Drum Brakes (Self-Actuating Brakes)

Drum brakes work with the help of brake shoes or brake pads that use frictional force between the pads and the drum to stop the wheel. The brake pads are pressed against the inner surface of the drum to create the frictional force.

The drum brake working principle was first patented by Louis Renault, a French industrialist, in the year 1902. However, the first drum brakes were used by Maybach in the year 1900.

Design:



A drum brake consists of various components:

• A pair of brake pads with linings made of materials having high frictional coefficient. The brake pads are hinged either at a single point or independent locations not far from each other. The other ends remain in contact with the pistons in the wheel cylinder.

• Wheel drum which rotates along with the wheels. It is usually made of cast iron.

• A wheel cylinder which has a pair of pistons for each brake pad. When brake pedal is depressed, oil pressure increases and acts on the pistons. The pistons force the brake pads to press against the wheel drum.

• Brake return springs allowing the brake pads to return to their original positions once the brake pedal is released.

• Drum brakes also have an emergency brake system which can be operated through a lever by a hand brake.

• Self adjusting system to automatically re-adjust the distance required for the brake pads to travel to press against the wheel drum when brakes are applied. Brake pads start wearing with time, so self adjusting mechanism will help in changing the distance. This is the reason why they are also called self actuating brakes.

Working:

When the brake pedal is depressed by the driver, the master cylinder pressurises the hydraulic oil and sends this hydraulic pressure to the wheel cylinder. This hydraulic pressure acts on the double-acting pistons which push the brake pads outside. The brake pads press against the inner surface of the wheel drum and create friction. This friction between the brake pads and the drum brings the wheels to rest.

Once the brake pedals are released, the return springs pull the brake pads back to the original position.

Self-Actuating Mechanism:

Drum brakes are also known as self-actuating brakes. As the brake pads wears out, they have to  travel a greater distance to press against the wheel drum. Therefore, self adjusting mechanism is used to automatically re-position the resting point of the hinges on which the brake pads are hinged, so as to get them closer to the drum.

Advantage of using a drum brake:

There is only one major advantage of drum brakes, it is cheap.

Disadvantages of a Drum brake:

• Complicated design, use of several components compared to a disc brake.


• Requires regular service, especially the brake pads which wear out quickly and needs to be replaced.