ identical principle for installation and constraining) • different side loads possible • the different arrangements of the MiniFLEXLOCK are discussed in chapter 1.4.1

Further information about the different MiniFLEXLOCK is provided in chapter 2 where the important mountingkits are described.

1.4.4.2 Rigidly clamped strutsThe rigidly clamped strut is the worst of all constrainers because must be adjusted very exactly to avoid verticalforce shunts. The main reason for using a rigidly clamped strut is that there is no clearance which could result indamage or at least wear in case of shocks or vibration.

The strut is simply a long horizontal rod which is clamped rigidly at both ends. This construction can be madevery stiff in the horizontal direction but flexible enough in the vertical direction. However, there are some designproblems: - The vertical stiffness can be reduced by making the rod length as big as possible

This is, however, counteracted by the fact that the longer the rod is the smaller the permissible load is. (Forslender rods, the permissible axial compression force is limited by the collapsing effect.)

- During installation the clamping has to be done with big care, because misalignment would result in arelatively big vertical reaction force.

- An arbitrary deformation of object or foundation could cause misalignment thus leading to arbitrary zeroerrors.

For choosing the right strut dimensions it is important to realize that the effect of a rigidly clamped strut is thesame as that of a pipe. Therefore you must calculate the allowed vertical stiffness of the strut. (Chapter 1.4.1provides a formula.)

Where to fix the constrainers to the object? - Avoid to use these struts out of the plane of the load cells!

If these constrainers are placed out of this plane and e.g. thermal dilatation deforms the distance to thisplane, you get zero shifts. They are caused by the deflection of the constrainer which then gives a verticalreaction on the object.

- Mount the struts horizontally!Avoid pretension during mounting. Therefore, make the alignment easy with spherical rings at both ends asis shown in the fig. 1.4-8. Use spring washers to make sure that the connection is tightened rigidly.

Fig. 1.4-8 Recommended fixation of rigidly clamped struts

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To make the use of rigidly clamped struts easier, we calculated some standard struts. If there is a need forhigher capacities, you can calculated the dimensions with the standard methods of strength analysis.

Material steel, zinc plated, property class 8.8length 500mmdiameters M12, M16, M20x1.5, M30x2

For each rod you need the following accessories:4 sets of spherical rings (DIN 6319)4 nuts4 spring washers

strut 12 strut 16 strut 20 strut 30thread M12 M16 M20x1.5 M30x2vertical stiffness (N/mm) 18 61 173 893effect of misalignment (N/°) 141 479 1359 7014collapsing load (kN) 1.3 7 15 75permissible deflection (mm) 9 6 5 3

REMARKS - The values above are calculated for a clamped length of 450mm. For other lengths L these values have to be

multiplied by for vertical stiffness (450mm/L)3

for misalignment effect (450mm/L)2

for collapsing load (450mm/L)2

- These struts can be used for installations with 3 load cells with a nominal capacity of at leastLn strut

100kg strut 12500kg strut 16 1t strut 20 5t strut 30

ExampleA weighing object with 3 load cells is to be constrained. The user expects shock loading and chooses rigidlyclamped struts for this purpose.load cell type PR 6201/24D1side forces 10kNconstrainer length 750mm

1) collapsing loadstrut 30

kN 10 > kN 27 = )750mm450mm

( kN 75 2•

2) vertical stiffnessstrut30Stiffness of the strut

mmN

192 = )750mm450mm

( mmN

893 = c3

CONS •

Stiffness of the load cell

mmkN

1177 = 20,000kg 3 0.5mm

s

m 9.81

= c2

LC ••

If you compare constrainer stiffness cCONS and load cell stiffness cLC, you see that the constrainer stiffness isnegligible.

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3) effect of vertical misalignmentstrut30

°•

°N

2525 = )750mm450mm

( N

7014 = f 2

In this case a vertical misalignment of 1° would mean a vertical displacement of 13mm. Suchdisplacements can exist if the supporting construction is not rigid enough.

1.4.4.3 FlexbeamsIn cases where high constraining forces (more than 20kN) are expected in the weighing installation the rockingpin, which can withstand 200kN, is a possible solution. However, if shock loading occurs, this is one cause ofwear of the rocking pins. Examples for shock loading are truck and railway weighbridges. Here frequentlyvehicles are braking on the platform, causing sudden horizontal forces on the platform and the constrainers.Therefore, on these installations regular maintenance is necessary to readjust the clearance of the rocking pinsafter the wear of their contact surfaces. We got good experiences with a special constrainer type, which isdesigned to solve the problem of the horizontal shock loading.

Fig. 1.4-9 Flexbeam

A U-shaped steel beam ending in two flat leaf springs is clamped at both ends at A and B. This construction isstiff in the constraining X-direction and it is flexible in the Y-direction and around the Z-axis and the X-axis. Atfirst sight this looks to be a good constrainer, but it can for that purpose only function correctly if it is also flexiblein the Z-direction and around the Y-axis. The solution for this is that both clamping points are made of I-shapedsupports, which are flexible in the Z-direction and around the Y-axis.

Fig. 1.4-10 Characteristic dimensions of flexbeams

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The table below suggests some standard flexbeams so that you need not to calculate them each time you mustuse them.

Fm

[kN]beamUNP

L[mm]

l[mm]

kY

[N/mm]Ym

[mm]HEA lS

[mm]kZ

[kN/mm]ktY

[kNm/rad]20 80 1,000 197 3.1 36.6 120 48 1.3 1.420 80 600 197 8.5 19.8 120 48 1.3 1.430 80 1,000 167 3.6 31.7 140 65 2.9 2.750 100 1,000 136 5.6 26.5 140 108 4.9 4.675 100 1,000 74 10.3 15.3 160 149 2.8 5.6

100 120 1,000 96 15.0 16.7 160 198 3.7 7.5150 160 1,000 92 25.7 15.0 180 298 3.8 9.9200 200 1,000 120 35.9 16.8 200 366 4.3 14.0300 240 1,500 101 31.6 20.0 240 476 4.8 23.0

How to use the table:1. you must make an estimate of the maximum constrainer force (e.g. braking force) which occurs in the

installation. Now, look up in the column Fm (the maximum allowed axial force) which combination UNP/HEAfulfills the job

2. a beam length L of 1000mm is practical in most cases3. reducing the beam length L increases the stiffness as can be seen in the case of Fm = 20kN. (N.B. You

cannot improve this by increasing the spring length l because this would decrease the permissible axialforce)

Influence of torsion (ktY)In the ideal case the stiffness ktY should be zero. To understand what happens because this is not so, we willlook to the platform with three flexbeams (fig. 1.4-11).

Fig. 1.4-11 Platform with three flexbeams

Without constrainer III the platform could rotate around point A. Moving point B over x mm in the indicateddirection gives a rotation of x/a rad. Then the supports of I and II are twisted, resulting in a momentum

kt a

x 2 = M y•

•

on the platform. This gives a reaction force at B of

kt a

2 =

aM

= F y2B •

per mm movement. Now, with constrainer III in its position but with a small movement at B (e.g. caused bythermal expansion) this results in the above calculated force in constrainers II and III.

Example:With 150kN flexbeam from table above, a = 2000mm the force in flexbeam III is 5N/mm at point B.Conclusion: the flexibility of the support is good enough.

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Influence of flexion (kz)In the ideal case this stiffness kz should be zero. To understand what happens because this is not so we look tothe platform with two flexbeams (fig. 1.4-12)

Fig. 1.4-12 Platform with two flexbeams

If by some reason point A moves over a distance of x mm in the indicated direction (e.g. thermal dilatation ofconstrainer II), both supports of the flexbeam I are deformed:

- twisting over an angle of x/L (L = beam length) gives a momentum M on the platform

[Nm] kt Lx

2 = M y••

This is reacted by a force in the flexbeam II of

]mmN

[ kt L

2 = f Y2t •

- flexion over a distance of x/2, resulting in a reaction force in the flexbeam II of

]mmN

[ k 0.5 = F Zf •

ExampleWith the 150 kN flexbeam from the table above we havektY = 0.99 × 107 Nmm/radkZ = 0.38 × 104 N/mmL = 1,000 mm

Then Ft + Ff = 1920 N/mmThe flexibility of the support is good enough.

Example: combined railway weighbridge (max. capacity 60t)

Fig. 1.4-13 Combined railway weighbridge

A braking force of 280 kN is expected. This should be taken up by two constrainers under the two main beams,to simplify the construction. For exact positioning of the rail ends on the bridge, it is necessary that there is alateral constrainer A at both ends of the weighbridge.

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Constrainer BWith the table above the constrainer for Fm = 150 kN could be chosen. Its technical data are

beam UNP 160, L = 1000 mm, l = 92 mmsupport HEA 180, lS = 298 mm, kY = 25.7 N/mm

Constrainer AThe designer wishes a shorter length than L = 1,000 mm. He chose UNP 140

L = 890 mm, l = 140 mmThe figures for vertical stiffness, maximum allowed force and the allowed maximum deflection are calculated.

kY = 15.2 N/mmFm = 92 kNYm = 20.5 mm

The supportsFor the supports he chose HEA 140.

lS = 140 mm, ts = 5.5 mm, hs = 133 mm - 2 × 8.5 mm = 116 mmWith an allowed shear stress t = 84 N/mm2 the axial force in the constrainer is limited to only 65 kN instead ofthe value for Fm as calculated above. The torsion stiffness around the vertical axis and the lateral stiffness are

ktY = 0.57 × 107 Nmm/radkZ = 0.3 × 104 N/mm

These stiffnesses are smaller than the corresponding figures in the table for a constrainer of 100 kN. So there isno need to repeat the kind of calculation as is done ...Concluding we can accept the chosen dimensions:

Fig. 1.4-14 Constrainer dimensions for railway weighbridge (example)

A solution with only one central longitudinal constrainer B is preferred from the viewpoint of measuring quality.In principle four constrainers are too many. The reason is explained in detail in chapter 1.4.1: you remember, atemperature difference of only 1°C between both constrainers B can cause an axial force in these constrainersof 0.23t. The result is a zero-error if the constrainers are not exactly horizontal. Therefore much attention has topaid to the horizontal position of all constrainers under this weighbridge.

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Flexbeams (calculation method)The calculation is divided into two paragraphs:

paragraph A describes the calculation of the beam itselfparagraph B explains how to dimension the supports.

A Calculation of the beam and its propertiesFigure 1.4-15 shows the mechanical model used for the flexbeam calculation: a stiff part ending in two flexibleleaf springs witha length l. These springs are held in a horizontal position at A and B. If B is moved downwardsrelative to A, there are working vertical forces FY and momenta M on the device at the points A and B.

Fig. 1.4-15 Flexbeam dimensions

1. vertical stiffness of the beamElasticity calculation teaches that

12t b

= I ,L l

I E 2 =

YF = k

3

2

YY

••

••

Spring dimensionsb widtht thicknessl lengthE Young's modulus (steel: E = 2.1 × 105 N/mm2)

2. allowed axial forceIn principle, this force is limited by two restrictions: either the stiff beam or the flexible beam can collapse.The calculation has to ensure the safety of both the cases.

a) collapsing of the beam

i

L = b

min

λ

for steel Fe 360 (St37) and λb < 100.8 the collapsing load we find

[N] A ) mm

N 1.154 -

mm

N (174.6 = F b22X •• λ

REMARKimin and A are to be found in the data for the used steel profile

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b) collapsing of the springs

tl 3.5

= s•

λ

for steel Fe 360 (St37) and λs < 100.8 the collapsing load will be

[N] t b ) mm

N 1.154 -

mm

N (174.6 = F s22X ••• λ

The permissible axial load is, of course, the smallest value of both the calculated values FX!

General remark about the slenderness ratioIn cases that λ > 100.8, the EULER formula must be used, resulting in a smaller permissible force. This

condition should be avoided.

3. permissible vertical constrainer deflectionThe limitation is caused by the permissible bending stress in the springs sb.

t l) + (Ll L

E 2 = Y

2b

••

•• σmax

B Calculation of the support and its properties

Fig. 1.4-16 Flexbeam support dimensions

1. permissible force in X- directionThis can be found from a shear calculation

[N] t l = F sssX ••τFor mild steel Fe360 the tables give us a value t = 92 N/mm2

2. stiffness around the Y-axis of the supportThis is expressed as a momentum per radian and found by

h

G l t = kt

ss

3s3Y •••η

η3 = 0.333G modulus of glidingFe 360 (St37): G = 81,000 N/mm2

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3. stiffness of the support in Z- directionThis is found by calculating the bending stress (, assuming that the ‘beam is clamped at both sides’).

12t l = I ,

h

I E 12 = k

3ss

3s

Z•••

1.4.4.4 Pivoting rod

The rod has two pivoting ends and is placed horizontally between the object and the fixed surroundings (orfoundation). In principle there are two effects, which could can a vertical reaction force on the object:1. bearing friction

Fig. 1.4-17 Reaction ∆∆G to an upward movement of the object

If the object would be moved upwards or downwards, the bearing will react with a friction momentum MF.This causes a vertical reaction force of

lM 2

=G F•∆

counteracting the movement. The friction momentum in the bearings is nearly proportional to the axialforce in the rod. Therefore also the disturbing vertical reaction is proportional to the axial force in the rod(and practically zero if there is no axial force).

2. non horizontal rod position

Fig. 1.4-18 Non horizontal pivoting rod

If the rod is placed under an angle a with the horizontal and a side force is working on the object, then theresultant of this force FH and the rod reaction is about 1.7% of FH per degree.

If heavy vibration or shock loading is expected, choose an overdimensioned type of constrainer. Also be surethat the pivoting rods are mounted with the smallest possible play, to avoid any hammering effects. Thisconcerns the bearings, their axles, and all the screw thread connections.

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1.4.4.5 Rocking pinsUnlike the above described types of constrainers rocking pins are mounted with a clearance. They cannot takeany shock loading.

Fig. 1.4-19 Principle of the rocking pin

The two end surfaces of each pin are part of a sphere with a diameter equal to the length of the pin. This spherecan roll between two flat vertical contact surfaces. In case of big horizontal forces during measurement, thecontact surfaces should be placed vertical within ±1° to avoid a disturbing vertical component.

The horizontal position of the rocking pin is not critical at all. The rocking pin is used in pairs, because it can onlytake up a compression and not a tensile force. The two pins of one pair shall always be mounted in one line andclose to each other. The latter to avoid clamping if the distance d (see fig. 1.4-19) changes by thermalexpansion or another deformation.

In cases of falling lumps of material, or of stirring devices in reactors, shocks can occur. The rocking pin is notsuitable in case of big shock loads because the pins must be mounted with an axial play. Big shocks willtherefore have a „hammering effect“ on the contact surfaces. This causes damage („wear“) on the long run.Therefore regular inspection and pin adjustment to the least possible play is necessary.

In case of big horizontal shocks a constraining device without play should be selected, e.g. rigidly clampedstruts.

1.4.5 Types of stopsIn general two different usages of stops must be separated.

Horizontal stops are often applied to replace constrainers, e.g. in weighbridges.Vertical stops, however, serve as lift-off protections and are applied additionally to constrainers in thehorizontal plane.

Both types have some properties in common: · Stops are safety devices. · Stops may not take forces during the weighing operation. · Stops only act in one direction (horizontal stops mostly take compression forces, vertical stops mostly tension

forces). · Stops are sensitive to frequent shocks and heavy vibration (‘hammering’ can damage their contact surface).

1.4.5.1 Horizontal stopsThough stops are no constrainers use the same rules for their positions in the horizontal plane. Keep in mindthat horizontal stops only take compression forces. Therefore, stops must be used in pairs to keep the weighingobject in its position.

Sometimes weighing objects are equipped with stops instead of constrainers. · If a suspended object is in a very stable equilibrium and under quiet conditions, sometimes constrainers are

omitted (see above chapter 1.4.2). This could be of an advantage, for high accuracy measurement, to avoidany possible friction.

· When weighing objects on compression load cells like weighbridges, constrainers are often replaced withstops. The restoring force caused by the spherical segments is thought to give a sufficient stable equilibriumand to assure the weighing object to be free from the stops during the weighing.

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Fig. 1.4-20 Weighbridge with 4 stops

In these installations, however, trouble can occur, usually if the maintenance is neglected: · excessive permanent movements of the weighing object can cause wear at the load cell billet top and

at the spherical segment · the stops can be damaged or destroyed by ‘hammering’ · the accuracy can decrease because of friction, non-vertical load cell position, and damage to the load

cell

A solution for this situation cannot always be found... This depends on a lot of circumstances: · the magnitude of the restoring force per mm object travel · the magnitude of the side forces acting · the maximum permissible side force on the load cell · the permissible measuring error and, therefore, the necessary vertical load cell position · the friction at the load cell top · the possible wear at the load cell contact points by the object movements.

To avoid the above mentioned negative effects (wear, stop damaged, accuracy decreased), the optimaladjustment of the stops is important. There are two opposite arguments for adjusting the gap distance of stops:1) wide gap distance

· useful to avoid friction between object and stopIf the object touches the stop, this could result in a reproducibility error by friction forces. Therefore, thegap between object and stop must be big enough.

· avoiding destruction at the stops · allowing a big object travel

Þ can cause not acceptably high restoring forces on the load cell and possible wear at the billet top

2) short gap distance · avoiding destructive hammering at the stop in case of sudden side forces (like braking forces)

1.4.5.2 Vertical stops (lift-off protections)Vertical stops (they are also named lift-off protections) are used in combination with horizontal constrainers: thehorizontal constrainers keep the weighing object in the right horizontal position and the vertical stops avoid itstilting or capsizing. Normally, the constrainers in the horizontal plane are mounted as suggested by the standardinstallation diagrams in fig. 1.4-2. Beside every load cell a vertical stops is installed.

