Issue 2/2009


03/02/09

Further development of proven suspension means in elevator construction

Steel – a material offering potential for enhanced performance and energy efficiency

Dr.-Ing. Ernst Wolf, Dr.-Ing. Andreas Franz
Energy efficiency, ecological efficiency, environmental compatibility – all issues which, as in countless other sectors, have become an established aspect of the elevator industry. While only a few years ago, increased speeds and higher elevators at the lowest possible prices were generally at the forefront of development objectives, today research tends to be driven by the achievement of greater economy and sparing use of resources. The design of the first machine roomless elevator in 1996 sparked a move towards ever smaller and more efficient drive systems which questioned existing concepts and consequently marked a departure into a number of new directions. Even counterweights have become surplus to requirements – a development which nobody would ever have envisaged only a few years ago.
Category: Issue 2/2009
Posted by: Editor

1. Outset situation

The design of any new elevator installation today increasingly focuses on the following criteria:
  • Efficient use of raw materials
  • Low energy consumption during manufacture and operation
  • Long service life of all components
  • Facility for recycling
  • Sustainability of production
Consideration of these criteria is linked to the ever more stringent demands placed on the means of suspension used in elevators. While a long service life has always been expected of elevator ropes, their influence on people and the environment during operation has now also become a priority issue. Low noise and the use of environmentally compatible lubricants are only two of a whole catalogue of requirements imposed on modern suspension means. Manufacturers address these challenges with a variety of different concepts. The steel rope still remains the most widespread means of suspension in use today.
The degree of energy required to manufacture steel as a raw material for producing steel ropes is relatively low. Also decisive is the fact that once produced, steel can be recycled with no loss of quality. It is here that the benefit of steel ropes is brought to bear in comparison to composite designs or suspension means made of other structural materials. The completely cyclical nature of its production loop makes steel is practically unique as a material. The ratio of scrap used in the overall steel production process has been growing steadily for years (Fig. 1), helping to save valuable energy and also to reduce CO2 emissions. To this extent, steel may be considered an environmentally friendly material whose potential is far from being exhausted. In the context of steel ropes used in elevators, there are two directions for possible optimization and further development:
  • Increasing the metallic cross-section of ropes
  • Increasing the strength of the individual wires
2. Increasing the metallic cross-section of ropes
In the field of elevator construction, generally speaking steel wire ropes with a fibre core in a 8x19 Seale or 8x19 Warrington strand construction are used. In step with the increasingly stringent demands imposed on ropes, ever more frequent use is being made of specialty products such as nine strand, full steel core and double parallel lay ropes.
Recognizing that compacted ropes are also used in crane engineering, Gustav Wolf launched a comprehensive research program as long as 2004/5 to test the suitability of this type of rope in elevator construction. The results were unveiled at the 2nd Stuttgart Rope Day in 2005.
It is a generally known fact that both complete ropes and also certain selected strands of a rope can be compacted using various processes such as drawing, hammering or rolling. Compaction always alters the shape of the wires while maintaining the same metallic cross-section. To avoid dropping below the specified rope diameter after compaction, outside wires with a larger diameter are used as a starting material. At the same time, a compacted rope demonstrates a higher metallic cross-section than a comparable non-compacted rope. In addition, the pressures exerted during compaction bring about a smoothing effect on the individual strand surfaces.
The research carried out by Gustav Wolf looked at the changed characteristics of compacted steel wire ropes by examining the answers to the following questions:
  • In ropes with fibre core, does compacting the outer strands only serve to increase service life?
  • How do the elongation properties change?
  • How does the higher metallic crosssection affect service life?
  • Are the strands more resistant to transverse pressure forces as a result of compaction?
Three rope types were selected to take part in the study. Alongside the standard 8x19 Seale rope with fibre core (8x19SNFC), the same construction but with a steel core (8x19S-IWRC), as well as a compacted rope with fibre core (8xK19SNFC) were subjected to testing. The precise rope parameters are shown in Fig. 2.
Fatigue bending tests were performed in accordance with the test method prescribed by the Institute of Mechanical Handling and Logistics (IFT) at the University of Stuttgart.
The bending zones were defined in accordance with the test rules laid down by OIPEEC (Organisation Internationale Pour l’Etude de l’Endurance des Câbles ). All the test ropes were subjected to the same rope tensile load for each of the test horizons. Tests were performed on three different test sheaves (round groove, undercut groove, V-groove) under five different tensile loads with a D/d ratio of 25. The ropes were set up with the lubrication supplied by the manufacturer and not regreased.
The results of the tests were used to determine:
  • Number of bending cycles till failure depending on sheave parameters
  • Number of bending cycles till failure as a factor of rope design
  • Cross section development and transverse pressure stability
  • Elongation properties
All the tests performed and their results were described in detail in the Liftreport 5/2005 [1] and Elevator World 3/2006 [2].
To summarize, the tests performed revealed the following findings: If ropes with fibre cores are compacted, an improvement of the rope service life is achieved. Ropes with steel cores demonstrate an even longer service life than compacted ropes with fibre core. This is a result of the higher metallic crosssection.
Transverse pressure stability is only marginally improved by compaction in fibre core ropes, while steel core ropes demonstrate substantially higher transverse pressure stability.
The study showed that compacted ropes demonstrate lower elongation compared to non-compacted ropes. Consequently their use entails a potential for savings in terms of operating costs.
The conclusion which can be drawn from the tests is that compacted ropes offer a potential alternative for use in elevator engineering. In almost all fields, their behavior is shown to be similar to steel core ropes.
With a view to substantiating and confirming the test results, a series of further-reaching tests was run in 2006/2007, the outcome of which are two new ropes. These were unveiled at the Interlift 2007 and met with an enthusiastic response from the specialist public. The special feature of both these new ropes is their compacted outer strands.
CompactTrac and PowerTrac – the result of targeted research and development
CompactTrac (Fig. 3) is an 8-strand rope with fi bre core and compacted outer strands. It offers a longer service life in comparison with popularly used conventional 8-strand fi bre core ropes. The “secret ” behind PowerTrac (Fig. 4) is the simultaneous use of compacted outer strands and a steel core. This combination ensures a marked increase in service life.
    
