Clutch Operation

Clutch Operation

The flywheel and the pressure plate are the drive or driving members of the clutch. The driven member connected to the transmission input shaft is the clutch disc, also called the friction disc. As long as the clutch is disengaged (clutch pedal depressed), the drive members turn independently of the driven member, and the engine is disconnected from the transmission. However, when the clutch is engaged, the pressure plate moves in the direction of the arrows and the clutch disc is bound 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 cast iron, which has a high graphite content to lubricate the engagement of the clutch. Welded to or pressed onto the outside diameter of the flywheel is the starter ring gear. The starter ring gear is replaceable on most flywheels. The large diameter of the flywheel allows for an excellent gear ratio of the starter drive to ring gear, which provides for ample engine rotation during starting. 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, which supports the front end of the transmission input shaft and maintains alignment with the engine¨s crankshaft. Sometimes a ball or roller needle bearing is used instead of a pilot bushing.

Clutch Disc

The clutch disc 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 friction facings. Molded friction facings are preferred because they withstand greater pressure plate loading force without damage. Woven friction facings are used when additional cushioning action 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, also called cushioning springs, which cause the contact pressure on the facings to rise gradually as the springs flatten out when the clutch is engaged. These springs eliminate chatter when the clutch is engaged and also reduce the chance of the clutch disc sticking to the flywheel and pressure plate surfaces when the clutch is disengaged. The wave springs and friction facings are fastened to the steel disc.

The clutch disc is designed to absorb such things as crankshaft 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. The number and tension of these springs is determined by engine torque and vehicle weight. Stop pins limit this torsional movement to approximately 3/8 inch.

Pilot Bushing

The purpose of the pilot bushing or bearing is 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 it 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 two-fold. 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 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 cast iron. The assemblies differ in the manner in which they move the pressure plate toward and away from the clutch disc.

Coil Spring Pressure Plate Assembly A coil spring pressure plate assembly uses coil springs and release levers to move the pressure plate back and forth. The springs exert pressure to hold the pressure plate tightly against the clutch disc. This forces the clutch disc against the flywheel. The release levers release the holding force of the springs. There are usually three of them. Each one has two pivot points. One of these pivot points attaches the lever to a pedestal cast into the pressure plate and the other to a release lever yoke bolted to the cover. The levers pivot on the pedestals and release lever yokes to move the pressure plate through its engagement and disengagement operations. To disengage the clutch, the release bearing pushes the inner ends of the release levers forward toward the flywheel. The release lever yokes act as fulcrums for the levers and the outer ends of the release levers move backward, pulling the pressure plate away from the clutch disc. This action compresses the coil springs and disengages the clutch disc from the driving members.


When the clutch is engaged, the release bearing moves backward toward the transmission. Without this force against the release levers, the coil springs are able to push the pressure plate and clutch disc against the flywheel with sufficient force to resist slipping. The following are common advantages of all pressure plate assemblies that use coil springs.

- They cleanly disengage the clutch at high engine speeds because of the high force exerted by the pressure plate springs.

- They offer great flexibility and can be used on various applications since the coil springs can be changed to increase or decrease pressure plate holding force.


Diaphragm Spring Pressure Plate Assembly The diaphragm spring pressure plate assembly relies on a cone-shaped diaphragm spring between the pressure plate and the pressure plate cover to move the pressure plate back and forth. The diaphragm spring (sometimes called a Belleville spring) is a single, thin sheet of metal that works in the same manner as the bottom of an oil can. The metal yields when pressure is applied to it. When the pressure is removed, the metal springs back to its original shape. The center portion of the diaphragm spring is slit into numerous fingers that act as release levers.


During clutch disengagement, these fingers are moved forward by the release bearing. The diaphragm spring pivots over the fulcrum ring (also called the pivot ring), and its outer rim moves away from the flywheel. The retracting springs pull the pressure plate away from the driven disc and disengage the clutch.


When the clutch is engaged, the release bearing and the fingers of the diaphragm spring move toward the transmission. As the diaphragm pivot over the pivot ring, its outer rim forces the pressure plate against the clutch disc so the clutch is engaged to the flywheel.


Diaphragm spring pressure plate assemblies have the following advantages over other types of assemblies.

- Compactness

- Less weight

- Fewer moving parts to wear out

- Little pedal effort required from the operator

- Provide a balanced force around the pressure plate so rotational unbalance is reduced

- Clutch disc slippage is less likely to occur. Mileage builds because the force holding the clutch disc to the flywheel does not change throughout its service life.


Clutch Release Bearing

The clutch release bearing, also called a throwout bearing, is usually a sealed, pre-lubricated ball bearing. Its function is to smoothly and quietly move the pressure plate release levers or diaphragm spring through the engagement and disengagement process.


The release bearing is mounted on an iron casting called a hub, which slides on a hollow shaft at the front of the transmission housing. This hollow shaft is part of the transmission bearing retainer.


To disengage the clutch, the release bearing is moved forward on its shaft by the clutch fork. As the release bearing contacts the release levers or diaphragm spring of the pressure plate assembly, it begins to rotate with the rotating pressure plate assembly. As the release bearing continues forward, the clutch disc is disengaged from the pressure plate and flywheel.


To engage the clutch, the release bearing slides to the rear of the shaft. The pressure plate moves forward and traps the clutch disc against the flywheel to transmit engine torque to the transmission input shaft. Once the clutch is fully engaged, the release bearing is normally stationary.


