汽车工程专业毕业设计论文外文翻译.

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

50、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。为了使您更深刻的理解，您可以比较一下加速所需马力和制动所需马力的大小。一辆结构简洁的75马力四缸发动机汽车从起动加速至时速60英里需要大约15秒钟，同样一辆汽车从时速60英里停下来所需的时间则不到6秒钟。这就是说，制动系统做同样的工作，但只需1/2的时间，效率很高。重量和速度的作用在汽车制动过程中，无论是客车还是货车，重量和速度是影响热量产生的重大因素。如果汽车的重量加倍，则动能转变成热能也加倍，如果汽车的速度加倍则使其制动需要近四倍的能量，制动系统的机械装置则需吸收和放出近四倍的热量。如上所述如果汽车的重

51、量和速度同时成倍增加，要使汽车停下来的马力必须增加为原来的八倍同时制动系统必须吸收和释放原来八倍的热量。制动系统的温度制动时所产生的热量通常大于机械装置所能吸收或放出的热量。因此，制动系统常出现温度过高，因此在制动装置内部应设法避免热量的积累。如果连续重复的刹车，制动系统内温度升高则能损坏制动套，制动鼓以及自动轴。如图48-2。在某些极端情况下，轮胎也可能因此而起火燃烧。图48-2制动力和轮胎摩擦力当汽车制动时，制动蹄受压与制动鼓和回转轴相连接，这样便可以阻止车轮的旋转，于是，轮胎与路面之间的摩擦力便能降低车速，实现汽车制动。不管怎样制动蹄与回转轴之间的摩擦力并不是持续的。然而它却会使其温度升

52、高。通过测试数据，制动套的摩擦系数在0.350.50范围内变化。路面上的摩擦系数大约为0.2。当然，该摩擦系数是随路面状况而改变的。接触面是测试的重要因素。使车轮旋转最有效彻底的停下来。一旦车轮被锁死，由于车轮与路面之间滑动摩擦力较小，汽车通常不会立即停下来，汽车防抱死原理便是由此产生的，它迅速制动然后立即放开，如此重复，就可以使汽车在不抱死和滑转的情况下迅速制动。制动距离通常汽车的行驶速度直接影响着汽车的制动距离，如表48-3中所示，汽车从时速20英里时开始刹车制动到停车，制动距离为45英尺，而从时速40英里开始制动到停车，制动距离为125英尺。如果从时速60英里开始，制动距离为272英尺，

53、相当于一个足球场那么大。注意在读表48-3过程中，应考虑在紧急制动时的额外反应时间，反应时间是指当遇见危险情况而开始反应，然后开始踩脚踏板，实现制动所需的时间。比如，当汽车的车速为时速20英里，在制动系统起作用之前也将驶出22英尺远。图48-3图48-4制动系统的动作图48-4是一个液压制动系统的简略曲线图，其中，制动踏板通过拉杆与制动主缸连接，制动主缸通过液压系统的液压软管与每个车轮的制动轮缸连接。整个制动系统都充满了专用液压油，制动液的流动由制动主缸的压力变化来控制。图48-5图48-6前车轮是盘式制动器。制动装置通过改变制动钳的夹紧力实现制动，以此降低车速或停车，如图48-5。后车轮制动

54、器则是制动鼓式的，通过改变制动蹄涨紧位置的机械装置的旋转使车轮内圈实现制动，以此使汽车降低行驶速度或停车，如图48-6。当制动踏板被踩下，它将改变制动主缸内活塞行程，使制动液通过整个制动系统分配到每个制动轮缸内。经制动主缸活塞驱动的压力油通过驱动制动钳或制动蹄来降低车轮的旋转，使汽车实现制动。图48-7实验的是踩下制动踏板如何实现增压。如图示情况下800单位的力施加在液压缸活塞0.8平方厘米上形成了1000帕的压强。汽车上每个前轮制动钳都有个1.5平方厘米的孔，为缸式。在每个单缸活塞上强制施加1500IB。每个后轮轮缸孔直径为1.0单后轮制动器具有双活塞，因此能产生2000IB的力。图48-7制动套的材料当前的汽车上常用的制动套有三种型式：无石棉的有机材料，金属和类似金属。在过去，制动套大多是不用棉材料制成。但由于其能吸收尘埃，容易对系统带来极大损害，现已很少使用。图48-8有机材