POWER TRAINS
UNIT-5
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POWER TRAINS:
General Arrangement of Clutch, Principle of Friction
clutches, Torque transmitted, Constructional details, Fluid
flywheel, Single plate, Multi plate and Centrifugal
clutches
Gear box: Necessity for gear ratios in transmission,
Synchronous gear boxes, 3,4 and 5 speed gear boxes,
Free Wheeling mechanism, Planetary gears systems,
over drives, fluid coupling and torque converters,
Epicyclic gear box, principle of automatic transmission,
calculation of gear ratios, Numerical calculations for
torque transmission by clutches.
Introduction
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A Transmission system uses a clutch, gear box,
propeller shaft and a differential gear to transmit
power from engine to the road wheels
The power may be transmitted to rear or front or
all four wheels depending on the drive used
The clutch and gear box varies the ratio of torque
output to torque input
The propeller shaft transmits final torque to the
rear axle from gear box
A differential gear equally distributes the final
torque between the road wheels.
Functions of Transmission System
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It disconnects engine from driving wheels when
required
The engine is connected to driving wheels when
required
It changes ratio of torque output to torque input, as
desired
It turns the drive through a right angle
Clutch
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A clutch is a mechanical
device which transmits motion
from one shaft to the other which are along a straight
line or same axis of rotation.
A Clutch is a machine member used to connect the driving
shaft to a driven shaft, so that the driven shaft may be
started or stopped when ever needed, without stopping the
driving shaft.
Location of The Clutch
Requirements of a clutch
1. Torque transmission.---The clutch must be able to transmit maximum
torque of the engine
2. Gradual engagement----The clutch should engage gradually to avoid
sudden jerks
3. Heat dissipation---The clutch should be able to dissipate large
quantity of heat which is generated during the clutch operation due
to friction
4. Dynamic balancing---The clutch should be dynamically balanced. This
is particularly required incase of high speed engine clutches.
5. Vibration damping ---The clutch should have suitable mechanism to
damp vibrations and to eliminate noise produced during power
transmitted.
6. Size---The clutch should be as small as possible so that it occupies
minimum space
7. Inertia -- clutch rotating parts should have minimum inertia
8. Clutch free pedal play – to reduce effective clamping load on
carbon thrust bearing
9. Ease of operation – as clutch transmits higher torque , its
disengagement should not tiresome the driver
10. Lightness
Used
Disconnecting the engine with gear box
 When starting the engine
 When changing the gears
 When stopping the engine
Principle of friction clutch operation
Principle of Operation
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The clutch works on the principle of friction.
When two surfaces are brought in contact with each other and
pressed they are united due to friction between them.
The friction depends on area of contact, pressure applied upon
them and coefficient of friction on the surface of the material.
The two surfaces can be separated and brought in contact when
required.
One is considered driving member and other driven member. The
driving member is kept rotating. When the driven member is
brought in contact with the driving member it also start rotating.
When the driven member is separated from the driving member,
it does not revolve .
T=µWR
Where, W- axial load applied
µ-Coefficient of friction
T- torque transmitted
R- effective mean radius of friction surface
Working of a clutch
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Clutch for a drive shaft:
The clutch disc (center)
spins with the flywheel
(left). To disengage, the
lever is pulled (black
arrow), causing a white
pressure plate (right) to
disengage the green
clutch disc from turning the
drive shaft, which turns
within the thrust-bearing
ring of the lever. Never
will all 3 rings connect,
with no gaps.
The Clutch
Types of Clutches
1. Friction Clutches:
a. Single plate clutch
b. Multi plate clutch----i)Wet ii)Dry
c. Cone clutch---i)External ii) Internal
2. Centrifugal Clutch
3. Semi-centrifugal clutch
4. Conical spring clutch or diaphragm clutch
a. Tapered finger type
b. Crown spring type
5. Positive Clutch—Dog and spline clutch
6.Hydraulic clutch
7.Electro magnetic clutch
8.Vaccum clutch
9. Over running clutch or free wheel clutch
Single Plate Clutch
Working
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It has only one clutch plate mounted on the splines of clutch shaft.