Fig. 1.4-21 Compression load cells, constrainers, and lift-off-protections

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Fig. 1.4-21 shows weighing installations on compression load cells with different numbers of constrainers andstops:

· installations with 5 constrainers (fig. 1.4-21, left sketch)The disadvantage of this type of design is not obvious: if the weighing objects expands under temperature,some of the constrainers in the second plane above the plane of the load cells can get clamped, transferdisturbing forces to the load cells and thus cause measuring errors.

· installations with 3 constrainers in the horizontal plane and lift-off protections beside them (fig. 1.4-21, rightsketch).This type of installation avoids the production of disturbing forces by constrainers in a second plane abovethe plane of the load cells. The vertical stops only make sure that the installation cannot tilt. The simplesttype of a vertical stops consists of a rod and two nuts. The width of the clearance is determined by max.permissible movement where the load cell does not leave its position. (PR 6201: 2mm)

Fig. 1.4-22 Simple lift-off-protection with a rod

A second type of lift-off protection is shown in fig. 1.4-23.

Fig. 1.4-23 Lift-off-protection

Most of our MiniFLEXLOCK mounting kits provide a threaded hole which makes the installation of a lift-offprotection very easy (fig. 1.4-20). If there is a need for a lift-off protection with a higher capacity, you can find thestrength of an appropriate rod by simply choosing from the table given below.

thread permissible forceM12 10.5kNM16 19.6kNM20 30.6kNM24 44.1kNM30 70.1kNM36 102.1kNM42 140.1kNM48 184.1kN

The properties of the bolts are assumed to fulfil at least the conditions belowlimit of elasticity 240N/mm2

ultimate load 400N/mm2

Such bolts are standard bolts and used in combination with steel type Fe360 or St37.

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1.5 Disturbing forcesThe load cell errors are specified and well-known by nearly everyone who uses load cells. Besides these errorsvarious other influences exist like mounting faults, environmental influences, and influences from the design couldaffect the measurement.

1. load cell errorsThese errors are normally intensively discussed in case of the malfunction of an installation. However, theirinfluence compared to mounting faults and external influences usually is small.

2. mounting faultsA lot of trouble concerning weighing installations is caused by improper mounting of the load cells:· side forces and momenta affect the load cell· bad application of the load to the load cell (use the right mounting parts)· non vertical load cell position (use spirit level)These errors are usually rather easy to detect and to correct.

3. external influencesThese influences must be taken into consideration during the design phase in order to choose the right loadcell type and the adequate way of constraining. Depending on the type of construction the errors listedbelow can occur• parasitic vertical forces on the installation• temperature on the load cell• dynamic forces on the installation (vibrations)• dust, snow, rain• wind forces• pipes, bellows

Advice concerning the items listed under position 2 is given in the chapters directly related to the load cellmounting (see section 2). This chapter 1.5 deals with the possible disturbing influences coming from theenvironment of an installation (item 3) and how to fight against them.

1.5.1 Environmental influences1.5.1.1 Wind forces

Wind forces influence mainly outdoor installations. They affect the strength of constrainers, lift-off protectionelements and the strength and the stability of the complete construction. These parts have to withstand theoccurring forces. Normally, wind forces do not affect the weighing process itself because the design load chosenappears only during a very heavy storm. While such a storm is blowing tank weighers or truck scales are out of use.

In order to determine the value of the wind forces acting on a vessel we must consider some basic facts on fluidmechanics:A. How can we describe the wind?B. How do the shape and the dimensions of the vessel influence the forces?C. How does the surrounding of the construction influence the forces?

A. Description of the windTaking a closer look at the blowing wind we find out that the description of the wind needs the knowledge of twofactors:a the velocity of the windb the properties of the air (density, viscosity)

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Fig. 1.5-1 Velocity distribution

The velocity of the wind is not constant in value when we consider different heights (see sketch on the right handside). Directly above the ground the velocity of the wind equals zero because of the friction between wind andground. The velocities increase according to a parabolic function of higher order. In most cases the parameters ofthe function describing this flow are unknown so that it is impossible to calculate the velocities. Therefore a morepractical approach has to be used: a rectangular velocity distribution is chosen to approximate a real measuredvelocity distribution. Such a choice is called the design load for this construction.

The state of the air depends on e.g. the barometric pressure, the air density and the temperature. The effect of thecompressibility of the air may be neglected. Even during a very heavy storm the velocity of the wind is rather slowcompared to the sonic speed. Therefore we can assume the properties of the air to be independent from thevelocity. For the calculation we need values for the properties density ρA and viscosity ηA. The barometric pressure pdecreases with an increasing height above the ocean. Therefore the barometric pressure at this level has is chosen.The same behaviour applies for the density ρA as you can find out from the ideal gas equation: p ~ ρA.

DESIGN LOADSValues for the design loads are specified either by the customer or by the national standards and regulations. Incase of wind the state for the design load is (usually) a very heavy storm with a wind velocity of 12 Beaufort orabove.

sm

vge 40.. =

The values for air density and air viscosity given are valid for Germany

26

21073.17)10(,25.1)10(

mNs

Cmkg

C AA−⋅=°=° ηρ

B. Shape and size of the vesselMany experiments were undertaken to measure the forces which the flowing air exerts on an upright standingvessel. The results show that the forces depend on - the shape of the vessel (circular vessel, square vessel, ...) - the smoothness of the vessel's surface - the size of the vesselThe two influences 'shape of the vessel' and 'smoothness of the surface' are assembled into one number that ischaracteristic for that particular installation. The number is called 'drag coefficient' or 'friction coefficient' and isreferred to as cw. For a normal weighed object the influence of the smoothness is negligible. Therefore the numberrepresents only the influence of the air velocity.

friction coefficient cw

upright standing circular vessel 0.8 ...1.0

There is another parameter influencing the magnitude of the wind force- the size of the vessel.

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In order to make the calculation as easy as possible an exposed area is defined

Definition exposed areaThe exposed area of the weighed object is calculated by multiplying its diameter and height.

HheightDdiameterAareaosedexp ⋅=The exposed cross sectional area was defined in the beginning of this century when the computers were notinvented.

C. Area of installationDepending on the area where the weighing object is placed you have to calculate with different air velocities.Example 1: weighing object in a town

Various buildings slow the air velocity down. This case is chosen for the standard design load.

Fig. 1.5-2 Weighing object near a town

Example 2: weighing object on a hill or near the seaThe wind blows without any hindrance at the weighing object. There is nothing that slows the air velocity down.Therefore you have to assume a higher design load as usual.

Fig. 1.5-3 Weighing object on a hill

wind velocity, impact pressureSupposed values for the wind velocity are not known, some useful assumptions have to be made. The table belowis intended to help you to find such assumptions.

Beaufort air velocity of wind impact pressure use

10 < 28.4 m/s 0.5 kN/m2

12 < 36.9 m/s 0.85 kN/m2

> 12 42.0 m/s 1.1 kN/m2 standard value near the town

> 12 45.6 m/s 1.3 kN/m2 standard value outside a town

> 12 51.4 m/s 1.65 kN/m2

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After having considered all relevant data the calculation scheme can be introduced.

calculation scheme for the wind forces acting on an upright standing vessel

Fig. 1.5-4 Schematic sketch for calculation

FW = cW • q • A FW wind force

cross sectional area of vesselA = D •• HD diameterH height

impact pressureq = 0.5 •• ρρL •• v²ρL density of the airv velocity of the air (rectangular velocity distribution)

drag coefficientcW = 0.8 ... 1 (upright vessel)

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Example 1. Horizontal forcesCalculate the horizontal forces for a paper pulp digester according to the sketch below. The diameter D is 3m, theheight H is 10m. Consider two cases:1. Beaufort 5 v = 8 m/s2. Beaufort 12 v = 40 m/s (hurricane)

Fig. 1.5-5 Paper pulp digester

The friction coefficient for this installation is assumed to be cw=0.8. The density of the air shall be ρA = 1.25 kg/m3.

case 1:q1 = 0.5 • 1.25 kg/m3 • (8m/s)2 = 0.04 kN/m2

Fw1 = 0.8 • 0.04 kN/m2 • 3m • 10m = 0.96 kN

q2 = 0.5 • 1.25 kg/m3 • (40m/s)2 = 1 kN/m2

Fw2 = 0.8 • 1 kN/m2 • 3m • 10m = 24 kN

Interpretation of the results:- this installation must be protected against capsizing - a vertical component of the wind force can cause a measuring error as the wind does not always blow exactly

horizontal; the error is found to be a temporary zero shift- errors bigger than 1‰ of the net weight can normally be expected only with very heavy wind ( > 5 Beaufort)

Example 2. A vertical lift-forceWind can cause an upthrust Fl on a lying tank. This is caused by the high air velocity at the upper side.Calculate for a horizontal cylindrical tank of 3m in diameter and 10m in length the `lift`-force. The tank is filled withsand (ρs= 0.5kg/dm³). Consider two cases:1. Beaufort 5 u= 8 m/s2. Beaufort 12 u= 40 m/s (hurricane)

Fig. 1.5-6 Horizontal tank

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Volume of the tank: VT = π/4 * 3² * 10 m³ = 70.7 m³Weight of the sand: FC = 70.7 m³ * 0,5 kg/dm³ * 9.81 m/s² = 347 kN

impact pressure q1 q1 = 0.5 • 1.25 kg/m3 • (8m/s)2 = 0.04 kN/m2

lifting force Fl1 Fl1 = 0.8 • 0.04 kN/m2 • 3m • 10m = 0.96 kN

impact pressure q2 q2 = 0.5 • 1.25 kg/m3 • (40m/s)2 = 1 kN/m2

lifting force Fl2 Fl2 = 0.8 • 1 kN/m2 • 3m • 10m = 24 kN

F1 = 0.96 kN = 2.8‰ FC

F2 = 29.04 kN = 6.9% FC

Interpretation of the results:- negligible under normal conditions

Example 3. A vertical ‘suction’-forceSometimes the vessel is protruding out of a building. E.g. to discharge into a truck (see figure below). If the wind isfree to blow under the building, it causes that the air pressure under the building is lower than indoors. A verticalforce Fv is the result. In this cases A denominates the cross section of the vessel.The vertical force is calculated by Fv = p • A.

Fig. 1.5-7 Discharging into a lorry

Calculate the forces for three vessels with diameters of 2m, 4m and 6m respectively. Assume a difference pressureof ∆p = 0.0025bar (= 2.5 cm H2O).

diameter of vessel vertical force Fv

2 m 78.5 kg

4 m 314 kg

6 m 707 kg

Interpretation of the results: - This effect can very seriously influence the measuring accuracy if you consider W&M installations. Non-

reproducibility error by arbitrary temporary zero shifts.

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Possible solutions to avoid these effects are: - make it impossible for the wind to blow under the building (e.g. by flexible doors) - make the opening in the floor where the vessel comes through much larger than the vessel. This makes the

pressure difference smaller.

1.5.1.2 Heat and heat transferHeat in its various forms can cause troubles to many installations, outdoors and indoors. Sometimes the reason isobvious, sometimes the reason is well hidden.Example 1:

A customer complains about a storage silo outdoors. The silo is standing next to a high production hall. Heobserved a change in weight throughout the day although the silo was neither loaded nor unloaded.In the morning, the silo stands in the shade. As the sun is rising, the shadow of the building wanders and in theafternoon the silo stands directly in the sun. In this example the reason for the varying weight values is quiteobvious: the weight changes because of the temperature sensibility of the load cells.

Example 2:A customer complains about a hopper scale indoors. The weigher is standing in a large hall together with manyother vessels. The temperature in the hall is kept constantly at 20°C. Accidentally the weight changes althoughthe contents of the hopper do not change.A visit at site produced the followings results: the outside wall had a door that was usually closed. For somereasons the door happened to be opened. This caused a draught of air which affected the load cells in such away that the weight indication changed.

In order to get a better understanding how heat and high temperatures can trouble load cells and scales thephysical effects are considered on the pages below. Three different physical mechanisms describe the heattransfer:- conduction- convection- radiation

mechanism 1: CONDUCTION

Fig. 1.5-8 Heat conduction

If the weighing object has a higher temperature than its surroundings, a heat flow through the load cell is forced.This flow can have different results.

1. damage of the load cellThe glue has a limited temperature stability. If a particular value is exceeded the glue softens and theapplication of the strain gauge to the measuring element loosens or even gets lost. Therefore the upper limitgiven for the storage temperature must not be exceeded.

2. change in characteristic valuesA heat flow through the load cell could result in the same effect as a too high rate of temperature change.

Possible solutions to overcome these complication are- use of high temperature load cells PR 6211 LT

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(compensated temperature range up to +180°C)- use of heat protection plates- sometimes: use of tension cells with long suspension rods

mechanism 2: CONVECTION

Fig. 1.5.9 Heat convection

apply flowing air with a high temperature to a vessel. This results in an increase in temperature in the vessel.Furthermore the load cells are affected. This can result in a high heat increase during a short time.

The detection of erroneous readings caused by conduction is often a little bit complicated since this errorhappens to appear under changing circumstances and at different times. If such an error is identified it may bedifficult to find a satisfying solution.

mechanism 3 RADIATION

Fig. 1.5-10 Heat radiation and protection shield

A heat flow caused by radiation depends on the ratio of two surface temperatures: the surface temperature of thehot object and the surface temperature of the heated object. The heat flow does not depend on any medium butonly on the different temperatures.

Errors caused by heat radiation are usually easy to detect. You find always an extremely hot object that causesthe complications. Possible sources for the radiation are- the sun in outdoor installations- an arc furnace or another container with hot object (e.g. molten metal)Mostly heat radiation can be fought by placing a sheet of metal between the object and the load cell.

As stated above all three heat transfer mechanisms can cause trouble to a weighing object. The heat convectionand the heat radiation affect only the load cell by changing its temperature. Because heat conduction exist normallywith hot weighing objects this mechanism affects both, the vertical position and the temperature stability of a loadcell.For this reason, the next paragraph introduces a calculation scheme for the temperature expansion of a beam.

The physical behaviour of such a beam is described by - the temperature difference between beam and its surrounding - a material dependent constant: the thermal expansion coefficient of the particular material

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material expansion coefficient k[mm/m/100K]

Fe 360 11

concrete 1.1

1.4301 1.6

aluminium 2.3

For easier comprehension and calculation the expansion coefficient is given in the form above whereas tablesnormally state it in the form k*10-6 K-1.

Calculation scheme for the thermal expansion of a beam

Fig. 1.5-11 Schematic sketch for calculation

∆l = l • k • ∆T ∆l thermal expansion

temperature difference To - Ta

To operating temperatureTa ambient temperature

thermal expansion coefficient

beam length

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Example 1: I-beam IPB 100Calculate the expansion of an I-beam IPB100 which is heated from an ambient temperature of 20°C to 70°C.What forces are generated internally in the beam if it is assumed to be clamped at both sides?

material constructional steel Fe 360beam length l = 2mambient temperature T = 20°C, operating temperature T = 70°Ccross sectional area A = 2,600mm2

Young's modulus E = 210,000 N/mm2

Solution1. strain εT = k • ∆T

εT = 1.1 * 50 mm/m = 0.55mm/m = 0.55*10-3

2. expansion ∆l = l • k •∆T∆l = 2m * 0.55mm/m = 1.1mm

3. force F = A • E • k • ∆TF = 2,600mm2*210,000N/mm*0.55*10-3 = 300kN

The force is caused in case of a rigidly clamped beam. Please observe that the values are higher thanusually expected. Such high forces could affect seriously the measuring result in that case the transducer isinfluenced by side forces because of the mounting principle. Remember, GLOBAL Weighing suggests for thisreason mounting kits where the load cell acts as articulated column.

Example 2 Tank weigherConsider the sketch below. The tank weigher is made from stainless steel. It is calibrated under ambienttemperature (20°C) and operated at a temperature of 250°C. Calculate the expansion under operatingconditions.

Fig. 1.5-12 Tank weigher

R = 1.25mstainless steel ⇒ k = 1.6 mm/m/100Ktemperature difference

∆T = 250°C-20°C = 230°CεT = 1.6mm/m * 230°C/100K= 3.68 mm/m

expansion of the radius∆r = 1.25 * 3.68 mm = 4.6mm

⇒ high expansionmax. permissible value for PR 6201: 10mmmax. permissible value for PR 6241: 3.9mm

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1.5.1.3 freezing environmental conditions (ice, snow)The load cells are specified to operate even at temperatures down to –30°C. However, if the air temperaturechanges continuously between e.g. +10°C and –10°C, ice could be deposited on the pivoting points of theweighing installation. Critical points are:1. load cell and mounting parts - contact point between the spherical load cell top and the hollow of the load button - contact point between spherical segment of the load cell and the base plate - the cardanic suspension of tension cells or load beams - the cardanic pivoting points of the constrainers2. installation - the gap between the weighing object and its surroundings

In all these cases the joint can cause a measuring error because the contact points freeze. If you have asuspicion that cold weather had affected one installation, you should check the whole installation on the pointsmentioned above. Anti-freezing agents, like glycol, can help you to overcome the situation.

What errors are caused if ice or snow is deposited on an outdoor vessel?If the silo is heated or held at a constant temperature above zero, additional loads caused by ice or snow do notneed to be observed. In all other cases you should be careful about them.In some countries legal regulations exist, which give design loads for ice and snow deposition. Theseregulations assume a thickness of the ice layer, e.g. 3cm. This equals a

27

mkN

liceforloaddesign i =

In the same way, the design loads for snow are given,

225.5...5.0

mkN

mkN

lsnowforloaddesign s =

Example: outdoor siloThe silo has a capacity of 50t.diameter D = 6mdesign load lS = 4kN/m2

Solution

)5.11(1134)6(4 2

2 tkNmkN

mms ≈=⋅⋅=π

The additional load is more than 20% of the silo capacity.

