The Clutch is the Lesson 2 of Manual Transmission and Drivetrain. A clutch is a mechanical device that allows the output shaft to be disconnected from the rotating input shaft. The clutch’s input shaft is typically attached to a motor, while the clutch’s output shaft is connected to the mechanism that does the work.
Most cars and trucks with a manual transmission use a dry clutch, which is operated by the driver using the left-most pedal. The motion of the pedal is transferred to the clutch using hydraulics (master and slave cylinders) or a cable. The clutch is only disengaged at times when the driver is pressing on the clutch pedal, therefore the default state is for the transmission to be connected to the engine. A “neutral” gear position is provided, so that the clutch pedal can be released with the vehicle remaining stationary.
Table of Contents
The Clutch : Lesson 2 – Manual Transmission and Drivetrain :
Clutch – General:
Objectives:
Upon completion of this lesson, you will be able to:
! Explain the purpose and function of a clutch.
! Describe a clutch.
! Identify the components of a clutch.
! Explain the theory and operation of a clutch.
Clutch system
Transaxle clutch assembly
- Slave cylinder
- Release bearing
- Pressure plate
- Clutch disc
- Pilot bearing
- Flywheel
- Clutch pedal
- Clutch master cylinder
The purpose of the clutch is to transfer torque from the engine to the transmission in a controlled manner. The clutch disconnects the engine from the transmission or transaxle when the driver pushes down the clutch pedal. As the driver allows the pedal to come up, the engine connects to the transmission/ transaxle, and the vehicle moves. The clutch must be designed so that this connecting (engaging) and disconnecting (disengaging) is smooth and can occur gradually.
It must not jump abruptly from disengagement to engagement. To move a vehicle, the engine must speed up to get enough power. It cannot in one moment bring the speed of the wheels up to the speed of the engine. Shifting gears in a moving car creates a similar situation.
The driving wheels are not turning at the same speed as the engine. To make smooth shifts between gears, the clutch slips a little, takes hold gently at first, and gradually grabs harder and harder. Thus the driving wheels start to move slowly and gradually pick up speed, until finally everything is turning at the same speed and the clutch is solidly engaged.
Clutch components
The size of clutch components varies based upon the type of vehicle in which they are used. Larger heavyduty vehicles use heavy-duty components so the clutch can handle the load of the vehicle. However, the typical clutch uses seven major clutch components.
The clutch components are:
! Flywheel
! Clutch disc assembly
! Pressure plate assembly (cover, plate, internal springs and levers)
! Clutch fork
! Release bearing
! Pilot bearing
! Hydraulic or mechanical linkage
Flywheel
The flywheel is the foundation on which the entire clutch is attached. It is bolted to the engine crankshaft and rotates with it. The flywheel is machined smooth to provide a uniform friction surface. The mass of the flywheel is designed to dampen the firing pulses of the engine.
Clutch disc
The clutch disc receives the torque of the engine and transfers it through a splined hub to the transmission input shaft. The disc has grooved friction material on both sides where it contacts the flywheel and the pressure plate. These grooves allow cleaner disengagement action and enhance air flow over the disc for cooling. Dampening springs in the hub are used to absorb engine pulses.
NOTE: Most vehicles use a single-disc clutch system. Some vehicles use a multipledisc clutch system for adequate torque transfer.
Pressure plate:
The pressure plate assembly is bolted to the engine flywheel. When engaged, it applies pressure against the clutch disc, holding it tightly against the surface of the flywheel. One side of the pressure plate is machined smooth. This side presses the clutch disc against the flywheel. On the other side of the pressure plate is the clutch cover. This cover bolts to the flywheel and provides the solid base for the pressure plate apply spring(s) to use to force the pressure plate against the clutch disc and flywheel.
Pressure plate types
Although all pressure plates perform the same function, the types of pressure plates vary.
Diaphragm spring pressure plate
The diaphragm spring pressure plate assembly uses a conical piece of spring steel to press the pressure plate against the friction disc and flywheel. The center portion of the spring is slit into numerous fingers that act as release levers.
When the clutch is disengaged, the release bearing is forced against the fingers of the diaphragm spring, which causes the outer rim of the spring to move away from the flywheel. The force on the pressure plate is then released, which releases the friction disc from the flywheel.
Many vehicles with diaphragm-type pressure plates are self-adjusting. During clutch replacement, the self-adjuster must be set before the pressure plate is installed on the vehicle.
Heavy-duty vehicles require that more force be applied to the clutch disc. These vehicles often use a coil-spring type pressure plate. On a coil-spring pressure plate, several coil springs are placed between the clutch cover and the pressure plate.
Some coil-spring pressure plates have weighted release levers that allow the centrifugal force of the rotating clutch to increase the force that the pressure plate applies to the clutch disc.
