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外文原文：
Passage A Power Train
The power train serves two functions: it transmits power from the engine to the drive wheels, and it varies the amount of torque. The power train includes:1.engine:that produces power;2.transmission:either manual or automatic;3.clutch:used only on manual transmission, or torque converter: used only on automatic transmission;4.drive shaft: that transmits the power from transmission to differential;5.that carries the power to the two wheel axles.See Fig.5-1.
Manual transmission
The function of a manual transmission, shown in Fig.5-2,is to transfer engine power to the drive shaft and rear wheels. Gears inside the transmission change the car’s drive-wheel speed and torque in relation to engine speed and torque. This keeps the engine’s output matched as close as possible to varying road speeds and loads.
A manual transaxle, shown in the Fig.5-3.，is a single unit composed of a manual ansmission, differential, and drive axles. Most front-wheel-drive（FWD） cars are equipped with a transaxle. Such transaxle are also found on some front-engined or rear-wheel-drive（RWD）,four-wheel-drive（4WD）cars and on rear-engined and rear-wheel-drive cars．
A manual transmission requires use of a clutch to apply and remove engine torque to the transmission input shaft. The clutch allows this to happen gradually so that the car can be started from a complete stop.
Manual transmission usually have four or five speeds, and often have "overdrive", which means that the output shaft can turn faster than the input Shaft for fuel economy on the highway.When you use it, it will reduce the engine speed by one-third, while maintaining the same road speed.
Clutch
Driving a car with a manual transmission, you depress the clutch, select a gear, and release the clutch while applying power to get the car to move.The clutch allows engine power to be applied gradually when a vehicle is starting out,and interrupts power to avoid gear crunching when shifting.Engaging the clutch allows power to transfer from the engine to transmission and drive wheel.Disengaging the clutch stops the power transfer and allows the engine to continue turning without force to the drive wheels.
The clutch basic components are:the flywheel,clutch disk,pressure plate,release bearing and linkage.See Fig.5-4.
The flywheel is bolted to the crankshaft of the engine.Its main function is to transfer engine torque from the engine to the transmission.
The clutch disk is basically a steel plate,covered with a frictional material that goes between the flywheel and the pressure plate.
A pressure plate is bolted to the flywheel.It includes a sheet metal cover,heavy release springs,a metal pressure ring that provides a friction surface for the clutch disk.
The release bearing is the heart of clutch operation.When the clutch pedal is depressed,the throw-out bearing moves toward the flywheel,pushing in the pressure plate’s release fingers and moving the pressure plate fingers or levers against pressure plate spring force.
The linkage transmits and multiplies the driver’s leg force to the fork of the clutch pressure plate.A mechanical clutch linkage usually consists of the clutch pedal,a series of linkage rods and arms,or a cable.A hydraulic clutch linkage typically includes a clutch master cylinder and reservoir,a hydraulic line and a slave cylinder.
Automatic transmission
Both an automatic transmission and a manual transmission accomplish exactly the same thing,but they do it in totally different ways.The key difference between a manual and an automatic transmissions is that the manual transmission locks and unlocks and different sets of gears to the output shaft to achieve the various gear ratios,while in an automatic transmission,the same set of gears produces all of different gear ratios.The planetary gear-set is the device that makes this possible in an automatic.
Automatic transmissions are used in many rear-wheel-drive and four-wheel-drive vehicles.Automatic transaxles are used in most front-wheel-drive vehicles.The major components of a transaxle are the same as those in a transmission,except the transaxle assembly includes the final drive and differential gears,in addition to the transmission.
An automatic transmission receives engine power through a torque converter,which is driven by the engine’s crankshaft.Hydraulic pressure in the converter allows power to flow from the torque converter to the transmission’s input shaft.The input shaft drives a planetary gear set that provides the different forward gears,a neutral position,and one reverse gear.Power flow through the gears is controlled by multiple-disk clutches,one-way clutches,and friction bands.
Passage B Power Train
Torque Converter
The key to the modern automatic transmission is the torque converter.It takes the place of a clutch in a manual transmission to send the power from the engine to the transmission input shaft.The torque converter offers the advantage of multiplying the turning power provided by the engine.
