11 September 2018

Disc brakes | Construction | Features

Construction : 

A disc brake can be made of a cast-iron disc bolted to the wheel hub and a stationary housing called calliper. The calliper is connected to some stationary part of the vehicle, like the axle casing or the stub axle and is cast in two parts, each part containing a piston. 

In between each disc and the piston, there is a friction pad held in position by retaining pins, spring plates etc. Passages are drilled for the fluid in the calliper to enter or leave each housing. These passages are also connected to another passage for bleeding. Each cylinder contains a rubber sealing ring between the cylinder and the piston. 

When the brakes are applied, hydraulically actuated pistons move the friction pads into contact with the disc, and later applying equal and opposite forces. On releasing the brakes, the rubber sealing rings act as return springs and retract the friction pad and pistons away from the disc. 

Disc brake

For this type of brake, 

T  = 2 µ p a R
µ = fluid pressure 
a = cross-sectional area of one piston 
R = distance of the longitudinal axis from the wheel axis of the piston 

For special types of disc brakes include the swinging calliper type and the sliding calliper type. 

In-swinging calliper type the calliper is hinged about a fulcrum pin and one of the friction pads is fixed to the calliper. The fluid under pressure presses against the disc to the other pad to apply the brake. The reaction the calliper causes it to move the fixed pad inward slightly, applying equal pressure to the other side of the disc. The calliper automatically adjusts its position by swinging around the pin. 

In the sliding calliper type, there are two pistons between which the fluid under pressure is sent which presses on friction pad directly onto the disc, whereas the other pad is pressed indirectly via the calliper. Both these types are adjusting and have resulted in a simpler and lighter construction. 

Features : 

The discs of the brakes have made of pearlitic grey cast iron. The material is anti-wear properties. Cast steel discs have also been employed in certain cases, which are still less and provided a higher coefficient of friction, yet the big drawback in their case is the less frictional behaviour. 

Recently material like ceramics and carbon fibre have also found their way into automotive brakes. The greatest advantage of these materials is the weight reduction which greatly improves vehicle dynamics and steering characteristics, due to the reduction in unsprung mass. 

Two types of discs have been employed in various makes of disc brakes, for example, solid or the ventilated type. The ventilated type no doubt provides better cooling. It is seen that using a ventilated disc results in a reduction of about 30% in the pad temperature, as compared to solid discs. This results in a longer pad life. Whereas the ventilated discs also have certain disadvantages. They are usually thicker and even sometimes heavier than the solid discs. 

Some disc brakes use such friction pads as contain metallic contact inside. The contact is exposed when the pad wears down to the specified thickness and thereby completes an electrical circuit which lights up a warning lamp on the instrumental panel, indicating to the driver that the pads need replacement. 

Some large pistons for disc brakes are made of hard plastic whereas in some other cases, these are precision ground and plated with nickel-chrome which provides them with a hard surface which is durable.