汽車工程專業(yè)畢業(yè)設(shè)計(jì)論文外文翻譯

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。 資料譯文：汽車制動(dòng)系統(tǒng)的原理汽車制動(dòng)裝置是通過摩擦力將動(dòng)能轉(zhuǎn)變成熱能的摩擦系統(tǒng)組件。當(dāng)汽車執(zhí)行制動(dòng)時(shí)，制動(dòng)系統(tǒng)是通過摩擦力將運(yùn)動(dòng)中的車輪的沖力轉(zhuǎn)變成為熱能。因此，汽車制動(dòng)系統(tǒng)是一套由機(jī)械和液壓組合而成的一種平衡裝置是通過摩擦阻力而使汽車運(yùn)動(dòng)緩慢。摩擦力摩擦力是使兩相接觸物體產(chǎn)生共同運(yùn)動(dòng)的一種力。它是由兩物體接觸表面的凸起部分和凹下部分之間的受力而產(chǎn)生的互鎖現(xiàn)象。因此，相對(duì)于光滑表面來說粗糙表面之間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條件的不同

49、而不同。兩種不同材料間的摩擦力要比相同材料間的摩擦力大，同樣，滑動(dòng)一側(cè)的摩擦力要比滾動(dòng)的摩擦力要大。摩擦系數(shù)摩擦力隨著兩接觸表面的正壓力的增加而增加。兩接觸表面的摩擦力相互獨(dú)立，而且它們相互間的摩擦力因摩擦系數(shù)不同而不同。摩擦系數(shù)是由物體本身重力在其表面的分力大小決定的。如圖48-1中。如果拉動(dòng)100lbs的物體需要60lbs的力則摩擦系數(shù)為60%。如果要拉100 lbs的物體只用35lbs的力時(shí)摩擦系數(shù)為35%。已經(jīng)存在的摩擦系數(shù)會(huì)隨著接觸表面的狀況而改變，潤滑劑能在很大程度上降低摩擦系數(shù)，這也是為什么要涂油脂，油液或者制動(dòng)液的原因。以至于十分潮濕的天氣也會(huì)引起摩擦系數(shù)的變化。圖48-1制動(dòng)

50、力強(qiáng)制車輛停下來所需要的力是巨大的。使車輛完全停下來所用的時(shí)間比使汽車升速時(shí)間要短的多。為了使您更深刻的理解，您可以比較一下加速所需馬力和制動(dòng)所需馬力的大小。一輛結(jié)構(gòu)簡潔的75馬力四缸發(fā)動(dòng)機(jī)汽車從起動(dòng)加速至?xí)r速60英里需要大約15秒鐘，同樣一輛汽車從時(shí)速60英里停下來所需的時(shí)間則不到6秒鐘。這就是說，制動(dòng)系統(tǒng)做同樣的工作，但只需1/2的時(shí)間，效率很高。重量和速度的作用在汽車制動(dòng)過程中，無論是客車還是貨車，重量和速度是影響熱量產(chǎn)生的重大因素。如果汽車的重量加倍，則動(dòng)能轉(zhuǎn)變成熱能也加倍，如果汽車的速度加倍則使其制動(dòng)需要近四倍的能量，制動(dòng)系統(tǒng)的機(jī)械裝置則需吸收和放出近四倍的熱量。如上所述如果汽車的重

51、量和速度同時(shí)成倍增加，要使汽車停下來的馬力必須增加為原來的八倍同時(shí)制動(dòng)系統(tǒng)必須吸收和釋放原來八倍的熱量。制動(dòng)系統(tǒng)的溫度制動(dòng)時(shí)所產(chǎn)生的熱量通常大于機(jī)械裝置所能吸收或放出的熱量。因此，制動(dòng)系統(tǒng)常出現(xiàn)溫度過高，因此在制動(dòng)裝置內(nèi)部應(yīng)設(shè)法避免熱量的積累。如果連續(xù)重復(fù)的剎車，制動(dòng)系統(tǒng)內(nèi)溫度升高則能損壞制動(dòng)套，制動(dòng)鼓以及自動(dòng)軸。如圖48-2。在某些極端情況下，輪胎也可能因此而起火燃燒。圖48-2制動(dòng)力和輪胎摩擦力當(dāng)汽車制動(dòng)時(shí)，制動(dòng)蹄受壓與制動(dòng)鼓和回轉(zhuǎn)軸相連接，這樣便可以阻止車輪的旋轉(zhuǎn)，于是，輪胎與路面之間的摩擦力便能降低車速，實(shí)現(xiàn)汽車制動(dòng)。不管怎樣制動(dòng)蹄與回轉(zhuǎn)軸之間的摩擦力并不是持續(xù)的。然而它卻會(huì)使其溫度升

