Rope/sheave contact wear: a fine tuning – Part 2
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  to check the real shape of U-undercut friction groove.
Figure 14 – V-Undercut Friction Groove: geometric parameter
Figure 15 – Real 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.
Figure 16 – Wearing 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  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  dealing with the subject of rope re-lubrication in service.
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.
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 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).
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