Lift Report

Rope elongation is one of the most frequently misunderstood terms about which there is a great deal of uncertainty. The reason for this is the circumstance that no unequivocal elasticity module exists for ropes which can be taken as approximately constant over its complete service life. Added to this is the fact that the question of rope elongation can arise in connection with elevators for different reasons, such as

The elongation behaviour encountered with even identical rope constructions is largely dependent on the competence of the manufacture and can vary tremendously.

The modulus of elasticity of ropes (E modulus) is frequently the subject of enquiry. The pictures below provide an explanation of the elongation behaviour of ropes, illustrating the individual steps of the process and the information which may be derived from it.

Measurements are performed on new ropes. The rope is subjected to continuously increased loads up to 10 % of its minimum breaking force. At the same time, the degree of elongation is recorded. The result of this loading process is the curve shown in the diagrams on the left. After releasing the load on the rope to the starting value, generally ten loading cycles are executed up to 50% Fmin with subsequent release of the load. The purpose of these loading cycles is to settle the ropes, and they are not recorded.

The final load, once again to 10% of the minimum breaking force, is fully documented in the same way as the first curve. This is shown in the right-hand section of the diagram.

In the following diagram, two measurements are entered for the purpose of comparison. These concern a 6-strand and an 8-strand rope with fibre core. The diagram clearly shows that the higher metallic cross-section of the 6-strand rope results in lower elongation under the same load conditions.

The pitch of the curves between two load points is frequently determined as the E modulus.

A comparison between regular and Lang lay ropes is provided in the following diagram. The significantly higher elongation of the Lang lay construction is clearly recognizable. This is particularly noteworthy in the case of extreme shaft heights/rope lengths.

The smallest elongation is provided by full steel ropes, which are illustrated in the final diagram in comparison to an 8-strand “fibre core rope”.

The horizontal distance between the two measurement curves is explained by the so-called initial elongation of ropes. This is an irreversible process. In an elevator, this elongation is the reason for the necessity to shorten ropes following installation.

A distinction must be drawn in this context depending on the rope construction. A class 6 x 19 + fibre core rope has a slightly lower permanent elongation (0.45 % to 0.75 %) than a class 8 x 19 + fibre core rope (0.55 % to 1.0 %). In the case of ropes with an independently manufactured steel rope core (IWRC), this value depends on the respective structure and generally lies between 0.15 % and 0.35 %.

This type of universally applicable guide value is subject to the proviso of a “normal” rope load. It is almost impossible for the rope manufacturer to provide any such data, as this can only be determined after operation of the elevator installation. The elongation behaviour of ropes depends on the load range in which they are operated. Where extreme lifting heights are involved , the intrinsic weight of the rope also has a role to play.

The pitch of the curves depends on the relevant E modulus of the ropes. Elongation is then only dependent upon the load. This characteristic of ropes provides a measure for the frequency of readjustment following car loading and unloading, and is termed elastic elongation. The competence of the rope manufacturer is tested by these two characteristic values. A high degree of permanent elongation generally gives rise to the need for several (costly) rope shortening processes.

“Direct” connection of the car to the traction sheave using ropes which demonstrate minimal elongation under load is synonymous for many elevator users with a high level of ride comfort. The transition from an 8 x 19 + FC construction rope to a rope with a steel wire core reduces elastic rope elongation by around 50 % if the other installation parameters are kept constant. This notable difference is the result firstly of the different E modulus, and secondly of the far larger metallic rope cross-section with steel core.

In addition to rope elongation, when considering spring deflection of car it should be borne in mind that the elasticity of the car frame and the impression of the springs exert an additional influence.

Elevator ropes are lubricated during manufacture in order to prevent corrosion and abrasion. However, the quantity of lubricant applied should only be great enough to ensure that elevators operate with just sufficient traction without slippage. As lubricants also tend to bind dust and abraded particles, how ever, this initial lubrication is hardly ever sufficient to be effective over the entire service life of the rope. It is consequently advisable to occasionally relubricate elevator ropes. As long as wiping a finger over the rope shows a faint smudge, there is no need for lubrication.

