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某载重汽车单级后驱动桥结构设计
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结构设计
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附录ADriveshafts and Drive AxlesDriveshaftsSee Figures 1 and 2 In a conventional longitudinally mounted front-engine/rear wheel drive vehicle, a driveshaft is used to transfer the torque from the engine, through the transmission output shaft, to the differential in the axle, which in turn transmits torque to the wheels. The driveshaft can be made out of steel or aluminum and can be either solid or hollow (tubular).A splined slip yoke assembly, either as an integral part of the shaft or utilizing a splined transmission output shaft, permits the driveshaft to move forward and rearward as the axle moves up and down. This provides smooth performance during vehicle operation.Figure 1 Cut-away view of a typical solid driveshaft and related components.On some four wheel drive vehicles, a front driveshaft connects the power flow from the transfer case to the front drive axle.The driveshaft uses flexible joints, called Universal joints (U-joints) or Constant Velocity joints (CV-joints) to couple the transmission/transaxle to the drive axle/drive wheels. Refer to the Universal and Constant Velocity joints section for more information.Front wheel drive vehicles also utilize driveshafts, although they are usually referred to as halfshafts. The halfshafts are usually equipped with CV-joints on each end which allow the wheels to turn as well as move up and down while still smoothly transferring engine power to the wheels. Front wheel drive vehicles typically use a transaxle (a combination TRANSmission and drive AXLE)Figure 2 Exploded view of a typical front wheel drive halfshaft assembly using CV joint components on both ends.Some rear and four wheel drive vehicles use halfshafts. These vehicles will usually have a rigidly mounted differential and an independent suspension with halfshafts linking the differential to the drive wheels. For example, the 1998 Chevrolet Corvette-not only does it use halfshafts to drive the rear wheels, the rigidly mounted transaxle is actually in the rear of the vehicle with a driveshaft connecting the front mounted engine to the transaxle! As another example, the four wheel drive Subaru models use a modified front wheel drive transaxle assembly with an additional power output. A driveshaft couples the front transaxle to the rear differential with four halfshafts driving the front and rear wheels.Universal and constant velocity jointsSee Figures 3, 4, 5, 6 and 7 Because of changes in the angle between the driveshaft or halfshaft and the axle housing or driven wheel, U-joints and CV-joints are used to provide flexibility. The engine is mounted rigidly to the vehicle frame (or sub-frame), while the driven wheels are free to move up and down in relation to the vehicle frame. The angle between the driveshaft or halfshaft and the axle housing or driven wheels changes constantly as the vehicle responds to various road conditions.Figure 3 U-joints are necessary to compensate for changes in the angle between the driveshaft and the drive axle.To give flexibility and still transmit power as smoothly as possible, several types of U-joints or CV-joints are used.The most common type of universal joint is the cross and yoke type. Yokes are used on the ends of the driveshaft with the yoke arms opposite each other. Another yoke is used opposite the driveshaft and, when placed together, both yokes engage a center member, or cross, with four arms spaced 90 apart. A bearing cup is used on each arm of the cross to accommodate movement as the driveshaft rotates.Figure 4 Exploded view of a typical cross and yoke universal assembly.The second type is the ball and