Untitled Document
* Rolling bearing
  * Ball bearings
  * Roller bearings
  * Bearing life
  * Minimum loads
  * Bearing
  * Radial location
of bearings
  * Bearing with
tapered bore
  * Axial location
of bearings
  * Housing and
shaft tolerance

Bearing Life

The theoretical life of a bearing can be calculated using the ISO standards. These calculations are usually based on the L10 life. This life is defined as the point at which 10% of a representative sample of identical bearings in a given application would have failed. Failure is defined as the stage when the first signs of fatigue occur on either the rolling element or raceway. Research shows that 50% of the total sample bearings would achieve five times of their calculated life and some continue to perform satisfactorily over even longer periods.

With these vast differences in life expectancy of rolling bearings, many other factors influence the bearing service life. Most important factors are the effectiveness of the oil film, bearing quality and contamination. SKF takes these factors into account when calculating the life expectancy of its bearings.

SKF has developed the infinite life theory, which states that under good operating conditions and provided the fatigue load limit is not exceeded; the bearing life will exceed that of the machine. It must be recognised that handling and contamination damage can dramatically reduce the bearings life. This is referred to as the bearing service life.

An important factor in the anticipated life of a roller bearing is that a doubling of the load on a significantly loaded roller bearing results in a life reduction of around 90%.

Fatigue, spalling or flaking
When rolling bearings are in operation the action of the rolling elements carrying the load through the load zone of the bearing results in cyclic stresses occurring in the sub-surface of the rolling element and raceway immediately below the load carrying surfaces. In normal operation with correctly sized and well-maintained bearings, the effect of these stress cycles is minimal. However, if the loads are excessive or damage occurs to the race way or rolling elements, these forces can increase the formation of sub-surface cracks which will progressively extend to the surface. As the rolling elements continually pass over the cracks, fragments of the material break away. This effect is called flaking or spalling and is progressive, eventually rendering the bearing unserviceable.

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