Issue 5/2005
09/02/05
Rope development for elevators
Dr.-Ing. Ernst Wolf, Dr.-Ing. Andreas Franz
Today, more than ever before, buyers around the world expect elevator ropes to offer long service lives while at the same time being available at competitive prices. The Gustav Wolf Seil- und Drahtwerke GmbH & Co. KG, Gütersloh, has been a reliable vendor to its customers for many years and is untiring in its efforts to develop new, cost-optimized ropes.
Category: Issue 5/2005
Posted by: Editor
Gustav Wolf produces steel wire and wire ropes today at five sites in Germany and abroad. Elevator ropes account for no insignificant part of the firm’s manufacturing output. A variety of equipment is available for on-site testing of wire and ropes. Included here are bending endurance testing units with up to six test pulleys. It was against this background that the service lives of various rope designs were examined, this to the customers’ benefit by identifying cost-favorable alternatives to previous designs.
Elevator ropes – State of the art
Normally used in elevator engineering are steel wire ropes in an 8x19 Seale or 8x19 Warrington design, with fiber cores in each case. The selection of the rope will depend on the use conditions. Variation options are presented by differing rated strengths for the outside wires (1180 N/mm², 1370 N/mm², 1570 N/mm² or 1770 N/mm²). To handle unusual circumstances one may also use special ropes with nine strands and an allsteel core or double parallel-lay rope.
One may differentiate between two fundamentally different situations in the use of elevator ropes: The ropes are either used as replacements, i.e. they are installed in preexisting lifts, or they are used for new lifts. In the latter case the engineering for the lift and its ropes can be attuned precisely one to another, thus optimizing the system as a whole. In the event of replacements, by contrast, the rope tensile force, referenced to diameter, is a given. It is possible to implement only minor variations while staying within the fixed constraints.
Compacting elevator ropes – Will this work in practice?
Being aware of the fact that compacted ropes are used in crane engineering, it seemed plausible to examine their suitability for elevator construction.
It is generally known that both complete ropes, or the stands alone, can be compacted by applying any of a number of processes such a drawing, hammering or rolling. During such processes the shape of the wires is changed but the metallic cross section remains the same. The rope or the strands are compacted.
Outside wires with larger diameters are used to avoid the rope falling below the required diameter after compaction. One further result is that the compacted rope exhibits a greater metallic cross section than a comparable, non-compacted rope. Moreover, the pressures exerted during compaction will smooth the surfaces of the individual strands.
Objective of the investigations
As regards ropes with fiber cores, the intention was to determine whether compacting the strands would in and of itself extend the rope’s operational life. Further aspects considered in testing were stretch properties and the influence of the greater metallic cross section on service life. Moreover, it was to be determined whether the strands, as a consequence of compaction, are more resistant to transverse pressures.
Selection of the test ropes
Three types of ropes were selected for examination. Tested in addition to the standard 8x19 Seale rope with fiber core (8x19S-NFC) were an identical design with a steel core (8x19S-IWRC) and a compacted rope with fiber core (8xK19S-NFC). The exact rope diameters are listed in Figure 1.
The limitation to standard designs was deliberate so as not to lose sight of the manufacturing costs.
The limitation to standard designs was deliberate so as not to lose sight of the manufacturing costs.
Description of the tests conducted
Endurance bending tests
Endurance bending testing was conducted following the test methods described by the Institute of Mechanical Handling and Logistics (IFT) at the University of Stuttgart. The bending segments were defined as per the rules for testing promulgated by the OIPEEC (Organisation Internationale Pour L’etude De L’endurance Des Cables). All test ropes were loaded at identical tension in each testing horizon. Testing was effected at three different test pulleys (round groove, undercut groove, and vee groove) at a D/d ratio of 25, with five different rope tension levels (Figure 2). The ropes were installed with the factory lubrication and were not re-lubricated.
Rope diameter
Changes in rope diameter were tracked during bending testing. The diameter was measured each time a certain number of alternations, specified in advance, had occurred.
Rope stretch
Stretch testing was conducted at between 7.3 % and 8.3 % of minimum breaking load. Thus the stresses occurring in the elevator due to the application and relief of loading were simulated. The first, second and tenth loads were recorded.
Test results
Number of bends to break, in comparison with earlier trials
A total of 47 endurance bending trials was conducted. Compared in Figures 3 to 5 are the test results with values calcu-lated as per Feyrer’s formula [1]. Calculation was made for the mean number of bends to break Nm.
Figures 3a and 3b show the measurement results for the round groove. Figure 4 depicts the results for the vee groove while Figure 5 shows them for the undercut groove.
As was expected, the test results for the ropes examined were significantly higher than the number of bends to break calculated after Feyrer. The reason is that Feyrer’s formula is based on a multitude of tests and the test population also included ropes with very low values for the number of bends to break.
In the undercut groove the loading level of 280 N/mm² appears to already lie beyond the Donandt point.
Number of bends to break as a factor of pulley parameters
The non-compacted rope with fiber core (8x19S-NFC) exhibits considerably poorer results when used in conjunction with the undercut groove and the vee groove than is the case with the round groove (Figure 6). The service life in the vee groove and the undercut groove falls to 1/4 or even 1/6 of the service life with the round groove. Woernle found similar results at relatively high tensions [1].