It has three parts that help multiply the power:an impeller（or pump）conn cted to the engine’s crankshaft,a turbine to turn the turbine shaft which is connected to the gears,and a stator（or guide wheel）between the two.See Fig. 5-6.
The torque converter is filled with transmission fluid that is moved by impeller blades.When the impeller spins above a certain speed,the turbine spins,driven by the impeller.
Planetary Gearing
Planetary gears provide for the different gear ratios needed to move a vehicle in the desired direction at the correct speed.A planetary gear set consists of a sun gear,planet gears,and a internal ring.See Fig. 5-7.
In the center of the planetary gear set is the sun gear.Planet gears surround the sun gear,just like the earth and other planets in our solar system.These gears are mounted and supported by the planet carrier and each gear spins on its own separate shaft.The planet gears are in constant mesh with the sun and ring gears.The ring gear is the outer gear of the gear set.Its has internal teeth and surrounds the rest of the gear set.Its gear teeth are in constant mesh with the planet gears.The number of planet gears used in a planetary gear set varies according to the loads the transmission is designed to face.For heavy loads,the number of planet gears is increased to spread the work load over more gear teeth.
The planetary gear set can provide a gear reduction or overdrive,direct drive or reverse,or a neutral position.Because the gears in constant mesh,gear changes are made without engaging or disengaging gears,as is required in a manual transmission.Rather, clutches and bands are used to either hold or release different members of the gear set to get the proper direction of rotation and/or gear ratio.
Different
On FWD cars,the differential unit is normally part of the transaxle assembly.On RWD cars,it is part of the rea axle assembly.Located inside the differential case are the differential pinion shafts and gears and the differential side gears. See Fig.5-8
The differential assembly revolves with the ring gear.Axle side gears are splined to the rear axle or front axle drive shafts.
When an automobile is moving straight ahead,both wheels are free to rotate. Engine power is applied to the pinion gear,which rotates the ring gear.Beveled pinion gears are carried around by the ring gear and rotate as one unit.Each axle receives the same power,so each wheel turns at the same speed. See Fig. 5-9.
When the car turns a sharp corner,only one wheel rotates freely.Torque still comes in on the pinion gear and rotates the ring gear,carrying the beveled pinions around with it.However,one axle is held stationary and the beveled pinions are forced to rotate on their own axis and "walk around"their gear.The other side is forced to rotate because it is subjected to the turning force of the ring gear,which is transmitted through the pinions. See Fig. 5-10.
Drive shaft
A drive shaft and universal joints（U-joints） connect the transmission to the rear drive axle on most rear-wheel-drive vehicles.Many four-wheel-drive vehicles also use drive shafts and universal joints,with one drive shaft between the transfer case and rear drive axle and a second drive shaft between the transfer case and the front drive axle. The drive shaft is sometimes called a propeller shaft.
The drive shaft and U-joints provide a means of transferring engine torque to drive axles.The universal joints allow the drive shaft to move up and down,to allow for suspension travel.Some drive shaft also have a slip joints that allows the drive shaft to make minor length changes as the vehicle suspension height changes.
Gears and gear drive
Gears are the most durable and rugged of all mechanical drives.They can transmit high power at efficiencies up to 98% and with long service lives. For this reason, gears rather than belts or chains are found in automotive transmissions and most heavy-duty machine drives. On the other hand, gears are more expensive than other drives, especially if they are machined and not made from power metal or plastic.
Gear cost increases sharply with demands for high precision and accuracy. So it is important to establish tolerance requirements appropriate for the application. Gears that transmit heavy loads or than operate at high speeds are not particularly expensive, but gears that must do both are costly.
Silent gears also are expensive. Instrument and computer gears tend to be costly because speed or displacement ratios must be exact. At the other extreme, gears operating at low speed in exposed locations are normally termed no critical and are made to minimum quality standards.
For tooth forms, size, and quality, industrial practice is to follow standards set up by the American Gear Manufactures Association (AGMA).
Tooth form
Standards published by AGMA establish gear proportions and tooth profiles. Tooth geometry is determined primarily by pitch, depth, and pressure angle.
