Lift Report

At the same time, the steel wire rope as a means of suspension influences the entire elevator system over all phases of its utilization period. This includes for instance the utilization characteristics of the elevator system. In respect of the suspension means, these depend on the rope design, but also on the installation and operational maintenance.

The few examples listed here already illustrate the closely-meshed network of dependency factors and mutual influences which affect the application of steel wire ropes. Given this complexity, it is hardly surprising that rope manufacturers advising elevator producers, elevator planners and operators are confronted with such wide-ranging and diverse issues which affect not only the rope as a machine element but increasingly also the entire elevator system. These issues, which have arisen over many years of work in the field of technical consultancy and sales at the company Pfeifer Drako Drahtseilwerk GmbH & Co. KG based in Mülheim an der Ruhr, have been collated and arranged systematically according to topic areas.

The six-part essay series entitled “Steel wire ropes for traction elevators” addresses these topic areas in the form of frequently asked questions and answers. In the first part of the series of essays, the emphasis is on questions relating to the fundamental building blocks of the rope such as the wire, strand, core and lubrication, the structure and design of the steel wire rope and the valid technical regulations. The following sections contain questions relating to

Finally, an explanation is provided on the right choice of steel wire rope for traction elevators.

Due to its construction and the structure made up of many individual steel wires, the steel wire rope  

Its benefits are

a) its redundancy and

b) The capacity to identify the possibility of the end of service life or preferably the correct time for discarding the rope before its condition becomes dangerous by means of externally visible criteria such as wire breakages.

When running over the traction sheave and the deflection sheaves, the wires in the ropes are exposed to a high complex of stress factors comprising tension, flexural stress, torsion and compression, which contribute towards material fatigue. During flexural stress, the wires bend in relation to each other. The friction created between the wires results in additional abrasive wear. Added to this is the influence of corrosive media. With increasing use, the abrasion characteristics become more pronounced, for example, the number of wire breakages over defined reference lengths increases. Regular inspection permits the correct time for exchanging the rope to be determined or the remaining service life to be estimated.

Redundancy actually means superfluity, a factor which is of extreme importance in the case of safety-relevant applications. A basic distinction is drawn here between active and passive redundancy. Active redundancy is provided by the interaction between wires laid jointly to create a rope or the multiple arrangements of suspension ropes in elevator systems. If one component fails, the remaining components take on its functions in line with their configuration. Passive redundancy relates for example to safety gears which only move into action in the event of an uncontrolled travel movement.

The helical structure (Fig. 1) addresses the fact that an elevator rope is bent over a sheave. This effect becomes evident if we imagine first a parallel wire bundle being bent over a sheave (Fig. 2). The inner wires lying on the sheave are too long and the outer wires too short.

 

Premature failure is the anticipated result. In a wire rope (Fig. 3), the areas with excess length and those with insufficient length lie one next to the other when running over a sheave, i.e. the strand only needs to shift marginally to achieve length compensation. For the individual wires in the strands, the same principle applies. When running over the sheave, all components – strand against strand and wire against wire – are in continuous movement.

The raw material for steel wire is unalloyed carbon steel with carbon content of 0.4 or better 0.6 to 0.8 % by weight. Other materials such as silicon and manganese are only present in minimal quantities as regulated by EN 10 016 [1].

Steel wires for elevators have nominal tensile strengths of 1370, 1570 and 1770 N/mm². Higher strength levels of up to 2500 N/mm² are possible with special approval. A steel wire achieves these extremely high strength levels by a process of manufacture which combines forming with heat treatment. This entails passing rolled wire with a diameter of between 5 and 10 mm through “nozzles” (wire drawing dies) by repeated drawing when cold to gradually reduce the dia meter. During this process, its tensile strength increases by a factor of 3 to 6. Between the drawing processes, the material is exposed to controlled heat treatment, which performs a process known as patenting. The relatively high tensile strength of the steel wires – characterized by extreme microstructure banding – is consequently not the result of factors such as a high content of alloying elements, but of material forming which has occurred when in a cold condition, Fig. 4 and 5.

    

Heat damages the wire. It is said that the high-strength banded forced microstructure regains its original strength of around 400 N/mm². The period of exposure to heat by fire, friction heat, radiated heat, light arcs; heat from welding etc. also exerts an effect on the residual serviceability properties of the wire. At a temperature of 480°C, a complete microstructure transformation takes place after 15 – 30 minutes. At higher temperatures, just seconds can be enough to cause permanent damage to thin wires of the kind used in products such as elevator ropes.

