chap94 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 94, the reader should
be able to:

•
•
•

Prepare for ASE Manual Drive Train and Axles
(A3) certification test content area “A” (Clutch
Diagnosis and Repair).

List the parts that are included in the clutch
system.
Describe how the clutch works.

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OBJECTIVES:
After studying Chapter 94, the reader should
be able to:

•
•

•

Explain the importance of proper clutch pedal
freeplay.
List the steps necessary to replace a clutch.
Describe the characteristic noise each clutch
component part makes when defective.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
Bushing
candlestick • clutch disc • coil spring clutch • cushioning

spring
front bearing retainer
lever style (pressure plate)
marcel spring
pilot bearing • pressure plate
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KEY TERMS:
quill • quill shaft
release bearing
slave cylinder
torsional dampers • throwout bearing

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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PURPOSE AND FUNCTION OF A CLUTCH
The clutch assembly is located between the engine and the 
transmission/transaxle. The purpose and function of a clutch include 
the following:

To disconnect the engine from the transmission/transaxle to 
permit the engine to remain running when the vehicle is stopped 
and to permit the transmission/transaxle to be shifted
Connect and transmit engine torque to the transmission/ 
transaxle

To dampen and absorb engine impulses and drivetrain vibration
To provide a smooth engagement and disengagement between 
the engine and the transmission/transaxle
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NOTE: The term transmission refers to rear­wheel­drive vehicles and the 
term transaxle usually refers to front­wheel­drive vehicles that have
a differential built into the unit. A separate differential is used with a 
transmission.

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COMPONENT PARTS AND OPERATION
OF A CLUTCH ASSEMBLY
A clutch assembly consists of a clutch disc that is splined to the input 
shaft of the transmission/transaxle. When the driver depresses the 
clutch pedal, a throwout bearing (release bearing) is forced against 
the fingers of the pressure plate, which is bolted to and rotates with 
the flywheel. 
When force is exerted on the center of the pressure plate, the pressure 
is released from the clutch disc that has been forced against the engine 
flywheel. With the pressure removed from the clutch disc, the engine 
can be operated without transferring torque to the transmission/ 
transaxle.
See Figure 94–1.

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Figure 94–1 Typical automotive clutch assembly showing all related parts. (Courtesy of LUK)

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When the driver reduces force on the clutch pedal, the pedal return spring and the pressure plate spring 
combine to return the clutch pedal to its at­rest position (clutch­engaged position).
When the clutch pedal moves up, the pressure on the throwout bearing is released and the force against the 
pressure plate spring(s) is released allowing the force of the pressure plate to clamp the clutch tightly 
between the flywheel and the pressure plate.

See Figure 94–2.

To summarize:
When the clutch pedal is up, the clutch is engaged.
When the clutch pedal is down, the clutch is disengaged.

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Figure 94–2 (a) When the clutch is in the released position (clutch pedal depressed), the clutch
fork is applying a force to the throwout (release) bearing, which pushes on the diaphragm
spring, releasing the pressure on the friction disc.

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Figure 94–2 (b) When the clutch is in the engaged position (clutch pedal up), the diaphragm
spring exerts force on the clutch disc, holding it between the flywheel and the pressure plate.

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Additional related parts include the pilot bearing (or bushing) that 
supports the front of the transmission input shaft. 

Figure 94–3 The transmission has just been
removed. Note that this type of transmission
incorporates the bell housing, which was
therefore removed at the same time as the
transmission. The clutch fork and throwout
(release) bearing also came off together. All
that remained attached to the engine was the
flywheel, clutch disc, and pressure plate.

The throwout bearing is often supported and rides on the 
transmission/transaxle front bearing retainer (called the quill).
NOTE: Most front­wheel­drive transaxles do not use a pilot bearing.
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Clutch Pedal Linkage
Levers and rods  Through a series of levers and rods, the 
release fork is forced against the throwout bearing. This method 
was commonly used on many older vehicles.
Cable operation  A cable is used similar to a brake cable used 
on a bicycle. 

Figure 94–4
A typical cableoperated clutch.

