Page 163 - Tribology in Machine Design

Friction, lubrication and wear in lower kinematic pairs 149

Using the same numerical data given in the previous section

or

If (L! + L 2 ) exceeds this value, skidding will occur.

The maximum retardation

Suppose that limiting friction is reached simultaneously on all the wheels,
so that

then, referring all the forces, for the system as a whole, to the centre of
gravity, G

or, more accurately, in accordance with eqn (4.152)

so that,

Hence from eqns (4.163) and (4.164)

and
maximum retardation —fg.

Under running conditions the braking torques on the front and rear axles
may be removed in a relatively short time interval, during which the
retardation remains sensibly constant. As L 2 is reduced, F 2 diminishes also
and passes through a zero value when L 2 = / 2a. For smaller values of L 2 it
becomes negative; when L 2 =0 the angular retardation of the wheels is due
entirely to the tangential friction force.
If, through varying conditions of limiting friction at either of the front
wheels, or because of uneven wear in the brake linings, the braking torques
on the two wheels are not released simultaneously, a couple tending to