Consideration is being given in different quarters to the possible use of alternative wire materials made of stainless steel. However, ropes made of these materials have little to recommend them for use in traction elevators due to their inferior fatigue bending properties compared to ropes made of carbon steel wires. They command an extremely high price and also come with a note of caution: The ropes supplied from stock by dealers generally lack a good geometry and the carefully controlled lubrication essential for elevator ropes.

The nominal tensile strength of wires can be set within broad limits. What is finally used depends on a range of factors, often also determined by traditional values. These include low sheave hardness levels and also locally applicable regulations and customs, Fig. 7. If the sheave has a low hardness level, it must be borne in mind that the hardness of the wire depends upon its tensile strength. Experience has shown that by using soft sheave materials together with “non-hard” wires, rope impressions can be avoided in the grooves. But in seeking an explanation, it is not sufficient to state that, for instance, wires with a nominal strength of 1370 N/mm² are simply not as hard as those with a strength of 1570 N/mm². In this case, the wire strength drops only from 470 HV (445 HB) to 410 HV (390 HB). Even the “softest” wire in a rope of strength class 1370/1770, i.e. having outside wires with a nominal tensile strength of 1370 N/mm², is still twice as “hard” as a good sheave with a hardness of between 210 and 230 HB.

One reason why low rope grades are customary in certain localities can be regionally applicable regulations permitting low rope safety factors. Due to high levels of contact pressure, a higher degree of groove wear or the effect of rope impressions occurs. This can be alleviated by using ropes with “non-hard” outer wires.

European and international elevator rope standards EN 12385 – Part 5 [2] also 4344 [3] have coined the term Rope grade to describe rope strength. It defines the nominal tensile strengths of the outer and inner wires, and assigns the rope a defined minimum breaking force. Rope grade 1370/1770 means that a rope has a “mixed strength” (termed “dual tensile” in ISO 4344) in which the outer wires of the outer strands have a nominal tensile strength of 1370 N/mm² and the inner wires of the rope have a strength of 1770 N/mm². Rope grades used for suspension ropes and governor ropes are summarized in Fig. 7. Based on a suitable wire material (carbon steel content and purity level matching the targeted wire nominal tensile strength), wires in the rated strength range of 1350 to 1800 N/mm² demonstrate practically the same fatigue bending properties under the same degree of stress.

For elevators in high-rise buildings with the greater rope weight from the need to have a large number of ropes at lower tensile strengths, then higher rope grades of 1770 are frequently used to reduce the number of ropes without reducing the safety factor. 1770 rope grades are also preferred for the operation of drum-driven elevators and roped hydraulic elevators.

In some cases, suspension ropes with wires of rope grade 1960 are manufactured. However, these are no longer regulated in accordance with EN 81-1/1998 [5] and require special approval (Certificate of Conformity). For governor ropes, these restrictions do not apply, and here 1960 grade ropes are used in combination with hardened sheaves.

Wire hardness rises on a linear basis with nominal wire strength (Fig. 6), which is lower in elevator ropes compared to for example crane ropes. The limited nominal wire strength and consequently limited wire hardness should protect the traction sheave against wear. However, Fig. 6 also shows that the wire is always far harder than the unhardened sheave (Brinell hardness HB). Measurement of the wire micro-hardness (Vickers hardness HV), which is occasionally requested by elevator producers in the Far East, only makes sense if soft sheave material and low rope safety factor necessitate the use of a “non-hard” wire material. Generally speaking, the correlation between wire tensile strength and wire hardness follows the progression shown in Fig. 6 for all carbon steel wires with a certain scatter range. More detailed information is provided in DIN 50 150 [4].

The elevator rope is customarily made from bright wires. The light lubricant coating on the wires in elevator ropes is generally sufficient as a protection against corrosion in dry lift shafts. For outdoor elevators, elevators operating in extremely damp or humid climates or in aggressive environments, the ropes should be made of galvanized wires. This type of rope has proven successful in lifts over decades. Water-resistant lubricants should be used in their manufacture and for relubrication. In the tropics, where torrential downpours of rain pose the everpresent risk of water penetration in the lift shaft, the governor rope should be galvanized even for indoor elevators. The only drawbacks of galvanized ropes are a price mark-up of around 10% and in some cases prolonged delivery periods. Due to the higher costs involved, the lower fatigue bending strength and so on, stainless steel ropes are little suited for use as elevator ropes.

