Issue 6/2008


11/02/08

Ropes and rope constructions

The six articles in a series entitled “Steel Wire Ropes for Traction Lifts” delve into questions of concern to rope makers and users in recent years. These articles are intended to help answer questions frequently asked and to support troubleshooting whenever a combination of elevator and suspension ropes behaves in an unexpected fashion. Many of the answers stem from research projects or from work carried out with lift owners or operators when searching for the cause of a problem.

At this juncture the authors would like to thank all those involved for their frankness and openness.
Category: Issue 6/2008
Posted by: Editor

When must elevator ropes be discarded?

Elevator ropes are normally discarded due to wire breakage, wear and diameter reduction. However, other discarding criteria such as corrosion or excessive elongation can also take effect.

How many wire breaks are admissible?

The number and distribution of externally visible wire breaks are the most important criteria when it comes to detecting when an elevator rope should be discarded. This is quantifi ed by a count of the maximum number of visible wire breaks over a reference length of the rope. According to ISO 4344 [3], the maximum number of wire breaks over one length of lay must be determined separately

  • For all outer strands and
  • For the two most heavily damaged outer strands

(Fig. 27, Fig. 28) and also evaluated separately. For 6- and 8-strand elevator ropes with fibre core, ISO 4344 provides an indication of the maximum admissible number wire breaks.

 

 

For all other elevator ropes, reference is made to the specific specifications of the relevant rope manufacturer. Taking the number of wire breaks at discard as a reference, it is then possible to determine whether the rope set should be discarded immediately, should be more intensively monitored in the future or should continue to be monitored normally (Fig. 29). To avoid strand breakages and the relevant consequential damage, the maximum admissible number of wire breaks must also be examined in accordance with ISO 4344 relative to the crown of a strand.

In old installations in compliance with TRA 102 [22], elevator ropes are classified, monitored and discarded under the highest gear drive group of DIN 15 020 [23] / ISO 4309 [24].

If the outer wires demonstrate heavy signs of abrasion (Fig. 27), the wires are likely to break at these points and in relatively quick succession. If wire breaks are evenly distributed as illustrated in Fig. 27, the residual service life can be relatively easily estimated.

The European Rope Standard EN 12 385 Part 3 [25] refers to ISO 4344 for elevator ropes in this context.

In non-European countries, the relevant national regulations apply to determining when a rope should be discarded.

The number of wire breaks can sometimes fail as a discard criterion under certain circumstances: Wire breaks due to external wear only occur when the sheaves, in particular the traction sheaves, in a  rope drive system are made of grey cast iron or steel. If only plastic sheaves are used, the safety directive for elevators “Plastic rope sheaves” must be observed, as here under certain circumstances inner wire breaks can occur rather than outer ones.

Fig. 29: Discard criteria for single layer ropes with fibre core according to visible wire breaks

At which wire diameter should ropes be discarded?

Due to external and internal wire wear, over long service periods a continuous diameter reduction can take place in elevator ropes. In ropes with fi bre core, this effect is exacerbated by the drying out and abrasion of the fi bre core.

With a diameter reduction of 6 % relative to the nominal diameter (a 13 mm rope reduces to 12.2 mm), the elevator rope should be discarded immediately, as

  • There is a risk of sudden rope breakage, e.g. due to inner wire breakage at the contact points between strands (Fig. 30),
  • The traction calculation is based on the fact that the rope fits precisely into the groove. Consequently the projected traction is no longer provided if the elevator ropes are too than and
  • Driving grooves which are abraded by ropes which are too thin are consequently too narrow for new ropes, which are then inevitably damaged.

What should be done in case of rust development in the strand gulleys?

If rust powder occurs in the strand gulleys, the rope diameter should be checked in the affected rope sections. In the case of diameter reductions of less than 4% relative to the nominal diameter, the rope can generally be restored by relubrication. Suspected causes for the formation of rust are insuffi cient lubrication, incorrect relubrication and a damp or aggressive shaft atmosphere.

