Surface Distress and Its Causes.

If the lubricant film between raceways and rolling elements becomes too thin, the peaks of the surface asperities will momentarily come into contact with each other. Small cracks then form in the surfaces and this is known as surface distress. These cracks must not be confused with the fatigue cracks that originate beneath the surface and lead to flaking. The surface distress cracks are microscopically small and increase very gradually to such a size that they interfere with the smooth running of the bearing. These cracks may, however, hasten the formation of sub-surface fatigue cracks and thus shorten the life of the bearing.
If the lubrication remains satisfactory throughout, i.e. the lubricant film does not become too thin because of lubricant starvation or viscosity changes induced by the rising temperature or on account of excessive loading, there is no risk of surface distress.

Appearance	Initially the damage is not visible to the naked eye. A more advanced stage is marked by small, shallow craters with crystalline fracture surfaces.
Cause	Inadequate or improper lubrication.
Action	Improve lubrication.


Figure 1 Surface distress in the form of a band encircling a roller from a spherical roller bearing.	Figure 2 The surface distress depicted in fig 1 – 100 x magnification.

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