1.5.1.4 rain, dustIt is unlikely that either rain or dust can form a layer, which is thick enough to influence the weighing: the dust isblown away by the wind, the rain drips down from the top of the vessel.

1.5.2 FrictionIn order to get an accurate result a weighing object has to stand free from its surroundings and any linksconnected to it must be as flexible as possible (refer to chapter 1.3). However, after some time of operation thelinks can harden and become stiff. If the scale is not cleaned regularly, dust can be assembled between theweighing object and its surroundings. Both effects can cause friction. Friction results in unpredictable zero shifts:non-reproducibility and hysteresis affect the accuracy.

Causes for friction - internal friction in the pivoting points - parts pressed against the object - internal friction in the hoses, tubes, cables, .

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For this reason, a regular check on the parts connected to the scale and a regular cleaning in a dusty area isrecommended.

1.5.3 Vibrations, shock loadingVibrations and shock loading (resulting from falling material) can severely influence a weighing object.

VibrationThe source of the vibration can be the scale itself, e.g. a mixing device. The second possibility is an externalsource, e.g. a motor. In this case, the vibrations are transmitted through the construction. To lower theamplitudes of the vibration you can install damping devices such as rubber mounting kits or additional cupssprings. The choice depends on the excitation frequency and the capacity of the load cells. For load cells withcapacities above 5t it is nearly impossible to find rubber mounting kits.Position of the damping elements: - if the vibrations come from outside of the scale, the damping element must be placed between the floor andthe load cell - if the vibrations come from the weighing object itself, the damping element must be placed between the loadcell and the weighing object

How can a possible influence be estimated?1. natural frequency f0 of the damping element

statxg

f ⋅=π2

10

2. ratio of excitation frequency fex and natural frequency f0

0ffex=η

3. max. vibration amplitude

21 η−= stat

peak

xs

Example: 900kg Platform with PR 6211/32D1 and rubber mounting kit PR 6011/03technical data of the rubber mounting kit: 1mm deflection under 2250N load

stiffness of the rubber element:

mmkN

mmN

cs 25.21

2250==

deflection under nominal load

mm

mmNN

cw

xs

LCstat 33.1

2250

3000===

natural frequency f0 of the damping element

)822(7.1333.1

81.9

2

1 2

0 rpmHzmmsm

f ==⋅=π

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min. frequency with 0 isolation

)1160(3.1920min rpmHzff ==⋅=

Vibrations produced by equipment like motors with higher speed than the min. frequency are damped. However,vibrations between 822rpm and 1160rpm are enlarged.

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1.7 The weighing results

The definitions given in paragraph 1.7.1 concern only the load cell properties.

1.7.1 Terminology for load cellserror difference between the measured value and the real value

HysteresisThe maximum difference between the loading and the unloading curve related to Cn.

Non-linearityMaximum deviation from the best straight line in relation to rated output. The best line goes through thecommon point of pre-load and the loading/unloading curve and splits these curves into two parts with thesame positive and negative values

Combined errorHalf distance between the limitation of the band, which covers loading and unloading curve, and where thecentre line goes through the common point of pre-load and the loading curve.

RepeatabilityThe difference between load cell output readings taken from consecutive tests under identical loading andenvironmental conditions.

CreepThe change in load cell output occurring with time while under constant load and with environmentalconditions and other variables also remaining constant.

Zero signal temperature coefficientRelative output signal variation without load related to rated output Cn and a temperature variation of 10K,where the variation of temperature per hour is maximum 5K/h.

Rated output temperature coefficientRelative variation of the real rated output Ci due to temperature variation related to Cn and a variation of 10Kin the nominal temperature range

Accuracy classThe accuracy is simply the worst value of following values• combined error• repeatability• creep (30min) Fcr30

• zero signal temperature effect• rated output temperature effect

1.7.1.2 load cell and weighing installationSome words describing the properties of load cells and weighing installations have different meaningsdepending on their actual context. Take i.e. the word „accuracy“.

load cell„accuracy“ is the worst value of the errors listed below• combined error• repeatability• creep (30 min)• zero signal temperature coefficient• rated output temperature coefficient

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weighing installation1) „accuracy“ refers to the O.I.M.L. curves for the different classes. It describes the deviation of the weight

indication from the real value of the weight.

2) The „accuracy“ of the weighing installation includes some more than only the load cell:• accuracy of the load cell• the quality of mounting• the steelworks, the concrete around the load cells• the cabling• the electronic

Direct influence is only possible on load cell and electronic. The other parts are done by the customer. In somecases he might need support from GLOBAL Weighing.

1.7.2 Influences from the constructionThe chapter on the design provides you with hints how to improve steelwork. As such constructions are verycomplicated, it is usually not possible to give an easy way to calculate the influences on a difficult design.Maybe a rule of thumb can help you:

calculate the following ratio

capacity cell load installederror weighing

If this value is about 0.001 ⇒ the installation should be in good condition.If this value is about 0.01 ⇒ the influence of the construction is dominant.

1.7.3 Approved installations1.7.3.1 W&M regulations

In most countries, legal regulations on the required accuracy are given by the national services of Weights &Measures in accordance with proposals of the O.I.M.L. (Organisation Internationale de Métrologie Légale) atParis.

The O.I.M.L. issues a lot of regulations concerning load cells and scales1. Test reports for load cells (these test reports are no approvals!)

Metrological regulation for load cells R 60The load cell test reports (according to OIML R60 or national regulations) contain values for Emax, nLC, Y, ...which are necessary for the design of the installation

2. Non-automatic weighing instruments (NAWI)Non-automatic weighing instruments R 76

GLOBAL Weighing mainly deals with non-automatic scales, i.e. truck scales, batching systems.

The W&M systems are subdivided into four precision classes:I precision speciale laboratoryII precision fine laboratoryIII precision moyenne weighbridgesIIII precision ordinaire used in factories where asphalt or concrete is madeThe classes C and D concern GLOBAL Weighing systems. These classes will be discussed in more detail.

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The permissible number of scale divisions isclass C 500 to 10000class D 50 to 1000

The permissible error for the installation during initial verification is shown in fig. 1.7-1 as well as the maximumerror for trade use.

Fig. 1.7-1 Permissible errors for W&M weighing systems (class III)

Fig. 1.7-2 Comparison between W&M systems (class III and class IIII)

· The technical data are valid for the temperature range from -10°C to +40°C. Outside this range there is anadditional deviation of 1d/5K permissible.

The certificates describe different types of scales, for instance · weighbridges · tank weighing · crane weighing · In-Motion-Weighing · ...The W&M authorities only certify complete ‘applications’, but no single load cells. A certificate contains all partswhich are necessary for the installation of the scales such as load cells and its accessories, special mechanicalconstructions for weighbridges, the indicator and its accessories, ...

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1.7.3.2 Weighbridges

1. Terminologyv (load cell verification interval)

the load cell interval used in the test of the load cell for accuracy classification (expressed in units ofmass)

vmin (minimum load cell verification interval)the smallest load cell verification interval into which the load cell measuring range can be divided

Emax maximum load

d (actual scale interval)value expressed in units of mass of- the difference between the values corresponding to two consecutive scale marks for analogue

indication, or- the difference between two consecutive indicated values for digital indication.

e (verification scale interval)value used for the classification and verification of an instrument (expressed in units of mass)

Calculation procedure for the values of an assizable weighing system (according to EN 45 501)1. Maximum capacity of the load cell

The maximum load of the scale must not exceed the sum of the capacities of the load cells

NR MaxQ

EMax

⋅⋅≥

N number of load cells in the scaleEmax maximum capacity of a single load cellR transmission factor of a lever system (for systems without levers R = 1)Q correction factor

scales with levers Q = 1.1...1.3scales without levers

1 - NN

+ L = Q De

MaxLDe dead loadMax maximum load to be measured

2. number of load cell divisionsn nLC ≥

3. smallest verification interval e

N R Y

E e or N R

v e ••

≥•≥ maxmin

4. number indicator divisionsn nind ≥

5. minimum output voltage

e NR

U E

S u exc •••≤∆

maxmin

S rated output, e.g. 1mV/VUexc supply voltage

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6. permissible input resistance

R N

R R LmaxLC

Lmin ≤≤

RLmin min. permissible input resistance of electronicsRLmax max. permissible input resistance of electronics

Example weighbridge for trucks (3000d)weighing range 60tweight of the deck 35t4 load cellsaccuracy 20kg

1. required load cell capacity

(T) 117,000kg 1.7 1

50,000kg 4 95,000kg

1.7 1 - 4

4 +

35,000kg + 60,000kg35,000kg

= Q

ť

•≤

≈

chosen: PR6221/54C3Emax = 14,000nLC = 3,000 ≥ 3000 = n

2. minimum permissible step for the load cell

kg7.1kg 4 1 14,000

50,000kg e 20≤≈•

•≥

minimum permissible step for indicator⇒ d ≥ 10kg

Therefore a step of 20kg is permissible.

V = 20kg 41

20V 50,000kg

VmV

u µ42

min •••≤∆

3. The indicator be a PR 1713 with nind = 5000.The minimum input voltage for PR 1713 is 1.2µV per d; this condition is fulfilled, too.

4. Permissible input resistance

(T) = 40

ΩΩ

≤Ω 270108

75

PR 1713 has a lower limit for the input resistance of 75Ω and no upper limit.

1.7.4 standard accuracy: non W&M applicationThe user often asks us to give some figures about “the accuracy of a process weighing system”. Properlyspeaking, there is no such thing as the accuracy of an installation. We can only speak of an installationaccording to e.g. ‘O.I.M.L. Class III, 3000d’, which means that the installation can pass some well defined tests.

However, we can speak about the weighing accuracy of a certain weight measurement. This gives informationon how far we can rely on the weighing result. It gives, in other words, the tolerance on the weighing result.

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To predict the possible error, it is not only necessary to know the measuring properties of the installation. Alsothe environmental situation and the loading situation for that special measurement must be known.

Some definitionstolerance zone of possible errorsinfluence factor specified load cell property

(result on output of disturbing influence or loading situation)disturbing influence condition which influences load cell output, independent of the force on the load cellloading situation history of loading until the moment of measurement

The error on the measured result of a weighing is built up by several aspects

1.7.4.1 Installation specificationThe load cell specification gives the primary information for the possible measuring errors. However, you alsoneed to know what part of the calibration curve is used in the particular installation. Or in other words, how theload cell output is transformed to a scale indication.

For a given installation the important parameters are:- installed load n • Ln (n: number of load cells)- dead load D- scale range S (the scale begins with zero)

Load cell errors expressed in relation to the scale range become:- a zero error of z% of Ln → error (n • Ln) • z%/S of scale range- a span error of s% → zero error D/S • s% of scale range

and a span error s% of the measured net load

The influence factors connected to the loading situation are specified as a% of Ln. The operational loads on theload cells, however, never are lower than D kg and mostly never will become more than D + S kg. Thereforethese errors will be smaller in a practical installation, as will be explained now:Normally the non-linearity can be described as quadratic deviation between the calibration curve and a straightline. That means, that for a smaller part of the calibration curve, the non-linearity error as a percentage of theused part is also smaller. Example:

If the non-linearity error were exactly quadratic, then with a specified non-linearity of 1% of Ln, this becomesonly S/(n • Ln) • 1% of the scale range. Of course, the slope of the calibration curve over the part S is not thesame as for the complete curve. Calibration of the installation, however, makes this not relevant.

Hysteresis will also be smaller than specified, if the load is only changing over a small part of the calibrationcurve. To simplify the matter for the error calculation we will use the specified figures for non-linearity, hysteresisand combined error with the remark that they are related to the scale range and not with Ln.

The specified creep is the effect that occurs if the load is changed from zero to Ln (the rated load of the loadcell), or inverse. Naturally, this effect is less, if the load is changed over a smaller part of the calibration curve.Also here we will use the specified figure for the error calculation, but with the remark that the figure is related tothe scale range instead of Ln.

Repeatability is always existing and does not change with the loading situation. Related to the scale range thefigure becomes bigger than the specified figure. Example:

If the specified repeatability is r% of Ln, this will be (n • Ln)/S • r% of the scale range.

1.7.4.2 Use of the weighing installationNot every error type mentioned above will always contribute to the final weighing result. That depends on theuse which is made of the installation. This will be illustrated by the following three typical examples.

Case 1 normal weighingThe aim is to make a rapid, accurate measurement of certain mass quantity. Before loading the platform orvessel (load carrier), the zero point on the scale is checked and/or adjusted. After the mass is placed and the

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indication is stable, the reading is done. The procedure eliminates all zero errors.

A variant to this procedure is ‘negative weighing’. In that case the mass to be weighed is taken out of the vessel.This also eliminates zero errors.

In both cases we have to do with1. TKc (temperature influence on span)2. Fu (hysteresis)3. Flin (non-linearity)4. Fv (repeatability)

Remarks:a) Knowledge of the conditions during the calibration of the installation can be important. If the temperature

is about the same as during calibration, the temperature effect is small. If the weighed loads are in theneighbourhood of test load during calibration, the non-linearity error will be very small. Excluding Flin

means that we have to calculate with Fu. In that case that the measurements are done in an arbitrary wayall over the scale range, Flin cannot be excluded and we can calculate with Fcomb as the combined effect ofhysteresis and non-linearity.

b) Repeatability is always existing. We have to it with twice.

Case II Storage silosHere the installation is used to measure how much material is present at different moments. The load ischanged continuously, but never made zero. In this case it is impossible to readjust the zero point before ameasurement. Moreover, the measurements are done over a very long period and under very differentenvironmental conditions. Therefore all influence factors play their role.

Remark:We have, of course, the case I again, if we use this installation for a short term measurement of the incomingor outgoing quantities.

Case III BatchingThe aim of this measurement is to weigh small quantities, (e.g. additives in a batch), which are added to orsubtracted from a big load. The loading therefore only changes in small steps, which excludes the hysteresiseffect. Also zero effects are not present, because the value of the small quantity is found by subtracting twosubsequent readings. Non-linearity plays ist role, because the slope of the calibration curve at the point of theactual load can be very different from the mean value. The error expressed in kg, however, will be very smallbecause the measured incremental quantity is very small. The conclusion is that in this case we have only to dowith the repeatability error Fv, and that twice!

Other error sourcesUntil now we discussed the weight indication for an ideal installation with:

- no mechanical disturbing influences, such as wind, pipes, friction etc- no electrical interference on the cabling- an ideal electronic equipment with negligible measuring errors- an exact reading of the weighing result with no human errors and no rounding off problems for

digitalisation

There are installations where not all of these sources of error can be eliminated or kept the desired minimum.Installation cost and technical problems can limit our possibilities! In that case it will be clear that all these errorsources should be taken into account if the accuracy of the final weighing result is discussed or even has to beguaranteed!

NoteThe rounding-off error in digital equipment can be very important. Per reading it can be ±0.5d at maximum.(d: scale division). If the weighing result is the difference of two readings this extra error will be ±1d at

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maximum (however, not probable ...). If the scales have 1000d this is ±0.1% of the scale range. Especiallyfor incremental weighing this could be a reason for choosing scales with many divisions.

Totalizing the partial errorsThis is an uncertain point in the error calculation. As is said already the specified influence factors are maximumand absolute values. We will call the maximum errors found with them the 'partial errors'. Now the problem isthat we want to come to the most probable total error. therefore we could totalize them by pure adding and findthe total error Ftot max (called ‘maximum’). But then we know that this Ftot max is certainly too big. The probabilitythat this maximum value is reached is practically zero.

Another approach is to take the root out of the sum of the squares of the partial errors. This could give a betterresult. However, there is no mathematical proof for that. (It could even be stated that such a calculation is onlyallowed for a big number of measurements made under the same circumstances. Here we have to do with onemeasurement under arbitrary conditions done with the aid of a small number of load cells out of a very bignumber of manufactured load cells! This is not the same.) We call this second approach Ftotp (‘probable’).Because there is no mathematical proof for the use of this approach, we do not use it.

Remark:If we use more than one load cell in the installation this has an ameliorating effect on the influence factors.For the combination the factors are the same as the specified ones if the first approach is used. With thesecond approach they are multiplied with 1/_n.

Example 1 normal weighingVessel with three load cells PR 6201/23D1

installed total 6000kgdead load 3000kgnet scale range 2000kg

The calibration was done at 1800kg net and at 15°C. What is the tolerance if a measurement is done of1500kg at a stable temperature of 35°C?

Technical dataTKc 0.03%/10KTK0 0.08%/10K *Fv 0.01% *Fcr30 0.02% *Fcr4h 0.05% *Flin 0.05% *Fu 0.03% *

In this case the relevant influence factors are TKc, Fu. The temperature has a difference of 20°C compared tocalibration.

The partial errors aretemperature on span (35°C - 15°C)/10°C • 0.0003 • 1500kg = 0.9kglinearity error 0.0005 • (2000kg/6000kg)2 • 6000kg = 0.33kgrepeatability at zero 0.0001 • 6000kg = 0.6kgrepeatability at load 0.0001 • 6000kg = 0.6kg

The total error is calculated asmaximum error 0.9 kg + 0.3 kg + 0.6 kg + 0.6 kg = 2.43 kg

REMARKFor a weight of 1,800kg, the maximum error is 2.37 kg

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Example 2 Storage siloWe use the same vessel as in example 1. What is the tolerance for a measurement of 1500kg at atemperature of 20°C?

Now all influence factors play a role.temperature effect on span

(20°C-15°C)/10°C • 0.0003 • (3000kg + 1500kg) = 0.68kgtemperature effect on zero

(20°C-15°C)/10°C • 0.0008 • 6000kg = 2.40kglinearity error

0.0005 • (2000kg/6000kg)2 • 6000kg = 0.33kgrepeatability at zero

0.0001 • 6000kg = 0.6kgcreep

0.0005 • (3000kg + 1500kg) = 2.25kg

The total error is calculated asmaximum 6.23 kg

Example 3 BatchingThe same vessel as in example 1. What is the accuracy with which we can measure an addition of 100 kg toa load of about 1500 kg?

