本科畢業(yè)設(shè)計（本科畢業(yè)論文）外文文獻(xiàn)及譯文文獻(xiàn)、資料題目：Clutch、Differential 、Governorsand Automobile transmission文獻(xiàn)、資料來源：建筑機械文獻(xiàn)、資料發(fā)表（出版）日期：2003院 （部）： 機電工程學(xué)院專 業(yè)： 機械工程及自動化班 級： 機職 041姓 名： 宋來兵學(xué) 號： 2004075354指導(dǎo)教師： 湯愛君翻譯日期： 2008.4.24山東建筑大學(xué)畢業(yè)設(shè)計外文文獻(xiàn)及譯文- 1 -外文文獻(xiàn)：ClutchA clutch is a device which allows two components, usually drive shafts, to be engaged or disengaged by its operation. Although it is commonly used on motor vehicles there are various types of clutch available for industrial applications. Electromagnetic, single dry plate, multi dry plate, multi –oil immersed plate, centrifugal and vane are just some of the various types of clutch available.The job of the clutch on the motor vehicle is to disconnect the engine from the road wheels while changing gear and then to allow the engine to pick up speed smoothly, which is especially important on starting. There are several designs used: the single dry plate type for vehicles with a manual gearbox, and the vane type fluid coupling or torque converter, or centrifugal clutch, for automatic transmissions.Single plate clutchIn a single plate clutch system, a flywheel made f cast iron is bolted to the rear end of the crankshaft. The face of the flywheel which touches the clutch plate is very smooth, so as not to promote wear. The clutch plate is a two-piece disc about eight inches (20 cm) in diameter. In the centre of the plate is a hole with splines (similar to gear teeth) init which correspond to the splines on the input shaft of the gearbox. The inner splined portion of the plate is connected to the outer friction part via ‘buffer’ springs which absorb the initial take-up shock. Both sides of the plate are covered with friction material on the outer one and a half inches (4 cm) of the diameter. This is a high friction, low wearing, heat resistant material, and is attached to the plate by rivets and bonding. The clutch cover consists of a pressed steel casing which houses a pressure plate backed up by several coil springs or a diaphragm spring, which provide the force to press the plate hard up against the flywheel.Vane and centrifugal clutchOn automatic gearboxes, a fluid type clutch is used.As with the dry clutch assembly, a large casing bolts onto the flywheel and contains all the parts. The casing is a casting which has impeller vanes attached to the inside of it. Another large wheel, which is attached to the gearbox input shaft, has the output vanes around the edge of it. It is fitted inside the outer casing, to allow 山東建筑大學(xué)畢業(yè)設(shè)計外文文獻(xiàn)及譯文- 2 -both parts to turn independently. The inside of the device is filled with oil, and sealed. As the flywheel rotates faster around the inner-output-vanes, the oil sets up a turbulence which makes the inner wheel rotate. This action now provides drive from the flywheel to the gearbox through the oil. The design allows the car to remain stationary when the engine is idling, but when the engine revs up, the