Transmission

TransmissionTransmission Complied by Ajay ‡ ‡ ‡ ‡ ‡ ‡ ‡ ‡ Typical Power Transmission In Automobile Types of gearbox Clutch Differential CV joint Types of gears & applications Materials used in transmission Leading manufacturers of gearbox The Goal of a Transmission Cars need transmissions because of the physics of the gasoline engine. First, any engine has a redline -- a maximum rpm value above which the engine cannot go without exploding. Second, engines have narrow rpm ranges where horsepower and torque are at their maximum. For example, an engine might produce its maximum horsepower at 5,500 rpm. The transmission allows the gear ratio between the engine and the drive wheels to change as the car speeds up and slows down. You shift gears so the engine can stay below the redline and near the rpm band of its best performance. Ideally, the transmission would be so flexible in its ratios that the engine could always run at its single, best-performance rpm value. Derailleur gears 6.Hub gears .Continuously-variable 5.Manual 2.Semi-automatic ‡ Tiptronic ‡ Direct-Shift Gearbox 4.Transmission types 1.Automatic 3. automatic transmissions feature clutch packs to select gear ratio. Transmissions which employ clutch packs but allow the driver to manually select the current gear are called semi. Some manuals are referred to by the number of gears they offer (e. but some. 5-speed) as a way of distinguishing between automatic or other available manual transmissions. Most automobile manual transmissions allow the driver to select any gear at any time. Manual transmissions are characterized by gear ratios which are selectable by engaging pairs of gears inside the transmission. although some do not. This second type of transmission is sometimes called a sequential manual transmission. Conversely. such as those commonly mounted on motorcycles and some types of race cars. only allow the driver to select the next-highest or next-lowest gear ratio.Manual transmission A manual transmission (also known as a stick shift or standard transmission) is a type of transmission used in automotive applications. . Manual transmissions often feature a driver-operated clutch and a movable gear selector. a 5-speed automatic transmission is referred to as a 5-speed automatic. Contemporary automotive manual transmissions are generally available with between 4 and 6 forward gears and one reverse gear.. In contrast.automatic transmissions. although manual transmissions have been built with as few as 2 and as many as 7 gears.g. Mercedes-Benz Actros. manual transmission . Mercedes-Benz C-class sport coupe, six-speed manual transmission, graphic illustration Other types of transmission in mainstream automotive use are the automatic transmission,semi-automatic transmission and the continuously variable transmission. Manual transmissions come in two basic types: simple unsynchronized systems where gears are spinning freely and their relative speeds must be synchronized by the operator to avoid noisy and damaging "clashing" and "grinding" when trying to mesh the rotating teeth, and synchronized systems that eliminate this necessity while changing gears. The clutch. which involved shifting the transmission into neutral and speeding up the gears. such transmissions are often easier to shift from one gear to another without the use of the clutch at all. otherwise the teeth would refuse to mesh. the speed of the gear driven by the engine had to drop to match the speed of the next gear. with multiple gear ratios available to the operator. and even had reverse. a technique called double declutching was used. In fact. as this happened naturally when the clutch was depressed. is only used for starting from a standstill. the gear driven by the engine had to be sped up to mesh with the output gear. so that the gears would be spinning at roughly the same speed when engaged. When upshifting to a higher gear. But the gears were engaged by sliding mechanisms or simple clutches.Unsynchronized transmission The earliest automotive transmissions were entirely mechanical unsynchronized gearing systems. it was just a matter of skill and experience to hear and feel when the gears could be persuaded to mesh. in these cases. which required a skilled operator who could use timing and careful throttle manipulation when shifting. requiring that the clutch be engaged so that the engine could be used to speed up the gears. then shifting from neutral into the lower gear. They could be shifted. when downshifting. However. . the action of all of these components is so smooth and fast it is hardly noticed. (One collar typically serves for two gears. the others being allowed to rotate freely. in the other direction selects the other) When the rings are bridged by the collar. but the gears are not directly rotationally connected to the shafts on which they rotate. that particular gear is rotationally locked to the shaft and determines the output speed of the transmission. the gears can freely rotate or be locked to the shaft on which they are carried. so arranged so that only one collar may be permitted to lock only one gear at any one time. The locking mechanism for any individual gear consists of a collar on the shaft which is able to slide sideways so that teeth or "dogs" on its inner surface bridge two circular rings with teeth on their outer circumference. Instead. sliding in one direction selects one transmission speed. The gearshift lever manipulates the collars using a set of linkages. thus greatly reducing the skill required to shift gears. when "shifting gears". one to the shaft. the teeth of the gears of all the transmission speeds are always in mesh and rotating. In a modern gearbox. In a synchromesh gearbox. in which all gears are always in mesh but only one of these meshed pairs of gears is locked to the shaft on which it is mounted at any one time.Synchronized transmission A modern car gearbox is of the constant mesh or synchromesh type. To correctly match the speed of the gear to that of the shaft as the gear is engaged. the collar initially applies a force to a cone-shaped brass clutch which is attached to the gear. the locking collar from one gear is disengaged and that of another engaged. one attached to the gear. . which brings the speeds to match prior to the collar locking into place. The collar is prevented from bridging the locking rings when the speeds are mismatched by baulk rings. For racing of production based transmissions. Being made of brass. six speeds and so on became universal by the 1960s. then four speeds. requiring only a single synchro and a simple linkage. however. known colloquially as "crashboxes". sometimes half the dogs on the synchros are removed to speed the shifting process. drivers' manuals in cars suggested that if the driver needed to shift from second to first. as there is only one reverse gear in the normal automotive transmission and changing gears in reverse is not required.The first synchronized transmission system was introduced by Cadillac in 1929. and the rotation of all the sets of gears at once results in higher frictional losses. In the early 1950s only the second-third shift was synchromesh in most cars. at the expense of much more wear. however. The modern cone system was developed by Porche and introduced in the 1952 Porche356. is not synchromesh. the process of shifting a synchromesh transmission is slower than that of shifting a nonsynchromesh transmission. for several reasons. With continuing sophistication of mechanical development. fully synchromesh transmissions with three speeds. synchronizers are prone to wear and breakage more than the actual gears. Even though automotive transmissions are universally synchromesh. five speeds. heavy trucks and machinery as well as dedicated racing transmissions are still usually nonsynchromesh transmissions. cone synchronizers were called "Porsche-type" for many years after this. it was best to come to a complete stop then shift into first and start up again. which are cast iron. . In addition. Reverse gear. Internals Like other transmissions, a manual transmission has both input and output shafts. Pairs of gears are attached to these shafts such that, when selected, will cause the output shaft to rotate at a given ratio of the input shaft speed. When a driver selects a gear, he is simply selecting a pair of these gears to be used; mechanical connections translate the driver's selection into an appropriate connection of gears and prevent more than one set of gears being engaged at any given time (as that would cause the transmission to lock). The teeth on gears of mass market automobiles are not straightcut, but are helically cut, in order to reduce gear whine. Reverse gear often is straightcut, however, leading to a characteristic whine from many cars when reversing. In racing vehicles (most commonly those involved in drag racing), sometimes a transbrake is incorporated, allowing the driver to lock the transmission into both first gear and reverse gear at the same time. This serves the purpose of allowing the driver to increase the engine speed without changing the vehicle's speed (much as one would do while in neutral, or while the clutch is disengaged), but being able to transfer as much of the resultant power to the tires in a shorter period of time. The input shaft of a manual transmission comes from the clutch, and is connected to a layshaft. The lay shaft has one gear on its input end and several on the output end, usually one per selectable gear. The output gears of the layshaft connect to the drive gears. These are fixed in place on the output shaft, which leads to the differential and tires. Manual transmissions are often equipped with 4, 5, or 6 forward gears. Nearly all have exactly one reverse gear. In three or four speed transmissions, in most cases, the topmost gear is "direct", i.e. a 1:1 ratio. For five speed or higher transmissions, the highest gear is usually an overdrive gear, with a ratio of less than 1:1. Older cars were generally equipped with 3-speed transmissions, or 4speed transmissions for high performance models and 5-speeds for the most sophisticated of automobiles; in the 1970s, 5-speed transmissions began to appear in low priced mass market automobiles and even compact pickup trucks, pioneered by Toyota(who advertised the fact by giving each model the suffix SR5 as it acquired the fifth speed). Today, mass market automotive manual transmissions are essentially all 5-speeds, with 6-speed transmissions beginning to emerge in high performance vehicles in the early 1990s, and recently beginning to be offered on some high-efficiency and conventional passenger cars. On earlier models with three or four forward speeds, the lack of an overdrive ratio for relaxed and fuel efficient highway crusing was often filled by incorporation of a separate overdrive unit in the rear housing of the transmission, separately actuated by a knob or button, often incorporated into the gearshift knob. A Very Simple Transmission To understand the basic idea behind a standard transmission, the diagram below shows a very simple two-speed transmission in neutral: Let's look at each of the parts in this diagram to understand how they fit together: The green shaft comes from the engine through the clutch. The green shaft and gear turn at the same rpm as the engine. so is the red shaft. The green shaft and green gear are connected as a single unit. so all of the gears on the layshaft and the layshaft itself spin as one unit. the engine and the transmission are disconnected so the engine can run even if the car is standing still. The green shaft and the red shaft are directly connected through their meshed gears so that if the green shaft is spinning. (The clutch is a device that lets you connect and disconnect the engine and the transmission. the engine and the green shaft are directly connected to one another.) The red shaft and gears are called the layshaft. . When you push in the clutch pedal. In this way. These are also connected as a single piece. the layshaft receives its power directly from the engine whenever the clutch is engaged. When you release the clutch pedal. the collar can slide left or right along the yellow shaft to engage either of the blue gears. However. the yellow shaft is spinning. The collar is connected. directly to the yellow shaft and spins with the yellow shaft. called dog teeth. the yellow shaft can turn inside the blue gears while the blue gears and the layshaft are motionless. fit into holes on the sides of the blue gears to engage them. The blue gears ride on bearings. If the engine is off but the car is coasting.The yellow shaft is a splined shaft that connects directly to the drive shaft through the differential to the drive wheels of the car. The purpose of the collar is to connect one