Lift Report

Research project no. 13 606 N “Lifetime of wire ropes in traction lifts with a combination of different groove profiles” was funded as part of the Promotion of Industrial Collective Research Program by the German Federal Ministry of Economics and Technology BMWi through the Confederation of Industrial Research Associations “Otto von Guericke” e.V. (AiF).

The service life and damage mechanisms of ropes running over a single sheave with semi-circular grooves without transmission of traction have already been intensively researched by Feyrer [1]. However, to date only little attention has been paid to the study of additional elevatorspecific damage factors. The dimensioning of elevator ropes taking into consideration specific loads is based largely on a study by Holeschak [2] dating back to 1987. To date, the correction factors fN3 calculated by Holeschak using statistical evaluations performed on actual lifts – which are based exclusively on ropes with fibre cores – have been used in order to take into consideration the more extensive damage sustained by ropes in traction sheave elevators with traction grooves, so reducing the service life of ropes in semi-circular grooves calculated according to Feyrer.

In addition, the major advances made in the development of elevator construction and the consequent trend towards compact gearless drive systems, modern lightweight elevator cars and the increasing use of rope constructions with steel core, all demand a review of the existing calculation rules.

To permit a fundamental analysis of different influencing factors and damage parameters – also in respect of the changed system parameters – a test program was developed involving different test series for detailed experimental analysis as part of the proposed research project.

The correction factors derived by Holeschak [2] are based on elevator installations equipped largely with the fibre core ropes which predominated at that time. Given that the requirements imposed on elevator ropes changed hand in hand with the technical advancement of elevator installations, with increased calls for lower elongation and greater nominal breaking force, over the past 15 years steel core ropes with substantially more complex designs have been used with increasing frequency. The research project consequently analyses not only typical fibre core constructions but also ropes with steel core, see Table 1, in order to permit a comparison of the different constructions in respect of their service life.

In elevator construction, special shaped grooves are used in single-wrap traction sheaves in order to reinforce traction. These grooves are shaped as undercut semi-circular U-grooves (right) or Vgrooves (centre), see Fig. 1. This results in increased traction capability but also – compared to normal semi-circular grooves – in substantially higher compressive stress on the rope. The grooves in deflection sheaves, by contrast, are exclusively semi-circular in shape, whereby an oversized groove radius r is customarily produced in order to prevent jamming of the rope.

As a rope runs over a traction sheave with shaped groove and over the deflection sheave with semi-circular groove, it is ovalized in different directions. However, to date no analysis had been performed of the extent to which this effect, illustrated by the diagram in Fig. 2, impacts on the service life of the rope.

The different masses acting on the two sides of the traction sheaves under the elevator car weight with payload and the counterweight result in different rope tensile forces on the two different sides, Fig. 3. Added to this are dynamic forces occurring during acceleration and deceleration processes. These traction forces have to be transmitted through friction grip between the traction sheave and the rope, resulting inevitably in slip.

Molkow [3] distinguishes here between three types of slip: Elongation slip, running radius slip and gliding slip. As a result of differing rope forces S1 and S2 on both sides of the traction sheave, the elastic elongation of the rope changes between the run-on and run-off point, creating elongation slip. The different penetration depth of the rope in the groove brings about running radius slip. Gliding slip is caused during acceleration and braking processes due to the inertia of masses where these cannot be fully compensated by rope elongation.

To allow the different damage factors to be quantified, different test series were developed. The test parameters were limited in accordance with the stipulations of the EN 81-1 [4] standard, and consequently correspond to the parameters occurring in real elevator applications. The tests were executed with a traction sheave diameter to rope diameter ratio of D/d = 40 and with a static maximum load on the test rope corresponding to a minimum safety factor of ν = 12.

The influence of different groove shapes and changing rope deformations on the service life of ropes was examined without traction and with S1/S2 = 1 on a continuous bending machine available at the IFT. As it is not possible to execute realistic continuous slip-afflicted fatigue bending tests using simple rope bending machines, a special large-scale test rig was designed and built within the framework of the research project [5]. This test rig is used for studying the additional elevatorspecific parameters traction and slip, allowing reproducible dynamic fatigue bending tests to be performed for the first time under different rope force conditions S1/S2 ≠ 1.

The data base for Feyrer’s service life formula [6] derives from tests with single sheaves and round grooves. To categorize the test ropes into the Feyrer data base, tests are performed up until rope breakage on single sheaves with round groove with D/d = 25 and a relative rope force of S/d² = 117 N/mm². In accordance with DIN 15 020 [7], an oversize of 6 % was selected for the round groove, corresponding to a ratio of r/d = 0.53.

