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Rope/sheave contact wear: a fine tuning – Part 2

 In Techniques

By Nicola Imbimbo – Read the author’s biography
Articolo apparso la prima volta su Elevatori n. 4/2009

Rope/sheave contact wear: a fine tuning – Part 2 (click here to read the first part)

5. Traction Groove

It is generally accepted that the friction groove is the element required to transmit the movement between the motor and the traction rope, but at the same time it is also the element that guarantees that a sufficient tractive force be developed between the sheave and the rope: depending on the geometric shape (V or U with or without undercut), on the constructional angles (g and b) and on the wearing state.

A study was conducted in the 70s by Petkov [10] to check the real shape of U-undercut friction groove.

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Figure 14 V-Undercut Friction Groove: geometric parameter

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Figure 15Real shape of “U-Undercut” friction groove

Over 300 measurements were taken from elevators in service.

What has been found is that: during the first period of service the groove shape change quickly and after that the diameter of the sheave is reduced, but more or less the shape remains almost the same.

All grooves shape inspected were more or less symmetrical, and their side profiles were more ellipses and hyperboles instead of circles as is mentioned in literature for friction calculation.

5.1 How wear affects the friction transmission

Even little changes in the original geometrical parameter could affect the friction performance, for this reason great care must be devoted to the maintenance in service of such element with periodical inspections, to check on a regular basis the amount of wear that develops [6,8].

In the relationship between rope and sheave, both of them are subject to wear during their use, but the rope wear is lesser compared to the sheave wear due to the following reasons: the rope is the hardest element and during one trip of the lift, one given point of the rope comes into contact with the traction sheave only once, but a point on the sheave comes in contact with the rope once each revolution. The design stage requires that the rope, considered as a “round” element, touch the traction sheave at the point indicated with A and B in the Figure 16.

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Figure 16Wearing process into “V” and “V-Undercut” friction

During the time the wear on the surface will change the geometrical contact points, and the rope will re-groove itself a more rounded surface: this phenomenon will reduce the pinching effect and consequently there will be a loss of traction. A certain point will be reached due to the continuous starting and stopping of the lift will cause the ropes to begin to slip. When this happen and is clearly detected, then replacement or a re-grooving operation of the traction sheave, to restore the original condition of traction must take place [6,9].

Some study conducted by Molkow [9] in the IFT Institute of Stuttgart University, demonstrated that a well lubricated rope will develop better traction conditions: when the rope is in a well lubricated state, it will be fit better into the traction sheave and from such good contact will develop a higher apparent friction coefficient. This means that if after replacement of old ropes the new ones display some slips, this would mainly be related to the condition of the friction groove being worn by the old dry ropes and the only cure is to replace or re-cut the grooves to suit the new nominal rope diameter.

Very rare are the cases that this effect is generated by ropes that have their original lubrication maintained. A paper has been published by Major [13] dealing with the subject of rope re-lubrication in service.

5.2 Sheave: raw material influence

It is not the aim of this paper to go into details of the manufacturing process of traction sheaves, but one basic rule applies about the raw material used. Driving sheaves are generally made of cast iron with the addition of steel scrap into the mixture, to give the required hardness surface.

In particular cases also cast steel could be added to such cast mixture, mainly to increase the performance of wear resistance and to reach higher surface hardness or to permit more harmonious results from heat treatment of the traction grooves [8]. When the design of lift is made, a strong assumption is taken into consideration: the set of ropes is equal tensioned. Unfortunately, for several reasons, this rarely happens and the ropes behave with different elastic and permanent elongation in the contact with traction grooves. In a short time this will lead to non-homogeneous wearing of the groove surface.

The rope itself is not a good re-cutting tool, and the shape of the traction groove will start to become irregular. Further to this, must be added the fact that the cast mixture of the sheave is a heterogeneous matrix and the rope, into this cutting activity, will meet harder and softer spots that again lead to a non-harmonious wear [6]. To maintain the wearing of the grooves to an acceptable value, it is necessary to provide enough bearing surface for the ropes that means that the “specific pressure” issue has to be taken into consideration.

5.3 Pinching effect of the “undercut”

To increase the friction performance of U-grooves, very often such geometrical groove shape come in combination with an “undercut”: this generates a pinching effect on the rope surface and creates an higher apparent friction coefficient. While there are benefits, we must also consider the negative effect and take into consideration that the real shape of a rope is not circular but is made of a certain number of strands: the larger is the “undercut” the higher will be the risk that the rope will be pinched into it [8,12].

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Figure 17 – Undercut 105°: Theoretical and deformed shape

The fibre core of the rope is an elastic element that has a certain degree of compression, but this behaviour is like a spring, the more it is compressed the higher the force will be on the fibre core which will push the strands against the groove surface. The fibre core will deform to its maximum causing the rope to be pinched by the Undercut and at this point the rope will start to leave its “Negative Imprint” to the sheave surface.

