Clutches

Objectives

• Describe the various clutch components and their functions.

• Name and explain the advantages of the different types of pressure plate assemblies.

• List the safety precautions that should be followed during clutch servicing.

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Objectives

• Explain how to perform basic clutch maintenance.

• Name the six most common problems that occur with clutches.

• Explain the basics of servicing a clutch assembly.

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Drive Trains

Front-WheelDrive Vehicle

Rear-WheelDrive Vehicle

Automotive Clutch

• Used to connect and disconnect the engine and manual transmission or transaxle

• Allows the driver to control power flow between the engine and transmission or transaxle

• Vehicles with manual transmissions require a clutch

• Power flow from one unit to another can be controlled with a drive disc and a driven disc

Clutch Theory

Clutch is engaged

Clutch is disengaged

Introduction

Clutches In order to change gear, the drive between the engine and the gearbox mustbe temporarily disconnected. The vehicle clutch enables the driver to do this.

A clutch is a component that is designed to connect together two rotating shafts. Clutches can be classified as one of two types - positive engagement (dog clutch) or gradual engagement (friction clutch). Positive engagement clutches normally use teeth in order to provide a positive connection, whereas gradual engagement clutches use friction

The clutch is situated between the engine and the transmission gearbox. In this example depressing the clutch pedal will engage and disengage the engine from the transmission. The clutch is designed to gradually and smoothly transmit power from the engine to the transmission to enable a vehicle to start off under full control.

Note: The ability of a clutch to transmit torque (clutch capacity) isnormally between 1.2 and 1.4 times the maximum torque of the engine. Commercial vehicles usually have a capacity between 1.5 and 2.5. Ifthe clutch is too light, slipping will take place and lead to prematurefailure. Too large a clutch will tend to cause the engine to stall and isinefficient

Requirements of a vehicle clutch

• it must connect power smoothly• it must transmit power without slipping• it must disengage quickly and smoothly• it must have good heat radiating properties• it must be well balanced• it must be trouble free and have a long service

life• it must be easy to inspect, adjust and repair.

Basic Clutch Parts

The clutch assembly consists of two major items, cover assembly (containing pressure plate and spring) and the clutch friction plate or disc which is trapped between the engine fly wheel and the cover assembly

Simplified Clutch

Clutch Components

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Clutch Action (Disengaged)

• When driver presses the clutch pedal, clutch release mechanism pulls or pushes on the clutch fork

• Fork moves the release bearing into the center of the pressure plate

• Pressure plate face pulls away from the clutch disc

• Clutch disc and transmission input do not turn

Clutch Action (Disengaged)

Clutch Action (Engaged)

• When driver releases the clutch pedal, spring pressure inside the pressure plate pushes forward on the clutch disc

• This locks the flywheel, disc, pressure plate, and transmission input together

• Engine rotates the transmission input

Clutch Action (Engaged)

Coil spring typeThe cover assembly is bolted to the flywheel and rotates at engine speed. The friction disc is splined to the gearbox input shaft. Trapped between the pressure plate and flywheel by spring pressure, the disc will transmit power to the transmission.

Depressing the clutch pedal will cause the lever to move the release bearing and relieve the pressure exerted by the spring on the disc. This will cause the disc to spin freely between the flywheel and the pressure plate. Power cannot therefore be transmitted to the transmission

Diaphragm spring typeThere are two main types of clutch cover assembly. The type illustrated usescoil springs to trap the friction disc between the pressure plate and the flywheel. The clutch cover uses a diaphragm spring

• Components:– pilot bearing– flywheel– clutch disc– pressure plate– release bearing– clutch housing (bell housing)– clutch release mechanisms– clutch start switch

Clutch Components

• Clutch housing– Connects engine

and transmission and houses the clutch assembly.

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Clutch (Bell) Housing

• Bolts to the rear of the engine

• Encloses the clutch assembly

• Made of aluminum, magnesium, or cast iron

• Transmission bolts to the back of the clutch housing

Clutch Components

• Flywheel– Acts as balancer for

the engine.– Adds inertia to the

rotating crankshaft.– Provides a surface

for the clutch to contact.

