Drum Brakes a Type of Mechanical Brake
The details of the drum brake a type manually operated mechanical brake system can be seen in the picture
In the motor vehicle, the wheel is attached to a drum. The drum encloses the entire brake mechanism and keeps out dust and moisture. The inner side of the drum is open. The backing plate at the open side of the brake drum completes the brake enclosure and holds the brake assembly. The backing plate is attached to the vehicle axle housing. The backing plate acts as the base for fastening the brake shoes and the operating mechanism. The wheel attaching bolts on the brake drum connect the wheel and the drum.The front coil spring suspension and the drum type brake mechanism in partly disassembled view can be seen in the picture
Construction and working of a drum brake system
The details of the drum brake system can be seen in the picture. The brake shoes which are roughly semicircular are made to contact the inside surface of the brake drum. In the most designs, two shoes are used with each drum to form a complete brake mechanism at each wheel. The brake shoes have brake linings on their outer surfaces. Each brake shoe is hinged at one end by an anchor pin. The anchor pin is fixed to the backing plate. The other end of each shoe rests on a cam. This can be turned by the camshaft which passes through a hole in the backing plate. The camshaft can be operated by the brake pedal through suitable linkages.
When the cam is turned, the brake shoes expand outward, the brake linings come into contact with the brake drum. Brake lining increases the coefficient of friction and also prevents wearing away of the metal. The force of friction is opposite to the direction of drum rotation. The friction between the drum surface and the shoe linings serves to stop or slow down the drum rotation and hence the wheel rotation.
Working of a drum brake
The friction force that comes into action also tends to make the shoes to revolve with the drum. The latter action is prevented by the pin and the cam. The pin is therefore called the anchorage pin.
The magnitude of the friction force, multiplied by the radius of the drum, gives the torque tending to stop the drum, that is, the braking torque. The entire mechanical linkage between the brake pedal and the shoes operates to transmit pedal force to the brake shoes, and to multiply. The pedal force through leverage produces effective braking force against the drum.
The retracting spring are held between the brake shoes. These retracting springs draw the shoes away from the drum when the cam is turned and moved to the release position.
This system in which the shoes are mounted to rub against the inside surface of the brake drum, is called internal expanding brake. In this system, each part of the linkage must be free to move. The joints must be properly lubricated to reduce friction and wear. Otherwise erratic and unequal braking action may result.