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1、Brake systemAn automotive brake mechanism is a friction device designed to change power into heat. When the brakes are applied, they convert the power of momentum of the moving vehicle (kinetic energy) into heat by means of friction。 The brake system, then，is a balanced set of mechanical and hydraul
2、ic devices used to retard the motion of the vehicle by means of friction。FrictionFriction is the resistance to relative motion between two bodies in contact。 It is caused by the interlocking of projections and depressions of the two surfaces in contact。 Therefore, there is less friction between poli
3、shed surfaces than between rough surfacesFriction varies with different materials and with the condition of the materials。 There is less friction between surfaces of different materials than between those of the same material。 There is less friction when one surface (tire tread) rolls over the other
4、 (pavement) than when it slides。Coefficient of FictionThe amount of friction created is proportional to the pressure between the two surfaces in contact It is independent of the area of surface contact。 The amount of friction developed by any two bodies in contact is said to be their coefficient of
5、friction (C.O.F.)。The coefficient of friction is found by dividing the force required to slide the weigh over he surface by the weight of the object。 See example in Figure 48-1。 If a 60 lb. pull is required to slide a 100 1b. Weight, then the C.O.F. would be 60 divided by 100 or 60. If only 35 lb. i
6、s required to slide the100 lb. weight, then the C.O.F. would be .35。It has been established that the coefficient of friction will change with any variation of the condition of the surfaces Any lubricant, of course, will greatly reduce the C.O.F, which is why it is so important to keep any grease oil
7、, or brake fluid from brake lining。 Even an extremely damp day will cause some variation in C.O.F.Figure48-1. The coefficient of friction is equal to the force required to slide a body across a surface divided by the weight of the bodyBraking ForcesTremendous forces are involved when braking a vehic
8、le。 The vehicle must be brought to a stop in a much shorter time than is required to bring it up to speed。 To better visualize this, compare horsepower required to accelerate a vehicle and horsepower needed to stop it。A compact vehicle with a 75 hp four cylinder engine requires about 15 sec. to acce
9、lerate to 60 mph. The same vehicle is expected to be able to stop from 60 mph in not more than six sec. Thai is: the brakes must do the same amount of work as the engine, but 2 1/2 times faster。 Effect of Weight and SpeedThe effect of weight and speed of the vehicle on braking is a big factor in hea
10、t generation in both passenger cars and trucks。 If the weight of the vehicle is doubled, the energy of motion to be changed into heat energy is doubled。 Also, the amount of heat to be absorbed and dissipated will be doubled。 The effect of higher speeds on braking is even more serious。 If the vehicle
11、 speed is doubled, four times as much stopping power must be developed。 Also, the brake mechanisms must absorb and dissipate four times as much heat。It follows that if both weight and speed of a vehicle are doubled, the stopping power must be increased eight times and the brakes must absorb and diss
12、ipate eight times as much heat。Brake TemperaturesThe amount of heat generated by brake applications usually is greater than the rate of heat absorption and dissipation by the brake mechanisms, and high brake temperatures result。 Ordinarily, the time interval between brake applications avoids a heat
13、buildup。 If however, repeated panic stops are made, temperatures may become high enough to damage the brake lining, Figure 48-2, brake drums or rotors, and brake fluid。 In extreme cases, the tires have been set on fire。Brake and Tire FrictionWhen brakes are applied on a vehicle, the brake shoes and
14、friction pads are forced into contact with the brake drums and rotors to slow the rotation of the wheels。 Then, the friction between the tires and the road surface slows the speed of the vehicle。However, friction between the shoes and drums and between the pads and rotors does not remain constant。 R
15、ather, it tends to increase with temperature。 From tests, the coefficient of friction of brake lining has been found to range from 0.35 to 0.50。The coefficient of friction of the tire on the road is approximately 02。 However, this varies with the road surface。 Surface contact is the determining fact
16、or。 The fastest stops are obtained with the wheels rotating。 As soon as the wheels become locked, there is less friction and the car will not stop as quickly or as evenly。 The anti-lock braking systems work on the principle of very rapid and repeated brake applications and releases to bring the vehi
