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Transmission |
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SPECIAL THANKS TO CHARLES OFRIA
HOW A TRANSMISSION WORKS
BY CHARLES OFRIA
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This article is broken down into five sections:
What is a transmission? There are two basic types of automatic transmissions based on whether the vehicle is rear wheel drive or front wheel drive.
On a rear wheel drive car,
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On a front wheel drive car,
There are a number of other arrangements including front drive vehicles
where the engine is mounted front to back instead of sideways and there are
other systems that drive all four wheels but the two systems described here
are by far the most popular. A much less popular rear drive arrangement has
the transmission mounted directly to the final drive at the rear and is
connected by a drive shaft to the torque converter which is still mounted on
the engine. This system is found on the new Corvette and is used in order to
balance the weight evenly between the front and rear wheels for improved
performance and handling. Another rear drive system mounts everything, the
engine, transmission and final drive in the rear. This rear engine
arrangement is popular on the Porsche.
The main components that make up an automatic transmission include:
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The
modern automatic transmission is by far, the most complicated mechanical
component in today's automobile. Automatic transmissions contain mechanical
systems, hydraulic systems, electrical systems and computer controls, all
working together in perfect harmony which goes virtually unnoticed until
there is a problem. This article will help you understand the concepts
behind what goes on inside these technological marvels and what goes into
repairing them when they fail.
the transmission is usually mounted to the back of the engine and is located
under the hump in the center of the floorboard alongside the gas pedal
position. A drive shaft connects the rear of the transmission to the final
drive which is located in the rear axle and is used to send power to the
rear wheels. Power flow on this system is simple and straight forward going
from the engine, through the torque converter, then through the transmission
and drive shaft until it reaches the final drive where it is split and sent
to the two rear wheels.
the transmission is usually combined with the final drive to form what is
called a transaxle. The engine on a front wheel drive car is usually mounted
sideways in the car with the transaxle tucked under it on the side of the
engine facing the rear of the car. Front axles are connected directly to
the transaxle and provide power to the front wheels. In this example, power
flows from the engine, through the torque converter to a large chain that
sends the power through a 180 degree turn to the transmission that is along
side the engine. From there, the power is routed through the transmission
to the final drive where it is split and sent to the two front wheels
through the drive axles.
Transmission
Components
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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 third or 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. The illustration on the right shows how the simple system described above would look 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, in this instance, 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.
. Many more combinations are possible using two or more planetary sets connected in various ways to provide the different forward speeds and reverse that are found in modern automatic transmissions. Some of the clever gear arrangements found in four and now, five-speed automatics are complex enough to make a technically astute lay person's head spin trying to understand the flow of power through the transmission as it shifts from first gear through top gear as the vehicle accelerates to highway speed. On newer vehicles, the vehicle's computer monitors and controls these shifts so that they are almost imperceptible.
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One-Way Clutch A common place where a one-way clutch is used is in first gear when the shifter is in the drive position. When you begin to accelerate from a stop, the transmission starts out in first gear. But have you ever noticed what happens if you release the gas while it is still in first gear? The vehicle continues to coast as if you were in neutral. Now, shift into Low gear instead of Drive. When you let go of the gas in this case, you will feel the engine slow you down just like a standard shift car. The reason for this is that in Drive, a one-way clutch is used whereas in Low, a clutch pack or a band is used.
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.
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. Since the '80s, in order to improve fuel economy, torque converters have been equipped with a lockup clutch (not shown) which locks the turbine to the pump as the vehicle speed reaches approximately 45 - 50 MPH. This lockup is controlled by computer and usually won't engage unless the transmission is in 3rd or 4th gear.
Oil Pump
Valve Body The most important valve, and one that you have direct control over is the manual valve. The manual valve is directly connected to the gear shift handle and covers and uncovers various passages depending on what position the gear shift is placed in. When you place the gear shift in Drive, for instance, the manual valve directs fluid to the clutch pack(s) that activates 1st gear. it also sets up to monitor vehicle speed and throttle position so that it can determine the optimal time and the force for the 1 - 2 shift. On computer controlled transmissions, you will 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.
Because of computer controls, sports models are coming out with the ability to take manual control of the transmission as though it were a stick shift, allowing the driver to select gears manually. This is accomplished on some cars by passing the shift lever through a special gate, then tapping it in one direction or the other in order to up-shift or down-shift at will. The computer monitors this activity to make sure that the driver does not select a gear that could over speed the engine and damage it. Another advantage to these "smart" transmissions is that they have a self diagnostic mode which can detect a problem early on and warn you with an indicator light on the dash. A technician can then plug test equipment in and retrieve a list of trouble codes that will help pinpoint where the problem is.
Governor, Vacuum
Modulator, Throttle Cable Of course, vehicle speed is not the only thing that controls when a transmission should shift, the load that the engine is under is also important. The more load you place on the engine, the longer the transmission will hold a gear before shifting to the next one.
There are two types of
devices that serve the purpose of monitoring the engine load: the
Throttle Cable
and the Vacuum Modulator.
A transmission will use one or the other but generally not both of these
devices. Each works in a different way to monitor engine load.
Seals and Gaskets A seal is usually made of rubber (similar to the rubber in a windshield wiper blade) and is used to keep oil from leaking past a moving part such as a spinning shaft. In some cases, the rubber is assisted by a spring that holds the rubber in close contact with the spinning shaft. A gasket is a type of seal used to seal two stationary parts that are fastened together. Some common gasket materials are: paper, cork, rubber, silicone and soft metal. Aside from the main seals, there are also a number of other seals and gaskets that vary from transmission to transmission. A common example is the rubber O-ring that seals the shaft for the shift control lever. This is the shaft that you move when you manipulate the gear shifter. Another example that is common to most transmissions is the oil pan gasket. In fact, seals are required anywhere that a device needs to pass through the transmission case with each one being a potential source for leaks. Spotting problems before they get worse Watch
for leaks or stains under the car
Be sensitive to
new noises, vibrations and
shift behavior
Adjustments
Reseal job
Replace accessible parts
Complete Overhaul
Replacement unit vs. overhaul existing unit |
Automatic transmissions contain many gears in various combinations. In a
manual transmission, gears slide along shafts as you move the shift lever
from one position to another, engaging various sized gears as required in
order to provide the correct gear ratio. In an 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. 
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 
The Hydraulic system is
a complex maze 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. This
is not unlike the human circulatory system (the fluid is even red) where
even a few minutes of operation when there is a lack of pressure can be
harmful or even fatal to the life of the transmission. 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 that is submerged
in anti-freeze in the radiator. Fluid passing through this chamber is cooled
and then returned to the transmission through the other steel tube. A
typical transmission has an average of ten quarts of fluid between the
transmission, torque converter, and cooler tank. 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 valve body is the
brain 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 servo 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. 
The
computer uses sensors on the engine and transmission to detect such things
as throttle position, vehicle speed, engine speed, engine load, stop light
switch position, etc. to control exact shift points as well as how soft or
firm the shift should be. Some computerized transmissions even learn your
driving style and constantly adapt to it so that every shift is timed
precisely when you would need it.
If there is a
persistent red oil leak that you are sure is coming from your car, you
should have your shop check to see if it is coming from your transmission or
possibly from your power steering system (most power steering systems also
use transmission fluid and leaks can appear on the ground in roughly the
same areas as transmission leaks.) If all you see is a few drops on the
ground, you may be able to postpone repairs as long as you check your fluid
level often (but check with your technician to be sure.) If transmission
fluid levels go down below minimum levels serious transmission damage can
occur (the same advice goes for power steering leaks as well.)