UNIT-III-TRANSMISSION SYSTEMS REAR AXLES

INTRODUCTION
• Engine could be engaged and disengaged with
the transmission system smoothly without
shock so that the vehicle mechanism is not
damaged and passengers do not feel
inconvenience. .
• A clutch is employed in automobiles for this
purpose

• The engines employed in automobiles are of
very high speed. Hence, a speed reduction is
necessary to reduce the speed to moderate
level as well as to get the required high torque
while moving from rest.
• For this purpose, a gearbox is employed in
automobiles.

• The motion of the crankshaft is transmitted to
the gearbox through the clutch.
• The gearbox consists of a set of gears to change
the speed according to the requirement.
• The motion is then transmitted to the propeller
shaft from gearbox through a universal joint.
• The purpose of universal joint is to connect two
shafts at an angle for power transmission.
• The power is transmitted to the differential unit
through another universal joint.
• Finally, power is transmitted from differential to
wheels through the rear end.
• The differential unit is used to provide the
relative motion between two run wheels while
the vehicle is taking a turn.

General arrangement of power
transmission

Functions or Purposes of Transmission
Systems
1. It enables the running engine to be connected or
disconnected from the driving wheel smoothly and
without shock.
2. It enables the leverage between engine and driving
wheel.
3. It enables the reduction of engine speeds.
4. It enables the turn of the drive round through 90°.
5. It enables the driving wheel to be driven at different
speeds.
6. It serves as a safety device by slipping when the torque
transmitted through exceeds a safe value, thus preventing
the breakage of parts in the transmission train.

CLUTCH
• Clutch is a mechanism used to connect or disconnect the engine
from the rest of transmission elements.
• It is located between engine and gearbox. During normal running
and stationary position, it is always in engaged condition.
• The clutch is disengaged when the driver processes the clutch
pedal.
• The clutch is disengaged for starting, changing gears stopping and
idling.
• When the clutch is engaged, the engine will be connected to the
transmission and Blows Bom engine to rem wheels through a
transmission system
• When the clutch disengaged by pressing the clutch pedal, the
engine will be disengaged from the transmission
• Thus, power does not flow to rear wheels while the engine is still
running

Functions of Clutch
(a) To permit the engagement or
disengagement of a gear when the vehicle is
stationery and engine is running.
(b) To transmit the engine power to rear wheels
smoothly without shocks to the transmission
system when the vehicle is in motion.
(c) To permit the engaging of the gears when the
vehicle is in motion without damaging gear
wheels.

Characteristics or Requirements of a
Clutch must have the following
requirements
• (a) It should be capable of transmitting maximum
torque of the engine.
• (b) The clutch should be able to engage gradually
and positively without the occurrence of sudden
jerks.
• (c) The clutch should ensure the dissipation of
heat sufficiently which is generated during
operation.
• (d) The clutch should be dynamically balanced to
the vibration in transmission system.

• (e) The size of the clutch should be as small as
possible so that it will occupy minimum space.
• (f) In order to reduce the effective clamping load
on the car thrust bearing as well as wear on it, a
provision should be made for clutch free pedal
play.
• (g) A suitable mechanism should be incorporated
within the clutch for damping of vibration and
elimination of noise produced during the
transmission.
• (h) The clutch must have non-tiresome operation
of disengagement for the driver for higher power
transmission

TYPES OF CLUTCHES
1. Friction clutch
(i) Single plate clutch
(ii) Multi-plate clutch
➢ Wet clutch
➢ Dry clutch
(iii) Cone clutch
➢ Internal cone clutch
➢ External cone clutch

2. Centrifugal clutch
3. Semi centrifugal clutch
4. Coil pressure spring clutch
5. Conical spring clutch or diaphragm clutch
➢ Tapered spring clutch
➢ Crown spring clutch
6. Positive clutch-dog and spline clutch
7. Hydraulic clutch
8. Vacuum clutch
9. Electromagnetic clutch
10. Over running clutch.

Single Plate Clutch
It is most commonly used in motor vehicle
such as cars, trucks and tractors.
The construction and different components of
this type of clutch is shown in Figure .
The single plate clutch consists of a flywheel,
clutch plate, friction or pressure plate, clutch
cover. release lever, bearing and clutch shaft.
 The various parts of the clutch are explained
below.

