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Pitting occurs when fatigue cracks are
initiated on the tooth surface or just below the surface.
Usually pits are the result of surface cracks caused by
metal-to-metal contact of asperities or defects due to low
lubricant film thickness. High-speed gears with smooth
surfaces and good film thickness may experience pitting due
to subsurface cracks. These cracks may start at inclusions
in the gear materials, which act as stress concentrators,
and propagate below and parallel to the tooth surface. Pits
are formed when these cracks break through the tooth surface
and cause material separation. When several pits join, a
larger pit (or spall) is formed. Another suspected cause of
pitting is hydrogen embrittlement of metal due to water
contamination of the lubricant. Pitting can also be caused
by foreign particle contamination of lubricant. These
particles create surface stress concentration points that
reduce lubricant film thickness and promote pitting. The
following guidelines should be observed to minimize the
onset of pitting in gear units:
-
Reduce contact stresses through load
reduction or by optimizing gear geometry.
-
Steel should be properly heat-treated to
high hardness. Carburizing is preferable.
-
Gear teeth should have smooth surfaces
produced by grinding or honing.
-
Use proper quantities of cool, clean,
and dry lubricant with the required viscosity.
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