Book Clutch

Transmission

Clutch

Preface

This training module introduces the concept ofclutch. The training module is part of a seriesof modules designed for the TransmissionBasic Training Program.

This module should be studied right after themodule: General Information.

While studying this module, you will have theopportunity to learn the function of a clutch, itsmain components as well as how thesecomponents work individually and within theclutch assembly.

ContentsClutch - Introduction 3For each type of vehicle, there is an ideal clutch,which is calculated in terms of: 4Clutch categories 5Hydraulic clutch 6Hydraulic clutch - principal of operation 7Mechanical clutch 8Mechanical clutch - main components 9Clutch disc 12Rigid disc 14Disc facing 15Clutch bell housing 17Clutch bearing 18Clutch operating system 19Clutch operating system - components 20Clutch servo 21Clutch operation 22Identification and classification of clutchpackages 31Some adjustments can be made in clutches: 32

Global TrainingTP95850 3

Clutch - Introduction

The clutch is one of the transmission components that enables the engagement anddisengagement of revolution power from the engine to the gearbox.

The clutch (1) is mounted between the engine and the gearbox. Some of its functions are:

- Interrupt the transmission of power from the engine to the gearbox during gearshifts.- Transfer torque from the engine to the gearbox and to the other transmission components.

Clutch action allows a smooth and progressive torque transmission from the engine to thegearbox.


For each type of vehicle, there is an ideal clutch, which is calculated in terms of:

- torque;- vehicle maximum weight (loaded);- gearshifts ratio;- type of application;- tire dynamic radius;- differential ratio.

These factors will determine the pressure plate diameter, its weight, as well as the type of disc tobe used.

The clutch should:

- transfer torque from the engine to the gearbox without slipping;- be resistant to high speeds and premature wear;- eliminate vibrations during the start;- enable a smooth and quick gearshifting- absorb engine vibrations.


Clutch categories

The many types of clutches may be divided into two main categories:

Hydraulic clutch (1)The torque is transferred from the engine to the gearbox with the help of a hydraulic fluid thatworks between the two main components of the hydraulic clutch: the pump rotor and the turbinerotor.

Mechanical clutch (2)Mechanical clutches are composed of one or more discs. Most of them are equipped with adiaphragm spring and they are hydraulically actuated by means of a hydropneumatic clutchservo. A pressure plate pushes the disc against the flywheel and transfers the torque from theengine to the gearbox.


Hydraulic clutch

A hydraulic clutch is basically composed of a pump rotor and a turbine rotor.

The rotors are made of propeller blades facing each other.

The pump rotor is installed in the engine output shaft and the turbine rotor is connected to thegearbox input shaft.


Hydraulic clutch - principal of operation

The engine starts to run and causes the pump rotor (1) to rotate. The hydraulic fluid betweenthe rotors is forced against the turbine rotor (2).

Pressure from the fluid causes the turbine rotor to rotate.

The faster the engine speed, the greater the fluid pressure in the turbine rotor.

The greater the pressure, the faster the turbine rotor rotates.

Through the turbine rotor rotation, the torque is transferred from the engine to the gearbox andthen the rest of the transmission system that makes the vehicle move.


Mechanical clutch

Mechanical clutches contain a diaphragm spring and are hydraulically controlled by a servo-aided cylinder.

This type of clutch is basically composed of:

1. Housing;2. pressure plate;3. disc;4. diaphragm spring;5. clutch bearing.


Mechanical clutch - main components

Compression plate

The compression plate is a mechanical component made up of:

1. spacing rivet2. leaf spring3. compression plate4. leaf spring attachment5. ring6. cover

The compression plate (3) is a strong steel ring that makes pressure against a disc by meansof helical springs or diaphragm springs.


There are two types of compression plates:

1. Helical spring loaded compression plate

This type of plate is not currently used in clutches mostly because it is not resistant to the highspeeds of late engines. Also, its pressure on the disc is dramatically reduced with facingwearing, thus increasing the clutch pedal effort.

2. Leaf spring loaded compression plate

Resistance to high engine speeds, together with a compact design that allows a smaller housing(essential for saving space), makes the leaf spring loaded plate a standard in practically allvehicles manufactured nowadays. Moreover, the leaf spring design makes it possible to workwith lower and nearly constant initial loads during the plate life thereby reducing the clutch pedaleffort.


There are two types of leaf springs: Push-Type and Pull-Type.

Volvo vehicles use the Pull-Type pressure plate.

The Pull-Type has many advantages over the Push-Type.The main difference between these two clutches is the support bearing displacement.

