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Transmission Components
The modern automatic transmission consists of many
components and systems that are designed to work
together in a symphony of clever mechanical,
hydraulic and electrical technology that has evolved
over the years into what many mechanically inclined
individuals consider to be an art form. We try to
use simple, generic explanations where possible to
describe these systems but, due to the complexity of
some of these components, you may have to use some
mental gymnastics to visualize their operation.
The main components that make up an automatic
transmission include:
Planetary Gear Sets which are the mechanical systems
that provide 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.
The 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.
Planetary Gear Sets
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.
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, six and even seven and eight-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 while the
vehicle accelerates to highway speed. On modern
vehicles (mid '80s to the present), the vehicle's
computer monitors and controls these shifts so that
they are almost imperceptible.
Clutch Packs
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 groves 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 groves 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.
One-Way Clutch
A one-way clutch (also known as a "sprag" clutch) is
a device that will allow a component such as ring
gear to turn freely in one direction but not in the
other. This effect is just like that of a bicycle,
where the pedals will turn the wheel when pedaling
forward, but will spin free when pedaling backward.
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.
Bands
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.
Torque Converter
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.
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.
Hydraulic System
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.
Oil Pump
The transmission oil pump (not to be confused with
the pump element inside the torque converter) 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 a pickup tube 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.
Valve Body
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 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 most important valve, and the 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.
Computer Controls
The computer uses sensors on the engine and
transmission to detect such things as throttle
position, vehicle speed, engine speed, engine load,
brake pedal position, etc. to control exact shift
points as well as how soft
or firm the shift should be. Once the computer
receives this information, it then sends signals to
a solenoid pack inside the transmission. The
solenoid pack contains several electrically
controlled solenoids that redirect the fluid to the
appropriate clutch pack or servo in order to control
shifting. Computerized transmissions even learn your
driving style and constantly adapt to it so that
every shift is timed precisely when you would need
it.
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
These
three components are important in the
non-computerized transmissions. They provide the
inputs that tell the transmission when to shift. The
Governor is connected to the output shaft and
regulates hydraulic pressure based on vehicle speed.
It accomplishes this using centrifugal force to spin
a pair of hinged weights against pull-back springs.
As the weights pull further out against the springs,
more oil pressure is allowed past the governor to
act on the shift valves that are in the valve body
which then signal the appropriate shifts.
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.
The Throttle Cable simply monitors the position of
the gas pedal through a cable that runs from the gas
pedal to the throttle valve in the valve body.
The Vacuum Modulator monitors engine vacuum by a
rubber vacuum hose which is connected to the engine.
Engine vacuum reacts very accurately to engine load
with high vacuum produced when the engine is under
light load and diminishing down to zero vacuum when
the engine is under a heavy load. The modulator is
attached to the outside of the transmission case and
has a shaft which passes through the case and
attaches to the throttle valve in the valve body.
When an engine is under a light load or no load,
high vacuum acts on the modulator which moves the
throttle valve in one direction to allow the
transmission to shift early and soft. As the engine
load increases, vacuum is diminished which moves the
valve in the other direction causing the
transmission to shift later and more firmly.
Seals and Gaskets
An
automatic transmission has many seals and gaskets to
control the flow of hydraulic fluid and to keep it
from leaking out. There are two main external seals:
the front seal and the rear seal. The front seal
seals the point where the torque converter mounts to
the transmission case. This seal allows fluid to
freely move from the converter to the transmission
but keeps the fluid from leaking out. The rear seal
keeps fluid from leaking past the output shaft.
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.
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