With a quadratic non-.linearity there could be calculated that the maximum deviation of the slope of thecalibration curve over the scale range will be ±0.07% of the mean slope. Taking this as a possible span error forthe measurement of the small quantity of 100 kg then we have the following partial errors:

span error due to non-linearity 0.0007 × 100 kg = 0.07kgrepeatability before the addition 0.0001 × 6000 kg = 0.6kgrepeatability after the addition 0.0001 × 6000 kg = 0.6kg

The total error is calculated asmaximum 1.3 kg

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1.8 Installation and commissioningThe possible measuring accuracy of a weighing installation depends on the positioning of the load cells andespecially on their mounting.

Load cells are accurate measuring devices and must be handled with care. Do not lift the load cell at its cable.Avoid shock loads on the load cell (falling goods, heavy shocks).During the building phase of a system the load cell should be replaced by a dummy to keep damages caused bywelding away from it.

1.8.1 Mechanical installationAligning the load cells is not necessary in case of only three load cells because such an installation ismechanically defined (refer to chapter 1.2). In case of more than 3 load cells in one installation the load cellsmust be adjusted in height to avoid waggling and to get an even load distribution. Each load cell of the particularinstallation shall take nearly the same load. This can be achieved by shimming: you insert thin sheets of metal(thickness: 0.5mm...2mm) between upper loading plate of the mounting kit and the construction. The criterionfor the best alignment is a load distribution which also under different loading conditions ensures enough loadon every load cell. To find out the actual load distribution, you must measure the forces on all load cellsseparately (this is especially advised in the case of a very big object with more than four load cells).

Alignment procedure1. Disconnect all load cell output leads2. Measure the outputs of the different load cells separately with a measuring instrument.3. Compare the indications.4. If an indication is too low, then you need another shim at that particular load cell.

1.8.2 Electrical installationLoad cell cabling instructionsFix the load cell cable in such a way that the load cell does not move when being pushed or pulled at the cable.

Shortening the load cell cable changes the factory calibration of the load cell. Therefore the shortening isforbidden.

The load cell cables and the extension cables should be carried in armoured steel conduits, which also have amagnetic shielding function.

All cables for measuring purposes have to to be mounted separately from other cables. They shall be put at adistance of at least 1m from all power cables. The permissible length between load cell and measuringinstrument mainly depends on local circumstances. Normally the limit for W&M purposes is 300m.

Ensure that no moisture enters the cables or the cable connections before and during the mounting, installationand operation.

The screens of the load cell cables should remain insulated and only interconnected at the ‘screen terminal’situated in the cable junction box.

The special GLOBAL Weighing cables PR 6135 or PR 6136 should be preferably used to interconnect the cablejunction box with the electronic measuring instrument. The screens of this cable should also be interconnectedat the ‘screen terminal’ in the cable junction box. Earthing of the total screen should be done at the ‘screenterminal’ of the measuring instrument.

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EarthingEarthing of an electronic weighing installation serves three purposes: safety, prevention of interference, anddamage prevention.

1. safetyAll electrical equipment connected to the mains must not cause a danger of life if being touched. The legalregulations are to observed.

2. InterferenceCapacitive coupling of the outer world to the measuring circuit can disturb the measurement. This can beavoided with cable screens which have to be at the same potential as the measuring circuit. Therefore thefollowing measures are to be taken

- provide armoured steel conduits for the cabling - least distance between power cables and measuring cables 1m.

Mind that all screens have to be earthed at one point only, to avoid that otherwise stray currents still couldchange their potential.

3. Damage preventionIt is a very dangerous situation if the strain gauge filament has a too big voltage difference to the load cellbody. For that reason the load cells have an earthing screw which can be interconnected with the ‘centralearth rail’. Heavy stray currents can be expected if e.g. the weighed object is situated outdoors or at a bigdistance from the electronic measuring equipment (weighbridge, big bunkers, etc.) In those cases you haveto take the following practical measures:a) An ‘earthing tube’ or ‘earthing plate’ with an earth resistance of <5Ω should be put into the ground in the

neighbourhood of the weighed object.b) This earthing electrode has to be connected with the ‘central earth rail’, mounted in the neighbourhood of

the cable junction box.c) now make the following connections:

(the thick lines in the wiring diagram)- all load cell earthing screws with the central earth rail with conductors of a t least 6mm2 copper- the ‘measuring earth terminal’ of the electronic measuring instruments with the central earth rail

(equalizing line)NOTES:• Mostly 16mm2 is enough. You can make a rough check, if you can estimate the possible current

flowing through the equalizing line. The voltage drop may not exceed 100V, which is themaximum allowed voltage between strain gauge filament and load cell body to avoid electricaldamage.

• The measuring cable PR 6135 is mounted in a steel pipe. This pipe could be used as a shuntresistance to the equalizing line to lower its resistance.Attention: Never use cable screenings for this purpose!

Fig. 1.8-1 Earthing of a weighing installation

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1.8.3 CalibrationReasons for a calibration procedure on siteWeighing is the determination of the mass of an object (refer to chapter 1.1). Mass is an invariable property ofan object. It does not change with its position on the earth nor in space. However weighing with load cellsmeasures a force. The weight FG is the force with which the earth attracts the mass. This force depends on theposition on earth (see table). The characteristic value for the weight is the gravitational field strength at thatspecific spot.

city gravitational field strength g [m/s2]

Hamburg 9.814

Helsinki 9.819

Johannesburg 9.786

Madrid 9.800

Melbourne 9.800

Milan 9.806

Paris 9.809

Singapore 9.781

Teddington 9.812

The weight is calculated with the formula

m g = FG •FG Force in Newtonm mass in kgg local gravitational field strength in m/s2

Please, notice the different values of g. The table shows a difference of 0.4% between the extremes in g. In theload cell factory in Hamburg the output of the load cells is calibrated within ±0.25%. If the desired weighingaccuracy must be better than ±0.25%, a calibration procedure on site is indispensable. Example: scales usedfor trade transactions.

Check on proper installationBefore the final calibration, it is necessary to check if all disturbing effects are reduced to a minimum. Alwayskeep in mind that errors are seldom generated by the load cells but are introduced by the mechanical orelectrical installation of the equipment.

1. Visual inspectionmechanically: pipes, hoses, dirt, stones, friction by touching the outer world, vibration of foundation,

bellows, wind, suction, etcelectrically: bad ground-connection, interference, moisture in cable connections

2. Stability testPut a stable load on vessel or platform and observe the stability of the weight indication. This should beexcellent.

3. ResolutionPut on a very small load of about 0.5d value and observe if the change in weight indication is big enough.Repeat this after taking the small test load away.

4. Hysteresis testCheck how the weight indication comes back after placing and removing a big load.

Errors with these tests should be well within the desired accuracy limits on different places of the scales.Possible causes of error can systematically be traced by:a. disconnecting or removing parts which link the weighing object and its surroundings (pipes, hoses and the

like)b. looking for changing material contents in pipes and hoses connected to the objectc. looking for changing gas pressure on bellows (see chapter 3.3)

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d. switching off electric motors on the object or in its neighbourhoode. measuring the insulation resistance in the cablingf. in case of severe doubt, the load cells can be checked electrically (see sheet 1.851)

Final calibrationThe purpose is to bring the indication on the measuring instruments (in mass units) in accordance with theweighed mass. This is done by changing the span of the electronic indicating instrument. The test weights usedfor the final calibration can be - calibrated test weights (weight stones with a small tolerance) - some material, weighed on another weighbridge - a known quantity of fluid of known specific gravity (and non volatile!).In all cases you should know (or have a good estimate of) the tolerance of the test weights, which should be atleast a factor 3 (O.I.M.L.) better, but preferably 5 or 10 times better than the desired installation accuracy.

If you have to check against an O.I.M.L.-tolerance curve, special attention must be given to the critical points at500d and 2,000d. Remark that correction of a small error at 500d, by changing the instrument span, has abigger influence at the higher scale values. Example:

a correction of 0.4d at 500d changes the indication at the 2,000d scale position by

1.6d = 0.4d 5002000

•

On the other hand a correction for an error of 0.4d at 2000d changes the indication at 500d only by

0.1d = 0.4d 2000500

•

1.8.3.1 Calibration of vessels of more than 5 tonsO.I.M.L. and most regulations require that a weighing installation is suitable for testing. This means among otherthings that a load carrier must be designed in such a way that test loads can applied in an easy and safe way. Inthe case of bunkers, hoppers, etc. a (detachable) carrier should be supplied for 10% of the weighing capacity,but at least for 5t of calibrated weights. There is no general rule for the special load carrier which has to beprovided. Depending on the local circumstances, this may be designed on top of the vessel, hooked around thevessel, or hanging under the vessel. However, be sure that a normal even distribution of the load is possible.

Calibration of the vessel may be done with a certain amount of calibrated weights (e.g. 5t) plus arbitraryweights. Remark: the arbitrary weights should have such a composition that their masses will not change duringthe calibration procedure (think of evaporation, hygroskopic effects, leakage, loss of material).

Calibration procedure for non- W&M installationsIf there are no legal requirements the procedure above described can be followed. However, if necessary asmaller amount of calibrated weights may be used.

Calibration procedure1. Adjust the weight indicator to read zero (compensation for tare weight)2. Put on the obligatory amount of calibrated weights; note the indicator reading3. remove the calibrated weights4. fill the vessel with material until exactly the previous reading5. repeat the steps 2...4 until 100% of the weighing capacity is reached6. adjust the span of the indicator, if necessary

1.8.4 Corner point adjustmentThe unloaded platform is put on the disconnected load cells. The load cells are supplied with 12V or 20Vdepending on the electronic used. The outputs of all load cells are measured separately. Shims are putbetween the upper loading plates and the platform until the difference between the readings is not more than0.2mV (this amount corresponds with about 5% of the load on the load cells due to the weight of the platform).

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O.I.M.L. and most national regulations ask for a corner point test. During this examination a test load (, which isdefined by these regulations) is placed successively on the corners of the ‘load carrying device’, e.g. theplatform. The different readings should be the same within prescribed limits. The reading is the result of theelectrical addition of the load cell outputs caused by the parallel circuiting of the load cells. The accuracy of thisaddition is influenced by the equality of the value Cn/Ra of the different load cells.

Cn rated output in mV/VRa output resistance in Ω

As for an example the output of two load cells in parallel can be calculated from

)RC L +

RC L(

R +RR R

a

22

a

11

aa

aa

2121

21 ⋅⋅⋅⋅

Normally the factory adjustments on the rated output Cn and the output resistance Ra of the load cells are goodenough to fulfil all legal requirements.

The corner point test results can be improved by looking for the load cell with the lowest output and thereafterdecreasing the output of all other load cells until that lowest output value.

Procedure1. Take the readings of the corner point test.2. Determine the lowest reading.3. Calculate the deviations of the other readings compared to the lowest reading.4. Insert a resistor in the supply line of the deviating load cells to lower the Cn- value

of these load cells.5. These resistors must have the value

][ impedance cell load [kg] load tcornerpoin actual

[kg] deviation = R Ω•

Note: The resistors should only be fitted with the instrument switched off!

Example of an adjustmentAccording to the data sheet the output resistance of the load cells is Ra = 610Ω. The test load of the corner pointtest is 10,000kg.

corner actual load [kg] deviation [kg]

1 9,994 -----2 10,016 +223 10,004 +104 10,000 + 6

Practical execution of the adjustmentThe resistors can be made of manganine wire. E.g. with 0.5mm the resistance is 476mm/ Ω.

corner resistor value manganine wire

load cell 1 no resistor ---load cell 2 1.3Ω 619 mm

load cell 3 0.6Ω 286 mm

load cell 4 0.4Ω 190 mm

The calculated resistors are mounted in the cable junction box at the indicated positions (see fig. 1.8-2).

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Fig. 1.8-2 Load cell wiring diagram

1.8.5 Load cell checkThe check described in this chapter is intended to give you a first idea what may have happened to a load cellwhich seems to be defective. A more detailed check is only possible under the conditions of a test stand, i.e. inthe factory in Hamburg. Before you do any checks at least the following questions must be answered:• What kind error was observed?

(occurrence of the error: accidentally, slowly increasing, suddenly, ...)• How long was the system in operation until the error occurred?The more you know about the error and the circumstances in the installation the easier you find right source ofthe problems (be it the load cell, be it the construction ...).

The load cell check consists of two parts1. a visual check2. a simple electrical check

Both the checks are important. Very often you can judge by the look of the load cell what caused the system tofail.

Visual checkThe visual check is quite simple: describe the load cell and its mounting parts. Especially focus on the pointsmentioned below. - load cell top and load cell bottom

size and shape of the black contact spots (large, round, ...)surface (smooth, ...)

- mounting partssize and shape of the black spotssurface

- cable sheathcuts ...

Simple electrical check on load cellsYou need the following instruments - 12V DC supply (e.g. indicator) - digital multimeter, resolution 10µV DC - display of weighing indicator

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A standard procedure is used to check the technical data of the load cell. It helps you to find the diagnosis whenthe load cell shows deviations. During the checks you measure the load cell data - tolerance on zero point D0

- input and output resistance (Rin, Rout) - insulation resistance (Riso)The load cell must be completely unloaded for all described checks.

Check 1 tolerance on zero point (D0)

Fig. 1.8-3 Tolerance on zero point

The permissible values depend on load cell type. You find them in the data sheets and the instruction manualsfor the particular type. For your convenience the values are calculated with a load cell supply voltage of 12V(see fig. 1.8-3) - PR 6201, PR 6221

< ±1% of 1mV/V• 12V = ±0.12mV - PR 6211, PR 6221, PR 6241, PR 6246

< ±1% of 2mV/V• 12V = ±0.24mVFor a supply voltage of 20V the values change to ±20mV or ±40mV, of course.

Check 2 bridge resistances

Fig. 1.8-4 Bridge resistances

The exact values differ with the load cell type and its accuracy class. Please refer to the data sheets or theinstruction manuals to find them out.Example: PR 6201/14C3 - input resistance Rin = 650Ω±6Ω

(measure red and blue core) - output resistance Rout = 610Ω±0.5Ω

(measure green and grey core)Additionally it is recommended to measure the further combinations

red and grey, red and green, blue and green, blue and grey

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Check 3 insulation resistance Riso

Measure each core and also the screen (if existing) with the housing. If you look into our data sheets, thespecification says „better than 5000MΩ“. But it should be not too easy to measure such values with a standarddigital multimeter because such an instrument displays “OL” (“Open Loop”) if the insulation resistance is higherthan 20MΩ.

Interpretation of the results:1. If a load cell fails in check 1, it was usually overloaded. Further information can be gained from check 2: if the

resistances between the cores (part 2 of check 2) differ significantlywas the load cell tilted?did the housing touch the bottom plate of the mounting kit?

2. A load cell passes check 1 but fails in check 2. In that case you can be sure that the strain gauge broken.Possible causes are· welding· lightning strokes· vibration· temperature too high for load cell

3. A load cell failed in check 3. An „insulation resistance“ between 10Ω and 50kΩ indicates that the straingauge and/or its insulation is destroyed. Values between 50kΩ and 200kΩ indicate the penetration ofmoisture in the load cell. In such a case you can measure a galvanic voltage between housing and cores.REMARK: the weight indication returns to a correct value when the load cell is completely insulated from

the ground potential (not valid in case of the penetration of moisture)

4. Perhaps the load cell passed all the tests above. It could be sensitive to knocking (use a light hammer, about100g). Connect the load cell like in fig. 1.8-3, put some paper or plastics between load cell and hammer, andknock slightly on the housing, the measuring element and the small connection box. Does the indicationremain stable?

Perhaps not all wires in the cable junction box are correctly connected? (bad soldering, screwed connection notfastened...)

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2. MOUNTING THE LOAD CELLSTo enable easy mounting combined with simple constraining, standard mounting kits are provided for all loadcell families in our programme. Most kits are available either in constructional steel or in stainless steel. - N- versions are always made of constructional steel (Fe360, St37), are zinc plated, and have a yellow

chromated surface - S- versions are always made of stainless steel (e.g. B.S. 304S15, X8CrNi 18 10, 1.4301)

Some mounting kits are integrated mounting kits: they combine a standard mounting solution with constraining,e.g. the MiniFLEXLOCK. Furthermore they have a threaded hole in the lower mounting plate which can be usedfor installing a lift-off protection. You can use following combinations of rod and lower mounting plate: - minimal property class of the rods 4.6

(i.e. limit of elasticity 240 N/mm2, ultimate load 400 N/mm2) - minimal rigidity of stainless steel A2

thread maximum tension force (kN)M6 2.5M8 4.5M12 10.5M16 19.6M20 30.6

The lift-off protection must not touch the weighing object during the execution of a weighing. It is a safety device,a stop and not a constrainer. Therefore the clearance, as shown in fig. 2.0-1, is important.

There are drilling templates (true-to-scale representations!) available for every compression cell mounting kit.They provide easy installation because you can already drill the holes for fixing the mounting kit in the workshopand need not to do it at site. Please, keep to the warnings on their covers: do not transmit the drilling templatesby telefax and do not copy them with a photocopier because the dimensions will not be correct on the copy.

Example: A distance of 65mm can vary between 62mm and 67mm depending on the type of telefax- orphotocopy- machine used.

Sometimes it may be necessary to protect the mounting bolts from losening, e.g. in an installation where heavyvibrations occur. This can be achieved by cementing the bolts to the nuts. A possible cement is LOCTITE 274.

Install the flexible copper strap which is delivered together with the load cell.

Make sure that before installing the load cell all welding at the construction is finished.Mount the load cell vertically so that the weighing result is not disturbed by the tilted load cell.

