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summer project pal
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Posts: 308
Joined: Jan 2011
10-01-2011, 09:56 PM



Transmission in automobiles is a unit which supplies the power from the clutch to the differential. The simplest transmissions are manual transmissions which consist of a set of gears which are in mesh when transmitting power. Manual transmission requires frequent shift between gears especially while driving in cities. Each time it requires engaging and disengaging clutch while shifting gears. This is a tiresome task. In order to make this task easier, automatic transmissions are evolved. The automatic transmission no more consists of a clutch unit. Thus there are two big differences between a manual transmission and an automatic transmission. They are:
• There is no clutch pedal in automatic transmission car.
• There is no gear shift in automatic transmission car. Once you put the transmission in the drive, every thing else is automatic.
Both the automatic and manual transmissions accomplish exactly the same thing, but they do it in totally different ways. Further, learning in a vehicle fitted with an automatic transmission is very easy. But even now the vehicles with automatic transmissions are not very popular in India. This is because the cost of such vehicles is comparatively high and low fuel economy. When automatic transmissions were introduced in India it was mainly used by persons having some kind of disable ness. Now that mind set has been changing rapidly because of many technological advancements making such vehicles more powerful and more economical ones.

.doc   AUTOMATIC TRANSMISSION REPORT.doc (Size: 1.27 MB / Downloads: 206)
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Transmission was invented in the year 1832 by W.H James. It consists of a set of interlocking gears. Here the wheels are arranged in such ways that by operating a lever, the driver can choose one of several ratios of speed between the input shaft and output shaft. In manual transmission, the number of gear ratios which is obtained is limited is in discrete values. The transmission system underwent many major changes as the technology developed. The automatic transmission was introduced in 1939, which switches to the optimum gear without driver intervention except for starting and going into reverse. The first vehicle fitted with automatic transmission was rolled out in the year 1940 by Oldsmobile. It featured a Hydra-Matic drive, making it as the first vehicle with fully automatic transmission. The first hydraulic automatics were introduced by General Motors,Chrysler,and Borg-Warner in the early 1950s.The automatic transmission was an instant success in all the European countries especially in U.S.A. The automatic transmission was a huge success because the concept was to make the automobile more user friendly. In the 1970s and 1980s so many design changes and improvements were introduced such like the addition of overdrive. In the 1990s so much improvements were came such as introduction of a separate clutch plate which seizes the entire automatic transmission and the drive is given to the rear wheels directly, use of continuously variable transmission etc.


The transmission is a device that is connected to the back of the engine and sends the power from the engine to the drive wheels. An automobile engine runs at its best at a certain r.p.m range and it is the transmission’s job to make sure that the power is delivered to the wheels while keeping the engine with in that range. It does this through various gear combinations. In first gear, the engine runs much faster in relation to the drive wheels while in high gear, the engine is loafing even though the car may be going in excess of 70mph.In addition to the various forward gears, the transmission has also a neutral position, reverse and a park position.

The modern automatic transmission is by far the most complicated mechanical component in today’s automobile. Automatic transmission contain mechanical system, hydraulic system, electrical system and computer controls, all working together in perfect harmony which goes virtually unnoticed until there is a problem.

Mercedes-Benz CLK, automatic transmission, cut-away model

There are two basic types of automatic transmissions based on whether the vehicle is rear wheel drive or front wheel drive.

In an automatic transmission, a set of gears produces all of the different gear ratios. The planetary gear set is the device that makes this possible in an automatic transmission.

The main components that make up the automatic transmission are;

• Planetary gear sets which are the mechanical systems that provides the various forward gear ratios as well as reverse.
• The hydraulic system which uses a special transmission fluid sent under pressure by an oil pump through the valve body to control the clutches and the bands in order to control the planetary gear sets.
• Seals and gaskets are used to keep the oil where it is supposed to be and prevent it from leaking out.
• Torque converter which acts like a clutch to allow the vehicle to come to a stop in gear while the engine is still running.
• The governor and the modulator or throttle cable that monitor speed and throttle position in order to determine when to shift.
• On newer vehicles shift points are controlled by computer which directs electrical solenoids to shift oil flow to the appropriate component at the right instant.