Where a diameter is reduced by more than 6% relative to the nominal diameter, generally speaking a rope change is necessary. In this case, the suspected cause for rust formation is excessive friction between the outer strands. Normally, the outer strands rest on the rope core, and friction between the outer strands is minimal. However, if the diameter of the rope core reduces due to rope wear, the outer strands begin to mutually support each other, with the result that greater friction occurs between them. The abraded particles produced by this process are not metallically bright but red-brown in colour (fretting corrosion). This abrasion process is known as “rope bleeding”, and the created rust powder is also termed “red dust” in the USA [26]. The risk lies in the possible resulting inner wire breaks which only become visible after load relief and extreme bending of the rope (Figs. 31 and 32). The long wire break ends are characteristic of this phenomenon.

Traction Sheaves

What types of different groove shape are there?

A distinction is drawn between shaped grooves (V-grooves, U-grooves with undercut) and round grooves. The groove shape exerts a signifi cant effect on the traction but also on the rope service life. The grooves can become worn with operation, and must be subjected to a special inspection when exchanging ropes. New ropes, perhaps those at the upper end of the diameter tolerance, will respond to worn and excessively small grooves with a shortened service life. The reason for this is excessive Hertz pressure when a rope with a nominal diameter d = 13 mm impacts on a worn groove with a diameter of, for example, 12.7 mm.

Another damaging influence on rope service life is a groove worn to an uneven depth, in particular in drive systems with double wrap drive. The ropes are conveyed at differing speeds in grooves with different active diameters. Path compensation takes place as a result of excessive slip, which is indicated in some cases by an audible creaking noise. In this case, the rope segments between the traction sheave and secondary sheave of the double wrap can be exposed to extremely high “strain tension”.

When changing the rope, consequently the groove profi les of unhardened traction sheaves should always be remeasured. The measurement gauges for the grooves should always be graduated in 1/10 mm steps. Fig. 33 illustrates an example of gauges produced internally by Pfeifer Drako. From which diameter difference between a new rope and worn groove the traction sheave should be exchanged depends on whether

  • There is too much traction being created by this pairing, and the car is drawn towards the shaft ceiling during the suspension, 
  • The unavoidable service life reduction diu to increased contact pressure is accepted and
  • The ropes in question have a fibre or steel core.

Remachining traction sheaves

During elevator maintenance, the discovery is frequently made that whilst a sheave may have worked without problems for between 10 and 15 years with its original rope set, after remachining the sheave when carrying out a rope change, abrasion or rope impressions occur at a far faster rate. In an analysis of the serviceability of this type of remachined sheave carried out already 25 years ago by one of the big car manufacturers of around 700 elevator installations, this type of behaviour was actually statistically proven to be typical of remachined sheaves. It was proven that 60% of such sheaves had to be exchanged again after only a year in operation. This refutes what would appear to be the obvious conclusion – that the new ropes are at fault. Not all new ropes can have resulted in the recorded cases of groove damage. It is conceivable that the cast iron in the groove area can have gradually become crumbly though decades of exposure to swelling pressure, and after repeated turning on a lathe, the ferritic top layer of the groove profi le which has been smoothed over time now begins to react sensitively.

What is an unhardened traction sheave?

The material used to make traction sheaves cannot be easily determined without further-reaching metallographic examination. However, the hardness of the traction sheave can be measured. Long-term studies have shown that at a hardness of up to 180 HB (Brinell hardness), regular rope impressions or higher levels of groove wear have occurred. At a groove hardness of between 180 and 195 HB the probability of this kind of damage occurring is reduced. With increasing groove hardness to above 200 HB, or better still over 210 HB, this type of damage pattern becomes highly unlikely.

For GG 25 grey cast iron, the hardness limit achievable using modern foundry methods is around 230 HB. The hardness test should only be executed at those points of the sheave at which the hard casting skin has been machined down by 2 mm or more, as otherwise the measured results attained with be excessively falsifi ed. Pressure against the test body must be sufficient to ensure that the thin surface layer compacted by the machining process is penetrated.