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Some flywheel have Ring gear for starter engagementSome vehicles have dual mass flywheel

Flywheel

• A mounting place for the clutch

• The pressure plate bolts to the flywheel face

• Flywheel face is precision machined to a smooth surface, where it contacts the clutch disc

• Normally made of iron for good wear and heat dissipation

Clutch Components

• Clutch (input) shaft– Projects from the front of the transmission.– Usually has a pilot that rides in a bearing or

bushing in the end of the crankshaft.– The clutch disc is

splined to the clutch shaft.

– When engaged, the clutch disc drives the input shaft.

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Clutch Disc

• Consists of a splined metal hub and a round metal plate covered with friction material (lining)

• “Splined” to the transmission input shaft

• Disc is free to slide back and forth on the shaft

Clutch Components

• Clutch disc– Is covered with friction material.– Transmits power from the engine

crankshaft to the transmission input shaft.– Torsional coil springs or rubber grommets

allow the disc to rotate slightly in relation to the pressure plate while they absorb the torque forces.

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Clutch Components

Clutch disc

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Clutch Disc

The central hub is splined to fit on the gearbox input shaft and is free to rotate a limited distance in relation to the friction faces. This movement is governed by torsion dampers made usually from coil springs or rubber inserts and reduces initial shock at power take up.

There is a tendency for the disc to stick to the flywheel or pressure plate when released in much the same way that two sheets of glass are hard to separate due to air pressure. Grooves in the friction faces help air to enter the gap forming when the pressure plate moves away from the disc.

Clutch Components

• Clutch disc (cont’)– The clutch facings are riveted to

wave springs, or cushioning springs, which cause the contact pressure on the facings to rise gradually when the clutch is engaged.

– These springs eliminate chatter by dampening the clutch engagement.

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Clutch Disc Torsion Springs

• Help absorb some of the vibration and shock produced by clutch engagement

• Small coil springs located between the clutch disc hub and the friction disc

Clutch Disc Facing Springs

• Flat, metal springs located under the disc’s friction material

• Springs have a light wave (curve)– allow the friction material to flex inward slightly

during clutch engagement– flexing smoothes engagement

Clutch Disc Friction Material• Made of heat-resistant substances• Grooves cut in the material aid cooling• Rivets are commonly used to bond the friction material to both sides

of the metal body of the disc• Leather ,cork,fabric,Asbestos

– Reybestos and ferodo > most suitable, Widely used

– Non asbestos clutch• Glass fibre mixed with special rubber• Kavalar with brass wire • Sintered metal friction

Pressure Plate

• Spring loaded device

• Locks or unlocks the clutch disc and the flywheel

• Clutch disc fits between the flywheel and pressure plate

• Two basic types:– coil spring– diaphragm spring

Clutch Components

Diaphragm Coil Spring Spring

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Components• Small coil springs

– clamp clutch disc between face and flywheel

• Face– large ring that contacts the clutch disc

• Release levers– allow the release bearing to move the face– hinged inside the pressure plate

Diaphragm Spring Pressure Plate

• Uses a single diaphragm spring instead of several coil springs

• Diaphragm spring is a large, round disc of spring steel which rides on a pivot ring part way in from the outer edge

• Pushing in on the center of the spring bends the outer edge away from the engine, releasing the clutch disc

Diaphragm Spring Pressure Plate

Clutch Components

• Release bearing– Is operated by the

clutch linkage.– Presses against the

pressure plate to release the clutch.

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Release Bearing

• Usually a ball bearing and collar assembly that reduces friction between the pressure plate levers and the clutch fork

• A few imported vehicles use a graphite bearing– ring shaped block of graphite presses on a

smooth, flat plate on the clutch release levers

Clutch Components

• Pilot bearings– The purpose of the

pilot bushing or bearing is to support the outer end of the transmission’s input shaft.

– Allows the input shaft to rotate in the crankshaft.

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Pilot Bearing

• Made of a solid bronze bushing, roller or ball bearing

• Prevents the transmission shaft and clutch disc from wobbling when the clutch is released

Clutch Linkage

• Clutch linkage transfers the clutch pedal movement to the release bearing.– Mechanical clutch

linkage uses shafts, levers, or a cable.

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Clutch Cable Mechanism

Clutch Cable Mechanism

• Uses a steel cable inside a flexible housing to transfer pedal movement to the clutch fork

• One end usually has a threaded sleeve for clutch adjustment

Clutch Linkage Mechanism

Clutch Linkage Mechanism

• Uses levers and rods to transfer motion from the clutch pedal to the clutch fork

• A bellcrank may be used to reverse the movement of the clutch pedal

• A release rod transfers motion to the fork and allows adjustment

Clutch Linkage

Hydraulic clutch linkage consists of a master cylinder, hydraulic tubing, and a

slave cylinder.