Pitch：Standards pitches are usually whole numbers when measured as diametral pitch P. Coarse-pitch gearing has teeth larger than 20 diametral pitch –usually 0.5 to 19.99. Fine-pitch gearing usually has teeth of diametral pitch 20 to 200.
Depth: Standardized in terms of pitch. Standard full-depth have working depth of 2/p. If the teeth have equal addenda(as in standard interchangeable gears) the addendum is 1/p. Stub teeth have a working depth usually 20% less than full-depth teeth. Full-depth teeth have a larger contract ratio than stub teeth. Gears with small numbers of teeth may have undercut so than they do not interfere with one another during engagement. Undercutting reduce active profile and weakens the tooth.
Mating gears with long and short addendum have larger load-carrying capacity than standard gears. The addendum of the smaller gear (pinion) is increased while that of larger gear is decreased, leaving the whole depth the same. This form is know as recess-action gearing.
Pressure Angle: Standard angles areand.Earlier standards include a 14-pressure angle that is still used. Pressure angle affects the force that tends to separate mating gears. High pressure angle decreases the contact ratio (ratio of the number of teeth in contact) but provides a tooth of higher capacity and allows gears to have fewer teeth without undercutting.
Backlash: Shortest distances between the non-contacting surfaces of adjacent teeth .
Gears are commonly specified according to AGMA Class Number, which is a code denoting important quality characteristics. Quality number denote tooth-element tolerances. The higher the number, the closer the tolerance. Number 8 to 16 apply to fine-pitch gearing.
Gears are heat-treated by case-hardening, through-hardening, nitriding, or precipitation hardening. In general, harder gears are stronger and last longer than soft ones. Thus, hardening is a device that cuts the weight and size of gears. Some processes, such as flame-hardening, improve service life but do not necessarily improve strength.
Design checklist
The larger in a pair is called the gear, the smaller is called the pinion.
Gear Ratio: The number of teeth in the gear divide by the number of teeth in the pinion. Also, ratio of the speed of the pinion to the speed of the gear. In reduction gears, the ratio of input to output speeds.
Gear Efficiency: Ratio of output power to input power. (includes consideration of power losses in the gears, in bearings, and from windage and churning of lubricant.)
Speed: In a given gear normally limited to some specific pitchline velocity. Speed capabilities can be increased by improving accuracy of the gear teeth and by improving balance of the rotating parts.
Power: Load and speed capacity is determined by gear dimensions and by type of gear. Helical and helical-type gears have the greatest capacity (to approximately 30,000 hp). Spiral bevel gear are normally limited to 5,000 hp, and worm gears are usually limited to about 750 hp.
Special requirements
Matched-Set Gearing: In applications requiring extremely high accuracy, it may be necessary to match pinion and gear profiles and leads so that mismatch does not exceed the tolerance on profile or lead for the intended application.
Tooth Spacing: Some gears require high accuracy in the circular of teeth. Thus, specification of pitch may be required in addition to an accuracy class specification.
Backlash: The AMGA standards recommend backlash ranges to provide proper running clearances for mating gears. An overly tight mesh may produce overload. However, zero backlash is required in some applications.
Quiet Gears: To make gears as quit as possible, specify the finest pitch allowable for load conditions. (In some instances, however, pitch is coarsened to change mesh frequency to produce a more pleasant, lower-pitch sound.) Use a low pressure angle. Use a modified profile to include root and tip relief. Allow enough backlash. Use high quality numbers. Specify a surface finish of 20 in. or better. Balance the gear set. Use a nonintegral ratio so that the same teeth do not repeatedly engage if both gear and pinion are hardened steel. (If the gear is made of a soft material, an integral ratio allows the gear to cold-work and conform to the pinion, thereby promoting quiet operation.) Make sure critical are at least 20% apart from operating speeding or speed multiples and from frequency of tooth mesh.
Multiple mesh gear
Multiple mesh refers to move than one pair of gear operating in a train. Can be on parallel or nonparallel axes and on intersection or nonintersecting shafts. They permit higer speed ratios than are feasible with a single pair of gears .
