How do I identify and prevent bearing fatigue?
Published: 07 March, 2017
The answer to this month’s trouble shooter is provided by Phil Burge, country communications manager at SKF.
A:
Only a tiny fraction of bearings – of the order of 0.5% – actually fail in use. The two most common causes for this are fatigue, and problems with lubrication. Each of these is responsible for around one-third of failures.
Fatigue itself is a change in the material structure of the bearing, caused by repeated stress at the contact areas between the rolling elements and raceways. It comes about in two ways:
• surface-initiated fatigue, which occurs when the surface of the bearing is distressed, leading to failure of the rolling contact material surface due mainly to inadequate lubrication; and,
• subsurface-initiated fatigue, where microcracks are generated below the surface of the raceway – which, when they spread to the surface, cause material to flake away (or spall).
A common cause of failure is overloading, and one of the main factors behind this is misalignment – caused by faulty mounting. A misaligned housing, for instance, can put excessive load on a small area at the edge of the bearing, which can initiate subsurface cracking. Similarly, impact damage during handling, storage and operation can lead to fatigue failures of both types.
Another common cause of fatigue failure is contamination, which can also be introduced during mounting – or may even be in place from a previous bearing failure.
Avoiding premature subsurface-initiated fatigue therefore requires three major conditions: clean bearing steel (from high quality bearings); good lubrication conditions (no contamination); and good load distribution (over the rolling elements and along the rolling element contact line).
These rules – along with a general proviso to treat bearing handling and mounting as a critical operation – apply equally to surface-initiated fatigue.
