Introduction to Drum brakes
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A drum brake is a brake that uses friction caused by a set of shoes or pads that press outward against a rotating cylinder-shaped part called brake drum.
Drum brakes are mainly located on the rear axle of a vehicle. There are usually no brake pads in drum brakes to produce friction. A drum brake consists of wheel cylinders attached to pistons. These pistons exert pressure on the inner wall of a spinning drum.
How Does a Drum Brake Work?
A drum brake system works as follows:
A driver applies brake pedals.A brake booster or servo system amplifies the power generated.There is a master cylinder that converts this power into hydraulic pressure, also known as oil pressure. There is a brake oil or brake fluid that conducts this pressure to the pistons on individual wheels.These pistons exert pressure on the brake linings.Brake linings produce friction against the inner surface of a brake drum.Since a brake drum rotates with vehicle’s wheels, it gradually slows them down and ultimately stops the vehicle.
Drum brakes consist of the following parts:
1. Backing Plate
The backing plate, also known as the torque plate, serves as a base for the other parts such as brake drums, wheel cylinders, and brake shoes. It plays a key role in absorbing the torque from the braking action. When a driver presses the brakes, maximum pressure is exerted on the backing plate.
Backing plates must be wear-resistant to lower the friction and operate as smoothly as possible. Additionally, it must be strong enough to support the other components. It protects the drum brake from dust and road debris as well as increases the rigidity of drum brakes. Recently, automatic brake adjusters and levers were added for emergency brakes to enhance their performance.
2. Brake Shoes
Brake shoes comprise two steel pieces welded together. It has a crescent-shaped piece containing holes and slots of different shapes. This supports the return springs, parking brake linkage, hold-down hardware and self-adjusting components. The friction-causing material is attached using adhesive or riveted to the inner lining.
The wheel cylinder passes on the force to the brake shoes through inner lining. They comprise two types of shoes, primary and secondary. There are three “V” – shaped notches called ribs on the edge of the lining table.
3. Wheel Cylinder
There is a separate wheel cylinder operating on each wheel. The leading shoe that is present closer to the front of the vehicle is known as the secondary shoe. On the contrary, the trailing shoe is the primary shoe.
At the end of each wheel, there are two different pistons operating the shoes. There is a master cylinder that generates hydraulic pressure on the piston cup, pushing them downwards. This forces them against the drum. The brake shoe restores them to their original positions as soon as a driver releases the brakes.
4. Brake Drum
It generally rotates along with the wheel and axle. Wear-resistant and heat-conductive cast iron is used for the construction of a brake drum. As soon as a driver presses the brakes, the lining exerts a radial force on the drum's inner surface. This ensures that the wheel axle slows down and ultimately stops. The friction generated during this process is strong enough to stop the vehicle.