oil is disturbed, thus giving a smooth take-off. Apart from the convenience of the automatic feature, this system eliminates a lot of moving parts. The only maintenance which is required is the periodic replacement of the oil.Centrifugal clutches contain a system of ‘bob’ weights which are attached by pivots to the flywheel. When the flywheel increases in speed, the weights are thrown out from the centre until they provide contact between the power source and the drive shaft. Such clutches are found on the simpler or semi-automatic systems.Other applicationsClutches are also used in industrial applications such as machine tools which have to stop and start without the motor being switched off. For this any of the methods described for a car, in a modified form, or an electromagnetic clutch can be used.The latter is a device which contains two electromagnets facing each other inside an outer casing. If the power is switched off, both halves of the clutch are independent. When current is passed through the magnets, they come together, with a self engaging coupling or friction plates in between, thus providing a straight through drive. This type of clutch provides no graduation of the power engagement, which is unnecessary in production machinery, for example.DifferentialThe differential is a gear assembly in a motor vehicle which allows the propeller shaft. or drive shaft ,to turn the wheels at different speeds when the vehicle is going around a curve. When a vehicle goes around a curve, the wheel on the inside of the curve travels less distance than the other, and so must turn more slowly, for safety in handling and to keep tyre wear to a minimum. A four wheel drive vehicle, such as a Jeep or a Land Rover, has two differentials. For maximum traction, a four wheel drive vehicle has been designed with three differentials, separating the front wheels, the rear wheels and the front from the rear, allowing each wheel to turn at its own speed under power. The only car which does not have a differential is the Daf car, built in Holland, 山東建筑大學(xué)畢業(yè)設(shè)計外文文獻(xiàn)及譯文- 3 -which has a belt drive system allowing slippage of the belt on the pulleys.The differential is encased in a casting, which is located on most cars (having rear wheel drive) in the middle of the rear axles between the wheels. (It is sometimes called the ‘cabbage head’ because of its bulbous appearance.). The drive shaft enters the casting in the front and one axle enters at each side. A pinion gear, which is splined into the end of the drive shaft, turns a beveled crown gear which is fastened onto the end of one of the axles. An assembly of four small beveled gears (two pinions and two star gears) is bolted to the crown gear and turns with it. The other axle is driven by the small pinion gear opposite the crown gear. The assembly drives both axles at the same speed when the vehicle is being driven in a straight line, but allows the axle opposite the crown gear to turn slower or faster, as