Page 152 - Tribology in Machine Design

138 Tribology in machine design

would have the effect of tightening the band, and the brake would be self-
locking.

Here, the direction off must be reversed to tighten the band on the drum.
/0
From the above conclusions it follows that if e >x/y> 1, downward
movement of the force P would tend to slacken the band. Hence for
successful action x must be less than y.
When the brake is used in the manner indicated above there is no relative
sliding between the friction surfaces, so that/is the limiting coefficient of
friction for static conditions. The differential tightening effect of the band
brake is used in the design of certain types of friction brake dynamometers.

4.11.2. The curved brake block

Figure 4.41 represents a brake block A rigidly connected to a lever or
hanger LE pivoted at E. The surface of the block is curved to make contact
with the rim of the flywheel B, along an arc subtending an angle 2\l/ at the
centre, and is pressed against the rim by a force P, at the end L of the lever.
In general, the normal pressure intensity between the contact surfaces will
vary along the length of the arc in a manner depending upon the conditions
of wear and the elasticity of the friction lining material of the brake block
surface. Let
p=the intensity of normal pressure at position 0, i.e. p is a
function of 0 and varies from 0=0 to 0 = 2i//,
a = the radius of the contact surfaces,
b = the thickness of the brake block,
K=the resultant force on the rim due to the normal pressure
intensity p,
$=the inclination of the line of action of R to the position 0=0.

Figure 4.41