Lift Report

At interlift 2007, Pfeifer DRAKO Drahtseilwerk GmbH presented steel wire ropes from the STX series with nominal diameters of d = 4 mm and d = 5 mm that have been issued with product type test certificates by TÜV Süd, Germany. This certification applies to elevator systems using traction sheaves with V-grooves of γ = 40° to γ = 50° and guide sheaves with round grooves at a diameter ratio D/d = 40. In a further development, however, and for the above-mentioned reasons of cost, space requirement and useable cabin fl oor area, the sheave diameter ratios were reduced to D/d = 30. The following article is a report about extensive development work on STX ropes with rope diameter d = 4 mm, ropes deployed with V-grooves (D/d = 40) and guide sheaves (D/d = 30). Our development work always reflects “system thinking” – which means de facto that the rope manufacturer no longer delivers just the rope but additionally provides the elevator manufacturer and/or the distributor with a components and methods service in order to make these advanced and future-ready thin steel wire ropes a safer and reliable system element.

As they run over the sheaves, steel wire ropes are exposed to a range of strains such as primary and secondary tensile stresses, bending stresses, pressure and torsional stresses – as well as wear and tear due to relative motions between rope and sheave and within the wires themselves. It is precisely because of this kind of attrition that the service life of the steel wire ropes cannot be computed direct even if there is adequate knowledge of the wire stresses; instead, that service life has to be ascertained via fatigue bending tests. The bending fatigue test rigs deployed for this purpose, the test procedures and the statistical evaluation are set out in detail in [1]. It may be noted here that – in addition to bending length, nominal wire strength and rope diameter – the diameter ratio of sheave to rope as well as the rope tensile force have a significant effect on rope service life. These parameters therefore play a key role in “Feyrer’s rope service life equation” with which the results of fatigue bending tests, i. e. the ascertained number of bending cycles up to the point of discard, can be described. In [1] it is standardized and/or frequently deployed rope constructions that tested and evaluated. Furthermore, [1] includes the presentation of a method by which the deviation of the test results can be taken into account. However, in the case of a special rope construction such as the Pfeifer DRAKO STX 4, the existing test results should not be accepted too readily. On the contrary, the corresponding basic principles for the service life equation have to be re-created – this by means of adjusted fatigue bending tests with realistic test parameters such as the diameter ratio D/d and groove form etc. At this juncture, it may be noted with some satisfaction that, from the service life angle, Steel Wire Rope STX 4 delivers higher values and, for deviation, lower values than the rope of a similar construction presented in [1].

STX 4 rope, the technical specifications of which are summarized in Table 1, will come to be the preferred choice in those elevator configurations where the highest- stressed rope section runs over the traction sheave with V-groove and two guide sheaves with round groove. By way of an example, we show in the following what service lives can be expected if a traction sheave with a V-groove and a Vangle of γ = 40° to 50° at D/d = 40 (sheave diameter D = 160 mm) and two deflection sheaves with round groove and D/d = 30 (sheave diameter D = 120 mm) are deployed. Fatigue bending tests were carried out, using the afore-mentioned parameters. Illustration 1 shows the bending fatigue test rigs in the Pfeifer DRAKO Technical Competence Center in Mülheim a. d. Ruhr, Germany. These bending fatigue test rigs are practically identical with the machines described in [1] and so, for this reason, we will dispense here with a detailed description and refer the reader to the literature. All that needs to be borne in mind here is that, during the fatigue bending tests, wire breaks in the bending zones were counted for reference lengths of l = 6 x nominal rope diameter d and l = 30 x nominal rope diameter d and that the rope diameter wasmeasured. The discard criteria for the STX 4 steel wire ropes to be applied are 12 wire breaks for l = 30 x d, 6 wire breaks for l = 6 x d and a diameter decrease by 6 % (which is intensified compared to DIN 15 020 with its 10 % against nominal rope diameter).

The test parameters for the fatigue bending tests are set in such a way that test duration can be kept within reasonable limits and that the information content in terms of subsequent elevator applications does not get submerged. For the tests with the V-grooves, we chose safety factors of S_f = 10. These safety factors do not actually occur in the subsequent application – i. e. the results of the fatigue bending tests have to be extrapolated to a certain extent. The same goes for the tests with the round grooves which were carried out with the relatively low safety factors S_f < 8. A comparison with the number of bending cycles as ascertained by Feyrer – for similar rope constructions,with the rope safety factors tested and with the D/d ratios – shows that the STX 4 rope achieves a fourfold higher number of bending cycles up to the pointof discard. However, to stay on the safe side, smaller augmentation factors are used for the further calculation of the number of journeys expected up to the point of discard.

