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.
