explaining the words

8/7/2019 Explaining the Words

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Explaining the Words, Terms and Jargon

This glossary is intended to define the most common terminology and language used in the

fastening industry.

A A joint Hard joint requiring 30o or less of rotation from snug to target

torque

Air motor Rotating cylinder with vanes in air tools to generate power fromcompressed air

Angle of rotation Angle measured from snug to target torque

B B joint Medium hard joint requiring about 180o of rotation to target

torque

Balancer Hanging device to hold the weight of tool at any height

C C joint Medium soft joint requiring about 360o of rotation to target

torque

cfm Air flow measured in cubic feet per minute

Clamp load Force holding two components together

Click mode Tester mode that displays reading upon the "click" action of aclick wrench

Clutch Mechanism to transfer torque from air motor to fastener

Continuous drive Tools that deliver torque continuously during its cycle

Controlled tool Tool controlled electronically with internal transducer

Controller Device that controls the torque output of a transducerized powertool

Cross threading When fastener threads bind with application threads

D D joint Soft joint requiring 720o or more of rotation from snug to target

torque

dB(A) Logarithmic measure of sound level in decibels. Each change in

3 dB(A) results in a change in sound level by a factor of two.

DC Tool A common type of controlled tool powered by a Direct Current

electric motor

Discontinuous drive Tools that deliver torque in increments during its cycle

Dynamic torque Torque measuring during fastening

E Ergo-Arm Mounted device that holds tool and absorbs torque reaction

Ergonomics Science of optimizing process to personnel

ESD Electro Static Discharge; concern of electronics assembly

FFirst peak mode Tester mode that displays the initial peak of torque regardless of

subsequent torque readings

Frequencyresponse

Filter setting of a torque analyzer

FRL Filter Regulator Lubricator; treats compressed air for use by air

tools


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H Hard joint Joint requiring 30o or less of rotation from snug to target torque

Hex drive Device to hold a screwdriver bit into the front of tool

J Joint Combination of a fastener, application, washers, gaskets, o-rings

Joint kit Joint simulator typically consisting of a stack of washers or a

spring

Joint simulator Device to emulate a fastener - application

K KDM Keypad Display Module; control panel for AcraDyne controller

kg-cm Metric measure of torque in kilogram centimeters

L Line control Using signals from tool to control aspects of the productionprocess

M Mag-O-Net Small magnetic ring place on screwdriver bit

Monitored tool Tool that reports the installed torque to a controller

Multiplier Device with many gears that multiplies torque output of anothertool

N NiCd Nickel Cadmium; a type of older technology tool battery

NiMH Nickel Metal Hydride; a type of newer technology tool battery

Nm Metric measure of torque in Newton meters

Nutrunner Tool to fasten nuts onto bolts; typically with angle head

P Parameter Set of conditions under which a cycle is considered acceptable

Peak mode Tester mode that displays highest torque reading of a cycle

Prevailing torque Torque required during the rundown of the fastener

Productivity Measurement of output of a process

Pulse tool Assembly tool with hydraulic chamber to generate torque

Q Quality Adhering to torque and quality specifications

R Ratchet wrench Tool used in tight spaces; rotates in increments

Reaction bar Strong engineered metal bar that absorbs torque reaction

Regulator Provides constant air pressure for use by air tools

Reliability Expected service life for a tool's intended purpose

Residual torque Torque remaining in the joint after tightening

Retractor Hanging device that holds weight of tool at specific heights

Rotary transducer Rotating device to measure torque during fastening

Rundown Portion of the cycle before the head of the fastener contacts thesurface

S Screw presenter Automation device to line up and present fasteners tomagnetized bit

Screwfeeder Automation device to feed fasteners to head of driver

Soft joint Joint requiring 720o or more of rotation from snug to targettorque

Solenoid valve Device to shut off air supply to tool


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Square drive Device to hold a socket into the front of tool

T Tension Force that stretches a fastener

Tightening Monitor Method to verify cycle completion and cycle time of assemblytools

Torque Force at a distance, or resistance to rotation

Torque analyzer Electronic device to analyze, display, measure torque

Torque reaction Kick or movement felt by operators during fastening

Track mode Tester mode that displays running torque with no memory

Transducer Device to measure torque

Tubenut wrench Tool to fasten tube nuts onto hydraulic or control lines

What is "Residual Mode" on Exacta

and Exacta 2 and how does it work?Considerations in Torque Auditing and Residual Mode inExacta Products

Torque auditing is one of the most challenging audits there is. It is an audit that doesnot fall neatly into the destructive versus non-destructive testing category. A torqueaudit changes the item being audited, just as indentation hardness testing or voltagechecking does. At the same time, it is usually necessary to perform the audit withoutdestroying the item (joint) being audited.Torque auditing in also difficult to perform accurately because of the numerous factorsthat affect the force (torque), and the fact that there are interactions among the

components in the joint and the forces involved.In this paper we will discuss some of the factors that affect the torque that is retainedin the joint by the initial tightening, and how they affect the audit and audit results.Note that this paper is not intended as a comprehensive discourse, nor is it in any waya substitute for performing torque audit experiments on the actual joints that you willbe auditing.