Fly wheel
The flywheel is the mounting surface for the
clutch.
The flywheel will rotate as the engine crankshaft
rotates.
The friction or pressure plate is bolted to the
flywheel face.
The clutch disc is clamped and held against the
flywheel by the spring action of the friction or
pressure plate.
The face of the flywheel is precision machined to
a smooth surface.
The face of the flywheel that touches the clutch
disc is made of iron.

Pilot Bearing
• The pilot bearing or bushing is pressed into the
end of the crankshaft to support the end of the
transmission input shaft.
• The pilot bearing is a solid bronze bushing but it
also may be a roller or ball bearing.
• The end of the transmission input shaft has a
small journal machined on its end.
• This journal slides inside the pilot bearing.
• The pilot bearing prevents the transmission shaft
and clutch disc from wobbling up and down when
the clutch is released.
• It also assists the input shaft center the disc on the
flywheel.

Clutch plate or disc plate
It is one of the important driving members of
a single plate clutch.
A clutch disc is the plate between flywheel
and friction or pressure plate.
It has a series of facings inverted on both sides
to increase the friction.
These clutch facings are made of asbestos
material. They are highly wear and heat
resistive.

The clutch plate consists of a central hub machined
with internal splines to limit the axial travel along the
splined gearbox during shaft.
A cushion drive clutch plate is provided with modern
motor vehicles.
It helps to provide a damping action against the
torsional vibrations or variations of the driving torque
between engine and transmission.
The damping action is achieved by coupling the splined
center hub to the driven plate with the help of a
flexible mounting.
The clutch plate is also provided with torsion springs.
The purpose of torsion springs is to cushion the clutch
engagement so as to eliminate the danger of the clutch
disc getting wrapped or broken.

Multi-Plate
Clutch
Multi-plate clutches are used in heavy
vehicles with racing cars and motorcycles for
transmitting high torque.
As compared to single plate clutch, this type
of clutch is smooth and easy to operate due
to their assembly of friction surface's contact.
It may be used where the space is very
limited.

The multi-plate clutch of small size transmits
approximately the same torque as a single
plate clutch of twice the diameter
These clutches may be wet or dry type. When
the clutch of this type is operated in a bath of
oil, it is called wet clutch.
But this oil immersed wet clutches are
generally used in conjunction with a part of the
automatic transmission.
The total number of clutch plates is divided
into two sets , One set of plate slides in
grooves on the flywheel and other one slides
on splines on the pressure plate hub.

Advantages
1. Increased torque transmission capacity
could be obtained.
2. The diameter is reduced as it has more
friction surface which reduces the size of
the clutch assembly.
3. It is highly reliable.
4. It is suitable for heavy vehicles.

 It consists of two cones having leather facings.
 These cones are known as male and female cones.
 One cone is fixed with the driving shaft i.e., flywheel
whereas the male member is splined on the driven
shaft i.e., gear box shaft
 When the clutch is in engaged position, the male cone
is fully inside the female cone in such a manner that
the friction surfaces are in perfect contact.
 It is done by the pressure springs which keep the male
cone pressed all time.
 Now, the torque is transmitted from the engine via the
flywheel and the male cone to the gearbox.
 When the clutch pedal is pressed, the male cone slides
against the spring force and the clutch is disengaged.
Hence, no power is transmitted.

This clutch is controlled by the engine speed
through an accelerator.
When the engine speed falls down, the clutch
will automatically disengage.
When the speed rises above the
predetermined value, the clutch is engaged.
Greater is the centrifugal force due to higher
engine speed, more will be powerful contact
between driving and driven members to
obtain the engagement

• These clutches are similar to the centrifugal
clutches with only difference of relatively light
clutch pressure springs exerting low pressure
at idling speed.
• In this clutch, the pressure between plates is
increased as the speed of rotation of the
clutch increases in proportion to the pressure
requirements
• This pressure increases as the weight is
increased.

Diaphragm Clutch
(a) Engaged position (b) Disengaged position

Positive Clutch (or)
Dog and Spline Clutch

The positive clutch consists of two components,
namely a driving member and a driven member.
It is quite simple in construction and operation.
It is generally used to lock two shafts together or
for locking a gear to shaft.
It consists of a sliding sleeve fitted with the
driving member having two sets of internal
splines.
The smaller diameter splines are used to slide on
a splined driving shaft whereas the bigger
diameter splines match with the external dog
clutch teeth on the driving shaft.