of the two blue gears to the yellow drive shaft. If the wheels are spinning. so they spin on the yellow shaft. Teeth on the collar. . through the splines. First Gear The picture below shows how. Meanwhile. the blue gear on the left is turning. which turns the blue gear on the right. the green shaft from the engine turns the layshaft. . This gear transmits its energy through the collar to drive the yellow drive shaft. Both of the blue gears freewheel on the yellow shaft at the different rates controlled by their ratios to the layshaft. When the collar is between the two gears (as shown in the first figure). the transmission is in neutral. but it is freewheeling on its bearing so it has no effect on the yellow shaft. the collar engages the blue gear on the right: In this picture. when shifted into first gear. From this discussion, you can answer several questions: When you make a mistake while shifting and hear a horrible grinding sound, you are not hearing the sound of gear teeth mis-meshing. As you can see in these diagrams, all gear teeth are all fully meshed at all times. The grinding is the sound of the dog teeth trying unsuccessfully to engage the holes in the side of a blue gear. The transmission shown here does not have "synchros" , so if you were using this transmission you would have to double-clutch it. Double-clutching was common in older cars and is still common in some modern race cars. In double-clutching, you first push the clutch pedal in once to disengage the engine from the transmission. This takes the pressure off the dog teeth so you can move the collar into neutral. Then you release the clutch pedal and rev the engine to the "right speed." The right speed is the rpm value at which the engine should be running in the next gear. The idea is to get the blue gear of the next gear and the collar rotating at the same speed so that the dog teeth can engage. Then you push the clutch pedal in again and lock the collar into the new gear. At every gear change you have to press and release the clutch twice, hence the name "double-clutching." You can also see how a small linear motion in the gear shift knob allows you to change gears. The gear shift knob moves a rod connected to the fork. The fork slides the collar on the yellow shaft to engage one of two gears. A Real Transmission The following animation shows you the internal workings of a four-speed transmission with reverse. . . . . . 000 5th 0.915:1 3. Here are some typical gear ratios: Gear Ratio RPM at Transmission Output Shaft with Engine at 3.000 rpm 1st 2.913 3rd 1.A five-speed transmission applies one of five different gear ratios to the input shaft to produce a different rpm value at the output shaft.295 2nd 1.568:1 1.278 .315:1 1.510 4th 1.000:1 3.195:1 2. Internally. it looks something like this: .The five-speed manual transmission is fairly standard on cars today. Looking at the shift rods from the top.There are three forks controlled by three rods that are engaged by the shift lever. first and second gear: . they look like this in reverse. Moving the knob forward and backward moves the collar to engage one of the gears. You can see that as you move the shifter left and right you are engaging different forks (and therefore different collars). When you push the knob forward to engage first gear. At all times.the dog teeth would never engage. you are actually pulling the rod and fork for first gear back. the blue reverse gear in this diagram is turning in a direction opposite to all of the other blue gears. However. Reverse gear is handled by a small idler gear (purple). they will make a lot of noise! .Keep in mind that the shift lever has a rotation point in the middle. it would be impossible to throw the transmission into reverse while the car is moving forward -. Therefore. The outer portion of the collar then slides so that the dog teeth can engage the gear. A synchro's purpose is to allow the collar and the gear to make frictional contact before the dog teeth make contact.Synchronizers Manual transmissions in modern passenger cars use synchronizers to eliminate the need for double-clutching. like this: The cone on the blue gear fits into the cone-shaped area in the collar. . This lets the collar and the gear synchronize their speeds before the teeth need to engage. but this is the general idea. and friction between the cone and the collar synchronize the collar and the gear. Every manufacturer implements transmissions and synchros in different ways. and by extension to the drive wheels. This allows for the transmission's gears to be independent of the engine (spinning purely through momentum or. * When the clutch pedal is fully released. * When the clutch pedal is fully depressed. the motion of the vehicle). one must dictate the speed of the other. so would be the engine. The clutch is what accomplishes this in manual transmissions. because the entire purpose the transmission serves is gear reduction. the clutch is fully disengaged. This allows for shifting without gear grinding. Without it. . the clutch "slips" to varying degrees. a coupling device is utilized to be able to separate the engine and transmission when necessary. and no torque is transferred from the engine to the transmission.Clutch In all vehicles utilizing a transmission . getting a vehicle to move from rest would be extremely difficult. This slippage allows for the slow introduction of power. the tires would dictate engine speed. Because the engine and tires are designed to be linked in order to drive. with less resistance introduced to the engine until enough momentum is built that the engine can operate normally without output reduction from the clutch. Clutch slippage is useful. * In between these extremes. and essentially all of the engine's torque is transferred. and anytime the vehicle is at a stop. for any engaged gear. the clutch is fully engaged. the engine and tires would at all times be inextricably linked. If there was no slippage. and as such. there are clutches in many things you probably see or use everyday: Many cordless drills have a clutch. chainsaws have a centrifugal clutch and even some yo-yos have a clutch! . In fact.How Clutches Work If you drive a manual transmission car. you may be surprised to find out that your car has more than one clutch in it. And it turns out that folks with automatic transmission cars have clutches. too. The clutch connects the two shafts so that they can either be locked together and spin at the same speed. In a drill.com/clutch1. In these devices. http://auto. the wheels need to be disconnected from the engine somehow. for instance. The clutch allows us to smoothly engage a spinning engine to a non-spinning transmission by controlling the slippage between them. one shaft is driven by a motor and the other is driving a drill chuck.howstuffworks. and the other shaft is driving another device. In a car. or be decoupled and spin at different speeds. one of the shafts is typically driven by a motor or pulley. you need a clutch because the engine spins all the time and the car wheels don't.Clutches are useful devices with two rotating shafts. In order for a car to stop without killing the engine.htm . . . you can see that the flywheel is connected to the engine. and the clutch plate is connected to the transmission. .Automobile Clutch In the figure below. . . . . . . . . http://auto.howstuffworks.com/clutch2.htm . . This locks the engine to the transmission input shaft.When your foot is off the pedal. causing them to spin at the same speed. which in turn presses against the flywheel. the springs push the pressure plate against the clutch disc. . a cable or hydraulic piston pushes on the release fork. http://auto. As the middle of the diaphragm spring is pushed in.htm When the clutch pedal is pressed.howstuffworks. This releases the clutch from the spinning engine. a series of pins near the outside of the spring causes the spring to pull the pressure plate away from the clutch disc.com/clutch3. which presses the throw-out bearing against the middle of the diaphragm spring.Force and Friction The amount of force the clutch can hold depends on the friction between the clutch plate and the flywheel. and how much force the spring puts on the pressure plate. . These springs help to isolate the transmission from the shock of the clutch engaging.Note the springs in the clutch plate. and they spin in sync. The clutch only wears while the clutch disc and the flywheel are spinning at different speeds. or the shoes of a drum brake -. Another problem sometimes associated with clutches is a worn throwout bearing.What Can Go Wrong? The most common problem with clutches is that the friction material on the disc wears out. you will wear out your clutch a lot faster.howstuffworks. The friction material on a clutch disc is very similar to the friction material on the pads of a disc brake. and eventually it won't transmit any power from the engine to the wheels.com/clutch4. So if you are the type of driver who slips the clutch a lot. It is only when the clutch disc is slipping against the flywheel that wearing occurs.after a while. http://auto. This problem is often characterized by a rumbling noise whenever the clutch engages. the clutch will start to slip.htm . When most or all of the friction material is gone. it wears away. the friction material is held tightly against the flywheel. When they are locked together. the pinion gears are free to rotate about their own axis when either of the side gears meets resistance. usually consisting of gears. which is transferred to two side gears by means of usually two opposing central pinion gears on a common shaft. such a method would result in too much damage to both the tire and road surface. In some vehicles such as karts. but when changing direction the outer wheel needs to travel farther than the inner wheel. When the ring gear and cage rotate. for supplying equal torque to the driving wheels. This works well enough when travelling in a straight line. torque is simply applied evenly to all driving wheels using a simple driveshaft. Differentials are typically composed of a gear mechanism in which a ring gear receives input power. the simple solution results in the inner wheel spinning. the pinion gears drive the side gears. For general road use. The pinion gear(s) are mounted to a cage which is affixed to the ring gear. Hence. . even as they rotate at different speeds.Differential (mechanics) In an automobile and other wheeled vehicles. a differential is a device. Another solution is the locking differential.One solution to this problem is the limited slip differential (LSD). as small differences in rotational speed between the front and rear of the vehicle cause a torque to be applied across the transmission. in the Antikythera mechanism. causing both wheels to turn at the same speed regardless of which has more traction. . paved roads in all wheel drive mode. used such a train to produce the difference between two inputs. With this differential. This phenomenon is known as "wind-up" and can cause damage to the transmission. one input related to the position of the sun on the zodiac. and the other input related to the position of the moon on the zodiac. On loose surfaces these differences are absorbed by the slippage of the road surface. The output of the differential gave a quantity related to the moon's phase. Vehicles without a center differential should not be driven on dry. this is equivalent to removing the differential entirely. one of the most common of which is the clutch-type LSD. A differential gear train can also be used to give the difference between two input axles. which employs a mechanism for allowing the pinion gear(s) to be locked. A four wheel-drive vehicle will have at least two differentials (one for each pair of wheels) and possibly a center differential to apportion power between the front and rear axles. The oldest known example of a differential. each of the side gears has a clutch which limits the speed difference between the two wheels. The first use of this technology on a production automobile was Honda¶s 1997Active Torque Transfer System on the Prelude SH. though they may be eliminated in the coming years. not an integrated system. including yaw rate. .Active differential A relatively new technology is the electronically-controlled active differential. A computer uses inputs from multiple sensors. Active differentials are common in the World Rally Championship. steering angle. Fully integrated active differentials are used on the 2005 MR Ferrari F430 and on all four wheels in the Acura RL. and lateral acceleration and adjusts the distribution of torque to compensate for undesirable handling behaviors like understeer. This "differential" was actually a planetary gearset placed next to an open front differential. invention later used on automobiles by Carl Benz.Antikythera mechanism contains a planar differential. 