How does an automobile drum brake work?

by:DEFUS     2020-05-26
Drum brakes have a large braking force, so they are mostly used in light trucks and heavy vehicles. The current car also uses the drum system, only in the form of a front disc and a rear drum. In addition to the low cost of the drum brake, it is also easy to integrate with the parking brake. For cars with drum brakes on the rear wheels, the parking brake is basically combined with the rear brake. The advantages of drum brakes During the braking process, due to the inertia, the load of the front wheels usually accounts for 70% to 80% of the total load of the car. The braking force of the front wheels is greater than that of the rear wheels, and the rear wheels play an auxiliary role. The cost of the drum-type automatic is low, the braking force is large, and the durability of the brake shoe is higher than that of the disc-type automatic. Therefore, it is mostly used for heavy vehicles with low speed and the rear brake of the car (the way of the front disc and the rear drum brake). Disadvantages The active heat dissipation of the drum system is poor, which can easily cause brake thermal exhaustion, and the braking force changes greatly when braking in different roads, which makes the braking performance worse. Brake powder is easy to produce during the use of the brake, and it needs to be cleaned regularly. There is also poor drainage, which affects the braking effect. Drum brakes are mainly composed of bottom plate, brake drum, brake shoe, wheel cylinder (brake cylinder), return spring, positioning pin, adjusting lever (ratchet) and other components. Working principle of the drum brake When braking, the lever principle is used to step on the pedal, so that the force of the push rod is applied to the master cylinder. , The pistons on both ends of the brake cylinder will simultaneously push the left and right brake shoes with equal force, and the other end of the two brake shoes is supported by the support rod (ratchet or pawl). The moving shoe expands outwards and forms a friction with the inner surface of the brake drum to achieve the purpose of braking. Since the wheels are rotating brake drums, the pressure acting on the automatic lift is asymmetrical, resulting in self-increasing and self-reducing forces. The friction torque of the booster brake shoe is 2-2.5 times that of the brake shoe, which causes the two brake shoes to wear differently. In the process of braking, the gap between the brake shoe and the brake drum will become larger due to wear. In order to improve the braking effect, it needs to be adjusted. Previously, it was manually adjusted, and the gap between the brake shoe and the brake drum was measured with a feeler gauge. Some cargo vehicles still need manual adjustment, and cars basically adjust the base clearance automatically. When the clearance increases, the brake shoe pushing amount exceeds a certain range, the clearance adjustment mechanism will adjust the adjustment lever (ratchet) Pull to the position where the next tooth of the adjustment tooth is jointed, thereby increasing the length of the connecting rod, returning the brake shoe to its original position, and the braking force disappears. It should be noted that when the brake shoe wears until the rivet is only 0.8mm, if the wear of the bonded brake shoe is only 1.6mm, the brake shoe should be replaced.
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