Lift Report

In the field of passenger transport, the dimensioning rules for steel wire ropes used in traction sheave lifts, shaft conveyor systems and cable railways are known. In shaft conveyor systems and cable railways, the sheave to rope diameter ratios D/d have been practically defined. In cable railways this also applies to a large extent also to the rope safety factor Sf, which describes the ratio of admissible load to minimum rope breaking force/computational rope breaking force. In shaft conveyor systems, somewhat surprisingly the safety factor is reduced with increasing depth. There are economic reasons for this, whereby this “loss of safety” is compensated by additional measures performed during inspections. The Technical Rules for Lifts TRA, and also the earlier draft of EN 81-1:1994 called for a diameter ratio of D/d ≥ 40 and Sf ≥ 12 against the minimum rope breaking force/practically the equivalent Sf ≥ 14 against the computational rope breaking force. Limits were laid down for maximum compressive forces. The drawback was that a combination (D/d; Sf) was defined without taking into account the bending sequence in the rope area most exposed to tensile, flexural and compressive forces. This gave rise to a risk of permitting rope drives with small rope service lives in which the discard age could not be reliably recognized between two inspection intervals. In the valid draft of EN 81-1:1998, Annex N presents a dimensioning rule in which an equivalent number of bends is determined for the bending sequence of the most exposed rope section, the sheave diameter and groove geometries, resulting in the calculation of a safety factor. The calculation is based on a number of trips until the discard age (N = 200,000 trips/year for 3 years), so allowing the discard age to be recognized between two inspection intervals in more highly frequented lift installations. This method draws on the findings described by Fey2000 and was described in detail for the fi rst time in Schi2002.

The definitions laid out in EN 81-1, Annex N result in a rope drive whose service life is limited and in many cases, for instance in the mid and high-rise sector, is considered low. It should be borne in mind here that the intention of this computation method is predominantly to satisfy safety objectives.

However, the world of lifts and elevators is highly diversified. In complete contrast to the highly frequented installations discussed above, in the field of residential lifts or when dealing with installations known to be little frequented, the definition provided by EN 81-1, Annex N is saidto be too “lavish”. When dealing with this type of lift, the obvious solution is to design the rope drive in such a way that the D/d ratio and safety factors are reduced in order to save traction-related costs and also permit optimization of the installation space. However, as might be expected, smaller D/d ratios and rope safety factors result in a reduced rope service life.

For the DRAKO steel wire rope type 250T d = 6 mm, d = 6,5 mm and d = 8 mm, the number of trips to discard is calculated for practically any optional sheave-torope diameter ratios, safety factors independently of the EN 81-1 calculation and differently shaped sheave grooves. This applies here for a case in which the rope section exposed to the highest levels of stress runs over the traction sheave andtwo deflecting pulleys. Both identical diameters in the traction and deflecting pulleys DTraction sheave = DDeflecting pulley and different diameters DTraction sheave ≠ DDeflecting pulley are analysed.

For the traction sheaves, V-grooves from γ = 35° to γ = 60° and U-grooves with undercut angles φ = 75° to φ = 105° are used. Reduction factors in accordance with EN 81-1, Annex N are applied. Unlisted reduction factors are determined by extrapolation.

EN 81-1 is based on a minimum number of trips to discard age, which in some casescannot be achieved with the above sheave-to-rope diameter ratios. The aim is now to demonstrate the number of trips which may be expected until discard age with the small D/d ratios for different rope safety factors Sf, differently shaped grooves and different sheave combinations.

As the parameters we are dealing with here are unconventional in the elevator construction sector, calculation errs on the side of safety, for instance by taking into consideration increases in rope force due to uneven rope tension, friction, acceleration and so on. Higher fatigue bending performance in DRAKO ropes compared to the total population according to Feyrer are taken “cautiously” into consideration.