required.Some units are designed to give a limited-slip or slip-lock differential, to equalize power between the wheels on a slippery or a soft road surface, providing safe handling and minimizing the likelihood of getting stuck in snow or soft earth.The gear ratio (ratio of the number of teeth on one gear to the number of teeth on the other) between the crown gear and the pinion gear is one of the factors that determines the performance characteristics of the car, such as acceleration and top speed.Early cars had pinion and crown gears with straight teeth on them, which resulted in noisy operation of the differential and allowed play in the gear teeth, causing undue wear. Today the pinion and crown gears are helical gears, which means that the toothed surfaces are beveled and the teeth themselves are curved. This design eliminates play between the teeth, because as the gears spin together one tooth is in full contact before the previous tooth leaves. A properly constructed differential should last the life of the car without any maintenance at all.In order to produce a particularly quiet differential, the pinion and crown gears are lapped together in a lapping machine which duplicates the operating conditions of the completed differential. After lapping, the two gears are kept together as a set. hey are inspected together in a machine in a quiet room ,which determines the exact thickness of shims（sheet metal discs used to ensure a close fit）required in the assembly to ensure quiet operation: then they go to the differential assembly line. All proper shimming is installed: then the unit is test-run, filled with a heavy oil and sealed. Quiet operation of the differential is essential in a vehicle with unit-body construction, as opposed to a separate body bolted to a frame, because noise from the differential 山東建筑大學(xué)畢業(yè)設(shè)計外文文獻(xiàn)及譯文- 4 -will be transmitted by the body itself.GovernorsA governor is an automatic regulating system designed to control the rotational speed of a prime mover such as a diesel engine (propelling a ship, for example) or, a steam turbine（driving an electrical alternator, for example).Feedback In the early days of steam, a speed-governed engine was easily identified by the spinning fly-ball device which spring by the action of centrifugal force. Less obvious was the mechanical connection between the sleeve and the throttle valve of the engine. Such a simple governor worked with the aid of EEDBACK; that is the system was adjusted continuously by using measurements of the quantity under control (in this case speed). The engineer could pre-load the sleeve spring to obtain the particular speed he wanted. Then an error from this desired speed brought about by an unpredictable change of output load on the engine in order to return the speed to its proper value.Hunting In 1868, James Clerk Maxwell gave the Royal Society a mathematical explanation of the undesirable‘dancing motion’，nowadays referred to as instability or hunting, meaning a