Automatic transmissions use a fluid clutch known as a
torque converter to transfer engine torque from the engine to the transmission.
The torque converter operates through hydraulic force
provided by automatic transmission fluid often simply called transmission oil.
The torque converter changes or multiplies the twisting motion of the engine
crankshaft and directs it through the transmission.
converter automatically engages and disengages power from the engine to the
transmission in relation to engine rpm. With the engine running at the correct
idle speed, there is not enough fluid flow for power transfer through the torque
converter. As engine speed is increased, the added fluid flow creates sufficient
force to transmit engine power through the torque converter assembly to the
Torque converters, or T/Cs, are either
one-piece, welded units that cannot be dismantled for repair, or they are
bolt-together units that can be broken down for servicing. The torque converter,
located between the engine and transmission, is a sealed, doughnut-shaped unit
that is always filled with automatic transmission fluid.
A special flex, sometimes called a flex disc,
is used to mount the torque converter to the crankshaft. The purpose of the flex
plate is to transfer crankshaft rotation to the shell of the torque converter
assembly. The flex plate bolts to a flange machined on the rear of the
crankshaft and to mounting pads located on the front of the torque converter
The flex plate also carries the starter motor
ring gear. A flywheel is not required because the mass of the torque converter
and flex disc acts like a flywheel to smooth out the intermittent power strokes
of the engine.
A standard torque converter consists of three
elements: the pump assembly, often called an impeller, the stator assembly, and
The impeller assembly is the input (drive)
member. It receives power from the engine. The turbine is the output (driven)
member. It is applied to the forward clutch of the transmission and to the
turbine shaft assembly. The stator assembly is the reaction member or torque
multiplier. The stator is supported on a roller race, which operates as an
overrunning clutch and permits the stator to rotate freely in one direction and
lock up in the opposite direction.
T/C EXTERIOR The exterior of the
torque converter shell is shaped like two bowls standing on end, facing each
other. To support the weight of the torque converter, a short stubby shaft
projects forward from the front of the torque converter shell and fits into a
pocket at the rear of the crankshaft. At the rear of the torque converter shell
is a hollow shaft with notches or flats at one end, ground 180 degrees apart.
This shaft is called the pump drive hub. The notches or flats drive the
transmission pump assembly. At the front of the transmission within the pump
housing is a pump bushing that supports the pump drive hub and provides rear
support for the torque converter.
T/C INTERIOR The impeller forms one
section of the torque converter shell. The impeller has numerous curved blades
that rotate as a unit with the shell. It turns at engine speed, acting like a
pump to start the transmission oil circulating within the torque converter
While the impeller is positioned with its
back facing the transmission housing. The turbine is positioned with its back to
the engine. The curved blades of the turbine face the impeller assembly.
The turbine blades have a greater curve than
the impeller blades. This helps eliminate oil turbulence between the turbine and
impeller blades that would slow impeller speed and reduce the converter's
The stator is located between the impeller
and turbine. It redirects the oil flow from the turbine back into the impeller
in the direction of impeller rotation with minimal loss of speed or force. The
side of the stator blade with the inward curve is the concave side. The side
with an outward curve is the convex side.
Transmission oil is used as the medium to
transfer energy in the T/C. As the pump impeller rotates, centrifugal force
throws the oil outward and upward due to the curved shape of the impeller
The faster the impeller rotates, the greater
the centrifugal force becomes. To harness some of this energy, the turbine
assembly is mounted on top of the impeller. Now the oil thrown outward and
upward from the impeller strikes the curved vanes of the turbine, causing the
turbine to rotate. (There is no direct mechanical link between the impeller and
turbine.) An oil pump driven by the converter shell and the engine continually
delivers oil under pressure into the T/C through a hollow shaft at the center
axis of the rotating torque converter assembly. A seal prevents the loss of
fluid from the system.
The turbine shaft is located within this
hollow shaft. As mentioned earlier, the turbine shaft is splined to the turbine
and transfers power from the torque converter to the transmission's main drive
shaft. Oil leaving the turbine is directed out of the torque converter to an
external oil cooler and then to the transmission's oil sump or pan.
With the transmission in gear and the engine
at idle, the vehicle can be held stationary by applying the brakes. At idle,
engine speed is slow. Since the impeller is driven by engine speed, it turns
slowly creating little centrifugal force within the torque converter. Therefore,
little or no power is transferred to the transmission.
When the throttle is opened, engine speed,
impeller speed, and the amount of centrifugal force generated in the torque
converter increase dramatically. Oil is then directed against the turbine
blades, which transfer power to the turbine shaft and transmission.
Types of Oil Flow
Two types of oil flow take place inside the
torque converter: rotary and vortex flow. Rotary oil flow is the oil flow
around the circumference of the torque converter caused by the rotation of the
torque converter on its axis. Vortex oil flow is the oil flow occurring
from the impeller to the turbine and back to the impeller.
The oil flow pattern as the speed of the
turbine approaches the speed of the impeller is known as the coupling point.
The turbine and the impeller are running at essentially the same speed. They
cannot run at exactly the same speed due to slippage between them. The only way
they can turn at exactly the same speed is by using a lockup clutch to
mechanically tie them together.
As mentioned earlier, the stator is a small,
wheel-like assembly positioned between the impeller and turbine. The stator has
no mechanical connection to either the impeller or turbine but fits between the
outlet of the turbine and the inlet of the impeller so all the oil returning
from the turbine to the impeller must pass through the stator.
The stator mounts through its splined center
hub to a mating stator shaft, often called a ground sleeve. The stator
freewheels when the impeller and turbine reach the coupling stage.
The stator redirects the oil leaving the
turbine back to the impeller, which helps the impeller rotate more efficiently.
Torque converter multiplication can only occur when the impeller is rotating
faster than the turbine.
A stator is either a rotating or fixed type.
Rotating stators are more efficient at higher speeds because there is less
slippage when the impeller and turbine reach the coupling point.
An overrunning clutch keeps the stator
assembly from rotating when driven in one direction and permits overrunning
(rotation) when turned in the opposite direction. Rotating stators generally use
a roller-type overrunning clutch that allows the stator to freewheel (rotate)
when the speed of the turbine and impeller reach the coupling point.
The roller clutch is a designed with a
movable inner race, rollers, accordion (apply) strings, and outer race. Around
the inside diameter of the outer race are several cam-shaped pockets. The
rollers and accordion springs are located in these pockets.
As the vehicle begins to move, the stator
stays in its stationary or locked position because of the difference between the
impeller and turbine speeds. This locking mode takes place when the inner race
rotates counterclockwise. The accordion springs force the rollers down the ramps
of the cam pockets into a wedging contact with the inner and outer races.
As vehicle road speed increases, turbine
speed increases until it approaches impeller speed. Oil exiting the turbine
vanes strikes the back face of the stator, causing the stator to rotate in the
same direction as the turbine and impeller. At this higher speed, clearance
exists between the inner stator race and hub. The rollers at each slot of the
stator are pulled around the stator hub. The stator freewheels or turns as a
If the vehicle slows, engine speed also slows
along with turbine speed. This decrease in turbine speed allows the oil flow to
change direction. It now strikes the front face of the stator vanes, halting the
turning stator and attempting to rotate it in the opposite direction.
As this happens, the rollers jam between the
inner race and hub, locking the stator in position. In a stationary position,
the stator now redirects the oil existing the turbine so torque is again