To experience such a phenomena, as usual, several parameters have to combine together: large undercut (b around 105°), high “specific pressure” value, elastic core of the rope, soft sheaves, low speed elevator, low change in the load rate into the car side (rope will not show any elastic elongation) [14].

An example can be seen in the following pictures (Figure 18).

Even with up to date knowledge, it is not clear which is the main parameter responsible that could lead to negative imprints, but it has been seen statistically that the above stated parameters are always present at the same time when such phenomena is displayed.

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Figure 18Large undercut with high specific pressure value, resulting in Negative Imprints

Some statistical analysis of the field survey, also demonstrate that the occurrence of negative imprints has higher probability when synthetic cores are used for ropes (due to the higher compression elasticity of such material); alternatively when a full steel core is used the probability is reduced due to the fact that the core has a lesser compressibility, and so the rope would be pinched less in the undercut.

Certainly other elements will participate to this behaviour of rope and sheaves, and not the scope of this publication to enter too deeply into such a technical description. When such “negative imprint” appears in service best advice is to seek the support of an expert.

5.4 Specific pressure

One of the worst “killers” of rope and sheaves in service is the “specific pressure”, as it was formulated for the first time in the 1927 by Hymans/Hellbronn [5].

This parameter represents the reciprocal force on which rope and sheave press against each other, and this will affect tremendously the surface abrasion of both due to the reciprocal movement acting between such two elements.

As it has been described in earlier paragraphs, the rope tension T1 and T2 on the sides of the traction sheave are different, this means that while running for such a sheave over its full rotation and travel, the rope will change the state of axial solicitation and in practical terms this means that the rope elongates and contracts like a “worm” on the surface of the sheave.Such reciprocal movement, connected with the actual value of the specific pressure, could be a good indication of surface wear.

It has also to be pointed out that the formulation of specific pressure is an indicative parameter, because it considers the rope as a “round” surface, but in reality this is a difficult hypothesis due to the fact that the ropes touch the sheave groove only on discrete points in correspondence to the strand crowns (the higher the number of strands the higher will be the number of contact points and the less the actual specific pressure).

The formulation of specific pressure is irrespective of the number of strands of the rope.

One very important factor pointed out from the original study of Hymans/Hellbronn [5], is the number of trips per hour or traffic analysis.

From the Figure 19 it can be seen that the higher the amount of traffic that the elevator is expected to take then the less will be the max Specific Pressure allowed, the same happens in respect of the rated speed of the rope.

The specific pressure is one of the key issues in determining the lifetime of ropes and sheaves, and very often is given too little consideration by lift designers.

Further to this the harmonized standard EN 81-1 since 1998 has cancelled any explicit reference to this calculation.

Some consideration is hidden in the calculation of the safety factor and number of equivalent sheaves, but it is a fact that lift configurations could be installed with a very high specific pressure value that will lead to shorter lifetime for ropes and sheaves compared with calculations under previous guidance.

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Figure 19 – Upper limits of acceptable pressure in lift traction grooves (Hymans/Hellbronn 1927)

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Figure 20Calculation of the specific pressure based on EN 81 not harmonized

In the design stage of an elevator it is always advisable to pay attention to a realistic value of the specific pressure issue, just to avoid trouble in service. As a reference it is good practise to still consider the former EN 81-1 not harmonized [1] in which formulas are presented in the Art.9 notes.

Even if such checking is not any longer obligatory, could be suggested that to have operational conditions that, respect such a limit, or will be not too much away from it.

6 Equal tensioning of the rope set: method of operation

One of the most important parameters when installing ropes on an lift is to be sure that all the ropes, comprising the set, are tensioning to an equal level with each other. The uneven distribution of the tension between the set of ropes could lead in very short time to surface wear of the traction sheave, with different pressure displayed in every groove. In practical terms this means that the slip phenomenon could occur in service.

When an Engineer starts to design a lift an assumption is made that all the ropes are loaded at the same level, but this assumption is in most of the cases a very optimistic consideration.

Common practise is for the tension to be checked only by the rule of thumb or hand, and this is a not a reliable “tool”. The real check on the tension needs to be made with a professional device, mechanical or maybe electronic device that is able with a great degree of accuracy, to check the real level of tensioning.

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Figura 21Verifica dell’equo tensionamento delle funi (dispositivo Weight-Watcher)

This is a good help to identify which rope end termination has to be tighten or loosed to reach an equal balancing between the set of ropes.

It is vital to check the rope tension during the installation of the lift (or during the replacement of the set of ropes) and then best practise to recheck the situation after few weeks of service at the latest after 4/6 weeks but not longer.

