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surfaces (face side) with irregular edges. ey
may appear thin and platelet-shaped — similar
to laminar particles but without the flattened
over-rolled appearance.
Rolling Traction Fatigue. is results
from a combination of rolling and sliding
under elasto-hydrodynamic lubrication (EHL)
conditions. Cyclic Hertzian loads radiate
downward, which propagates crack forma-
tion and subsurface fatigue from repeated
flexing. Combined sliding produces traction
forces, dislocating particles from the socket of
the bruised area. Subsequent rolling contact
produces downward flaking at edges, causing
the damaged area to spall-out further.
Premature fatigue often occurs due to the
over-rolling of hard contaminant particles (e.g.,
silica), which induces surface indentations and
berms (stress risers). ese are initiation sites
for cracking, flaking and spall-out conditions.
Chunklike particles from rolling traction
fatigue typically exhibit fractured or torn
surfaces and edges. Some may show a less
chunky appearance, such as a thick platelet
with varying shapes and profiles.
Fatigue particles generated by a ball
bearing may resemble a rose petal shape. Torn
or feathered edges are common. ey are
frequently subjected to over-rolling (rework),
Figure 2 Gears often form a network of
deep checkered cracks across or near the
pitch line. When they get deep enough and
converge, small pieces are pulled out by
sliding traction to form chunklike particles,
leaving spalled surface damage behind.
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