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
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.