With additional oversizes of 3 % (r/d = 0.515) and 1 % (r/d = 0.505), the impact of smaller groove radii on the achievable number of bending cycles was examined. Based on the small amount of test data gained within the framework of very extensive testing periods, it was not possible to discern any significant influence on the determined number of bending cycles to break. At the suggestion of the working group monitoring the project, tests were performed with an even greater groove oversize of 20 % (r/d = 0.6), which is beyond that customary in practice. Ropes with a fibre core returned almost the same number of bending cycles to break here as was the case for round groove oversizes of r/d = 0.505 to 0.53, while the number of bending cycles to break in steel core ropes fell in this case to the factor fN3(SES) = 0.77. This reduction in service life approximately corresponds to the known value for a round groove oversize of 20 % according to Feyrer of fN3(SES) = 0.66 [1].

The shaped groove tests carried out with a relatively low rope tensile force of one twelfth of the minimum breaking force S = Fmin/12 and a ratio of D/d = 40 resulted in extremely long test periods. Consequently the tests were not continued up until rope breakage but only until discard age.

In order to determine the number of bends to discard ZAm, the discard criteria used are of decisive importance. Feyrer [8] defines the number of bends to discard ZAm as a distance from the number of bends to break Zm. The number of bends to discard ZAm for a reference length of 30 x d (BA30)/6 x d (BA6) originates from the evaluation of a large number of fatigue bending tests. In contrast to the number of broken wires to discard in accordance with DIN 15 020 [7], Feyrer does not differentiate between Seale and Warrington constructions, but instead distinguishes between ropes with different core. According to Feyrer, a considerably higher number of broken wires to discard are possible in ropes with steel core than with fibre core.

In practice, elevator ropes are assessed on the basis of discard criteria defined in DIN 15 020 [7] or ISO 4344 [9]. Alongside the number of externally visible broken wires, additional factors such as diameter reduction are also used as discard criteria. The criterion “rope discard at a 10 % diameter reduction relative to nominal rope diameter d” defined in DIN 15 020 set at an even more severe level of 6 % by ISO 4344. In particular in the case of thin ropes (d < 8 mm), the criterion rope diameter reduction is frequently taken as a basis within the framework of individual approvals in practical application. Consequently, this discard criterion was also taken into consideration in this research project.

In Table 2, various discard criteria are listed which were applied in the evaluation of fatigue bending tests with shaped grooves and slip-afflicted fatigue bending tests.

During fatigue bending tests with V and U grooves, very high numbers of bends to discard were frequently achieved. Due to the extremely long test duration involved in some cases with over 3.5 million bending cycles and the associated test running times of over 3 months, it was only possible in some cases to determine the discard criterion BA30 by extrapolation. In 56 test results with shaped grooves, there were only 4 outliers which fell below the correction factors valid to date for the reduction of service life in shaped grooves according to Feyrer [1] and EN 81-1 [4]. In comparison to ropes with fibre core, the steel core ropes demonstrated a tendency towards smaller correction factors fN3, in other words greater reduction of the service life. The rope wear discovered in the discarded ropes is due exclusively to radial slip on run-on and run-off of the rope. In contrast to real elevator applications, during tests performed on bending  machines no transmission of traction forces takes place and consequently no relative movement in the tangential direction between the rope and sheave. The repeated stress on the rope at the same points of contact between the outer arcs of the wire and the uniform radial slip stress result in marked wear with pit formation at the groove flanks. The lubrication is displaced and cannot be guided into the contact points by the relative movement between the sheave and rope which otherwise occurs during tangential slip.

When a section of rope runs over a shaped groove and is subsequently bent over a semi-circular groove, both bending cycle numbers according to Feyrer [1] can be collated using the damage accumulation hypothesis put forward by Palmgren and Miner as independent loads. Contrary to expectation, it was not possible to prove additional damage to the rope due to changing rope ovalization.

The test rig developed specifically for assessing the impact of rope slip on the service life of elevator ropes corresponds to the principle of an elevator with one traction and one deflection sheave, as illustrated in Fig. 4. The elevator car and counterweight masses were applied using two frames with variable weight plates G1 and G2 moving along vertical guide rails. Elevator operation was simulated by alternate lifting and lowering of the test weights. The static rope tensile forces S1 and S2 can be set with the aid of the weight plates in such a way that the tests can be run with the same rope safety level ν = 12 independently of the rope constructions and diameters.

While the suspension of traction sheave elevators always involves more than one elevator rope, only one suspension rope is used at both test points of the test rig. In this way, it is possible to keep the weights required to generate the required rope tensile forces relatively small. In addition, this arrangement avoided the test results being influenced by positive slip as a result of different running radii of the ropes in the grooves. The wrap angle of the sheave was 155°.

