Page 125 - Tribology in Machine Design

Friction, lubrication and wear in lower kinematic pairs 111

4.5. Plate clutch - A long line of shafting is usually made up of short lengths connected
mechanism of operation together by couplings, and in such cases the connections are more or less
permanent. On the other hand, when motion is to be transmitted from one
section to another for intermittent periods only, the coupling is replaced by
a clutch. The function of a clutch is twofold: first, to produce a gradual
increase in the angular velocity of the driven shaft, so that the speed of the
latter can be brought up to the speed of the driving shaft without shock;
second, when the two sections are rotating at the same angular velocity, to
act as a coupling without slip or loss of speed in the driven shaft.
Referring to Fig. 4.16, if A and B represent two flat plates pressed together
by a normal force R, the tangential resistance to the sliding of B over A is
F =fR. Alternatively, if the plate B is gripped between two flat plates A by
the same normal force R, the tangential resistance to the sliding of B
between the plates A is F = 2fR. This principle is employed in the design of
disc and plate clutches. Thus, the plate clutch in its simplest form consists of
an annular flat plate pressed against a second plate by means of a spring,
one being the driver and the other the driven member. The motor-car plate
clutch comprises a flat driven plate gripped between a driving plate and a
presser plate, so that there are two active driving surfaces.

Figure 4.16

Multiple-plate clutches, usually referred to as disc clutches have a large
number of thin metal discs, each alternate disc being free to slide axially on
splines or feathers attached to the driving and driven members respectively
(Fig. 4.17). Let n = the total number of plates with an active driving surface,
including surfaces on the driving and driven members, if active, then;
(n— l) = the number of pairs of active driving surfaces in contact.
If F is the tangential resistance to motion reduced to a mean radius, r m,
for each pair of active driving surfaces, then

The methods used to estimate the friction couple Fr m, for each pair of active
surfaces are precisely the same as those for the other lower kinematic pairs,
such as flat pivot and collar bearings. For new clutch surfaces the pressure
intensity is assumed uniform. On the other hand, if the surfaces become
worn the pressure distribution is determined from the conditions of
Figure 4.17 uniform wear, i.e. the intensity of pressure is inversely proportional to the