trunnion universal, a T-shaped shaft that is enclosed in the body of the joint. The trunnion ends are each equipped with a ball mounted on needle bearings that move freely in grooves in the outer body of the joint, in effect creating a slip-joint. This type of joint is always enclosed.Figure 5 Cut-away view of a typical enclosed ball and trunnion type U-joint.A conventional universal joint will cause the driveshaft to speed up or slow through each revolution and cause a corresponding change in the velocity of the driven shaft. This change in speed causes natural vibrations to occur through the driveline necessitating a third type of universal joint - the double cardan joint. A rolling ball moves in a curved groove, located between two yoke-and-cross universal joints, connected to each other by a coupling yoke. The result is uniform motion as the driveshaft rotates, avoiding the fluctuations in driveshaft speeds.Figure 6 Exploded view of a typical double cardan U-joint assembly.The CV-joints, which are most commonly associated with front wheel drive vehicles, include the Rzeppa, the double offset, Tri-pod and Birfield joint.The Rzeppa and double offset are similar in construction. They use a multi-grooved cross which is attached to the shaft. Balls ride in the cross grooves and are retained to the cross by a cage. The entire assembly then slides into an outer housing which has matching grooves for the balls to ride in.Figure 7 Exploded view of a CV-joint equipped halfshaft. CV-joints shown are the Rzeppa/double offset style and the Tri-pod.The Tri-pod design is similar to the ball and trunnion design, except it has three needle bearing mounted balls inside the housing spaced evenly apart (thus its name).The newest of the CV-joints is called the Birfield. This joint is primarily found on import vehicles although some domestic vehicles are starting to use it as well. Front-wheel driveSee Figure 8 Front-wheel-drive vehicles are the more common arrangement for most cars and mini-vans these days. These vehicles do not have conventional transmissions, drive axles or driveshafts. Instead, power is transmitted from the engine to a transaxle, or combination of transmission and drive axle, in one unit. Refer to the Automatic or Manual Transmission/Transaxle Section for more information on the transaxle.A single transaxle accomplishes the same functions as a transmission and drive axle in a front-engine/rear-drive axle design. The difference is in the location of components.In place of a conventional driveshaft, a front wheel drive design uses two driveshafts, usually called halfshafts, which couple the drive axle portion of the transaxle to the wheels. Universal or constant velocity joints are used just as they would be in a rear wheel drive design.Figure 8 Example of a typical transverse engine, front-wheel drive system. Notice that the components are similar to the rear-wheel drive systems, except for location.Rear-wheel driveSee Figure 9 Rear-wheel-drive vehicles are mostly trucks, very large sedans and many sports car and coupe models. The typical rear wheel drive vehicle uses a front mounted engine and transmission assemblies with a driveshaft coupling the transmission to the rear drive axle. The rear axle assembly is usually a solid (or live) axle, although some import and/or performance models have used a rigidly mounted center differential with halfshafts coupling the wheels to the differential.Figure 9 View of the typical rear-wheel drive axle system with leaf springs.Some vehicles do not follow this typical example. Such as the older Porsche or Volkswagen vehicles which were rear engine, rear drive. These vehicles use a rear mounted transaxle with halfshafts connected to the drive wheels. Also, some vehicles were produced with a