Pressure plate is bolted to the flywheel through clutch springs and is
free to slide on the clutch shaft when the clutch pedal is operated.
When the clutch pedal is pressed, the pressure plate moves back
against the force of the springs and clutch plate becomes free
between the flywheel and the pressure plate.
Thus, the flywheel remains rotating as long as the engine is running
and the clutch shaft speed reduces slowly and finally stops rotating.
As soon as the pedal is pressed, clutch is said to be disengaged
otherwise it remains engaged due to spring forces.
Used in Ford V8 truck, Tata trucks, Premier President, Hindustan
Ambassador, Bajaj Matador, Mahindra Jeep
Cover assembly of single plate clutch
Multiplate Clutch
Working
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Multiplate clutch consist of a number of clutch plates, instead of
only one clutch plate as incase of single plate clutch.
As the number of clutch plates increased number of friction surfaces
which increases the capacity of the clutch to transmit torque.
The plates are alternately fixed to the engine shaft and gearbox
shaft.
They are firmly pressed by strong coil springs and assembled in a
drum.
Each of the alternate plate slides in the grooves on the flywheel and
other slides on the splines on the pressure plate. Thus each alternate
plates have inner and outer splines.
The multi-plate clutch works in the same way as single plate clutch,
by operating the clutch pedal. Multiplate clutch is used in heavy
vehicles, racing cars etc, where more torque is to be transmitted.
Cone clutch
Contact surface is in the form of cone
Centrifugal clutch
Working
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The centrifugal clutch uses centrifugal force instead of spring force for
keeping it in the engaged position. Also it does not require clutch pedal to
operate it.
The clutch is operated automatically depending upon the engine speed. The
engine can be stopped in gear without stalling the engine.
Similarly vehicle can be started in any gear pressing the accelerator pedal.
It consists of weight A pivoted at B. When the engine speed increases the
weights fly off due to the centrifugal force, operating the bell crank lever P
which presses the plate C.
The movement of the plate C presses the spring E Which ultimately presses
the clutch plate D on the flywheel against spring G. This makes the Clutch
engaged.
The spring G keeps the clutch disengaged at low speeds at about 500 rpm.
Stop H limits the movement of the weights due to the centrifugal force.
Semi-Centrifugal clutch
Clutch operation methods
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Mechanically
Electromagnetic operation
Hydraulic operation
Vacuum operation
Clutch by wire
Electromagnetic clutch
Hydraulically operated single plate clutch
Vacuum operation clutch
Clutch by wire
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In this system there is no mechanical link between the
clutch and the pedal
The clutch pedal is electronically controlled by means of
an Electronic Control Unit (ECU) and an actuator
A sensor on the pedal measures its exact position which
is transmitted to ECU
Information about the car’s behavior from other sensors
is also transmitted to ECU, which then operates the
clutch through the actuator
Advantages are improved drive ability, better pedal
feel and less wear due to absence of mechanical
linkage
Wet clutch
Description
Wet clutch
Dry clutch
Heat dissipation
Life
Torque
transmission
Clamping force
Coefficient of
friction
Good
Longer life
Lower
Bad
Short
Higher
More
Less due to oil
presence
Less
More
Coil spring single plate clutch
Diaphragm clutch
Advantages of diaphragm w.r.t coil spring
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More compact means of storing energy
Less affected by the centrifugal forces, it can
withstand higher rotational speeds whereas coil
springs distort transversely at higher speeds
Diaphragm acts as both clamping spring and
release lever
Clutch Components