GEARBOX
 Gearbox is a speed and torque changing device.
 It changes speed and torque between engine and
driving wheels.
 The mechanism through which the driving torque of
the engine is transmitted to the driving wheel is the
gearbox.
 Torque is not directly transferred but it can be done in
the form of power using a suitable device.
 It is done between engine of the automobile and
driving wheels.
 During this, the gear ratio varies between engine and
rear wheels.

Purposes of
Gearbox
 It helps the engine to disconnect from driving wheels.
 It helps the running engine to be connected to the
driving wheel smoothly and without shock.
 It provides the leverage between engine and driving
wheels to be varied.
 It helps to reduce the engine speed in the ratio of 4: 1
in case of passenger cars and in a greater ratio in case
of heavy vehicles such as trucks and lorries.
 It helps the driving wheels to be driven at different
speeds.
 It gives the relative movement between engine and
driving wheels due to flexing of the road springs.

TYPES OF
GEARBOX
1. Manual transmission
(a) Sliding mesh gearbox
(b) Constant mesh gearbox
(c) Synchromesh gearbox
2. Epicyclic gearbox
3. Automatic transmission
(a) Hydramatic gearbox
(b) Torque converter gearbox.

Among the manual gear transmissions, this
sliding mesh type is simple in construction. In this
gearbox, spur gears are used.
Figure shows the construction of a sliding mesh
type transmission having three forward and one
reverse speeds.
There are three gears (1,6 and 5) attached on the
main shaft and four gears (2, 3, 4 and 7) on the
counter shaft or lay shaft.
The two gears on the main shaft (6 and 5) can be
slide by a shafting yoke and mesh with the gears
(3 and 4) on counter shaft.
Therefore, it is called sliding mesh gearbox.
A separate idler gear (8) is mounted on the idler
shaft.

First or low speed gear Second gear
Third or top gear Reverse gear

First gear Second gear
Third or top gear Reverse gear

Advantages of epicyclic gearbox
1. The planetary gears are in constant mesh. Hence, dog
clutches or sliding gears are not used.
2. External contracting hand brackets or multi-plate clutches of
relatively small dimensions are used for changing the gears.
3. It is a more compact unit because the planetary gear
operates within a ring gear with its external surface of
cylindrical form.
4. It is distributed over several gear wheels instead of having the
load on only one pair of gears.
5. A greater area of gear tooth contact can be obtained due to
distribution of loads.
6. In comparison to the three or four speed gearbox, gears and
gear housings of this gearbox can be made smaller in overall
dimensions.

Gear Shift or Selector Mechanism
• For gear changing and selecting a particular
gear train to operation, the selector
mechanism is implemented to obtain the
same easier.
• In this case, the gears are shifted by a gear
shift lever. In manually operated selective
transmission systems, the gear shift lever is
mounted either on steering column or on the
floor board.

Steering column mounted gear shift
mechanism

OVERDRIVES
• Overdrive is a device which is used to step up
the gear ratio in the car.
• It is mounted is between transmission and
propeller shaft.
• It provides a high cruising speed with a
comparatively low engine speed (up to about
20-25% less) on long journeys.
• Due to this, less wear of the engine parts,
vibration and noise are produced.

TRANSFER BOX
Transfer box is a part of a four-wheel-drive system used
in four-wheel-drive or all wheel-drive vehicles.
It is also called as "transfer gear case", "transfer
gearbox", "transfer box", "jockey box" or "T-case".
The function of transfer box is to distribute the torque
generated in engine to all four wheels of the vehicle.
 The transfer box is connected to the engine, front
axles and rear axles through drive shafts.
This shifting mechanism is placed in the transfer case.
The transfer gear box is controlled by the driver.
The control is located in the vehicle compartment.
It is either in the form of a transfer lever or a button.

Position of transfer case or transfer
box

Functions of transfer box
(a) It receives power from the transmission and
sends to both front and rear axles.
(b) It locks the front and rear axles mechanically
when required.
(c) The on-road transfer case coordinates the
difference between rotation of the front and rear
wheels.
(d) It provides low and high range of speeds.
Types of transfer box
1. Gear-driven type transfer box
2. Chain-driven type transfer box
3. Housing type transfer box
4. Transfer case shift type.

