Page 151 - Tribology in Machine Design

Friction, lubrication and wear in lower kinematic pairs 137

Again, suppose x and y are the perpendicular distances of the fulcrum F
from the lines of action of T 2 and Tj respectively. It is assumed that the
brake is used in such a manner as to prevent the rotation of the drum when
the crane is carrying a load Q attached to a rope passing round the
circumference of the barrel. If a force P at leverage d is necessary to support
this load, then

The relation between the effective tensions TI and T 2 is given by

where/is the coefficient of friction for the contact surfaces and 0 is the angle
of wrap of the band round the drum. Hence, combining the above
expressions

and

so that

To study the effect of varying the ratio x/y on the brake action, no\
consider the following cases:

Here the line of action of T 2 passes through F and a downward movement
of the force F produces a tightening effect of the band on the drum.

In this case there is no tightening effect since the displacements of A and B in
the directions of T 2 and T 1 are equal in magnitude. Hence, to maintain the
load the band would have to be in a state of initial tension.

For this ratio a small movement of the lever in the negative direction of P,