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Clutch plate
Clutch facing
Pressure plate
Springs
Bearing
Clutch plate
Torsional springs and cushioning springs
Clutch facing
Requirement of a good clutch facing
 Good wearing properties
 Presence of good binders in it
 Cheap and easy to manufacture
 High co-efficient of friction
 High resistance to heat
Types of Friction materials
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Millboard type- asbestos sheet treated with certain impregnants
Moulded type- mixed asbestos fibers with a binding material
Woven type- cloth with certain binders
Solid woven variety- cloth is woven to required thickness
Laminated variety- layers of cloth one upon the other are held
together by a binder. Stitches are provided in addition to binders
Common clutch facing materials are;
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Leather- coefficient of friction- 0.27
Cork -0.32
Fabric-0.4
Asbestos -0.2
Reybestos and Ferodo- 0.35
Non-asbestos clutch facing
Design details of single plate clutch
Uniform Pressure Intensity: P= constant
 Total axial load on clutch W= πPr(ro2 -ri2 )
 Total Torque Transmitted
T= 2 /3 µ W(ro3-ri3)/(ro2-ri2)
T= µWR
 Effective Mean Radius R=2/3(ro3-ri3)/(ro2-ri2)
Uniform Rate of Wear: Pr= constant
 Axial load on clutch W= 2πPr(ro -ri)
 Torque Transmitted
T= µ W(ro+ri)/2
T= µWR
 Effective Mean Radius R= (ro+ri)/2
T=2µWR
Problem-1
The engine of a car employing a single plate friction
clutch develops maximum torque of 150Nm. External
diameter of the clutch plate is 1.2 times its internal
diameter. Determine the dimensions of the clutch plate
and the axial force provided by the springs. The
maximum allowable pressure intensity for the clutch
facings is 100KPa and coefficient of friction is 0.3.
Assume uniform wear.
Problem-2
An automobile clutch has a clutch plate of 160mm
inside and 240 mm outside diameter. Six springs in
the clutch provide a total force of 4.8KN, when the
clutch is new and each spring is compressed 5 mm. the
maximum torque developed by the automobile engine
is 250 Nm. Determine
i.
Factor of safety for the new clutch and
ii. The amount of wear of the clutch facing that will
take place before the clutch starts slipping.
Assume coefficient of friction for the facing is 0.3
Problem-3
Determine the size of the clutch plate suitable for an
Ambassador car employing a single plate type of
friction clutch and developing 37.5 KW at 4200rpm.
The inside diameter of the clutch plate is 0.6 times its
outside diameter and it is to be ensured that even
after a loss of 30% of the engine torque due to wear
of the clutch facing, the clutch does not slip. The
intensity of pressure on the facing is not to exceed
70kPa. Assume µ=0.3
Problem-4
Single plate friction clutch is to be designed for an Ashok
Leyland bus developing 80.85 kW at 2400 rpm. The
maximum torque developed however is 376 Nm at 1600
rpm. A maximum wear of clutch facing of 3 mm is to be
allowed, when the clutch must transmit at least 25%
excess torque. A pressure intensity of 190kPa can be
safely allowed and ratio Inside dia/ outside dia= 0.6 is
considered reasonable.
a) Calculate clutch plate dimensions
b) If 9 springs are used and initial spring force is to be
1.2 times the spring force after allowable wear of 3
mm, find out the spring stiffness. Assume µ=0.35.
Design details of Multi plate clutch
If n= total number of friction plates in the multi plate
clutch, then number of pairs of contact surfaces= (n-1)
then, Torque T= (n-1) µWR
i) For uniform pressure intensity,
T= (n-1)2 /3 µ W(ro3-ri3)/(ro2-ri2)
ii) For uniform wear rate,
T=(n-1)W(ri +ro)/2
Problem on Multi plate clutch
A multi plate clutch is to be designed for a motor
cycle whose engine develops maximum torque of
13Nm at 3500rpm. The external diameter of the
clutch facings is limited to 100mm and the inner
diameter may be assumed to be 0.2 times the
external diameter. The maximum intensity of pressure
may be taken as 80kPa and µ=0.3. Calculate the
number of plates.