Bending Wire Rope Over Sheaves & Drums
Sheaves, drums and rollers must be of a correct design if optimum service is to be obtained from both the equipment and the wire rope. Because there are many different types of equipment and many different operating conditions, it is difficult to identify the one specific size of sheave or drum most appropriate for every application. The guideline to follow is this: the most practical design is the one that most closely accommodates the limiting factors imposed by the equipment, the operating conditions and the wire rope.
All wire ropes operating over sheaves and drums are subjected to cyclic bending stresses, thus the rope wires will eventually fatigue. The magnitude of these stresses depends—all other factors being constant—upon the ratio of the diameter of the sheave or drum to the diameter of the rope. Frequently, fatigue from cyclic, high-magnitude bending stress is a principal reason for shortened rope service. In order for a rope to bend around a sheave, the rope's strands and wires must move relative to one another. This movement compensates for the difference in diameter between the underside and the top side of the rope, the distance being greater along the top side than it is on the underside next to the groove. Proper rope movement (and service) is adversely affected if the wires cannot adjust to compensate for this length differential. Also, there can be additional limitations to wire movement because of excessive pressure caused by a sheave groove diameter which is too small, or by lack of rope lubrication. Avoid changing the bending direction from one sheave to another as this reverse bending further accelerates wire fatigue.
The relationship between sheave diameter and rope diameter is a critical factor that is used to estimate the rope's fatigue resistance or relative service life. It is expressed in the D/d ratio mentioned earlier in which D is the pitch diameter of the sheave and d is the diameter of the rope. Table 1 lists suggested minimum D/d values for various rope constructions. Smaller values can affect rope life. Table 2 (on next page) shows the effect of rope construction and D/d on service life.