Safety Assessment en 13001-3-2

Fakultät Maschinenwesen Institut für Technische Logistik und Arbeitssysteme Professur für Technische Logistik

SCIENTIFIC REPORT PROFESSORSHIP OF LOGISTICS ENGINEERING PROF. DR.-ING. HABIL. THORSTEN SCHMIDT MATERIALS HANDLING TEAM

Safety Assessment - EN 13001-3-2

Prof. Dr.-Ing. habil. Thorsten Schmidt Dipl.-Ing. Martin Anders

July 2015 FE-TL-35

Safety Assessment - EN 13001-3-2

Content 1 

Subject of Evaluation............................................................................................................. 3 

2 

Rope drive design – previous approaches............................................................................. 3 

3 

Service life and lifetime of running wire ropes ...................................................................... 3 

4 

Verification procedure with EN 13001-3-2 ............................................................................ 3 

5 

Occurring problems............................................................................................................... 4 

6 

5.1 

Possible misinterpretation of wtot – values ..................................................................... 4 

5.2 

Danger of early rope failure............................................................................................ 4 

Recommendations ................................................................................................................ 6  6.1 

Option 1: Changing the designations of lr and wtot and implement limitations for lr ...... 6 

6.2 

Option 2: Classification of the rope force history parameter sr ...................................... 6 

6.3 

Remarks to multilayer spooling ...................................................................................... 6 

7 

Conclusion............................................................................................................................. 7 

8 

References ............................................................................................................................ 7 

Appendix 1: Exemplary calculation

Report – Safety Assessment - EN 13001-3-2 Prof. Dr.-Ing. habil. Thorsten Schmidt, Dipl.-Ing. Martin Anders

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1

Subject of Evaluation

In 2014 the standard EN 13001 part 3-2 “Limit states and proof of competence of wire ropes in reeving systems” [1] became effective. This standard is to be used together with the standard 1 and 2 of EN 13001 “Cranes – General Design”. The following evaluation gives a short overview about the calculation sequence and the differences to other standards.

2

Rope drive design – previous approaches

The dimensioning of wire ropes and rope drive components (sheaves, drums) can be performed according to the standards DIN 15020 [2], ISO 4308 [3] or ISO 16625 [4]. The procedure with c-values or Zp-values and the given limits which consider the frequency of utilization and the load spectrum lead in any case to a sufficient service life of the wire ropes.

3

Service life and lifetime of running wire ropes

The service life (time until discard) and the lifetime (time until rope failure) of wire ropes running over sheaves/drums can be expressed as no. of bending cycles. In several decades of research and testing ways to estimate the no. of bending cycles that wire ropes can endure under different levels of stress until discard or rope failure were developed (see Stuttgart method or Leipzig method acc. to VDI 2358 [5]). Experiences with the applications of wire ropes, especially cranes, as well as in-house tests of wire rope manufacturers correspond to the magnitude of estimated numbers of bending cycles. The lifetime of wire ropes is basically depending on the following parameters: rope forces, D/d ratio, rope construction and further rope drive design parameters.

4

Verification procedure with EN 13001-3-2

With this new standard the design of a rope drive with its components by calculating minimum values for rope and sheave diameters in order to ensure adequate service life is no longer possible. With chosen rope drive parameters now two verifications have to be performed, firstly the proof of static strength and secondly the proof of fatigue strength. In both cases a design rope force is to be compared with a limit design rope force. The proof of static strength ensures a minimum rope safety factor under consideration of dynamic factors and the design of the rope drive (diameters, reeving system, breaking load). This proof does not consider any operation condition like utilization or load spectrum. It ensures a minimum real safety factor against the minimum breaking load with insufficient endurable number of bending cycles if the maximum rope force is considered in lifetime estimations. The level of safety can be compared with crane designs according to the group classification of the mechanism M1 according to ISO 4301 [6] or “Triebwerkgruppen” 1Em and 1Dm according to DIN 15020-1. The proof of fatigue strength considers the operation parameters such as load spectrum and frequency of utilization. This consideration is based on rope spectrum factor kr which equals the load spectrum factor Km according to ISO 4301 and the number of operation cycles C of the crane during its projected lifetime integrated in the relative number of bending cycles νr. Both influences – load spectrum and utilization – are combined in the rope force history parameter sr. The value νr is again depending on a “number of bending cycles during the design life of a rope” wtot, which is mainly defined by a number of ropes lr which are to be defined for the design life of the crane. The Annex B of EN 13001-3-2 gives values for lr as a guideline depending on the type of crane. Report – Safety Assessment - EN 13001-3-2 Prof. Dr.-Ing. habil. Thorsten Schmidt, Dipl.-Ing. Martin Anders

