When rolling bearings and gears are lubricated with a clean lubricant the i
nitial metal to metal contact through the oil film will normally stop in le
ss than a million loading cycles if the surfaces are at all able to run-in.
If, after the running-in, the temperature is increased or the speed is dec
reased, metallic contact through the oil film will start again and continue
until the surfaces have run in again at the new running conditions. The ru
nning-in continues as long as some asperities reach each other through the
oil him, and the run-in state is characterized by the minimum separation be
tween the highest roughness tops being very close to zero. The surface stru
cture is changed by running and if that change is slow and well controlled,
the surfaces will run in until they become smooth enough to no longer wear
. This running-in thus determines the running conditions and the functional
ity of the surfaces. If the lubricant contains particles, and these are lar
ge and hard enough, the running-in of the surfaces can be destroyed and the
y will never become smooth enough to be separated by an oil him. This leads
to wear and/or spalling failure of the surfaces. Whether the surfaces will
wear or spall depends on the properties and the concentration of contamina
nt particles. For some levels of contamination the surfaces can first wear
and then spall when the load distribution over the contact area has been ch
anged by the wear. To ensure that the surfaces can work in a stable mode wi
thout wear or spalling, the running conditions have to be ideal for each po
int on the lubricated surface all the time. (C) 2000 Elsevier Science Ltd.
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