17、cle to a stop without locking or skidding。Stopping DistanceAverage stopping distance is an important consideration directly related to vehicle speed。 As charted in Figure 48-3, a vehicle that can be stopped in 45 ft. from 20 mph will require 125 ft. to stop from 40 mph。 At 60 mph, the vehicle will r
18、equire 272 ft. to stop; almost the length of a football field。Note in reading the chart in Figure 48-3, you need to consider reaction time in addition to the time required to make a sudden stop。 It is the time you need to react to a warning of danger, move your foot, and apply the brakes。 For exampl
19、e, when the vehicle is going 20 mph, it will travel 22 ft before the brakes are actually applied。Braking System OperationA simplified drawing of an automotive hydraulic brake system is shown in Figure 48-4。 Typically, the brake pedal is connected to a master cylinder by a push rod。 The master cylind
20、er is connected to the service brakes at each wheel by brake lines and hoses。 The entire hydraulic system is filled with a special brake fluid, which is forced through the system by the movement of the master cylinder pistons。The front brakes are disc type. wherein friction pads in a brake caliper a
21、re forced against machined surfaces of a rotating disc (rotor) at each wheel to slow and stop the vehicle, Figure 48-5。The rear brakes are “drum” type, wherein internal expanding brake shoe assemblies are forced against the machined surface of a rotating drum at each wheel to slow and stop the vehic
22、le, Figure 48-6。As the brake pedal is depressed, it moves pistons within the master cylinder, forcing hydraulic brake fluid throughout the brake system and into cylinders at each wheel。 The fluid under pressure causes the cylinder pistons to move which, in turn, forces the brake shoes and/or frictio
23、n pads against the brake drums and/or rotors to retard their movement and stop the vehicle。Figure 48-7 shows how the force applied to the brake pedal is multiplied。 In this instance, 800 lb. of force is applied to a master cylinder piston area of 0.8 sq. in resulting in a pressure of 1000 psi(800 0
24、8) in the hydraulic brake system。Each front brake caliper bore has a piston area of 15 sq m。 Since the caliper is single piston type. a force of 1500 1b ( 1000 x 1.5) is applied to the brake friction pads。 Each rear wheel cylinder has a piston area of 1.0 sq. in。 Since each rear wheel cylinder has t
25、wo pistons, a total force of 2000 1b (1000 x 1.0 x 2) is produced。Brake Lining MaterialsThere are three basic types of brake lining in current original equipment use: non-asbestos organic, metallic, and semi-metallic。 In the past, asbestos was used almost exclusively in the manufacture of brake lini
26、ng。 Then it was discovered that breathing dust containing asbestos fibers can cause serious bodily harm。Organic lining usually consists of a compound of non-asbestos friction materials, filler materials, and high temperature resins。 These elements are thoroughly mixed, formed into shape, and placed
27、under heat until a hard, slate-like board is formed。 The material is cut and bent into individual segments and attached to drum brake shoes, or it is cut into individual pads and attached to disc brake shoes。 See Figure 48-8。Metallic brake lining is made of sintered metal。 It is composed of finely p
28、owdered iron or copper, graphite, and lesser amounts of inorganic fillers and friction modifiers。 After thorough mixing, a lubricating oil is usually added to prevent segregation of different materials。 The mixture is then put through a briquetting process and compressed into desired form。The non-as
29、bestos organic type brake lining or semi metallic lining is used for conventional brake service。 Under extreme braking conditions (police cars, ambulances, sports cars), the metallic type lining is used。 Under severe usage, the frictional characteristics of the metallic lining are more constant than
30、 that of the organic lining。Brake Rotor and Drum MaterialsA disc brake rotor is defined as the parallel-faced circular rotational member of a disc brake assembly。 Generally, rotors are made of cast iron with ventilating fins separating the two braking surfaces。 See Figure 48-9。 Venting makes the rot
31、ors run cooler and provides quicker cooldown after a brake application。Disc brake rotor braking surfaces are precisely machined for quality of finish, thickness, parallelism, and absence of lateral runout。 Some rotors have a groove machined in the braking surfaces to help reduce brake noise。The use
32、of cast iron for the braking surface of brake drums is almost universal。 The drums are either solid cast iron or steel with an inner lining of cast iron。 Some all steel drums were used in the past。 However, cast iron has a higher coefficient of friction than steel so it generally is the first choice
33、 of the car manufacturers。 The steal/cast iron brake drums are used on heavier vehicles because the assembly has the strength of steel and the frictional properties of cast iron。Some brake drams are made of aluminum with a cast iron liner for the braking surface。 Since aluminum has a higher conducti