Automatic transmission contains many gears in various combinations. In manual transmissions gears slide along the shaft as one move the shift lever from one position to another, engaging various sized gears as required in order to provide the correct gear ratio. In automatic transmission, however, the gears are never physically moved and are always engaged to the same gears. This is accomplished through the use of planetary gear sets. The basic planetary gear set consists of a sun gear, a ring gear and two or more planet gears, all remaining in constant mesh. The planet gears are connected to each other through a common carrier which allows the gears to spin on shafts called "pinions" which are attached to the carrier.
One example of a way that this system can be used is by connecting the ring gear to the input shaft coming from the engine, connecting the planet carrier to the output shaft, and locking the sun gear so that it can't move. In this scenario, when we turn the ring gear, the planets will "walk" along the sun gear (which is held stationary) causing the planet carrier to turn the output shaft in the same direction as the input shaft but at a slower speed causing gear reduction (similar to a car in first gear.
If we unlock the sun gear and lock any two elements together, this will cause all three elements to turn at the same speed so that the output shaft will turn at the same rate of speed as the input shaft. This is like a car that is in high gear. Another way that we can use a Planetary gear set is by locking the planet carrier from moving, then applying power to the ring gear which will cause the sun gear to turn in the opposite direction giving us reverse gear.
In any epicyclic gearing system, one of the three basic components is held stationary; one of the two remaining components is an input, providing power to the system, while the last component is an output, receiving power from the system. The ratio of input rotation to output rotation is dependent upon the number of teeth in each gear, and upon which component is held stationary. That is either the sun gear or the planet gear or the ring gear can be the input, the output, or can be held stationary. Choosing which piece plays which role determines the gear ratio for the gear set. Let's take a look at a single planetary gear set.
For a ring gear with 72 teeths and sun gear with 30 teeths different gear ratios can be obtained as follows.
Input Output Stationary Calculation Gear Ratio
A Sun (S) Planet Carrier © Ring ® 1 + R/S 3.4:1
B Planet Carrier © Ring ® Sun (S) 1 / (1 + S/R) 0.71:1
C Sun (S) Ring ® Planet Carrier © -R/S -2.4:1

Also, locking any two of the three components together will lock up the whole device at a 1:1 gear reduction. The first gear ratio listed above is a reduction -- the output speed is slower than the input speed. The second is an overdrive -- the output speed is faster than the input speed. The last is a reduction again, but the output direction is reversed. There are several other ratios that can be gotten out of this planetary gear set, but these are the ones that are relevant to our automatic transmission.
More planet and sun gear units can be placed in series in the same ring gear housing (where the output shaft of the first stage becomes the input shaft of the next stage) providing a larger (or smaller) gear ratio. This is the way some automatic transmissions of present time work.
With two of these gearsets in a row, we can get the four forward gears and one reverse gear.
This automatic transmission uses a set of gears, called a compound planetary gearset, that looks like a single planetary gearset but actually behaves like two planetary gearsets combined. It has one ring gear that is always the output of the transmission, but it has two sun gears and two sets of planets.

Left to right: the ring gear, planet carrier, and two sun gears
The following figure shows the inside of the planet carrier.


On automatic transmissions, the torque converter takes the place of the clutch found on standard shift vehicles. It is there to allow the engine to continue running when the vehicle comes to a stop. The principle behind a torque converter is like taking a fan that is plugged into the wall and blowing air into another fan which is unplugged. If you grab the blade on the unplugged fan, you are able to hold it from turning but as soon as you let go, it will begin to speed up until it comes close to the speed of the powered fan. The difference with a torque converter is that instead of using air, it uses oil or transmission fluid, to be more precise.
A torque converter is a large doughnut shaped device (10" to 15" in diameter) that is mounted between the engine and the transmission. It consists of three internal elements that work together to transmit power to the transmission. The three elements of the torque converter are the Pump, the Turbine, and the Stator. The pump is mounted directly to the converter housing which in turn is bolted directly to the engine's crankshaft and turns at engine speed. The turbine is inside the housing and is connected directly to the input shaft of the transmission providing power to move the vehicle. The stator is mounted to a one-way clutch so that it can spin freely in one direction but not in the other. Each of the three elements have fins mounted in them to precisely direct the flow of oil through the converter
With the engine running, transmission fluid is pulled into the pump section and is pushed outward by centrifugal force until it reaches the turbine section which starts it turning. The fluid continues in a circular motion back towards the center of the turbine where it enters the stator. If the turbine is moving considerably slower than the pump, the fluid will make contact with the front of the stator fins which push the stator into the one way clutch and prevent it from turning. With the stator stopped, the fluid is directed by the stator fins to re-enter the pump at a "helping" angle providing a torque increase. As the speed of the turbine catches up with the pump, the fluid starts hitting the stator blades on the back-side causing the stator to turn in the same direction as the pump and turbine. As the speed increases, all three elements begin to turn at approximately the same speed.