Series trains:Overall ratio is input shaft speed divided by output speed ,also the product of individual ratios at each mesh ,except in planetary gears .Ratio is most easily found by dividing the product of numbers of teeth of driven gears by the product of numbers of teeth of driving gears.
Speed increasers (with step-up rather than step-down ratios) may require special care in manufacturing and design. They often involve high speeds and may creste problems in gear dynamics. Also, frictional and drag forces are magnified which, in extreme cases , may lead to operational problems.
Epicyclic Gearing:Normally, a gear axis remains fixed and only the gears rotates. But in an epicyclic gear train, various gears axes rotate about one anther to provide specialized output motions. With suitable clutchse and brakes, an epicyclic train serves as the planetary gear commonly found in automatic transmissions.
Epicyclic trains may use spur or helical gears, external or internal, or bevel gears. In transmissions, the epicyclic (or planetary) gears usually have multiple planets to increase load capacity.
In most cases, improved kinematic accuracy in a gearset decreases gear mesh excitation and results in lower drive noise. Gearset accuracy can be increased by modifying the tooth involute profile, by substituting higher quality gearing with tighter manufacturing tolerances, and by improving tooth surface finish. However, if gear mesh excitation generaters resonance somewhere in the drive system, nothing short of a “perfect” gearset will substantially reduce vibration and noise.
Tooth profiles are modified to avoid interferences which can result from deflections in the gears, shafts, and housing as teeth engage and disendgage. If these tooth interferences are not compensated for by profile modifications, gears load capacity can be seriously reduced. In addition, the drive will be noisier because tooth interferences generate high dynamic loads. Interferences typically are eliminated by reliving the tooth tip, the tooth flank, or both. Such profile modifications are especially important for high-load , high-speed drives. The graph of sound pressure levelvs tip relief illustrates how tooth profile modifications can affect overall drive noise. If the tip relief is less than this optimum value, drive noise increases because of greater tooth interference; a greater amount of tip relief also increase noise because the contact ratio is decreased.
Tighter manufacturing tolerances also produce quietier gears. Tolerances for such parameters as profile error, pitch AGMA quality level. For instance, the graph depicting SPL vs both speed and gear quality shows how noise decreases example, noise is reduced significantly by an increase in accuracy from an AGMA Qn 11 quality to an AGNA Qn 15 quality. However, for most commercial drive applications, it is doubtful that the resulting substantial cost increase for such an accuracy improvement can be justified simply on the basis of reduced drive noise.
Previously, it was mentioned that gears must have adequate clearance when loaded to prevent tooth interference during the course of meshing. Tip and flank relief are common profile modifications that control such interference. Gears also require adequate backlash and root clearance. Noise considerations make backlash an important parameter to evaluate during drive design. Sufficient backlash must be provided under all load and temperature conditions to avoid a tight mesh, which creates excessively high noise level. A tight mesh due to insufficient backlash occurs when the drive and coast side of a tooth are in contact simultaneously. On the other hand, gears with excessive backlash also are noisy because of impacting teeth during periods of no load or reversing load. Adequate backlash should be provided by tooth thinning rather than by increase in center distance. Tooth thinning dose not decrease the contact ratio, whereas an increase in center distance does. However, tooth thinning does reduce the bending fatigue, a reduction which is small for most gearing systems.
