Fourwheel drive and allwheel drive 505 Driveline arrangements 506 Transmission with gear transfer 507 Transfer case with chain drive 510 Centre differentials 514 Transaxle centre differentials 514 Viscous couplings 514 Transfer unit with chain and viscous coupling 516 Suspension arrangements 519 Fourwheeldrive service 521 Allwheel drive 524 Types of allwheeldrive systems 524 Technical terms 530 Review questions 530
Trang 1Four-wheel drive and
all-wheel drive
Chapter 25
Drive-line arrangements
Transmission with gear transfer
Transfer case with chain drive
Trang 2Four-wheel-drive vehicles have drive lines to both
front and rear axles so that all four wheels of the
vehicle can be driven Some vehicles are designed for
optional off-road use, where four-wheel drive is
selected for rough, soft or sandy conditions Other
vehicles, mainly passenger cars, have all-wheel drive
which can be used full time for better on-road
performance.
Drive-line arrangements
In general, there are three different arrangements for
four-wheel drive for passenger and light commercial
vehicles:
1 Larger passenger and light commercial vehicles.
These have a normal drive line to the rear wheels
and an additional drive line to the front wheels.
2 Smaller passenger vehicles and those with transaxles These have a transaxle with a normal front-wheel-drive arrangement They also have a drive line to the rear wheels.
3 Vehicles with rear engines These have their normal rear-wheel drive and an additional drive line to the front wheels of the vehicle.
While some vehicles have four-wheel drive as an option, there are others which are made with only four- wheel drive.
Larger passenger and light commercial vehicles
Figure 25.1 shows the basic arrangement for a wheel-drive vehicle with its engine mounted longitudinally This has a four-speed or five-speed transmission of conventional design behind the engine, with a transfer case beside it (Figure 25.2) The transfer case is actually a smaller transmission with two speeds that provide a high and a low range It has output shafts at both front and rear The front and rear axles are both rigid axles.
four-Propeller shafts are connected to the output shafts
to carry the drive from the transfer case to the front and rear axles The axles then transfer the drive through the final-drive gears, the differential and the axle shafts to drive the wheels.
Figure 25.3 is another arrangement The main transmission has an output shaft for the rear propeller shaft, and the transfer case has an output shaft for the front propeller shaft.
figure 25.1 Basic arrangement of a larger
four-wheel-drive vehicle ROVER AUSTRALIA
figure 25.2 One arrangement of a transmission and transfer case for four-wheel drive ROVER AUSTRALIA
Trang 3chapter twenty-five four-wheel drive and all-wheel drive 509
Smaller passenger vehicles
and those with transaxles
The four-wheel-drive arrangement for a smaller
vehicle with a transverse engine is shown in Figure
25.4 This has a transaxle and a normal front-wheel
drive with open drive shafts As well as this, it has a
transfer case attached to the transaxle The transfer
case has an output shaft to the rear of the vehicle.
A two-piece propeller shaft with a centre bearing is
used to carry the drive from the output shaft of the
transfer case to the rear axle The rear axle has a final
drive and differential assembly mounted to the body of
the vehicle, and swing axles connect it to the
rear-wheel hubs.
Vehicle with rear engine
Figure 25.5 shows the arrangement of a vehicle with a
rear engine The transaxle is at the front of the engine
and open drive shafts are used to drive the rear wheels.
It is a normal type of manual transmission, except that
it has extra gearing and an output shaft at the front.
A propeller shaft connects the output shaft to the final
drive in the front axle.
The front final drive includes a viscous coupling,
final-drive gears and a differential Open drive shafts
carry the drive to the front wheels.
Transmission with gear transfer
A transmission and transfer case are shown in Figure 25.6 with the parts identified The transmission has four speeds that can be selected with the gear lever The transfer gearing has two speeds that are selected with a separate lever This provides a high range and a low range With the four speeds of the main trans- mission and the two speeds of the transfer, eight different forward gear ratios are available to the driver The main transmission is similar to a transmission for a two-wheel-drive vehicle, except that it has an extended mainshaft with some of the transfer gears.
