Clutch Assembly section 13

Everyone knows that a car has an engine to power it, but not everyone is familiar with a clutch or how it works. This mechanism both engages and disengages your power transmission from the driving shaft to the driven shaft. It connects rotating shafts, and there can be two or more of these under your hood. If you drive a manual transmission, the clutch is connected to both the shaft coming from the engine and the shafts that turn the wheels. While the motor is going to spin constantly, you dont want the wheels continually spinning. Frictional forces are what clutches rely on to operate. Frictional clutches connect one moving member to another one that is moving at a different speed, or not at all, to get it moving at the same speed so that there is no slippage. Various materials are used to create this friction.

start

• 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.

After studying Chapter 94, the reader should be

able to:

OBJECTIVES:

Continued

• 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.

After studying Chapter 94, the reader should be

able to:

OBJECTIVES:

candlestick • clutch disc • coil spring clutch • cushioning spring

front bearing retainer

lever style (pressure plate)

marcel spring

pilot bearing • pressure plate

KEY TERMS:

Continued

quill • quill shaft

release bearing

slave cylinder

torsional dampers • throwout bearing

KEY TERMS:

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:

Continued

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

NOTE: The term transmission refers to rear-wheel-drive vehicles and the term transaxle usually refers to front-wheel-drive vehicles that have

a differential built into the unit A separate differential is used with a

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/

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

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 at-rest position (clutch-engaged 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.

Continued

To summarize:

When the clutch pedal is up, the clutch is engaged.

When the clutch pedal is down, the clutch is disengaged.

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

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.

Continued

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 front-wheel-drive transaxles do not use a pilot bearing.

NOTE: Most front-wheel-drive transaxles do not use a pilot bearing.

Clutch Pedal Linkage

Continued

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

cable-operated clutch.

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.

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

Continued

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.

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.

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.

High-Performance Clutch Disc Another type of friction material is

a ceramic and metallic mixture This creates a hard, long-lasting

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 ceramic-metallic 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.

Figure 94–7 A racing or high-performance clutch disc lacks the features of a stock clutch disc

that help provide smooth engagement.

Stock Clutch Disc Around the center hub of the clutch disc are

torsional dampers that absorb the initial shock of engagement

and help dampen engine-firing 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

Continued

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

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

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

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

Coil spring style This style of pressure plate uses coil springs

and three release arms A coil-spring-style 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. Continued

Diaphragm spring style This style is the most commonly

used pressure plate design One large, round, spring-steel

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 one-piece diaphragm spring does the job of all the release

levers and coil springs in a coil spring clutch.

See Figure 94–12

Figure 94–12 Typical diaphragm-style pressure plate that uses a Belleville spring.

Continued

The driver must push harder on the clutch pedal to disengage a coil

spring clutch than a comparable diaphragm spring design The

pedal effort for a coil spring clutch increases the farther down the

pedal is pushed The pedal effort for a diaphragm spring clutch

decreases during the second half of pedal travel.

In many cases, aftermarket clutch parts manufacturers supply a

diaphragm spring pressure plate assembly when an original coil

spring assembly must be replaced If so, carefully compare the new pressure plate to the old one to make sure it is the correct

replacement.

Physical characteristics, such as dimensions and bolt-hole patterns,

must be identical for the assembled clutch to operate properly

Check that the inner edges of the diaphragm spring fingers are the

Flywheels The engine flywheel serves four basic purposes:

Continued

Smoothes out or dampens engine power pulses.

Absorbs some of the heat created by clutch operation.

Provides the connection point for the starter motor to

crank the engine.

Provides application surface for the clutch friction disc.

A flywheel is heavy, or has a large mass, which creates inertia

The inertia provided by the flywheel mass tends to keep

crankshaft speed more constant.

The flywheel absorbs some of the heat created by clutch operation

by acting as a heat sink for the clutch friction disc.

An external ring gear is pressed or welded onto the flywheel

along its outer circumference When the starter motor is engaged,

the starter-drive gear meshes with the flywheel ring gear.

Through gear reduction, the flywheel transfers starter motor

rotation to the crankshaft to crank the engine.

The face on the transmission side of the flywheel has a smooth,

machined area that creates the application surface for the clutch

friction disc This surface must be properly finished to allow

adequate slippage as the clutch engages and disengages, and to

prevent slippage when the clutch is engaged

Continued

Figure 94–13 A flywheel

after it has been machined

(ground) to provide the

correct surface finish for the

replacement clutch disc.

The flywheel is constructed of cast iron and attaches to the end

of the engine crankshaft The carbon content of the cast iron

(about 3%) provides a suitable surface for the clutch disc.

The carbon, in the form of graphite, acts as a lubricant to provide

a smooth engagement of the clutch.

The weight of the flywheel helps to absorb and smooth out

engine-firing impulses.