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Clutch Pedal Linkage
Hydraulic  A small master cylinder and a slave cylinder 
located near the throwout bearing is a very common method
of connecting the clutch pedal to the release fork on vehicles 
equipped with manual transmission. 

Figure 94–6 A typical clutch master cylinder
and reservoir mounted on the bulkhead on
the driver’s side of the vehicle. Brake fluid is
used in the hydraulic system to operate the
slave cylinder located on the bell housing.

See also Figure 94–5 following.
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Figure 94–5 A hydraulic clutch linkage uses a master cylinder and a slave cylinder.

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Clutch Disc  The clutch disc is round with a splined center hole 
that slips over the splines of the input shaft of the transmission/ 
transaxle. Friction material is riveted to both sides of the clutch 
disc—one side touches the flywheel of the engine and the other 
side touches the friction surface of the pressure plate.
Friction material is either woven or molded from a mixture of other 
materials. Woven materials are softer and help cushion clutch 
engagement, but they may not last as long as moldedmaterials.
Friction materials operate in high heat and pressure. For many 
years, asbestos was the most common material used in both clutch 
and brake linings. Today, clutch friction material may contain 
paper, cotton, and bits of copper or brass wire with resin holding 
the mixture together.
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CAUTION: Always take precautions when working around clutch lining 
material and assume that it contains asbestos. These precautions should 
include wetting the friction disc before removal and preventing any dust 
from the lining from becoming airborne.

High­Performance Clutch Disc  Another type of friction material is 
a ceramic and metallic mixture. This creates a hard, long­lasting 
lining, but is more expensive and does not cushion clutch 
engagement as much as a softer lining.
Instead of a full circle of softer friction material, the disc may have 
only a few segments or buttons of this ceramic­metallic material. 
Clutches that use these discs, which are sometimes called “button 
clutches,” are found in racing applications where strength and 
durability are a greater concern than smooth engagement.
See Figure 94–7.
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Figure 94–7 A racing or high-performance clutch disc lacks the features of a stock clutch disc
that help provide smooth engagement.

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Stock Clutch Disc  Around the center hub of the clutch disc are 
torsional dampers that absorb the initial shock of engagement 
and help dampen engine­firing in pulses being transmitted into
and through the transmission/transaxle.
See Figure 94–8.
The torsional dampers are either coil springs or made of rubber.
In the space between the friction surfaces is a wavy spring steel 
material called a cushioning spring or marcel spring.
The marcel spring also helps to absorb the initial shock of 
engagement and allows for a smooth engagement of the clutch. 
See Figure 94–9. 
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Figure 94–8 A typical stock clutch friction disc that uses coil spring torsional dampers.

Figure 94–9 A marcel is a wavy spring that is placed between the two
friction surfaces to cushion the clutch engagement.

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HINT: The larger center hub section of the disc must be installed with the 
thicker portion facing the pressure plate

Figure 94–10 Cutaway of the center section of a clutch plate showing the various layers of steel
plates used in the construction.

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Pressure Plates  The purpose of the pressure plate is to exert a 
force on the clutch disc so that engine torque can be transmitted 
from the engine to the transmission/transaxle.
A strong spring(s) is required to provide adequate clamping force 
on the clutch disc. However, a strong pressure plate spring force 
must be released by the force of the driver’s foot to disengage the 
clutch. The pressure plate, like the flywheel, is usually made of 
nodular cast iron. A smooth, machined area on one side forms the 

friction disc contact surface.
When the clutch engages, spring force pushes the pressure plate 
toward the flywheel so the friction disc is clamped between the 
flywheel and the pressure plate. Engine torque flows through the 
clutch to the transmission input shaft. 
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Coil spring style  This style of pressure plate uses coil springs 
and three release arms. A coil­spring­style pressure plate is also 
called a lever style because it uses levers to compress the 
springs. 

Figure 94–11 A coil spring (lever style) clutch pressure plate.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Diaphragm spring style  This style is the most commonly 
used pressure plate design. One large, round, spring­steel 
spring is used to apply even force on the clutch disc. 
These tend to be smaller assemblies, weigh less, and have 
fewer parts than coil spring assemblies.
The one­piece diaphragm spring does the job of all the release 
levers and coil springs in a coil spring clutch.
See Figure 94–12. 

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