front engine, rear transaxle setup with a driveshaft connecting the engine to the transaxle, and halfshafts linking the transaxle to the drive wheels.Four-wheel driveSee Figure 10 When the vehicle is driven by both the front and rear wheels, two complete axle assemblies are used and power from the engine is directed to both drive axles at the same time. A transfer case may be attached to, or mounted near, the rear of the transmission/transaxle and directs the power flow to the rear and/or front axles through two driveshafts. Since the angles between the front and rear driveshafts change constantly, slip joints are used on the shafts to accommodate the changes in distance between axles and transfer case.Another form of four or All Wheel Drive (AWD) design may use a front mounted engine and modified front wheel drive transaxle assembly with an additional power output. Two halfshafts connect the front wheels to the transaxle. Some models may have a transfer case connected to the transaxles additional power output. A driveshaft couples the front transaxle or transfer case to the rear differential with two halfshafts driving the rear wheels.Shifting devices attached to transfer cases disengage the front drive axle when four wheel drive capability is not needed. However, some newer transfer cases are in constant mesh and cannot be totally disengaged. These are known as full-time four wheel drive and are just what the name says, four wheel drive operating all the time. This is made possible by either a differential in the transfer case or through the use of a hydraulic viscous coupling.Jeep& vehicles use a full-time system called Quadra-Trac, which is full-time four wheel drive with a limited slip differential in the transfer case. All you have to do is drive.Figure 10 Typical transmission and transfer case design four wheel drive system. The shaded area represents the power flow.Viscous coupling transfer caseBack in the early 80s, American Motors created a full-time four wheel drive system that requires no action by the driver to activate the system, and take advantage of the improved traction and handling of four wheel drive.Since this time other similar systems have been implemented both in domestic and import vehicles.The heart of these systems is a transfer case, which distributes the torque between front and rear axles by means of a viscous or fluid coupling. The coupling provides a slip-limiting action and absorbs minor driveline vibrations, giving smoother and quieter operation.When the front and rear driveshafts turn at the same speed, as they do when the vehicle drives straight down the road, there is no differential action. In a turn or other maneuvers where front and rear wheels must travel slightly different distances, differential action is required because the driveshafts must be able to rotate at slightly different speeds. When this happens, the fluid in the coupling-a liquid silicone-permits normal differential action.Greater variations in speed between the driveshafts, such as occur when a wheel or pair of wheels encounter reduced traction and tend to spin, bring the viscous couplings slip-limiting characteristics into action. The action of the viscous coupling is velocity-sensitive, permitting the comparatively slow movements typical of normal differential action but quickly building up resistance and effectively transmitting available torque to the axle with the best traction.The action of the fluid between the plates in the coupling could be compared to the action of water against a body when wading across a pool. In waist-deep water, a person can walk with comparatively little effort as long as he moves slowly and gently. However, when he tries to hurry, the additional effort that is required is proportionate to the