1st century BC .first use of differential on an Australian steam car by David Shearer.James Starley of Coventry invents chain-drive differential for use on bicycles. . [Sources: Britannica Online and [1]] 1832 . 1897 . in ancient times.modern automotive differential patented by watchmaker Onésiphore Pecqueur (1792-1852) of the Conservatoire des Arts et Métiers in France for use on a steam car. Here are some of the milestones in the history of this device. at least in some places.History There are many claims to the invention of the differential gear. 1810 . 2634 BC according to legend . a differential for road locomotives.Rudolph Ackerman of Germany invents a four-wheel steering system for carriages.Richard Roberts of England patents 'gear of compensation'.South Pointing Chariot used by the Yellow Emperor Huang Di in China. 1827 . which some later writers mistakenly report as a differential. 1876 . but it is likely that it was known. If the left side gear (red) encounters resistance or is immobile. In an automobile and other wheeled vehicles. input torque is applied to the ring gear (blue). in turn applying extra rotation to the right side gear (yellow). The pinion gear (green) applies power to both side gears (red and yellow). If both wheels turn at the same rate. which in turn may drive the left and right wheels. the pinion gear does not rotate. a differential is a device. the pinion gear (green) rotates about the left side gear.In this differential. usually consisting of . or sun gear. Typically. rotating about a central. the planet gears are mounted on a movable arm or carrier which itself may rotate relative to the sun gear. or planet gears. The planet gear carrier (green) is driven by an input torque. The sun gear (yellow) provides the output torque. Note the red marks both before and after the input drive is rotated 45° clockwise. which meshes with the planet gears.Epicyclic gearing Epicyclic gearing or planetary gearing is a gear system that consists of one or more outer gears. Epicyclic gearing is used here to increase output speed. while the ring gear (red) is fixed. Epicyclic gearing systems may also incorporate the use of an outer ring gear or annulus. . Each wheel travels a different distance through the turn. and that the inside wheels travel a shorter distance than the outside wheels. the front wheels travel a different distance than the rear wheels. Also.Why You Need a Differential Car wheels spin at different speeds. Since speed is equal to the distance traveled divided by the time it takes to go that distance. . especially when turning. the wheels that travel a shorter distance travel at a lower speed. the back wheels on a front-wheel drive car -. That force would have to be transmitted through the axle from one wheel to another. But the driven wheels are linked together so that a single engine and transmission can turn both wheels.this is not an issue.For the non-driven wheels on your car -. putting a heavy strain on the axle components. This would make turning difficult and hard on your car: For the car to be able to turn. There is no connection between them. one tire would have to slip. a great deal of force is required to make a tire slip. the wheels would have to be locked together. so they spin independently.the front wheels on a rear-wheel drive car. . forced to spin at the same speed. With modern tires and concrete roads. If your car did not have a differential. . . allowing each output to spin at a different speed.What is a Differential? The differential is a device that splits the engine torque two ways. . and also in many all-wheel-drive (full-time four wheel-drive) vehicles. These all-wheel-drive vehicles need a differential between each set of drive wheels. and they need one between the front and the back wheels as well. .The differential is found on all modern cars and trucks. because the front wheels travel a different distance through a turn than the rear wheels. Part-time four-wheel-drive systems don't have a differential between the front and rear wheels. This is why these vehicles are hard to turn on concrete when the four-wheel-drive system is engaged. they are locked together so that the front and rear wheels have to turn at the same average speed. instead. Spinning at Different Speeds The image below labels the components of an open differential. . slowing the rotational speed of the transmission one final time before it hits the wheels ‡To transmit the power to the wheels while allowing them to rotate at different speeds (This is the one that earned the differential its name.How Differentials Work The differential has three jobs: ‡To aim the engine power at the wheels ‡To act as the final gear reduction in the vehicle.) . then the ring gear has 4.htm . and none of the pinions within the cage are rotating -. this is the last gear reduction in the car. both drive wheels are spinning at the same speed. Note that the input pinion is a smaller gear than the ring gear.10 times as many teeth as the input pinion gear.both side gears are effectively locked to the cage. If the final drive ratio is 4. You may have heard terms like rear axle ratio or final drive ratio. http://auto.10.com/differential3.howstuffworks. These refer to the gear ratio in the differential. The input pinion is turning the ring gear and cage.When a car is driving straight down the road. com/differential3. you can see that the pinions in the cage start to spin as the car begins to turn. http://auto. while the outside wheel spins faster.howstuffworks. allowing the wheels to move at different speeds. the wheels must spin at different speeds.htm In the figure above. The inside wheel spins slower than the cage.When a car makes a turn. . howstuffworks.com/differential4.htm .Open Differential http://auto. you may know of a trick that makes acceleration easier: If you start out in second gear. There are two factors that determine how much torque can be applied to the wheels: equipment and traction. when there is plenty of traction. If you give the car more gas after the wheels start to slip. there needs to be enough traction to transmit that torque to the ground. In dry conditions. even though a car may be able to produce more torque. This will make it easier to accelerate without spinning the wheels. the amount of torque is limited to the greatest amount that will not cause a wheel to slip under those conditions. or even third gear. instead of first.On Thin Ice The open differential always applies the same amount of torque to each wheel. . the wheels will just spin faster. So. the amount of torque applied to the wheels is limited by the engine and gearing. such as when driving on ice. because of the gearing in the transmission you will have less torque available to the wheels. If you've ever driven on ice. in a low traction situation. Now what happens if one of the drive wheels has good traction. and the maximum amount of torque is limited to the greatest amount that will not make the wheels slip. It doesn't take much torque to make a tire slip on ice. And when the wheel with good traction is only getting the very small amount of torque that can be applied to the wheel with less traction. your car isn't going to move very much. Remember that the open differential always applies the same torque to both wheels. . and the other one is on ice? This is where the problem with open differentials comes in. locking differential and Torsen differential. with an open differential on both the front and the back. including the clutch-type LSD. and you won't be able to move at all. Now. remember -.Off Road Another time open differentials might get you into trouble is when you are driving off-road. The next few sections will detail some of the different types of limited slip differentials. sometimes called positraction. The solution to these problems is the limited slip differential (LSD). the open differential always applies the same torque to both wheels. If one of the front tires and one of the back tires comes off the ground. When a wheel slips. Limited slip differentials use various mechanisms to allow normal differential action when going around turns.as we mentioned on the previous page. or an SUV. the viscous coupling. they will just spin helplessly in the air. they allow more torque to be transferred to the non-slipping wheel. . If you have a four-wheel drive truck. you could get stuck. .Clutch-Type Limited Slip The clutch-type LSD is probably the most common version of the limited slip differential. but it adds a spring pack and a set of clutches. This type of LSD has all of the same components as an open differential. Some of these have a cone clutch that is just like the synchronizers in a manual transmission. The result is that you can move forward. although still not with the full power of your car. wanting both wheels to go the same speed. even though the wheel on the ice is not able to transmit much torque to the ground. The torque supplied to the wheel not on the ice is equal to the amount of torque it takes to overpower the clutches. The clutches fight this behavior. Both side gears spin with the cage when both wheels are moving at the same speed. the other wheel will still get the torque it needs to move. which are attached to the cage. as in a turn. . The stiffness of the springs combined with the friction of the clutch determine how much torque it takes to overpower it. it must first overpower the clutch.the only time the clutches step in is when something happens to make one wheel spin faster than the other. and the clutches aren't really needed -.The spring pack pushes the side gears against the clutches. If one wheel wants to spin faster than the other. Getting back to the situation in which one drive wheel is on the ice and the other one has good traction: With this limited slip differential. The viscous coupling has two sets of plates inside a sealed housing that is filled with a thick fluid. Under normal conditions.the wheels that are not slipping. When a car is turning. The faster the plates are spinning relative to each other. perhaps because it is slipping. One set of plates is connected to each output shaft. When one set of wheels tries to spin faster. It is commonly used to link the back wheels to the front wheels so that when one set of wheels starts to slip. torque will be transferred to the other set. the more torque the viscous coupling transfers. this also highlights a disadvantage of the viscous coupling: No torque transfer will occur until a wheel actually starts slipping. The coupling does not interfere with turns because the amount of torque transferred during a turn is so small. tries to catch up with the faster disks. as shown in below. The viscous fluid. both sets of plates and the viscous fluid spin at the same speed. stuck between the plates. However. This transfers more torque to the slower moving wheels -.Viscous Coupling The viscous coupling is often found in all-wheel-drive vehicles. the set of plates corresponding to those wheels spins faster than the other. the difference in speed between the wheels is not as large as when one wheel is slipping. dragging the slower disks along. . . . howstuffworks.http://auto.com/differential9.htm . A simple experiment with an egg will help explain the behavior of the viscous coupling.applied force to the shell. If you set an egg on the kitchen table. but the yolk will quickly catch up. speeding it up. the force is applied between the fluid and the sets of plates in the same way as between the yolk and the shell.between the still-moving yolk and the shell -. the shell will be moving at a faster speed than the yolk for a second. To prove that the yolk is spinning. that friction -. we used the friction between the shell and the yolk to apply force to the yolk. causing it to speed up. When we stopped the shell. In a viscous coupling. once you have the egg spinning quickly stop it and then let go -. the shell and the yolk are both stationary. If you suddenly spin the egg. . In this experiment.the egg will start to spin again (unless it is hard boiled). . pneumatic or hydraulic mechanism to lock the two output pinions together. but adds an electric. This type of differential has the same parts as an open differential.Locking and Torsen The locking differential is useful for serious off-road vehicles. if a particular Torsen differential is designed with a 5:1 bias ratio. the Torsen is superior to the viscous coupling because it transfers torque to the stable wheels before the actual slipping occurs. For instance. The bias ratio determines how much torque can be transferred. if one set of wheels loses traction completely. . they are often used to transfer power between the front and rear