continuous oscillation of the speed of such governors. This heralded the development of the classical theory of automatic control, without the application of which these simple governors would have continued to behave in an erratic and unpredictable manner.It was soon realized that, since the fly-ball device displaced the sleeve spring proportionately to the square of the speed (because of the nature of centrifugal force) instead of directly proportional to the speed itself, the sensitivity of the system was bound to change with the speed demanded of a particular system. So a system could be too sluggish at low speed and yet hunt at high speed. Fortunately in most applications, with the notable exceptions of engines used for road and rail transportation, the desired speed is not required to vary much if at all. It follows that, although it is now quite a simple matter to replace the mechanical fly-ball device with an electrical tacho-generator, capable of giving an output electrical potential directly proportional to speed, many modern governors are still built around a device utilizing centrifugal force, although 山東建筑大學(xué)畢業(yè)設(shè)計外文文獻(xiàn)及譯文- 5 -this is scarcely visually recognizable as such.Load sensing The simple design of governor suffers from a further drawback known as droop, that is, a temporary or permanent change from the desired speed following a change in the output load imposed on the prime mover. An ideal governor, which does not suffer from droop, is called isochronous (meaning constant speed).One area where this matters particularly is in the generation of electrical power, where droop directly affects the frequency of the alternating supply. Governors for such systems may be fitted with a load-sensing device in which the load on the engine is measured and fed back as well as the speed. Such measurement of the load may be done by a coupling between the engine and the driven load or, where the load is an alternator, by measuring the electrical current in the machine. Inevitably this results in greeter complication and a more difficult design problem.Automobile transmissionThe automobile transmission uses a series of gears which enables the engine to continue to operate at maximum efficiency motion requires a large amount of power to overcome the inertia of the vehicle’s weight. This process requires high engine speed, needed for high power, and a gradual increase in a vehicle’s speed to avoid a jerky start. To do this, a low gear ratio allows the crankshaft to revolve several times in order to turn the rear axle once. The low gear ratio is used for starting, climbing steep slopes, and other situations in which maximum power is required. As power needs are reduced, a second, higher gear ratio is used which rotates the rear axle with fewer revolutions of the crankshaft. As the car’s speed increases, successively higher gear ratios are used until the drive from the engine to the rear wheels passes through the transmission without reduction. Two