What are the characteristics of disc brakes and drum brakes?

Author: Workplace novice

Link: https://www.zhihu.com/question/468608940/answer/3310059330

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The structure of the fixed components in the massage wiping pair is different, and the disc brake is divided into two types: caliper disc and full disc. The fixed friction element of the caliper disc brake is the brake block, which is installed in the brake caliper connected to the axle and cannot rotate around the axle axis. The contact surface between the brake pads and the brake disc is very small, and the center angle they occupy on the disc is generally only 30 'to 50', so this type of disc brake is also known as a point disc brake. The rotating and fixed components of the friction pair in a full disc brake are both disc shaped, and during braking, all the friction surfaces of the discs come into contact. The working principle is similar to that of a clutch, so it is also called a clutch brake. The most commonly used type of full disc brake is the multi disc full disc brake. Multi disc full disc brakes can be used as both wheel brakes and retarders. There are several types of caliper type brakes depending on the structure of the brake caliper. The fixed caliper type is shown in Figure A, with the brake caliper fixed and hydraulic cylinders on both sides of the brake disc. When braking, only the brake pads in the hydraulic cylinders on both sides move towards the disc surface. This form is also known as opposed piston or floating piston. The floating caliper sliding caliper type is shown in Figure B. The brake caliper can slide axially relative to the brake disc, with only a hydraulic cylinder located on the inner side of the brake disc, and the outer brake block fixed on the caliper body. During braking, the piston is hydraulically pressed against the brake disc by the movable brake block, while the reaction force pushes the brake caliper body and the fixed brake block towards the other side of the brake disc until the two brake blocks are subjected to equal force. The swinging caliper type is shown in Figure C, which is also a single-sided hydraulic cylinder structure. The brake caliper body is hinged with a support fixed on the axle. To achieve braking, the caliper body does not slide but swings in a plane perpendicular to the brake disc. Obviously, the brake pads cannot wear completely and evenly. For this reason, it is necessary to pre shape the lining blocks into wedges (the inclination angle of the friction surface on the back is about 6 °). During use, the lining blocks gradually wear out until the remaining thickness is uniform (usually around 1mm), and should be replaced immediately. The advantages of fixed clamp type are: there are no other sliding parts except for the piston and brake block, which is easy to ensure the stiffness of the clamp; The structure and manufacturing process are not much different from general brake wheel cylinders; Easy to achieve modification from drum to disc; Very adaptable to the requirements of different circuit drive systems (can use a three hydraulic cylinder or four hydraulic cylinder structure). The disadvantages of fixed caliper type include: at least two hydraulic cylinders are placed on both sides of the brake disc, so they must be connected by internal or external oil pipes that cross the brake disc. This increases the radial and axial dimensions of the brake, making it more difficult to arrange on the car. On the other hand, it increases the chances of heating, causing the brake fluid to overheat and vaporize; A fixed caliper brake must also serve as a parking brake, and an additional set of auxiliary brake calipers must be attached to the main brake calipers for parking brake use, or a disc drum combination brake as shown in the following figure can be used. The auxiliary brake caliper has a relatively simple structure and a small friction pad area. In the combination of disc and drum brakes, the diameter size of the drum brake is relatively small, and a bidirectional force increasing drum brake is often used. Compared with the auxiliary brake caliper type, it can generate reliable parking brake force. The advantages of a floating caliper brake are: there is only a hydraulic cylinder on the inner side of the disc, so the axial size is small, and the brake can be further closer to the wheel hub; There is no oil passage or oil pipe crossing the brake disc, and the cooling conditions of the hydraulic cylinder are good, so the possibility of brake fluid vaporization is low; Low cost; The brake pads of the floating caliper can also be used for parking brake. The installation position of the brake caliper can be before or after the axle. As shown in the figure below, the brake caliper located behind the axle can reduce the composite load F of the wheel hub bearing during braking; If the brake caliper is located in front of the axle, it can prevent the tire from splashing mud into the caliper. If the brake caliper needs to be applied more than ten times before it can be restored. The braking torque is independent of the direction of car movement. Easy to form a dual circuit braking system, making the system highly reliable and safe. Small size, low quality, and good heat dissipation. The pressure is evenly distributed on the brake pads, so the wear of the pads is also uniform. Replacing the lining blocks is simple and easy. The gap between the pad and the brake disc is small (0.05-0.15mm), which shortens the braking coordination time. Easy to achieve automatic gap adjustment. The main disadvantage of disc brakes is that they are difficult to completely prevent dust and rust (except for enclosed multi disc full disc brakes). When used as a parking brake, the additional hand drive mechanism required is quite complex. Boosters must be installed in the brake drive mechanism. Due to the small working area of the lining blocks, they wear out quickly and have a low service life, requiring the use of high material lining blocks. Disc brakes are widely used on the front wheels of cars.