Drum Brake System: Description and Operation

Application of the brakes causes the wheel cylinder piston to force the leading edge of the primary shoe and lining assembly into contact with the rotating drum. The shoe tries to rotate with the drum and transfers force to the secondary shoe and lining assembly through the star-wheel adjuster. The secondary shoe's lining leading edge "bites" into the drum and tries to rotate, just like the primary shoe. Since the shoes cannot rotate, they wedge themselves into the drum. The rotating torque from the shoes increases the braking force applied by the wheel cylinder. Because of this wedging action, the design is a duo-servo, as opposed to a single-servo design where the wheel cylinder pressure alone is the source of braking force. The torque from the brake shoes is transferred through the backing plate to the axle flange. Brake adjustments are automatic and occur during reverse brake applications.