When bearings raotate , the inner and outer rings and rolling elements are constantly loaded . This produces material fatigue and eventually bearing failure . The total number of revolutions before a failure occurs is called the basic rating life .
Life of individual bearings varies considerably , even if they are of the same size , same material , same heat treatment and are under the same operating conditions .
Statistically , the total number of revolutions reached or exceeded by 90% of a sufficiently large group of apparently identical bearings before the first evidence of material fatigue occurs is called the basic rating life .