A starter ring gear is welded or pressed onto the outside diameter

of the flywheel

Often the ring gear can be replaced separately without having to

replace the flywheel in the event of a failure, as shown below.

Continued

The pilot bearing is often installed in the center of the flywheel (or in the end of the crankshaft) to support the end of the input shaft of the transmission.

Figure 94–14 The starter motor will spin but the engine will not crank if the ring gear on the

flywheel is broken

Dual-Mass Flywheels High-performance vehicles and vehicles

equipped with diesel engines, use a dual-mass flywheel, to dampen engine vibrations and keep them from being transmitted to the

passenger compartment through the transmission and shift linkage.

A dual-mass flywheel consists of two separate flywheels attached

with damper springs, friction material, and ball bearings to allow

some movement between the primary and secondary flywheel.

By allowing a very slight amount of movement between the two

flywheels, the damper absorbs engine torque peaks and normal

vibration to provide smoother drivetrain operation.

The damper assembly is completely sealed, because it also

contains a fluid or lubricant, typically silicone-based, which also

Figure 94–15 A cutaway of a dual-mass

flywheel used on a Ford diesel pickup truck.

Continued

The starter ring gear mounts on

the primary flywheel

Power from the starter motor

does not have to flow through the

damper assembly to reach the

engine crankshaft

The ring gear and pilot bearing

are usually attached to the

primary flywheel.

The clutch friction surface is

usually the secondary flywheel

Shim It or Replace It

Whenever replacing a clutch, most experts agree that the flywheel should

be removed from the engine and resurfaced.

When material is removed from the surface of the flywheel, the geometry (relationship) of the clutch parts changes because the pressure plate is

now closer to the rear of the engine by the amount removed from the

flywheel.

Ask your parts supplier for a shim equal in thickness to the amount of

material removed during resurfacing Generally, these round shims are

available in 0.020’’ to 0.100” thicknesses.

The shim is installed between the crankshaft flange and the flywheel If a shim is not used, the flywheel may have to be replaced to properly restore proper clutch operation and service life.

NOTE: If the dual-mass flywheel fails, the symptom is the same as a

slipping clutch The torque-limiting friction material connecting the

primary and secondary flywheels can fail This failure requires the

replacement of the flywheel assembly.

NOTE: If the dual-mass flywheel fails, the symptom is the same as a

slipping clutch The torque-limiting friction material connecting the

primary and secondary flywheels can fail This failure requires the

replacement of the flywheel assembly.

If engine oil or transmission lube gets onto the friction surface of the clutch, the clutch will chatter when engaged This grabbing and releasing of the

clutch is not only harmful to the drivetrain (transmission, driveshaft, etc.)

but also is disturbing to the driver when the vehicle vibrates and shakes

while driving.

To avoid the possibility of a chattering clutch, always repair oil leaks as

soon as possible Rocker (valve) cover gaskets, intake manifold gaskets, oil galley plugs, rear main seals, as well as the input shaft seal on the

transmission/transaxle itself can all lead to clutch contamination

Repair the Oil Leaks Before Replacing the Clutch - Part 1

Figure 94–16 (a) Before replacing the clutch, the bell housing should be cleaned and the clutch fork pivot lightly lubricated

Figure 94–16 (b) The input shaft seal should

also be replaced to prevent the possibility of

getting transmission lubricant on the friction

surfaces of the clutch.

Repair the Oil Leaks Before Replacing the Clutch - Part 2

Pilot Bearing or Bushing The transmission input shaft from the

transmission, through the clutch assembly, to the engine

Transmission bearings support the transmission end of the shaft.

Depending on the length of the input shaft, it may also need a pilot bearing or bushing to support it at the engine end.

A front-wheel-drive vehicle with a stepped flywheel, compact

clutch assembly, and a transaxle has a short input shaft that may

not reach all the way to the flywheel.

This design does not need a pilot bushing or bearing to support the engine end of the input shaft.

See Figure 94–17.

Continued

Figure 94–17 A transaxle assembly has been removed to replace the clutch Note the short

input shaft This vehicle did not use a pilot bearing (bushing)

Other designs, such as a rear-wheel-drive vehicle with a flat

flywheel, have a much longer transmission input shaft At the

engine, the transmission input shaft rests inside a small bore in the

flywheel or crankshaft flange.

The pilot bearing or bushing, which is pressed into this bore,

supports the engine end of the input shaft and provides a

low-friction surface for the shaft to ride on This keeps the shaft and

friction disc perfectly aligned with the flywheel and pressure plate.

The pilot bearing or bushing rotates with the crankshaft when the

engine is running At times, the input shaft does not rotate when the engine is running The pilot bearing or bushing lowers the friction

between these two moving parts Pilot bushings are usually a

sintered metal sleeve pressed into the bore in the crankshaft flange.

Continued