Healthy sheaves equals better wire rope life
- 4 days ago
- 3 min read
In the majority of wire rope applications, the rope bends around, or comes into contact with, sheaves. This means that the condition, orientation and shape of the sheave has a direct impact on how long the wire rope lasts in service.
Therefore, when a thorough examination of wire ropes takes place, it is important to assess the sheave condition as well. The following are aspects of sheave conditions and orientation which should be considered and checked:
Sheave groove diameter
The diameter of the sheave groove has a significant effect on rope life. Firstly, wire rope design specifications provide for an oversize tolerance. Normally allow for about 2.5%. Secondly, as the wire rope bends around the sheave it tends to flatten slightly and this ovality effectively also increases the rope diameter of the wire rope in the sheave groove.
Therefore, when a new rope is fitted, the sheave groove size should be between 4% and 7.5% greater than the nominal rope diameter. Undersize grooves can significantly reduce the life of wire rope not only due to abrasive action of the sheave on the rope, but also by the tight groove nipping the rope, preventing individual wires from sliding on one another as the rope bends.
This causes unequal loading of the individual wires, some being overloaded, leading to premature fatigue and early failure. Undersize grooves can also induce corkscrews in the wire rope. With an oversize sheave groove the wire rope is not well supported and the resulting point loading on the wires can lead to plastic deformation of the wires and fatigue failure.
diameter, use is made of sheave gauges. Check whether the size indicated on the gauge includes the oversize allowance. LEEASA has recently launched their innovative stainless steel sheave gauges. This set includes a full range of gauges from 8 mm rope diameter to 58 mm diameter, neatly positioned in a well-designed container.
Each gauge size indicates both the minimum oversize (4%) as well as the maximum oversize (7.5%). The sheave groove diameter is measured by scraping the gauge along the bottom of the groove (best to shine a torch behind it). The gauge should make contact with the complete arc of the sheave groove.
Sheave diameter
The diameter of the sheave has a direct impact on the fatigue life of a wire rope. If the sheave diameter is too small, wire fatigue failures (square ends) occur. When using a 6X36/F wire rope the recommended sheave diameter is at least 19 times the wire rope diameter. An 18 strand non spin should not be made to bend around a sheave of less than 26 times the rope diameter. For high performance ropes, check with the manufacturer for recommended minimum sheave diameter.
Sheave Groove
Rifling Corrugated sheave grooves worn by wire ropes under high pressure (or soft sheave material) can damage a wire rope running in the sheave.
Sheave Alignment
As the rope leaves the drum and enters the sheave, an angle is formed with the sheave. This angle is called the fleeting angle. If a sheave is not aligned correctly, the resulting excessive fleeting angle may cause the wire rope to come into contact with the sheave flange. This causes premature wear of the wire rope and sometimes induces turn into the rope with subsequent waviness. If the wire rope is well lubricated, misalignment is often indicated by a buildup of grease on one flange of the sheave.
Broken flanges Any sheaves with broken flanges should be replaced immediately. This not only can damage the wire rope but is a potentially dangerous condition. There are other aspects to consider such as sheave material, angle of flare and bearings. These can all affect the service life of wire ropes.
Add value to your inspections by investigating the reasons for premature discard of wire ropes and help to eliminate the causes. It is what professionals do!
LEEASA,
+27 (0) 84 740 7875,
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