Subject: Automobile Engineering

Topic: Brakes, Steering & Front Axles

Difficulty: Low

A disc brake consists of a cast iron disc bolted to the wheel hub & a stationary housing called caliper. The caliper is connected to some stationary part of the vehicle, like the axle casing or the stub axle & is cast in two parts, each part containing a piston. In between each piston & the disc, there is a fraction pad held in position by retaining pins, spring plates etc. Passages are drilled in the caliper for the fluid to enter ar leave each housing. These passages are also connected to another one for bleeding. Each cylinder contains a rubber sealing ring between the cylinder & piston.(fig.3.1).
When the brakes are applied, hydraulically actuated pistons move the piston friction pad into contact with the disc, applying equal & opposite force on the later. On releasing the brakes, the rubber sealing rings act as return spring & retract the pistons & the friction pads away from the disc. For a brake of this type,
$T=2\,\mu\, p\, a\, R$
where : p= fluid pressure a= cross- sectional area of one piston R= distance of longitudinal axis of piston from wheel axis.