Auto Module 01

7/31/2019 Auto Module 01

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Clutch Automobile Engineering Prof. Pradeepkumar Suryawanshi

DON BOSCO INSTITUTE OF TECHNOLOGY, MUMBAI

Department of Mechanical EngineeringAutomobile Engineering (B.E. Sem- VIII)

Module 01

CLUTCH1.1 Clutch:A clutch is a device used to transmit the rotary

motion of one shaft to another when desired. Theaxes of the two shafts are coincident. It can also

be described as a machine member used to

connect a driving shaft to a driven shaft so thatthe driven shaft may be started or stopped at will,

without stopping the driving shaft. Thus it is an

interruptible connection between two shafts.

1.2 Requirements of Clutch:

i) Torque Transmission: The clutch should

be able to transmit maximum torque of

engine under all working conditions. It is

usually designed to transmit 125 to 150 percent of maximum engine torque.

ii) Gradual Engagement: The clutch shouldpositively take the drive gradually without the

occurrence of sudden jerks.

iii) Heat Dissipation: During clutch operation

large amount of heat is generated. The

rubbing surfaces should have sufficient area

and mass to absorb the heat generated. Theproper design of clutch should ensure proper

ventilation or cooling for adequate dissipation

of heat.

Clutch temperature is the major factor limiting

the clutch capacity. This requires that theclutch facing must maintain a reasonable

coefficient of friction with mating surfaces

under all working conditions. Moreover the

friction material should not crush at hightemperatures and clamping loads.

iv) Dynamic balancing: The clutch should bebalanced dynamically particularly in high

speed clutches.

v) Vibration Damping: Suitable mechanism

should be incorporated within the clutch toeliminate noise produced in the transmission.

vi) Size: The size of the clutch must besmallest possible so that it should occupy

minimum amount of space.

vii) Inertia: The clutch rotating parts should

have minimum inertia. Otherwise when theclutch is disengaged for gear changing, theclutch plate will keep on spinning, causing

hard shifting and gear clashing in spite of

synchronizer.

viii) Clutch free pedal play: To reduce

effective clamping load on the carbon thrust

bearing and wear thereof, sufficient Clutchfree pedal play must be provided in the clutch.

ix) Ease of operation: For higher torquetransmission the operation of disengaging the

clutch must not be tiresome to the driver.

1.3 Classification of Clutches:

Clutches can be classified as follows:

Positive Clutches:a) Square Jaw Clutch

b) Spiral Jaw Clutch

Friction Clutches:a) Single Plate Clutch

b) Diaphragm spring type singleplate clutch

c) Multiplate Clutch

d) Cone clutch

e) Semi-centrifugal Clutchf) Centrifugal Clutch

g) Wet Clutch

BE Mech Sem-VIII/Auto Engg/Module:01 Page 1


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Fluid Flywheel

1.4 Positive Clutches:

The positive clutches are used when a positive

drive is required. This type of clutch is designed

to transmit torque without slip. It is the simplestof all shaft connectors, sliding on a keyed shaft

section or a splined portion and operating with ashift lever on a collar element. The simplest type

of a positive clutch is a jaw or claw clutch. The

jaw clutch permits one shaft to drive another

through a direct contact of interlocking jaws. Itconsists of two halves one of which is

permanently fastened to the driving shaft by a

sunk key. The other half of a clutch is movableand it is free to slide axially on the driven shaft,

but it is prevented from turning relatively to itsshaft by means of feather key. The jaws of theclutch may be of square type or spiral type as

shown in Fig. 1.1

Fig. 1.1 Positive clutches. (a) Square-jaw clutch. (b)

Spiral-jaw clutch.

Fig. 1.2 Square-jaw clutch.

A square jaw type is used where engagement and

disengagement in motion and under load is notnecessary. This type of clutch will transmit power

in either direction of rotation. The spiral jaws may

be left-hand or right-hand, because power

transmitted by them is in one direction only. Thistype of clutch is occasionally used where the

clutch must be engaged and disengaged while inmotion. Engagement speed should be limited to

10 rpm for a square-jaw clutch and 150 rpm for a

spiral-jaw clutch. If disengagement under load isrequired, the jaws should be finish-machined and

lubricated. The use of jaw clutches are frequently

applied to sprocket wheels, gears and pulleys. In

such a case the non-sliding part is made integralwith the hub. Fig 1.3 shows the photographic

views of square and spiral jaw clutches.

