The life of a rolling bearing depends on the total number of stress cycles and the loads incurred by rolling elements and raceways. The standardized calculation method for dynamically stressed bearings is based on material fatigue (pitting) causing the damage. Normal fatigue manifests itself by flaking or spalling of the rolling surfaces. An increasing local stress may result in fracture of the ring. If the bearing fails earlier than predicted by the life calculation, it should be checked for overloading. With this failure cause excluded, faulty mounting or poor maintenance or wear might be the cause for the damage. The following pages describe some of the more common
forms of bearing damage and their causes.

Why Does a Bearing Fail?
Faulty Mounting
Local damage to the raceways, such as nicks, score marks or indentations suggest faulty mounting. This type of damage occurs, if, for instance, the inner ring of a cylindrical roller bearing is inserted outof-square into the outer ring, or if the mounting force is applied through the rolling elements. Surface damage is also caused, when foreign particles enter the bearing and are cycled. The damage can be recognized for instance by a louder running noise;
in the long run, it may lead to premature fatigue of the functional surfaces. The typical sign for surface damage are the raised edges of the indentations.

Contamination
Foreign particle indentations in the functional surfaces may lead to premature fatigue (see chapter 6.1.1). Foreign particles with abrasive effect, however, accelerate bearing failure due to wear. The surfaces are roughened and look dull. Progressive wear causes excessive clearance.

Corrosion
Corrosion in rolling bearings may occur in various forms and have different causes. The damage shows in an uneven and loud running noise. The rust abraded by the rolling elements causes wear.

Passage of Electric Current
Continuous passage of electric current causes brownish flutes parallel to the axis over the entire circumference of one or both raceways as well as on the rolling elements.

Imperfect Lubrication
Starved lubrication is caused by an insufficient lubricant supply or by use of an improper lubricant. If the lubricating film does not sufficiently separate the parts in rolling contact, sliding motion and wear result. Since maximum material stressing occurs at the raceway surfaces, micro pits and consequently large-area superficial flaking is produced.