Learn Automotive Clutches Explained – Flywheel, Clutch Disc, Pressure Plate (Coil & Diaphragm Spring), Release Bearing, Fork and Linkage Systems (Shaft/Lever, Cable, Self Adjusting, Hydraulic, Internal Slave Cylinders)

 

The Clutch System:

The clutch is a fundamental component in manual transmission vehicles, positioned between the transmission and the engine to provide a mechanical coupling between the engine’s flywheel and the transmission’s input shaft. The driver operates the clutch through a linkage that extends from the passenger compartment to the bell housing, also called the clutch housing, which sits between the engine and the transmission. All manual transmissions require a clutch to engage or disengage the transmission from the engine. If the vehicle lacked a clutch and the engine remained permanently connected to the transmission, the engine would stop every time the vehicle was brought to a stop. The clutch allows the engine to idle while the vehicle is stationary and also enables smooth shifting between gears. These principles apply equally to manual transaxles. The clutch engages the transmission gradually by permitting a controlled amount of slippage between the transmission’s input shaft and the flywheel. The components required to achieve this include the flywheel, clutch disc, pressure plate assembly, clutch release bearing or throwout bearing and the clutch fork.

The clutch system is an mechanical assembly that enables manual transmission vehicles to start from a stop, idle without stalling, and shift gears seamlessly. From the flywheel and friction disc to the various pressure plate designs and evolving linkage mechanisms, each component is engineered to work in harmony to control power delivery. Whether relying on traditional shaft and lever systems, modern cable setups, self adjusting ratchets, hydraulic circuits or internal concentric slave cylinders, the underlying principles remain consistent.

Operation:

The basic principle of clutch operation involves the pressure plate and flywheel serving as the drive or input members of the assembly. The clutch disc, also referred to as the friction disc, functions as the driven or output member and connects to the transmission’s input shaft. As long as the clutch is disengaged with the clutch pedal depressed, the drive members turn independently of the driven member, effectively disconnecting the engine from the transmission. However, when the clutch is engaged and the clutch pedal is released, the pressure plate moves toward the flywheel. The clutch disc is then squeezed between these two revolving drive members and is forced to turn at the same speed, enabling power transfer.

The Flywheel:

The flywheel serves as both a critical engine component and the main driving member of the clutch. It is normally constructed from nodular or gray cast iron, which contains a high graphite content that helps lubricate the engagement of the clutch. Welded or pressed onto the outside diameter of the flywheel is the starter ring gear. The large diameter of the flywheel provides an excellent gear ratio between the starter drive and the ring gear, ensuring ample engine rotation during the starting process. The rear surface of the flywheel acts as a friction surface and is machined very flat to guarantee smooth clutch engagement. Additionally, the flywheel absorbs torsional vibration from the crankshaft and provides the necessary inertia to rotate the crankshaft through the four strokes of the engine cycle.

The flywheel has two distinct sets of bolt holes drilled into it. The inner set is used to fasten the flywheel to the crankshaft, while the outer set provides a mounting plate for the pressure plate assembly. A bore located 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 proper alignment with the engine’s crankshaft. In many applications, a ball or roller needle bearing is used instead of a traditional pilot bushing. Some transaxle designs feature a short, self centering input shaft that does not require a pilot bushing or bearing at all.

Dual Mass Flywheel:

A few cars and light trucks utilize a dual mass flywheel. These flywheels are employed to reduce vibrations transmitted through the transmission, provide smoother shifting, and reduce gear noise. Dual mass flywheels can significantly decrease the oscillations of the crankshaft before they move through the transmission. The flywheel consists of two rotating plates connected by a spring and damper system. The forwardmost portion of the flywheel bolts to the end of the crankshaft and works to smooth out the crankshaft’s oscillations. The pressure plate of the clutch is bolted to the rearward portion of the flywheel. Engine torque moves from the front plate through the damper and spring assembly to the rear plate before it enters the transmission. Some dual mass flywheel designs feature a torque limiting feature that prevents damage to the transmission during peak torque loads. The rotation of the two flywheel plates can differ by as much as 360 degrees, allowing the forward plate to absorb torque spikes and prevent them from being transmitted through the transmission.