Transfer gearing
The arrangement of the gearing is shown in Figure 25.7 This has to do three things:
1 provide drive for high range
2 provide a gear ratio for low range
3 transfer power to the front drive.
The transfer gearing is shown in neutral in the diagram To obtain drive, the sliding sleeves are moved along their splines to engage with the dog teeth
on one of the gears The sliding sleeves and the dog teeth on the adjacent gears form a dog clutch With the teeth engaged, the sleeve locks the gear to the shaft.
figure 25.3 Arrangement of the drive line of a four-wheel-drive vehicle TOYOTA
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Trang 4High range
For high range, the front sleeve is moved forwards to
engage with the dog teeth on the input gear This
connects the input gear with the rear output shaft, and
drive is carried straight through the transfer case.
There is no gear reduction.
Low range
For low range, the front sleeve is moved to the rear.
This engages the dog teeth on the rear drive gear and
locks it to the rear output shaft.
Drive from the input gear is now through the
countergears to the rear drive gear and the rear output
shaft A gear reduction is provided by the countergears.
Transfer
For front drive, the rear sliding sleeve is moved
forwards This engages the dog teeth of the transfer
drive gear and locks it to the rear output shaft Rotation
of the rear output shaft is now transferred by the
transfer drive gear, via the idler gear, to the front drive
gear and the front output shaft.
Front drive can be selected to give four-wheel drive whether the transfer is in high range or low range.
Transfer case with two output shafts
The transfer case in Figure 25.8 has a different arrangement It has two output shafts, one for the front propeller shaft and one for the rear propeller shaft.
Gearshift lever positions
In manually selected transmissions with four-wheel drive, there are two gear levers, one for the normal transmission and one for the transfer case (Figure 25.9) The main transmission can have four or five gears The transfer case has a high range and a low range for four-wheel drive (H4 and L4), a high range for two- wheel drive (H2), and also a neutral position (N) Neutral is used if auxiliary equipment is being driven from the transfer case.
The main gearshift lever is shown with common selector positions, and the transfer shift is shown with one arrangement However, there are variations to
figure 25.4 Arrangement of four-wheel drive for a vehicle with a transaxle and transverse engine FORD
Trang 5chapter twenty-five four-wheel drive and all-wheel drive 511
figure 25.5 Arrangement of a rear-engine vehicle with four-wheel drive VOLKSWAGEN
507-532_May 2chap 25 13/9/06 4:17 PM Page 511
Trang 6those shown The transfer positions will be different
for a vehicle with full-time four-wheel drive.
Transfer case with chain drive
Figure 25.10 shows a transfer assembly that has a
chain drive The gearing in the transfer case provides
a gear reduction of approximately 2.5:1 when low
range is selected The chain has no effect on the gear
ratio Its purpose is to transfer drive from a sprocket
on the rear output shaft to a sprocket on the front
output shaft.
The arrangement of the chain can be seen in Figure
25.11 The chain is a silent type, which has a number
of links across its width that form teeth in the chain.
The links are of normal tooth shape when the chain is
flat, but are shaped so that the teeth spread as the chain
passes around the sprockets The spread teeth fill the space between the sprocket teeth so that there is no clearance between the chain teeth and the sprocket teeth – this reduces chain noise.
■ Because of the design of the teeth, the chain is much quieter than other chains and that is why it
is called a silent chain.