increase in speed he attempts to achieve. Therefore, it is with the viscous coupling. However, instead of water, there is liquid silicone with a viscosity nearly the consistency of honey.This four wheel drive system is more efficient than other automatic four wheel drive systems because there is no open differential (as opposed to a limited-slip differential) between the driveshafts. In the open differential system, the loss of traction at one wheel results in no torque being delivered to the other wheels, since it is the nature of the open differential to deliver motion to the easy shaft-the one that is slipping. When using a viscous coupling, the loss of traction at one wheel on the rear axle brings the slip-limiting character of the viscous coupling into action, causing drive torque to be transferred to the front axle.In addition to the differential function, the viscous coupling also improves braking effectiveness. It acts as a skid deterrent, tending to equalize drive-shaft speeds when the wheels at one end or the other want to lock and slide.Drive axle/differentialAll vehicles have some type of drive axle/differential assembly incorporated into the driveline. Whether it is front, rear or four wheel drive, differentials are necessary for the smooth application of engine power to the road.Powerflow See Figure 11 The drive axle must transmit power through a 90 angle. The flow of power in conventional front engine/rear wheel drive vehicles moves from the engine to the drive axle in approximately a straight line. However, at the drive axle, the power must be turned at right angles (from the line of the driveshaft) and directed to the drive wheels.This is accomplished by a pinion drive gear, which turns a circular ring gear. The ring gear is attached to a differential housing, containing a set of smaller gears that are splined to the inner end of each axle shaft. As the housing is rotated, the internal differential gears turn the axle shafts, which are also attached to the drive wheels.Figure 11 Component parts of a typical driven axle assembly.Differential operationSee Figure 12 The differential is an arrangement of gears with two functions: to permit the rear wheels to turn at different speeds when cornering and to divide the power flow between both rear wheels.The accompanying illustration has been provided to help understand how this occurs. The drive pinion, which is turned by the driveshaft, turns the ring gear (1).The ring gear, which is attached to the differential case, turns the case (2).The pinion shaft, located in a bore in the differential case, is at right angles to the axle shafts and turns with the case (3).The differential pinion (drive) gears are mounted on the pinion shaft and rotate with the shaft (4).Differential side gears (driven gears) are meshed with the pinion gears and turn with the differential housing and ring gear as a unit (5).The side gears are splined to the inner ends of the axle shafts and rotate the shafts as the housing turns (6).When both wheels have equal traction, the pinion gears do not rotate on the pinion shaft, since the input force of the pinion gears is divided equally between the two side gears (7).When it is necessary to turn a corner, the differential gearing becomes effective and allows the axle shafts to rotate at different speeds (8).As the inner wheel slows down, the side gear splined to the inner wheel axle shaft also slows. The pinion gears act as balancing levers by maintaining equal tooth loads to both gears, while allowing unequal speeds of rotation at the axle shafts. If the vehicle speed remains constant, and the inner wheel slows down to 90 percent of vehicle speed, the outer wheel will speed up to 110 percent. However, because this system is known as an open differential, if