Fig. 2.0-1 Some indispensable parts and accessories

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2.1 Mounting the compression load cell PR6201The mounting kits are intended to be used together. They are equal in height for the load cells in the rangebetween 0.5t up to 50t. The way how to arrange the mounting kits is discussed in chapters 1.3 and 1.4. (3 loadcells installed with PR6143/.., the others with PR6145/..).

Sometimes the stainless steel version, especially the base plate and the load disc, are checked with magnetsand found to be magnetic; and that is correct: both articles are magnetic, although they are made of stainlesssteel. So do not tell the customer all parts from stainless steel are non-magnetic! Furthermore, even non-magnetic stainless steel can become magnetic after special treatment (e.g. deep drawing, pressing).

2.1. 1 Mounting kit PR 6145The standard mounting plate kit PR6145 permits easy and service-friendly installation of the PR6201/.. load cellseries.

type order number capacityPR 6145/00 N 9405 361 45001 0.5...50tPR 6145/00 S 9405 361 45002 0.5...20tPR 6145/08 N 9405 361 45081 100tPR 6145/10 N 9405 361 45101 200t

REMARK:If PR 6201/24D1, PR 6201/24C3, PR6201/34 and PR6201/54 are used with stainless steel versionPR6145/00S, you must order the additional stainless steel bottom plate PR6143/54S.

ASSUMPTIONS· Ensure that the foundation is horizontal (check using spirit level), flat and rigid for the expected loads.· Ensure even load distribution. The foundations should be at equal height and the contact surfaces of the

object (vessel or platform) should be located in parallel.· The bore holes of top and bottom mounting plate must coincide.

MOUNTING INSTRUCTIONS· Mount lower and upper mounting plate at foundation or object support with screws.

It is indispensable to ensure that the plates are located in parallel and vertically upon each other

· Link upper and lower mounting plate with the flexible copper strap (fig. 2.1-1, pos.2) packed with the loadcell. Two screws M10x15 with washer A10 are included in the mounting kit

· Clean the load cell seating in the mounting plate from dirt.

· Grease load cell top and bottom with the included grease.

· Insert the load cell only, when all welding work near the load cell and mounting work at the object is finished.

· When using PR6145/00S, the bottom disc marked with ‘SS’ and the light beige O-ring have to be used. Thestandard bottom disc (without marking) and the dark grey O-ring are not required

· Position the load button correctlytype capacity position of load button

PR 6201 0.5t ... 10t, 20t L small hollow down, circular groove upPR 6201 20tD1/C3, 30t, 50t small hollow up, circular groove down

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INSPECTION AFTER MOUNTING· check if the mounting plates are vertical upon each other, and in parallel· the load cell must be in vertical position (if necessary, release the mounting screws slightly and correct the

position of the mounting plates, without obstructing the holes for the bolts)· ensure the right position of the load button

Fig. 2-1-1 Dimensions of mounting plate kit PR 6145/..

Type Nominal load Dimensions in mma b c d e f g h i

PR 6145/00N 0.5t...50t 15 190.5 15 150 115 14 65 100 18PR 6145/00S 0.5t...50t 15 190.5 15 150 115 14 65 100 18PR 6145/08 100t 30 290 30 180 145 18 95 130 18PR 6145/10 200t 40 385 40 220 185 24 135 180 14

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2. 1.2 Mounting kit MiniFLEXLOCK PR 6143

MiniFLEXLOCK PR6143 is a mounting kit for PR6201 load cells with a load rating of 0.5t to 50t, which alsoprovides horizontal constraining of the object to be weighed. The kit is prepared for construction of a protectionagainst lifting.

In order to ensure the required space for movement of the weighing facility, max. three MiniFLEXLOCK kits maybe used for constraining an object. When using a higher number of load cells, the remaining load cells must beinstalled with mounting kit PR6145/00.

type order number load cellPR 6143/00N 9405 361 43001 0.5t...50tPR 6143/00S 9405 361 43002 0.5t...20tPR 6143/10N 9405 361 43101 0.5t...50tPR 6143/10S 9405 361 43102 0.5t...50t

ASSUMPTIONS· ensure that the foundation is horizontal (check using spirit level), flat and rigid for the expected load· ensure even load distribution

the foundations should be at equal height and the contact surfaces of the object (vessel or platform) shouldbe located in parallel

· the bore holes of top and bottom mounting plate must coincide.

MOUNTING INSTRUCTIONS· insert the dummy load cell or, if no dummy is provided, the load cell with the load button

· position the load button correctlytype capacity position of load button

PR 6201 0.5 ... 10t, 20t L small hollow down, circular groove upPR 6201 20t D1/C3, 30t, 50t small hollow up, circular groove down

· When using a stainless steel MiniFLEXLOCK (version S), the bottom disc marked with „SS“ and the food-resistant O-ring (light beige) must be used. In this case, the standard bottom disc (without stamp) and thedark grey standard O-ring are not required.

· Connect upper and lower mounting plate with flexible copper strap (packed with the load cell). Two bolts M8and washers for locking are delivered with the mounting kit.

· Re-mount the upper mounting plate at the auxiliary plate.

· Mount lower and upper mounting plate by means of bolts and washers to foundation and weighing objectsupport (vessel, platform, etc.). For constraining unit PR6143/00, bolts M12, min. rigidity class 5.8 (to ISO898), and for PR6143/10, bolts M14, min. rigidity class 8.8 are required. It is indispensable to ensure that theplates are located in parallel and vertically upon each other

· When all constraining units and mounting kits are mounted at the weighing facility, remove the auxiliarymounting plates (tighten the relevant bolts in the threaded holes of the mounting plates) and align themounting plates so that the dummies stand untiltedly and in exact vertical position. If required, release themounting bolts slightly and adjust the mounting plates. For this purpose, the holes offer some free space.

· tighten the mounting bolts crosswisely in the order 1...6 (fig. 2.1-2).

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Fig 2.1-2 Tighten the bolts in the order 1...6

Bolt Property class Mounting torquePR 6143/00N M12 5.8 104NmPR 6143/00S M12 A2 104NmPR 6143/10N M14 8.8 165NmPR 6143/10S M14 A2 165Nm

· Only when all welding work near the load cell and mounting and alignment at the object to be weighed arefinished, lift the object and replace the dummy by the load celltake care that the load button is positioned correctly. The load cell must stand vertically and untiltedly afterinstallation.

INSPECTION AFTER MOUNTING· Check if the mounting plates are vertical upon each other, and in parallel· The load cell must be in vertical position (if necessary, release the mounting screws slightly and correct the

position of the mounting plates, without obstructing the holes for the bolts)· Ensure the right position of the load button· Check, if the constrainer is free of load, i.e. if it can be rotated around its longitudinal axis. If this is not the

case, release the counter-nuts (Fig. 14, part 5) of the constrainer, and turn the hexagon (6), until theconstrainer is free of load. Subsequently, re-tighten the counter nuts and position the swivel bearing eyesvertically.

· Check, if there is vertical space for movement and the required space for thermal expansion. The permissibletolerances without considerable effect on the accuracy are given in Fig. 15. The space for movementrequired for displacement of the object to be weighed due to thermal expansion, vibration, etc. can be usedonly if load cell and constraining unit are installed exactly.

To prevent vertical force shunts, all mechanical connections (pipes, cables, bellows) of the object to be weighedto its surrounding construction must be as flexible as possible. The overall load must be supported by the loadcells.

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Fig. 2.1-3 Lift-off protection with PR 6143

SERVICE INSPECTION- Check if the constrainer is not clamped.- Check if the safety clips are present and if the axle is fixed

Fig. 2.1-4 Safety clip and axle

Fig. 2.1-5 Dimensions of MiniFLEXLOCK PR 6143/00

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Fig. 2.1-6 Dimensions of MiniFLEXLOCK PR 6143/10

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2.3 Mounting the ultra flat PanCake load cell PR 6251

PanCake is the ideal load cell for level-by-weight (cf. 3.5). Its performance is better than the often used tankradar system or ultrasonic level measuring devices. PanCake is easy and simple to install (“just bolt it”) underthe foot of a tank. There is no need for additional expensive mounting kits.

If you have no access from the upper side of the foot to the load cell, you may apply the transition plate tomount the PanCake (fig. 2.3-1).

Fig. 2.3-1 Transition plate PR 6051/00S for PanCake

All supporting and connecting plates (for foundation and vessel) must be horizontal, flat and rigid.

The plate which is in contact with the load cell top must be tempered to a hardness of 42 HRc. The standardbase plates PR 6051/1x fulfil this requirement. The little pins help to position the load cell correctly, but they arenot designed to withstand horizontal side loads.

Fig. 2.3-2 Base plates PR 6051/10S, .../11S for PanCake

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2.4 Mounting bending beamsA safety instruction for suspended loads

If a break in suspension, support, load cell, or mounting part etcrepresents a hazard to the life and health of men or animals, or ifgoods may be damaged, additional safety devices must be provided.

Observe the overload protection as the beams have quite low capacities and are easy to overload! Keep in mindthat a 10kg load cell will be damaged if it is loaded with 30kg. In other words, no one must step onto it;otherwise it is damaged.

Fig. 2.4-1 Mounting kit PR 6007

The use of the mounting kit PR 6007 ensures a simple and safe suspension of the weighing object. The stablecover protects the thin and sensible bellows. By turning the protection cover to 180°, the load end of the cell iseither protected or freely accessible. The pre-stretched steel rope provides easy restoring to the stable positionand a counteracting against side forces and torque momenta of the object. Spherical washers minimize the riskof tilting at the suspension points. The overload stop protects the load cell against damages due to overload.

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MOUNTING INSTRUCTIONS - Already mount load cell and mounting kit in the workshop

Pay attention to the arrow on the front side of the load cell, showing the direction of the load.

- Clean the support surface and the mounting surface of the load cell and mount the load cell by means of twobolts.

- Mount the kit on the supporting construction and fasten the plate with four bolts.

- Lift the object to be weighed and mount the steel rope. Take care that the spherical washers are correctlypositioned. The castle nut has to be secured by a split.

- If the object is suspended by more than 1 load cell, adjust the mounting kits in such a way that it is level.

- Adjust the overload stop!Load the object with 130% full load. The overload stop must touch the load cell under these conditions. Lockthe overload stop safely.

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2.5 Mounting the S-type load cells

2.5.0 Differences between PR 6246 N and PR 6241D1, PR 6246D12.5.0.1 General information about the replacement

PR 6246N had been developed as a universal load cell for tension and compression use. For the W&M versionof class C3 it proved to be necessary to separate tension from compression type. Therefore it was decided tocreate two different families: one for tension applications (PR 6246) and the other for compression use (PR6241). In some countries there was a certain demand for a load cell useful for W&M systems of class IIII. So,the N versions were upgraded to D1 versions.

Direct replacement of PR 6246 N in an existing installation is possible as you see from the table belowinstalled load cell replacement for

tension modereplacement for

compression mode

PR 6246 N, 100kg...500kg PR 6246 D1, 100kg...500kg(PR 6241 D1, 100kg...500kg)

PR 6241 D1, 100kg...500kg(PR 6246 D1, 100kg...500kg)

PR 6246 N, 1t...3t PR 6246 D1, 1t...3t PR 6246 D1, 1t...3tPR 6241 D1, 3t

The above table shows that the small versions of PR 6241 D1 and PR 6246 D1 may be used in either mode ifthey are not in an W&M system. But you may not use the compression type load cells PR 6241 with capacitiesof 1t up to 5t in tension mode. This applies for all installations, W&M and non- W&M.

In new installations the following load cells should preferably be usedtension mode PR 6246 D1, 100kg...3tcompression mode PR 6241 D1, 100kg...5t

You have to use the load cells from the list below for approved installationstension mode PR 6246 D1, 100kg...3tcompression mode PR 6241 D1, 100kg...5tYou may not use the types with the small capacities in other modes as indicated by the two arrows on the loadcell.

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2.5.0.2 Colour codes of the load cell cableThe colour code of the load cell cable wires was also changed from PR 6246 N to PR 6241 D1, PR 6246 D1types. PR 6246 N was defined as a „universal load cell“; its preferred operation direction was „compressionmode“. Now the definitions are - PR 6241 D1 is a compression load cell. The output signal is positive if the load cell is loaded with a

compression load.- PR 6246 D1is a tension load cell. The output signal is positive if the load cell is loaded with a tension load.

Fig. 2.5-2 Connection for load cell operated in specified mode

Fig. 2.5-3 Connection for load cell operated in reverse mode

2.5.1 Mounting kits for compression load cell PR 6241

2.5.1.1 Mounting kits PR 6041/30, PR 6041/40Mounting kits PR 6041/30 N + S and PR 6041/40 N + S are used for unconstrained installation of GLOBALWeighing series PR 6241 compression load cells. The kit can be used for load cell types PR 6241 D1 andPR 6241 C3.Mounting kits PR 6041/30 and PR 6041/40 N + S comprise an upper and a lower mounting plate and a loadbutton set.

Fig. 2.5-4 Mounting kit PR 6041/30

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The load cell is installed with the load buttons fitted between the mounting plates. Due to their spherical headsurface, the load buttons permit limited load cell inclination, in order not to prevent free movement with thermalexpansion of the object (vessel, etc.) to be weighed.

During transport and mounting, the mounting kit is fixed by means of an auxiliary mounting plate, which must beremoved after installation. The mounting kit includes a load cell dummy, which is used for installation andalignment of the mounting kit and replaced by the load cell after finishing mounting, welding and alignment.

Before installing the mounting kit with the load cell, ensure that the foundation is horizontal (check with spiritlevel), flat and rigid for the expected load. In order to ensure even load distribution, the foundations should be atequal height and the supporting surfaces of the object to be weighed (vessel or platform) should be in parallel.The bore holes of upper and lower mounting plate must coincide.

MOUNTING INSTRUCTIONS· Screw lower and upper mounting plate (with auxiliary plate fitted and load cell dummy in position) at

foundation and support of object to be weighed. When all mounting kits or constraining units are mounted atthe weighing facility, remove the auxiliary mounting plates (screw the relevant screws in the threaded holesof the mounting plates) and align the mounting plates so that the load cell dummy is uncanted and exactlyhorizontal. If necessary, release the mounting screws somewhat and adjust the mounting plates (the screwholes offer some space). Now, tighten the mounting screws correctly.

· Connect upper and lower mounting plate using the flexible copper strap delivered with the load cell; twomounting screws M8 x 10 with spring washers are included in the mounting kit.

· Replace the load cell dummy with the load button set only after finishing the mounting and alignment at theobject to be weighed including all welding near the load cell. When installing, take care that the mountingparts are exempt of dirt (sand, etc.).The load cell shall be positioned centrally between the limiting pins and shall be vertical and untilted afterinstallation.

Fig. 2.5-5 Mounting kit PR 6041/40

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2.5.1.2 MiniFLEXLOCK PR 6043/30, PR 6043/40Mounting kits PR 6043/30 N + S and PR 6043/40 N + S are used for constrained installation of GLOBALWeighing series PR 6241 compression load cells. The kit can be used for load cell types PR 6241 D1 and PR6241 C2 + C3.To ensure an invariably high weighing accuracy, the foundation for the mounting kit must be horizontal (usespirit level), flat and rigid for the loads to be supported. In order to prevent partial overloading of the load cells,the load must be distributed as symmetrical as possible. Therefore, the foundations of the mounting kits must bealigned and the supporting surfaces of the weighing object (vessel or platform) must mounted in parallel.

Fig. 2.5-6 MiniFLEXLOCK PR 6043/30

MOUNTING INSTRUCTIONS

· Connect upper and lower mounting plate with flexible copper strap (packed with the load cell). Two bolts M8and washers for locking are delivered with the mounting kit.

· Mount lower and upper mounting plate by means of bolts and washers to foundation and weighing objectsupport (vessel, platform, etc.).

· When all constraining units and mounting kits are mounted at the weighing facility, remove the auxiliarymounting plates (tighten the relevant bolts in the threaded holes of the mounting plates) and align themounting plates so that the dummies stand uncantedly and in exact vertical position. If required, release themounting bolts slightly and adjust the mounting plates. For this purpose, the holes offer some free space.

· Only when all welding work near the load cell and mounting and alignment at the object to be weighed arefinished, lift the object and replace the dummy by the load cell. Take care that the pressure piece ispositioned correctly. The load cell must stand vertically and untiltedly after installation.

· Check, if the constrainer is free of load, i.e. if it can be rotated around its longitudinal axis. If this is not thecase, release the counter-nuts of the constrainer, and turn the hexagon, until the constrainer is free of load.Subsequently, re-tighten the counter nuts and position the swivel bearing eyes vertically.

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Fig. 2.5-7 MiniFLEXLOCK PR 6043/40

After mounting, check · if the load cell stands untiltedly in the mounting kit, · if the upper mounting plate is positioned horizontally, and · if there is vertical space for movement and the required space for thermal expansion. The space for

movement required for displacement of the object to be weighed due to thermal expansion, vibration, etc.can be used only if load cell and constraining unit are installed exactly.

To prevent vertical force shunts, all mechanical connections (pipes, cables, bellows) of the object to be weighedto its surrounding construction must be as flexible as possible. The overall load must be supported by the loadcells.

2.5.2 Mounting kits for the tension load cell PR 6241

2.5.2.1 Swivel bearing kit PR 6046A safety instruction

If a break in suspension, support, load cell, or mounting part etc represents ahazard to the life and health of men or animals, or if goods may bedamaged, additional safety devices must be provided.

The swivel bearing kit is used for suspension of objects to be weighed on load cells PR 6246. The kit is easy toinstall and permits free suspension of the object with short pendulum length and very low friction which mayrender constraining unnecessary.

PR 6246/22...52 PR 6246/13...33

PR 6046/00 possible impossiblePR 6046/11 impossible possible

· When fitting the swivel bearing bolts into the load cell thread, following the hints in the operating instructionsconcerning screw-in depth, maintaining of the load cell and torque is indispensable.