7. Conclusions

Ropes and sheaves are not only commodity items, but sometimes they provide an indicator of some faulty consideration on the whole project: negative imprints, rapid wear, vibration on ropes, etc, are clear signs that the lift is not performing its the working conditions.

Beside that it must be understood that the forces acting between ropes and sheaves are generated by physical laws and if a reasonable service life of the lift is expected, they have to be carefully considered in every aspect. Designing a lift not only means applying the EN 81 rules, but also other considerations that have to be addressed.

Further to this, new studies are going on in the field of new developments and ropes, or traction member, will play an important role in the near future.

New developments in the concept of the complete lift will allow the possibility of using motors with even smaller traction sheaves, that means less torque which in turn use less current from the power supply. This will reflect also on the size of the motors that will become lighter and on the size of the inverters.

This “down-sizing” process must be made with reliable technology for the traction member production giving the total guarantee of their performance in service.

If this is the direction being followed, certainly ropes and sheaves must no longer be considered just as a commodity, but as key elements in the new development, based on high quality material performance and correct specification.

Only by involving experts in the proven field of ropes and traction can these new technological developments be supported and successful in use.

8. Notes

The aim of this paper was mainly to go back to the basics on the main parameter affecting the lifetime of ropes and sheaves.

In practical terms it has been a good opportunity to offer this dissertation with simple and understandable topics in a way to allow the widest audience to understand it.

For sure it has not been the intention to offer a comprehensive discussion and in some cases the advice of an expert has to be sought. Most of the hints described in this paper have come from practical experience and are intended to offer a cross reference of thinking within the industry and not be classed as a directive.

Bibliographic references

[1] EN 81, Safety rule for the construction and installation of lifts and service lifts, Part I, Electric Lifts, Edition 2, European Standard, Brussels, 1985.

[2] EN 81-1/1998, Safety rules for the construction and installation of lifts, Part I: Electric Lifts.

[3] EN 12385-5:2003, Steel wire rope safety – Part 5: Stranded ropes for lifts.

[4] ISO 4309:2004, Cranes-Wire rope-Care, maintenance, installation examination and discard.

[5] Hymans, F./Hellbronn, A.V., Der neuzeitliche Aufzug mit Treibscheibenantrieb, Springer Verlag, Berlin 1927 (The modern elevator with traction drive).

[6] Hymans, F., Electric Elevators books I &II, International Textbook Company, Scranton PA. 1931.

[7] Paolelli, R., Ascensori e montacarichi ad azionamento elettrico,Collana di studi e documenti sulla prevenzione No.49, ENPI, Roma 1969 (Electric lifts for passengers and freight).

[8] Molkow, M., Wire ropes in elevators, Pfeifer Drako brochure, Mulheim an der Rhur 2002.

[9] Molkow, M., Driving capability of traction drives, Technical Bullettin No.5, Elevator World, Mobile AL.

[10] Petkov, K.D., Theoretical and experimental study of sheave and rope traction drive, International Lift Symposium, Amsterdam 1984.

[11] Janovsky, L., Elevator mechanical design Third Edition, Elevator World, Mobile Al. 1999.

[12] Scheunemann, W./Vogel, W./Barthel,T., Steel wire ropes in elevators, PFEIFER DRAKO brochure, Mulheim an der Rhur 2008.

[13] Major, D., Lubrication and maintenance of steel wire ropes on lifts, Elevation magazine, Dartford Kent 2008.

[14] Verrett, R., Negative impressions of the rope’s surface in the grooves of sheaves and drums, Wire Rope Technology brochure, Aachen 2006.

[15] Usabiaga, H./Madoz, M.A./Ezkurra, M./Pagalday, J.M., Mechanical interaction between wire ropes and sheaves, OIPEEC Conference “Trends for ropes”, Athens 2006.

[16] Urchegui, M.A./Madoz, M.A./Tato, W./Gomez, X., Wear characterisation techniques for fatigued wire ropes, OIPEEC Conference “Trends for ropes”, Athens 2006.

[17] Schonherr, S./Wehking, K.H., Reduction in service life of wire ropes running over sheaves with angular offset, OIPEEC Conference “Trends for ropes”, Athens 2006.

[18] Imbimbo, N., “Teoria e Pratica: Il controllo delle funi metalliche di sospensione”, Elevatori, n. 6/2005, pag. 68, Milano 2005 (clicca qui per leggerlo inline).

[19] Imbimbo, N., “Nuove tendenze per gli MRL”, Elevatori, n. 2/2007, pag. 50, Milano 2007.

The end

To learn more, read also:

Nicola Imbimbo – Rope/sheave contact wear: a fine tuning – First part

Nicola Imbimbo – Control of suspension metal ropes

Nicola Imbimbo – Wire rope discard criteria

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