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.