Fastener MotionTo audit torque it is necessary to move the installed fastener. Traditionally, this has ledto one of three methods of auditing.

Method 1 - First Motion in the Tightening DirectionIn this method, a variable-reading type torque wrench (dial, beam, or digital) is

used to apply force to the fastener in the tightening direction until motion isdetected. The detected peak torque is regarded as and recorded as theresidual torque.

Method 2 First Motion in the Loosening DirectionIn this method a variable-reading type torque wrench (dial, beam, or digital) isused to apply force to the fastener in the removal direction until motion isdetected. The detected peak torque is regarded as and recorded as theresidual torque.


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Method 3 Comparison of Rotation EndpointIn this method the location of a point on the fastener in relation to an item inthe grip of the fastener is marked. The fastener is then loosened, and thetorque required to rotate the fastener back to that same point in relationship tothe marked item in the grip is measured and recorded as the residual torque.

Each of these methods has numerous risks and sources of error. Method 1 is the mostpopular and most commonly-used method for a variety of reasons, and is the methodwe will examine the most here.The fastener does not engage the torque wrench directly. There is a fastenerengagement device attached to the torque wrench, and it is this device that connectsthe two. When this device is a socket, there is literally a hidden source of error in themeasurement of the residual torque.The socket usually covers the fastener head completely. When the fastener beingaudited is a nut that is on a bolt or stud, it is not possible to see the top of the nutbecause the nut is covered by the socket. As a result, it is common practice to markthe socket and the item(s) in the grip of the joint to assist in detecting motion of thenut. The auditor engages the torque wrench to the nut, marks the socket and gripitem(s) with an almost continuous line, and starts applying torque. When the line onthe socket begins to rotate in relationship to the line on the item(s) in the grip, firstmotion of the nut is visually detectable.Unfortunately this tells us nothing of the relationship between the nut and the bolt orstud. It is the rotation of the nut in relationship to the bolt or stud that creates theclamping force that holds the joint together, and the nut can rotate in relationship tothe items in the grip without rotating in relationship to the bolt or stud.This disparity is possible partly because of something called bolt windup. Bolt windupis a twisting of the shank of the bolt itself. This phenomenon occurs in some, but notall joints. Whether it occurs or not is a function of a number of variables and theirinteractions. Research shows that the relationship between bolt diameter and boltlength is among the variables and interactions having the greatest effect. A short,large-diameter bolt will tend to wind up less than a long slender bolt.

If the bolt winds up during the torque audit, it is possible to rotate the nut inrelationship to the item(s) in the grip without rotating the fastener in relationship to thebolt. When a socket is used on the nut, the auditor cannot tell whether the nut movedin relationship to the bolt/stud or grip item(s), or both.Further compounding the error possible from this source is the possibility that the pointat which bolt/stud windup occurs may fall inside the torque specification, rather thanoutside. This could lead to windup affecting some audit torque readings and notothers.Other factors that may affect windup include bolt hardness, thread pitch andcoefficient of friction. We cannot address all of the possibilities here, but instead drawto your attention that this is a potential source of variation and torque audit error. If youwish more in-depth material, one source for such information would be An Introductionto the Design and Behavior of Bolted Joints, Third Edition, by John H. Bickford. It is

available from Marcel Decker Press.

Perceived Motion and Human ResponseWhen a joint torque audit is being conducted, the auditor may rely on a visual clue thatthe fastener has moved (see above), or the auditor may rely on feedback from the feelof the torque wrench, or both. In either case there is a dynamic of increasing pressureon the tool, a change in the fasteners to which the force is being applied, and asensing of the change. The sensing of the change in the fastener from static to


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dynamic from at rest to in motion takes time to occur. During that time the auditoris sensing the change and responding to it by reducing or releasing pressure on thetool.This elapsed time typically takes several tenths of a second. Human nervous systemstend to average about 0.3 seconds for a relatively simple response such as this. Thisresponse time is a variable, and has all of the typical characteristics of a human

variable. In addition to lot-to-lot variation (variation among auditors) there is within-lotvariation (variation in response from a single individual). The length of time it takes anauditor to realize that the fastener has rotated and to stop increasing pressure on thewrench (and increasing the torque) leads to variation in the audit results.

