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