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Stick-slip. To the unaided eye the motion of
sliding objects appears steady. In reality this motion is
jerky or intermittent because the objects slow during shear
periods and accelerate following the shear. This process is
continuously repeated while the objects are sliding.
During shear periods, the static friction
force F controls the speed. Once shearing is completed, the
kinetic friction force Fs controls the speed and
Fk the object accelerates. This effect is known
as stick-slip.
In well lubricated machinery operated at the
proper speed, stick-slip is insignificant, but it is
responsible for the squeaking or chatter sometimes heard in
machine operation. Machines that operate over long sliding
surfaces, such as the ways of a lathe, are subject to
stick-slip. To prevent stick-slip, lubricants are provided
with additives to make Fs less than Fk
.
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