英文翻译-动力传动系A.doc

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 te