Spheroidal graphite cast iron GGG 60 demonstrates better material characteristics than GG 25, but is also more costly. Experience and analyses gained from expert consultation demonstrate that it is not only the hardness of the groove which determines resistance to wear, but also the alloy component such as the copper (CU), which substantially increases wear resistance. Wear characteristics are also infl uenced by the formation and distribution of the graphite particles in the cast iron. The fact that the sheaves are no longer stored prior to utilization but machined and mounted immediately after casting can also exercise a detrimental effect.

What is hardened sheave?

Hardening V-grooves has been customary since around 1967. Hardened, undercut U-grooves have been a familiar feature since around 1978. When using hardened rope grooves, it should be noted that:

  • The profile of the different grooves and groove depth must be correctly matched ("If the groove is hardened, the rope is no longer able to help correct the groove"),
  • The edges of the undercut must be well rounded as otherwise two deep wear lines will appear in the ropes,
  • In hardened V-grooves, no dual tensile ropes with "soft" outer wires may be used, but only ropes made of wires with nominal tensile strengths of 1570 and 1770 N/mm²,
  • The ropes must be relubricated without fail,
  • Ropes which become too thin - more so than is the case with unhardened traction grooves which share in the wearing process - run onto the edges of the undercut, resulting in insufficient traction and
  • U-grooves with a 105 ° undercut should be avoided where possible, as once the necessary rounding of the undercut edges has been carried out, no apppreciable rope seat is left, and the rope becomes deformed, in particular when using a 8 x 19 + fibre core rope construction. In the worst case scenario, the rope will then run onto the edges of the undercut and react by premature failure.

Traction sheave with plastic core or made fully our of plastic

A plastic or plastic-core traction sheave in which traction can be radically increased by the plastic is a practically unknown phenomenon in Europe. It is important to bear in mind when pairing that determination of the discard age by externally visible wire breaks can be impeded. However, it is also true to say that these sheave materials work successfully in other countries.

While TRA 003 and EN 81/1986 still stipulatea binding requirement for grey cast iron or steel traction sheaves, specifying a coeffi cient friction of μ = 0.09, in EN 81- 1/1998 the required coefficient of friction is specified relative to the nominal speed of the installation. As a standard is recommendatory in character, evidence of equality in terms of safety leaves the door open for the use of alternative traction sheave materials.

What is contact pressure?

As an authoritative characteristic value for the service life of both the rope and traction sheave, too little attention is paid by elevator constructors to the factor of the contact pressure occurring between these two components. By adjusting contact pressure, for example to the frequency of use, it is possible to exert an instrumental influence on load and consequently on the service life of the rope. However, in EN 81-1/1998 there is no mention of the contact pressure calculation which was featured in the predecessor version of the standard. From the point of the view of the rope manufacturer, this is a major emission. Contact pressure is “indirectly” included in the calculation of the safety factor in accordance with Annex N of EN 81-1. Although the focus has correctly been placed on a minimum rope service life, the standard neglects to include a explicit verification of contact pressure. It is possible to state that a configuration in compliance with EN 81-1/1998 permits significantly higher contact pressure than was admissible according to EN 81-1/1986. The fundamental correlation between contact pressure and serviceability was established as long ago as the standard reference work on traction [30] published back in 1927, Fig. 34.

Regulations

Rope manufacturers only ever have the opportunity to see machine rooms if the rope service life is shorter than the operator has anticipated. In many cases, it becomes evident that although the design has been performed in accordance with EN 81-1, in other words a safe minimum service life has been calculated, this should not be confused with an elevator which is balanced to achieve maximum economic efficiency. It frequently occurs that the parameters which determine the service life of a rope are maximized to their limits, which in turn brings about a corresponding reduction of  service life. To increase user satisfaction, there should be better communication between partners at the pre-planning stage of the installation to determine the expectations placed on service life. This should increase awareness of the fact that a long service life is associated with costs.