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· Take care that the load cell is mounted in correct position, the connecting cable must not be weighedtogether with the load. The letters must be upright.

· Secure the swivel bearing bolt by counter nuts to avoid self-releasing under the circumstances of theapplication. If heavy vibration must be expected, the threads must be protected with e.g. Loctite 274.

Fig. 2.5-8 Swivel bearing kits PR 6046/00 and PR 6046/11

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2.7 Mounting the compact load cell PR 6211The load buttons are available as spare parts.

load cell capacity order code

PR 6211/31...32 30kg...300kg 5312 693 98068PR 6211/52...53LT 500kg...5t 5312 693 98069PR 6211/52...53D1 500kg...10t 5312 693 98085

2.7.1 Mounting kits for the small type (30...300kg)

2.7.1.1 Mounting kit PR 6011/00Mounting kit PR 6011/00 is designed especially for easy and reliable installation of small PR 6211/31.../32series compression load cells with a nominal load of 30 kg to 300 kg. By means of a rod, 3 nuts and 2 washers,the mounting kit facilitates the construction of a protection against lifting.

Mounting kit comprises two mounting plates. The load cell is seated in a recess of the lower mounting plate. Apressure piece is located between load cell head and upper mounting plate. Its teflon surface permits easymovement within the recess in the upper mounting plate. Thereby, thermal expansions and small displacementsof the object to be weighed can be absorbed without a measurement error.

Fig. 2-7.1 Mounting kit PR 6011/00

The space for movement is limited by the size of the recess. If the load button presses against the edge of therecess with the constraining missing, horizontal forces in this direction are compensated via the load cell. In thiscase, heavy horizontal forces cause an inclination of the load button and may cause measurement errors andeven mechanical damage. Therefore, take care that the load buttons are located centrally in the recess with theobject in rest.

Before installing the mounting kit, ensure that the foundation is horizontal (check with spirit level), flat and rigidfor the expected load. To ensure even load distribution, the foundations should be equally high and thesupporting surfaces of the object to be weighed (vessel or platform) should be in parallel. It can be used formounting kit and constraining unit. The holes in upper and lower mounting plate must coincide.

To facilitate load cell and mounting kit installation and alignment, dummies can be used. These dummies areinserted when installing the mounting kits. As their head fits exactly into the recess of the upper mounting plate,

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they ensure accurate alignment of mounting kits and weighing installation. The dummies are replaced by theload cells only, when all mechanical and alignment work, as well as welding near the load cell are finished.

MONUNTING INSTRUCTIONS · Link upper and lower mounting plate using the flexible copper strap packed with the load cell. Two mounting

screws M6 x 6 with fan washer are included in the mounting kit.

· Screw lower and upper mounting plate to foundation or object support. Taking care that the plates are inparallel and vertically upon each other is indispensable.

The above-mentioned dummy ensures that they are centered.

· Replace the dummy by the load cell only, when all welding work near the load cell and mounting at the objectare finished. Handle the load cell carefully to prevent damage to the membrane on the bottom. Place theobject to be weighed carefully on to the load cells and take care that the load button is seated correctly in therecess of the upper mounting plate.

· Check, if the load button is seated centrally in the recess of the upper mounting plate. For this, checkdistance A (Fig. ???) to protective ring around the load cell: it must be6 mm. If necessary, release the mounting screws by a few turns and shift the mounting plates as far aspermitted by the holes for the screws.

Only if load cells and mounting kits are installed exactly, the space for movement, which is required for objectdisplacement due to thermal expansion, vibration, etc., can be used fully without limiting the measurementaccuracy.To prevent vertical force shunts, all mechanical connections of the object to be weighed to the surroundingconstruction (pipes, cables, bellows) must be as flexible as possible. The overall load must be supported only bythe load cells.

Tighten all screws and nuts well. When heavy vibrations are expected, we recommend using a protection e.g.by means of Loctite 274.

A protection against lifting, can be realized using a rod M6 (rigidity ≥ 4.6), three nuts and two washers arerequired. Note that the rod must have sufficient space for movement in bore hole.

2.7.1.2 Rubber mounting kit PR 6011/03Rubber mounting kit PR 6011/03 is used where the load cell is subjected to shock and vibration which cancause measurement errors. In order to minimize these effects, screw the rubber mounting kit on to the uppermounting plate of mounting kit PR 6011/00 or constraining unit PR 6011/20 with shock or vibration originatingfrom the object to be weighed (vessel with stirring unit, shaking facility, etc. or filling with bulk material).With vibrations issued from the foundation, fit the rubber mounting kit below the lower mounting plate. In caserubber mounting kits are used, all load cells must be fitted with such a kit. The rubber mounting kit, screwed onto or below mounting kit or constraining unit, must not be used for compensation of horizontal forces.

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Fig. 2.7-2 Rubber mounting kit PR 6011/03

Bolt the rubber mounting kit to the mounting plate of the mounting kit using three screws M6 x 10 with springwasher. A threaded pin M12 is provided for fixing at the object to be weighed (or foundation).

MOUNTING INSTRUCTIONS · Link weighing object and foundation using the flexible copper strap packed with the load cell.

· Rubber mounting kits must be screwed to the relevant mounting plate of the mounting kit before mounting(see above). The required screws M6 x 12 and fan washers are included. Max. tightening torque for thescrews: 7Nm. Subsequently, mount the overall kit with dummy, if possible.

· The rubber mounting kit must not be canted.

To prevent vertical force shunts, all mechanical connections of the object to be weighed to the surroundingconstruction (pipes, cables, bellows) must be as flexible as possible. The overall load must be supported only bythe load cells.

Tighten all screws and nuts well. When heavy vibrations are expected, we recommend using a protection e.g.by means of Loctite 274.

2.7.1.3 MiniFLEXLOCK PR 6011/20Constraining unit PR 6011/20 MiniFLEXLOCK is designed especially for easy and reliable installation of smallPR 6211/31.../32 series compression load cells with a nominal load of 30 kg to 300 kg.The constraining unit constrains the object to be measured in longitudinal direction, i.e. the unit absorbshorizontal forces of max. 450 N. The required space for normal thermal expansion is ensured in Y-direction withthe constraining unit and in all directions with the mounting kit. To ensure the required space for movement ofthe measuring facility, max. three MiniFLEXLOCK units PR 6011/20 may be used for constraining a vessel.When using 4 or more load cells, the remaining load cells must be installed using mounting kit PR 6011/00 inorder not to prevent compensation for expansion and to obtain an equal mounting height.

By means of a rod, 3 nuts and 2 washers, the constraining unit facilitates the construction of a protection againstlifting.

Constraining unit comprises two mounting plates. The load cell is seated in a recess of the lower mounting plate. A load button is located between load cell head and upper mounting plate. Its teflon surface permits easymovement within the recess in the upper mounting plate. Thereby, thermal expansions and small displacementsof the object to be weighed can be absorbed without a measurement error.

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The space for movement is limited by the size of the recess. If the load button presses against the edge of therecess with the constraining missing, horizontal forces in this direction are compensated via the load cell. In thiscase, heavy horizontal forces cause an inclination of the load button and may cause measurement errors andeven mechanical damage.Therefore, take care that the load buttons are located centrally in the recess with the object in rest position andthat horizontal forces are absorbed by the constrainers of three constraining units PR 6011/20.

Fig. 2.7-3 MiniFLEXLOCK PR 6011/20

Before installing a constraining unit, ensure that the foundation is horizontal (check with spirit level), flat andrigid for the expected load. To ensure even load distribution, the foundations should be equally high and thesupporting surfaces of the object to be weighed (vessel or platform) should be in parallel. It can be used formounting kit and constraining unit. The holes in upper and lower mounting plate must coincide.

To facilitate load cell and mounting kit installation and alignment, dummies can be used. These dummies areinserted when installing the mounting kits. As their head fits exactly into the recess of the upper mounting plate,they ensure accurate alignment of mounting kits and weighing installation. The dummies are replaced by theload cells only, when all mechanical and alignment work, as well as welding near the load cell are finished.

MOUNTING INSTRUCTIONS · To facilitate installation of the mounting plates of the constraining unit, the constrainer can be removed. For

this, just withdraw the swivel eyes from the swivel balls.

· Link upper and lower mounting plate using the flexible copper strap packed with the load cell. Two mountingscrews M6 x 6 with fan washer are included in the mounting kit.

· Screw lower and upper mounting plate to foundation or object support. Taking care that the plates are inparallel and vertically upon each other is indispensable.The above-mentioned dummy ensures that they are centred.

· Re-mount the constrainers of the constraining units. With deviating length of the constrainer, release thecounter-nuts and adjust the length by turning the hexagon. Re-tighten the counter nuts.

· Replace the dummy by the load cell only, when all welding work near the load cell and mounting at the objectare finished. Handle the load cell carefully to prevent damage to the membrane on the bottom. Place theobject to be weighed carefully on to the load cells and take care that the load button (1) is seated correctly inthe recess of the upper mounting plate.

· Check, if the load button is seated centrally in the recess of the upper mounting plate. For this, checkdistance A to protective ring around the load cell: it must be

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6 mm. If necessary, release the mounting screws by a few turns and shift the mounting plates as far aspermitted by the holes for the screws. The distances B and C, measured in longitudinal constrainer direction,must not differ by more than 3mm. I.e. the load button must not press against the recess edge. If necessary,adjust the constrainer length as described above. Tighten the mounting screws well.

Only if load cells and mounting kits are installed exactly, the space for movement, which is required for objectdisplacement due to thermal expansion, vibration, etc., can be used fully without limiting the measurementaccuracy. To prevent vertical force shunts, all mechanical connections of the object to be weighed to thesurrounding construction (pipes, cables, bellows) must be as flexible as possible. The overall load must besupported only by the load cells.

Tighten all screws and nuts well. When heavy vibrations are expected, we recommend using a protection e.g.by means of Loctite 274.

A protection against lifting, can be realized using a rod M6 (rigidity ³ 4.6), three nuts and two washers arerequired. Note that the rod must have sufficient space for movement in bore hole.

2.7.2 Mounting kits for the big type (500kg...10t)

2.7.2.1 Mounting kit PR 6011/10Mounting kit PR 6011/10 was specially designed for non-constrained installation of PR 6211/52.../14 series loadcells with 500kg...10t nominal capacity. This easy-to-install mounting kit permits reliable mounting..

The load button is located between the load cell head and the upper mounting plate. A slide plate cemented intoa recess on the upper mounting plate rests on the teflon-plated top of this load button. The load button canmove easily in the recess so that thermal expansions and little displacements of the weighing object can beabsorbed without causing a measurement error.

The space for movement is limited by the dimensions of the recess. If the load button presses against therecess edge, horizontal forces in this direction are induced into the load cell. In this case, strong horizontalforces will cause an inclination of the load button, which can result in a measurement error or even inmechanical damage. Therefore, ensure that the load buttons are located centrally in the recess at rest positionof the weighing object, and that constraining and/or horizontal limitations are provided, when strong horizontalforces must be taken into account.

Fig. 2.7-4 Mounting kit PR 6011/10

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Before installing the mounting kit with the load cell, ensure that the foundation is horizontal (check using spiritlevel), flat and rigid for the expected load. To ensure even load distribution, the foundation should be at equalheight and the contact surfaces of the object (vessel or platform) should be located in parallel. The bore holes oftop and bottom mounting plate must coincide.

MOUNTING INSTRUCTIONS · Mount lower and upper mounting plate at foundation or object support with screws. It is indispensable to

ensure that the plates are located in parallel and vertically upon each other. If necessary, adjust using asuitable U-shaped profile.

· Link upper and lower mounting plate with the flexible copper strap packed with the load cell; two screws M8 x10 with washer are included in the mounting kit.

· Only when all welding work near the load cell and mounting work at the weighing object is finished, clean theload cell seating in the mounting plate from dirt, remove the load cell from the styrofoam packing, andposition it in the recess. Handle the load cell carefully to prevent damaging the membrane on the bottom ofthe load cell.

Put the weighing object carefully on the load cells and ensure that the load button is positioned correctly in therecess of the upper mounting plate. The upper and lower mounting plate must be vertical upon each other.

· Check, if the upper mounting plate is seated centrally. For this, check distance A around the load cell toprotection ring. If necessary, release the mounting screws slightly and correct the position of the mountingplates, without obstructing the holes for the bolts. Any accidentally installed limitations must be correctedaccordingly.

2.7.2.2 MiniFLEXLOCK PR 6011/30Constraining unit PR 6011/30 N + S MiniFLEXLOCK is a mounting kit for the GLOBAL Weighing seriesPR 6211 load cells for compression measurements up to a nominal load from 0.5 to 10t with horizontalconstraining of the object to be weighed. Permitting quick and simple installation with a load cell, the unitcompensates horizontal forces up to 5 kN in longitudinal constrainer direction whilst giving sufficient space in Y-direction for movement of the object due to normal thermal expansion.

Fig. 2.7-5 MiniFLEXLOCK PR 6011/30

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To ensure the required space for movement of the measuring facility, max. three MINI FLEXLOCK unitsPR 6011/30 may be used for constraining a vessel. When using 4 or more load cells, the remaining load cellsmust be installed using mounting kit PR 6011/10 in order not to prevent compensation for expansion and toobtain an equal mounting height.

Using a rod, 3 nuts and two washers, the constraining unit permits easy installation of a lift-off protection.

Constraining unit PR 6011/30 comprise two mounting plates and a constrainer. The load cell is seated in arecess of the lower mounting plate. A load button is located between load cell head and upper mounting plate.Its teflon surface permits easy movement within the recess in the upper mounting plate. Thereby, thermalexpansions and small displacements of the object to be weighed can be absorbed without a measurement error.

The space for movement is limited by the size of the recess. If the load button presses against the edge of therecess with the constraining missing, horizontal forces in this direction are compensated via the load cell. In thiscase, heavy horizontal forces cause an inclination of the load button and may cause measurement errors andeven mechanical damage.Therefore, take care that the load buttons are located centrally in the recess with the object in rest position andthat horizontal forces are absorbed by the constrainers of three constraining units PR 6011/30.

Before installing the constraining unit, ensure that the foundation is horizontal (check with spirit level), flat andrigid for the expected load. To ensure even load distribution, the foundations should be equally high and thesupporting surfaces of the object to be weighed (vessel or platform) should be in parallel. The holes in upperand lower mounting plate must coincide. Mounting the constraining unit is done by means of threaded bolts M8.

To facilitate load cell and mounting kit installation and alignment, dummies can be used. These dummies areinserted when installing the mounting kits. As their head fits exactly into the recess of the upper mounting plate,they ensure accurate alignment of mounting kits and weighing installation. The dummies are replaced by theload cells only, when all mechanical and alignment work, as well as welding near the load cell are finished.

MOUNTING INSTRUCTIONS · Remove the upper screws of the auxiliary plate. Lift the upper mounting plate , remove the plastic pipe and

insert the load cell dummy or if not available the load cell into the recess of the lower mounting plate.Position the upper mounting plate onto the load cell head and fix the screws of the auxiliary plate.

· Link upper and lower mounting plate using the flexible copper strap packed with the load cell. Twomounting screws M8 x 10 with spring washer are included in the load cell delivery.

· Use bolts M8 to mount bottom and top mounting plate at foundation and object support. When all mountingkits are fixed remove auxiliary plate. Screw the screws for the auxiliary plate into the thread holes of themounting plates.

· Tighten the fixing bolts crosswisely. Take care that the plates are in parallel and vertically upon each other.The above-mentioned dummy ensures that they are centred.

If the load cell is used for the alignment, check - if the upper mounting in non-constrained direction (Y axis) is positioned centrally to the bottom one in order

to allow movement due to thermal expansion during operation. Distances A and B between protective ringand load cell must be equal.

- if the constrainer is without load, i.e. if it can be turned by hand.If this is not possible: · Release the fixing bolts of the mounting plates and lock nuts of the constrainer. · Turn the central hexagon, until the constrainer is free of load. · Re-tighten lock nuts and fixing bolts crosswisely. · Check, if the constrainer is free of load.

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Distances C and D, measured in longitudinal constrainer direction, may not differ by more than 3.5mm, i.e. load button must not be pressed against the edge of the recess in the upper mounting plate.Check, if distances A and B in Y direction are equal.If not repeat above mentioned adjustments.

· Replace the dummy by the load cell only, when all welding work near the load cell and mounting at theobject are finished. Handle the load cell carefully to prevent damage to the membrane on the bottom. Placethe object to be weighed carefully on to the load cells and take care that the load button is seated correctlyin the recess of the upper mounting plate.

Check, if the distances A and B between protective ring and load cell are equal.

Check, if the constrainer is without load, i.e. if it can be turned by handIf not perform above mentioned adjustments.

Only with load cell and constraining unit installed correctly, full use can be made of the space required formovement of the object due to thermal expansion, vibration, etc., without limitation of the measurementaccuracy.Tighten all screws and nuts well. When heavy vibrations are expected, we recommend using a protection e.g.by means of Loctite 274. To prevent vertical force shunts, all mechanical connections of the object to itssurrounding construction (pipes, cables, bellows) must be as flexible as possible. The overall load must besupported only by the load cells.

A protection against lifting can be realized using a rod M12 (rigidity ≥ 4.6), three nuts and two washers. Notethat the rod must have sufficient space for movement in bore hole. Note that 1mm (±0.5mm) space for lifting isindispensable.

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2.8 Accessories for the installation

Load cell cable, cable junction box and extension cable transmit the weight (a DC signal of some millivolts) fromthe load cell to the electronic indicator. The importance of excellent load cell and cable data is obvious foreveryone. Best weighing results, however, also require a careful design of the cable junction box.

Commercially available standard boxes from companies like Rose and Weidmüller which are often applied inelectrical installations suffer from various shortcomings, e.g. insufficient insulation resistance, low protectionclass, low resistance to irradiation...