one wheel should become stuck (as in mud or snow), all of the engine power can be transferred to only one wheel.Figure 12 Overview of differential gear operating principles.Limited-slip and locking differential operationSee Figure 13 Limited-slip and locking differentials provide the driving force to the wheel with the best traction before the other wheel begins to spin. This is accomplished through clutch plates, cones or locking pawls.The clutch plates or cones are located between the side gears and the inner walls of the differential case. When they are squeezed together through spring tension and outward force from the side gears, three reactions occur. Resistance on the side gears causes more torque to be exerted on the clutch packs or clutch cones. Rapid one wheel spin cannot occur, because the side gear is forced to turn at the same speed as the case. So most importantly, with the side gear and the differential case turning at the same speed, the other wheel is forced to rotate in the same direction and at the same speed as the differential case. Thus, driving force is applied to the wheel with the better traction.Locking differentials work nearly the same as the clutch and cone type of limited slip, except that when tire speed differential occurs, the unit will physically lock both axles together and spin them as if they were a solid shaft.Figure 13 Limited-slip differentials transmit power through the clutches or cones to drive the wheel having the best traction.Identifying a limited-slip drive axleMetal tags are normally attached to the axle assembly at the filler plug or to a bolt on the cover. During the life of the vehicle, these tags can become lost and other means must be used to identify the drive axle.To determine whether a vehicle has a limited-slip or a conventional drive axle by tire movement, raise the rear wheels off the ground. Place the transmission in PARK (automatic) or LOW (manual), and attempt to turn a drive wheel by hand. If the drive axle is a limited-slip type, it will be very difficult (or impossible) to turn the wheel. If the drive axle is the conventional (open) type, the wheel will turn easily, and the opposing wheel will rotate in the reverse direction.Place the transmission in neutral and again rotate a rear wheel. If the axle is a limited-slip type, the opposite wheel will rotate in the same direction. If the axle is a conventional type, the opposite wheel will rotate in the opposite direction, if it rotates at all.Gear ratio See Figure 14 The drive axle of a vehicle is said to have a certain axle ratio. This number (usually a whole number and a decimal fraction) is actually a comparison of the number of gear teeth on the ring gear and the pinion gear. For example, a 4.11 rear means that theoretically, there are 4.11 teeth on the ring gear for each tooth on the pinion gear or, put another way, the driveshaft must turn 4.11 times to turn the wheels once. Actually, with a 4.11 ratio, there might be 37 teeth on the ring gear and 9 teeth on the pinion gear. By dividing the number of teeth on the pinion gear into the number of teeth on the ring gear, the numerical axle ratio (4.11) is obtained. This also provides a good method of ascertaining exactly which axle ratio one is dealing with.Another method of determining gear ratio is to jack up and support the vehicle so that both drive wheels are off the ground. Make a chalk mark on the drive wheel and the driveshaft. Put the transmission in neutral. Turn the wheel one complete turn and count the number of turns that the driveshaft/halfshaft makes. The number of turns that the driveshaft makes in one complete revolution of the drive wheel approximates the axle ratio.Figure 14 The numerical ratio of the drive axle is the number of the teeth on the