Deflection and diversion sheaves

Deflection and diversion sheaves should be made of the same high-grade cast iron as traction sheaves. The grooves of deflection and diversion sheaves only rarely wear to such a degree that new ropes could be damaged as a result. Despite this, however, the grooves of the deflection and diversion sheaves should be included in the inspection when changing ropes. The frequently voiced opinion that a sheave which has a minimal wrap angle is consequently exposed to minimal stress is a misconception. Contact pressure, in other words the force per millimetre of wrap length, is just as great as if the sheave had a wrap angle of, for instance, 180°. Here too, the degree of contact pressure determines the extent of sheave and rope wear. According to elevator manufacturers, the use of universal sheaves for a range of rope diameters has not proven successful. Deflection sheaves can be made of plastic, for example polyamide. Their use is regulated in Germany by the Safety Guidelines for Lifts SR plastic sheaves [31]. There is no concern whatsoever regarding the use of plastic sheaves in conjunction with a steel or grey cast iron sheave. The discard age is clearly indicated by symptoms such as externally visible wire breaks, which occur as a result of running over the traction sheave.

Groove wear in the form of rope impressions (braid formation)

If braid formation (Fig. 35) occurs as a form of groove wear evenly in all grooves and is highly pronounced, then in all probability the sheaves used have an insufficient hardness level.

However, if the sheave hardness is in fact sufficient according to past experience, then there are a number of other circumstances which can result in the occrrence of rope impressions. These include: 

  • Uneven rope tension levels,
  • Dry ropes (lack of relubrication) and
  • Grooves which are excessively narrow, e.g. following a rope change or change of a worn traction sheave.

In each instance, primarily the quality of the cast iron is instrumental. It is highly likely that a correlation exists between the form of groove wear and rope elasticity. Dips of this type in the groove must have been fi led out by a twisting movement of the rope as it runs over the sheave. Experience has shown that 8-strand ropes with a fi bre core made of polypropylene are found to be mounted in a disproportionately high number of cases when rope impressions have been discovered in grooves. Conversely, ropes with steel wire core, i.e. ropes with a substantially reduced longitudinal elasticity, are only very seldom found to be responsible for the occurrence of rope impressions in grooves, provided the sheave has suffi cient hardness. Experience has shown that where traction sheaves of inferior cast iron quality and hardness are used, it is possible to avoid excessive groove wear by selecting a “non-hard” rope. In this type of rope, the outer wires of the outer strands comprise wires of a relatively low wire tensile strength of between around 1180 and 1370 N/mm².

Bibliography: 

[1] EN 10 016, Non-alloy steel rods for drawing and/or cold rolling

[2] EN 12 385 – Part 5 (2003), Steel wire ropes – Safety Part 5: Stranded ropes for lifts

[3] ISO 4344 (published 2004), Steel wire ropes for lifts – Minimum requirements

[4] DIN 50 150, Conversion table for Vickers hardness, Brinell hardness, Rockwell hardness and tensile strength, December 1976, Beuth Verlag GmbH, Berlin

[5] EN 81-1/1998, Safety rules for the construction and installation of lifts - Part 1: Electric lifts

[6] TRA 003, Technical rules for elevators – calculation of traction sheaves, September 1981, Verein der Technischen Überwachungsvereine e. V., Essen

[7] DIN EN 81, Safety rules for the construction and installation of lifts – Particular applications for passenger and goods passenger lifts Part 1: Electric lifts, October 1986

[8] EN 12 385 – Part 1 (published 2003), Steel wire ropes. Safety. – Part 1: General requirements

[9] ASME A 17.1 Safety Code for Elevators and Escalators. The American Society of Mechanical Engineers, New York

[10] EN 13 411 – Part 4 (2002), Terminations for steel wire ropes. Safety. Part 4: Metal and resin sockets.

