The use of high tensile fibre ropes in elevators

There is a large body of knowledge on the service life of free-running steel wire ropes. Consequently, their use in traction sheave lifts ensures a high degree of operational safety.

However, the use of steel wire ropes at conveyance heights over 500 m is not economically viable. Consequently, alternative materials for use in traction sheave applications are being tested at the Institute of Mechanical Handling and Logistics (IFT) of the University of Stuttgart.

Architecture is continuing to develop – buildings are getting higher all the time. Passenger lifts are currently limited by the high deadweight of their lifting gear (steel wire ropes) to a conveyance height of about 500 m. Consequently, at these conveyance heights, lift groups located above each other are required. This means users have to change, which takes time, is inconvenient and also greatly increases operating costs.

Proved as lifting tackle

The use of low-strength fibres, such as polyester, has already proven its value in sport, the maritime sector, forestry and as lifting tackle. High tensile fibres made of aramid, thermotropic liquid crystal polymer (TLCP) and high molecular polyethylene (HMPE) represent the current state of the art.

In particular, ropes made of high tensile fibres can compete with steel wire ropes and exceed the properties of the latter considerably in several respects, thus opening up new possibilities in industrial applications. Compared to wire ropes, they provide one-and-a-half to three-times greater tensile strength and simultaneously a seven to eight-fold lower length-related rope weight. As a result, greater conveyance heights can be realised and the serial connection of lifts is unnecessary.

Studies on high tensile fibre ropes

Why are steel wire ropes primarily used in conveyance technology despite the advantages of high tensile fibre ropes? This is due to the lack of knowledge and studies on high tensile fibre ropes. Due to this research deficit, above all regarding service life and reliable detection of the replacement state of wear, using fibre ropes is frequently not permitted.

For example, among other things, steel wire ropes or steel chains are prescribed as lifting gear for cars, counterweights or take-up blocks in the current DIN EN 81-20.

To rectify this, traction-based continuous bending experiments with high tensile fibre ropes were conducted at the Institute of Mechanical Handling and Logistics (IFT) of the University of Stuttgart (see box). A large scale test rig with traction sheave drive and lifting height of 8 m is available for this purpose at the IFT (Figure 1). The aim of the experiments there is to evaluate the high tensile fibre ropes used in terms of operational safety, reliability, convenience and economic efficiency.

Stresses produce material fatigue

Ropes running across sheaves are subjected to stress from a totality of expanding bending and tensile stress, compression and relative movements of the individual rope elements to each other. Moreover, the supporting ropes of a traction sheave drive are subject to frictional wear, arising from the slippage between rope and groove.

Since ropes are not permanently attached mechanical elements, these stresses produce material fatigue, with increasing wear towards the end of the service life or even tearing of the rope (Figure 2). Therefore, it is all the more important in technical systems to detect the replacement state of wear or wear replacement criteria and replace the rope if necessary. [1], [2]

Other rope materials and rope designs

The investigations at the IFT have yielded initial important findings on the use of high tensile fibre ropes in traction sheave lifts. It was demonstrated that the material behaviour of the high tensile fibres, slippage occurring and behaviour of the rope-groove combination were controllable factors in conducting the experiment.

The focus of future studies will be above all other rope materials and rope designs as well as the dimension, groove form and material of traction sheaves used on a large scale. Other practice-relevant research topics are increasing the nominal speed and acceleration as well as studying the rope oscillation and vibration occurring at great conveyance heights.

Marco Testa
The author is academic assistant in the Rope Technology Department at the IFT.

www.uni-stuttgart.de/ift

[1] Berner, Oliver; Lebensdauer von Stahlseilen beim kombinierten Lauf über Treib- und Ablenkscheiben unterschiedlicher Rillenform, Stuttgart, 2011, Stuttgart.
[2] Wehking, Karl-Heinz; Laufende Seile: Bemessung und Überwachung; 3. Auflage, Renningen. Expert-Verl., 2005.

Since its establishment in 1927, the Institute of Mechanical Handling and Logistics (IFT) of the University of Stuttgart has been engaged in research in the field of rope technology. The rope technology department concentrates on wire ropes, fibre ropes, personal protective equipment, cableway technology and rope use.

It provides studies to determine rope service life in continuous bending fatigue and extension experiments, static and dynamic experiments, drawing up damage expert opinions, safety and risk analyses, cableway technology and destructive and non-destructive rope testing.

Furthermore, the IFT advises industrial companies as well as the operators of installations and buildings on the specific use of ropes. The department is recognised worldwide as a testing and expert opinion authority. The over 1300 m² experiment hall at the IFT, the rope laboratory, is equipped with testing machinery and installations for rope testing, in part developed at the laboratory itself.