FINAL DRIVE
INTRODUCTION
The power developed by the engine is transferred to the
wheels through clutch, gear box, universal joints, propeller shaft, final
drive, differential and rear axles. Description of universal joints, propeller
shaft, final drive, differential and rear axles has been given in this unit.
UNIVERSAL JOINTS
Universal joint is used to connect two shafts at an angle
for transmitting torque. In the transmission shaft of an automobile, two
universal joints are used – one between main transmission shaft and propeller
shaft and another between another end of propeller shaft and the differential.
Therefore, the universal joints make the joints flexible so that power can be
transmitted at an angle.
A universal joint takes care of rising and falling motion of
the rear end of the propeller shaft which is connected to differential. Two
universal joints are shown in Figure 5.1 along with the propeller shaft.
TYPES OF UNIVERSAL JOINTS
(a) Cross or spider joint (variable velocity joint).
(b) Ball and trunnion joint (variable velocity joint).
(c) Constant velocity joints.
PROPELLER SHAFT
The propeller shaft is a shaft that transmits power from
transmission to the differential. Propeller shaft transmits the rotary motion
of main transmission shaft to the differential so that rear wheels can be
rotated. A sliding joint, is also fitted between universal joint and propeller
shaft on transmission side which takes care of axial motion of propeller shaft.
Propeller shaft is made of a steel tube which can withstand torsional stresses
and vibrations at high speeds.
FINAL DRIVE OR FINAL REDUCTION
The final drive is composed of a bevel gear and crown wheel.
The level pinion is connected to propeller shaft. The pinion is in mesh with
the crown wheel. Crown wheel is part of differential. Final drive provides
fixed speed reduction. Because the crown wheel has more number of teeth and it
is connected to rear axles and level pinion has less number of teeth
SLIP JOINT
The rear axle housing with wheel and differential is
attached to the frame of automobile through springs. As the vehicle moves over
uneven surface, this whose assembly moves up and down due to expansion and
compression of springs. This changes the length of propeller shaft because it
is connected to differential and gear box. Slip joint (Figure 5.3) allows for
the change in length of propeller shaft. When spring is compressed propeller
shaft shortens and when spring is expanded, propeller shaft returns to original
length.
DIFFERENTIAL
When a four-wheeler takes a turn, the outer wheel turns
faster than inner wheel. Thus, there is relative movement between inner and
outer wheel. The function of the differential is to permit the relative
movement between inner and outer wheels when vehicle negotiates (takes) a turn.
The torque transmitted to each rear wheel is equal in this case, although their
speed is different.
The differential is made up of a system of gears which
connect the propeller shaft and rear axles. It is a part of inner axle housing
assembly. The assembly consists of differential, rear axles, wheels and
bearings.
Construction and Working
It consists of sun gears, planet pinion, a cage, a crown
wheel and a bevel pinion. A sun gear is attached to inner end of each rear
axle. A cage is attached on left axle. A crown gear is attached to the cage and
the cage rotates with the crown gear. The crown gear is rotated by the bevel
pinion. Crown gear and cage remain free on the left rear axle. Two planet
pinions are on a shaft which is supported by the cage. The planet pinions mesh
with the sun gears. The rear wheels are attached to outer ends of two rear
axles. When the cage rotates, sun gears rotate. Thus, the wheels also rotate.
In case one inner wheel runs slower than other when the vehicle takes a turn, the
planet gears spin on their shaft, transmit more rotary motion to outer wheel.
When vehicle runs in straight line, the crown gear, cage, planet pinions and
sun gears turn together as a unit. Thus, there is no relative motion.
TYPES OF DIFFERENTIALS
There are three types of differentials:
(a) Conventional type,
(b) Non-slip or self-locking type, and
(c) Double reduction type.
Conventional Type
Conventional type differential described in Section 5.6
delivers same torque to each rear wheel. If any of the wheels slips due to any
reason the wheel does not rotate and vehicle does not move.
Non-slip or Self-Locking Type
Non-slip or self-locking type differential overcomes this
drawback. It construction is similar to that of conventional type differential.
But, two sets of clutch plates are provided additionally. Also, the ends of
planet shafts are left loose in notches provided on the differential cage.
Double Reduction Type
Double reduction type differential provides further speed
reduction by additional gear. This type of differential is used in heavy duty
automobiles which require larger gear reduction between engine and wheels.
REAR AXLES
Rear axle transmits power from differential to the wheels so
that vehicle may move. Rear axle is not a single piece but it is in two parts
which are connected by the differential. This is shown in Figure 5.5. Each part
of rear axle is called the half shaft. Outer end of the rear axle carries the
wheel while inner end is connected to sun gear of the differential. In vehicles
which employ rear wheel drive, rear wheels are driving wheels. However, in
front wheel drive vehicles, front wheels are driving wheels. Rear axles and
differential are completely enclosed in a housing to protect them from dust,
dirt, water and any possible damage.
Functions of Rear Axle
(a) To transmit power from differential to the wheels. This
is main function.
(b) To carry weight of automobile.
TYPES OF REAR AXLES
Rear axles differ on the basis of method of supporting them
and mounting of rear wheels. On this basis, these axles can be classified into
three types:
(a) Half floating axle shown
(b) Three-quarter floating axle
(c) Fully floating rear axle shown
Half Floating Axle
In a half floating rear axle, the axle is at the centre of
the axle casing and the bearings are inside the axle casing. The weight of
vehicle is transmitted first to suspension spring, then to axle casing, then to
axle and finally to ground.
Three-quarter Floating Axle
In three-quarter floating rear axle, bearings are on the
outer side of axle casing, i.e. between casing and wheel. In this case, major
part of vehicle weight is taken by axle casing and not by axle. This is the
main advantage of three-quarter floating type over half floating type. Thus,
axle breakdown is less in this case compared to the previous type.
Fully Floating Rear Axle
In fully floating rear axle, the bearings are provided
between axle casing and the wheel. In this case, all the vehicle weight is
transmitted to ground through axle case and wheel. The axle is not supported by
bearings but it is supported at both ends. This type of axle is very strong and
therefore, it is used for heavy duty vehicles. In the event of breakdown of
axle, wheel cannot come out. This, it is safer but costly.