Torque converter-sectional view

Dismantled view of torque converter


A clutch pack consists of alternating disks that fit inside a clutch drum. Half of the disks are steel and have splines that fit into grooves on the inside of the drum. The other half have a friction material bonded to their surface and have splines on the inside edge that fit grooves on the outer surface of the adjoining hub. There is a piston inside the drum that is activated by oil pressure at the appropriate time to squeeze the clutch pack together so that the two components become locked and turn as one.
In this transmission there are four clutches. Each clutch is actuated by pressurized hydraulic fluid that enters a piston inside the clutch. Springs make sure that the clutch releases when the pressure is reduced. Below you can see the piston and the clutch drum.

One of the clutches in a transmission
The following figure shows the alternating layers of clutch friction material and steel plates. The friction material is splined on the inside, where it locks to one of the gears. The steel plate is splined on the outside, where it locks to the clutch housing. .

The clutch plates


A band is a steel strap with friction material bonded to the inside surface. One end of the band is anchored against the transmission case while the other end is connected to a servo. At the appropriate time hydraulic oil is sent to the servo under pressure to tighten the band around the drum to stop the drum from turning.
In the figure below, it can be seen that one of the bands in the housing of the transmission. The metal rod is connected to the piston, which actuates the band.

In the below figure, it can see that the two pistons actuating the bands. Hydraulic pressure, routed into the cylinder by a set of valves, causes the pistons to push on the bands, locking that part of the gear train to the housing.


The Hydraulic system is a complex mix of passages and tubes that sends transmission fluid under pressure to all parts of the transmission and torque converter. The diagram at left is a simple one from a 3-speed automatic from the '60s. The newer systems are much more complex and are combined with computerized electrical components. Transmission fluid serves a number of purposes including: shift control, general lubrication and transmission cooling. Unlike the engine, which uses oil primarily for lubrication, every aspect of a transmission's functions are dependant on a constant supply of fluid under pressure. In order to keep the transmission at normal operating temperature, a portion of the fluid is sent through one of two steel tubes to a special chamber which consists of a radiator. Fluid passing through this chamber is cooled and then returned to the transmission through the other steel tube. In fact, most of the components of a transmission are constantly submerged in fluid including the clutch packs and bands. The friction surfaces on these parts are designed to operate properly only when they are submerged in oil.

The passageways shown in the above figure route fluid to all the different components in the transmission. Passageways molded into the metal are an efficient way to route fluid; without them, many hoses would be needed to connect the various parts of the transmission.

The transmission oil pump is responsible for producing all the oil pressure that is required in the transmission. The oil pump is mounted to the front of the transmission case and is directly connected to a flange on the torque converter housing. Since the torque converter housing is directly connected to the engine crankshaft, the pump will produce pressure whenever the engine is running as long as there is a sufficient amount of transmission fluid available. The oil enters the pump through a filter that is located at the bottom of the transmission oil pan and travels up directly to the oil pump. The oil is then sent, under pressure to the pressure regulator, the valve body and the rest of the components, as required.
Automatic transmissions have a neat pump, called a gear pump as shown below.

The inner gear of the pump keyed to the housing of the torque converter, so it spins at the same speed as the engine. The outer gear is turned by the inner gear, and as the gears rotate, 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 governor is a clever valve that tells the transmission how fast the car is going. It is connected to the output, so the faster the car moves, the faster the governor spins. Inside the governor is a spring-loaded valve that opens in proportion to how fast the governor is spinning -- the faster the governor spins, the more the valve opens. Fluid from the pump is fed to the governor through the output shaft.
The faster the car goes, the more the governor valve opens and the higher the pressure of the fluid it lets through.

To shift properly, the automatic transmission has to know how hard the engine is working. There are two different ways that this is done. Some cars have a simple cable linkage connected to a throttle valve in the transmission. The further the gas pedal is pressed, the more pressure is put on the throttle valve. Other cars use a vacuum modulator to apply pressure to the throttle valve. The modulator senses the manifold pressure, which drops when the engine is under a greater load.
The manual valve is attached to the shift lever. Depending on which gear is selected, the manual valve feeds hydraulic circuits that inhibit certain gears. For instance, if the shift lever is in third gear, it feeds a circuit that prevents overdrive from engaging.
Shift valves supply hydraulic pressure to the clutches and bands to engage each gear. The valve body of the transmission contains several shift valves. The shift valve determines when to shift from one gear to the next. For instance, the 1 to 2 shift valve determines when to shift from first to second gear. The shift valve is pressurized with fluid from the governor on one side, and the throttle valve on the other. They are supplied with fluid by the pump, and they route that fluid to one of two circuits to control which gear the car runs in.

