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The graph
shows the several regimes of friction in the journal bearing shown in (a).
The friction force is plotted as a function product: (relative velocity x viscosity)/load.
At low values of this quantity, the friction force is almost independent
of its value and the lubrication film is only a few atom layers in thickness.
As the value increases into the intermediate region, the friction force
drops rapidly and reaches a minimum value. The lubricant thickness in this
region has increases to 10 to 50 atom layers. It is found that the wear rate
of the bearing is low if it is operated in this region. As the parameter is
increased further, the lubricating film exhibits hydrodynamic behavior and
the friction force increases slowly. The lubricating film is now of macroscopic
thickness and behaves as a continuum.
In many cases the viscosity
and load on a bearing will be determined by the device design and shaft velocity
will be the controlling factor in bearing behavior. The graph shows that
for these conditions there is an optimal shaft speed for minimum friction
force and low bearing wear. |
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