Depending on the travel dynamic, each section of the test rope is subject to a specific traction force when it rolls over the traction sheave. This results from the different rope forces occurring in the rope sections as they run on and off the two sides of the traction sheave, whereby the individual stress of a rope position x is independent of the direction of movement. Fig. 5 illustrates the determined drive force value |FT| of each rope position for different static rope force ratios S1/S2.

A rope force ratio S1/ S2 = 1 results in symmetrical stress on the rope in the region of the acceleration and deceleration phases of weights G1 and G2. Conversely, when the rope force ratio is uneven, the rope zone between 6000 mm and 8000 mm is exposed to the greatest degree of stress. During acceleration which takes place on upward travel of the heavier weight G1 and the deceleration which takes place on downward travel of G1, this section of the rope runs over a traction sheave.

The influence of the individual traction force value |FT| on the rope service life can be examined by comparison with an experimentally determined individual case of rope damage, for instance using the broken wires occurring in each rope section.

Using a computer-aided wire breakage recording system, it was possible to generate a detailed picture of wire breakage development. This was done by visually inspecting the entire rope length at regular intervals and electronically storing the occurring broken wires. The relevant wire break position was determined by means of a rope transducer on the basis of the position of the weights, and saved in a file. The benefit of this method is that different discard criteria can be applied when evaluating the wire break values.

Fig. 6 shows an example of wire break development over a reference length of 30d (B30) and 6d (B6) for rope B observed during test rig trials.

The test was carried out with a static rope tensile force of S1/S2 = 1,39 and a traction sheave with 98° undercut. Wire break development at the relevant rope positions x clearly correlates with the traction force value |FT| determined in Fig. 5.

Fig. 6 can be used to determine the individual number of bending cycles ZAi for each position x of the rope up to the point at which the discard age is reached. Fig. 7 illustrates six different tests with the rope B. Each point in the diagram represents a single test rope section with a length of l = 30 x d. The achieved number of bending cycles to discard for the rope section is shown in this graph relative to the dynamic rope tensile force ratio S1*/S2* occurring in the relevant rope section while rolling over the traction sheave.

The 35° V-groove (green) shows a relatively small number of bend cycles to discard, irrespective of the rope tensile force ratio. With this V shape, the rope is predominantly damaged by the high specific compression in the V-groove, while sliprelated wear contributes hardly at all to the rope damage by comparison. With a very high rope force ratio S1*/S2* > 2, the number of bends to discard even increases slightly. This is due to the diminishing mean compressive stress which occurs with the lower adjusted weight G2 where very high rope force ratios are selected.

In the tests with 105° U-grooves (blue) and 98° U-grooves (red), the rope stress is lower than is the case with the 35° Vgroove, while instead the dynamic rope force ratio S1*/S2* exerts a significantly greater impact on the achieved number of bending cycles to discard. The rope slip and the associated rope wear have a greater impact on service life in comparison to the 35° V-groove tests. While up to 700,000 bending cycles to discard can be achieved in the 98° U-groove with a rope force ratio of S1*/S2* = 1, in the rope sections with rope force ratios of S1*/S2* > 1.45 bending cycles to discard reach only ZAi < 300,000. With this result, it was possible for the first time to experimentally quantify the dependency of rope service life on the rope force ratio.

The tendency already observed during tests with shaped grooves without traction force transmission, namely that the service life of ropes with steel core is reduced compared to ropes with fibre core under the same rope safety conditions, was even more clearly evident during the slip-afflicted fatigue bending tests.

Fig. 8 shows the correction factors fN3 determined for each rope section for ropes A (blue) and B (green) in the 35° Vgroove.

If, as customary in practical application, the number of broken wires to discard independently of fibre or wire core is used as a discard criterion in accordance with DIN 15 020, then the correction factors determined for rope B actually come in below the previously valid value of fN3 = 0.054 for 35° V-grooves.

For rope A with fibre core, when applying DIN 15 020, a far higher service life results.

Also in the case of the tests illustrated by Fig. 9 with a 98° V-groove, rope B with steel core (green) demonstrates a significantly longer life compared to rope A with fibre core (blue).

With a rope force ratio of S1*/S2* = 1, both ropes are above the correction factor according to Holeschak of fN3 = 0.12 for a 98° V-groove, with fN3 > 0.45 (rope A blue) and fN3 > 0.25 (rope B green). While rope A does not exceed the existing correction factor fN3 = 0.12 for a 98° V-groove even in the higher rope force ratio range, rope B achieves values below this limit already from a rope force ratio of S1*/S2* = 1.12 fN3.

Under the selected test conditions, other strand constructions with steel core also demonstrate a reduced service life: For instance the Warrington rope D also falls substantially short of the existing correction values. Additional series of tests with different rope constructions and rope diameters should be performed in order to quantify and provide statistical confirmation of these findings.