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5

Occurring problems

5.1

Possible misinterpretation of wtot – values

For rope drives verified with the standard EN 13001-3-2 considering the guidance for lr in annex B of the standard, the real endurable number of bending cycles is much lower than the calculated “number of bending cycles during the design life of a rope” wtot. Exemplary lifetime estimations for wire ropes with acknowledged techniques such as Stuttgart and Leipzig method show that wtot-values are 7 to 10 times higher (see Figure 1 and Appendix 1). 5.400.000

5.000.000

real no. of bending cycles for discard N with max. load

4.800.000

real no. of bending cycles for discard N with load spectrum

4.200.000

no. of bending cycles during the design life of a rope w_tot

3.600.000 3.000.000

2.500.000

2.400.000

1.666.667 1.250.000 1.000.000 833.333

494.719 270.416

250.991

132.080

1

2

189.244

90.356

3

146.665

68.891

4

120.240

55.760

5

102.161

1.800.000

714.286

88.973

46.880

6

40.464

7

1.200.000 625.000

78.908

35.608

8

555.556

70.961

31.802

9

500.000 64.520

600.000 0

28.737

10

Figure 1: design and real number of bending cycles exemplary calculation Appendix 1

The wtot-values are the result of an arbitrarily chosen number of wire ropes for a given crane project but not a product of an endurance calculation. The name however may imply that it represents an actual number of bending cycles of wire ropes in crane operation. This can lead to a misinterpretation of the service life with the danger of false specification of service intervals in order to check for discard criteria. 5.2

Danger of early rope failure

An even more serious problem is the lack of limits for the proof of fatigue strength. Due to an arbitrarily chosen number of wire ropes lr the influences of the load spectrum can be terminated by simply choosing higher number of ropes in order to reach a small value for sr. The following chart shows the principle procedure of both verifications (static and fatigue) with certain remarks about the possibility of manipulation. The standard allows the possibility to have a heavy load spectrum with much too small safety values due to the fact that the proof of static strength can become decisive. That can lead to false rope drive design with unexpected, early rope failure.

Report – Safety Assessment - EN 13001-3-2 Prof. Dr.-Ing. habil. Thorsten Schmidt, Dipl.-Ing. Martin Anders

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EN 13001-3-2: Cranes – General design – Limit states and proof of competence of wire ropes in reeving systems

Proof of static strength Design rope force

-

Dynamic factor Reeving efficiency Non parallel falls Horizontal forces

≤

Proof of fatigue strength Design rope force

Limit design rope force

-

-

Minimum breaking load Minimum rope resistance factor (static) depending on D/d ratio

≤

Dynamic factor Reeving efficiency Non parallel falls Horizontal forces

Proof of static strength Rope force history parameter sr

Leads to a minimum of real wire rope safety against M.B.L. with insufficient endurable no. of bending cycles

- Rope spectrum factor kr represents the load spectrum of the crane - vr is the relative number of bending cycles with reference point wD = 5·105

- sr can be manipulated significantly by changing wtot which is defined by the free definable no. of ropes specified for the design life of the crane lr - In the present standard there are no limitations for sr

The proof of fatigue strength can be easily manipulated to be positive if lr is chosen properly. If it is positive with reserve, the proof of static strength becomes decisive in any case. The influence of load spectrums can be avoided.

Limit design rope force

-

Minimum breaking load Minimum rope resistance factor (fatigue)

-

further influences Rope force history parameter

Further influences ff particularly ff1 with - ff1 should consider the diameters of drum and sheaves and is limited by minimum D/d ratio 11,2 and ff1 ≥ 0,75 - ff1 can be manipulated by changing wtot which is defined by the free definable no. of ropes specified for the design life of the crane lr - therefore the logical basis for ff1 can not be seen

Number of bending cycles during the design life of a rope wtot

-

- wi …number of bends per movement - imax … no. of movements per rope - C … total no. of working cycles during design life of the crane - nmov … no. of movements per working cycle lr … The number of ropes specified for the design life of a rope. lr can be chosen freely. The guidance for lr which is given in Annex B provides arbitrary values for different types of cranes. These given values result in much too high values for wtot comparing to real endurable number of bending cycles.