Power flow with chain drive
The diagrams in Figure 25.12 show the power flow through the transfer gearing and chain There are three positions:
1 two-wheel drive, high range
2 four-wheel drive, high range
3 four-wheel drive, low range.
figure 25.6 Transmission and transfer case assembly TOYOTA
Trang 7chapter twenty-five four-wheel drive and all-wheel drive 513
figure 25.7 Arrangement of the gearing in a transfer case
H high range, L low range, F front drive
figure 25.8 Manual transmission with a transfer case TOYOTA
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Trang 8There are two selector forks connected to the fer shift lever These move a sleeve which engages or disengages the dog clutches to alter the power flow through the transfer gearing.
trans-Two-wheel drive, high range
When the transfer selector lever is moved to the 2H position, the front sleeve is moved forwards This connects the transmission main output shaft to the transfer rear output shaft In the transfer case, this is a straight-through drive.
Drive from the transfer case is transmitted through the propeller shaft to the rear wheels only There is no gear reduction in the transfer case and no power to the front output shaft.
Four-wheel drive, high range
When 4H is selected, the rear sleeve is moved forwards This connects the chain drive sprocket to the rear output shaft Drive is taken through the transfer chain to the sprocket on the front output shaft.
figure 25.9 Transmission and transfer selector positions
– there are also other arrangements TOYOTA
figure 25.10 Transfer case in which a chain is used to transfer the drive FORD
Trang 9chapter twenty-five four-wheel drive and all-wheel drive 515
The front propeller shaft carries the drive from the
front output shaft to the front axle, and the rear
propeller shaft carries the drive from the rear output
shaft to the rear axle.
Four-wheel drive, low range
When 4L is selected, the front sleeve is moved wards This introduces the transfer countergear into the gear train Instead of the drive going directly from the
rear-figure 25.11 Drive chain linkage DAIHATSU
figure 25.12 Power flow through a transfer case with a drive chain DAIHATSU
507-532_May 2chap 25 13/9/06 4:17 PM Page 515
Trang 10input shaft to the transfer rear output shaft, the
counter-gear provides a reduction.
The countergear is driven by a constant-mesh gear
on the input shaft, and it also drives a constant-mesh
gear on the transfer rear output shaft When the sleeve
is moved, it connects the constant mesh gear to the
output shaft Drive now passes through the countergear
so that the rear output shaft is driven at reduced speed.
The chain also drives the front output shaft at this
reduced speed.
Centre differentials
Some transmissions are provided with a third
differential This is located in the transmission Its
purpose is to equalise the drive between the front and
rear axles It acts in the same manner as a normal
differential.
With a differential in the drive line, four-wheel
drive can remain engaged, and the centre differential
will prevent transmission wind up This is a condition
that can occur if a vehicle without a centre differential
is driven in four-wheel drive over hard-surfaced roads
or through tight turns Tyre scuffing and transmission
damage could result.
When a vehicle without a centre differential is
operated off-road, the difference in wheel speeds is
taken care of by the loose or softer surfaces, and
wind-up does not become a problem.
■ A centre differential compensates for variations in
speeds between the front and rear wheels.
Centre-differential lock
A lock is provided on some centre differentials This
allows the differential to be locked when driving in
conditions where there would be wheel spin Without a
differential lock, one wheel in loose conditions could
spin and immobilise the vehicle With the differential
locked, the rear wheels continue to drive, even if one
front wheel has no traction.
Transaxle centre differentials
The arrangement of a lockable centre differential for a
transaxle is shown in Figure 25.13.
The transaxle centre differential and the front
differential are combined into a single compact unit
which allows full-time four-wheel drive The centre
differential is always in the drive line, but can be
locked when necessary for bad driving conditions.
The front differential is the normal transaxle ential with side gears and pinions The side gears drive the front-wheel drive shafts in the usual way.
differ-The centre differential is of a different design It has a simple planetary gear system which consists of internal teeth inside the final-drive ring gear, a planet carrier with pinions, and a sun gear The internal teeth are part of the final-drive ring gear, and the sun gear is connected to an intermediate gear With this arrange- ment, the final-drive ring gear is connected to the intermediate gear by the planetary gearing.