Using a specially developed test rig at the IFT, fatigue bending tests were carried out with dynamically changing rope force ratios S1*/S2* ≠ 1. This permitted the rope stresses acting in traction sheave elevators to be analysed for the first time realistically in reproducible tests. In order to examine the specific damage mechanisms, the results of slip-afflicted tests were compared to the results achieved using fatigue bending machines without traction force transmission. Analysis of the test results has permitted the following conclusions to be drawn:

- In tests with traction force transmission, the ropes demonstrate a substantially reduced service life as against the comparative results obtained in normal fatigue bending machines. With an undercut 98° V-groove, a rope force ratio of S1*/S2* = 1.2 and α = 155° traction sheave wrap angle, for instance, the rope service life diminishes to below 50 % of the life achieved in test rigs without traction transmission.

- Alongside the rope damage brought about by the traction groove shape, the rope force ratio S1/S2 also exerts a substantial impact on the achievable number of bend cycles to discard. In traction rope drives with groove shapes which give rise to relatively low compression stress (e. g. round grooves with small undercut angles), the rope force ratio is the factor which deter- mines service life. The new type of testing facilities described here should be used in the future to ensure that the rope force ratio is included in the calculation factors when assessing rope service life for traction sheave elevators.

Realistic results which reflect the actual stress acting on ropes in traction sheave elevators can only be determined using tests with traction force transmission. When using lightweight elevator cars for energy-efficient elevators, the rope force ratio and the damage factor rope slip will also gain in importance as calculation factors.

- Contrary to expectation, it was not possible to discover any significant influence exerted by changing rope ovalization on the service life of ropes when running the rope over a combination of shaped and round grooves.

- As a maximum static load in accordance with the rope safety factor ν = 12 was always selected irrespective of the groove shape and the wire construction used, a far higher stress level resulted for the ropes with steel core in the tests. Due to the higher minimum breaking force of the steel core rope, the relevant bending tests were performed with higher rope tensile forces and consequently with higher specific groove pressure levels compared to the fibre core ropes. This test design resulted in a diminished service life of ropes with steel core: in some cases the steel core ropes only achieved a quarter of the number of trips compared to fibre core ropes. In the tests performed with additional traction stress, the correction factors fN3 of the steel core ropes were actually regularly lower than the factors currently used in accordance with EN 81-1 Annex N.

- However, this test design coincides with current practice in the industry. Even in the currently valid design rules specified in EN 81-1 Annex N, no distinction is made between fibre and steel core ropes. The minimum admissible rope safety is determined independently of the used rope construction, although the used factors are based on an analysis of elevator installations using fibre core ropes. The executed tests with steel and fibre cores refute the validity of this generalized approach. What the test results actually show is that the effective rope tensile force and consequently the specific compression must be taken into consideration in any prognosis of rope service life in traction sheave elevators.

- The trend towards ever smaller rope diameters and the use of extremely high-strength wires give rise to the need for further-reaching tests in respect of correlations between specific compression in shaped grooves and rope service life. Future analysis of the service life of ropes in traction systems should consequently be extended to include a diameter below 8 mm.

[1] Feyrer, K.: Drahtseile. Bemessung, Betrieb, Sicherheit. (Wire ropes. Tension, Endurance, Reliability) SpringerVerlag 2000

[2] Holeschak, W.: Die Lebensdauer von Aufzugsseilen und Treibscheiben im praktischen Betrieb (Service life of elevator ropes and traction sheaves in practical operation). University of Stuttgart, Dr.-Ing. Dissertation 1987

[3] Molkow, M.: Die Treibfähigkeit von gehärteten Treibscheiben mit Keilrillen (Traction capacity of drive sheaves with hardened V-grooves). University of Stuttgart, Dr.-Ing. Dissertation 1982

[4] DIN EN 81-1: Safety rules for the construction and installation of lifts – Part 1: Electric lifts, February 1999

[5] Berner, O.: Prüfstand für praxisnahe schlupfbehaftete Dauerbiegeversuche. Aufzüge der Zu kunft – Visionen, Grenzen und Betrieb. (Test rig for practical slip-affl icted fatigue bending tests. Lifts of the future – visions, limits, operation) VDI report no.:1937 (2006)

[6] Feyrer, K.: Bruchbiegewechselzahl von Parallel schlagseilen (Number of bending cycles to break in parallel lay ropes), Draht 35 (1984) 11, p. 566–570

[7] DIN 15 020: Principles relating to rope drives, part 1: Calculation and construction, 1974-02, part 2: Supervision during operation, 1974-04

[8] Feyrer, K.: Ablegedrahtbruchzahl von Parallel schlagseilen (Number of bending cycles to break in parallel lay ropes), Draht 35 (1984) 12, p. 611-615

[9] ISO 4344: Steel wire ropes for lifts – Minimum requirements, February 2004