Clutch fork
A clutch fork is used on some clutch assemblies to move the release bearing into and out of the fingers or levers of the pressure plate. It mounts to a pivot ball on the transmission and uses mechanical leverage for application and release of the pressure plate.
Clutch forks are commonly used on transmissions with mechanical-type linkages. However, clutch forks are also used in combination with some hydraulic clutch systems.
- Clutch fork
- Release bearing
Release bearing
The release bearing is a sealed ball bearing that acts upon the diaphragm fingers or release levers of the pressure plate to disengage the clutch.
The release bearing is mounted to either a release lever or hydraulic cylinder. When the driver presses the clutch pedal down, the release bearing is forced into the pressure plate release fingers or levers, forcing them inward. The pressure plate force is released and disengages the clutch. Often the release bearing is mounted in a carrier.
Many release bearings are designed to be in contact with the pressure plate fingers even when the clutch pedal is fully released.
Typical release bearing
- Bearing carrier
- Release bearing
Pilot bearing
The pilot bearing is used on many vehicles. It mounts either in the center of the flywheel or in the rear of crankshaft. Its purpose is to support the input shaft of the transmission while allowing the shaft to rotate independently of the crankshaft.
Some front-wheel drive vehicles do not use a pilot bearing in their clutch systems.
Clutch linkage
The connection between the clutch pedal and the release bearing is the clutch linkage. There are two basic types of clutch linkage:
! Manual linkage with self-adjuster
! Hydraulic linkage
Manual linkage
A cable linkage with self-adjuster is used on some vehicles. This type of linkage connects the clutch pedal to the release fork. At the top of the clutch pedal where the cable attaches is a self-adjuster that adjusts the cable as the clutch disc wears.
During operation there is a slight preload applied to the release bearing by a spring-loaded ratcheting pawl. This pawl engages into a toothed wheel (quadrant) that is mounted at the clutch pedal pivot point. When the clutch is disengaged, the pawl engages a tooth on the quadrant. As the clutch wears, the slack in the cable allows the pawl to move to the next tooth of the quadrant, automatically taking the slack out of the cable and maintaining the correct adjustment on the clutch.
Manual clutch linkage
- Pawl tension spring
- Clutch cable
- Pivot
- Clutch pedal
- Quadrant tension spring
- Quadrant
- Pawl
Hydraulic system
Basic hydraulic system
- Clutch master cylinder reservoir
- Clutch master cylinder
- Clutch slave cylinder with release bearing
Hydraulic clutch controls use hydraulic pressure to move the release bearing against the release fingers or levers of the pressure plate. Similar to a brake system, the hydraulic clutch has a master cylinder, hydraulic tubing, and a slave cylinder.
When the driver presses the clutch pedal, a linkage arm connected to the master cylinder forces the master cylinder piston down its bore.
Pressure is applied to the brake fluid contained in the master cylinder, which sends this pressure to the slave cylinder.
The piston in the slave cylinder changes this pressure into mechanical force by moving outward.
This mechanical action forces the release bearing into the pressure plate release fingers or levers, disengaging the clutch.
Operation
Clutch operation
Most cars and light trucks use a single dry-disc clutch. Basically, this system has one plate squeezed tightly between two other plates. The middle plate is driven. A strong spring or set of springs forces the two driving members together. This tightens their grip on the middle plate until all are turning together as one unit.
The other driving member is called the pressure plate. The pressure plate is a heavy ring of cast iron that is smooth on one side. The pressure plate is fastened to the clutch cover, which is bolted to the flywheel, so they all turn together.
The driven plate is a flat disc of steel with friction material facing on each side. The disc is fastened by splines to the input shaft of the transmission. Since the clutch disc has internal splines, it fits on the transmission input shaft and must rotate when the input shaft rotates. The clutch disc is free to move back and forth on the input shaft due to its straight splines.
Clutch disengaged
- Flywheel
- Clutch disc
- Pressure plate
- Spring
- Transmission input shaft
- Crankshaft
The engine flywheel is used for one of the driving members. The surface of the flywheel is machined very smooth where the friction plate pushes up against it.
When the driver pushes down the clutch pedal (clutch disengaged), the pressure plate is forced away from the flywheel. Since the clutch disc is no longer held against the flywheel, the engine no longer drives the clutch disc and transmission input shaft.
Clutch disengagement allows the transmission input shaft to stop rotating, so the vehicle can be stopped without stalling the engine. If the vehicle is moving, releasing the torque on the input shaft allows smooth
shifts because the transmission/transaxle gears are not under a load.
When the clutch pedal is up (clutch engaged), the pressure plate assembly squeezes the clutch disc against the flywheel. This action forces the clutch disc to turn with the flywheel and drive the transmission input shaft.
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