The Clutch Disc:

The clutch disc is splined to the transmission’s input shaft and receives the driving motion from the flywheel and pressure plate assembly, transmitting that motion to the transmission input shaft. There are two types of friction facings used on clutch discs. Molded friction facings are preferred because they can withstand greater pressure plate loading force without sustaining damage. Woven friction facings are utilized when additional cushioning action is required for clutch engagement. Until recently, the material molded or woven into these facings was predominantly asbestos. Due to the health hazards associated with asbestos, modern clutches use alternative materials such as paper-base compounds and ceramics. Particles of cotton, brass, rope, and wire are added to the material to prolong the life of the clutch disc and provide torsional strength.

Grooves are cut across the face of the friction facings to promote clean disengagement of the driven disc from the flywheel and pressure plate, while also facilitating 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 reduce chatter during engagement 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 engineered to absorb crankshaft vibration, abrupt clutch engagement, and driveline shock. Torsional coil springs allow the disc to rotate slightly in relation to the pressure plate while absorbing the torque forces. The number and tension of these springs are determined by engine torque and vehicle weight.

Pilot Bushing and 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 to the transmission when the clutch is engaged. The transmission end of the input shaft is supported by a large bearing in the transmission case, but because the input shaft extends unsupported from the transmission, a pilot bushing is necessary to keep it correctly positioned. By supporting the shaft, the pilot bushing ensures the clutch disc remains centered in the pressure plate.

The Pressure Plate Assembly:

The pressure plate assembly has a twofold purpose. First, it must squeeze the clutch disc onto the flywheel with sufficient force to efficiently transmit engine torque. Second, it must move away from the clutch disc so the disc can stop rotating, even while the flywheel and pressure plate continue to turn. There are two basic types of pressure plate assemblies: those with coil springs and those with a diaphragm spring. Both types feature a stamped steel cover that bolts to the flywheel and acts as a housing to hold the parts together. Both also incorporate the pressure plate itself, which is a heavy, flat ring made of nodular or gray cast iron. The assemblies differ in how 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 and flywheel, forcing the disc against the flywheel. The release levers, usually three in number, release the holding force of the springs. Each lever has two pivot points: one attaches the lever to a pedestal cast into the pressure plate, and the other attaches to a release lever yoke or keybolt bolted to the cover. The levers pivot on the pedestals and release lever yokes to move the pressure plate through engagement and disengagement. To disengage the clutch, the release bearing pushes the inner ends of the release levers forward toward the flywheel. The release levers are class one levers, meaning the fulcrum lies between the effort and the load, so each end of the lever moves in the opposite direction. When force pushes one end down, the other end moves up.

In a coil spring pressure plate, the release lever yokes act as fulcrums, and the outer ends of the release levers move backward, pulling the pressure plate away from the clutch disc. This action compresses the springs and releases the clamping force on the disc. When the clutch is engaged, the release bearing moves away from the pressure plate, allowing the springs to push the pressure plate and clutch disc against the flywheel, permitting power transfer. Several advantages and disadvantages characterize the coil spring pressure plate. Clamping pressure can be increased or decreased by changing the number of springs and their tension. However, these assemblies require more pedal effort than diaphragm pressure plates. Additionally, as the clutch disc wears, the coil springs expand and their clamping force is reduced. Because of these disadvantages, passenger cars and light trucks are now almost exclusively equipped with diaphragm spring clutches.

Diaphragm Spring Pressure Plate Assembly:

The diaphragm spring pressure plate assembly relies on a cone-shaped diaphragm spring positioned between the pressure plate and the pressure plate cover to move the plate back and forth. The diaphragm spring, sometimes called a Belleville spring, is a single, thin sheet of metal that operates similarly to the bottom of an oil can; the metal yields when pressure is applied and springs back to its original shape when the pressure is removed. The center portion of the diaphragm spring is slit into numerous fingers that function 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, disengaging the clutch. When the clutch is engaged, the release bearing and the fingers of the diaphragm spring move toward the transmission. As the diaphragm pivots over the pivot ring, its outer rim forces the pressure plate against the clutch disc, engaging it to the flywheel.
Diaphragm spring pressure plate assemblies offer several advantages. They are compact, weigh less, and have fewer moving parts prone to wear. They require little pedal effort from the operator and provide a balanced force around the pressure plate, reducing rotational unbalance. Clutch disc slippage is less likely to occur, and the force holding the clutch disc to the flywheel does not change throughout its service life, contributing to better mileage buildup.

