Drum Brake

Function

In comparison with the disk brake, the drum brake has smaller actuating forces due to a larger self reinforcement with the same braking power. Other advantages of the drum brake are the longer service intervals and a longer durability of the linings.

How it works

Inside the brake drum manufactured from cast iron or steel – in this case featuring notches for the pulse generator of the ABS – there are two sickle-shaped brake shoes that can be pressed out by a spreading mechanism. The brake shoes are connected to the suspension via the brake anchor plate, this way the braking forces are transferred to the wheel. A double acting wheel cylinder (simplex drum brake) operates the brake shoes, pressing the friction lining of both the leading and trailing brake shoe against the inside of the brake drum.
In the picture on the very top the brake drum is dismantled. In the middle, one recognizes the wheel brake cylinder which transfers the pressure of the brake pedal as force via two pistons to the respective brake shoe. Each brake shoe is firmly tied to the drum brake at the bottom, thus they are opened out and pressed against the friction lining. Parallel to the wheel brake cylinder the return springs are situated, providing the reset of the brake shoes after braking. A special feature of this simplex drum brake is that the two brake shoes affect their respective friction linings differently. Under the assumption that the driving direction is to the left, the left shoe is the leading shoe, braking stronger than the trailing shoe to the right. This is, because the leading shoe functions as a wedge with the clockwise rotating drum. Especially if the vehicle is equipped with ABS, this effect is undesirable, because it is essential to loosen the brakes quickly. To counter this effect and to obtain even wear, the right trailing shoe has clearly a bigger lining surface. At the right shoe the hand-brake lever for the hand-brake can be recognized. The hand-brake cable is visible, too, coming from the left, it is hooked in with the hand-brake lever. The hand-brake lever supports itself through a connecting rod against the left shoe. Additionally, the connecting rod carries the mechanism for the automatic self-adjustment. This devise keeps a constant clearance between the brake shoes and the brake drum. The self-adjustment devise is equipped with an adjusting screw and an adjusting dog, which moves out the adjusting screw by approximately 0.02 mm per brake procedure. If the brake is used frequently and exceeds a temperature of 80°C, a bimetal stripe prevents the automatic adjustment, thus offsetting the expansion of the brake drum due to the temperature.