2.8.1 Cable junction boxes

Since the junction boxes PR 6130/08 and PR 6130/6X are especially designed for weighing purposes, theyovercomes the a.m. disadvantages and some more:- The printed circuit board inside the box has some recesses which restrain the propagation of parasitic

currents; in this way, the extremely high insulation resistances of some thousands of MegOhms ismaintained.à The user gets a stable and reliable weighing result, even under harsh conditions. Dirt and dust inside

the junction box do not influence the weighing accuracy.- The cable junction boxes are protected against ingression of dust and water acc. to IP 65 (NEMA 4X).à The cable junction box may be installed in harsh environment.

- A valve allows that moisture which penetrated the box to escape easily.à Reduction of humidity ensures reliable operation.

- A drilling template comes together with the box, making installation quite an easy matter.- All cable glands are now according to the new regulation. à metric cable glands

2.8.1.1 Plastic cable junction box PR 6130/08Cable junction box PR 6130/08 is designed for the use in most industrial applications. It is easy to mount andconnects up to 8 load cells in parallel to an extension cable PR 6135 or to a corresponding cable with approx.8...12mm outside diameter.The cable junction box is made from plastic and meets the protection class IP65 (or NEMA 4X). The unit issuitable for installation at a vertical wall, whereby the cable entries are positioned at the bottom side.The two M25 cable glands are for four load cell cables each; please close the open entries with the plugssupplied with the box. The M20 cable entry is fitted for the extension cable.

This cable junction box may not be used in hazardous area application.

Fig. 2.8-1 Cable junction box PR 6130/08 (dimensions)

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- The cable is preferably introduced from the bottom side.

Fig. 2.8-2 Connection of extension cable and load cell cables- Fit the cable screenings with crimps and connect them to the screw terminals marked K5/K6; they are not

connected electrically to the housing.- The screening of the extension cable must be connected to earth (protective earth or potential equalisation)

on the side of the weight indicator.

2.8.1.2 Stainless steel cable junction box PR 6130/60 (and PR 6130/68)Cable junction boxes PR 6130/60, PR 6130/68 are suitable for all industrial and all W&M weighing systems(corner adjustment built-in). They may also be installed in hazardous areas.

Fig. 2.8-3 Cable junction boxes PR 6130/60, PR 6130/68 (dimensions)

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- The cable is preferably introduced from the bottom side.- Connect the cores to the terminals according to the colour markings.- Fit the cable screenings with crimps and connect them to the screw terminals marked yellow; they

are not connected electrically to the housing.- The screening of the extension cable must be connected to earth (protective earth or potential

equalisation) on the side of the weight indicator.- The housing earth connection or potential equalisation cable must be fitted below the

earth screw (fig.2.8-3, part 1) on the housing outside.- The adhesive label packed with the instrument must be fitted on the cover below the type label.- The cable junction box is suitable for connection of intrinsically safe circuits. The circuits are:

- the connected load cells (passive)- the extension cable to one interface with one (active), intrinsically safe circuit, e.g. PR 1626/60

in connection with an evaluating instrument, e.g. indicator PR 1713.The intrinsically safe circuit comprises power supply, sense and measurement voltage circuit.

- Connecting several active, intrinsically safe circuits in the cable junction box is not permissible!

2.8.2 Extension cables PR 6135 (PR6136)The installation cable connects the cable junction box and the weighing electronic over long distances (severalhundreds of metres). In all industrial and W&M applications, the grey cable PR 6135 is used, for hazardous areaapplications the special cable PR 6136 is recommended (blue sheath).

The cable supply the voltage necessary for the operation of the load cell, and transfer the output back to theweighing electronic. The 6-wire-technique eliminates errors caused temperature changes and ensures aconstant supply voltage. The overall screen guarantees a high degree of resistance against errors resulting fromelectric fields. The additional separate screening of the measuring wires is a second barrier for the mV outputsignal against fields.

Some important properties of the extension cable:- UV resistant, no silicon, flame proof, no halogen- multiple screening ensures highly reliable signal transmission- guarantees the safe transmission of the measuring signal over long distances

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2.9 Constraining devicesGLOBAL Weighing delivers mounting kits and MiniFLEXLOCK for every load cell family. It is recommended touse these mounting kits because they include the two functions CONSTRAINING and STANDARDIZEDINSTALLATION. If for some reason additional constraining is needed, one of the units listed below may beapplied.

2.9.1 Horizontal constrainers PR 6152/02

Fig. 2.9-1 Horizontal constrainer PR 6152/02

GLOBAL Weighing horizontal constrainers PR 6152/02 are used for constraining weighing installations. Theyare particularly suitable for platform weighers with nominal capacities from 10t and more. Correct constraining ofthe object prevents measurement errors due to horizontal cross-forces or due to displacement of the centre ofgravity. Moreover, constraining protects against damage caused by horizontal forces.

When installing, take care to provide sufficient space for thermal expansion: any restraint in vertical directionmay lead to important measurement errors. Horizontal constrainers PR 6152/02 eliminate forces up to 200kN inpressure direction. In order to absorb horizontal forces acting in opposite direction a second horizontalconstrainer must be installed. Therefore PR 6152/02 is only available in pairs.

Fig. 2.9-2 Operating principle of horizontal constrainer

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In order to avoid absorption of vertical forces, the horizontal constrainer should be mounted in a correcthorizontal position. With the weighing installation unloaded, the flange at the platform may be somewhat higher,in order to be positioned horizontally or slightly below the flange at the foundation when the platform is loaded.Thereby, a minimum symmetrical deviation of the axial line from the horizontal line is provided. Optimum spaceand force absorption are ensured when the constrainer is installed without compression or extension, i.e. if thethrust piece in the middle can just be turned manually so that there is virtually no space left for shockgeneration.

Mounting the PR 6152/02The figure suggests two consoles for mounting a pair of rocking pins.

- the consoles have an angle of exactly 90°. They are mounted on a horizontal plane, and therefore thenecessary vertical position of the contact surfaces is easily obtained.

- The consoles can be bolted to the construction to allow for a very small axial play. Preferred is a value of0.5 mm; the pins can just be 'rotated' by hand.

- If necessary, the centre piece, which is bolted th the object, can be adjusted for verticallity with shims.- As explained in chapter 1.4.4.5, the horizontal position of the rocking pin is not critical at all.- the pins are fixed with

2 bolts M10 x 501 threaded rod M10 x 70

Fig. 2.9-3 Mounting example for horizontal constrainer PR 6152/02

The simple consoles can only be used if the mounting height of the load cells used is smaller than about200 mm. In cases with bigger load cells or if the horizontal force which has to be taken is > 20 kN, we have todo with a very heavy and big object. In those cases there should be made a special design for the mounting ofthe constrainers, fitting to the rest of the installation optimally. Fig. x suggests a heavier type of console forhorizontal loads up to about 100 kN. The design has to fulfil the following requirements:

- both ends of the PR 6152/02 have to be mounted against smooth and clean surfaces which must bevertical within ±0.5°

- the consoles have to be adjustable in the horizontal direction, in such a way that PR 6152/02 has an axialplay of about 0.5 mm

- the deformation the axial direction of the constrainer under maximum horizontal load has to be smallerthan 0.5 mm

- if the consoles are bolted, the bolts have to exert enough pression of the console against the underlyingconstruction that friction can take the horizontal force

- if the consoles are welded, provisions have to be made (e.g. shims between the PR 6152/02 and the verticalsurfaces) to make the adjustment of the 0.5 mm axial play possible

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2.9.2 Constrainer PR 6143/80 and PR 6143/83

GLOBAL Weighing constrainer PR 6143/8. can be used universally for constraining of weighing installations.Correct constraining of an object prevents measurement errors and protects against damage due to horizontalforces, without impairing the freedom of the object to move in vertical direction.

Fig. 2.9-2 Constrainer PR 6143/8x

Note that thermal expansion and displacement may affect the freedom of movement of the object wherebyconsiderable measurement errors may be caused. (cf chapter 1.4)

constrainer type max. constrainer forcePR 6143/80 2 kNPR 6143/83 20 kN

In order to avoid absorption of vertical forces, install the constrainer in a correct horizontal position whereby adeviation of < 1° (= height difference < 8mm) is negligible.

The nominal distance between the axles of PR 6143/8. is 500mm. After installation, this distance is adjustable±6mm by turning the constrainer, to provide optimum freedom of movement. Distance K (see drawing) must notexceed 10mm.

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3. Tank weighing

3.1 OverviewThe general aspects of weighing are discussed in chapter 1. Some aspects are especially of interest for tankscales, hopper scales and the like:

chapter 1.3 General recommendations on the designchapter 1.4 Constrainingchapter 1.5 Disturbing influenceschapter 1.7.4 Standard accuracy: non W&M application

This chapter defines three types of weighing installations and discusses their accuracyaspects.

The standard mounting parts for the load cells are described in chapter 2.

This chapter 3 discusses some special aspects which appear only in tank scales, hopper scales etc. A firstsubject are the connections of tanks with pipes to other objects of a process. Wrong dimensioning can causemeasuring errors. Chapter 3.4 explains how to avoid them.

Sometimes load cells are replaced by pivots to save money. Chapter 3.5 explains how to design pivots in orderto avoid errors.

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3.2 Application examples

3.2.1 Some hints for installations pivotsSuppose there is a vessel with 3 point bearing, two bearings have pivots and the third one has a load cell. In whichcase do you need additional constraining?From the mechanical point of view, a pivot is a clamped connection. No degrees of freedom are left for a rigidobject, neither rotational nor translational. If the object is rigid enough and no side forces act on the object, you donot need additional constrainers, whereas in case of a long non-rigid tank additional constrainers are absolutelynecessary.

Additional constraining is recommended in particular if the load cell is tilted by side forces.

Further recommendations - Links like pipes, hoses etc. should be placed at the side of the pivot to avoid errors resulting from thermal

expansion of these links. - Pivots and load cells together should be used with liquids only.

Fig. 3.2-1 Horizontal tank with pivot and load cell

3.2.2. Installations with load cells only

- Installations including more than four load cells to weigh an object should be carefully designed so that the loaddistribution is as even as possible. The load cells should preferably placed at equal distances on thecircumference. (see fig. 3.2.2)

Fig. 3.2-2 Distribution of more than four load cells

- Large vessels usually demand constrainers for heavy side forces, which are caused e.g. by wind. Standardconstrainers like PR 6152/02 can withstand forces up to 200kN. Higher forces can be employed e.g. toflexbeams. (see chapter 1.4.4.3)

- Suspended installationsIf a break in suspension, support, load cell, or mounting part etc. representsa hazard to the life and health of men and animals, or if goods may bedamaged, additional safety devices have to be provided.

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3.4 Pipes, bellows

All vessels are integrated in continuous and discontinuous industrial processes as e.g. storage or blendingvessels. Pipes convey the materials from the storage to the process bins, which are equipped with a weighingsystem. The reading informs about the actual state and the progress of the process. In order to achieve therequired accuracies, the system design must ensure that no undesired external forces confuse the output signal: - environmental influences on vessel and load cell (temperature, air pressure etc.) - support or suspension points - each device connected to the vessel, e.g. pipes and hoses, must be examined carefully if its movements canaffect the measuring result. Accurate design avoids undesirable force shunts and hence errors in themeasurement. Chapter 1.3 presents some information about the piping design (as far as the weighing point ofview is concerned) whereas this chapter covers the topics · influences by pipes · models for systems with vessels and pipes · how to use bellows · calculation of pipe stiffness.

3.4.1 Influences of pipe connectionsIn many installations the pipes are directly, rigidly fixed at the vessels and at other supporting points in the building.This creates no real problem under stable ambient conditions, e.g. constant temperature, and in case of rigidsupporting points under vessel and pipes. The latter condition is yet quite often not fulfilled since many weighersare installed on platforms or inside frameworks, which must be regarded as elastic. At least all outdoor vessels aresubjected to temperature changes between day and night. Both thermal expansions and movements between thesupporting points of pipe and vessel cause reaction forces in the load cells, thus leading to incorrectmeasurements (e.g. zero point errors, span errors). These errors cannot be compensated neither mechanically norelectrically because they depend on the actual state of the complete system including loading and environmentalconditions.

Fig. 3.4-1 illustrates the possible forces and displacements: a rigid pipe can lift some weight off the vessel andcause a displacement (δP). A force FP along the pipe can push the vessel to the side and exert the forces ∆F whichneed not to be of the same magnitude. The charging makes the vessel grow in diameter as it is not stiff.

Fig. 3.4-1 Vessel with pipe connections

As long as the ambient conditions remain stable, pipes can fix a vessel on a rigid foundation in its position,meaning that they can replace constrainers. On the other hand, varying temperatures influence the dimensions ofthe pipes: increasing temperatures lengthen stiff pipes and widen the vessel, causing a change in the height of theconnection point. As a result some of the weight can be lifted off the load cells when pipes and vessel are rigidlyconnected. This effect decreases the span and lowers the weight indication below the indication achieved withoutpipes. During the calibration procedure the instrument can be re-adjusted to overcome this error. The pipestiffnesses, however, cannot be assumed a constant value: they change during operation, causing a span error,an additional non-linearity error and a non-reproducibility error.

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Beside changes at the connecting point of pipe and vessel, the height of the supporting points of the pipe canchange by some reason like temperature dilatation, deformation of the foundation and interconnections betweenvessels (refer to the design chapter 1.3). With too stiff pipes this results in a zero shift.

An installation with rigid pipe connections in combination with other standard constrainers can be staticallyundefined (overdetermined), i.e. they (constrainers and pipes) can clamp the vessel. A pipe outside the plane ofthe constrainers exerts a moment on the weighing object (see chapter 1.4), which increases or decreases theloads on the load cells. In case of thermal expansion, forces of unknown magnitude arise producing a poormeasuring result.

Conclusion:It is evident to decide in advance whether to constrain the vessel by pipes or by standard constrainers only. In thefirst case, all rules written down in chapter 1.4 for constrainers apply to pipes in the same way whereas in thesecond case the pipes must be flexible enough not to clamp the weighing system. Expansion devices like bellowsare installed to achieve this.

In most cases the latter method will be preferred for ease of mounting and a clear, transparent design.

Fig. 3.4-2 Thermal dilatation Fig. 3.4-3 deformation of foundation

Fig. 3.4-4 The pipe is connected to another vessel with flexible foundation

The following examples illustrate the possible effects and show their magnitude.

Example 2 vessels interconnected (fig. 3.4-4)

1. Influence of a pipe on the spanWe assume a vessel standing on a rigid floor. A pipe with vertical stiffness of C = 216 N/mm is connected. Thereare three load cells of 10t, which means a load cell stiffness of 300,000 N per 0.5mm. The influence of the pipe onthe span is therefore

%.%

mmN

mmN

0360 100 600000

216=⋅

⋅

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A span error is no problem if the circumstances do not change; otherwise the indication will change depending onthe pipe loading.

2. Influence of the vessel temperature on zeroWe assume that the vessel is made of aluminium, the height is 5m, and the temperature of the vessel changesfrom 10°C to 100°C. Then the vessel will grow vertically over

( )5 000 100 10 24 10 6 110 8, .mm C C

C mm⋅ ° − ° ⋅ ⋅ −

°=

The pipe will deflect the same amount causing a vertical force Fv

N 2,333 = 216 mm810 = FmmN

.v

⋅

Assuming a net scale range of 12t (or about 120 kN) this means a zero error of

%.%,

941 100 kN 120

N 3332=⋅

3. Zero error if the pipe is connected to a second vesselWe assume that the second vessel is put on load cells and the weight in that vessel changes over e.g. 50% oftheir totalized nominal load. then the second vessel moves over at least 50% of 0.5mm or 0.25mm in verticaldirection. (Possibly this movement is bigger by elasticity of the foundation of the second vessel.)Pipe deflection over 0.25mm causes a vertical force of

NmmN

mm. 54216250 =⋅

which means a zero error on the first vessel of

%.%N,

N0450100

000120

54=⋅

3.4.2 Describing systems with vessels and pipesThe objective of this paragraph is to show how to transform a system consisting of vessels, pipes, load cells andaccessories into a mechanical model. After the identification of all important influences which can cause incorrectmeasurements, their description enables the user to calculate the reactions of the system. A clear and completesystem description can only be obtained if the supporting points and the foundations are taken into considerationbeside vessel and pipes...

Two different types of foundations are distinguished: (i) flexible foundations like steelworks and (ii) stiff foundationslike concrete foundations. The flexible foundation is characterized by the property that it deflects under load. Thelonger the supporting beams are, the higher their deflection under load since the deflection increases with the thirdpower of the beam length. As a rule of thumb you can say that the deflection can rise up to 0.25% of the beamlength. On the other hand, a stiff foundation does not respond to vertical forces with vertical deflections except forcase of damage. Thus the foundation can completely be described by the possible vertical deflection of thesupporting points. They can only move if the foundation is not rigid, if the permissible load is exceeded, or if theload distribution differs from the expected one.

Vessels and pipes are made of steel and therefore are elastic so that they can be deformed. Since the variouselements are interconnected, all the deflections can influence the correct weighing result. Some examples illustratethis mutual influence:Material in a pipe tries to pull it down to the ground. Insufficient supporting conveys these forces to the vessel andcauses a reaction force in the load cells.A vessel filled to its limits widens and tries to pull all connected pipes downwards. Stiff pipes, however, can liftsome of the weight off the vessel.

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So you must expect expansions of the vessel and vertical movements of the pipe but also axial expansions of thepipes. The vertical pipe movements are described by the vertical displacements of the supporting / suspendingpoints, by its elasticity and geometric dimensions. The pipe supports can deflect when traffic passes; but usuallythe pipe deflects first. The pipe expands under temperature and bends under deflection of the supporting points. Ifthe pipe does not hang freely, it transfers forces.