The Clutch Release Bearing:

The clutch release bearing, also called a throwout bearing, is typically a sealed, prelubricated 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 a hub that 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 on its shaft by the clutch fork. As the bearing contacts the release levers or diaphragm spring of the pressure plate assembly, it begins to rotate with the rotating 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. Once the clutch is fully engaged, the release bearing is normally stationary. In self-adjusting clutch linkages, used on many vehicles, just enough tension is applied to the clutch control cable to keep constant light pressure against the release bearing. Consequently, the release bearing remains in contact with the release levers or diaphragm spring of the rotating pressure plate assembly and rotates with the pressure plate.

The Clutch Fork:

The clutch fork is a forked lever that pivots on a support shaft or ball stud located in an opening in the bell housing. The forked end slides over the hub of the release bearing, while 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 Systems:

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

Shaft and Lever Linkage:

Found on older vehicles, the shaft and lever clutch linkage has many parts and pivot points. It transfers the movement of the clutch pedal to the release bearing via shafts, levers, and bell cranks. On some vehicles, the pivot points were equipped with grease fittings, while others had low-friction plastic grommets and bushings at their pivot points. A typical assembly includes a release lever and rod, an equalizer or cross shaft, a pedal-to-equalizer rod, an assist or over center spring, and the pedal assembly. Depressing the pedal moves the equalizer, which in turn moves the release rod. When the pedal is released, the assist spring returns the linkage to its normal position and removes pressure on the release rod, causing the release bearing to move away from the pressure plate.

Cable Linkage:

A cable linkage performs the same controlling action as the shaft and lever linkage but with fewer parts. The clutch cable system is compact and offers the advantage of flexible installation, allowing it to be routed around power brake and steering units. These advantages make it the most commonly used clutch linkage. The clutch cable is made of braided wire; the upper end connects to the top of the clutch pedal arm, and the lower end fastens to the clutch fork. It is designed with a flexible outer housing fastened at the firewall 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, moving the clutch fork to disengage the clutch. The pressure plate springs and springs on the clutch pedal provide the force to move the cable back when the pedal is released.

Self Adjusting Clutch Mechanism:

Self adjusting clutch mechanisms monitor clutch pedal play and automatically adjust it when necessary. Usually, the mechanism is a ratcheting unit 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 keeps the pawl in contact with the toothed segment. The pawl allows the toothed segment to move in only one direction relative to the pawl. The clutch cable is guided around and fastened to the toothed segment, which is free to rotate in one direction independently of the clutch pedal. The tension spring pulls the toothed segment backward. When the clutch cable develops slack due to stretching and clutch disc wear, the cable is adjusted automatically when the clutch is released. The tension spring pulls the toothed segment backward and allows the pawl to ride over to the next tooth, effectively shortening the cable by reeling in the slack through the repositioning of the toothed segment. This self-adjusting action takes place automatically throughout the clutch’s operational life.

Hydraulic Operated Clutch Linkage:

Frequently, the clutch assembly is controlled by a hydraulic system, where hydraulic pressure transmits motion from one sealed cylinder to another through a hydraulic line. Like the cable linkage, the hydraulic linkage is compact and flexible, allowing engineers to place the release fork in various locations for greater body design flexibility. Additionally, the hydraulic pressure developed by the master cylinder decreases pedal effort and provides a precise method of controlling clutch operation. Brake fluid is commonly used as the hydraulic fluid in these systems.
The hydraulic clutch master cylinder uses a pushrod to move the piston and primary cup, creating hydraulic pressure. A snapring restricts piston travel. A secondary cup at the snapring 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 may have a bleeder valve to bleed air from the system for efficient operation. The cylinder body is threaded for a tube and fitting at the fluid entry port. Rubber seal rings seal the hydraulic pressure between the piston and the slave cylinder walls.

A piston retaining ring restricts piston travel to a set distance. Piston travel is transmitted by a pushrod to the clutch fork, and a pushrod boot keeps contaminants out of the slave cylinder. When the clutch pedal is depressed, the piston and primary cup develop 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 disengaged 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 residual pressure in the system when the clutch assembly is in the engaged position.

Internal Slave Cylinders:

An internal concentric slave cylinder is found on some cars and light trucks. These units combine the slave cylinder and the clutch release bearing into one assembly. By positioning the slave cylinder directly behind the release bearing, the movement of the bearing is linear. In other linkage designs, the release bearing moves through an arc during engagement and disengagement. An internal slave cylinder is a doughnut-shaped unit that mounts to the front of the transmission, with the transmission’s input shaft passing through it. The slave cylinder is either bolted to the transmission’s front bearing cover or held by a pressed pin. In many cases, the transmission must be removed to properly diagnose and service this type of slave cylinder.