Issue 2/2010
The impact of reverse bending in the rope drives of traction lifts
Dr.-Ing Wolfram Vogel
The steel wire ropes used in traction lifts are exposed to bending fatigue as they pass over the traction sheaves and deflection pulleys. As regards the bending sequence, attention must be paid to the occurrence of simple and reverse bending processes, which exert a greater or lesser impact on rope service life. In traction lifts, reverse bending processes occur in steel wire ropes when travelling over traction sheaves and deflection pulleys where the traction is arranged above or below next to the travel path. Reverse bending processes in steel wire ropes also have to be assumed in situations where an additional deflection is needed to increase the wrapping angle of the traction sheave and consequently the traction capability.
Due to the small size of traction and deflection sheaves admissible for DRAKO ropes in accordance with type testing certificate G515, which can be as low as D/d=25, when retrofitting new systems with small drives in existing shafts, deflection processes involving reverse bending of the wire ropes can occur with regularity. Fig. 1 illustrates rope guidance examples involving reverse bending processes in the rope drives of traction lifts.
Reverse bending of steel wire ropes and the impact this has on rope life when using sheaves with round grooves has been analysed and described in FeyJah1991, Jeh1985 and Mül1965. To date, there has been no research reported on the special case in which reverse bending takes place between a traction sheave with shaped grooves and a deflection sheave, which can also involve different diameter ratios. The following article discusses calculation of the service life relative to implementation of reverse bending under the special conditions described above.
Current state of research
The history of research in this field is relatively brief. Practically the only analyses performed on the subject of reverse bending in running steel wire ropes to date are by Mül1961, Jeh1985 and FeyJah1991. Based on the results from fatigue bending tests performed by Mül1961 and confirmed by Jeh1985, the service lifereducing impact of reverse bending was determined in comparison to simple bending with factor 2 in DIN 15 020. These results were determined through testing with a restricted parameter field. In their tests, FeyJah1991 varied a number of parameters within broad limits: diameter ratio from sheave to rope, rope tensile force S and the ropes themselves. The results show that the number of cycles to breakage or discard age for reverse bending depends heavily on the number of bending cycles to breakage or discard when bending takes place in the same direction. By means of linear multiple regression, suitable equations have been derived from the test results. As the number of simple bending cycles reduces and the D/d ratio increases, i.e. with diminishing fl exural stress, the ratio of reverse bending cycles to simple bending cycles increases.
The bending tests performed by the above authors were performed with extremely small distances between the sheaves. With increased spacing apart of the sheaves between which reverse bending of the rope takes place, the possibility that the ropes escape the effects of reverse flexural stress cannot be excluded. This would have the positive consequence of increasing service life. Practice has shown that this must be evaluated on a case-by-case basis. In EN 81-1, the assumption of reverse bending is only made where the distance between sheaves is smaller than 200 times the nominal rope diameter. The service life-reducing impact of reverse bending is taken into consideration here by assigning the factor 4 to the number of pulleys with reverse bending compared to pulleys where bending takes place in the same direc-tion.
Definition of reverse bending and simple bending
For ropes running over sheaves, a distinction is made between bending in the same direction (simple bending) and reverse bending. With simple bending processes, the rope assumes the status straight-bent-straight. Where two sheaves follow on from each other, the bending direction of the rope must be the same. Where reverse bending takes place, which involves at least two sheaves, the rope assumes the statuses straight-bent-straight-reverse bent. A detailed illustration of both forms of bending is provided, with inclusion of OIPEEC Recommendation 3, in Fey2000.
Reverse bending in traction lifts
The tests and evaluations mentioned above regarding the service life behaviour of ropes subjected to reverse bending were performed with sheave combinations using the same D/d diameter ratios and with round grooves. In traction lifts with adjacent drives, however, the mounting situation which occurs is that
- The traction sheave is confi gured with a shaped
- The traction sheave and deflection sheave between which the reverse bending process occurs regularly have different diameter ratios D/d.
A calculation of the service life must take account of the bending sequence in the rope section subjected to the greatest level of stress. Once the bending sequence and bending elements are known, it is possible to attribute the infl uences of reverse bending and of the shaped groove. A number of bending cycles must be calculated for each bending ele-
ment, and collated to the number of trips using the damage accumulation hypothesis put forward by Palmgren and Miner. Attention must be paid when using this bending element model not to under or over represent the infl uence of either reverse bending or shaped groove, both of which factors exert an extreme impact on rope life.
The bendingsequence applicable when using an adjacent drive is illustrated in Fig. 2. While the handling of bending elements 1 and 4 is not critical, bending elements 2 and 3 must be subjected to close scrutiny to determine which element is assigned which influence. The variant of joining the traction sheave and reverse bending together in a single bending element results in extremely small calculated cycle numbers.
