1、 附录 1 Hydraulic Brake Systems The braking system is the most important system in cars. If the brake system fail, the result can be disastrous. When you step on the brake pedal, you expect the vehicle to stop. The brake pedal operates a hydraulic system that is used for two reasons. First, fluid unde
2、r pressure can be carried to all parts of the vehicle by small hoses or metal lines with out taking up a lot of room or causing routing problems. Second, the hydraulic fluid offers a great mechanical advantage-little foot pressure is required on the pedal, but a great deal of pressure is generated a
3、t the wheels. The brake pedal is linked to a piston in the brake master cylinder, which is filled with hydraulic brake fluid. The master cylinder consists of a cylinder containing a small piston and a fluid reservoir. Modern master cylinders are actually two separate cylinders. Such a system is call
4、ed a dual circuit, because the front cylinder is connected to the front brakes and the rear cylinder to the rear brakes. (Some vehicles are connected diagonally.) The two cylinders are actually separated, allowing for emergency stopping power should one part of the system fail. The entire hydraulic
5、system from the master cylinder to the wheels is full of hydraulic brake fluid. When the brake pedal is depressed, the pistons in the master cylinder are forced to move, exerting tremendous force on the fluid in the lines. The fluid has nowhere to go, and forces the wheel cylinder pistons (drum brak
6、es) or caliper pistons (disc brakes) to exert pressure on the brake shoes or pads. The friction between the brake shoe and wheel drum or the brake pad and rotor (disc) slows the vehicle and eventually stops it. Also attached to the brake pedal is a switch that lights the brake lights as the pedal is
7、 depressed. The lights stay on until the brake pedal is released and returns to its normal position. Each wheel cylinder in a drum brake system contains two pistons, one at either end, which push outward in opposite directions. In disc brake systems, the wheel cylinders are part of the caliper (ther
8、e can be as many as four or as few as one). Whether disc or drum type, all pistons use some type of rubber seal to prevent leakage around the piston, and a rubber dust boot seals the outer ends of the wheel cylinders against dirt and moisture. When the brake pedal is released, a spring pushes the ma
9、ster cylinder pistons back to their normal positions. Check valves in the master cylinder piston allow fluid to flow toward the wheel cylinders or calipers as the piston returns. Then as the brake shoe return springs pull the brake shoes back to the released position, excess fluid returns to the mas
10、ter cylinder through compensating ports, which have been uncovered as the pistons move back. Any fluid that has leaked from the system will also be replaced through the compensating ports. All dual circuit brake systems use a switch to activate a light, warning of brake failure. The switch is locate
11、d in a valve mounted near the master cylinder. A piston in the valve receives pressure on each end from the front and rear brake circuits. When the pressures are balanced, the piston remains stationary, but when one circuit has a leak, greater pressure during the application of the brakes will force
12、 the piston to one side or the other, closing the switch and activating the warning light. The light can also be activated by the ignition switch during engine starting or by the parking brake. Front disc, rear drum brake systems also have a metering valve to prevent the front disc brakes from engag
13、ing before the rear brakes have contacted the drums. This ensures that the front brakes will not normally be used alone to stop the vehicle. A proportioning valve is also used to limit pressure to the rear brakes to prevent rear wheel lock-up during hard braking. 1. Friction materials Brake shoes an
14、d pads are constructed in a similar manner. The pad or shoe is composed of a metal backing plate and a friction lining. The lining is either bonded (glued) to the metal, or riveted. Generally, riveted linings provide superior performance, but good quality bonded linings are perfectly adequate. Frict
15、ion materials will vary between manufacturers and type of pad and the material compound may be referred to as: asbestos, organic, semi-metallic, metallic. The difference between these compounds lies in the types and percentages of friction materials used, material binders and performance modifiers.
16、Generally speaking, organic and non-metallic asbestos compound brakes are quiet, easy on rotors and provide good feel. But this comes at the expense of high temperature operation, so they may not be your best choice for heavy duty use or mountain driving. In most cases, these linings will wear somew
17、hat faster than metallic compound pads, so you will usually replace them more often. But, when using these pads, rotors tend to last longer. Semi-metallic or metallic compound brake linings will vary in performance based on the metallic contents of the compound. Again, generally speaking, the higher
18、 the metallic content, the better the friction material will resist heat. This makes them more appropriate for heavy duty applications, but at the expense of braking performance before the pad reaches operating temperature. The first few applications on a cold morning may not give strong braking. Al
19、so, metallic and semi-metallic are more likely to squeal. In most cases, metallic compounds last longer than non-metallic pads, but they tend to cause more wear on the rotors. If you use metallic pads, expect to replace the rotors more often. When deciding what type of brake lining is right for you,
20、 keep in mind that todays modern cars have brake materials which are matched to the expected vehicles performance capabilities. Changing the material from OEM specification could adversely affect brake feel or responsiveness. Before changing the brake materials, talk to your dealer or parts supplier
21、 to help decide what is most appropriate for your application. Remember that heavy use applications such as towing, stop and go driving, driving down mountain roads, and racing may require a change to a higher performance material. Some more exotic materials are also used in brake linings, among whi
22、ch are Kevlar and carbon compounds. These materials have the capability of extremely good performance for towing, mountain driving or racing. Wear characteristics can be similar to either the metallic or the non-metallic linings, depending on the product you buy. Most race applications tend to wear
23、like metallic linings, while many of the street applications are more like the non-metallic. 2. Brake fluid On a disk brake, the fluid from the master cylinder is forced into a caliper where it presses against a piston. The piston, in-turn, squeezes two brake pads against the disk(rotor)which is att
24、ached to the wheel, forcing it to slow down or stop. This process is similar to a bicycle brake where two rubber pads rub against the wheel rim creating friction. With drum brakes, fluid is forced into the wheel cylinder which pushes the brake shoes out so that the friction linings are pressed against the drum which is attached to the wheel, causing the wheel to stop.
