Factors Affecting Bending Fatigue

 

Since fatigue is an inherent phenomenon, it cannot be eliminated, however it can be slowed down by adopting, when possible, particular features with respect to rope design and system layout.

 

With respect to rope design, the most effective way to reduce fatigue evolution is by avoiding the use of extremely high strength wires (over 2160 kN/mm2). As already mentioned, this improves steel ductility allowing a better resistance to repeated bending cycles.

Contrary to expectation, rope composed of many small wires may not have higher fatigue resistance, especially when working at low safety factor. In terms of system design, there are several strategies that can be adopted to preserve rope life.

The first tool is to increase the bending ratio of the component over which the rope is running. This can have some practical limitations, especially when dealing with large size ropes, howeverit has to be emphasised that the typical recommended ratio for good fatigue performance is approximately 20.

Another approach which can be adopted without major expense is the selection of proper groove size. The recommended value is approximately 1.08 times the nominal rope diameter, depending on rope type and possible fleet angle.

Fleet angle must be always considered and limited, as it creates a stress within the structure of the rope and contributes to fatigue build up: it should not exceed 2°, or 4° for ropes having plastic impregnated cores.

In case of fleet angle, groove oversize should be increased to 1.10 or more in order to facilitate the passage of the rope through the groove.