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.

The torque 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 transmission.

Design

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 shell.

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.

Components

A standard torque converter consists of three elements: the pump assembly, often called an impeller, the stator assembly, and the turbine.

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 shell.

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 efficiency.

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.

Basic Operation

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 housing.

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.

Overrunning Clutch

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 unit.

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 multiplied.