ring gear divided by the number of the teeth on the pinion gear. Driveline maintenanceSee Figures 15 and 16 Maintenance includes inspecting the level of and changing the gear lubricant, and lubricating the universal joints if they are equipped with zerk-type grease fittings. Apply high temperature chassis grease to the U-joints. CV-joints require special grease, which usually comes in a kit along with a new rubber boot.Most modern universal joints are of the extended life design, meaning that they are sealed and require no periodic lubrication. However, it is wise to inspect the joints for hidden grease plugs or fittings, initially.Also, inspect the driveline for abnormal looseness, whenever the vehicle is serviced.Figure 15 Some U-joints are equipped with grease (zerk) fittings. Lubricate these using a grease gun.Figure 16 Recommended driveshaft and differential service locations for rear-wheel drives.CV boot inspectionSee Figures 23 and 24 It is vitally important during any service procedures requiring boot handling, that care be taken not to puncture or tear the boot by over tightening clamps, misuse of tool(s) or pinching the boot. Pinching can occur by rotating the CV joints (especially the tripod) beyond normal working angles.The driveshaft boots are not compatible with oil, gasoline, or cleaning solvents. Care must be taken that the boots never encounter any of these liquids.The ONLY acceptable cleaning agent for driveshaft boots is soap and water. After washing, the boot must be thoroughly rinsed and dried before reusing.Many manufacturers recommend inspecting the CV boots at every oil change (every 3,000 miles or 4,800 km). However, a good rule of thumb is that, if the vehicle needs to be raised for any procedure, check the CV boots. Noticeable amounts of grease on areas adjacent to or on the exterior of the CV joint boot is the first indication that a boot is punctured, torn or that a clamp has loosened. When a CV joint is removed for servicing of the joint, the boot should be properly cleaned and inspected for cracks, tears and scuffed areas on the interior surfaces. If any of these conditions exist, boot replacement is recommended.Figure 23 Inspect CV boots periodically for damage.Figure 24 A torn boot should be replaced immediately.Basic drive axle problems Drive axle problems frequently give warnings in the form of abnormal noises. Unfortunately, they are often confused with noise produced by other parts.First, determine when the noise is most noticeable.Drive noise: Produced during vehicle acceleration. Coast noise: Produced while the vehicle coasts with a closed throttle. Float noise: Occurs while maintaining constant vehicle speed on a level road. Second, make a thorough check to be sure the noises are coming from the drive axle, and not from some other part of the car.Road noiseBrick or rough concrete roads produce noises that seem to come from the drive axle. Road noise is usually identical whether driving or coasting. Driving on a different type of road will tell whether the road is the problem.Tire noiseTire noises are often mistaken for drive axle problems. Snow treads or unevenly worn tires produce vibrations seeming to originate elsewhere. Temporarily inflating the tires to 40 psi will significantly alter tire noise, but will have no effect on drive axle noises (which normally cease below about 30 mph).Engine or transmission noiseDetermine at what speed the noise is most pronounced and then stop the vehicle in a quiet place. With the transmission in Neutral, run the engine through speeds corresponding to road speeds where the noise was noticed. Noises produced with the vehicle standing still are coming from the engine or transmission.Front wheel bearingsWhile holding the vehicle speed steady, lightly apply the foot brake; this will often decrease bearing noise, as some of the load is taken from the bearing.Drive axle noisesEliminating other possible sources can narrow the cause to the drive axle, which normally produces noise from worn gears or bearings. Gear noises tend to peak in a narrow speed range, while bearing noises will usually vary in pitch with engine speeds.Troubleshooting basic driveshaft problems 附录B 传动轴和驱动桥传动轴见图1和图2在传统纵向安装前置后驱汽车，驱动轴是用来传输发动机扭矩的，通过输出轴的传递，传到桥的差速器上，从而传递扭矩到车轮。