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Hydraulic Clutch Release Mechanism

• Uses a simple hydraulic circuit to transfer clutch pedal action to the clutch fork

• Components:– clutch cylinder– hydraulic line– slave cylinder

Clutch Cylinder

• Produces hydraulic pressure for the system

• Mounted to the firewall or cowl

• Push rod links the clutch pedal and the cylinder piston

• Most systems use brake fluid

Clutch Cylinder

Slave Cylinder

Uses hydraulic pressure to cause

clutch fork movement

Multiplate Clutch Assembly

Drum Piston Seals Return Springs

Unlined Plates

Snap Ring

Lined Discs

Pressure Plate

A typical clutch assembly contains the apply piston, piston return springs, and the clutch plates.

Common Clutch Problems

• Clutch slippage– Definition: With the clutch engaged, engine

speed increases but vehicle speed does not.– Before disassembly:

• Check linkage.• Check for worn or binding parts.• Check engine mounts.

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Common Clutch Problems

• Other causes for clutch slippage– Oil-soaked or worn disc facings– Warped flywheel or pressure plate– Weak pressure plate springs– Contact between the release bearing and the

fingers of the pressure plate

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Common Clutch Problems

• Clutch chatter– Definition: Shaking or shuddering when

clutch is engaged.– Before Disassembly:

• Check engine mounts.• Check for leaks from

rear main seal, transmission input shaft seal, and clutch slave cylinder.

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Common Clutch Problems

• Clutch noises– These noises are usually caused by bad

bearings or bushings.– To diagnose, determine whether the noise

changes with the clutch engaged or disengaged.

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Common Clutch Problems

• Causes for damaged release bearings– Misalignment– Overheating– Slippage– Component damage

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Common Clutch Problems

• Clutch vibrations– Are usually caused by worn engine mounts,

loose bolts, excessive flywheel runout, or flywheel and/or pressure plate assembly imbalance.

• Dragging clutch– Is usually caused by linkage problems,

incorrect pedal adjustments, or defective clutch assembly.

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Common Clutch Problems

• Pulsating clutch pedal– Is usually caused by broken or bent release

levers, misaligned bell housing, or warped pressure plate, flywheel, or clutch disc.

• Binding clutch– Is usually caused by binding linkage or

cables, defective clutch assembly, or improper installation.

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Flywheel service

• Visually inspect for hot spots, grooves, scoring and cracks

• Check with dial indicator for run out and crank end play

• Most flywheels can (should) be resurfaced– Must remove dowel pins

• Inspect ring gear

Clutch Service Checks

Measuring flywheel runout and Crankshaft endplay

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Clutch Service Checks

Measuring clutch housing face runout

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Hydraulic Clutch Problems

• Soft clutch pedal or excessive pedal travel– Is usually caused by low fluid level.– May be caused by a faulty master or slave

cylinder.

• Hard pedal– Is commonly caused by binding linkage or

swollen cups in the master or slave cylinder.

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Clutch disc service

• Inspect for wear similar to brake pads

• Common to have loose or broken springs

• Common failure due to oil contamination– Must find cause BEFORE repair

Clutch disc service• Clutch disc will normally have a flywheel

side

• Disc must be centered with clutch alignment tool before pressure plate bolted down

• Make sure new disc fits on input shaft of the transmission before you install it

Pressure plate service

• Mark relation to flywheel if reused• Remove bolts slowly in criss-cross pattern• Inspect fingers for parallelism, cracking or

uneven wear• Inspect friction surface for hot spots, cracks

etc and with straight edge for warpage• Must be torqued to specs and in sequence• Uses hard bolts

Release bearing service• Very common failure - isolated by lightly

applying clutch and listening

• Inspect for looseness or roughness

• Most are sealed but some were lubeable

• Clutch fork must be lubed at pivot point and bearing contact points

• Bearing retainer and input shaft should be lubed

• Clutch fork must be secured at the pivot point

Pilot servicing

• Should always be changed with clutch

• Can be isolated by applying clutch in gear

• Special pullers may be used for removal– Can be removed with grease

• Should be lightly lubed on installation