Joint HardnessJoint hardness refers to the rate at which tension rises as torque is applied. A hard

joint is one in which very few degrees of fastener rotation are needed to go from 10%of the desired bolt preload (tension) to 100% of bolt preload. This would be typical of a

joint in which two hard steel plates are bolted together. A soft joint is one in which ittakes many degrees of rotation to move from 10% of the desired preload to 100% ofthe desired preload. An example of this would be a joint in which there was a thick soft

gasket between two hard steel plates. The additional rotation is needed because thejoint rate (slope of torque-tension line) is changed; the gasket material compressesand necessitates more fastener rotation.How hard or soft the joint is interacts with the torque wrench and the auditor,combining in largely unpredictable manners.

Dynamic Installation and Static AuditThe inertial difference between fastener tightening during installation and joint torqueauditing is another source of variation in audit results.When a fastener is being tightened during installation, the torque is normally dynamic.Whether the installation tool is powered or manual, the fastener is being rotated untilthe torque control device stops rotating it. The opposite is true in a joint torque audit;the fastener is at rest and motion must be started in order for the torque to bemeasured.The need to overcome inertia, to get the fastener to move, contributes to error both inthe measurement and in the understanding of the results.The first diagram here shows torque versus time during fastener installation. Thevertical axis is torque and the horizontal axis is time. The first diagram shows thetorque rising over time, as occurs during installation.


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When the torque application tool has attained the specified torque, force stops beingapplied and the initial preloaded torque is present. This is shown in the diagramimmediately below.

Once force is no longer applied, any joint settling that will occur can begin. The jointsettling may be occasioned by any of a number of factors that include, but are notlimited to: bolt windup, flow of paint or gasket materials, unevenness of mating

surfaces, surface finish of mating surfaces, perpendicularity of bolt/stud shank tounderside of bolt/stud head, perpendicularity of grip item(s) to underhead of bolt,bolt/stud installation seating or perpendicularity error.Joint settling takes time. Even though it may begin immediately (or later, or not at all),the amount of time between the installation and the audit means that inconsistent timebetween installation and audit may contribute to variation in the joint torque auditresults.The diagram below shows a slight amount of settling having occurred after installation.This leaves the actual residual torque slightly below the installation torque.


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Having established what the actual torque is at the time of the audit, we can now lookat what happens during the audit process and some of how measurement erroroccurs. The diagram immediately below establishes where the actual torque is in the

joint with a green horizontal line.

The diagram below shows the beginning of the joint torque audit. Using a standardvariable reading torque wrench that detects, or is set to detect, the peak torque. As theauditor begins applying force, the torque reported by the tool rises. The auditor

continues applying torque until the actual torque is achieved as shown in this diagram.


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Even though the actual torque has been reached, the force required to overcomeinertia to make the fastener rotate has not been achieved. This requires additionalforce additional torque. This is shown in the next diagram immediately below.

This torque is not the actual torque, but it is the peak torque that will be measured andrecorded by the torque wrench during audit IF no other errors enter the process. But itis not what the real torque is. It is the sum of the actual torque and the torque requiredto overcome inertia to go from static torque to dynamic torque.

In the transition to dynamic torque the torque actually drops to, or extremely close to,the actual torque. This transition is shown by the blue section of the torque line in thegraph below.


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If we assume the operator senses the fastener motion and reacts to it very quickly, theinstalled torque will still be changed by some amount. This is a rise in torque shown bythe second blue segment below, and assumes the operator stopped quickly enoughthat the peak reported by the tool (peak torque finger, pointer, or digits) did notchange.

In the scenario just depicted, there is a zone of error that is equal to the differencebetween the peak torque reported by the torque tool and the actual torque on the joint

when the audit was conducted. This error can be substantial and misleading.In this example we assumed that the operator stopped applying force very quicklywhen the fastener moved. Yet we know that the auditors response is a variable, andthat the variation can result in an even higher reported value.A further assumption related to the auditor and audit a hidden assumption - is thatthe force was applied steadily so that no torque spike affected the peak torque. Anerratic force application, particularly at or very near the point where the static torquetransitions to dynamic torque and fastener movement, can increase the error in thereported torque.


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We also assumed a minimal amount of joint settling occurred between the timethe dynamic torque ceased being applied in the installation process and the auditbegan was very little. The greater the amount of joint settling, the lower the reported

joint audit torque will be. It is entirely possible to have a lower reported audit torquethan was actually applied to the joint during assembly. The softer the joint and themore variation in the components within it, the more likely this is to occur.

