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Although not as obvious as plate or thrust
bearings, the operation of journal or sleeve bearings is an
example of hydrodynamic lubrication. When the journal is at
rest, the weight of the journal squeezes out the oil film so
that the journal rests on the bearing surface. As rotation
starts, the journal has a tendency to roll up the side of
the bearing. At the same time fluid adhering to the journal
is drawn into the contact area. As the journal speed
increases an oil wedge is formed. The pressure of the oil
wedge increases until the journal is lifted off the bearing.
The journal is not only lifted vertically, but is also
pushed to the side by the pressure of the oil wedge. The
minimum fluid film thickness at full speed will occur at a
point just to the left of center and not at the bottom of
the bearing. In both the pivoting shoe thrust bearing and
the horizontal journal bearing, the minimum thickness of the
fluid film increases with an increase in fluid viscosity and
surface speed and decreases with an increase in load.
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