这个传动轴可以是钢或铝制作，可以是实心的或空心的（管） 。一个可滑动的花键组合, 无论是作为轴的一个组成部分或利用花键套轴输出，允许传动轴跟着车桥向前向后向上和向下移动。这样车辆就可以顺利运行。图1一个典型的固体传动轴及相关配件的剖视图。对一些四轮驱动的车辆，前驱动轴的动力是通过分动器连接到前驱动桥的。驱动轴使用灵活的关节，称为万向节（ U型接头）或等速万向节（CV型接头）用于连接变速器/驱动桥/驱动车轮。可参照的等速和不等速万向节章节以获取更多信息。前轮驱动车辆也要用到传动轴,虽然他们通常被称为半轴。这个传动轴之间通常用等速万向节连接，它允许车轮向上和向下移动，同时还能平稳地将发动机的动力传递给车轮。前轮驱动车辆通常使用一个驱动桥(结合变速器和驱动桥)。图2一个典型的两端使用等速万向节的前轮驱动轴装备爆炸视图。一些后轮和四轮驱动的车辆使用半轴 。这些车辆将通常有刚性安装的差速器和独立悬架来通过半轴连接差速器和驱动轮。例如， 1998年的雪佛兰克尔维特它不仅利用半轴来驱动后轮，刚性安装的驱动桥实际上是后轮驱动车辆通过一个传动轴连接到前面的发动机到驱动桥！另一个例子，在四轮驱动斯巴鲁车型使用修改过的前轮驱动桥总成增加动力输出。一个驱动轴联合后面的差速器和驱动桥通过四个半轴驱动前轮和后轮。万向节和等速万向节见图3 ， 4 ， 5 ， 6和7因为半轴和传动轴还有桥壳和驱动轮之间的角度改变,不等速万向节和等速万向节是用来提供了灵活性的。发动机是固定安装到车架上的(或者副车架),从而和车架形成可以自由的上下移动的关系。半轴和传动轴还有桥壳和驱动轮之间的角度可以随着不同的车辆路况作出反应而改变。图3 万向节是必要的，以弥补传动轴和驱动桥之间的夹角变化。几种不同的万向节和等速万向节的使用，可以尽可能地给予动力传输灵活性和平顺性。最常见的万向节类型是十字轴和叉架型。万向节叉的使用使驱动轴的两端通过节叉臂相互对应。另一种节叉是驱动轴的对应，当放在一起，这两个节叉啮合成一个中心部件，或者十字轴，通过4个节臂分隔成90 。十字轴上的每个臂使用一个轴承套以容纳驱动轴的运动和旋转。图4一个典型的十字轴和节叉通用装配爆炸视图。 第二种类型是球面和普遍十字头式， T型轴在万向节的主体中是封闭的。在接头的外部主体沟槽机构中，十字头两端都装配有一个球在滚针轴承中自由移动，实际上建立一个滑动关节。这种类型的万向节始终是封闭的。图5 一个典型的封闭球和十字头式万向节的刨视图一个传统的不等速万向节会导致驱动轴加快或减慢通过各个转速并引起从动轴相应的速度变化。这一速度的变化会造成传动系统自然振动的发生,使必要有第三种万向节双向万向节。滚动球在曲线槽中移动,位于两个十字轴和节叉之间的万向节,通过一个联接叉连接。结果传动轴的旋转是匀速运动,避免传动轴速度的波动。图6一个典型的双向等速万向节装配爆炸视图 这个等速万向节,这在前轮驱动车辆中是最常见的,包括Rzeppa型、双偏移,Tri-pod和Birfield型万向节。这个Rzeppa和双偏移式的结构是相似的。他们用一多槽十字轴连接到轴上。球在十字槽保持到一个笼子里。整个组合就滑入一个具有安装球匹配的凹槽的外壳中。图7配备等速万向节的半轴爆炸视图。等速万向节是Rzeppa /双偏移式和Tri-pod式 Tri-pod式设计类似球笼式万向节的设计，除了它有三个球滚针轴承均匀的间隔安装在壳体内（因此而得名） 。最新的等速万向节被称为Birfield 。这种万向节，主要是安装在进口车辆，虽然一些国产车也开始使用它的。前轮驱动见图8现时，前轮驱动的车辆更常见的安排在大多数轿车和小型货车。这些车辆没有传统的传动，驱动桥或传动轴。相反，动力从发动机传递到桥，或变速器到驱动桥组合，都在一个部件。参考自动或手动变速器/驱动桥章节以获取更多驱动桥的信息。在前轮和后轮驱动桥设计中，一个单一的桥完成传递和驱动车轴相同的功能。所不同的是，组成部分的位置。用以代替传统的传动轴，一个前轮驱动设计采用两个传动轴，通常称为半轴 ，他连接驱动桥中的传动轴和车轮。在后轮驱动的设计中万向节或等速万向节的应用就像他们。图8一个典型发动机横置，前轮驱动系统实例。注意，该组件与后轮驱动系统类似，除了位置外 后轮驱动见图9后轮驱动的车辆大多是卡车，大型轿车和许多跑车和轿跑车型。典型的后轮驱动的车辆使用前置发动机和变速箱总成，通过传动轴连接传递到后驱动桥。后轴总成通常是刚性的（或活动的）轴，但也有一些进口和/或高性能车型在差速器中心通过半轴连接车轮和差速器采用了刚性安装。图9典型的后轮驱动桥钢板弹簧系统一些汽车不遵循这个典型的例子。如老款的保时捷或大众汽车是后置后驱。这些车辆使用后驱动桥使半轴连接到驱动轮。另外,一些车辆制造前置后轮驱动装置,它通过一个传动轴连接动力到驱动桥,和半轴连接驱动桥到驱动轮。四轮驱动见图10当汽车是前后两个车轮驱动时，两个完整的轴总成在同一时间都使用来自发动机的动力。这里附加一个分动器，或就近安装，这样变速器动力通过两个传动轴，直接流向后桥和前桥。由于前和后传动轴角度的变化不断，这里用柔性万向节，以适应车轴和分动器之间距离的变化。另一种形式的四轮或全轮驱动（全时四轮驱动系统）的设计使用前置发动机和改装过的前轮驱动桥能增加的动力输出。两个半轴连接前轮到前桥。有些型号可能有分动器连接两驱动桥之间以增加动力输出。一个驱动轴连接前桥和分动器和后差速器通过两个半轴驱动后轮。当不需要四轮驱动性能的时候，连接到分动器的转换装置会使前驱动桥脱离。然而，一些新的分动器是在经常啮合和不能完全脱离接触。这些被称为“全职”四轮驱动和如它的名字所说，四轮驱动所有时间都在运转。这一点要归功于分动器的差速器或通过使用一种液压粘性联轴器。吉普车使用全时系统称为Quadra-Trac，它在分动器中带有全时四轮驱动的限滑差速器。您需要做的仅仅是驾驶。图10典型四轮驱动系统的变速器和分动器设计。阴影部分表示动力流向 粘性联轴分动器早在80年代，美国汽车公司创立了一个全时四轮驱动系统，无需采取行动来启动系统，并利用改进的牵引力和操作四轮驱动。自那以后类似系统已经在国内和进口汽车上应用。在这些系统中核心是分动器，它通过粘性或液力偶合器分配转矩给前后桥。耦合提供了限滑，并吸收传动系统较小的振动，使运转更平滑和安静。当前传动轴和后传动轴以同样的速度行使时，因为车辆在路上是直线行使的，这样就没有差速行为。在转弯或其他在前轮和后轮必须行使不同距离时，差速器需要起到作用，因为传动轴必须要能够在稍微不同的速度下旋转。在这种情况下，液力耦合器有正常的差速行为。传动轴之间更大的速度变化，如一个或一对车轮遇到牵引力减小和附带的旋转，使粘性联轴器的限滑特性起到作用，粘性联轴器的这种特性作用是速度感应的，在差速作用中允许较低的速度，不过很快就恢复阻力和有效地传递现有的转矩，传递给驱动桥最好的牵引力。在联轴器之间液体的这种作用，可以拿身体穿过水池来做对比。在齐腰的水深中，一个人在水中可以顺畅地行走，只要他移动得缓慢和轻柔。然而，当他试图更快,当他以增加速度去努力实现时，阻力随着他增加的速度成比例增加。所以，对于粘性联轴器。这不是水，而是一种接近蜂蜜粘度的粘性硅树脂。这种限滑四轮驱动系统比其它自动四轮驱动系统更有效能，因为在传动轴之间是没有“开放”差速器（与限滑差速器形成对比）。在“开放式”差速器系统中，一个车轮损失的牵引力结果并没有分到其它的车轮，因为在开放式差速器中，在车轮滑行时，动力自然地传递到更“容易”旋转的轴。当使用的是一个粘性联轴器，在后桥失去牵引力的一个车轮促使粘性联轴器的限滑装置起到作用，使转矩转移到前桥。除了增加差速器的功能外，粘性联轴器还改善了制动效能。当一端车轮或者其它车轮锁止和滑动的时候，它作为一个防滑装置，使传动轴的速度趋向相等。驱动桥/差速器所有的车辆
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