It is not possible to provide any definitive statement in respect of relubrication intervals, as they depend on:

Relubrication using fluid lubricants can be carried out using an oil can and paintbrush or decorator’s roller. Oil spray cans should only be used for small rope lengths. In any case, only very minimal quantities should be applied, after which the elevator should execute several complete round trips, paying attention to observe the slip behaviour. Afterwards, additional lubricant can be added if necessary.

If in any doubt whether the rope still has adequate traction after relubricating, before and after relubricating carry out a complete round trip (car completely up, make a joint chalk mark across the rope and elevator, car completely down and then back up again). There should be no major offset between the chalk marks).

Permanent lubrication devices can make for problems when used continuously and in installations with only minimal traction reserve.

The lubricant should not be too low in viscosity, but have sufficient creep capability that it is able to penetrate the inside of the rope.

The most suitable lubricants appear to be rope lubricants diluted with solvent. When used with caution (good ventilation) and carefully metered (a solvent which has not quite evaporated compromises traction) this has proven the ideal combination.

In some countries, however, relubrication agents containing solvent are prohibited for occupational safety reasons. Hydraulic oils or worm gear oils are unsuitable. Lubricants with a solids content (such as molybdenum sulphide or Teflon particles) are also unsuitable for traction elevators, as these agents can reduce the friction between rope and groove to an admissibly high degree.

Ropes for roped hydraulic elevators and compensating ropes may and indeed should be more heavily lubricated.

Consequently, this type of rope, and only this type, may be relubricated with suitable lubricating grease, as in this case the precise amount of lubricant is not as critical as is the case with traction sheave ropes. Generally speaking, however, the customary lubricating oils for traction sheave ropes would also be used in these applications.

No special precautions, only more frequent checks. If applicable, use galvanized ropes.

Apart from installations in extremely dry climates, use galvanized elevator ropes. The basic lubrication provided when the ropes are manufactured should not be normal Vaseline in this case, which can be washed away by water, but a water-resistant medium. Special types of Vaseline exist for this purpose. Relubrication, which is also essential for galvanized ropes, should be performed in this case without fail using lubricants containing solvents. These should be applied during cooler weather (the solvent should not evaporate as quickly) and after extended dry periods.

As hardly any installations exist which have to operate permanently in a very hot or very cold environment, no special measures are required for temperatures ranging between 0° and 50° C.

Where temperatures are constantly between 40° and 50° C, the condition of the lubrication should be checked at more frequent intervals, as the lubricant becomes less viscous and is consequently used up more quickly. The lubrication effect is also less pronounced.

Requests are sometimes voiced for the fibre core to be provided with sufficient lubricant during manufacture to provide a type of life-time lubrication for the entire rope, possibly even over a period of decades. It would be an easy matter for a rope manufacturer to inject a generous helping of grease (for instance 25 %) into the fibre core. However, far from the desired effect of providing a gradually metered lifetime lubrication, the excess grease would seep out within just weeks of hanging the rope. However, the main reason for carefully limiting the grease content of the fibre core becomes evident on studying Fig. 26, which illustrates the cross-section of a new, unloaded 8 x 19 Seale + FC elevator rope. The outer strands are supported on the fibre core; the rope diameter is consequently determined by the volume on the inside of the rope (= fibres + grease).

As the life of an elevator rope is closely linked to its effective rope diameter, it is essential for the fibre core to maintain its volume for as long a period as possible. Consequently relubrication should be performed from the outside in such a way that lubricant also penetrates the fibre core.

One of the possible causes of excessive slippage of ropes on the sheave can be overlubrication of the rope. Under no circumstances should an attempt be made to wash down ropes using cleaning agents or solvents. The solvent penetrates the rope and draws an ever greater amount of lubricant towards the outside.

The following method of external degreasing was developed over 30 years ago and has been used in the meantime with success in solving problem situations over the years. It uses a very fine, neutral reacting powdery quartz fl our which can be obtained under the brand name Florideal.

This powder can be applied for instance by forming a funnel shape with gloved hands, filling them with the powder and slowly dusting the ropes in a downward direction from the traction sheave (machine positioned at the top). The powder absorbs the oil/grease. The dried mass then crumbles away. Subsequently brush away what remains of the powder/grease mixture using a wire brush. The sheaves should also be cleaned, possibly even using solvent.

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