How does the airbrake differ from the hydraulic?

Hydraulic brakes are applied by liquid pressure. The pedal force is transmitted to the brake shoe by means of a confined liquid through a system of force transmission. The force applied to the pedal is multiplied and transmitted to all the brake shoes by a force transmission system.

This system is based upon Pascal's principle, which states that "The confined liquids transmit pressure without loss equally in all directions".

The brake system essentially consists of two main components — master cylinder and wheel cylinder. The master cylinder is connected by tubing to the wheel cylinders at each of the four wheels. The system is filed with the liquid under light pressure when the brakes are not in operation.

The liquid is known as brake fluid and is usually a mixture of some components. Each wheel brake consists of a cylinder brake drum which is mounted on the inner side of the wheel and revolves with it and two brake shoes which are mounted inside the brake drums and do not rotate.

The shoes are fitted with a heat and wear resisting brake lining on their surfaces.The brake pedal is connected to the master cylinder piston by means of a piston rod. When the brakes are to be applied, the driver depresses the pedal, the piston is forced into the master cylinder, thus increasing the pressure of the fluid in the master cylinder and in the entire hydraulic system.

This pressure is conducted instantaneously to the wheel cylinders of each of the four brakes, where it forces the wheel cylinder pistons outwards.

These pistons, in turn, force the brake shoes out against the brake drums. Thus the brakes are applied. When the driver releases the brake pedal, the master cylinder piston returns to its original position due to the return spring pressure, thereby lowering the pressure of the liquid thereby releasing the brakes.

Power brakes are also known as power assisted brakes. The above hydraulic system is applicable for small vehicles. But in the case of vehicles fitted with disc brakes and heavy commercial vehicles, a greater braking force is required. Power assisted brakes are used to equate the limited strength of the driver with the great braking force.

Source : The Hindu