Construction

As shown in figure, a disc brake consists of a cast iron disc bolted to the wheel hub and a stationary housing called caliper. The caliper 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 Piston and the disc, there is a friction pad held in position by retaining pins, spring plates etc. Passages are drilled in the caliper for the fluid to enter or leave is housing. These passages are also connected to another one 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, applying equal and opposite forces on the later. On releasing the brakes, the rubber sealing rings act as return springs and retract the piston and the friction pads away from the disc.

Constructional features

1. Calipers :- Brake caliper is the Assembly which houses the brake pads and pistons. Basically there are two types of calipers viz., the fixed and the floating types. A fixed caliper does not change its position relative to the disc. It employs one or more pairs of pistons to clamp each side of the disc. The floating caliper may further be classified as the swinging Caliper type and the sliding caliper type. In swinging caliper type the caliper is hinged about a fulcrum pin and one of the friction pads is fixed to the caliper. The fluid under pressure presses the other pad against the disc to apply the brake. The reaction on the caliper causes it to move the fixed pad inward slightly, applying equal pressure to the other side of the disc. The caliper automatically adjusts its position by swinging about the pin. In the Sliding Caliper 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 calipers. Both these types are self adjusting and have resulted in simpler and lighter construction.

2. Pistons :- The most common brakes use a single hydraulically actuated Piston within a cylinder in the caliper. Some high-performance brakes may have as many as eight pistons. Pistons are generally made of aluminium or chrome plated iron.

Some large piston 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.

3. Discs :- The discs of the brakes have been conventionally made of pearlitic grey cast iron. The material is cheap and has good antiwear properties. Cast steel discs have also been employed in certain cases, which wear less and provide higher coefficient of friction, yet the big drawback in their case is the less uniform frictional behaviour.

Recently materials like ceramics and carbon fibre have also been found their way into Automotive brakes. 'Porsche Carrera' is the first road car in which these materials have been used for brake disc, though these have been used commonly in racing cars. 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 disc have been employed in various makes of disc brakes that is the solid or the ventilated type. The ventilated type, no doubt, provides better cooling. It is seen that using a ventilated disc results in reduction of about 30% in the pad temperature, as compared to solid discs. This results in longer pad life. However, the ventilated discs also have certain disadvantages. They are usually thicker and even sometimes heavier than the solid disc. In case of severe breaking conditions, they are liable to wrap and moreover, the dirt accumulates in the vents, which, apart from affecting cooling, results in wheel imbalance.

4. Brake pads :- The brake pads are designed for high friction.

In many vehicles a provision is made to alert the driver when the need to replace the brake pads due to wear arises. In some there is a thin piece of soft metal which causes the brakes to squeal when the pads become two thin, while 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 instrument panel, indicating to the driver that the pads need replacement.