[11] DIN 3093, Wrought aluminium alloy ferrules; Part 1 and 2, December 1988, Beuth Verlag GmbH, Berlin

[12] EN 13 411 – Part 3 (2003), Terminations for steel wire ropes. Safety – Part 3: Ferrules and ferrulesecuring

[14] EN 13 411 – Part 1 (2002), Terminations for steel wire ropes. Safety – Part 1: Thimbles for steel wire rope slings

[15] DIN 15 315, Wire rope grips for elevators, May 1983, Beuth Verlag GmbH, Berlin

[16] EN 13 411 – Part 7 (2004), Terminations for steel wire ropes. Safety – Part 7: Symmetric wedge socket

[17] DIN 1142, Wire rope grips for rope terminations, January 1982, Beuth Verlag GmbH, Berlin

[18] EN 13 411 – Part 5 (2003), Terminations for steel wire ropes. Safety – Part 5: U-bolt wire rope grips

[19] EN 13 411 – Part 6 (2003), Terminations for steel wire ropes. Safety – Part 6: Asymmetric wedge socket

[20] Czitary, E., Seilschwebebahnen [Cable pulleys], Springer Verlag, Vienna, 1951

[21] Wyss, Th., Stahldrahtseile der Transport- und Förderanlagen [Steel wire ropes in transport and conveyor systems] Schweizer Druck- und Verlagshaus AG, Zürich 1956

[22] TRA 102, Technische Regeln für Aufzüge – Prüfung von Aufzuganlagen, [Technical rules governing lifts – inspection of lift systems] April 1981, Verein der Technischen Überwachungsvereine e. V., Essen

[23] DIN 15 020, Principles Relating to Rope Drives sheet 2, monitoring of rope installations, April 1974, Beuth Verlag GmbH, Berlin

[24] ISO 4309, Wire ropes for lifting appliances - Code of practice for examination and discard, 1990

[25] EN 12 385 – 3 (2003), Steel wire ropes - Safety – Part 3: Information for use and maintenance

[26] Wire Rope Users Manual, American Iron and Steel Institute, Washington

[27] Babel, H., Metallische und nichtmetallische Futterwerkstoffe für Aufzugscheiben [Metallic and non-metalic fi ller materials for elevator sheaves], Dissertation University of Karlsruhe, 1979

[28] Hafenbautechnische Gesellschaft e. V., Hinweis für den Einsatz von Seiltrieben mit Kunststoff-Seilrollen in Kranen, fördern und heben 33 (1983) [Notes on the use of rope traction systems with plastic sheaves in cranes, transport and lifting], No. 1, p. 33

[29] DIN 15 063, Lifting appliances; sheaves, technical conditions, see explanations on 5.4 December 1977, Beuth Verlag GmbH, Berlin

[30] Hymans, F./Hellbronn, A. V., Der neuzeitliche Aufzug mit Treibscheibenantrieb [The modern lift with traction drive], Springer Verlag

[31] SR Kunststoffrollen, Sicherheitstechnische Richtlinien für Aufzüge – Seilrollen aus Kunststoff [SR plastic sheaves, safety guidelines for lifts – plastic sheaves], December 1984, Carl Heymanns Verlag KG, Köln, Berlin

[32] Molkow, Michael, Stahlseile und neuartige Tragmittel [Steel ropes and new means of suspension], Lift-Report 27. year of publication (2001), Volume 5, p. 6-12

Authors

Dr.-Ing. Wolfgang Scheunemann is Technical Director and Head of the Technical Competence Center at Pfeifer DRAKO Drahtseilwerk GmbH & Co. KG

Dr.-Ing. Wolfram Vogel is Head of Research and Development at Pfeifer DRAKO Drahtseilwerk GmbH & Co. KG

Dipl.-Ing. Thomas Barthel is Head of Testing for Elevator Technology at Pfeifer DRAKO Drahtseilwerk GmbH & Co. KG

6/2008

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