1、 The elasticity of the piston return seal ring is insufficient, which causes drag. Unlike the drum brake, the disc brake has a special brake shoe return spring. Its return depends on the deformation elasticity of the piston seal ring in the wheel cylinder. After return (as shown in the figure), the unilateral clearance between the friction plate and the brake disc is 0.05-0.5mm, while the brake clearance of the drum brake is generally 0.03-0.6mm. When the piston seal ring has a problem with its material, such as hardness, It is specified that 70+5 degrees. If low-quality piston seal rings are used, low hardness will affect piston return, causing drag, severe wear on the brake disc surface, and deep wear marks and grooves. When braking, the contact surface will not be well contacted, causing jumping and vibration during braking, which will cause squeal.
2、 The damping gasket falls off or fails
Generally, a damping gasket is attached between the lining plates of two friction blocks and the piston of the disc brake. The damping gasket is formed from 0.5-0.8 mm quenched cold rolled steel plate, and the rubber layer is coated on both sides to cushion and reduce the vibration frequency of the friction block during braking. If the damping gasket falls off or fails, the brake squeal will increase
3、 Glazed surface of friction block
The brittle and bright glaze layer on the friction block surface has a lower friction coefficient than the normal friction block, which will not only produce squeal, but also reduce the braking efficiency. In general, glaze gloss is caused by frequent emergency braking (low-quality friction materials), or because the friction block surface is polluted by oil.
For vehicles with "front disc and rear drum" braking configuration, the braking force of the front wheel disc brake accounts for about 60% - 80% of the total braking force (the ratio of front wheel driven vehicles is high), so the brake squeal of the front wheel (especially unilateral squeal) should be paid attention to.
The following reasons
a. When the brake disc rotates, the working surface runout exceeds 0.05mm, the brake disc is deformed or the surface is defective;
b. The brake oil leaks and pollutes the friction block surface due to the damage of the piston sealing ring of the cylinder block of the brake caliper assembly and the crack of the cylinder block.
C. The special pipe is not used for the exhaust of the caliper body assembly, which causes the friction block to be stained when the brake oil escapes.
d. The hub oil seal is damaged, causing the lubricating grease to contaminate the friction block.