Page 126 - Tribology in Machine Design

112 Tribology in machine design

radius. Let r l and r 2 denote the maximum and minimum radii of action of
the contact surfaces, R =the total axial force exerted by the clutch springs
and n a = (n— l) = the number of pairs of active surfaces.

Case A, uniform pressure intensity, p

Case B, uniform wear; pr — C
If p 2 is the greatest intensity of pressure on the friction surfaces at radius r 2,
then

Comparing eqns (4.37) and (4.39), it is seen that the tangential driving force
F =fR can be reduced to a mean radius, r m, namely

Numerical example
A machine is driven from a constant speed shaft rotating at 300r.p.m. by
means of a friction clutch. The moment of inertia of the rotating parts of the
2
machine is 4.6 kgm . The clutch is of the disc type, both sides of the disc
being effective in producing driving friction. The external and internal
diameters of the discs are respectively 0.2 and 0.13m. The axial pressure
applied to the disc is 0.07 MPa. Assume that this pressure is uniformly
distributed and that the coefficient of friction is 0.25.
If, when the machine is at rest, the clutch is suddenly engaged, what
length of time will be required for the machine to attain its full speed.