1、 附录 1 Brake Systems 1.Drum vs. Disc Brake technology, just like suspension technology and fuel-system technology, has come a long way in recent years. 1） Drum Brakes Early automotive brake systems, after the era of hand levers of course, used a drum design at all four wheels. They were called drum b
2、rakes because the components were housed in a round drum that rotated along with the wheel. Inside was a set of drum that, when the brake pedal was pressed, would force the shoes against the drum and slow the wheel. Fluid was used to transfer the movement of the brake pedal into the movement of the
3、brake shoes, while the drum themselves were made of heat-resistant friction material similar to that used on clutch plates. This basic design proved capable under most circumstances, but it had one major flaw. Under high braking conditions, like descending a steep hill with a heavy load or repeated
4、high-speed slow downs, drum brakes would often fade and lose effectiveness. Usually this fading was the result of too much heat build-up within the shoes. Remember that the principle of braking involves turning kinetic energy (wheelmovement) into thermal energy (heat). For this reason, drum brakes c
5、an only operate as long as they can absorb the heat generated by slowing a vehicles wheels. Once the brake components themselves become saturated with heat, they lose the ability to halt a vehicle, which can be somewhat disconcerting to the vehicles operator. 2） Disc Brakes Disc brakes are used on t
6、he front wheels of most cars and on all four wheels on many cars. A disc rotor is attached to the wheel hub and rotates with the tire and wheel. When the driver applies the brakes, hydraulic pressure from the master cylinder is used to push friction linings against the rotor to stop it. In the disc
7、brake rotor assembly, the rotor is usually made of cast iron. The hub may be manufactured as one piece with the rotor or in two parts. The rotor has a machined braking surface on each face. A splash shield, mounted to the steering knuckle, protects the rotor from road splash. A rotor may be solid or
8、 ventilated. Ventilated designs have cooling fins cast between the braking surfaces. This construction considerably increases the cooling area of the rotor casting. Also, when the wheel is in motion, the rotation of these fan-type fins in the rotor provides increased air circulation and more efficie
9、nt cooling of the brake. Disc brakes do not fade even after rapid, hard brake applications because of the rapid cooling of the rotor. The hydraulic and friction components are housed in a caliper assembly. The caliper assembly straddles the outside diameter of the hub and rotor assembly. When the br
10、akes are applied, the pressure of the pistons is exerted through the shoes in a clamping action on the rotor. Because equal opposed hydraulic pressures are applied to both faces of the rotor throughout application, no distortion of the rotor occurs, regardless of the severity or duration of applicat
11、ion. There are many variations of caliper designs, but they can all be grouped into two main categories: moving and stationary caliper. The caliper is fixed in one position on the stationary design. In the moving design, the caliper moves in relation to the rotor. Most late-model cars use the moving
12、 caliper design. This design uses a single hydraulic piston and a caliper that can float or slide during application. Floating designs float or move on pins or bolts. In sliding designs, the caliper slides sideways on machined surfaces. Both designs work in basically the same way. In the single pist
13、on floating caliper, the single-piston caliper assembly is constructed from a single casting that contains one large piston bore in the inboard section of the casting. Inboard refers to the side of the casting nearest the center line of the car when the caliper is mounted. A fluid inlet hole and ble
14、eder valve hole are machined into the inboard section of the caliper and connect directly to the piston bore. The caliper cylinder bore contains a piston and seal. The seal has a rectangular cross section. It is located in a groove that is machined in the cylinder bore. The seal fits around the outs
15、ide diameter of the piston and provides a hydraulic seal between the piston and the cylinder wall. The rectangular seal provides automatic adjustment of clearance between the rotor and shoe and linings following each application. When the brakes are applied, the caliper seal is deflected by the hydr
16、aulic pressure and it inside diameter rides with the piston within the limits of its retention in the cylinder groove. When hydraulic pressure is released, the seal relaxes and returns to its original rectangular shape, retracting the piston into the cylinder enough to provide proper running clearan
17、ce. As brake linings wear, piston travel tends to exceed the limit of deflection of the seal； the piston therefore slides in the seal to the precise extent necessary to compensate for lining wear. The top of the piston bore is machined to accept a sealing dust boot. The piston in many calipers is st
18、eel, precision ground, and nickel chrome plated, giving it a very hard and durable surface. Some manufacturers are using a plastic piston. This is much lighter than steel and provides for a much lighter brake system. The plastic piston insulates well and prevents heat from transferring to the brake
19、fluid. Each caliper contains two shoe and lining assemblies. They are constructed of a stamped metal shoe with the lining riveted or bonded to the shoe and are mounted in the caliper on either side of the rotor. One shoe and lining assembly is called the inboard lining because it fits nearest to the
20、 center line of the car. The other is called the outboard shoe and lining assembly. The application and release of the brake pressure actually causes a very slight movement of the piston and caliper. Upon release of the braking effort, the piston and caliper merely relax into a released position. In
21、 the released position, the shoes do not retract very far from the rotor surfaces. As the brake lining wears, the piston moves out of the caliper bore and the caliper repositions itself on the mounting bolts an equal distance toward the car. This way, the caliper assembly maintains the inboard and o
22、utboard shoe and lining in the same relationship with the rotor surface throughout the full length of the lining. Sliding calipers are made to slide back and forth on the steering knuckle support to which it is mounted. There is a V shaped surface, sometimes called a rail, on the caliper that matches a similar surface on the steering knuckle support. These two mating surfaces allow the caliper to slide in and out. The internal components of the caliper are the same as those previously described.