Rotating Release Bearing Self-adjusting clutch linkages, used on many vehicles, apply just enough tension to the clutch control cable to keep a constant light pressure against the release bearing. As a result, the release bearing is kept in contact with the release levers or diaphragm spring of the rotating pressure plate assembly. The release bearing rotates with the pressure plate.


Clutch Fork

The clutch fork is a forked lever that pivots on a ball stud located in an opening in the bell housing. The forked end slides over the hub of the release bearing and the small end protrudes from the bell housing and connects to the clutch linkage and clutch pedal. The clutch fork moves the release bearing and hub back and forth during engagement and disengagement.


Clutch Linkage

The clutch linkage is a series of parts that connects the clutch pedal to the clutch fork. It is through the clutch linkage that the operator controls the engagement and disengagement of the clutch assembly smoothly and with little effort.

Clutch linkage can be mechanical or hydraulic. Mechanical clutch linkage can be divided into two types: shaft and lever linkage, and cable linkage.


Shaft and Lever Linkage The shaft and lever clutch linkage consists of the various shafts, levers, adjustable rods, and pivots that transmit clutch pedal motion to the clutch fork. A rod connects the clutch pedal to the lever and shaft assembly. When the upper lever is moved by the clutch pedal, the shaft rotates and moves the lower lever, which is connected to a pushrod that is attached to the clutch fork. The linkage assembly is located between the chassis and bell housing, near the lower rear part of the engine block. These linkages are not commonly used on modern vehicles, simply because there are so many wear points.


Cable Linkage A cable linkage can perform the same controlling action as the shaft and lever linkage but with fewer parts. The clutch cable system does not take up much room. It also has the advantage of flexible installation so it can be routed around the power brake and steering units. These advantages help to make it the most commonly used clutch linkage.

The clutch cable is made of braided wire. The upper end is connected to the top of the clutch pedal arm, and the lower end is fastened to the clutch fork. It is designed with a flexible outer housing that is fastened at the fire wall and the clutch housing.

When the clutch pedal is pushed to the disengaged position, it pivots on the pedal shaft and pulls the inner cable through the outer housing. This action moves the clutch fork forward to disengage the clutch. The pressure plate springs and springs on the clutch pedal provide the force to move the cable back when the clutch pedal is released.


SELF-ADJUSTING CLUTCH Self-adjusting clutch mechanisms monitor clutch pedal play and automatically adjust it when necessary.

Usually the self-adjusting clutch mechanisms is a ratcheting mechanism located at the top of the clutch pedal behind the dash panel. The ratchet is designed with a pawl and toothed segment, and a pawl tension spring is used to keep the pawl in contact with the toothed segment. The pawl allows the toothed segment to move in only one direction in relation to the pawl.

The clutch cable is guided around and fastened to the toothed segment, which is free to rotate in one direction (backwards) independently of the clutch pedal. The tension spring pulls the toothed segment backwards.

When the clutch cable develops slack due to stretching the clutch disc wear, the cable is adjusted automatically when the clutch is released. The tension spring pulls the toothed segment backwards and allows the pawl to ride over to the next tooth. This effectively shortens the cable. Actually, the cable is not really shortened; but the slack has been reeled in by the repositioning of the toothed segment. This self-adjusting action takes place automatically during the clutch's operational life.


Hydraulic Clutch Linkage Frequently, the clutch assembly is controlled by a hydraulic system. In the hydraulic clutch linkage system, hydraulic (liquid) pressure transmits motion from one sealed cylinder to another through a hydraulic line. Like the cable linkage assembly, the hydraulic linkage is compact and flexible. Cable linkages also allow engineers to place the release fork anywhere that gives them more flexibility in body design. In addition, the hydraulic pressure developed by the master cylinder decrease required pedal effort and provides a precise method of controlling clutch operation. Brake fluid is commonly used as the hydraulic fluid in hydraulic clutch systems.

A hydraulic clutch master cylinder's pushrod moves the piston and primary cup to create hydraulic pressure. The snap ring restricts the travel of the piston. The secondary cup at the snap ring end of the piston stops hydraulic fluid from dripping into the passenger compartment. The piston return spring holds the primary cup and piston in the fully released position. Hydraulic fluid is stored in the reservoir on top of the master cylinder housing.

The slave cylinder body has a bleeder valve to bleed air from the hydraulic system for efficient clutch linkage operation. The cylinder body is threaded for a tube and fitting at the fluid entry port. Rubber seal rings are used to seal the hydraulic pressure between the piston and the slave cylinder walls. The piston retaining ring is used to restrict piston travel to a certain distance. Piston travel is transmitted by a pushrod to the clutch fork. The pushrod boot keeps contaminants out of the slave cylinder.

When the clutch pedal is depressed, the movement of the piston and primary cup develops hydraulic pressure that is displaced from the master cylinder, through a tube, into the slave cylinder. The slave cylinder piston movement is transmitted to the clutch fork, which disengages the clutch.

When the clutch pedal is released, the primary cup and piston are forced back to the engaged position by the master cylinder piston return spring. External springs move the slave cylinder pushrod and piston back to the engaged position. Fluid pressure returns through the hydraulic tubing to the master cylinder assembly. There is no hydraulic pressure in the system when the clutch assembly is in the engaged position.

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