wheels. Both wheels will continue to spin at the same speed as if nothing had changed. the difference in torque causes the gears in the Torsen differential to bind together. These devices are often used in high-performance all-wheel-drive vehicles. the other wheel won't know or care. As soon as one wheel starts to lose traction. it is capable of applying up to five times more torque to the wheel that has good traction. the Torsen differential will be unable to supply any torque to the other set of wheels. In this application. However. The design of the gears in the differential determines the torque bias ratio. and when activated. Like the viscous coupling. and five times zero is zero. If one wheel ends up off the ground. both wheels will spin at the same speed.This mechanism is usually activated manually by switch. The Torsen (from Torque Sensing) works as an open differential when the amount of torque going to each wheel is equal. The Torsen differential* is a purely mechanical device. clutches or viscous fluids. it has no electronics. Front universal joint 2. . The universal joints are required because the differential is offset and at an angle to the transmission.Driveshaft There are three main components to a basic drive shaft system: 1. and 3. Rear universal joint The purpose of the shaft is to transfer power from the motorcycle transmission to the differential. Drive shaft. This is a property due to the design of the joint. they can be phased so they cancel each other out and no vibration is felt. At angles less than 1/2 degree. This angle is necessary in order to keep the needle bearings contained in the caps rolling. causing vibration and eventually failure. Whenthere are two joints. So the next question is: "If universal joints vibrate. Vibration All universal joints vibrate. the joints are always in pairs. the needles stay locked in the same position and wear into the cap. . then why does my car not vibrate?" When a drive shaft is designed for application in a car.Driveshaft Angle All universal joints are designed to have a minimum of 1/2 degree of working angle. it will create vibrations. Because the drive shaft is at some angle to the output shaft. At the other end of the driveshaft. and vice versa. . This causes the rotating speed of the drive shaft to fluctuate on every turn. The rear end moves up and down. When we look at the opposite side of the universal joint the other two caps on the universal joint must turn about the center of the drive shaft. This produces a constant shaft speed at the differential shaft. so the drive shaft can never be perfectly straight. then slowing to slightly below the output shaft speed.The universal joints work as follows: When the output shaft turns. This joint is timed to the front universal joint in order to be exactly opposite to it. When the drive shaft speeds up from the action of the front universal joint. Automotive drive shafts are not straight for the reasons explained above. If this effect is not counteracted with a second universal joint. the two caps of the front universal joint must turn around the center of the output shaft. at first speeding up slightly faster than the output shaft. the action of the rear universal joint slows it down. there is a second universal joint. the cross of the universal joint must wobble back and forth to allow the bearing caps to trace these circles out while rotating. and then. Since there are fewer components used in this drive shaft than a CV-type drive shaft (which we will talk more about later).. As time went by you started making your truck yours. this will be the most inexpensive type of drive shaft.Pictured is a typical drive shaft. bumpers. Chances are that suddenly your truck wasn't as pleasant to drive on the highway as it once was. a lift kit. the slip spline allows for the changing distance between the transfer case output and the axle. Nagging vibrations keep you from enjoying the drive. When your 4x4 came from the factory it should have come to you with little or no driveline vibration. What happened?! . adding nerf bars.. This type of drive shaft is probably what was used on your vehicle when it came from the factory. As the suspension on your vehicle allows your axle to move as you travel down the road or trail. It uses a single u-joint at each end of the drive shaft. and a slip spline. When there is some operating angle on the u-joint. as you drive down the road the suspension will allow the axle to move. When your driveline has an operating angle of zero (if the output of the transfer case is lined up perfectly with the input to the differential). This is never the case though. Even if the transfer case and differential were lined up perfectly. The u-joints are now being forced to operate beyond the limits that will ensure a smooth operating drive shaft. the path that the u-joint travels in is no longer a perfect circle. the u-joint always has a small operating angle.Driveline vibration is most likely the culprit. . To understand why it is that a u-joint creates so much more vibration when the operating angle is increased you need to look more closely at the way the u-joint works. The lift kit you installed changed the operating angles of your driveline. continually changing the operating angle of the driveline. it is an ellipse. the u-joint is operating under the best possible conditions. they are bolted together! Path of u-joint operating through an angle . the u-joint is actually speeding up and slowing down twice per revolution! Imagine with me.In the drawing to the right you can see an example of the path the u-joint travels when there is some operating angle present. it has to travel a longer distance than the output of the transfer case does. Since the output of the transfer case is traveling in a perfect circle. it should create no vibration... The blue ellipse represents the path that the u-joint must follow when operating at an angle. so the u-joint is traveling FASTER than the output of the transfer case! After all they both must arrive at point B in the same amount of time. If the driveline of your vehicle is traveling at a constant speed. the black circle will represent the path the output of the transfer case is traveling in. Now look at the path of the u-joint as it travels from point A to point B. and at a constant speed. a perfect circle... The u-joint must travel a lesser distance since it follows the blue path of the ellipse. or it falls apart on the trail because the slip spline is not long enough for your vehicle's needs. If they are not in phase it will create unnecessary driveline vibration. therefore the u-joint is traveling SLOWER than the output of the transfer case.Now look between point B and point C. Spicer Recommends 3 degrees Maximum Also notice the proper geometry is to have the transfer case output and axle exactly parallel . Care must be taken when putting the two pieces back together so that the u-joints are in phase. and from point D to point A it again travels SLOWER. From point C to point D the u-joint again travels FASTER. Each one must speed up and slow down at exactly the same time as the other. The u-joint actually must speed up and slow down twice per revolution when it is operating at some angle. This is something to consider if you ever take your drive shaft apart. Because of the changing speed of the drive shaft the two u-joints (one at each end of the drive shaft) must be in phase. which will also decrease the life of the u-joint. but it will likely cause unwanted driveline vibration. The u-joint angle must be considered when the suspension is extended as far as possible. etc) you are also changing the elliptical path that the u-joint must travel in. The greater the operating angle. Take this into consideration when designing your suspension and driveline. it will depend on the shape of the yokes being used to hold the u-joint. If your u-joint is normally operating near the absolutely maximum the joint can handle. In this case you would be able to use the u-joint at the maximum angle that the u-joint will still be able to function. The angle could also be increased by grinding away some of the material in the yokes. and the greater the torsional vibration caused by the u-joint. It is for this reason Spicer recommends an operating angle of less then 3 degrees for a u-joint. . shimming the rear axle.As you adjust the operating angle of the u-joint (by lifting the vehicle. and FAILURE IS CERTAIN. This absolute angle is not a set angle. a very conservative number. The absolute limiting factor for the operating angle of a u-joint is when the yokes holding the u-joint in place contact with each other. the less like a circle the path of the ellipse. and only at slow speeds then driveline vibration is not a concern. The u-joint will then be forced to operate beyond what is physically possible. right where they contact each other. If the vehicle is used only on the trail. when you take the vehicle on the trail the u-joint will have to withstand an even greater operating angle because the suspension will let the axle extend away from the vehicle. It is possible to use the u-joint at an operating angle greater than 3 degrees. These are simple to make and can be tremendously strong. where a cross-shaped metal pivot sits between two forked carriers (These are not strictly CV joints as they result in a variation of the transmitted speed except for certain specific configurations). notably Alfa-Romeo. and are still used to provide a flexible coupling in the propeller shafts. . where there is not very much movement.Constant-velocity joint Constant Velocity Joints or CV joints allow a rotating shaft to transmit power through a variable angle. although some rear wheel drive cars. without an appreciable increase in friction or play. and need regular maintenance. they become "notchy" and difficult to turn when operated at extreme angles. at constant rotational speed. for a total of ten CV joints. Early front wheel drive systems such as those used on the Citroen Traction Avant and the front axles of Land Rover and similar four wheel drive vehicles used Hardy-Spicer joints.Audi Quattros use them for all four half-axles and on the front-to-rear drive shaft (propeller shaft) as well. and could only be used in rigid axle designs.BMW. However.Porche. They also need more complicated support bearings when used in drive axles. They are mainly used in front wheel drive cars. and Volvo use them as part of the rear axle and all-wheel-drive. Based on a design by Alfred Rzeppa in 1928. At the "inboard" end. a "Triax" joint is used. This has a three-pointed yoke attached to the shaft.As front wheel drive systems became more popular. Constant Velocity joints solved a lot of these problems. Since there is only significant movement in one axis. Two different types of CV joint are used on the driveshafts of modern cars. the shortcomings of Hardy-Spicer joints in front axles became more and more apparent. which has barrel-shaped rollers on the ends. with cars such as the Mini using compact transverse engine layouts. These fit into a cup with three matching grooves attached to the differential. where the shaft only moves up and down with the movement of the suspension. this simple arrangement works well. Driveshafts using CV joints are self-supporting along their length. They allowed a smooth transfer of power despite the wide range of angles they were bent in. . and do not need additional supports (although very long shafts such as the right-hand driveshaft on the Citroen CX or Peugeot 205 have an intermediate bearing that supports the inboard joint). This has a large steel ball attached to the end of the shaft. the CV joint may disjoin causing the vehicle to stop moving. The joint will then pick up dirt. and road deicing salt and cause the joint to overheat and wear. In worst case. . water.At the "outboard" of the shaft. and can accommodate the large changes of angle when the front wheels are turned by the steering system. These are held in place by a bronze or steel cage. and the grease can also contaminate the brakes. This joint is extremely flexible. Maintenance is usually limited to checking that the rubber gaiter (dust/weather boot) that covers them is secure and not split. and fit into a grooved cup similar to the triax joint. the MoS2-molybdenite grease that the joint is packed with. Damaged CV joint gaiters will usually cause a car to fail a safety inspection. with grooves machined in it to take (usually six) large steel balls. will be thrown out. These joints are very strong. a slightly different unit is used. and are usually highly overspecified for a given application. If the gaiter is damaged. when lifting off the throttle. you can clean the joint carefully. which in turn is usually caused by the outer joint gaiter having split. left