Advantages of fluid flywheel
1. It gives smoother power take up than the centrifugal type
when the engine is accelerated.
2. The fluid in the coupling behaves as a cushioning agent
between engine and gearbox in order to absorb shocks during
braking or coasting down on incline.
3. No need of separate pedal or lever to operate.
4. The driving member acts as a flywheel on the crankshaft.
Thus, it is smoothened out the torque variation.
5. There is no wear on moving parts.
6. No need of adjustment is required.
7. No maintenance is necessary, except maintaining required oil
level.
8. It is simple in design.
9. There is no jerk on transmission when the gear engages. It
damps all shocks and strains.
10. No skill is required for operating.

Disadvantages of fluid flywheel
• 1. There is a drag on the gear box-shaft even
the slip is 100%.
• 2. It has the gear changing difficult with the
ordinary crash type gearbox. So, the fluid
flywheel is generally used with epicyclic
gearbox which avoids this difficulty.

PROPELLER SHAFT
The propeller shaft is connected between transmission shaft
(i.e., gearbox output shaft) and pinion shaft (i.e., input shaft)
of the differential at the wheel axle.
As it propels the vehicle forward, it is called propeller
shaft. The propeller shaft is also called driveline shaft or drive
shaft.

Functions of Propeller Shaft
• 1. It transmits rotary motion of the gearbox
output shaft to the differential and then
through the axle shafts to wheels.
• 2. It transmits motion at an angle which
frequently varies.
• 3. It allows some changes in length between
gearbox and rear axle

SLIP JOINTS
• A slip joint is used between propeller shaft
and universal joint connecting the propeller
shaft to compensate for this change of length
and it helps to transmit power

UNIVERSAL JOINTS
• Universal joints are mainly used to make a flexible
connection between two rigid shafts at an angle.
• It permits the transmission of constantly varying
power.
• It is used to connect the propeller shaft with the gear
box shaft to transmit rotary motion.
Types of Universal Joints:
Universal joints are classified as follows.
(a) Variable velocity joints,
(b) Constant Velocity (CV) joints.

Ring type universal joint
In this case, both the driven and driving shafts do not turn at the same
speed even though each part of a revolution is at the same spin.
The driven and driving shafts should be placed in a straight line to turn
at the same speed through each part of a revolution.

Constant velocity joint
In this type of joint, the driven shaft is turned at the same speed
as the driving shaft turns through each part of revolution at any
degree of flex. These joints are mainly used in front drive axles
for transmitting power through a large angle required.

Major Components of Differential
• 1. Drive pinion or Bevel pinion.
• 2. Ring gear or Crown wheel.
• 3. Differential case.
• 4. Differential side gear or sun gears.
• 5. Differential pinions (or) planet gears.
• 6. Axle shafts or half shafts.
• 7. Pinion shaft or cross pin (or) spider.

Types of Differential
• 1. Conventional differential
• 2. Limited slip differential
• 3. Non slip differential
• 4. Double reduction differential

REAR AXLE
• Power is transmitted through rear axles to driving
wheels. It includes revolving shafts for driving wheels.
The rear axles are basically of two types such as
• 1. dead axles and
• 2. live axles
Driving axle supports the following.
(a)Total weight of the rear wheel causes bending and also
the shearing action.
(b) Bending load and also an end thrust or pull caused by
side thrust on the wheels when cornering.
(c) Driving torque.

Semi-floating axle
In a semi-floating axle, the various forces acting on the half shaft are as follows.
1. Side thrusts when a vehicle negotiates a corner.
2. Shear force and bending movement due to weight of the vehicle.
3. Twisting caused due to driving and braking torques.

Three-Quarter floating axle
The three-quarter floating axle sustains the following loads.
1. Bending load due to side thrust, when the vehicle is cornering, and
2. Twisting caused due to driving and the braking thrusts.

Full-floating axle
The full-floating axle is used in most of the trucks.

UNIT-III-TRANSMISSION SYSTEMS REAR AXLES

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UNIT-III-TRANSMISSION SYSTEMS REAR AXLES