The rope with the steel core (8x19S-IWRC) also shows the best results with the round groove (Figure 7). The absolute number of alternating bends – with the round groove and for a load level of 117 N/mm² – is N = 380,000, and that is considerably greater than the value for the fiber core rope (approx. N = 200,000). With the vee and undercut grooves it also falls to one sixth of the value found for the round groove.
The compacted rope with fiber core (8K19S-NFC), running in the round groove, achieved about N = 280,000 at loading of 117 N/mm² and thus also attained greater service life than the 8x19S-NFC rope (Figure 8). There is also a great reduction in service life on other groove profiles.
The decline in the service live with vee and undercut grooves, when compared with the round groove, has already been described by Feyrer [2] and Holeschak [3]. Feyrer has provided detailed correction factors.
Number of bends to break as a factor of rope design
Figure 9 compares the number of flexures to break, in the round groove, for all the rope designs tested. At identical tensile loading the rope with the steel core (8x19S-IWRC), and that means the one with the greatest metallic cross section, attained the highest number of bends. This fact has already been taken into account in the elevator industry, evidenced by the increasing use of steel-core ropes. The compacted rope with fiber core (8xK19S-NFC), with its 12 % greater metallic cross section when compared with the non-compacted rope (8x19S-NFC), achieves something like a 30 % improvement in service life.
Cross section development and transverse pressure stability
If a rope runs over a pulley regularly, then wear zones will arise at the contact surface. These differ for ropes with steel cores and those with fiber cores (Figure 10). Thanks to its stability when exposed to transverse pressure, the steel-core rope, when compared with the fiber-core version, will not be deformed as severely when passing over the pulley. The fiber-core ropes experience a more or less pear-shaped deformation when running in vee grooves and this will trigger secondary bending stresses.
The descent of the ropes into the vee groove was recorded over the service life (Figure 11).
Thiemann [4] notes that the ropes, when running through the vee groove, are subjected to high transverse pressures and thus are prone to deformation. Steel-core ropes are, according to Thiemann, not sensitive to this. The trials confirm this hypothesis. The 8x19S-IWRC rope exhibited the expected degree of stability even after 2,000 flexure cycles.
Fiber-core ropes (8x19S-NFC, 8xK19S-NFC) by comparison slip even deeper into the pulley after as few as 10,000 or 20,000 flexural cycles.
Stretch properties
The rope with the steel core (8x19S-IWRC) exhibits the least stretch of all three rope designs (Figure 12), at identical tensile load. The non-compacted rope with fiber core (8x19S-NFC) exhibits the most stretch and thus the smallest coefficient of elasticity (Young’s modulus). In comparison with non-compacted rope, the compacted rope with fiber core (8xK19S-NFC) showed a distinctly higher coefficient of elasticity.
This means that when installing replacement ropes in elevators, i.e. when substituting compacted ropes or steel-core ropes for non-compacted fiber-core ropes, the rope shortening factor is less.
Summary
Ropes with fiber cores are widely used in elevator engineering, all around the world. Compacting these ropes will result in an improvement in the service lives. Ropes with steel cores, at identical tensile loading, offer an even longer service life than compacted ropes with fiber cores. This is a result of the greater metallic cross section. A further advantage is a reduction in rope stretch and this will cut costs in mass production.
Resistance to transverse pressures is not improved significantly by compacting fiber-core ropes. By contrast, steel-core ropes exhibit a markedly greater stability when subjected to transverse pressures. It was found that compacted ropes were less subject to stretch than non-compacted ropes. Thus there are savings potentials to be found in the use of compacted ropes.
It can be concluded from these trials that compacted ropes represent a potential alternative for use in elevator engineering. They show properties in all areas which are similar to steel-core ropes. Field testing will be required to verify and confirm the results of laboratory testing.
References:
[1] Feyrer, Klaus. Drahtseile – Bemessung, Betrieb, Sicherheit. Springer Verlag, Berlin. 2nd edition, revised and expanded, 2000.
[2] Feyrer, Klaus. Laufende Drahtseile, Benennung und Überwachung. Renningen – Malmsheim: Expert Verlag, 2nd edition, completely revised, 1998.
[3] Holeschak, Wolfgang. Die Lebensdauer von Aufzugseilen und Treibscheiben im Drahtseil. Dissertation, Stuttgart University of Applied Sciences, Institute of Mechanical Handling, 1987.
[4] Thiemann, Hans. Aufzüge – Betrieb, Wartung und Revision. Verlag Technik, Berlin. 6th edition, revised, 1982.
About the authors
Dr. Ernst Wolf is CEO at Gustav Wolf Seil- und Drahtwerke GmbH & Co. KG.
Dr. Andreas Franz has been employed by Gustav Wolf Seil- und Drahtwerke GmbH & Co. KG since 1998 and as Technical Director is responsible for the steel wire rope
division. He is a member of the Technical Commissions of the DSV and the EWRIS.
5/2005