52、高。通過測試數(shù)據(jù)，制動(dòng)套的摩擦系數(shù)在0.350.50范圍內(nèi)變化。路面上的摩擦系數(shù)大約為0.2。當(dāng)然，該摩擦系數(shù)是隨路面狀況而改變的。接觸面是測試的重要因素。使車輪旋轉(zhuǎn)最有效徹底的停下來。一旦車輪被鎖死，由于車輪與路面之間滑動(dòng)摩擦力較小，汽車通常不會(huì)立即停下來，汽車防抱死原理便是由此產(chǎn)生的，它迅速制動(dòng)然后立即放開，如此重復(fù)，就可以使汽車在不抱死和滑轉(zhuǎn)的情況下迅速制動(dòng)。制動(dòng)距離通常汽車的行駛速度直接影響著汽車的制動(dòng)距離，如表48-3中所示，汽車從時(shí)速20英里時(shí)開始剎車制動(dòng)到停車，制動(dòng)距離為45英尺，而從時(shí)速40英里開始制動(dòng)到停車，制動(dòng)距離為125英尺。如果從時(shí)速60英里開始，制動(dòng)距離為272英尺，

53、相當(dāng)于一個(gè)足球場那么大。注意在讀表48-3過程中，應(yīng)考慮在緊急制動(dòng)時(shí)的額外反應(yīng)時(shí)間，反應(yīng)時(shí)間是指當(dāng)遇見危險(xiǎn)情況而開始反應(yīng)，然后開始踩腳踏板，實(shí)現(xiàn)制動(dòng)所需的時(shí)間。比如，當(dāng)汽車的車速為時(shí)速20英里，在制動(dòng)系統(tǒng)起作用之前也將駛出22英尺遠(yuǎn)。圖48-3圖48-4制動(dòng)系統(tǒng)的動(dòng)作圖48-4是一個(gè)液壓制動(dòng)系統(tǒng)的簡略曲線圖，其中，制動(dòng)踏板通過拉桿與制動(dòng)主缸連接，制動(dòng)主缸通過液壓系統(tǒng)的液壓軟管與每個(gè)車輪的制動(dòng)輪缸連接。整個(gè)制動(dòng)系統(tǒng)都充滿了專用液壓油，制動(dòng)液的流動(dòng)由制動(dòng)主缸的壓力變化來控制。圖48-5圖48-6前車輪是盤式制動(dòng)器。制動(dòng)裝置通過改變制動(dòng)鉗的夾緊力實(shí)現(xiàn)制動(dòng)，以此降低車速或停車，如圖48-5。后車輪制動(dòng)

54、器則是制動(dòng)鼓式的，通過改變制動(dòng)蹄漲緊位置的機(jī)械裝置的旋轉(zhuǎn)使車輪內(nèi)圈實(shí)現(xiàn)制動(dòng)，以此使汽車降低行駛速度或停車，如圖48-6。當(dāng)制動(dòng)踏板被踩下，它將改變制動(dòng)主缸內(nèi)活塞行程，使制動(dòng)液通過整個(gè)制動(dòng)系統(tǒng)分配到每個(gè)制動(dòng)輪缸內(nèi)。經(jīng)制動(dòng)主缸活塞驅(qū)動(dòng)的壓力油通過驅(qū)動(dòng)制動(dòng)鉗或制動(dòng)蹄來降低車輪的旋轉(zhuǎn)，使汽車實(shí)現(xiàn)制動(dòng)。圖48-7實(shí)驗(yàn)的是踩下制動(dòng)踏板如何實(shí)現(xiàn)增壓。如圖示情況下800單位的力施加在液壓缸活塞0.8平方厘米上形成了1000帕的壓強(qiáng)。汽車上每個(gè)前輪制動(dòng)鉗都有個(gè)1.5平方厘米的孔，為缸式。在每個(gè)單缸活塞上強(qiáng)制施加1500ib。每個(gè)后輪輪缸孔直徑為1.0單后輪制動(dòng)器具有雙活塞，因此能產(chǎn)生2000ib的力。圖48-7制動(dòng)套的材料當(dāng)前的汽車上常用的制動(dòng)套有三種型式：無石棉的有機(jī)材料，金屬和類似金屬。在過去，制動(dòng)套大多是不用棉材料制成。但由于其能吸收塵埃，容易對(duì)系統(tǒng)帶來極大損害，現(xiàn)已很少使用。圖48-8有機(jī)材