When the ball or roller of
rolling-contact bearings rolls, contact stresses occur on the inner
ring, the rolling element, and on the outer ring.
Because the curvature of the contacting
elements in the axial direction is different from that in the radial
direction, the equations for these stresses are more involved than in
the Hertz equations.
If a bearing is clean and properly
lubricated, is mounted and sealed against the entrance of dust and
dirt, is maintained in this condition, and is operated at reasonable
temperatures, then metal fatigue will be the only cause of failure.
Inasmuch as metal fatigue implies many
millions of stress applications successfully endured, we need a
quantitative life measure. Common life measures are
• Number of revolutions of the inner
ring (outer ring stationary) until the first tangible evidence of
fatigue
• Number of hours of use at a
standard angular speed until the first tangible evidence of fatigue
The commonly used term is bearing life,
which is applied to either of the measures just mentioned. It is
important to realize, as in all fatigue, life as defined above is a
stochastic variable and, as such, has both a distribution and
associated statistical parameters.
The life measure of an individual
bearing is defined as the total number of revolutions (or hours at a
constant speed) of bearing operation until the failure criterion is
developed. Under ideal conditions, the fatigue failure consists of
spalling of the load carrying surfaces.
The American Bearing Manufacturers
Association (ABMA) standard states that the failure criterion is the
first evidence of fatigue. The fatigue criterion used by the Timken
Company laboratories is the spalling or pitting of an area of 0.01
in2.
Timken also observes that the useful
life of the bearing may extend considerably beyond this point. This
is an operational definition of fatigue failure in rolling bearings.
The rating life is a term sanctioned by the ABMA and used by most
manufacturers.
The rating life of a group of nominally
identical ball or roller bearings is defined as the number of
revolutions (or hours at a constant speed) that 90 percent of a group
of bearings will achieve or exceed\ before the failure criterion
develops. The terms minimum life, L10 life, and B10 life are also
used as synonyms for rating life.
The rating life is the 10th percentile
location of the bearing group’s revolutions-to-failure
distribution. Median life is the 50th percentile life of a group of
bearings. The term average life has been used as a synonym for median
life, contributing to confusion. When many groups of bearings are
tested, the median life is between 4 and 5 times the L10 life.
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