The Exacta Residual ModeThe existence of the above problems in joint torque auditing led to the development ofthe Residual Mode for the Exacta line of digital torque wrenches.The question posed was straightforward: Can we reduce the zone of error in jointtorque auditing for most of the joints our customers encounter in their assemblyoperations?The answer to this question is a qualified Yes.In the Exacta line we use high-speed processors to measure the torque. Theseprocessors look at the analog output from the torque transducer many thousands oftimes per second. Since we know the dynamic of the joint torque audit we were able todevelop an algorithm that looks at a string of torque values and compares them to the

known pattern of torque for a static joint torque audit after installation.The residual mode is an algorithm that looks for an increase in torque followed by arapid dropoff that is followed by a similar rate of increase in torque as occurred prior tothe dropoff. It then reports as the residual torque the torque at the bottom of thedropoff before the second rise in torque.This mode is represented by a symbol that depicts quite well the principle applied toobtain the residual torque value. The symbol is shown to the right.The reported residual torque value is reported as the value that corresponds to theline with the arrow; the valley created by the resumption of dynamic torque in movingthe fastener.The actual algorithm used will not capture the exact residual torque on any joint,except by pure chance. It will, however, reduce the zone of error in the joint torqueauditing of many or most of the joints our customers audit. The amount of reduction

can be quite significant in many audit applications.It is not possible to store an infinite variety of algorithms on the tool. In fact, we canonly keep, store, and use exactly one algorithm. Since most of the joints our customerbase audits are hard or fairly hard joints, the algorithm is designed to work best with

joints of high hardness (fairly few degrees of rotation to move from 10% to 100% ofdesired preload).Even with an algorithm designed primarily for this type of joint, perfect torque detectionis not possible. There will still be a small discrepancy between the reported torque andthe peak dynamic torque applied during installation.Joint settling that occurs after the installation process and before the audit process willstill affect the audit results. Where a great deal of settling has occurred, thediscrepancy between the reported residual torque and the installed dynamic torquecan be large enough to cause concern. As a rule of thumb, the softer the joint themore settling is likely to take place and the greater the discrepancy is likely to be.Another source of variation still lies with the operator. Even though the electronics cansense, measure, compare, and report the torque many times faster than a humanbeing with a different torque wrench can, there are still limits to the capabilities of thetool, and these capabilities can be overwhelmed by incorrect or erratic audittechnique.Erratic force application during the audit will create a rise/fall/rise pattern such as thealgorithm uses to determine the residual torque. When this occurs, the tool will report


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exactly the torque it found, even though the cause was erratic force applicationinstead of fastener movement.Note that softer joints, such as those including painted components or gasketmaterials in the grip can also cause this rise/fall/rise pattern to occur when flow of thesofter material takes place during the audit. Even a set of components with a roughsurface finish can flow or reflow to some degree when the torque rises during the audit

process. This is particularly true as the applied torque approaches the actual torque.It is possible for either the joint or the operator to introduce a discrepancy between theinstalled torque and the audited torque.

Recommendations1. Conduct experiments on the actual joints to be audited to determine howclosely the Residual Mode will come to the reported dynamic torque from theinstallation process.2. Insure the experiments take into account the variables discussed here,particularly those pertinent to joint hardness and joint settling or relaxation.3. Use the results of the experiments to refine the joint audit process. Thismay include such refinements as auditor training, audit time windows for the

elapsed time between assembly torque application and conducting the audit,and other changes as your data indicates will be beneficial

Class Outline

Objectives

What Is Torque?

What Is Preload?

What Is Clamping Force?

The Importance of Torque

The Effects of Overtightening and

Undertightening

Factors Affecting Torque

Determining Torque How Torque Is Derived

Applying Torque

Proper Torque: An Example

How Torque Is Measured

Inspecting Bolted Joints

Inspecting Torque Tools

Summary

Class Objectives

Define torque.

Explain the importance of preload.

Explain the importance of clamping force.

Explain the importance of torque.

Describe the effects of overtightening.

Describe the effects of undertightening.

Describe factors that can affect torqueaccuracy.

Describe factors to consider when determiningtorque.

Explain how torque is derived.

Describe methods for applying torque.

Solve for the torque value.

Describe methods for measuring torque.

Describe methods for inspecting bolted joints.

Explain the importance of inspecting torquetools.

