Rolling bearing failure index

2.0 Fatigue failure

Click on photograph to enlarge
	Figure 2a
	Figure 2b
	Figure 2c
	Figure 2d
	Figure 2e
Main Characteristics	The life of a rolling bearing is defined as the time the first detectable fatigue pit forms on any of the contacting surfaces (figures 2a, b). This is a somewhat random effect, determined by metallurgical imperfections in the steel and, while modern steel-making techniques have produced steels that are significantly cleaner and have more uniform structures, crystal lattice imperfections or other defects still provide the loci for fatigue. This can occur on either the races or the rolling elements, depending on the design of the bearing, the direction of loading and variations in steel quality.
Cause	Once a fatigue pit has formed, it sets up a point of weakness and fatigue damage progresses round the component (figures 2c, d). In this condition, although the bearing may have become noisy, it is still performing its function of guiding the shaft and cannot really be said to have failed in the conventional usage of the term. Collapse of the bearing occurs when the component fractures from a stress raiser set up by the fatigue pits (figure 2e). Practical experience suggests that once the first fatigue pits are formed the bearing has only used up 70-80% of its life to collapse.
Note
Possible Confusion with Other Types of Damage
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