Fig. 5 provides a comparison of the achievable service life of CompactTrac, Power- Trac and conventional 8-strand fibre core rope on the basis of the number of bending cycles till failure.
The two new ropes are characterized in detail by improvements in the following areas:
  • Longer service life due to higher number of bending cycles till failure, and improved wear resistance due to the larger surface area of the outer strands.
  • The larger surface area of the outer strands leads to a better seat in the groove of the traction sheave resulting in a reduction of the concentrated (point) load on the elevator ropes which commonly occurs in convention- al elevator ropes. The surface pressure is more evenly distributed over the ropes. This results not only in minimization of wear but also substantially lower noise and vibrations.
  • Reduced elongation due to the higher metallic cross-section
The clearly improved elongation properties of the rope also bring a reduction in maintenance work. When different forces are acting on the car and counterweight side, slippage of the rope on the traction sheave is reduced by the higher e-modulus of the rope, so reducing friction and consequently also wear.
  • Smaller diameters are possible due to higher breaking loads
The associated material savings mean lower costs for new installations. With its CompactTrac and PowerTrac products, Gustav Wolf has developed a new generation of elevator cables characterized by a longer service life and simpler, less costly maintenance. Tests are currently being run to determine the effect of increasing the metallic cross section on the compaction of steel rope cores.
Heart and soul …
It is generally accepted that the quality and consequently service life of a wire rope depend crucially on its core. This vital importance is what gives the core its German name “Seele”, which translates as “soul”, and indeed any rope is only as good as its core. Its support function must be durable and effective if it is to absorb the high apex pressures applied by the surrounding strands and prevent premature strand contact. Given this vital function, looking to improve core strength would appear an obvious approach to increasing the service life of the rope.
In an age when elevator installations are required to operate over ever greater heights, there is a consensus that ropes with steel cores are taking over from natural fibre cores, even though fibre core wires certainly still have a valuable role to play in certain applications. Symptoms reported from the early days of the steel core such as loosening or basket formation of the outer strands are now distant history. Rope manufacturers today have now mastered the art of coordinating contact ratios between the steel core and the outer strands.
Nowadays, 8 and 9-strand ropes are used almost exclusively in elevator installations. This is because their contact surface in the groove is greater than is the case with 6-core ropes, which ensures lower Hertzian pressure. Particularly in the case of 8 and 9-strand ropes, a correctly dimensioned core takes on a special significance. The “strand camber”, which is greater compared to a 6-strand core, must be radially well supported without strand contact taking place.
Compacted rope cores
Elevator manufacturers continuously endeavour to push out the boundaries of what can be achieved, for instance by using ever diminishing traction sheave sizes . However, the resulting reduction in the D/d ratio and higher tractive forces result in higher surface area pressure both between the rope and the sheave and also inside the rope itself. In ropes with a steel core, this inner pressure is greater than is the case with fibre core ropes. However, if the surface of the wire is flattened, in other words the strands are compacted, this pressure is reduced.
The notching effect is appreciably reduced in strand wires with smoother services. A compacted steel core with its increased metallic cross-section also offers far greater dimensional stability, slowing down the process of diameter reduction and notching between the outer strands.
Comparative fatigue bending tests under equal load show that ropes with compacted steel cores demonstrate around 10% higher bending resistance than those without a compacted steel core.
Bending resistance can be further increased by ensuring sufficient and regular relubrication. Fig. 6 illustrates the increase in bending resistance brought about by regular lubrication during bending tests on a fibre core rope, a steel core rope and a combined or mixed core rope.
Fig. 6 clearly shows that the degree by which bending resistance increases as a result of relubrication is greatest in the case of steel core ropes, indicating that the benefits of a steel core rope are only brought to bear providing it is regularly lubricated.
On the other hand, despite the controversy of the debate surrounding the subject, it is an indisputable fact that fibre cores store lubricant and give it off to the rope over the course of its service life.
A mixed core rope ideally combines the benefits offered by pure fibre cores with those of metal cores, reducing not only friction between the wires themselves, but also between strand wires of the steel core and the wires of the outer strands. At the same time, the strands of the steel core benefit from soft support, and pressure between the steel core and the outer cores is reduced. If the strands of the core are additionally compacted as in the PAWO F3 IWRC(K) (Fig. 7) and PAWO F7 IWRC(K) (Fig. 8) ropes, the conditions for an optimum service life have been created.
    