Class Vocabulary

bolt A cylindrically shaped, threaded device used for fastening parts. Bolts usuallyhave blunt ends and mate with a nut.

breakaway torque The torque value that registers when the fastener is first turned.

calibration The act of adjusting a measuring instrument against a standard to ensure itsaccuracy.

clamping force The compressive force that a fastener exerts on a joint.
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Class Vocabulary

click wrench A type of wrench that measures the amount of torque applied to a fastener. Aclick wrench indicates the torque value with the use of a digital display.

clutch A coupling found inside a motorized device that connects or disconnects parts todrive the mechanism.

compression A pushing or pressing force that is directed toward the center of an object.

dial wrench A type of wrench that measures the amount of torque applied to a fastener anddisplays the torque value on a dial.

distance The amount of space between two points. Torque is determined by multiplyingforce times distance.

embedding To be enveloped by or surrounded by another material or object. A fastener isembedded into the base component. Embedding can cause changes in torque.

English system A standard system of measurements based on the inch, second, pound, andFahrenheit degrees. English measurements are primarily used in the United Statesand England.

fatigue strength The maximum stress a fastener can withstand for a specific number of repeatedapplications prior to its failure.

force The push or pull that gives energy to an object. Torque is determined bymultiplying force times distance.

friction A force that resists motion between two components that are in contact with eachother. Friction is an obstacle to overcome when accounting for torque.

friction stabilizer A device used in bolted joints that regulates the amount of friction in the joint.Friction stabilizers help engineers to maintain the proper torque.

hard joint A joint in which it takes the fastener less than a 30 turn to get the fastener from

fitting snugly to reaching torque.

impact wrench A screwdriving tool that uses a discontinuous drive clutch and compressed air toachieve torque. Impact wrenches are often used to tighten lug nuts on cars whenchanging a tire.

inspection torque The amount of preload in a fastened joint.

load The overall force that is applied to a material or structure. A fastener thatsupports the weight of an object is withstanding a load.

metric system A standard system of measurements based on the meter, second, kilogram, andCelsius degrees. The metric system is internationally recognized.

preload The tension created in a fastener when it is first tightened. Preload must be

greater than the external forces applied to the assembly to prevent joint failure.

repeatability The ability of an object to deliver accurate and dependable results over a longperiod of time.

residual torque The amount of tension that remains in a joint after fastening a threaded fastener.

screw A threaded device used for fastening parts or transferring motion. Screws usuallyhave pointed ends.
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Residual torque is nearly always measured with an electronic or mechanical manual torque wrench.The challenge is that these tools are not readily capable of measuring residual torque. The chart

above illustrates a torque-time trace recorded when a fixed DC nutrunner with an external torque-angle transducer performed a torque audit during a competitive engine assessment. The annotated

points of the trace show the key milestones of the recording.

Prior to A, torque increases continuously at a rapid rate as the tightened screw resists the torque

being applied to it. At A, there is a brief dip before thetorque climbs again at a slightly lesser rate to the

maximum torque at B, after which it drops sharply. Thefastener being audited was a high-strength screw with

a large length-to-diameter ratio.

The dip at A coincided with the friction under the head being overcome suddenly and the headbreaking free. In the brief period between A and B, the body of the fastener is winding up until the

threads finally break free at B, causing the torque to dip in segment C. Because the static frictionwas greater than the dynamic friction, the torque required to rotate the threads at D (the residualtorque) is less than what was required to break the threads free at B.

As the fastener continues to rotate along segment E, torque rises more gradually because thefastener is not as stiff when it is being elongated. The peaks at A and B are not always present, andare usually associated with a hard interface. In fact, this effect usually makes its presence knownwith an audible creaking sound as the energy stored in the bolt is suddenly released.

By examining the figure, the problem of measuring residual torque by using the peak valuedisplayed by a standard torque wrench should become apparent. The auditor could very likely think

the movement at A caused by the head breaking free was movement of the threads and stopshortly after. Even if the audit was halted very close to the desired point D, the peak value thatwould be read would be the torque at B rather than D.

The most common scenario is that the pull does not stop until somewhere up the segment E,

leading to higher values than are actually present. This outcome occurs whether the spikes at A andB are present or not. So, whats the best solution?

When careful and consistent auditing is used, these limitations are usually not critical, as long as

values are compared only to past audits to ensure fasteners werent missed and the installationprocess appears in control. However, when residual values are expected to be accurate relative to

the initial installation torque, more advanced auditing tools may be warranted.

Picking points off a trace is very accurate, but its often not practical for a production environment.As a result, audit wrenches have been developed specifically for measuring residual torque. These

wrenches use a number of different techniques for automatically selecting the start of threadrotation.

To see a review of these wrenches, including a report on their effectiveness, click

www.archetypejoint.com/comparisontest. A

explaining the words

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