Page 200 - Tribology in Machine Design

Sliding-element bearings 185

depend entirely upon the oil pressure at the inlet, and that this pressure will
largely govern the carrying capacity of the bearing. For the purposes of
design calculation it may be assumed that the film is in a state of simple
torsional shear.

Numerical example
A vertical shaft of 75 mm diameter rests in a footstep bearing and makes
750 r.p.m. If the end of the shaft and the surface of the bearing are both
perfectly flat and are assumed to be separated by a film of oil 0.025 mm
thick, find the torque required to rotate the shaft, and the power absorbed
in overcoming the viscous resistance of the oil film. The coefficient of
3
viscosity of the oil is // =40 x 10 ~ Pa s.

Solution
Let

then

and

Considering an elementary ring of radius r and width dr,

and so

or