The shift circuit

The valve body is the control center of the automatic transmission. It contains a maze of channels and passages that direct hydraulic fluid to the numerous valves which then activate the appropriate clutch pack or band to smoothly shift to the appropriate gear for each driving situation. Each of the many valves in the valve body has a specific purpose and is named for that function. For example the 2-3 shift valve activates the 2nd gear to 3rd gear up-shift or the 3-2 shift timing valve which determines when a downshift should occur.
On computer controlled transmissions, it also have electrical solenoids that are mounted in the valve body to direct fluid to the appropriate clutch packs or bands under computer control to more precisely control shift points.

Electronically controlled transmissions, which appear on some newer cars, still use hydraulics to actuate the clutches and bands, but each hydraulic circuit is controlled by an electric solenoid. In addition to monitoring vehicle speed and throttle position, the transmission controller can monitor the engine speed, if the brake pedal is being pressed, and even the anti-lock braking system.
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

The above figure shows how the simple system described above would work in an actual transmission. The input shaft is connected to the ring gear (Blue), The Output shaft is connected to the planet carrier (Green) which is also connected to a "Multi-disk" clutch pack. The sun gear is connected to a drum (yellow) which is also connected to the other half of the clutch pack. Surrounding the outside of the drum is a band (red) that can be tightened around the drum when required to prevent the drum with the attached sun gear from turning.
The clutch pack is used here is to lock the planet carrier with the sun gear forcing both to turn at the same speed. If both the clutch pack and the band were released, the system would be in neutral. Turning the input shaft would turn the planet gears against the sun gear, but since nothing is holding the sun gear, it will just spin free and have no effect on the output shaft. To place the unit in first gear, the band is applied to hold the sun gear from moving. To shift from first to high gear, the band is released and the clutch is applied causing the output shaft to turn at the same speed as the input shaft.

The transmission output shaft is splined outside as shown in the figure. The transmission output shaft fits inside a casing that has a parking mechanism built in it. It may seem like a simple thing to lock the transmission and keep it from spinning. When the shift lever is placed in park, the rod pushes the spring against the small tapered bushing. The bushing thus operates the parking mechanism causing it to sit between the notches.

The automatic transmission technology has advanced a lot and it is slowly making its presence felt even in developing countries. The automatic transmission is going to stay in the scene for a long time and in future it may phase out the manual transmission for ever.

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summer project pal
Active In SP

Posts: 308
Joined: Jan 2011
10-01-2011, 10:06 PM


.ppt   AUTOMATIC TRANSMISSION1.ppt (Size: 1.04 MB / Downloads: 295)

Two types- manual and automatic transmissions
Manual transmission- needs frequent shift between gears
Automatic transmission
- no clutch pedal
- automatic shifting between gears
- most suitable for beginners
First transmission in 1832 by W.H.James
First automatic transmission introduced in 1939
1940 Oldsmobile features a hydra-matic automatic transmission
First hydraulic automatics were introduced by GM,Chrysler and Borg-Warner in early 1950’s
Greater developments in 1990’s


Mercedes-Benz CLK, automatic transmission, cut-away model

Main components of automatic transmission are
Torque converter
Planetary gear system
Hydraulic system
Seals and gaskets
Electronic controls
Main components of torque converter are;

Turbine and


Dismantled view of torque converter

Planetary gear system has got three main components
Ring gear

Sun gear

Planet gear

Left to right: the ring gear, planet carrier, and two sun gears

The main components are;
Clutch plates

Clutch discs


Piston and seal

One of the clutches in a transmission

The clutch plates
Used for holding or releasing the drum
Most complex part of the transmission

Purposes - shift control
- general lubrication
- transmission cooling

Hydraulic system of a 1960 model vehicle

Gear type oil pump

Responsible for producing oil pressure

Keeps circulating the oil
Fitted to the transmission output shaft

Speed sensing valve

Mainly three types of valves;
Throttle valve or modulator

Manual valve

Shift valve
Collection of channels and passages
Made of two or more number of pieces
Used between two mating parts

Two main external seals: front seal and rear seal

Seals: between parts having relative motion between them

Gasket: between two stationary parts
Common in most modern cars
Electronic control monitors;
-vehicle speed
-engine speed
-brake pedal position
-throttle position

Electronically controlled transmissions can do things like:
Downshift automatically when going downhill

Upshift when braking on a slippery surface

Inhibit the upshift when going into a turn on a winding road


The automatic transmission technology has advanced a lot and it is slowly making its presence felt even in developing countries. The automatic transmission is going to stay in the scene for a long time and in future it may phase out the manual transmission for ever.


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