34、vity of heat than cast iron, brake drums of the aluminum/cast iron construction will operate at much lower temperatures than solid cast iron drums。 Regardless of the material used in brake drum construction, drums occasionally are provided with cooling fins。Disc BrakesSingle piston, sliding or float
35、ing caliper disc brakes have been used on the front wheels of passenger cars for many years。 See Figure 48-10。 In the past, fixed calipers with four pistons per caliper actuated the friction pads to stop the rotors and the vehicle。 The two caliper housings were fixed in place。 There was no lateral m
36、ovement as with the single piston caliper。With single piston disc brake calipers, Figure48-11, the caliper slides or floats on mounting bolts or on sleeves on mounting bolts or pins to apply friction pads to the machined surfaces of a rotating disc。 Disc brakes are self adjusting。 The caliper piston
37、 seals are designed to retract the piston enough to allow the friction pad to lightly contact the rotor without any drag。 Generally, when front-wheel drive moved into prominence, some modifications of the single came necessary。 Chrysler, for example, introduced an assembly featuring a sliding calipe
38、r and adapter setup utilizing pins, bushings, and sleeves。 See Figure 48-12 。GM Rear Disc BrakesSome General Motors cars have disc brakes front and rear。 Rear disc brakes, like front disc brakes, operate by means of a single piston caliper applying friction pads to a rotating disc or rotor。 In addit
39、ion, however, each GM rear disc brake caliper is equipped with a parking brake actuator mechanism which, ill turn is operated by a series of cables connected to the parking brake pedal。 The GM parking brake mechanism on the rear caliper Figure 48-13, consists of a lever and screw setup whereby the s
40、crew is threaded into a nut built into the caliper piston assembly。 The lever is actuated by a series of cables connected to the parking brake pedal。 The parking brake pedal assembly is a ratcheting mechanism that must be pumped (up to 3 1/2 strokes) to set。 When the parking brake pedal is depressed
41、, the lever turns the screw, moving the caliper piston outward and causing the caliper to slide inward。 The resulting clamping action of the friction pads on each rear rotor locks the brakes。 This action causes the rotor to reduce its speed and therefore the car speed is reduced。Typically, the GM pa
42、rking brake will release automatically when the transmission selector lever is placed in reverse or any drive position with the ignition ON。 The automatic release system utilizes a vacuum diaphragm on the parking brake pedal assembly, a vacuum switch on the transmission range selector, and connectin
43、g vacuum hoses。The GM parking brake system uses four separate cables。 The front cable joins the intermediate cable at the adjuster screw。 Front there, the intermediate cable extends to the rear of the car where right and left rear cables connect by means of an equalizer。Ford Rear Disc BrakesFords fo
44、ur wheel disc brake system uses a dual master cylinder, hydraulic brake booster, and a two-way pressure control valve to balance front and rear braking action。The rear disc brake caliper assembly is similar to Fords pin slider front brake caliper, except for the addition of a parking brake mechanism
45、。 The parking brake lever on the back of the caliper is cable operated by the parking brake pedal。The caliper assembly consists of a housing, piston, parking brake mechanism, inboard and outboard friction pads, wear indicator, anti-rattle clip, and anchor plate。 See Figure 48-14。 The caliper assembl
46、y slides on two greased locating pins (attaching bolts) between the caliper and anchor plates。 Rubber insulators keep the pins from direct contact with the caliper housing。The parking brake lever is attached to the operating shaft。 When the parking brake is applied, the cable rotates the lever and s
47、haft。 Three steel balls roll between ramps formed in pockets on the opposing heads of the operating shaft and thrust screw。 The steel balls force the thrust screw away from the operating shaft, forcing the friction pads against the rotor。The parking brake is self adjusting。 An automatic adjuster in
48、the piston moves on the thrust screw to compensate for lining wear。 资料译文：汽车制动系统的原理汽车制动装置是通过摩擦力将动能转变成热能的摩擦系统组件。当汽车执行制动时，制动系统是通过摩擦力将运动中的车轮的冲力转变成为热能。因此，汽车制动系统是一套由机械和液压组合而成的一种平衡装置是通过摩擦阻力而使汽车运动缓慢。摩擦力摩擦力是使两相接触物体产生共同运动的一种力。它是由两物体接触表面的凸起部分和凹下部分之间的受力而产生的互锁现象。因此，相对于光滑表面来说粗糙表面之间具有的摩擦阻力更大。摩擦力因不同的材料及不同材料的使用条件的不同
49、而不同。两种不同材料间的摩擦力要比相同材料间的摩擦力大，同样，滑动一侧的摩擦力要比滚动的摩擦力要大。摩擦系数摩擦力随着两接触表面的正压力的增加而增加。两接触表面的摩擦力相互独立，而且它们相互间的摩擦力因摩擦系数不同而不同。摩擦系数是由物体本身重力在其表面的分力大小决定的。如图48-1中。如果拉动100LBS的物体需要60LBS的力则摩擦系数为60%。如果要拉100 LBS的物体只用35LBS的力时摩擦系数为35%。已经存在的摩擦系数会随着接触表面的状况而改变，润滑剂能在很大程度上降低摩擦系数，这也是为什么要涂油脂，油液或者制动液的原因。以至于十分潮湿的天气也会引起摩擦系数的变化。图48-1制动力强制车辆停下来所需要的力是巨大的。使车辆完全停下来所用的时间比使汽车升速时间要短的多。为了使您更深刻的
