Laws of Sliding Friction lubricated Surfaces
(a) The frictional resistance is almost
independent of the pressure (normal force per unit area) if
the surfaces are flooded with oil.
(b) The friction varies directly as the
speed, at low pressures; but for high pressures the friction
is very great at low velocities, approaching a minimum at
about 2 ft/sec linear velocity, and afterwards increasing
approximately as the square root of the speed.
(c) For well lubricated surfaces the
frictional resistance depends, to a very great extent, on
the temperature, partly because of the change in viscosity
of the oil and partly because, for journal bearings, the
diameter of the bearing increases with the rise in
temperature more rapidly than the diameter of the shaft,
thus relieving the bearing of side pressure.
(d) If the bearing surfaces are flooded
with oil, the friction is almost independent of the nature
of the material of the surfaces in contact. As the
lubrication becomes less ample, the coefficient of friction
becomes more dependent upon the material of the surfaces.