3. Increasing the strength of the individual wires
Although the nominal strength of the elevator ropes is prescribed by DIN EN 12 385-5, an increase in the strength of certain wires is possible. The standard actually states: “The nominal strengths of core wires, filler and core wires must be defined by the manufacturer”. This gives the green light for manufacturer to use, for instance, inner wires with a higher strength rating.
If high-strength wires are also used in the top layer, these elevator ropes may only be used subject to a separate type examination certificate. Examples of this type of special rope are the PAWO 8x19W-IWRC 6.0 mm and 6.5 mm, which are Elevator, and the PAWO F3 6.0 mm, used by LM Liftmaterial in its JADE elevator.
Alongside the positive repercussions of an increase in strength, such as a higher breaking load, reduced diameter, possible reduction of the traction sheave diameter and savings in terms of material and energy, the less positive aspects must naturally also be taken into account. These include, for instance, the possibility of higher wear occurring with traction sheaves made of soft materials. Only hardened sheaves should consequently be used. However, lined sheaves are also familiar from the field of shaft hoisting plants, and the possibility of using plastic sheaves can also be considered, although some thought would have to be given to the problems inherent in recognizing the discard age of ropes running on plastic sheaves.
The production of high-strength wires calls for extensive indepth expertise.
Manufacture of high-strength wires
Steel rod manufacturers today offer a selection of well over 200 qualities. Rope wires are generally manufactured from unalloyed steel with a carbon content of around 0.4 to 0.9 % by means of cold forming. Steel rod complying to DIN EN 10 016 with a diameter of 5 to 10 cm is generally used as a starting material.
Selection of the steel and its method of processing to produce a wire are contributory factors in achieving the greatest possible degree of resistance to pressure coupled with optimum bending characteristics. The currently held consensus is that a fine-grade steel microstructure offers the optimum conditions. The grain size is influenced by introducing certain additives to the melt and in particular by the subsequent processing method used to produce the rope wire.
The strength and other properties of the wire are determined in the main by the carbon content of the steel, the degree of deformation occurring during the drawing process and the types of heat treatment (patenting) used. As the proportion of carbon and the degree of deformation increase, the wire gains in strength, while its elasticity, bending properties and torsional flexibility generally diminish.
Rope wires made of micro-alloyed steel qualities (e.g. alloyed with chrome or vanadium ) demonstrate good physical properties coupled with high strength. Steel with a 0.92 % carbon content and micro alloyed chrome currently provide the highest steel rod quality.
Differing opinions have been voiced in the past on the influence of increasing wire strength on fatigue bending properties and consequently on the service life of a rope. One finding by Woernle [3] stated that while the application of constant rope tractive force causes the rope life to remain constant for wire strengths ranging from 1280 to 1960 N/mm², an increase in wire strength coupled with a constant rope safety factor brings about a drop in service life. Müller [4] discovered that ropes manufactured from wires with nominal strengths of 1570 to 1960 N/mm² achieve approximately the same number of bending cycles till failure with application of identical tractive forces.
For the first time, Wolf [5] has examined a large number of ropes to determine the influence of wire strength on service life, and established the fatigue bending life of wires as a parameter influencing the service life of the rope. The results of the research established a correlation between the increase in rotating bending fatigue and the increase in the service life of wire ropes if other parameters such as a particularly large or small core mass are left out of account.
Fig. 9 illustrates a wire rotation bending machine of the type used for quality assurance purpose at Gustav Wolf. The tested wire is clamped between two collet chucks in a 180° arch. One of the two chucks is driven, the other moves in easyrunning coupled motion. The distance between the chucks can be adjusted as required, allowing the load and consequently the flexural stress to be changed. The speeds achieved under different levels of flexural stress are recorded. The measured values are used to produce a service life curve (Wöhler curve) (Fig. 10), which allows a statement to be made regarding the dynamic efficiency of the wire.
   