中文譯文：
動(dòng)力傳動(dòng)系A(chǔ)
動(dòng)力傳動(dòng)系有兩個(gè)作用：它把動(dòng)力從發(fā)動(dòng)機(jī)傳送到驅(qū)動(dòng)輪上，并且改變扭矩的大小。動(dòng)力傳動(dòng)系包括：1.發(fā)動(dòng)機(jī)：制造動(dòng)力；2.變速器：不是手動(dòng)就是自動(dòng)；3.離合器：僅用在手動(dòng)變速器或者液力變矩器；4.驅(qū)動(dòng)軸：把動(dòng)力從變速器傳到差速器；5.差速器：將動(dòng)力傳到兩個(gè)驅(qū)動(dòng)軸上。
手動(dòng)變速器
手動(dòng)變速器的作用是，把發(fā)動(dòng)機(jī)動(dòng)力傳送到傳動(dòng)軸和驅(qū)動(dòng)輪。變速器內(nèi)的齒輪，改變車輛驅(qū)動(dòng)輪和發(fā)動(dòng)機(jī)之間轉(zhuǎn)速和扭矩的比例。這樣保持發(fā)動(dòng)機(jī)的輸出盡可能的靠近改變路面速度和最低速度。
一個(gè)手動(dòng)的驅(qū)動(dòng)橋，是一個(gè)由手動(dòng)變速器，差速器，傳動(dòng)軸組成的。大多數(shù)前輪驅(qū)動(dòng)汽車裝有一個(gè)驅(qū)動(dòng)橋。這樣的驅(qū)動(dòng)橋也能在一些前置引擎或者后輪驅(qū)動(dòng)，四輪驅(qū)動(dòng)的汽車，在后置引擎和后輪驅(qū)動(dòng)的汽車上看到。
一個(gè)手動(dòng)變速器包括使用一個(gè)離合器來消除發(fā)動(dòng)機(jī)扭矩到變速器輸入軸。離合器允許這樣漸漸發(fā)生以至于汽車能夠啟動(dòng)。
手動(dòng)變速器通常有四到五個(gè)檔位，而且一般有“超速檔”，對(duì)于在路上的燃油經(jīng)濟(jì)性這樣就意味著輸出軸比輸入軸轉(zhuǎn)的更快。當(dāng)你使用變速器的時(shí)候，要維持同樣的速度，將減少發(fā)動(dòng)機(jī)轉(zhuǎn)速的三分之一。
離合器
駕駛手動(dòng)擋汽車，你踩下離合器，嚙合了齒輪，然后松掉離合器而汽車會(huì)適應(yīng)動(dòng)力前進(jìn)。離合器可使車輛啟動(dòng)后發(fā)動(dòng)機(jī)的動(dòng)力被逐漸的加載，并可通過切斷動(dòng)力防止換擋時(shí)齒輪被咬碎。離合器嚙合時(shí)把發(fā)動(dòng)機(jī)動(dòng)力傳送到變速器和驅(qū)動(dòng)輪。離合器分離停止動(dòng)力傳輸，在沒有動(dòng)力傳到驅(qū)動(dòng)輪上的情況下，發(fā)動(dòng)機(jī)可以持續(xù)運(yùn)轉(zhuǎn)。
離合器基本的部件是：飛輪，離合器盤，壓力盤，分離軸承和聯(lián)接裝置。
飛輪被螺栓固定在發(fā)動(dòng)機(jī)的曲軸上。它的主要作用是傳送發(fā)動(dòng)機(jī)扭矩從發(fā)動(dòng)機(jī)到變速器。
離合器盤基本就是一塊鋼板，在飛輪和壓力盤中間覆蓋了一種耐摩擦材料。
離合器盤螺栓連接在飛輪上。它包括一張薄片金屬封蓋，彈簧，一個(gè)給離合器盤提供摩擦表面的金屬壓力環(huán)。
分離軸承是離合器操縱機(jī)構(gòu)的中心。當(dāng)離合器踏板踩下時(shí)，分離軸承指向飛輪，壓盤推進(jìn)釋放了擋板然后移動(dòng)壓盤彈簧片到壓盤彈簧彈力頂。
聯(lián)接裝置成倍地傳送駕駛員腿部力量到離合器壓盤的膜片。一個(gè)機(jī)械離合器連接裝置通常由離合器踏板，一系列連接桿臂或者一組電纜。一個(gè)液壓離合器聯(lián)接裝置大體上是由一個(gè)離合器制動(dòng)缸和儲(chǔ)存器，一組液壓管路和一個(gè)從動(dòng)缸。
自動(dòng)變速器
自動(dòng)變速器和手動(dòng)變速器嚴(yán)格的講都能完成一樣的工作，但他們完成工作的方法完全不一樣。手動(dòng)變速器和自動(dòng)變速器的之間的根本區(qū)別在于手動(dòng)變速器鎖與不鎖在不同的輸出軸來實(shí)
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