Proof of fatigue strength

Report – Safety Assessment - EN 13001-3-2 Prof. Dr.-Ing. habil. Thorsten Schmidt, Dipl.-Ing. Martin Anders

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6

Recommendations

Reflecting the described discrepancies and in order to minimize the effort for making adjustments, the following options are recommended. Either option 1 or option 2 may give a solution. In addition the consideration of multilayer spooling is evaluated in 6.3. 6.1

Option 1: Changing the designations of lr and wtot and implement limitations for lr

Performing the proof of fatigue strength with help of the guidance for lr given in annex B of EN 13001-3-2 the results show acceptable safety factors for most cases. However the chosen number of ropes lr influences the proof of fatigue strength significantly. It is therefore recommended to limit the number of ropes in annex B. Experiences and service life estimations for wire ropes show that the resulting number of bending cycles for the design life of a rope wtot (calculated with lr-values in Annex B [1]) exceeds the real endurable number of bending cycles of wire ropes significantly. For this reason it is recommended to designate the values lr and wtot explicitly as theoretical values in order to avoid misinterpretation of wtot. 6.2

Option 2: Classification of the rope force history parameter sr

In order to avoid possible manipulation as described in the chart in section 5.2 a fixed link between load spectrum, crane utilization and the proof of fatigue strength has to be ensured. This could be implemented by classifying sr analogous to the stress history parameter s in EN 13001-1 (see Figure 2). Therefore the relative number of bending cycles has to change into a relative number of rope stress cycles or likewise in order to represent the crane utilization. This way the sr-value is no longer depending on arbitrary values lr and wtot respectively and a safe rope drive design can be achieved.

Figure 2: S-classification of the stress history parameter in EN 13001-1 [7]

6.3

Remarks to multilayer spooling

Part of the proof of fatigue strength is the consideration of further influences by implementing additional factors ff1 to ff7 in section 6.4 of the standard [1]. Factor ff5 shall consider the reduced rope life in multilayer spooling drums and is depending on the number of movements and the rope force spectrum (see table 8 in [1]). Depending on all other parameters for a crane project it is possible that in the end the safety against the minimum breaking load is lower than defined Report – Safety Assessment - EN 13001-3-2 Prof. Dr.-Ing. habil. Thorsten Schmidt, Dipl.-Ing. Martin Anders

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general safety factors for multilayer spooling based on experience in present standards such as ISO 16625 (ZP-values). Due to the complexity of the influences on the wire rope stresses and deterioration in multilayer spooling a general sufficient safety factor ensures a reduction of rope damage. Therefore it is recommended to implement corresponding safety factors for multilayer spooling already in the proof of static strength in order to ensure a direct connection to the minimum breaking load of the wire ropes.

7

Conclusion

In the new standard for rope drives EN 13001-3-2 the basic principle from designing steel structures as in EN 13001-1 was transferred for the proof of competence of wire ropes. In contrast to the well-defined S-classification for stresses in part 1, the approach with decisive value lr in part 3-2 poses a loophole for any user of the standard in order to proof the fatigue strength as positive. The possible variation of the proof of fatigue strength eliminates the logical influence of the detailed composition of the further influences in EN 13001-3-2, sections 6.4 and 6.5 (factors ff1 to ff7). The present form of the standard can lead to an undermining of basic safety principles, which are ensured in other present standards (ISO 4308, DIN 15020, ISO 16625) for wire ropes and rope drive due to the use of c-values or ZP-values.

8

References

[1]

EN 13001-3-2: Cranes – General Design, Limit states and proof of competence of wire ropes in reeving systems. 2014.

[2]

DIN 15020-1: Lifting appliances; basic principles for rope reeving components; computation and design. 1974.

[3]

ISO 4308-1: Cranes and lifting appliances – Selection of wire ropes – Part 1: General. 2003.

[4]

ISO 16625: Cranes and hoists – Selection of wire ropes, drums and sheaves. 2013.

[5]

VDI 2358: Wire ropes for materials-handling equipment. 2012.

[6]

ISO 4301-1: Cranes and lifting appliances – Classification – Part 1: General. 1986.

[7]

EN 13001-1: Cranes – General Design, General principles and requirements. 2015.

Dresden, 07/21/2015 place, date

Director of institute

Report – Safety Assessment - EN 13001-3-2 Prof. Dr.-Ing. habil. Thorsten Schmidt, Dipl.-Ing. Martin Anders

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Appendix 1 – exemplary calculation supported by Steinbach, G. – VDI; TSU e.V. - E.- M.- Arndt - Str. 14; D-04425 Taucha

Report – Safety Assessment - EN 13001-3-2 Prof. Dr.-Ing. habil. Thorsten Schmidt, Dipl.-Ing. Martin Anders

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