How a Clutch Operates
The purpose of the clutch is to connect and disconnect the power of your engine power. It does this at the transmission. A lot of torque is required to move the weight of the engine, the body, all your vehicle's internal components, your passengers and any cargo. Because a gas engine does not produce a lot of torque starting out, so it must be able to run without any load until it builds up enough torque to move the vehicle. The clutch allows the engine to build up torque by disconnecting the transmission from the engine and temporarily relieving the engine of any resistance or load.
The transfer of engine power to the transmission must be gradual and smooth. If it is not, your drivetrain components would rapidly break or wear out. A gradual power transfer occurs when you slowly and evenly release the clutch pedal.
Note: this is a general description. Consult a repair manual or other reference for an exact description of your vehicle's clutch.
The pressure plate and clutch disc link the engine and transmission. When you release the clutch pedal, the plate and disc make contact (you "engage" the clutch). This physically joins the transmission and the engine. When you push in the pedal, the plate and disc separate (the clutch is "disengaged"), which disconnects the transmission from the engine.
Most clutch assemblies consist of the following parts: a flywheel, a clutch disc, a pressure plate, the throw out bearing and fork, actuating linkage, and a pedal.
The pressure plate and flywheel (the "driving members") are connected to the engine's crankshaft and rotate with it. The clutch disc, located between the pressure place and the flywheel, is attached to the transmission shaft. The pressure plate rotates and drives the clutch disc on contact and, in so doing, turns the transmission shaft.
There is a circular diaphragm spring within the pressure plate cover. In a "relaxed" state, i.e., when you fully release the clutch pedal, this spring is convex; that is, it is "dished" outward toward the transmission. Pushing in the clutch pedal actuates the attached linkage. The throw-out fork is connected to the end and holds the throw-bearing. When you depress the clutch pedal, the clutch linkage pushes the bearing and fork forward to contact the diaphragm spring of the pressure plate. The spring's outer edges are secured to the pressure plate and are pivoted on rings. When the throwout bearing compresses the center of the spring, the outer edges bow outward and by doing so, pull the pressure plate in the same direction -- away from the clutch disc. This separates the disc from the plate, disengaging the clutch and allowing you to shift the transmission into another gear.
A coil-type clutch return spring attached to the clutch pedal arm permits full release of the pedal. Releasing the pedal pulls the throw-out bearing away from the diaphragm spring, which results in a reversal of spring position. As bearing pressure is gradually released from the center of the spring, the outer edges of the spring bow outward, pushing the pressure plate into closer contact with the clutch disc. As the disc and plate move closer together, friction between the two increases and slippage is reduced, until when full spring pressure applied (when you fully release the pedal) the speed of the disc and plate is the same. This stops all slipping, creating a direct connection between the disc and the plate, transferring power from the engine to the transmission. The clutch disc is now rotating with the plate at engine speed and, because it is attached to the transmission shaft, the shaft now turns at the same engine speed.