When the clutch pedal is applied, the support bearing is displaced towards the steering gearcasing and the gearbox.The ends of the diaphragm spring and the pressure plate move away from the disc disengagingthe engine and the clutch.The performance of the Pull-Type is superior due to a greater capacity to transfer a highertorque.Moreover, due to its simpler design, the Pull-Type clutch is faster to install and more easilymaintained.Its new design also reduces the weight to the clutch assembly, which means less weight to thevehicle.It also results in a reduced stress and more uniform plate pressure, even after long use, whichmeans extended system life.


Clutch disc

The clutch disc (1) is the main element contacting the flywheel engine (2) and the plate (3).When engaged, the clutch disc transfers torque from the engine to the gearbox and from theother transmission components to the wheels, making it possible for the vehicle to run.


The disc is made of steel. Clutch disc (1), damping springs (2), a grooved hub (3), and afacing (4).

The dry clutch disc is fiber-faced on both surfaces.Facings are built in non-asbestos materials resistant to the high temperatures produced whenthe disc is in operation. They are fastened to the steel disc with rivets.

The steel disc is connected to the hub by means of a friction element with damping springs.These springs absorb the engine peaks and vibrations.The gearbox input shaft is inserted into the splined hole of the disc hub.When the hub rotates, the input shaft also turns and transfers the torque to the gearbox.To keep the load on the clutch disc facings as uniform as possible, the disc is equipped with acorrugated plate. This design provides a smooth clutch action and reduces the risk ofoverheating.


There are two types of clutch discs: a rigid disc and a torsion-damping disc

Rigid disc

In its simplest design, a rigid disc is made of a grooved hub (1) that slides over the clutch pilotshaft, a drag disc (2), two facings (3) responsible for the friction between the flywheel engineand the pressure plate, and attachment rivets (4).In a more modern design, a rigid disc may be provided with segment springs between thefacings. The segment springs absorb axial motions of the disc and permit slight sliding, resultingin smoother engagement.


Disc facing

Facings are components that provide friction between the plate and the flywheel engine.Mostly manufactured with organic-based materials (with or without asbestos), the facings shouldpresent the following basic qualities:

- a high friction coefficient;- a constant friction coefficient as the temperature increases;- resistance to premature wear;- resistance to high temperatures;- resistance to high speeds;- do not cause vibrations.

In late model vehicles, facings practically without asbestos have been used, due to their highresistance to wear, when compared with the traditional asbestos facings.

Inorganic-based facings (sintered, ceramic) are restricted to vehicles where a smooth start isnot very important (tractors, special trucks, racing trucks). The main qualities of these facingsare the high resistance to wear, high friction coefficient and resistance to high temperatures.

ATTENTION: The use of materials with asbestos is currently prohibited in many countries dueto the risk of cancer.


Clutches may have one or two discs.- Single disc clutch (1)- Double disc clutch (2)

Double disc clutches operate the same way as single clutches except for the presence ofanother clutch disc and an intermediate pressure plate.The intermediate pressure plate is located between the two clutch discs. It is installed in anintermediate ring bolted between the flywheel engine and the clutch body.

A double disc clutch has a larger friction area and can transfer a greater torque than the singledisc clutch.For this reason, this type of clutch is used in heavy-duty vehicles requiring a larger torque.

IMPORTANTWhile replacing the clutch disc, lubricate the grooves in the gearbox main shaft with a thin layerof heat-resistant grease before installing the new disc.


Clutch bell housing (1)

The function of the clutch bell housing is protecting the clutch from contamination by water anddirt. It forms the front of the gearbox, connecting it to the engine casing.Lever and disengaging shaft (2)

The lever (3) and the disengaging shaft (2) are part of the clutch control. They are responsiblefor the transmission of movement from the clutch driving system to the clutch bearing.


Clutch bearing

The function of the bearing (1) is to transfer the movement from the disengaging shaft to thediaphragm spring.

The inner part of the bearing is attached to the revolution direction by the disengaging shaft andthe intermediate part is attached to the diaphragm spring and rotates together with the clutchbody and the engine.

The clutch bearing is provided with a ring-like spring (2) whose function is to fix the bearingproper to the disengaging shaft when the gearbox is installed.


Clutch operating system

The clutch is driven by a servo-aided system called Hydropneumatic driving system.This system consists of a hydraulic master cylinder connected to the clutch pedal and a servohydraulic cylinder.

