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Module 1 - More on Drum and Disc Brakes (Page 2 of 3)

Drum Brakes:

Drum brakes use semi-circular brake shoes mounted so that they will contact the inner surface of a metal drum. During operation, the brake shoes are forced against the inner surface of the drum. Since the brake shoes are stationary and the drum is rotating, the contact between the two members creates friction. The kinetic (moving) energy of the vehicle is transformmed into heat and the vehicle slows and enventually stops.

Drum brakes are normally designated according to their operating characteristics. Some of the most common drum brake characteristics are:

1. Self Energizing

The two shoes in a drum brake system are designated as "leading" or "trailing." The shoe that is facing the front of the vehicle is the leading shoe or primary shoe. The shoe that is facing the rear of the vehicle is the trailing shoe or secondary shoe.

In the diagram below, when the shoes contact the rotating drum, friction causes the rotational forces to be transmitted to the leading shoe. The leading shoe cannot rotate because of the anchor. So, the leading shoe is jammed tightly against the drum because of lever action. Thus the leading shoe is self-energizing in the forward direction - it multiplies the application force and forces the shoe tightly against the drum.

Study the diagram below. Raise your hand and ask the teacher for assistance if needed.

2. Servo Action

The above-mentioned self-energizing brakes uses a fixed anchor at the bottom of the backing plate. Servo drum brakes differ by placing the fixed anchor at the top of the backing plate (refer to the diagram below).

When the self energizing action accors in a servo brake drum assembly, the rotational force is transmitted to the trailing (secondary) shoe through the link / adjuster at the bottom of the shoe assemblies.

Therefore the primary or leading shoe applies the secondary shoe through servo action. The secondary shoe performs most of the braking. Therefore, the secondary shoe is usually larger (more friction material) than the primary shoe.

3. Duo Servo

Duo servo drum brakes are perhaps the most popular style of drum braking systems. Quite simply, duo servo drum brakes simply means that one brake shoe helps to apply the other one.

For example, when braking in the forward direction, the primary and secondary shoe is applied by the wheel cylinder. Friction and the rotation of the brake drum causes the primary shoe to jam itself against the secondary shoe. The top of the secondary shoe wedges against the anchor at the top of the backing plate. Therefore the primary shoe helps to apply the secondary shoe in the forward direction. This is how duo servo brakes operate.

Things to know about Duo Servo brakes:

  1. The anchor is always at the top of the backing plate
  2. The self-energizing feature of duo servo brakes works in both directions, forward and backwards
  3. The bottom of the brake shoes are "floating" - that is, they are not solidly mounted to the backing plate.


Study the diagrams below of duo-servo brake operation.

Module 1 (Page 2 of 3)