and right. Wear in the inner joints shows up as a "clunk" when applying power. Kits which include the grease. Some universal gaiters are split lengthwise enabling them to be fitted without having to disassemble the wheel hub and CV joint. It is caused by the joint overheating. Wear in the outer joint usually shows up as vibration at certain speeds. To determine if the joint is worn. If caught in time. The two main failures are wear and partial seizure. or if severe. allowing the joint to throw out its grease. . Worn joints will make a rhythmic clicking or cracking noise. a bit like the vibration caused by an unbalanced wheel. find a big empty car park and drive the car slowly in tight circles. gaiter and retaining clips are available from most motor factors.Faultfinding and diagnosis Constant velocity joints are usually reliable and largely trouble-free. Partial seizure causes a strange "pattering" sensation through the suspension. repack with grease and replace the gaiter. with no regard to the vibration the drive shaft would cause at high speed. . the best choice would be a typical drive shaft with a single u-joint at each end. if you wanted a drive shaft that would operate at the maximum possible angle. This joint contains a centering kit to ensure that each u-joint always has the same amount of angle. Double Cardan Driveshaft It is also important to note that the maximum operating angle of a double cardan joint is LESS than the maximum operating angle of a single u-joint. unlike the typical driveshaft that we talked about above. Again.Constant Velocity Drive Shafts Constant Velocity (CV) drive shafts are named so because they do rotate at a constant speed. The most common CV joint for drive shafts is the double cardan. to split the difference of the operating angle where the drive shaft connects to the output of the transfer case. The double cardan uses two u-joints. Due to the increased number of parts used in making the CV driveshaft. For a CV drive shaft the ideal operating condition is to have the pinion pointed at the output of the transfer case. In the image to the right. Using a CV shaft also requires you to adjust the pinion angle. notice the change in pinion angle (the angle of the pinion flange on the differential) in relation to the output of the transfer case. so that when the axle rotates up while under use the pinion angle will be correct. . then it may be necessary to set the pinion low. For example if the vehicle has significant axle wrap that will allow the pinion to point up higher than desired when accelerating or maintaining freeway speeds. a CV drive shaft is a must. so that the lower u-joint has no operating angle.For any vehicle with a steep drive shaft angle. they will of course cost more than a typical driveshaft. Each vehicle will have its own quirks that need to be taken into account when setting pinion angle. that must be able to comfortably and reliably drive at freeway speeds. you can purchase inexpensive axle shims from your local 4wd parts shop. and come in increments of 1 degree. If your vehicle uses control arms to locate the axle beneath the vehicle (like a Jeep TJ) you may have to purchase adjustable length control arms to be able to adjust pinion angle. as well as the angle at which the axle sits. buy a pair of shims with the angle that you need. loosen the u-bolts that hold the springs and axle together. Axle shims can be found as large as 6 degrees (possibly even larger). but if you're in need of that extreme of a change it may be time to consider cutting the spring mounts off your axle. and welding new ones on at the desired angle. Instead. place the shims between the spring and axle. . These adjustable links will give you the ability to control where the axle sits beneath the vehicle. Shims should be readily available. To install the shims.To adjust the pinion angle of a vehicle with leaf springs. DO NOT use multiple shims to get the angle you need. and retighten the u-bolts. and will also keep you driveshaft higher up. it may be best to return the pinion angle to stock and consider other options. Consider calling a shop like South Bay Driveline who is willing to help you find a solution. which could starve the pinion gear and bearing for oil damaging them. and will include free shipping to your continental US address. This will reduce the operating angle of your driveline considerably.Adjusting the pinion angle of the front axle with shims may have a large effect on steering geometry and wheel caster. with the pinion rotated up the gear oil level at the pinion will be less. A shop can cut through the factory welds. For some vehicles it is also possible to modify the axle to rotate the pinion up. and out of harm's way. but leave the stock steering geometry and alignment in place. Also note. The two most common types of drive shafts have been shown here. If adjusting the pinion angle adversely effects the steering or throws the alignment far off from the specified values. It may be necessary to overfill the axle so that the pinion receives the necessary oil. rotate the knuckles. but it is best to contact a driveline shop to discuss your particular needs. but costly alternative is a high-pinion differential. If your local driveline service is not willing to help you find a solution to your unusual driveline problem. and weld the knuckles back in place. More creative solutions to extreme driveline problems exist. . Used with either rotated spring perches or axle shims this will rotate the pinion up. you may have to search elsewhere for your driveshaft. A common. . . . . . . Custom designed and built performance driveshafts and CV joint grinding. direct from one of Europe¶s leading specialist aftermarket production facilities. . . . . Cross-section through a typical outer CV joint . . or the position where no gears are engaged. backward. so reverse will be less likely to be accidentally chosen when downshifting from 5th to 4th (or by someone used to a 6-speed transmission and trying to shift from 5th to the non-existent 6th). gear selector. and right allows the driver to select any given gear. which is technically a sixth gear on this transmission. In this configuration. A common layout for a 5-speed transmission is shown below. although the shifter is usually equipped with springs so that it will return to the N position if not left in another gear. the entire horizontal line is a neutral position. There is usually a mechanism that only allows selection of reverse from the neutral position. the gear lever must be pushed laterally before it is pushed longitudinally. Moving this lever forward.Gear selection Floor-mounted shifter In most modern cars. N marks neutral. The R denotes reverse. The shift pattern for a specific N transmission is usually printed on the shifter knob. 1 3 5 This layout is called the shift pattern. left. In reality. gear lever. or simply shifter. gears are selected through a lever attached to the floor of the automobile²this selector is often called a gearstick. 2 4 R . so that he does not accidentally select it when trying to find first gear.Another common five-speed shift pattern is: R 1 3 I N I 4 5 2 Transmissions equipped with this shift pattern usually feature a lockout mechanism that requires the driver to depress a switch or the entire gear lever when entering reverse. . rear-wheel drive cars have a transmission that sits between the driver and the front passenger seat. A 4-speed floor shifter is sometimes referred to as "Four on the Floor". Floor-mounted shifters are often connected directly to the transmission. Front-wheel drive and rear-engined cars often require a mechanical linkage to connect the shifter to the transmission. Most front-engined. although shifting occurs in a vertical plane instead of a horizontal one. First gear in a 3-speed is often called "low. Column shifters are mechanically similar to floor shifters. but manual column shifters are no longer common. .Column-mounted shifter Some older cars feature a gear lever which is mounted on the steering column of the car. A 3-speed column shifter is sometimes referred to as "Three on a Tree". Column shifters also generally involve additional linkages to connect the shifter with the transmission." while third is usually called "high. trucks. and vans produced with manual transmissions during the 1950s and 1960s. Note that reverse in a car with a column shift is in nearly the same position as park (P) is on a car with a column-mounted gear selector with an automatic transmission. of course." There is. Many automatic transmissions still use this placement. R 2 N 1 3 The 3-speed shift pattern is typical of American cars. no overdrive. Sequential transmissions are generally controlled by a forward-backward lever. these are connected mechanically to the transmission. these controls are attached to sensors which instruct a transmission computer to perform a shift²many of these systems can be switched into an automatic mode. These transmissions often provide clutch control.Sequential manual Some transmissions do not allow the driver to arbitrarily select any gear. foot pedal. but the clutch is only necessary when selecting first or reverse gear from neutral. . although the shift pattern is modified slightly for safety reasons. In some cases. In many modern examples. much like an automatic transmission. Most gear changes can be performed without the clutch. where the computer controls the timing of shifts. or set of paddles mounted behind the steering wheel. Motorcycles typically employ sequential transmissions. Instead. the driver may only ever select the next-lowest or next-highest gear ratio. Semi-manual Some very new transmissions (BMW's Sequential Manual Gearbox (SMG) and Audi's Direct-Shift Gearbox (DSG), for example) are conventional manual transmissions with a computerized control mechanism. These transmissions feature independently selectable gears but do not have a clutch pedal. Instead, the transmission computer controls a servo which disengages the clutch when necessary. These transmissions vary from sequential transmissions in that they still allow nonsequential shifts: BMWs SMG system, for example, can shift from 6th gear directly to 4th gear when decelerating from high speeds. How Automatic Transmissions Work If you have ever driven a car with an automatic transmission, then you know that there are two big differences between an automatic transmission and a manual transmission: There is no clutch pedal in an automatic transmission car. There is no gear shift in an automatic transmission car. Once you put the transmission into drive, everything else is automatic. Both the automatic transmission (plus its torque converter) and a manual transmission(with its clutch) accomplish exactly the same thing, but they do it in totally different ways. It turns out that the way an automatic transmission does it is absolutely amazing! Location of the automatic transmission Some Basics Just like that of a manual transmission, the automatic transmission's primary job is to allow the engine to operate in its narrow range of speeds while providing a wide range of output speeds. Mercedes-Benz CLK, automatic transmission, cut-away model while in an automatic transmission. . So the transmission uses gears to make more effective use of the engine's torque. the engine would be screaming along near the red-line. and that ratio would have to be selected to allow the car to travel at the desired top speed. the same set of gears produces all of the different gear ratios. cars would be limited to one gear ratio. and at high speeds. and to keep the engine operating at an appropriate speed. If you wanted a top speed of 80 mph. you'd quickly find out that you had almost no acceleration when starting out. The key difference between a manual and an automatic transmission is that the manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios. The planetary gearset is the device that makes this possible in an automatic transmission. If you did. You've probably never tried driving a manual transmission car using only third gear. A car like this would wear out very quickly and would be nearly undriveable.Without a transmission. then the gear ratio would be similar to third gear in most manual transmission cars. Among other things. you see: ‡An ingenious planetary gearset ‡A set of bands to lock parts of a gearset ‡A set of three wet-plate clutches to lock other parts of the gearset ‡An incredibly odd hydraulic system that controls the clutches and bands ‡A large gear pump to move transmission fluid around . you find a huge assortment of parts in a fairly small space.Planetary Gearsets When you take apart and look inside an automatic transmission. planet carrier. this one part creates all of the different gear ratios that the transmission can