To calculate the number of journeys, the number of bending cycles up to the point of discard for the steel wire ropes STX 4 in the round groove and V-angle groove versions was taken, this assuming that the highest-stressed section of rope runs over the traction sheave with V-groove (D/d = 40) and over two guide sheaves with round groove (D/d = 30) with single bending – Figure 2.

According to the Palmgren-Miner rule, the number of journeys to be expected is

Planned, for example, is the deployment of 12 parallel ropes at weights of cabin F = 800 kg, load capacity Q = 630 kg and suspension cable HK = 30 kg. Rope safety is then S_f = 22.4. Rope safety requirements as per EN 81-1 have thus been met. To show which service lives can be expected, even if the number of ropes and, therefore, rope safety is reduced, different calculations were carried out up to S_f = 12. On the one hand, this should show which limit the system has and, on the other, which service life can be expected with the known passenger frequency rates. For rope safety S_f = 22.4, one can expect a number of journeys to the point of discard that lies over the basis of EN 81-1, Annex N with 600,000 journeys.

The steel wire rope plays the central role in any elevator or lift. As a bearer cable, it has done full justice to this task for over 100 years. Its active role in ply with the drive unit reflects just another aspect. Further aspects can be seen in Figure 3.

From the rope user’s point of view, the rope and additional components can solve different problems. In addition to rope assemblies already realized in practice – cutting, affixing non-detachable end terminations, providing accessories etc. – other auxiliary services may include, amongst many others …

New elevator concepts require a risk analysis which also covers proof of safe suspension . In the case of ropes outside the usual standards – such as DIN EN 12 385-5, ISO 4309 – proof of equivalent safety must be furnished. If a relevant certificate from a recognized Notified Body is available, then the elevator manufacturer need not run elaborate tests. In that respect, Pfeifer DRAKO has been providing elevator manufacturers with active support for many years now.

When it comes to the actual installation, other services are also available. On the one hand, the rope can be delivered on reusable modular reels; on the other hand, there now exists a whole range of installation and guide clips to make the handling of any rope package all the easier.

The end terminations as per DIN EN 13 411 used in the elevation construction industry have proven their worth for current rope diameters and are deployed in many systems. However, in terms of smaller rope diameters and, consequently, thecompact drive solutions now feasible, the current end terminations are too large. The installation space required to fasten the ropes makes imperative a widening of the ropes towards the connecting points. A linear rope guidance is not possible [and] the resulting deviation re duces the expected service life of the ropes. For this reason, the hardwood (for example) rope grips used block any compensation for length, Figure 5.

New and compact end terminations (Figure 6) permit a straight path for the ropes without any deflection. In addition, the termination system offers a highly simple and time-saving possibility of rope installation. Thanks to variable rope distances from one another in the end terminations, these can be guided parallel to one another in the entire elevator system. Adjustment screws permit the rope tensions to be set exactly. Moreover, as previously, it is also possible to shorten the rope after the first operational settings.

A large number of ropes is a big challenge when it comes to setting equal rope tension. There are any number of different accessories and tools available to do this. We show one particularly efficient measuring device in Figure 7. Here, with just the one adjustment procedure, all ropes can be set to the same tension. The previous method – which consisted of individual ropes being (re)measured and then (re)adjusted and subsequently tested – can be simplified to a great extent.

The calculations made for different elevator configurations lead almost inevitably to different numbers of ropes. A traction sheave developed on a modular basis allows a configuration as required and, also , means that a change of traction sheave can be carried out with little or no difficulty.

When up and running, ropes should be monitored at predefined intervals with a view to establishing the point of wire rope discard. As rope diameters get smaller, it is recommended that methods be used alternative to the previous methods of “counting the number of broken wires” or “measuring rope diameter”.

Other significant details – sometimes concealed beneath the basic requirements – include uniform rope tension within the ropes set out in parallel and the deviation of the rope properties within a given batch over a longer period of observation. Moreover, rope manufacturers must also be able to answer questions such as what influence does the deployment of plastic guide sheaves have on rope service life and, above all, on establishing the point of discard. Let us single out deviation by way of an example.

From one production batch of STX 4 rope measuring approx. 8,000 m, we took rope specimens at five different places and subjected them to fatigue bending tests. The tests were all conducted on the samefatigue test rig using constant test parameters. The number of bending cycles up to the point of discard so ascertained were then recorded on a probability graph – Illustration 8. It can be stated that the number of bending cycles up to the pointof discard can be described exceptionally well on the basis of logarithmic normal distribution. The deviation or the standard deviation – characterized by the steep course of the regression line – is with lgs = 0.038 relatively small and lies at the lower margin of the deviation given in [1] for several tests based on the same batch.

Growing global competition requires – and is facilitating – new system approaches. In addition to alternative means of suspension, new system concepts are also making their mark, systems that both complement and upgrade traditional steel wire rope. A reduction in the number of mechanical and logistical interface points for the elevator manufacturers also means product developments optimized for cost and time.