Plain bearing failure - Fatigue - Summary

PLAIN BEARING FAILURE

Plain bearing failure index

Fatigue

3.4 Fatigue Failures: Summary

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	Figure 'a': Section through fatigued white metal showing crack penetrating to the backing layer, but not to the backing metal
	Figure 'b': Fatigued white metal showing lack of tin layer on steel backing
Main Characteristics	Fatigue damage is readily distinguished from other failure mechanisms. It manifests itself by crazy cracking on the surface; the cracks propagate through the white metal to the tin lining on the bearing shell where they continue until joining up with another crack and form a loose piece that detaches from the backing. These loose pieces have an aspect ration of about 3:1 to 5:1 (Fig. 'a'). The full fatigue strength of the white metal lining depends on a sound metallurgical bond between the backing material and the white metal that is obtained by tinning the backing before pouring on the molten white metal. If a sound metallurgical bond is not created, the fatigue strength is markedly reduced, making the bearing susceptible to premature fatigue failure. Normal and premature fatigue can be distinguished by examining the bottom of the fatigue pit; this should show a coating of tin on the backing, not steel or a rusty surface (Fig. 'b'). Fatigue failure is normally confined to journal bearings and rarely occurs with thrust bearings. The loose pieces of white metal can remain in place, held in by the close fitting journal, or disappear by getting trapped so that they rub and wipe'.
Possible Causes	Fatigue in journal bearings can result from inadequate design, but is usually the result of excessive alternating loading.
Actions	Where fatigue is the result of excessive load, the only remedy is to change to a more fatigue resistant bearing material (but check that the shaft hardness is at least 3 times the hardness of the new bearing material, if not then there is an enhanced risk of shaft damage in the event of a bearing failure). Where it is a consequence of vibration, the vibration level has to be reduced either by improved balancing, changing the operating conditions, or changing the bearing geometry (clearance, profile).