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Pitting wear is due to surface failure of a
material as a result of stresses that exceed the endurance
(fatigue) limit of the material. Metal fatigue is
demonstrated by bending a piece of metal wire, such as a
paper clip, back and forth until it breaks. Whenever a metal
shape is deformed repeatedly, it eventually fails. A
different type of deformation occurs when a ball bearing
under a load rolls along its race. The bearing is flattened
somewhat and the edges of contact are extended outward. This
repeated flexing eventually results in microscopic flakes
being removed from the bearing. Fatigue wear also occurs
during sliding motion. Gear teeth frequently fail due to
pitting.
While pitting is generally viewed as a mode
of failure, some pitting wear is not detrimental. During the
break-in period of new machinery, friction wears down
working surface irregularities. This condition is considered
to be nonprogressive and usually improves after the break-in
period. However, parts that are continuously subjected to
repeated stress will experience destructive pitting as the
material’s endurance limit is reached.
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