produce.The center of attention is the planetary gearset. An automatic transmission contains two complete planetary gearsets folded together into one component. About the size of a cantaloupe. Everything else in the transmission is there to help the planetary gearset do its thing. From left to right: the ring gear. and two sun gears . We can get lots of different gear ratios out of this gearset. Choosing which piece plays which role determines the gear ratio for the gearset. Let's take a look at a single planetary gearset. One of the planetary gearsets from our transmission has a ring gear with 72 teeth and a sun gear with 30 teeth.Planetary Gearsets & Gear Ratios Any planetary gearset has three main components: ‡The sun gear ‡The planet gears and the planet gears' carrier ‡The ring gear Each of these three components can be the input. the output or can be held stationary. . the output speed is slower than the input speed. Notice that the first gear ratio listed above is a reduction -. The second is an overdrive -.4:1 Also. There are several other ratios that can be gotten out of this planetary gear set.Input Output Stationary Calculation Gear Ratio A Sun (S) Planet Carrier (C) Ring (R) 1 + R/S 3. but these are the ones that are relevant to our automatic transmission. but the output direction is reversed.71:1 C Sun (S) Ring (R) Planet Carrier (C) -R/S -2.4:1 B Planet Carrier (C) Ring (R) Sun (S) 1 / (1 + S/R) 0.the output speed is faster than the input speed. The last is a reduction again. . locking any two of the three components together will lock up the whole device at a 1:1 gear reduction. http://auto. With two of these gearsets in a row.Animation of the different gear ratios related to automatic transmissions Click on the buttons on the left in the table above.htm So this one set of gears can produce all of these different gear ratios without having to engage or disengage any other gears. we can get the four forward gears and one reverse gear our transmission needs.howstuffworks.com/automatic-transmission3. . It has one ring gear that is always the output of the transmission. that looks like a single planetary gearset but actually behaves like two planetary gearsets combined. called a compound planetary gearset. and two sun gears . but it has two sun gears and two sets of planets. planet carrier.Gears This automatic transmission uses a set of gears. Let's look at some of the parts: How the gears in the transmission are put together Left to right: the ring gear. Notice how the planet on the right sits lower than the planet on the left. Planet carrier: Note the two sets of planets.The figure below shows the planets in the planet carrier. Only the planet on the left engages the ring gear. The planet on the right does not engage the ring gear -.it engages the other planet. . The shorter gears are engaged only by the smaller sun gear.Next you can see the inside of the planet carrier. The longer planets are engaged by the bigger sun gear and by the smaller planets. Inside the planet carrier: Note the two sets of planets. . htm . http://auto.The animation below shows how all of the parts are hooked up in a transmission.com/automatic-transmission4.howstuffworks. the smaller sun gear is driven clockwise by the turbine in the torque converter.4:1. but is held still by the one-way clutch (which only allows rotation in the clockwise direction) and the ring gear turns the output.First Gear In first gear. The first set of planets engages the second set.this is where the trick with the two sets of planets comes in. The planet carrier tries to spin counterclockwise. but because that clutch is released. the bigger sun gear is free to spin in the opposite direction of the turbine (counterclockwise). the gear ratio is: Ratio = -R/S = . The small gear has 30 teeth and the ring gear has 72.4:1 So the rotation is negative 2. But the output direction is really the same as the input direction -. this combination reverses the direction. . so referring to the chart on below. and the second set turns the ring gear.72/30 = -2. which means that the output direction would be opposite the input direction. You can see that this would also cause the bigger sun gear to spin. we multiply the first stage by the second. and the ring gear fixed. planet carrier as output. 2. with the sun as input.47:1 reduction. This may sound wacky.67:1 To get the overall reduction for second gear. . the formula is: 1 + R/S = 1 + 36/30 = 2.2 x 0. but it works. The first stage of the planet carrier actually uses the larger sun gear as the ring gear. the planet carrier acts as the input for the second planetary gear set. It acts like two planetary gearsets connected to each other with a common planet carrier.Second Gear This transmission does something really neat in order to get the ratio needed for second gear. So the first stage consists of the sun (the smaller sun gear). the larger sun gear (which is held stationary) acts as the sun.2:1 The planet carrier turns 2. the planet carrier. At the second stage.2 times for each rotation of the small sun gear. and the output is the planet carrier. the ring gear (large sun gear) is held stationary by the band.67. and the ring gear acts as the output. so the gear ratio is: 1 / (1 + S/R) = 1 / (1 + 36/72) = 0. and the ring (the larger sun gear). The input is the small sun gear. For this stage. to get a 1. . If both sun gears turn in the same direction. producing a 1:1 ratio. With the arrangement in this gearset it is even easier -all we have to do is engage the clutches that lock each of the sun gears to the turbine. You'll remember from the previous section that all we have to do to get a 1:1 output is lock together any two of the three parts of the planetary gear. This locks the ring gear to the planets and causes everything to spin as a unit.Third Gear Most automatic transmissions have a 1:1 ratio in third gear. the planet gears lockup because they can only spin in opposite directions. This allows cars to drive at freeway speed while the engine speed stays nice and slow. The small sun gear freewheels. a shaft that is attached to the housing of the torque converter (which is bolted to the flywheel of the engine) is connected by clutch to the planet carrier. Ratio = 1 / (1 + S/R) = 1 / ( 1 + 36/72) = 0. .67:1 So the output spins once for every two-thirds of a rotation of the engine.Overdrive By definition.the opposite of a reduction. this time with the planet carrier for input. the output speed is 3000 RPM. In this transmission. an overdrive has a faster output speed than input speed. engaging the overdrive accomplishes two things at once. the sun gear fixed and the ring gear for output. the only input comes from the converter housing. Nothing is connected to the turbine. Let's go back to our chart again. If the engine is turning at 2000 rotations per minute (RPM). It's a speed increase -. and the larger sun gear is held by the overdrive band. In order to improve efficiency. when overdrive is engaged. In this transmission. some cars have a mechanism that locks up the torque converter so that the output of the engine goes straight to the transmission. we have: Ratio = -R/S = 72/36 = 2. the bigger sun gear is driven.Reverse Reverse is very similar to first gear.0:1 So the ratio in reverse is a little less than first gear in this transmission. The planet carrier is held by the reverse band to the housing. except that instead of the small sun gear being driven by the torque converter turbine. and the small one freewheels in the opposite direction. . So. according to our equations from the last page. In the next section. inputs and outputs: Gear 1st Input 30-tooth sun 30-tooth sun Planet carrier Output 72-tooth ring Planet carrier 72-tooth ring Fixed Planet carrier 36-tooth ring 36-tooth sun Gear Ratio 2. . Let's summarize the gear ratios. This is done by a series of clutches and bands inside the transmission. you are probably wondering how the different inputs get connected and disconnected.0:1 After reading these sections.4:1 2.67:1 -2.47:1 3rd 30.0:1 OD Reverse Planet carrier 36-tooth sun 72-tooth ring 72-tooth ring 36-tooth sun Planet carrier 0.Gear Ratios This transmission has four forward gears and one reverse gear.and 36-tooth suns 72-tooth ring 1.67:1 2nd Total 2nd 1.2:1 0. we'll see how these work. For instance. Let's take a look at a band. with different clutches and bands engaging and disengaging. a shaft that is attached to the housing of the torque converter (which is bolted to the flywheel of the engine) is connected by clutch to the planet carrier. and the larger sun gear is held by the overdrive band. when we discussed overdrive. we discussed how each of the gear ratios is created by the transmission. lots of things have to be connected and disconnected by clutches and bands. The small sun gear freewheels. The big sun gear is held to the housing by a band so that it could not rotate. Nothing is connected to the turbine. The planet carrier gets connected to the torque converter housing by a clutch. Each gear shift triggers a series of events like these. when overdrive is engaged. we said: In this transmission. The small sun gets disconnected from the turbine by a clutch so that it can freewheel. the only input comes from the converter housing. . To get the transmission into overdrive.Clutches and Bands In the last section. They are actuated by hydraulic cylinders inside the case of the transmission.Bands In this transmission there are two bands. . you can see one of the bands in the housing of the transmission. One of the bands In the figure above. The gear train is removed. The bands in a transmission are. which actuates the band. steel bands that wrap around sections of the gear train and connect to the housing. literally. The metal rod is connected to the piston. The pistons that actuate the bands are visible here. causes the pistons to push on the bands. routed into the cylinder by a set of valves. Above you can see the two pistons that actuate the bands. locking that part of the gear train to the housing. . Hydraulic pressure. One of the clutches in a transmission . Springs make sure that the clutch releases when the pressure is reduced.Clutches The clutches in the transmission are a little more complex. Each clutch is actuated by pressurized hydraulic fluid that enters a piston inside the clutch. Notice the rubber seal on the piston -. In this transmission there are four clutches.this is one of the components that is replaced when your transmission gets rebuilt. Below you can see the piston and the clutch drum. where it locks to the clutch housing. The hydraulic system controls which clutches and bands are energized at any given moment. The friction material is splined on the inside. These clutch plates are also replaced when the transmission is rebuilt. where it locks to one of the gears. .The next figure shows the alternating layers of clutch friction material and steel plates. The steel plate is splined on the outside. The clutch plates The pressure for the clutches is fed through passageways in the shafts. The mechanism that does all this is pretty neat. . You have to be able to engage the mechanism even if the lever does not line up with the gear. something has to prevent the lever from popping up and disengaging. Let's look at some of the parts first.When You Put the Car in Park It may seem like a simple thing to lock the transmission and keep it from spinning. Once engaged. but there are actually some complex requirements for this mechanism: You have to be able to disengage it when the car is on a hill (the weight of the car is resting on the mechanism). The parking-brake mechanism engages the teeth on the output to hold the car still. the car can't move. This is the section of the transmission that hooks up to the drive shaft -. .so if this part can't spin.The output of the transmission: The square notches are engaged by the parkingbrake mechanism to hold the car still. The empty housing of the transmission with the parking brake mechanism poking through. This helps to disengage the parking brake when you are parked on a hill -. . Notice that it has tapered sides. as it does when the car is in park Above you see the parking mechanism protruding into the housing where the gears are located.the force from the weight of the car helps to push the parking mechanism out of place because of the angle of the taper. This rod actuates the park mechanism. . This rod is connected to a cable that is operated by the shift lever in your car. the bushing holds down the lever so that the car will not pop out of park if it is on a hill. the rod pushes the spring against the small tapered bushing.it has to roll a little for the teeth to line up to where the parking mechanism can drop into place. If the park mechanism is lined up so that it can drop into one of