The driving system components are as follows:1. fluid reservoir2. master cylinder3. hydraulic pipe4. pneumatic pipe5. clutch servo6. register tube7. clutch fork

The clutch servo increases the hydraulic force generated in the master cylinder, therebynoticeably reducing the clutch pedal effort.


Clutch operating system - components

Master cylinder

The hydraulic master cylinder is installed next to the clutch pedal and is responsible for the feedpressure to the clutch servo.The master cylinder is also responsible for the generation of the hydraulic power transferred tothe clutch servo through the pneumatic pipe.The master cylinder requires two basic adjustments for the clutch pedal to operate properly.

1. The top adjusting bolt (1) adjusts the play between the master cylinders piston rod and thepiston. This play assures that the piston seal returns to a position just above the hole connectingto the fluid reservoir. This _eliminates any residual pressure in the hydraulic piping.

2. The bottom adjusting bolt (2) adjusts the clutch pedal stroke and, consequently, the clutchservo piston stroke.


Clutch servo

The clutch servo is located beside the gearbox.The clutch servo function is to convert the master cylinders pressure into a movement.

This action takes place because the hydraulic pressure generated by the master cylinder movesthe reaction piston upward in the clutch servo.

The clutch servo also contains a wear indicator valve (1).


Clutch operation

When the clutch is not applied, that is, the clutch pedal is released (1), the plate presses theclutch disc (2) against the flywheel engine.

The clutch is connected to the flywheel (3) and transfers engine movement to the gearboxmain shaft (4).

All the clutch assembly turns in the same engine speed and transfers the torque from the engineto the power transfer system.

The support bearing is forced backwards by the spring in the disengaging fork (5).


Picture showing the whole system.

(1) The pilot valve closes the compressed air passage inside the cylinder and allows the air toenter through the exhaust hole.

(2) The air exhaust hole is open.The pressure in the system is equal to the atmospheric pressure.


The clutch pedal is not applied. The engine is running or not but the vehicle is at rest and thegearbox is in neutral position. Or, else, the vehicle may be moving with one the gears engaged.


Now, lets see what happens when the clutch pedal is applied.

The master cylinder piston displacement causes an increase in the system hydraulic pressure.

Due to this increase in the pressure, the reaction piston in the clutch servo starts upward:

(1) the reaction piston is pushed forward, ....

(2) the compressed air passage opens, and ...

(3) the exhaust hole closes and prevents the air from escaping.


Due to the compressed air pressure in the pneumatic chamber (1), the main piston (2) startsoutward.

When the piston (2) moves outwards, the clutch starts to disengage.

The flywheel (4) remains rotating if the engine is running.

The leaf spring (5) and the plate (6) are pulled backwards.

The clutch disc (7) is no longer in contact with the flywheel engine (4) and the plate (6).


When the main piston reaches the end of its upward stroke, the hydraulic fluid occupies itsspace and reduces the hydraulic pressure (1).

Remember: the piston is displaced by the compressed air pressure and not by the hydraulicpressure.

Now the clutch is fully disengaged (2).


A disengaged clutch makes it possible to start the engine. The engine movement, however, isnot transferred to the gearbox main shaft because the clutch disc is free, without pressure.

This way, the engine does not transfer torque to the power transfer system.

The driver is then free to shift gears.


When the clutch pedal is released, the hydraulic pressure reduces to almost atmosphericpressure. The pressure spring (4) starts the reaction piston (1) to move downward and backto its original position and allows the pilot valve (2) to close the compressed air passage and,at the same time, to open the passage for the air exhaust hole (3).

Thus, air comes out of the pneumatic chamber (5) and the main piston (6) starts downwardby the spring action (7) on the clutch fork.


Now the clutch is engaged with the flywheel engine again.

The plate presses the disc against the flywheel engine.

The engine transfers torque to the transmission.


Identification and classification of clutch packages

The type of clutch used in a vehicle is identified on the identification plate attached to the clutchbell housing.

Clutches are arranged as follows:

VersionDisc external diameter (in inches)Number of discsPull-type clutch / Push-type clutchManufacturer

Clutches are sorted out according to the support bearing driving direction: pull-type or push-type.


Some adjustments can be made in clutches:

- (1) pedal free valve (clutch pedal)- (2) driving stroke (clutch servo)

Some inspections should be made regularly:

- in the clutch hydraulic system- in the clutch pneumatic system- in the clutch disc(s)- in the flywheel- in the pressure plate

Follow the instructions in the Service Manual on Clutches, according to model of vehicle andtype of clutch.

Documents

Book Clutch