principal types of transmission are used, manual and automatic.Manual transmission This system permits the driver to select the desired gear ratio by manipulating a shift lever. Besides the forward speed gears, additional gearing is incorporated to permit the vehicle to operate in reverse. Manually operated passenger car transmissions used in the United States and Canada usually have three speeds forward and one in reverse. Trucks, tractors, buses, and other heavy-duty vehicles have as many as 10 forward speeds and 2 in reverse. These units are 山東建筑大學(xué)畢業(yè)設(shè)計外文文獻(xiàn)及譯文- 6 -basically 10 forward speeds and 2 in reverse. These units are basically five-speed transmissions with a two-speed auxiliary gearbox. Transmissions that are to be shifted with the vehicle in motion incorporate sysnchromesh units to prevent gears from clashing as they are meshed. The synchromesh unit synchromizes the speed of the gears so that they revolve at the same speed as they slide into engagement. Automatic transmission This system changes gear ratios automatically in response to changes in engine speed or throttle dously after world war Ⅱ , and they are installed in more than half of the automobiles produced in the united states. Automobiles equipped with an automatic transmission have a control lever which allows the driver to select neutral, low, drive, and reverse. The engine is started in neutral, and the lever is moved to “drive” for normal operation when moving forward. In “drive” position the vehicle can accelerate from rest to maximum speed by simply depressing the accelerator. The “l(fā)ow” position prevent the transmission from shifting out of the lower gear ratios. It is used for climbing steep grades, in mud, or at other times when maximum power is needed. Some units have a “park” position, which locks the transmission to prevent a parked car from rolling. The automatic transmission makes it easier to drive a car, but it is less efficient than a manually shifted unit and increases gasoline consumption. For this reason, the automatic transmission makes it easier to drive a car, but it is less efficient than a manually shifted unit and increases gasoline consumption. For this reason, the automatic transmission is not as common in Europe, where economy of operation is a prime sales factor.Four basic types of automatic transmissions have been developed to the point where they have been installed in production vehicles. The first consists of a standard mechanical transmission and clutch which is automatically shifted by pneumatic, hydraulic, or electric power units. The second type uses a hydraulic torque converter plus a planetary gear system to increase engine torque. The third system combines a hydraulic coupling with an automatically shifted mechanical gearbox to provide torque. The third system combines a hydraulic coupling with an automatically shifted mechanical gearbox to provide torque amplification. The fourth type uses 山東建筑大學(xué)畢業(yè)設(shè)計外文文獻(xiàn)及譯文- 7 -one or more stages of hydraulic torque conversion to provide torque multiplication.Automatic transmissions shift in response to signals from speed sensing and throttle position sensors. The units incorporating hydraulic torque converters use the hydraulic fluid, under pressure, to engage and disengage planetary gear trains.