Repair 7

How Does an Auto Clutch Work

In a car with a manual transmission, the function of the clutch is to interrupt the transfer of power between the engine and the drivetrain. This interruption, along with the gradual reapplication of power as the clutch is engaged, allows the car to be started smoothly from a stop and prevents the grinding of gears during shifting. The clutch mechanism consists of several parts that work together to perform this function.

Flywheel

The flywheel is a large metal disc attached the crankshaft of the engine. It is turned by the action of the engine, and it is the point of contact through which power is transferred from the running engine to the transmission and drivetrain. The flywheel has teeth around its circumference, which are engaged by the starter motor in order to turn the engine over; it also has a smooth outer surface that is gripped by the clutch disc when the clutch is engaged.

Clutch Disc and Pressure Plate

The clutch disc is a metal circle covered with a high-friction coating; the disc is pressed against the flywheel when the clutch is engaged. The pressure plate is a mechanism that uses a system of springs and levers to clamp the clutch disc against the flywheel when the clutch is engaged and to release the clutch disc when the clutch pedal is depressed. The hub of the clutch disc is attached to the input shaft of the transmission, and when the flywheel, clutch disc and pressure plate are squeezed together, power is transferred from the engine, causing the transmission shaft to rotate.

Clutch Linkage

The clutch pedal is connected to the clutch mechanism via a linkage system that can be either mechanical or hydraulic. A mechanical linkage consists of rods or cables connecting the clutch pedal to the clutch, while a hydraulic linkage uses lines filled with hydraulic fluid. The function of the linkage is to move the throw-out bearing -- a sliding bearing that moves the pressure plate away from the clutch disc -- when the pedal is depressed.

Automatic Transmissions

Automatic transmissions use a much more complicated mechanism in which sensors and electronics control when the clutch is engaged and disengaged. Hydraulic pressure activates either a number of steel plates and friction discs called a clutch pack -- much like the clutch in a manual transmission -- or a series of transmission bands that tighten and release around the clutch housing. When either of these systems is engaged, power is transferred from the engine to the transmission, and the car moves.