Page 135 - Tribology in Machine Design

Friction, lubrication and wear in lower kinematic pairs 121

then

where Wis the weight of each shoe and r is the radial distance of the centre
of gravity of each shoe from the axis.
At the commencement of engagement R =0 and the angular velocity of
rotation is

so that

At a speed of 750 r.p.m.,

The couple due to each shoe

and finally,

4.9. Boundary Under boundary lubrication conditions the surfaces are considered to be
lubricated sliding technically dry or only slightly lubricated, so that the resistance to relative
bearings motion is due to the interaction between the highest asperities covered by
the boundary film. Then, frictional force F =fR, where / is the kinetic
coefficient of friction. The magnitude of the friction couple retarding the
motion of the journal is determined by the assumed geometric conditions of
the bearing surface.

Case A. Journal rotating in a loosely fitting bush
Figure 4.25 represents a cross-section of a journal supporting a load Q at
the centre of the section. When the journal is at rest the resultant from
pressure will be represented by the point A on the line of action of the load
Figure 4.25 Q, i.e. contact is then along a line through A perpendicular to the plane of