When the brake shoe does not work in its original position, there should be an appropriate gap between the friction pad and the brake drum. Its setting value is stipulated by the automobile manufacturer, generally between 0.25 and 0.5 mm. If the clearance in any brake friction pair is too small, it will not be easy to ensure that the brake is completely released, resulting in friction pair traction. Too large will also make the brake pedal travel too long, so that the driver is inconvenient to operate, and will delay the brake start to work. However, during the working process of the brake, the continuous wear of the friction disc will cause the brake clearance to increase gradually. When the situation is serious, even if the brake pedal is stepped down to the limit position, it will not produce enough braking moment. Most cars are equipped with brake clearance self-adjusting device, but some trucks still use manual adjustment.

Brake clearance adjustment is an important item in automobile maintenance and repair. According to the different working process, it can be divided into two types: primary alignment and step adjustment.

The limiting friction ring 2 is used to define the inner limit position of the brake shoe without braking, which is installed in the ring groove of the inner end of the cylinder piston 3. The width of the ring groove or spiral groove on the piston is larger than the thickness of the limiting friction ring. The maximum axial displacement of the piston relative to the friction ring is the gap between the two. The clearance should be equal to the cylinder piston stroke required for complete braking when the brake clearance is a set standard value.

When braking, the cylinder piston moves out. If the brake clearance increases beyond the set value due to various reasons, the piston moves out to 0 and can not achieve complete braking, but as long as the cylinder pushes the piston along with the friction ring, until the complete braking is realized. In this way, when the brake is released, the brake shoe can only be restored until the piston contacts the limit friction ring in the new position, which means that the brake clearance is a set value.

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