Understanding the relationship between sheave size and rope life

JUNE 2015

Inappropriate selection of the sheaves can lead to premature failure of wire rope by fatigue mechanisms.

Wire rope and cabling is very widely used throughout industry in a variety of applications.  Failures of such ropes however continue to take place, often due to misuse or lack of understanding of the constraints under which wire ropes operate.  Users are referred to the appropriate suppliers who can advise them of the optimum operating conditions, but what is often abused is the size of the sheave wheel selected for a particular wire rope diameter.  Many suppliers specify that the ratio of sheave diameter to rope diameter should be in the range 20 to 30 (i.e. D/d = 20 to 30).  However at small rope diameters (6 to 10mm), this stringency can be relaxed to ratios closer to 10.  Research studies conducted, have shown that for a 13mm diameter wire rope, simply changing a pulley diameter from the recommended 300mm to 150mm diameter led to a six fold decrease in fatigue life of the rope – a significant loss.  Manufacturers’ tables should be consulted for all applications and their advice headed (they are seldom overly conservative).  

In addition, wear or incorrect machining of the grooves of the sheave is also a common problem, and worn sheaves contribute to significant wear of wire ropes and should be re-machined as soon as any wear (as measured using a suitable groove gauge) becomes apparent.  Similarly inappropriate fleet angles (angle of wire running from the drum to a guide pulley/sheave) can also cause significant wear and shorten fatigue life.  Fleet angles are specified at typically less than 1-2 degrees and should be adhered to for optimum fatigue life.

Published in Technical Tips by Origen Engineering Solutions on 1 June 2015