HMK

Crane Service LifeClassification of Cranes

Service Life of Cranes A Brief Explanation of the Basic TermsClassification of Cranes according to FEM 1.001

Two crucial aspects need to bekept in mind when weighing upthe options before purchasingmaterial handling equipment,such as a Mobile Harbour Crane:the maximum handling rates ofthe crane and its total intendedduration of use (service life).

While great importance is attachedto the maximum handling rates inthe decision to purchase, too littlefocus is placed on an assessmentof the cranes service life.

For this reason, the basic interre-lationships and factors influencingthe service life of a MobileHarbour Crane are explainedbelow.

What Do we Really Mean byService Life?When a component or an entire unit, such as a crane, is excessivelyoverloaded, it will fail at once.However, by far the more relevantcause of the damage is failure of the crane due to use beyond itsservice life.

Each individual work cycle of a crane(even if the load lifted is far belowthe cranes maximum lifting capacity)contributes to component fatigue. Ifa load is applied often enough, thecomponent fails even if the individualloads alone did not lead to the failure.

The Paper Clip ExperimentTake a paper clip in your hand andtry to tear it apart. It will not bepossible. However, if you bend itback and forth several times, even a relatively small force is sufficient tobreak it. As a result of the repeatedloading, the useful life of the paperclip is exceeded because, as withmost components in technicalapplications, it was designed for afinite service life that is not based on fatigue endurance.

With the help of this simpleexperiment it is easy to understandthat both the intensity of the stress(i.e. how powerfully the paper clipwas bent back and forth) and thenumber of bends (work cycles) are relevant for the time at which the failure occurs.

However, how would a paper clipmade of thicker wire behave?Experience from everyday life teachesus that it would have lasted longer.Another aspect for evaluating theservice life is thus the durability(loading capacity) for which acomponent has been designed.

Two fundamental elements are decisive for evaluating the service lifeof a component or a crane: the loadand the loading capacity. The servicelife (Figure 1) is a result of the interplay of these two components.

Effects of LoadingThe loads acting on a Mobile HarbourCrane during operation are notconstant. A distinction is madebetween different modes of operation(heavy load operation, containeroperation, grab operation, etc.), in which very different loads occur(empty containers full containers,empty grabs full grabs, etc.).

It is, therefore, not possible to assume aconstant load for the crane. As a result,the standards according to which acrane is designed (e.g. FEM 1.001) definethe loading spectrum by means ofwhich an attempt is made to describethe actual load applied to a crane.

How is a Loading SpectrumDetermined?To accurately determine a loadingspectrum for a particular crane, astatistical random check is used.Readings are taken, for example,thousand cycles for a containerhandling crane and a crane equippedwith grab (50 t lifting capacity in eachcase) as shown in Figure 2.

Figure 1

On the basis of these random checks,it is then possible to determine theloading spectrum of the craneexamined. However, since the exactdistribution of loads in practice is notknown in advance, crane manufac-turers must apply different criteriawhen designing a crane.

The standard on which the cranecalculation is based, i.e. FEM 1.001,makes it possible to estimate thefuture loading of a crane and selectone of four standardised loadingspectra. The standardised loadspectrum classes range from Q1(crane predominantly lifts low loadsand only rarely maximum permissibleloads) to Q4 (crane frequently liftsloads equal to its maximum lifting

capacity). Therefore, the decisivefactor for selection of a spectrumclass is how frequently a crane has to lift heavy loads (in comparison tothe maximum load in the mode ofoperation examined).

The diagram in Figure 2 clearlyshows that a crane equipped with a grab, for example, is subjected to significantly higher loads than acontainer-handling crane becauseheavy loads occur at a higherfrequency (the crane driver almostalways fills the grab completely withbulk material). A crane fitted with a grab must therefore always beclassified in the highest spectrum class.