If you exaggerate, you can compare a vessel with a balloon: it widens when filled.

A mechanical model which describes the movements and elasticities of the system can be built from the standardelements “spring” (stiffness c) and “rigid connection” (stiffness = 4). The springs work in compression and/orbending mode. (load cells: compression or tension; pipes: bending).

3.4.3 Calculation of the pipe stiffnessThe calculation of the stiffness is based on the mechanical model shown in fig. 3.4-5.

Fig. 3.4-5 Model for the stiffness calculation of the pipe

Like in many strain calculation the main influencing factors are - Young's modulus E - the length l of the pipe - the cross sectional area of the pipe, cross section A and moment of inertia I

Young's modulus is a constant and typical for the each material.material Young's modulus

steel 210,000 N/mm2

copper 110,000 N/mm2

aluminium 70,000 N/mm2

The cross sectional area A of a pipe and the moment of inertia I are easy to calculate:

[ ]22 - 2D 4

= mmdA

⋅

π [ ]44 - 4D 64

= mmdI

⋅

π

The formula for the pipe stiffness is achieved by applying the elasticity calculation:

⋅

⋅mmN

l

IEKC

3

=

Substituting the expression for the momentum of inertia I results in

( )

⋅⋅⋅mmN

lE

dDKC3

44 - 64

= π

K is called the clamping factor and characterizes the mounting conditions of the pipe. This factor has to bedetermined for some interesting cases. The theory claims that the clamping factor K equals 12 if the pipe isstraight and clamped at one side and free at the other one. Experiments, however, show that a value of K = 10is more realistic.

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In some installations and special designs other values for the clamping factor K can be assumed:

1. Bend in the vertical planeh/l K0.2 80.5 6.31 4.85 3.4

Fig. 3.4-6 Pipe with a bend in the vertical plane

2. Bend in the horizontal planeb/l K0.2 7.10.5 4.31 1.85 0.06

Fig. 3.4-7 Pipe with a bend in the horizontal plane

3.4.4 Calculation of the influence on the measuring resultThe actual stiffness is compared with a permissible stiffness under the condition that the pipe must not influencethe weighing result. The permissible stiffness is calculated using the load cell stiffness.

]mmN

[Enhg

C maxa ⋅⋅=

Emax nominal load of one load cell [kg]n number of load cells -----h max. deflection under load [mm]g gravitational field strength ≈9.81m/s2

The actual stiffness Ct is the sum of the calculated values C for all pipes connected to the vessel to be weighed.Compare the totalized pipe stiffness Ct and the allowed stiffness Ca

pit CC Σ=A

CC a

t ≤

If the totalized pipe stiffness Ct is bigger than Ca, measures have to be taken.

- The allowed influence on the ‘span’The stiffness of the pipe reduces the span of the installation. a typical value for the allowed influence is 1 percent. This means that the totalized pipe stiffness should not be more than 1 per cent of the total of all load cellstogether. The resulting decrease in span of 1 per cent can easily be corrected with the measuring instrumentduring the calibration procedure.

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If the pipe stiffnesses were constant there would not be a measuring error caused by the pipes. However, achange in pipe stiffness causes a span error.

Example:Assuming that the pipe stiffness is 1 per cent of the permissible stiffness. A 10 per cent change would resultin a span error of 0.1 per cent. For high accuracy weighing, it is recommended to choose a value lower than1 per cent for the permissible influence.

If the calculated totalized pipe stiffness Ct is bigger than the permissible pipe stiffness Ca then the followingmeasures could be taken:- localize the stiffest pipe- lower its stiffness

You could try to make the pipe longer or try to make the clamping less rigid. You could suggest to installbellows.

3.4.5 BellowsPipes are often fixed with bellows to avoid destructive forces. They are mainly used to compensate for axial,lateral, and angular expansion due to temperature. This is achieved by transforming the force in the pipe to amovement. Four types of bellows are distinguished: · axial type

Axial bellows compensate thermal expansion along the axis of the pipe

Fig. 3.4-8 Axial bellows

· lateral typeLateral bellows compensate movements in the plane rectangular to the axis of the pipe.

Fig. 3.4-9 Lateral bellows

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· angular typeAngular bellows must be used in groups of two or three. Two angular bellows substitute one lateral bellows.Three angular bellows form a 3 point system.

Fig. 3.4-10 Angular bellows

· universal typeUniversal bellows compensate thermal expansions along the axis and rectangular to the axis of the pipe.

When using bellows you must observe some restrictions. · Bellows must not be loaded with torsional momenta. · Only low frequency vibrations are permitted · The number of movements is limited (e.g. 5000).

If the pipe is made flexible at both ends, the stiffness is reduced to a minimum because there is no bending inthe pipe itself. Practically this is done with bellows.

3.4.6 Influences of gas pressureIf the contents of a weighed vessel are under gas pressure, the pipe connections ask for some extra care. Gaspressure in the pipe gives: · no influence in flexible horizontal pipes · even no influence if that pipe is vertically connected to the vessel with a stiff part · however, if the connection is made via bellows, a vertical disturbing force can arise. If the effective area of

the bellows is A, then this force Fv is

ApFv ⋅∆=

example 1:With an pressure of ∆p = 2 bar and a diameter D = 150mm the weight indication is increased withFv = 3.5 kN.

kN.m.mN

mmbarFv

531504

102

1504

2

222

5

22

=⋅⋅⋅=

⋅⋅=

π

π

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example 2:A hopper is filled with dusty material. Bellows are provided to protect the environment. During the fillingprocess the internal pressure increases temporarily by ∆p = 100 Pa. The diameter of the bellows is 1.5m.The weight indication changes with Fv = 177N.

Nm.mN

m.PaFv

177514

10

514

100

222

2

22

=⋅⋅=

⋅⋅=

π

π

3.4.7 Influence of vertical bellowsThe contents of this hopper are not correctly measured because the bellows reduce the weight by the greyshaded column above. The material inside this column is supported by the outlet pipe and not by the weighingobject.

Fig. 3.4-11 Hopper with vertical bellows

If you exchange the positions of valve and bellows, you get correct results for charging the hopper. Whendischarging the above described effect happens to the measurement again: the (grey shaded) column ofmaterial above the bellows is immediately subtracted from the contents of the hopper.

Conclusion:Such a bellows installation should be avoided. For this reason, place the bellows horizontal.

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3.5 Level control using pivots

Fig. 3.5-1 Schematic diagram of a system for level control

In order to reduce the cost price of a weighing installation sometimes not all bearings are equipped with load cells.Some of them are replaced by pivots. Two combinations are possible: · one load cell and two pivots in case of a three point bearing · two load cells and two pivots in case of a four point bearingBecause of this replacement you cannot expect a too high accuracy. So, level control can be used if the customeronly requires level control.

Level control is a method which determines the momentum instead of the weight. The weight FG is found bycalculating the momentum equilibrium around the pivoting axis.

Gla

F ⋅=

This equation shows which effects can disturb the measuring result: · the centre of gravity wanders

A change in the position of the centre of gravity changes the distance a to the pivoting axis. · not carefully designed pivots give disturbing moments

· the distances a and l could change· the friction moment could be too big· the stiffness of the pivot could be too big

· Horizontal forces on the object, out of the plane of the pivots, disturb the moment M.

Some aspects must be observed when designing a weighing system with pivots: · the weighing vessel should have a big diameter and a low height · the centre of gravity must have a constant distance to the pivots · the weighing object must be stiff · the temperature must be constant (constant distance of the centre of gravity)

The following sketches show situations in which the designer of the weighing system failed.

Fig. 3.5-2 Wandering centre of gravity Fig. 3.5-3 Wandering centre of gravity

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3.5.1.1 Standardized pivotsTo avoid the necessity to calculate pivots each time you need them, two standard types are proposed here. Theirlength is 200mm; their other dimensions are given in fig. 3.5-4. Two different I-beam types were chosen:

HEB 100 maximum permissible vertical load: 16 tonsHEM 100 maximum permissible vertical load: 33 tons

Remark: Risk of buckling makes that load decreases if a big horizontal force is expected. The flexibility of thepivots is small enough to use them in combination with load cells with a nominal load of 200 kg or more.

The main properties of an ‘I-beam’-pivot are: · maximum allowed vertical load

This has nothing to do with the measuring properties, but only a question of strength of the device. The beamshould be strong enough to bear the load.

· Influence of side force on the permissible loadThe pivot could be used as a constraining element. With the normal load cell mounting parts there are no forces

due to temperature expansion and the ‘internal’ side forces on the pivot will be small. With external influences likewind or a calamity, however, the side force on the pivot could be the reason for its collapse if we do not take thisside force into account in the strength calculation. In the table, the effect of the side force is expressed in aformulae for the maximum allowed load.

· the stiffness of the pivotthe torsional stiffness of the beam around the pivoting axis will, in principle, influence the measurement. This is

because a vertical load cell movement results in a reacting moment of the pivot and therefore in a disturbingvertical reaction on the load cell. In the table, the beam stiffness is given as the reacting moment per bendingangle of the I-beam (expressed in Nmm/rad).

To get a better idea of of how this works out in a practical case, also a disturbing ‘reaction force’ is listed. The listedvalue will be generated in the following practical case:

distance between the load cell and the pivot: d=2000mmvertical movement of the load cell x=0.5mm

The bending stress in the beam for that case is also put in the table.

Putting it generally:reaction = (x/d²) x stiffnessstress is proportional to the bending angle of the I-beam

Example: with a distance of 4000mm and a movement of 1mm the reaction will be the half of the value in the tableand the stress remains the same.

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In practice you could claim that the pivot is good for measuring forces bigger than 500 x this listed reaction.Example: we take two standard pivots HEB 100 (of 200mm) for weighing a small object with one load cell.

As there are two pivots the total reaction will be twice 1.7N. Therefore the set-up is suitable formeasuring with the load cell a mass bigger than

500 x ( 2 x 1.7N))/10 = 170kgIf all three supporting points take the same load, the installation is good for an object with aminimum total net mass of

3 x 170kg or about 500 kg

I-beam type HEB 100 HEM 100

beam length 200 mm a [m] 200 mm a [m]

weight [kg] 4.1 20.4 × a 8.4 41.8 × a

height [mm] 100 100 120 120

flange width [mm] 100 100 106 106

body thickness [mm] 6 6 12 12

pivot stiffness [Nmm/rad] 1.35 × 107 6.75 × 107 × a 1.08 × 108 5.4 × 108 × a

reaction [N] 1.7 8.4 × a 13.5 67.5 × a

stress [N/mm2] 2.8 2.8 5.6 5.6

permissible load [t] L+28H < 16.8 L+28H < 84 × a L+14H < 33.6 L+14H < 168 × a

Remarks · A standard minimum length of 200mm of the pivoting beams is chosen for ease of mounting and to have a wide

application range for HEB 100. A smaller length (down to a minimum of 100mm) is possible, but makes thedevice less robust.

· If the object is very small the construction could be simplified by combining two pivots to just one pivot of morethan 200mm length.

· Also if there are extreme side forces, (e.g. example 1 on page 3) we are forced to choose a bigger length than200mm.

· We chose profiles for the smallest possible height and a relatively thick body. This makes the buckling tendencyvery low. Profiles with a bigger height will be more flexible around the pivoting axis, but will be less resistantagainst a horizontal force.

· Sometimes we meet a beam with welded strengthening plates as shown in the sketch (designed in an attempt todecrease the buckling tendency). This is not necessary with the two proposed standard pivots. The design ismore expensive and the local stresses during bending of the beam body will be bigger than without these plates.

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3.5.1.2 Mounting recommendations for level control with pivots

1) For an adequate pivot function the two pivoting beams under one object should be positioned in line as accurateas possible. Together they form a knife edge.

Fig. 3.5-4 Installation of two pivots in line

2) The pivots should be bolted to the object and to the foundation to take all possible horizontal forces

3) excessive bending of the pivoting beams should be avoided during the mounting procedure of the object

4) The easiest way to mount the load cell is using the mounting kit PR 6145. Sometimes it may be recommended touse mounting kit MiniFLEXLOCK PR 6143.

3.5.2 Calculation of an I-beam pivotIn chapter 3.5.1 we suggested to design the pivot as an I-beam and gave some standardized pivots. Supposedthere is no chance to use a standard pivot a special one has to be calculated. For the calculation you need to knowsome characteristic values: - the thickness of the stick - the height of the stick

Fig. 3.5-5 characteristic values of an I beam

For the calculation you have to determine the slenderness ratio λ

1

153th

. ⋅=λ

With this ratio the permissible stress for the material is found. The material was chosen as Fe 360 because its oftenused and usually easy to get. The higher the stick the higher the danger that the buckling effect affects the pivot. Forthis reason the permissible force lowers with higher stick.

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Fig. 3.5-6 Loading of an I beam

We did not take a possible side force into account. However a side force perpendicular to the pivot, acting at thesame time, decreases the permissible vertical force. In this chapter a method is presented which takes both forcesinto account. Moreover, the calculation method for the pivot stiffness of an I-beam is described.In principle the pivot is loaded by forces and momenta (refer to fig. 3.5-6) - a vertical compression force Fv

- a horizontal force F1 (caused by external forces on the object)- a pivoting moment M (caused by e.g. load cell deflection)

Fig. 3.5-7 Permissible stress for an I beam

The permissible horizontal force F1max is limited by the allowed bending stress σb in the beam.

1

2

3 htl

H bmax ⋅

⋅⋅=

σ

Note: σb should be kept low, because this stress is additional to the compression stress caused by the vertical force.

ltFv

b ⋅−=140σ

The permissible vertical force can be calculated on buckling. The permissible buckling stress s as taken from thegraph should, however, be decreased by the bending stress σb, caused by the side force.

213

tlhH

b ⋅⋅⋅

=σ

Because this can reduce very strongly the permissible vertical force, it is advised to choose an I-beam for whichλ<30 and therefore σ=140N/mm².

In those cases: bending stress + compression ≤ 140 N/mm²

221 140

3

mmN

tlF

tlhH

≤⋅

+⋅

⋅⋅

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Example: for HEM 100 with l = 250 mmh1 = 56 mmt = 12 mm

Now, with this equation: Fv + 14 F1 < 420 kN

The pivot stiffness can be defined as the moment in Nmm, which is necessary th rotate the pivot over 1 radian. Thiscan be calculated from

1hEI

k p

⋅=

To get a better insight in this value, we could measure this stiffness as the vertical reaction on a vertical movementof a point with a horizontal distance of one mm to the pivoting line. The pivot stiffness is then expressed as thevertical reaction force Ra in newton, when that point deflects vertically over z mm.

The pivoting action deforms the beam, giving a bending stress s1. This can be calculated from:

Φ⋅⋅⋅= Eht

11 2

1σ

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Appendix A

A Eaccuracy class 71 earthing 36, 80-81accuracy aspects 71-79 effect of horizontal force 129-130acoustical impedance 34 electrical check 85-87alignment 16, 80 electrical damage 86

electrical shunt 37B equilibrium 17base plate 89-90 error 71batching accuracy 77 error types 71bellows 125-126 error estimation 75-79braking force 50buckling effect 131 F

falling material 33, 69-70C feeling constrainers 42-43cable 35, 80-81 flexible object 27cable junction box 112-114 freedom of movement 39cable length 80 freezing 68cable protection 35 friction 68-69cable screen 36, 80-81 flexbeams 48-54cabling 35 flexbeam supports 53-54calculation pivoting beam 131-133 flexbeam dimensions 49calculation of flexbeams 52-54 flexbeams in practice 50-51calibrated weights 82-83 flexbeam calculation 52-54calibration 82-83capsizing , 56-57 Gcardanic pivoting points 68 gas pressure 126-127central earth rail 36, 81centre of gravity 15 Hclamped constrainer 42-43 hammering 39clamping factor 123-124 heat 64-65collapsing load 47, 52 heat protection 31, 64-65collision 26 horizontal stop 55-56conduction 64 hysteresis 71constrainer 39-57constrainer pattern 39 Iconstrainer plane 15 ice 68constrainer number 39 impact 33constrainer, too many 40 installation design 21-38constraining 39-57 insulation 87constraining and temperature 41 insulation resistance 87corner adjustment 83-85 interference 36, 81corner point test 83-85

LD lifting force 62-63design 21-38 lift-off-protection 56-57destructive load 30 links 28-29dimensioning a pivoting beam 131-133 load distribution 84dirt 68 load cell tilt 8disturbing effects 58-70 load cell type and capacity 29dynamic overload 33-35 load cell cabling 36

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M stray current 35

measuring error 71 strut stiffness 47measuring range suction force 63measuring accuracy 75-79 suspended object 8-12, 43-44MiniFLEXLOCK 46 suspension rod 10-12moisture 35, 80 swivel bearing 102-103mounting kit

TN temperature 64-67natural frequency 17-20 terminology 71-75nominal load 30 thermal expansion 66non-linearity 71 tilting 8, 56-57non-reproducibility 71 torque on tension cell 8-12

O Vomitting constrainers 39 vibration 69-70overload 31-35overload protection 32 Wobject stability 15-16 waggling 16

weak foundation 23P weight 82pendulum length 18 weights & measures (W&M) 72-75pipe 120-127 welding 37-38pipe calculation 123-125 wind 58-63pipe stiffness 123-125 wind velocity 58-59pivot stiffness 130, 133pivot weighing recommendations 119 Zpivoting rod 54-55 zero error 121-122preload 32principle of flexbeams 48

Rradiation (heat) 65railway weighbridge 50-51reproducibility 71restoring force 17-20rigid mounting 6-7rocking pin 55rod suspension 10

Ssafety device 39shim 7shock 33-35shock wave 33-35shock loading 33-35side load 39stability 16-17stable equilibrium 15standardized pivots 130static overload 31-32statically undefined 16stiffness of flexbeam support 53-54stop 39, 55-57

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