Drum Brake

A brief introduction to drum brakes

General characteristics of drum brakes:

Plus (+)

Smaller overall dimensions than "common" disc brakes for a specific braking torque.
No radial forces to consider during design.
Some drum brakes are free from play, which is an important feature if high-precision positioning is required. One such example is simplex brakes.

The brakes consist of a primary and a secondary brake shoe. Which shoe is which depends on the braking direction. The primary shoe takes up most of the torque and its lining therefore wears fastest. Braking often in one direction results in different degrees of wear to the brake shoes.
Each brake shoe is attached at its bottom end to a separate fixed pivot attached to the backing plate.
99% per cent of mechanical brakes are simplex brakes.
The wheel cylinder is bolted in place, but does not take up any forces.
One way of transferring force at the top of the shoes is to use a wheel cylinder with two separate pistons, each pushing outwards on its own brake shoe.
One or more external return springs pull back the shoes to release the applied braking torque.
Simplex brakes are cheaper to manufacture than servo brakes, the design is lighter and changes in friction are not amplified.

DUPLEX brakes are able to handle a slightly higher braking torque.
In principle there are two primary brake shoes.
There is a separate wheel cylinder for each shoe, and the rear of the wheel cylinder acts as a stop. Each brake shoe is naturally attached at its bottom end to a separate fixed pivot attached to the backing plate, a distinguishing feature of Simplex brakes.

Each brake shoe is linked to the other and the brake shoes are floating. The torque is taken up by a wheel cylinder or mechanical stop.
Servo brakes must not be fitted to steering front wheels since the braking action can vary depending on external conditions. This design is sensitive to variations in the coefficient of friction.
Servo brakes are self-amplifying.
Higher torque than simplex brakes for the same size of wheel cylinder.
Servo brakes are generally chosen for slow-moving vehicles that do not travel faster than 30 km/h, but they should not be ruled out for faster vehicles.
More expensive to manufacture than simplex brakes and a more complex design.
Typical applications: Heavy vehicles, fork lift trucks, used on trailers at speeds in excess of 30 km/h.