What is?

It's a component of the braking system that automatically corrects the gap between the brake's shoes and the drums, as the shoes wear down due to use.

Advantages

Besides ensuring fast, efficient and safe answers on breaking, the automatic brake adjuster has other advantages:

Minor wearings;
More saving and less consumed air, which increases life span of the compressor;
The components of the air-cooling system, the brake system and the tires have more durability;
The vehicle stays less time in repair.

How does it work?

A box has its extremities linked to the master cylinder's wire, and transmits the movement to a gear, that is installed inside it and fixed to the S-cam shaft. An adjusting lever (control unit) compares the distance covered by the box after the brake pedal is pushed and corrects the exceeding gap.

This process is divided in three stages

1st - Regular stroke: indicates the gap between the brake's shoes and drums, allowing the wheels to spin freely while the brake pedals are not pressed.
2nd - Excessive stroke: happens due to the wearing of the brake's shoes and drums and is automatic and constantly corrected by the adjuster.
3rd - Elasticity stroke: this stroke appears when the brake pedal is pressed, and its purpose is to compensate the drum's elasticity and expansion, caused by friction.

IMPORTANT: The regular stroke is determined by the vehicle's manufacturer and can vary in each vehicle. If the measured stroke is different from the recommended by the manufacturer, it means that the adjuster is not functioning correctly.

Initial position:

Traffic conditions: On this position the brakes are not operating and internally the rack end (C) is leaned on the slot's upper edge (A) of the control unit (U).

Braking start:

1st Stage: Regular stroke
On this movement the box moves and inside it the rack end leans on the slot's lower edge of the control unit. The shoes expand and must reach the drum.

2nd Stage: Excessive stroke (gap larger than recommended)
When this movement continues, it means that the gap is bigger than it should be. The rack cannot advance because it is leaned on the low corner of the control unit's disc groove. The adjuster's movement pushes the one-way gear beyond the rack's teeth. The gear can rotate freely over the drive spring. In this movement, the "S" shaft expands the shoes until the shoes reach the drum.

3rd Stage: Elasticity stroke
When the brake shoes are progressively pressed against the drum, the torque on the worm shaft and the gear increases quickly. This power transmitted by the gear presses the worm, that compresses the compression spring, pushing away the worm shaft.

Elasticity Rotation (compensation of elasticity and the drum's dilation):
While the adjuster is spinning, the rack cannot move, as it's anchored on the control unit's disc groove. Now, the gear movement over the rack makes the gear set spin freely, because the worm shaft is separated from the set. This is necessary for the adjustment not be affected by elasticity and brake drum dilation.

(3rd Stage) Return rotation:
At this stage the torque on the adjuster end, which starts returning, decreases. The compression spring pushes the worm until it reaches the coupling ring. Return springs keep the rack end in contact with the lower part of the control unit's disc groove. Then, the rack reverses the stroke.

(2nd Stage) Return rotation:
That's when the gap adjustment takes part. The rack cannot move, as its end is pressed against the lower edge of the control unit's disc slot. While the adjuster returns, the rack turns the coupling to the only direction that the worm turns, making the gear and the "S" shaft spin with the shaft, advancing the "S" shaft angle and correcting the shoes excessive gap.

(1st Stage) Return rotation:
The adjuster keeps returning and the rack stops, as its end is not in contact with the control unit's disc groove.