Construction of a Drum Brake


1. Construction

The drum brake consists of the following components.

(1) Wheel cylinder

(2) Brake shoe

(3) Brake lining

(4) Return spring

(5) Brake drum

(6) Piston

(7) Piston cup

2. Operation

The drum brake stops the tire from rotating by using the hydraulic pressure transmitted from the master cylinder to the wheel cylinder to press the brake shoe against that brake drum, which is rotating together with the tire. When the hydraulic pressure to the wheel cylinder disappears, the force of the return spring pushes the shoe away from the inner surface of the drum and returns it to is original position. As the brake shoe is surrounded by the brake drum, it is difficult for the generated heat to dissipate. This type of brake is less resistant to heat.

3. Leading shoe and trailing shoe

When hydraulic pressure is applied to the wheel cylinder, the shoes on both sides of the drum are pressed against the inside of the drum with a force commensurate with the hydraulic force applied by the piston. As shown in the figure at left, different compression forces are generated in the right and left shoes. The frictional force causes the shoe on the left side to bite into the drum in the direction of rotation, while conversely the shoe on the right side receives the repulsion force of the rotation drum, which reduces the compression force. The action that increases the frictional force by biting into the drum is called the self energizing function, and the shoe receiving this function is called the leading shoe, and the shoe not receiving this function is called the trailing shoe.

4. Types of the drum brake

Drum brake has different types, depending on the combination of the leading and trailing shoe. Use properly, depending on the purpose, with the feature that is generated by the leading and trailing shoe.

• Leading-and-trailing type

•Two-leading type

• Uni-servo type

• Duo-servo type

Blue arrow:

Rotational direction of the wheel

Pink arrow:

Direction that the piston moves

5. Clearance Adjustment

(1) Auto adjustment type The lining affixed to the surface of the brake shoe is worn down as the brakes are used. The gap between the drum and lining must be periodically adjusted to maintain the proper pedal depression stroke. Auto adjustment type brakes automatically adjust the gap. The auto adjustment type operates when the parking brake is applied or during braking, and in either case an adjuster is turned by an adjust lever to adjust the gap. (2) Manual adjustment type Measure the inner diameter of the brake drum. Turn the adjuster to adjust the outer diameter of the brake shoes so it is approximately 1mm (0.039 in.) smaller than the inner diameter of the brake drum. Using a screwdriver, turn the adjusting nut and expand the shoes until the drum locks. Turn the adjusting nut back the specified number of notches. Refer to the repair manual for the specified number of notches.

Brake Pedal Height Adjustment

Use a ruler to measure the brake pedal height. If it is out of the prescribed range, adjust the pedal height. Strokes are required to attain the proper braking force. Adjust brake so that they are not working when pedal is not depressed. With the engine stopped, depress the brake pedal several times in order to deactivate the brake booster. Then, gently press the pedal with your finger and measure the pedal freeplay with a ruler.

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