Loading capacityWhile loading is a characteristic of crane operation, the loadingcapacity is a characteristic of thecrane construction. When a crane isdesigned, a certain crane classificationis selected by the manufacturer andthe crane is designed according tothis classification.

FEM 1.001 offers a choice of eightgroup classifications (A1 to A8). Inthe example above with the servicelife of a paper clip, we noted that thethickness of the wire is a determining

factor for the service life. In the steelstructure of a crane the same applies.To put it in very simplified form:

Service LifeIf the loading capacity (crane group)and the expected load (spectrumclass) are known, the service life(class of utilisation) of the crane canbe determined on that basis.

According to FEM 1.001, this willgive us a class of utilisation from U0 to U9 and thus an indication ofhow many work cycles the crane willmanage before the end of its servicelife is reached. Towards the end ofthe service life, the probability offailure increases rapidly with furtheroperation. The relationship betweenthe sizes, crane group, spectrum class and service life is illustrated inFigure 3 and Table 1. If we nowcompare a crane in crane group A8to a crane in crane group A6 withregard to service life, it is easy todetermine on the basis of the diagramhow long the useful life of each onewill be.

The higher the number of the cranegroup, the thicker the steel sheet andtubing used and the more robust thecrane.

If both cranes are used for the sametype of application, the same spectrumclass applies to both cranes. If thisclass is specified as Q3, it can beseen that the crane classified in A8will have a service life of more than4,000,000 work cycles whereas thecrane classified according to A6 willreach the end of its service life afteronly 500,000 work cycles.

The crane with the higher classificationthus has a useful life that is over fourtimes longer given the same loads!As a rule of thumb, one can state onthe basis of the diagram in Figure 3.

Cranes that have been classified inthe highest crane group, A8, aredesigned such that the loads on thecomponents during operation are solow that theoretically no fatigue iscaused by such operation.

Consequently, a crane classified incrane group A8 does not have alimited service life and will have anunlimited fatigue endurance.

Given the same loads, an increase inthe crane group by one level (e.g.from A6 to A7) corresponds to adoubling of the expected useful life.

10 0 012 0 014 15 016 181 018 63 3520 30 022 43 024 45 026 42 028 37 030 45 032 64 034 86 336 138 738 101 1340 57 3042 37 4044 12 8746 3 17048 1 31550 0 300

No. of cyclesLoad on ropes [t]

Crane fittedwith grab

Containerdischarging

crane350

300

250

200

150

100

50

010 14 18 22 26 30 34 38 42 46 50

Weight [t]

Num

ber

of c

ycle

s

Figure 2: Determination of the loading spectra

Container operation

Grab operation

Subjects to change without notice

Gottwald Port Technology GmbH P.O. Box 18 03 43 40570 Dsseldorf, GermanyPhone: +49 211 7102-0 Fax: +49 211 7102-3651 info@gottwald.com www.gottwald.com

U8

U7

U6

U5

U4

A1 A2 A3 A4 A5 A6 A7 A8

Q1

Q2

Q3

Q4

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

Serv

ice

life

(in m

illio

n w

ork

cycl

es)

U0 U1 U2 U3 U4 U5 U6 U7 U8 U9

16,000 32,000 63,000 125,000 250,000 500,000 1 m 2 m 4 m > 4 m

Q1 A1 A1 A1 A2 A3 A4 A5 A6 A7 A8

Q2 A1 A1 A2 A3 A4 A5 A6 A7 A8 A8

Q3 A1 A2 A3 A4 A5 A6 A7 A8 A8 A8

Q4 A2 A3 A4 A5 A6 A7 A8 A8 A8 A8

Class of utilisation and duration of use (number of work cycles)Loadingspectrum

class

Figure 3: Relationship between crane group and useful life

Table 1

Cla

ssifi

catio

n 01

/07.

03 U

K S

+S

...

fatigue endurance rangefatigue endurance range

range of fatigue strength for finite liferange of fatigue strength for finite life

Crane group