The clutch is located between the transmission and engine where it provides a mechanical coupling between the engine’s flywheel and the transmission’s input shaft. All manual transmissions require clutch to engage and disengage the transmission.
The clutch engages the transmission gradually by allowing a certain amount of slippage between the transmission’s input shaft and the flywheel.
The basic principle of clutch operation is shown in figure. The pressure plate and flywheel are input members of the assembly. The clutch disc is the output member and is connected to the transmission input shaft. As long as the clutch is disengaged (clutch pedal depressed), the engine is disconnected from the transmission. However, when the clutch is engaged (clutch pedal released), the pressure plate moves toward the flywheel and the clutch disc is squeezed between the two revolving drive members and forced to turn at the same speed.
The flywheel, an important part of the engine, is also the main driving member of the clutch. It is normally made of nodular or gray cast iron, which has a high graphite content to lubricate the engagement of the clutch. Welded or pressed onto the outside diameter of the flywheel is the starter ring gear. The rear surface of the flywheel is a friction surface machined very flat to ensure smooth clutch engagement. The flywheel also provides some absorption of torsional vibration of the crankshaft. It further provides the inertia to rotate the crankshaft through the four strokes.
The flywheel has two sets of bolt holes drilled into it. The inner set is used to fasten the flywheel to the crankshaft, and the outer set provides a mounting plate for the pressure plate assembly. A bore in the center of the flywheel and crankshaft holds the pilot bushing or bearing, which supports the front end of the transmission input shaft and maintains alignment with the engine crankshaft.
- Clutch Disc (see photo):
The clutch disc is splined to the transmission’s input shaft and receives the driving motion from the flywheel and pressure plate assembly and transmits that motion to the transmission input shaft.
There are two types of the friction facings of the clutch disc. Molded friction facings are preferred because they withstand greater pressure plate loading force without damage. Woven friction facings are used when additional cushioning is needed for clutch engagement. Until recently, the material that was molded or woven into facings was predominantly asbestos. Now, because of the hazards associated with asbestos, other materials such as paper-base and ceramics are being used instead. Particles of cotton, brass, rope, and wire are added to prolong the life of the clutch disc and provide torsional strength.
Grooves are cut across the face of the friction facings. This promotes clean disengagement of the driven disc from the flywheel and pressure plate; it also promotes better cooling. The facings are riveted to wave springs which cause the contact pressure on the facings to rise gradually as the springs flatten out when the clutch is engaged. The wave springs and friction facings are fastened to the steel disc. The clutch disc is designed to absorb such things as crank shaft vibration, abrupt clutch engagement, and driveline shock. Torsional coil springs allow the disc to rotate slightly in relation to the pressure plate while they absorb the torque forces.
- Pilot Bushing/Bearing:
The pilot bushing or bearing is sometimes used to support the outer end of the transmission’s input shaft. This shaft is splined to the clutch disc and transmits power from the engine (when the clutch is engaged) to the transmission. The transmission end of the input shaft is supported by a large bearing in the transmission case. Because the input shaft extends unsupported from the transmission, a pilot bushing is used to keep in position. By supporting the shaft, the pilot bushing keeps the clutch disc centered in the pressure plate.
- Pressure Plate Assembly:
The purpose of the pressure plate assembly is twofold. First, it must squeeze the clutch disc onto the flywheel with sufficient force to transmit engine torque efficiently. Second, it must move away from the clutch disc so the clutch disc can stop rotating, even though the flywheel and pressure plate continue to rotate.
Basically, there are two types of pressure plate assemblies: those with coil springs and those with a diaphragm spring. Both types have a stamped steel cover that bolts to the flywheel and acts as a housing to hold the parts together. In both, there is also the pressure plate, which is a heavy, flat ring made of nodular or gray cast iron. The assemblies differ in the manner in which they move the pressure plate toward and away from the clutch disc.