the notches in the output gear section. but the lever will not lock into place until the car rolls a little and the teeth line up properly. then the spring will push on the tapered bushing. .Top view of the park mechanism When the shift lever is placed in park. This is why sometimes your car moves a little bit after you put it in park and release the brake pedal -. Once the car is safely in park. the tapered bushing will push the mechanism down. If the mechanism is lined up on one of the high spots on the output. If you put the transmission in second gear. unless you move the shift lever. the transmission will downshift to the next lower gear. If you floor the gas pedal. If you move the shift selector to a lower gear. You may not realize how many different ways it operates. shifts will occur at lower speeds than if you accelerate at full throttle. If the car is going too fast. it will never downshift or upshift out of second. For instance. the transmission will automatically select the gear based on vehicle speed and throttle pedal position. If you accelerate gently.Hydraulic System The automatic transmission in your car has to do numerous tasks. the transmission will downshift unless the car is going too fast for that gear. even from a complete stop. it will wait until the car slows down and then downshift. here are some of the features of an automatic transmission: If the car is in overdrive (on a four-speed transmission). . .You've probably seen something that looks like this before. The passageways you can see route fluid to all the different components in the transmission. without them. It is really the brain of the automatic transmission. we'll discuss the key components of the hydraulic system. many hoses would be needed to connect the various parts of the transmission. Passageways molded into the metal are an efficient way to route fluid. managing all of these functions and more. then we'll see how they work together. First. and as the gears rotate.The Pump Automatic transmissions have a neat pump. It draws fluid from a sump in the bottom of the transmission and feeds it to the hydraulic system. . fluid is drawn up from the sump on one side of the crescent and forced out into the hydraulic system on the other side. The outer gear is turned by the inner gear. Gear pump from an automatic transmission The inner gear of the pump hooks up to the housing of the torque converter. so it spins at the same speed as the engine. The pump is usually located in the cover of the transmission. It also feeds the transmission cooler and the torque converter. called a gear pump. It is connected to the output. the more the governor valve opens and the higher the pressure of the fluid it lets through. Inside the governor is a spring-loaded valve that opens in proportion to how fast the governor is spinning -. so the faster the car moves. Fluid from the pump is fed to the governor through the output shaft.the faster the governor spins.The Governor The governor is a clever valve that tells the transmission how fast the car is going. the more the valve opens. the faster the governor spins. The governor . The faster the car goes. the more pressure is put on the throttle valve. it feeds a circuit that prevents overdrive from engaging.Valves and Modulators Throttle Valve or Modulator To shift properly. Depending on which gear is selected. if the shift lever is in third gear. the automatic transmission has to know how hard the engine is working. For instance. The modulator senses the manifold pressure. the manual valve feeds hydraulic circuits that inhibit certain gears. . The further the gas pedal is pressed. There are two different ways that this is done. which drops when the engine is under a greater load. Manual Valve The manual valve is what the shift lever hooks up to. Other cars use a vacuum modulator to apply pressure to the throttle valve. Some cars have a simple cable linkage connected to a throttle valve in the transmission. and they route that fluid to one of two circuits to control which gear the car runs in. They are supplied with fluid by the pump. The shift valve determines when to shift from one gear to the next. The shift valve is pressurized with fluid from the governor on one side. and the throttle valve on the other.Shift Valves Shift valves supply hydraulic pressure to the clutches and bands to engage each gear. the 1 to 2 shift valve determines when to shift from first to second gear. The shift circuit . The valve body of the transmission contains several shift valves. For instance. When the car accelerates quickly. Since the car is accelerating at light throttle. Each shift valve responds to a particular pressure range. This means that the pressure from the governor has to be higher (and therefore the vehicle speed has to be faster) before the shift valve moves over far enough to engage second gear. As car speed increases. Let's discuss what happens when the car accelerates gently. so when the car is going faster. the shift will occur at a lower speed. because the pressure from the governor is high enough to trigger that valve. the throttle valve applies more pressure against the shift valve.The shift valve will delay a shift if the car is accelerating quickly. . If the car accelerates gently. the pressure from the governor builds. the throttle valve does not apply much pressure against the shift valve. and the second gear circuit opens. the 2-to-3 shift valve will take over. This forces the shift valve over until the first gear circuit is closed. Electronically controlled transmissions have even more elaborate control schemes.electronically controlled transmissions can do things like: ‡Downshift automatically when going downhill to control speed and reduce wear on the brakes ‡Upshift when braking on a slippery surface to reduce the braking torque applied by the engine ‡Inhibit the upshift when going into a turn on a winding road . In addition to monitoring vehicle speed and throttle position. but each hydraulic circuit is controlled by an electric solenoid.Electronic Controls Electronically controlled transmissions. Using this information and an advanced control strategy based on fuzzy logic -. still use hydraulics to actuate the clutches and bands. This simplifies the plumbing on the transmission and allows for more advanced control schemes. and even the anti-lock braking system. the transmission controller can monitor the engine speed.a method of programming control systems using human-type reasoning -. In the last section we saw some of the control strategies that mechanically controlled transmissions use. which appear on some newer cars. if the brake pedal is being pressed. Some automatic transmissions with advanced control systems can detect this situation after you have gone around a couple of the curves. Let's say you're driving on an uphill. when you take your foot off the gas. When you come to a curve you slow down. winding mountain road. Then when you accelerate out of the curve.Let's talk about that last feature -. Most transmissions will upshift to third gear. and "learn" not to upshift again. they will downshift again. . the transmission shifts into second gear to give you enough acceleration and hill-climbing power. you would probably leave the car in the same gear the whole time. When you are driving on the straight sections of the road. But if you were driving a manual transmission car. or even overdrive. taking your foot off the gas pedal and possibly applying the brake.inhibiting the upshift when going into a turn on a winding road. Each pulley comprises of 2 pieces of disc. As a result. with slope surface. the other pulley must adapt itself inversely since the length of the belt is fixed. that equals to a big gear. Continuous Variable Transmission is an ideal design . the belt runs in an orbit with relatively small diameter. that equals to a small gear of conventional gearbox. the belt is pushed outside and runs in an orbit of large diameter. at any time the most suitable ratio can be chosen so that performance and energy efficiency are both optimized.CVT (Continuous Variable Transmission) In theory.it varies the transmission ratio continuously so that you can say it is an automatic transmission with infinite no. of ratios. The theory of CVT is very simple. You might simply understand it from the picture beside. When one pulley is varied. The core of CVT consists of a driving belt running between two pulleys. one connect to the engine output and one to the drive shaft. the transmission ratio can be varied by pushing or easing the discs. When the discs are pushed towards together. As a result. This multiply the change of transmission ratio. When the discs are positioned far away from each other. too. . c.c. use a metallic belt developed by Netherlands' Van Doorne Transmissie BV.000 c. I'm afraid many automatic makers will lose a big slice of market share. As the belt is the highly stressed member. but implementation is difficult. In the 80s. the longitude ones hold the transverse plates and deal with strain. This belt consists of hundreds of transverse metal plates and longitude metal tapes. Therefore it was bounded to Ford Fiesta.c. The transverse ones are used to grip the pulley. Honda introduced it into the 1600 c. class ! Hopefully in the next few years. it must be very strong and grip very well on the pulleys. As the belt improved gradually. Most CVTs. In then.c. Civic. . class. including Honda Civic's. then Nissan even applied it to the 2.300c. all of them had less than 1.000 c. Fiat Uno 60 Selecta and Subaru Justy. CVT failed to be popular because belts were not strong enough to handle the torque from larger engines.Difficulties The theory is ideal. CVT will invade 3. size and reliability. improvements in design have made CVTs more common. so you didn't see them in production automobiles. . These days. The Toyota Prius is a hybride car that uses a CVT. CVTs could not compete with four-speed and five-speed transmissions in terms of cost. The transmission is connected to the engine through the clutch.CVT A CVT has a nearly infinite range of gear ratios. The input shaft of the transmission therefore turns at the same rpm as the engine. In the past. are designing their drivetrains around CVTs. Indeed. Today. including General Motors. the technology has been refined and improved. which Leonardo da Vinic conceptualized more than 500 years ago and is now replacing planetary automatic transmissions in some automobiles. Honda and Nissan. Audi. several car manufacturers. But the continuously variable transmission (CVT). Nissan HR15DE engine with Xtronic CVT . ever since the first toroidal CVT patent was filed in 1886.Some say you can't teach an old dog new tricks. is one old dog that has definitely learned a few new tricks. continuously variable transmissions don't have a gearbox with a set number of gears.CVT Basics Unlike traditional automatic transmissions. Ford Freestyle Duratec engine with CVT . which means they don't have interlocking toothed wheels. The most common type of CVT operates on an ingenious pulley system that allows an infinite variability between highest and lowest gears with no discrete steps or shifts. broadly speaking. remember that. . they are still described as having low and high "gears" for the sake of convention. Although CVTs change this ratio without using a set of planetary gears. a gear refers to a ratio of engine shaft speed to driveshaft speed.If you're wondering why the word "gear" still appears in the explanation of a CVT. Most CVTs only have three basic components: ‡A high-power metal or rubber belt ‡A variable-input "driving" pulley ‡An output "driven" pulley CVTs also have various microprocessors and sensors. but the three components described above are the key elements that enable the technology to work. Pulley-based CVT . By comparison. and you'll see a complex world of gears.Pulley-based CVTs: The Parts Peer into a planetary automatic transmission. brakes. clutches and governing devices. a continuously variable transmission is a study in simplicity. The variable-diameter pulleys are the heart of a CVT. the belt rides higher in the groove. the belt rides lower in the groove. When the two cones of the pulley are far apart (when the diameter increases). V-belts get their name from the fact that the belts bear a V-shaped cross section. CVTs may use hydraulic pressure. and the radius of the belt loop going around the pulley gets smaller. Each pulley is made of two 20degree cones facing each other. which increases the frictional grip of the belt. Vbelts are preferred if the belt is made of rubber. and the radius of the belt loop going around the pulley gets larger. centrifugal force or spring tension to create the force necessary to adjust the pulley halves. When the cones are close together (when the diameter decreases). . A belt rides in the groove between the two cones. . the driven pulley transfers energy to the driveshaft. The driving pulley is also called the input pulley because it's where the energy from the engine enters the transmission. known as the drive pulley (or driving pulley).Pulley-based CVTs: Creating "Gears" Variable-diameter pulleys must always come in pairs. The second pulley is called the driven pulley because the first pulley is turning it. As an output pulley. One of the pulleys. is connected to the crankshaft of the engine. the belt rides higher and the pitch radius increases. When the pulleys are far apart. the belt rides lower and the pitch radius decreases. then the rotational speed of the driven pulley decreases. they create an infinite number of gear ratios -. the other decreases its radius to keep the belt tight. resulting in a higher ³gear. then the rotational speed of the driven pulley increases. a CVT has an infinite number of "gears" that it can run through at any time.