中文譯文：離合器離合器是通過其運轉(zhuǎn)使兩個元件，通常是傳動軸，嚙合或分離的一種裝置。雖然離合器一般用在機動車輛上，但是各種類型的離合器也適于工業(yè)上的應(yīng)用。常用的一些離合器的種類有：電磁式、單片干式、多片干式、多片油浸式、離心式和葉片式。 機動車輛的離合器的作用是：在換擋時將發(fā)動機與行走輪分離，然后讓發(fā)動機平穩(wěn)加速，這在啟動時尤其重要。常用的集中離合器的結(jié)構(gòu)型式有：裝手動變速箱的機動車上用的那種單片干式離合器和自動換擋用的葉片式液力離合器，或液力變矩器，或離心式離合器。單片離合器在單片離合器裝置中，用螺栓將鑄鐵飛輪固定在曲軸后端。與離合器片接觸的飛輪表面非常平滑，其目的是減少磨損。離合器片是一套直徑約為 8 英寸（20 厘米）的兩個圓盤。圓盤中心孔有花鍵（與齒輪相似） ，該花鍵槽與齒輪箱輸入軸上的花鍵相嚙合。離合器片的內(nèi)緣花鍵槽部分通過“緩沖”彈簧與其外援摩擦部分相連接，緩沖彈簧緩沖（離合器）剛接合時的沖擊。離合器片的兩側(cè)邊緣鑲蓋著寬為 1.5 英寸（4 厘米）的摩擦材料。這些摩擦材料是一種摩擦力大，磨損小而且耐熱的材料，與離合器片鉚接在一起。離合器外殼是壓制成的鋼板殼，此鋼殼內(nèi)裝有壓板，此壓板由幾個螺旋彈簧或一種膜片彈簧所支承，彈簧的彈力將壓片緊緊地壓在飛輪上。葉片離心式離合器在自動變速箱上，用了一種液力離合器。液力離合器同干式離合器的裝配形式一樣，一個大外殼用螺栓緊固在飛輪上，殼內(nèi)裝有各種零件。這一外殼是鑄造成的，殼內(nèi)裝有一些葉輪片。以使這兩組葉片各自獨立轉(zhuǎn)動。向該裝置內(nèi)充滿油后再封好。當(dāng)飛輪帶動內(nèi)輸山東建筑大學(xué)畢業(yè)設(shè)計外文文獻(xiàn)及譯文- 8 -出片迅速旋轉(zhuǎn)時，油就被擾動，并帶動內(nèi)輪轉(zhuǎn)動。這一動作是通過油將飛輪的轉(zhuǎn)動傳給變速箱。該裝置可允許在發(fā)動機空轉(zhuǎn)時汽車保持不動，但是，當(dāng)發(fā)動機加速時，油被擾動，從而使起動平穩(wěn)。該裝置除了自動變速方便這一特點之外，還減少許多運動機件。所要求的唯一保養(yǎng)就是要定期換油。離心式離合器有一“懸動”重量體系，該體系與飛輪由樞軸連接。當(dāng)飛輪轉(zhuǎn)速加快時，此“懸動”重量從中心向外甩出，使動力源的輸出軸與傳動軸之間相連接為止。這種離合器用于比較簡單的或半自動的變速裝置上。其他方面的離合器 離合器也用于工業(yè)上，如在機床上，當(dāng)需要機床停車或再開動而不讓電機停車時就需要離合器。為此，對前述的汽車上用的任何一種離合器，只要稍加改動一下形式或者使用電磁離合器，即均可適于機床的使用。電磁離合器由兩個電磁元件組成，這兩個電磁元件在離合器外殼內(nèi)相互緊貼著。若電源斷開，則離合器的輸入輸出兩軸則可各自獨立自由轉(zhuǎn)動。當(dāng)電流通過電磁鐵時，磁體之間就以自體嚙合的聯(lián)接方式，即如摩擦盤一樣使兩磁鐵貼合在一起，從而使傳動可靠。這種離合器不能使傳動分級嚙合，例如，在專用機械中就不需要這種離合器。差速器差速器是裝在激動車輛上的一套齒輪裝置。當(dāng)車輛轉(zhuǎn)彎時，差速器使轉(zhuǎn)動軸以不同的速度驅(qū)動各驅(qū)動輪。當(dāng)車轉(zhuǎn)彎時，彎路內(nèi)側(cè)的車輛行走的距離要比另一側(cè)車輪行走的距離短些。因此，為了駕駛安全和最大限度的減少輪胎的磨損，內(nèi)側(cè)車輪必須轉(zhuǎn)的慢些。一臺四輪驅(qū)動的車輛，像吉普車或越野車，都裝兩套差速器。為了得到最大的牽引力，四輪驅(qū)動車已設(shè)計成裝有三套差速器，分別裝在兩個后輪間及后輪與前輪間，容許每個輪在動力作用下以各自的速度轉(zhuǎn)動。唯一不裝差速器的是荷蘭造的“達(dá)夫”小汽車，該車有一套皮帶傳動裝置，容許皮帶在皮帶輪上打滑。差速器裝在一個鑄成的齒輪包內(nèi)，該齒輪包安裝在后軸的兩個車輪之間，因為大多數(shù)汽車都是后軸驅(qū)動的。 （因為它那圓圓的樣子，有時叫它“甘藍(lán)頭” ）傳動軸從鑄包的前面裝進(jìn)，后軸從兩側(cè)裝進(jìn)。小傘齒輪與主傳動軸末端以花鍵相連接，并帶動固定在一后軸軸端的那個冠形大齒輪轉(zhuǎn)動。用螺栓連接把四個小傘齒輪（兩個小齒輪和行星齒輪）與冠形大齒輪裝配在一起，并隨大傘齒輪傳動。大傘齒輪對面的那個小齒輪驅(qū)動另一后軸。該傳山東建筑大學(xué)畢業(yè)設(shè)計外文文獻(xiàn)及譯文- 9 -動機構(gòu)以同速驅(qū)動機構(gòu)兩后軸時，汽車便直線行駛。但是，當(dāng)需要時，大傘齒輪對面的那一后軸可慢轉(zhuǎn)或快轉(zhuǎn)。有些汽車的總成設(shè)計，提出一種止滑或帶滑動閉鎖裝置的差速器，以使在滑路面或軟路面上兩后輪間的傳動動力大小相等，從而能安全操作，并使車陷入雪地里或軟地理的可能性最小。傘齒輪與游星齒輪之間的傳動比（一齒輪齒數(shù)以另一齒輪齒數(shù)之比）是決定汽車使用性能，如加速度和最高行速的因素之一。早期汽車的大小傘齒輪是直齒，曾導(dǎo)致差速器運轉(zhuǎn)時發(fā)出噪聲，并容許齒輪齒間有間隙存在，從而加劇了磨損。當(dāng)今的大小傘齒輪都是螺旋齒，即齒輪齒面為錐面，齒輪本身為曲面形。這種齒輪結(jié)構(gòu)形式消除了齒輪間隙，因為在大、小齒輪同時旋轉(zhuǎn)時，其任一齒在其前一齒離開之前是處于完全接觸狀態(tài)的。結(jié)構(gòu)合理的差速器，其壽命應(yīng)該與汽車的壽命相同，而根本不必維修。為生產(chǎn)無噪聲的差速器，就要將大、小傘齒輪一起放進(jìn)研磨機中按照成品差速器的運轉(zhuǎn)條件精研，反復(fù)進(jìn)行研磨。研磨后，把兩齒輪裝成一組，再將它們裝在機器中，在安靜的房間里進(jìn)行檢查。依次決定裝配所要求的墊片（用于緊配合的金屬圓薄片）的精確厚度，以確保無噪聲運轉(zhuǎn)。然后把大、小傘齒輪放入差速器組裝線上。該差速器中的所有齒輪都裝在滾柱軸承上，并裝上各種合適的墊片，充滿重油密封好后進(jìn)行試驗運轉(zhuǎn)。