Suspension ropes for traction elevators are regularly produced using Seale, Warrington and Filler strand constructions. The diagrams illustrate the strand constructions for a Seale (1-9-9), a Warrington (1-6-6+6) und a Filler (1-6-6F-12) rope, each with 19 strands. Less commonly used, and then generally for larger rope diameters, are Warrington-Seale strands.

The above listed strands in a so-called parallel strand construction are characterized by the fact that the lay length of the wires in the wire plies is identical, with one wire from the outer ring positioned in linear formation in the channel provided between two wires below. No wires cross over each other in the strands, so markedly reducing the incidence of abrasion.

In standard strand constructions known today as cross lay constructions, wires cross over each other in the strand. In these strands, the wires make contact with each other at specific pressure points, resulting in high levels of pressure between the wires and secondary flexural stress. Due to the increased wear and the risk of internal wire breakage, the standard construction is little suited for elevator ropes, but is still found in some cases in the form of thin ropes, for example in dumb waiters and speed limiters.

When designing a strand, it is important to take into consideration the fact that most wires in the strand cross-section appear in the form of ellipses. Consequently the process of designing and monitoring the structure of high-performance elevator ropes is performed nowadays using the latest data processing methods.

The world’s most frequently used strand construction for elevator ropes is the 19- wire Seale strand (1-9-9). Because of the thick outer wires, the Seale strand offers a higher degree of resistances against external wear in use when running over the traction sheave and the deflection points.

The Warrington strand features far thinner wires in the outer wire circle than a Seale strand. This makes for a marked reduction in flexural stress. During fatigue bending tests on round grooves, ropes made of Warrington strands with a 1-6- 6+6 construction achieve around a 20 to 40% longer service life than comparable ropes made using Seale strands. Ropes made from Warrington strands are popularly used in traction elevators with double wrap drives and in roped hydraulic elevators. Consequently both Seale and Warrington are encountered as strand constructions for elevator ropes in countries such as Germany and the UK.

Ropes made using the filler strand construction also offer very good fatigue bending properties. Based on fatigue bending tests, the 8 x 21 filler strand with fibre core (strand: 1-5-5F-10) has been adopted by Canadian elevator standards. Elevator ropes with a diameter of over 16 mm (5/8”) should be designed with a filler construction (1-6-6F-12) due to their improved flexibility, see illustration. The filler construction is particularly well suited for 6-strand ropes. The filler strand is sensitive to geometrical distortion. This applies in particular in where the wire diameter deviates from the nominal diameter. In the case of ropes with rope diameters lower than 10 mm, a filler construction is not advisable due to the extreme thinness of the filler wires.

Warrington-Seale strands are used where large rope diameters are involved in which the outer wires of a Seale strand would become excessively thick, but a high abrasive resistance is imperative. This applies in the case of compensating ropes with a diameter of around 24 mm and for suspension ropes with a diameter around 22 mm. It is advisable to change over to this strand construction when using rope diameters in this range. In some cases, well lubricated ropes with a 6x26 Warrington-Seal construction (strand structure 1-5-5+5-10) have proven the ideal solution for elevator drive systems with a large number of sheaves positioned closely with one behind the other and reverse bending. Ropes produced using a Warrington- Seale construction is sensitive to disturbances to the rope geometry and/or running on traction sheaves with V-grooves or U-groove with undercut. They should preferably be used with round grooves.

Dr.-Ing. Wolfgang Scheunemann is Technical Director and Head of the Technical Competence Centre 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

[1] EN 10016, Non-alloy steel rods for drawing and/or cold rolling

[2] EN 12385 – 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 50150, 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 12385 – 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 13411 – 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 13411 – Part 3 (2003), Terminations for steel wire ropes. Safety – Part 3: Ferrules and ferrule-securing

[14] EN 13411 – Part 1 (2002), Terminations for steel wire ropes. Safety – Part 1: Thimbles for steel wire rope slings

[15] DIN 15315, Wire rope grips for elevators, May 1983, Beuth Verlag GmbH, Berlin

[16] EN 13411 – 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 13411 – Part 5 (2003), Terminations for steel wire ropes. Safety – Part 5: Ubolt wire rope grips

[19] EN 13411 – 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 Aufzugsanlagen, [Technical rules govern ing lifts – inspection of lift systems] April 1981, Verein der Technischen Überwachungs vereine e. V., Essen

[23] DIN 15020, 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 12385 – 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-metallic 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 15063, 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], Dezember 1984, Carl Heymanns Verlag KG, Köln, Berlin

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