´ Thus. The ratio of the pitch radius on the driving pulley to the pitch radius on the driven pulley determines the gear.´ When the pitch radius is large on the driving pulley and small on the driven pulley. at any engine or vehicle speed. in theory. when the pitch radius is small on the driving pulley and large on the driven pulley. For example. When the pulleys are close together. As the two pulleys change their radii relative to one another. . resulting in a lower ³gear.The distance between the center of the pulleys to where the belt makes contact in the groove is known as the pitch radius. When one pulley increases its radius.from low to high and everything in between. The introduction of new materials makes CVTs even more reliable and efficient.Pulley-based CVTs: Applications and Advances The simplicity and stepless nature of CVTs make them an ideal transmission for a variety of machines and devices. snowmobiles and motor scooters. . not just cars. the transmissions have relied on high-density rubber belts. bow-tie-shaped pieces of metal. These flexible belts are composed of several (typically nine or 12) thin bands of steel that hold together high-strength. thereby reducing their efficiency. CVTs have been used for years in power tools and drill presses. One of the most important advances has been the design and development of metal belts to connect the pulleys. which can slip and stretch. In all of these applications. They've also been used in a variety of vehicles. including tractors. .Metal belt design Metal belts don't slip and are highly durable. enabling CVTs to handle more engine torque. They are also quieter than rubber-belt-driven CVTs. Nissan Extroid toroidal CVT .the toroidal CVT system -.Toroidal CVT Another version of the CVT -.replaces the belts and pulleys with discs and power rollers. Here's how it works: ‡One disc connects to the engine. . transmitting power from one disc to the other.Although such a system seems drastically different. located between the discs act like the belt. This is equivalent to the driving pulley. This is equivalent to the driven pulley. or wheels. ‡Another disc connects to the drive shaft. all of the components are analogous to a belt-and-pulley system and lead to the same results -.a continuously variable transmission. ‡Rollers. They spin around the horizontal axis and tilt in or out around the vertical axis.. When the wheels touch the driving disc near the rim.e. resulting in an increase in speed and a decrease in torque (i. incrementally changes the gear ratio.The wheels can rotate along two axes. overdrive gear). low gear). providing for smooth.e. A simple tilt of the wheels. nearly instantaneous ratio changes. they must contact the driven disc near the rim. then. resulting in a reduction in speed and an increase in torque (i. When the wheels are in contact with the driving disc near the center. they must contact the driven disc near the center. .. which allows the wheels to touch the discs in different areas. . Then. known as a hydrostatic CVT. the fluid flow is converted back into rotational motion.Hydrostatic CVTs Both the pulley-and-V-belt CVT and the toroidal CVT are examples of frictional CVTs. The pump converts rotational motion into fluid flow. In this type of transmission. which work by varying the radius of the contact point between two rotating objects. the rotational motion of the engine operates a hydrostatic pump on the driving side. that uses variable-displacement pumps to vary the fluid flow into hydrostatic motors. There is another type of CVT. with a hydrostatic motor located on the driven side. which is why they are common in agricultural tractors and all-terrain vehicles. power is transmitted mechanically. and at a high speed. Hydromechanical transmissions transfer power from the engine to the wheels in three different modes.Often. Between these extremes. . power is transmitted hydraulically. a hydrostatic transmission is combined with a planetary gearset and clutches to create a hybrid system known as a hydromechanical transmission. At a low speed. Hydromechanical transmissions are ideal for heavy-duty applications. the transmission uses both hydraulic and mechanical means to transfer power. CVT Benefits Continuously variable transmissions are becoming more popular for good reason. They boast several advantages that make them appealing both to drivers and to environmentalists. The table below describes some of the key features and benefits of CVTs. . Manual transmissions normally do not require active cooling. This is because manuals generally involve a clutch instead of a torque converter. These comparisons are general guidelines and may not apply in certain circumstances. the recent popularity of semi-manual and semi-automatic transmissions renders many of these points obsolete. This results in both better acceleration and fuel economy.Manual transmissions are typically cheaper to build than automatic transmissions. 5. whereas automatics have many clutch packs.Manual transmissions are typically more efficient than automatic transmissions. because not much power is dissipated as heat through the transmission.It is generally easier to build very strong manual transmissions than automatic transmissions.A driver has more direct control over the state of the transmission with a manual than an automatic.Comparison with automatic transmissions Manual transmissions are typically compared to automatic transmissions. which can cause significant power losses. Advantages 1. 3. Additionally. as the two represent the majority of options available to the typical consumer. 4. Manual transmissions usually have only one clutch. 2. . Manual transmissions require more driver interaction than automatic transmissions. potentially causing damage to the engine and transmission as well as compromising safety. or in vehicles equipped for disabled drivers. cars where a floor-shifter is inconvenient.Manual transmissions require more controls than automatic transmissions.Disadvantages 1. 4.The smooth and quick shifts of an automatic transmission are not guaranteed when operating a manual transmission. 2. .Manual transmissions are more difficult to learn to drive as one needs to develop a feel for properly engaging the clutch. 6. 3.A driver may inadvertently shift into the wrong gear with a manual transmission.Manual transmissions make it especially challenging to start when stopped upward on a hill. This is an issue in cramped cockpits. 5. especially for newer drivers. . are nearly universally equipped with automatic transmissions in the US. Small economy cars predominantly feature manual transmissions because they are relatively cheap and efficient. Most luxury cars are unavailable with a manual transmission. Very heavy trucks also feature manual transmissions because they are efficient and. such as rental cars and taxis. Conversely. more importantly. manual transmissions are no longer popular in many classes of cars sold in North America. Some cars. Economy cars are also often powered by very small engines. and automatic transmissions can make them comparatively very slow. the manual transmission is the base equipment. and family cars and large trucks are sold predominantly with automatics. In situations where automatics and manual transmissions are sold side-by-side. and the automatic is optional²although the automatic is sometimes available at no extra cost.Applications and popularity Many types of automobiles are equipped with manual transmissions. Sports cars are also often equipped with manual transmissions because they offer more direct driver involvement and better performance. Nearly all cars are available with an automatic transmission option. can withstand the severe loads encountered in hauling heavy loads. although many are optionally equipped with automatics. Off-road vehicles and trucks often feature manual transmissions because they allow direct gear selection and are often more rugged than their automatic counterparts. They have straight teeth. Spur gears . Sometimes.Spur Gears Spur gears are the most common type of gears. many spur gears are used at once to create very large gear reductions. and are mounted on parallel shafts. dancing monster.windup alarm clock. the teeth collide. like the electric screwdriver. most of the gears in your car are helical.oscillating sprinkler. This is because the spur gear can be really loud. But you won't find many in your car. Each time a gear tooth engages a tooth on the other gear. and this impact makes a noise. It also increases the stress on the gear teeth. . To reduce the noise and stress in the gears.Spur gears are used in many devices .washing machine and clothes dryer. until the two teeth are in full engagement. the contact starts at one end of the tooth and gradually spreads as the gears rotate.Helical Gears The teeth on helical gears are cut at an angle to the face of the gear. Helical gears . When two teeth on a helical gear system engage. One interesting thing about helical gears is that if the angles of the gear teeth are correct. Because of the angle of the teeth on helical gears. helical gears are used in almost all car transmissions. Crossed helical gears . Devices that use helical gears have bearings that can support this thrust load. For this reason.This gradual engagement makes helical gears operate much more smoothly and quietly than spur gears. they create a thrust load on the gear when they mesh. they can be mounted on perpendicular shafts. adjusting the rotation angle by 90 degrees. as each tooth engages. spiral or hypoid. They are usually mounted on shafts that are 90 degrees apart.Bevel Gears Bevel gears are useful when the direction of a shaft's rotation needs to be changed. but can be designed to work at other angles as well. The teeth on bevel gears can be straight. it impacts the corresponding tooth all at once. Bevel gears . Straight bevel gear teeth actually have the same problem as straight spur gear teeth -. Spiral bevel gears .Just like with spur gears. These spiral teeth engage just like helical teeth: the contact starts at one end of the gear and progressively spreads across the whole tooth. the solution to this problem is to curve the gear teeth. On straight and spiral bevel gears. This allows the input pinion to be mounted lower than the axis of the ring gear. Hypoid bevel gears in a car differential This feature is used in many car differentials. making more room for people and cargo. can engage with the axes in different planes. If you were to extend the two shafts past the gears. Since the driveshaft of the car is connected to the input pinion. they would intersect. The ring gear of the differential and the input pinion gear are both hypoid. Figure shows the input pinion engaging the ring gear of the differential. The hypoid gear. the shafts must be perpendicular to each other. on the other hand. This means that the driveshaft doesn't intrude into the passenger compartment of the car as much. this also lowers the driveshaft. . but they must also be in the same plane. the friction between the gear and the worm holds the worm in place. One other very interesting usage of worm gears is in the Torsen differential. Worm gear Many worm gears have an interesting property that no other gear set has: the worm can easily turn the gear. . and even up to 300:1 or greater.Worm Gears Worm gears are used when large gear reductions are needed. but the gear cannot turn the worm. which is used on some highperformance cars and trucks. This is because the angle on the worm is so shallow that when the gear tries to spin it. This feature is useful for machines such as conveyor systems. in which the locking feature can act as a brake for the conveyor when the motor is not turning. It is common for worm gears to have reductions of 20:1.