The Brake System:
Rotating & Non Rotating Units
The brake drum is mounted directly onto the wheel and provides the rotating braking surface. The brake shield, sometimes known as the backing plate or dust shield, is mounted on some fixed structure such as the axle housing. The brake shield forms a support for the non rotating braking surface (brake shoes) and its operating mechanism.
The brake shoes may be anchored to the brake shield by separate pins or the same pin. Springs or clips are usually used to hold the shoes close to the brake shield and to prevent them from rattling. A fairly strong retracting spring is hooked between the shoes to pull them away from the drum when the brakes are released.  With a mechanical hookup, pressure can be applied to the brake shoes by means of a cam, toggle, or double-lever arrangement.
A cam turned by a small lever is the method most frequently used. Turning the cam by the lever tends to spread the brake shoes and push them outward against the drum. With the hydraulic system, pressure is applied to the brake shoes by means of a cylinder and pistons. |