Comparisons of calculated and observed actual rope service life have produced good approximations, where reverse bending is calculated with a mean dia eter formed from the diameter of the traction sheave and the defl ection pulley and based on a round groove. Additional refinement of reverse bending by taking into account the different diameter of the traction sheave and counterweight with half a bending cycle each increases the work involved in the damage accumulation hypothesis and the accuracy. The procedure which uses the mean diameter is justified if the diameters of the traction and deflection sheave differ only minimally. The influence of the traction sheave groove on the number of trips is taken into consideration in the "half bending cycle" on the driving sheave (marked "3" in Fig. 2). Here, the dimin ution factors from DIN 81-1, Annex N are applicable.
The rope tensile force used in calculation of the bending cycles depends on the arrangement of the elevator car and the counterweight. The rope force increases arising from the shaft efficiency and from the acceleration, and an estimation of uneven rope tension levels must be applied accordingly. In addition, the transport sequence, i.e. the pro-rated load must be known. If better findings are not available from observation etc., then the specification provided in VDI Guideline 4707 governing pro-rated loads can be used as a basis for calculating trip requirements.
Extending the scope of type testing certificate G515 to include the impact of reverse bending
As a result of the type testing certificate from Pfeifer DRAKO, the DRAKO 250T wire rope is admissible for lifts both inside and outside the requirements of EN 81-1. The safety factor Sf and/or the traction sheave to deflection pulley diameter ratio D/d may deviate from the requirements of EN 81-1 for Pfeifer DRAKO ropes. If the safety factor Sf falls within the requirements of EN 81-1 Annex N, the lift in question is in compliance with the standard. No additional measures are needed for operation. If the safety factor Sf lies outside the requirements of EN 81-1 Annex N, the lift in question is one with a "reduced number of trips". Additional measures are required for operation, such as a reliable trip counter.
The anticipated number of trips is estimated in G515 for cases where the rope zone exposed to the greatest stress runs over the traction sheave and two deflecting pulleys in the same bending direction (with the bending length determined on the safe side corresponding to two distances between storeys) for different diameter ratios D/d and safety factors Sf in accordance with available graphs, whereby "single variety D/d" and different D/d ratios for the traction and deflecting pulley were taken into consideration. Intermediate values can be interpolated using a provided method. Finally, the range of validity of the type testing certificate G515 is summarized in Table 1. On the basis of the findings presented in the above, type testing certifi cate G515 is now extended to include the case of reverse bending between the traction and deflecting sheave.
Summar
With type testingcertifi cate G515 from Pfeifer DRAKO, the field of low and medium-frequented traction lifts is now covered for the first time in terms of approval by a notified body. Areas such as retrofitting with unavoidable reverse bending of ropes will be regulated in future by a general approval procedure on the basis of the type testing certificate G515 from Pfeifer Drako and the findings presented when considering the effect of reverse bending processes in calculation of the service life of steel ropes.
Bibliography
Fey2000 Feyrer, K.: Drahtseile. Bemessung, Betrieb, Sicherheit (Wire ropes. Tension, Endurance, Reliability). SpringerVerlag 2000
FeyJah1991 Feyrer, K; Jahne, K.: Seillebensdauer bei Gegenbiegung (Rope life in the event of reverse bending). DRAHT 42 (1991) 6, p.433-438
Mül1961 Müller, H.: Das Verhalten der Drahtseile bei Wechselbiegung (Behaviour of wire ropes in the event of alternating bending) Drahtwelt 47 (1961) 3, p.193-201 and dhf 8 (1962 2, p.49-52
Jeh1985 Jehmlich, G.: Anwendung und Überwachung von Drahtseilen (Application and monitoring of wire ropes). Berlin: VEB Verlag Technik 1985
VogScheu2010 Vogel, W.; Scheunemann, W.: Auslegung von Seiltrieben in Treibscheibenaufzügen außerhalb der EN 81-1 (Baumusterprüfbescheinigung G515) (Design of wire ropes in traction sheave lifts outside of EN 81-1 (type testing certificate G515). Lift-Report 1/2010, p. 8
DIN15020 Lifting Appliances; Principles Relat-ing to Rope Drives; Calculation and Construction. 1974-02
EN 81-1 Safety rules for the construction and installation of lifts. Teil 1: Electric lifts, German draft1998
OIPECC recommendation 1-6OIPEEC Bulletin 56, Torino 11/1988
VDI 4707 Aufzüge Energieeffizienz (Lifts Energy efficiency). VDI Guideline March 2009
G515 Baumusterprüfbescheinigung G515 - Seiltrieb zur Verwendung als Teil des Triebwerks für Treibscheibenaufzüge mit und ohne verringerte Fahrtenzahl vom 21.08.2009 Type testing certificate G515 Rope drive for use as part of the driving mechanism for traction sheave lifts with and without reduced number of trips dated August 21, 2008 (issued by TÜV Süd Industrie Service GmbH Lifts and Machinery Special buildings / Department Lift and Safety Components Filderstadt)