 
Continuous in-process tests on produced rope wires have resulted in optimization of the wire drawing process at Gustav Wolf. By implementing the tests described above and using the findings, today suitable high-strength ropes can to be produced in compliance with the specific requirements of each individual rope customer .
Ropes from high-strength wires
One product of the research work performed at Gustav Wolf is a 4.0 mm elevator rope with wire strength in excess of 2000 N/mm² and outstanding fatigue bending properties. This rope has been in successful use for a considerable period in the Kone Maxispace and Re-Generate elevator systems (Fig. 11).
Using a rope of this extreme thinness allows the elevator manufacturer to substantially reduce the diameter of the traction sheave in comparison to the sheave required for a 8.0 mm rope. This in turn reduces the necessary drive output and permits the use of smaller, energy-saving motors. But substantial savings can also be made in the use of high-grade raw materials for rope production. If a rope with a strength of 1570 N/mm2 had been used in a comparable installation, the weight of the rope would have increased by around 40 kg in each case. Based on a not unrealistic assumption of 1000 lift installations per annum, this would mean a calculated saving of 40 tons of steel.
Where thicker ropes are involved, the potential for material and energy savings through the use of higher-strength wires becomes even more evident. The 9-strand rope 19.0 mm PAWO F10 HT, for example, has a breaking force currently matched only by 22.0 mm ropes of a comparable design. If we compare the weight by metre, a saving of around 40% is achieved in the high-strength 19.0 mm rope compared to its 22.0 mm equivalent. As the 9-strand ropes are used particularly in high-rise installations involving an extremely long running length, every last gram of intrinsic weight counts. Alongside the material savings, the valuable energy saved by the reduced drive output is also considerable.
4. Summary
If the available potential for influencing steel characteristics is selectively utilized during the processing of steel to produce wires and during wire further processing operations, considerable improvement of the performance of steel ropes as suspension means in elevator construction can still be achieved. This approach not only reduces the use of precious natural resources but also saves energy. As the result of targeted research and development, Gustav Wolf has succeeded in developing and marketing new energyefficient steel ropes with a high performance capacity. Continuous investment in production plants used to manufacture wires and ropes guarantees the very highest product standard while reducing environmental impact, energy requirement and the consumption of raw materials. Gustav Wolf is committed to the pursuit of this type of sustainable production process.
Literature
[1] Wolf, E., Franz, A.: Seilentwicklung für Aufzuganlagen (Rope development for elevator installations) Lift-Report, Heft 5/2005, 28–32
[2] Wolf, E., Franz, A.: Rope Development for Elevators; Elevator World, March 2006, 122–126
[3] Woernle, R.: Ein Beitrag zur Klärung der Drahtseilfrage (Towards clarifi cation of the wire rope question) VDI 72 (1929) 13, 417–426
[4] Müller, H.: Drahtseile im Kranbau, Auswahl und Betriebsverhalten (Wire ropes in crane construction, selection and operating behaviour) VDI Report no. 98, reprint dhf 12 (1966) 11, 714–716 and 12, 766–773
[5] Wolf, E.: Seilbedingte Einflüsse auf die Lebensdauer laufender Drahtseile (Rope-related influences on the service life of running wire ropes) Diss. Universität Stuttgart 1987
Lecture delivered at the Heilbronner Aufzugstage 2009.
2/2009

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