The working principle of a drum brake system involves converting hydraulic pressure from the brake pedal into mechanical force to stop or slow down the wheel.
1. Pressing the Brake Pedal
When the driver presses the brake pedal, it activates the master cylinder.
The master cylinder pushes brake fluid through the brake lines towards the wheel cylinders in each drum brake.
2. Hydraulic Pressure in the Wheel Cylinder
The brake fluid creates hydraulic pressure in the wheel cylinder, located inside the drum brake assembly on each wheel.
The wheel cylinder contains two pistons positioned at either end.
3. Expanding Brake Shoes
Hydraulic pressure pushes the pistons outward, causing the brake shoes to move away from each other.
The brake shoes are lined with friction material and are mounted with a slight arc, allowing them to contact the drum surface evenly.
4. Contact with the Brake Drum
The outward movement of the brake shoes forces them to press against the inner surface of the drum, which rotates along with the wheel.
Friction is generated between the brake shoes and the rotating drum. This friction slows down the rotation of the drum and, consequently, the wheel attached to it.
5. Self-Energizing Effect
In a drum brake, the frictional force between the shoes and the drum often creates a self-energizing effect.
As the drum rotates, it pushes the leading edge of one shoe harder against the drum, amplifying the braking force with minimal pedal effort.
6. Deceleration and Stopping
The friction force generated between the shoes and the drum converts the vehicle's kinetic energy into heat, slowing down the wheel.
The braking process continues until the wheel slows down to the desired speed or stops completely.
7. Releasing the Brakes
When the driver releases the brake pedal, the hydraulic pressure in the wheel cylinder drops.
Return springs attached to the brake shoes pull them back to their original positions away from the drum.
This releases the friction between the brake shoes and drum, allowing the wheel to rotate freely again.
8. Heat Dissipation
During braking, a large amount of heat is generated due to friction.
Unlike disc brakes, drum brakes are enclosed, which limits airflow and makes them less effective at dissipating heat.
This can lead to brake fade in situations where continuous braking is required, as the drum brake’s friction surfaces can overheat and lose effectiveness.
Advantages:
Economical to manufacture and maintain.
More effective as a parking brake due to the self-energizing action.
Usually has a longer lifespan under light braking conditions.
Disadvantages:
Poor heat dissipation compared to disc brakes, leading to brake fade.
Requires more effort to stop the vehicle, especially under heavy braking.
More prone to wear in high-heat or heavy-use conditions.
Applications:
Commonly found on the rear wheels of some cars and in many heavy vehicles.
Often used in applications where cost and durability are more critical than braking performance.