對于車體為整體結(jié)構(gòu)的汽車來說，差速器的無噪聲運轉(zhuǎn)是極為重要的。因為差速器發(fā)出的噪聲會從車體本身傳出來。這與用螺栓連接在車架上的那種可分式車體結(jié)構(gòu)根本不同。調(diào)速器調(diào)速器是用于控制原動機轉(zhuǎn)速的一種調(diào)速裝置，像柴油機（如船用柴油機）或者汽輪機（如驅(qū)動交流發(fā)電機的汽輪機）等。調(diào)速器的反饋作用在蒸汽機時代的初期，根據(jù)旋轉(zhuǎn)飛球裝置就很容易辨認(rèn)出速度可調(diào)的蒸汽機。此旋轉(zhuǎn)飛球裝置靠著離心力的作用，在其滑套克服彈簧的壓力而產(chǎn)生位移時來調(diào)節(jié)速度。該滑套與蒸汽機的節(jié)閥之間的機械聯(lián)結(jié)機構(gòu)不太明顯。這種簡單的調(diào)速器是借助于反饋作用進(jìn)行調(diào)速的，也就是說，用所控制的測量值（在這種場合為速度）連續(xù)地調(diào)節(jié)這個系統(tǒng)。工程山東建筑大學(xué)畢業(yè)設(shè)計外文文獻(xiàn)及譯文- 10 -師可預(yù)先給滑套彈簧加壓，以得到所要求的具體速度值。由蒸汽機輸出負(fù)荷不可預(yù)知的變化所引起的偏離預(yù)定速度的誤差會調(diào)節(jié)蒸汽機的供汽量，以使速度恢復(fù)到何時的值。轉(zhuǎn)速的波動1868 年，詹姆斯·克拉克·馬克斯韋爾為英國皇家學(xué)會就那種令人討厭的“舞蹈運動”作了精確的解釋。現(xiàn)今這種運動被稱為不穩(wěn)定性運動或擺動。意思是指這種調(diào)節(jié)器速度的等幅震蕩。這一解釋預(yù)示了自動控制經(jīng)典理論的發(fā)展，如果當(dāng)時不應(yīng)用該經(jīng)典理論，這些簡單的調(diào)速器就會仍然以無規(guī)律的，不可預(yù)測的方式運轉(zhuǎn)。人們不久就認(rèn)識到了，由于飛球裝置使滑套彈簧位移與速度平方成正比（因離心力的特性引起） ，而不是直接與速度本身成正比，因此，該調(diào)節(jié)裝置的靈敏性就一定會隨著對具體裝置所要求的速度而變化。但此調(diào)速裝置在低速時不太靈敏，而在高速時又不甚穩(wěn)定。幸運的是，除鐵路、公路運輸中所用的發(fā)動機外，對大多數(shù)發(fā)動機的轉(zhuǎn)數(shù)幾乎不要求有很大變化。即使有，也極小。由此可見，雖然現(xiàn)在用一電動轉(zhuǎn)數(shù)傳感器（能輸出一直接與轉(zhuǎn)速成正比的電壓值）來代替那種機械的旋轉(zhuǎn)飛球裝置已是一個很簡單的問題，但是，許多現(xiàn)代的調(diào)速器，仍是根據(jù)離心力的裝置為基礎(chǔ)制造成的，而像這樣的裝置用肉眼幾乎是看不出來的。載荷的傳感調(diào)速器的原始設(shè)計患有進(jìn)一步被稱為速度“呆滯”的這一缺陷。即由于施加給原動機的輸出載荷的變化引起的偏離預(yù)定的速度所呈現(xiàn)的暫時性或永久性的變化。等時（意思是恒速）調(diào)速器由于沒有速度“呆滯”的缺陷，被認(rèn)為是一種理想的調(diào)速器。受其影響最大的是在發(fā)電領(lǐng)域，其調(diào)速器的速度“呆滯”直接影響交流電流的頻率。該領(lǐng)域所用的調(diào)速器可裝上一種負(fù)荷傳感器，這樣，不僅是發(fā)動機的轉(zhuǎn)速，而且還有其載荷均可被測量，被反饋。發(fā)動機載荷的測量值，是通過發(fā)動機與被驅(qū)動的載荷之間的連接器測得；如果該載荷是一臺交流發(fā)電機，則可通過測量發(fā)電機的電流而測到載荷的大小。這必然會導(dǎo)致結(jié)構(gòu)更復(fù)雜因而設(shè)計更困難。汽車傳動系汽車傳動系使用一系列齒輪，在車速改變時，該系統(tǒng)能使發(fā)動機繼續(xù)以高效率運轉(zhuǎn)。起動汽車需要的功率大以克服車子本身的慣性。啟動過程要求發(fā)動機的轉(zhuǎn)速高，即大功率所必需的轉(zhuǎn)速，同時還要求汽車逐步加速，以免造成顛簸起動。為此，低速比可使曲軸轉(zhuǎn)動數(shù)軸而后輪軸只轉(zhuǎn)一周。低速比用于起動，爬陡坡及其他需要最大功率的場合。當(dāng)功率需要減小時，就用較高的變速比，此變速比使曲軸以較少的轉(zhuǎn)數(shù)帶動后輪軸。隨著車速的山東建筑大學(xué)畢業(yè)設(shè)計外文文獻(xiàn)及譯文- 11 -增大，要連續(xù)地使用更高的變速比，直至發(fā)動機到后輪軸的傳動在經(jīng)過傳動裝置時無需減速為止。傳動裝置主要有兩類：手動的和自動的。手動傳動系該傳動系要求司機熟練地操縱變速桿已選擇所需的變速比。汽車一般除裝有前進(jìn)變速齒輪外，還裝有輔助齒輪裝置，以便倒車。在美國和加拿大，客車的手動系統(tǒng)有三個前進(jìn)檔和一個倒車檔?？ㄜ嚒⑼侠瓩C、公共汽車及其他重型車輛都有十個前進(jìn)檔和兩個倒車檔。這些機構(gòu)主要有五速傳動機構(gòu)及一個二速輔助齒輪箱。那些使汽車在行進(jìn)中可變速的傳動系統(tǒng)都裝有同步齒輪變速機構(gòu)，以防止齒輪嚙合時碰撞。同步齒輪變速機構(gòu)使齒輪速度同步，這樣，在這些齒輪進(jìn)入嚙合時，能同速轉(zhuǎn)動。自傳動系統(tǒng)該裝置隨發(fā)動機的速度變化或調(diào)速氣門的變化而自動地改變速比。第二次世界大戰(zhàn)后，應(yīng)用自動換擋的車輛迅速增加。美國現(xiàn)今生產(chǎn)的汽車一半以上都裝有自動換擋裝置。裝自動換擋的汽車都有一操縱桿，司機可用它選擇中速、低速、行駛及倒車速度。發(fā)動機以中速起動，操縱桿移至“行駛”檔為正常向前運動。掛“行駛”檔時，只要踩下加速踏板，汽車便可從靜止加速到最大速度。低檔位置可防止變速機構(gòu)變速超出較低的變速范圍。低檔用于爬坡、泥路或其他需要大功率的場合。有的而車還裝“停車”檔，它可鎖住變速機構(gòu)以防止已停的汽車溜動。使用自動換擋裝置使駕車容易，然而，它沒有手動變速效率高，而且使其油耗量增大。因此自動變速機構(gòu)在歐洲用的并不普遍，因為在那用經(jīng)濟實惠使汽車銷售的首要因素。現(xiàn)已研制出四種基本自動換擋裝置，并已達(dá)到在汽車上安裝使用的程度。第一種是標(biāo)準(zhǔn)的機械傳動裝置及靠氣動力，液力或電力裝置自動換檔的離合器。第二種是液力變矩器加一行星齒輪機構(gòu)，以增加發(fā)動機的扭矩。第三種是把液力聯(lián)軸器與自動換檔機械齒輪箱結(jié)合在一起以增大扭矩。第四種是用一級或多級的液力變矩器以增大扭矩。自動換檔是隨著來自速度傳感器和調(diào)速氣門位置傳感器的變化而變速的。該裝置與液力變矩器相合并，利用高壓液流使行星齒輪機構(gòu)嚙合或分離。