A friction clutch is part of a machine, such as a car, which uses direct contact and friction to operate another part of the machine, generally by causing it to rotate. In the case of an automobile, the clutch allows the transmission and the engine to rotate at the same speed, which transmits rotational movement to the wheels, and the car moves forward. The friction between the flywheel on the engine and the clutch plate or clutch disc from the transmission creates the force which rotates the wheels and moves the car. Clutches fall into either the wet or the dry category depending on whether they are lubricated, and may also be divided into a push or pull type depending on how they disengage. When the friction clutch is engaged, friction between the flywheel and the clutch plate creates movement, and when the clutch is disengaged, the lack of friction causes the wheels to lose their forward momentum and eventually stop turning all together.
The friction clutch operates with the help of six major parts. When the driver of a manual transmission car pushes on the clutch pedal, it presses on a part known as the release fork, which in turn pushes on a throwout bearing. The throwout bearing's job is to press on a spring known as the diaphragm spring. The diaphragm spring is the key to engaging and disengaging the clutch. The pressure the diaphragm spring applies on the clutch’s pressure plate forces it away from the flywheel, located on the engine, and once the pressure from the pressure plate is gone, the clutch plate is no longer held against the flywheel and the clutch is then disengaged.
Due to the amount of wear and tear caused by the friction between the clutch plate and the flywheel, certain machines use wet clutches. In a wet clutch, liquid lubricant helps the parts turn smoothly and stops them from wearing out as quickly from the constant friction. Dry clutches lack lubrication between parts. While wet clutches last longer and create smooth, stable performance, the slick lubricant can also cause slipping. Many wet clutches use multiple discs stacked together to prevent this.
Friction clutches can also be divided into a push or pull category. Push clutches are the most commonly used in cars. For this type of friction clutch to work, the diaphragm spring needs to be pushed inward to disengage the clutch. Some cars use a pull clutch instead of a push clutch. This type of friction clutch requires the diaphragm spring be pulled back to release the clutch.