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.
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
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
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
- 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.
- 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
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
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.
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
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
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
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
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
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.