Clutch type clutch replaced an earlier type with coil

10.1.1 Clutch type clutch replaced an earlier type with coil

springs as it has a number of advantages when

A clutch is a device for disconnecting and con-

used on light vehicles:

necting rotating shafts. In a vehicle with a man- ● not affected by high speeds (coil springs can ual gearbox, the driver pushes down the clutch

be thrown outwards);

when changing gear to disconnect the engine ● low pedal force making for easy operation; from the gearbox. It also allows a temporary neu-

● light and compact;

tral position for, say, waiting at traffic lights and ● clamping force increases or at least remains

a gradual way of taking up drive from rest. constant as the friction lining wears. The clutch is made of two main parts, a pres-

sure plate and a driven plate. The driven plate, The method of controlling the clutch is quite often termed the clutch disc, is fitted on the shaft,

simple. The mechanism consists of either a cable which takes the drive into the gearbox. When the

or hydraulic system.

clutch is engaged, the pressure plate presses the driven plate against the engine flywheel. This allows drive to be passed to the gearbox. Pushing

10.1.2 Manual gearbox

down the clutch springs the pressure plate away, The driver changes the gears of a manual gear- which frees the driven plate. Figure 10.1 shows

box by moving a hand-operated lever called a

Clutch cover assembly and driven discs

Components for automatic transmissions

V 4470 clutch facing for limited slip differential

Clutch cover assembly

Compact clutch module

Electro-mechanical actuator for compact automatic clutch

Hydraulic clutch cylinders

Clutch release bearings

Figure 10.1 Clutch components (Source: Valeo)

248 Advanced automotive fault diagnosis

Final drive gear

Interlock

Figure 10.2 Five-speed gearbox (FWD)

Layshaft

(Source: Ford)

gear stick! All manual gearboxes have a neutral loaded to sit centrally on the synchromesh hub. position; three, four, or five forward gears; and a

When the outer ring (synchroniser sleeve) is made reverse gear. A few even have six forward gears

to move by the action of the selector mechanism now! The driver puts the gearbox into neutral as

the cone clutch is also moved because of the lock- the engine is being started, or when a car is

ing keys. The gear speeds up as the cones touch, parked with the engine left running.

thus allowing the dog clutches to engage smoothly. Power travels in to the gearbox via the input

A baulking ring is fitted between the cone on the shaft. A gear at the end of this shaft drives a gear on

gear wheel and the synchroniser hub. This is to pre- another shaft called the countershaft or layshaft. A

vent engagement until the speeds are synchronised. number of gears of various sizes are mounted on

A detent mechanism is used to hold the selected the layshaft. These gears drive other gears on a

gear in mesh. In most cases this is just a simple third motion shaft also known as the output shaft.

ball and spring acting on the selector shaft(s). The gearbox produces various gear ratios by

Figure 10.4 shows a rear wheel drive gearbox with engaging different combinations of gears. For

the detent mechanism marked. Gear selection reverse, an extra gear called an idler operates

interlocks are a vital part of a gearbox. These are to between the countershaft and the output shaft. It

prevent more than one gear from being engaged at turns the output shaft in the opposite direction to

any one time. On the single rail (one rod to change the input shaft. Figure 10.2 shows a front wheel

the gears) gearbox shown in the figure, the inter- drive gearbox.

lock mechanism is shown at the rear. As the rod is Figure 10.3 shows the power flows through

turned (side to side movement of the gear stick) this box in each of the different gears. Note how

towards first–second, third–fourth or fifth gear in each case (with the exception of reverse) the

positions, the interlock will only engage with gears do not move. This is why this type of gear-

either the first–second, third–fourth or fifth gear box has become known as constant mesh. In other

selectors as appropriate. Equally when any selec- words the gears are running in mesh with each

tor clutch is in mesh the interlock will not allow other at all times. Dog clutches are used to select

the remaining selectors to change position. which gears will be locked to the output shaft. These clutches which are moved by selector levers, incorporate synchromesh mechanisms.

10.1.3 Driveshafts and wheel

A synchromesh mechanism is needed because

bearings

the teeth of the dog clutches would clash if they met at different speeds. The system works like a

Light vehicle driveshafts now fall into one of two friction type cone clutch. The collar is in two parts

main categories, the first being by far the most and contains an outer toothed ring that is spring-

popular.

Transmission systems 249

Figure 10.3 Gearbox power flows (Source: Ford)

● Driveshafts with constant velocity joints

Front bearings

(FWD) transmit drive from the output of the final drive to each front wheel. They

Seal

Keeps out dirt and water, and keeps in the

must also allow for suspension and steering

grease lubrication

movements.

Spacer

Ensures the correct positioning of the seal

● Propshaft with universal joints (RWD)

transmits drive from the gearbox output to the Supports the weight of the vehicle at the

Inner bearing

front, when still or moving.

final drive in the rear axle. Drive then contin-

Ball bearings are used for most vehicles

ues through the final drive and differential,

with specially shaped tracks for the

via two half shafts to each rear wheel. The

balls. This is so the bearings can stand

propshaft must also allow for suspension

side loads when cornering

movements.

Swivel hub

Attachment for the suspension and steering as well as supporting the bearings

Wheel bearings are also very important. They

Outer bearing

As for inner bearing

allow smooth rotation of the wheel but must also

be able to withstand high stresses such as from

Drive flange

Runs inside the centre race of the bearings.

load in the vehicle and when cornering. The wheel is bolted to this flange

250 Advanced automotive fault diagnosis

Interlock

Synchro hub

Detent

Propshaft flange

Speedo drive

Gear selector fork

Helical gears

Input on first motion shaft

Baulking ring

Gearbox casing Layshaft

Figure 10.4 Rear wheel drive gearbox

Rear bearings

about 4:1; in other words when the gearbox out- put is turning at 4000 rev/min the wheels will turn

Stub axle Solid mounted to the suspension arm, this

at 1000 rev/min.

stub axle fits in the centre of the two

Many cars now have a transverse engine, which

bearings

drives the front wheels. The power of the engine

Seal Keeps out dirt and water, and keeps in the

therefore does not have to be carried through a right

grease lubrication

angle to the drive wheels. The final drive contains

Inner bearing Supports the weight of the vehicle at the

ordinary reducing gears rather than bevel gears.

rear, when still or moving. Ball bearings are used for most vehicles with specially shaped

The differential is a set of gears that divide the

tracks for the balls. This is so the bearings

torque evenly between the two drive wheels. The

can stand side loads when cornering

differential also allows one wheel to rotate faster

Spacer To ensure the correct spacing and pressure

than the other does when necessary. When a car

between the two bearings

goes around a corner, the outside drive wheel

Drum For the brakes and attachment of the wheel

travels further than the inside one. The outside

Outer bearing As for inner bearing

wheel must therefore rotate faster than the inside

Washer The heavy washer acts as a face for the nut

one to cover the greater distance in the same time.

to screw against

Some higher performance vehicles use limited

Castle nut and Holds all parts in position securely. With

slip differentials. The clutch plates are connected

split pin this type of bearing, no adjustment is made

to the two output shafts and hence if controlled

because both bearings are clamped on to the

will in turn control the amount of slip. This can be

spacer. Some older cars use tapered

used to counteract the effect of one wheel losing

bearings and adjustment is very important

traction when high power is applied.

Grease Retains grease, but should not be over retainer cap

Differential locks are used on many off-road type vehicles. A simple dog clutch or similar device prevents the differential action. This allows far better traction on slippery surfaces.

packed. Also keeps out the dirt and water