If the action of the planetary gearing of the centre differential is ignored for the time being, the drive through the unit is:
1 from the output pinion of the transaxle to the drive ring gear
final-2 then through the planetary gearing to the mediate gear
inter-3 from the intermediate gear to the idler gear
4 then through the crown wheel and pinion to the pinion’s shaft.
The pinion shaft is connected by the propeller shaft to the rear axle, which also has a differential, and so the drive goes to the rear wheels.
The side gears of the front differential drive the front-wheel drive shafts (not shown) in the usual way for a transaxle.
Planetary action
The planetary gearing is located between internal teeth of the ring gear and a sun gear attached to the intermediate gear With equal load on the front and rear wheels of the vehicle, there is no planetary action The gearing merely transfers drive from the final-drive ring gear to the intermediate gear without any change in speed.
Whenever there is a difference in speed between the front and rear wheels, planetary action occurs This allows the ring gear and the intermediate gear to rotate
at different speeds.
While the gears are arranged differently, the action
is similar to that of the gears in a normal differential – the planetary gearing carries drive between the final- drive ring gear and the intermediate gear while still allowing movement between the two gears.
Viscous couplings
A viscous coupling can be used with a centre differential, with a rear differential, or with a front
Trang 11chapter twenty-five four-wheel drive and all-wheel drive 517
differential Its purpose is to limit the difference in
speed between the front and rear drives It acts like a
limited-slip clutch for the centre differential It allows
a small difference in speed between the front and rear
drives, but acts automatically to prevent any great
difference in speeds.
Figure 25.14 shows the construction of a viscous
coupling It consists of a number of plates, with one set
of plates splined to the coupling casing and the other
set splined to its hub Spacer rings locate the plates in
the casing The coupling contains silicone oil and is
sealed with X-shaped seals These retain the oil under
operating pressure.
The coupling operates whenever there is a
differ-ence in rotational speed between the hub and the
casing Slots in the plates cause shearing of the silicone
oil, and this creates a fluid-coupling effect.
■ The plates of a silicon coupling do not come into
contact with each other; they are separated by the
silicon oil.
Centre viscous coupling
Figure 25.15 shows a viscous coupling fitted to the centre-differential assembly of a transaxle This is the same type of transaxle as in Figure 25.13, but with a viscous coupling added.
Transaxle with crown wheel and pinion and viscous coupling
The transaxle in Figure 25.16 is used with a tudinally mounted engine It has a crown wheel and pinion, a centre differential and a viscous coupling The front drive shafts are connected to the differ- ential assembly at the front of the transmission in the normal way The propeller shaft to the differential
longi-at the rear of the vehicle is connected to splines on the output shaft that extends from the rear of the transaxle.
The viscous coupling and the differential are,
in effect, an extension of the countershaft of the transmission.
figure 25.13 Transaxle centre differential combined with the front differential – the centre differential is lockable FORD 507-532_May 2chap 25 13/9/06 4:17 PM Page 517
Trang 12Rear differential with viscous coupling
Figure 25.17 illustrates a rear differential with a
viscous coupling This is a conventional design of
differential assembly and final drive, but it has a
viscous coupling between the two output shafts.
Viscous couplings can be located anywhere in the
drive line between the front and rear drives They are
generally fitted wherever it is most convenient for the
particular design of transmission and transfer unit.
Front final drive with viscous coupling
The front-axle final-drive unit in Figure 25.18 has a
viscous coupling This automatically distributes the
driving forces to the front and rear wheels It is
mounted on the pinion shaft so that the pinion splines
engage with splines in the hub of the coupling The
coupling casing is connected to the drive flange by
the coupling shaft.
Power is transmitted from the flange through the
coupling to the pinion, then to the crown wheel and
Trang 13chapter twenty-five four-wheel drive and all-wheel drive 519
figure 25.15 Centre differential with a viscous coupling and a differential lock FORD
figure 25.16 Full-time four-wheel-drive transaxle has a centre differential and viscous coupling – it also has a crown
wheel and pinion for the front-wheel drive
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