Page 136 - Tribology in Machine Design

122 Tribology in machine design

the section. When rotating commences, we may regard the journal as
mounting the bush until the line of contact reaches a position C, where
slipping occurs at a rate which exactly neutralizes the rolling action. The
resultant reaction at C must be parallel to the line of action of Q at 0, and the
two forces will constitute a couple of moment Q x OZ retarding the motion
of the journal. Further, Q at C must act at an angle 0 to the common normal
CN and, if r is the radius of the journal

hence,

The circle drawn with radius OZ = r sin 0 is known as the friction circle for
the bearing.

Case B. Journal rotating in a closely fitting bush

A closely fitted bearing may be defined as one having a uniform distribution
of radial pressure over the complete area of the lower part of the bush (Fig.
4.26). Let

p=the radial pressure per unit area of the bearing surface,
Q— the vertical load on the journal,
Figure 4.26 I —the length of the bearing surface.
Then,

and substituting for Q,

For the purpose of comparison take case A as the standard, and assume
boundary conditions of lubrication /= 0.1, so that

and

In general, we may then express the friction couple in the form/VQ, where/'
is defined as the virtual coefficient of friction, and for the closely fitting bush