Fig 1.3 Photographic views of square and spiral jaw

clutches

1.5 Principle of Friction Clutches:

The principle of friction clutch may be explainedby means ofFig.1.4



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Fig.1.4 Principle of Friction Clutches

Let disc C keyed to shaft A, rotates at some

speed, say N rpm. Initially when clutch is not

engaged shaft B and disc D keyed to it arestationary. Now apply some axial force W to the

disc D so that it comes in contact with disc C. As

soon as the contact is made the force of friction

between C and D will come into play andconsequently the disc D will start rotating. The

speed of D depends upon friction force present,

which in turn, is proportional to the force Wapplied. If W is increased gradually, the speed of

D will be increased consequently till the stage

comes when the speed of D becomes equal to thespeed of C. Then the clutch is said to be fully

engaged.

1.6 Single Plate Clutch:

A single disc or plate clutch shown in Fig.1.5consists of a clutch plate C whose both sides are

faced with a friction material G.

Fig.1.5 Single disc or plate clutch

It is mounted on a hub which is free to move

axially along the splines of the driven shaft D andis held between the flywheel A and the pressure

plate E. The pressure plate is mounted inside the

clutch body which is bolted to the flywheel. Both

the pressure plate and flywheel rotate with engine

crankshaft or the driving shaft. The pressure platepushes the clutch plate towards the flywheel by a

set of coil springs S. The springs are arrangedcircumferentially which provide axial force to

keep the clutch in engaged position. The three

levers known as release levers or fingers arecarried on pivots suspended from the case of the

body as shown in Fig 1.6. These are arranged in

such a manner that the pressure plate moves away

from the flywheel by the inward movement of athrust bearing. A pedal is provided to pull the

pressure plate against the spring force whenever itis required to be disengaged. Ordinarily clutchremains in engaged position.

Fig.1.6 Single disc or plate clutch with linkage

When the clutch pedal is pressed down, its

linkage forces the thrust release bearing to movein towards the flywheel and pressing the longerends of the lever inward. The levers are forced to

turn on their suspended pivot and the pressure

plate moves away from the flywheel by the knife

edges, thereby comprising the clutch springs asshown in Fig 1.6 a).



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Fig 1.6 a) Operation of the release levers

With the movement of the pressure plate, the

friction plate is released and clutch is disengaged.On the other hand, when the foot is taken off from

the clutch pedal, the thrust bearing is moved back

by the levers. This allows the springs to extendand thus the pressure plate pushes the clutch plate

back towards the flywheel.

The axial pressure exerted by the spring providesa frictional force in the circumferential direction

when the relative motion between driving and

driven members tends to take place. If the torquedue to this frictional force exceeds the torque to

be transmitted, then no slipping takes place and

power is transmitted from the driving shaft to thedriven shaft.

Fig 1.7 shows exploded photographic view and

Fig 1.8 shows cut away section of clutchassembly of single plate clutch.

Fig 1.7 Exploded photographic view of single plate clutch

Fig 1.8 Cut away section of clutch assembly of single plate

clutch

Advantages:

1. As compared to cone clutch, the pedalmovement is less in this case which leads to easier

gear changing.

2. It is more reliable compared to cone

Clutch.

Disadvantages:

As compared to cone clutch, the springshave to be stiffer and this means greater force

required to be applied by the driver while

disengaging.

1.7 Diaphragm spring type single plate clutch:

The construction of this type of clutch is similarto the single plate type of clutch described above

except that here diaphragm springs (also knownas Belleville springs) are used instead of theordinary coil springs. In the free condition, the

diaphragm spring is of conical form, but when

assembled, it is constrained to an approximately

flat condition because of which it exerts a loadupon the pressure plate. A diaphragm spring type

clutch is shown in Fig 1.9 and Fig 1.10 shows a

diaphragm spring in free condition.



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Fig 1.9 A diaphragm spring type clutch

Fig 1.10 A diaphragm spring in free condition

Advantages:

This type of clutch has now virtually

superseded the earlier coil spring design in many

countries in clutch sizes ranging upto 270 mm indiameter. However in case of heavy vehicles, the

coil spring type clutches are still being used,

because of the difficulty to provide sufficientclamping force by a single diaphragm spring. The

diaphragm spring offers following distinct

advantages.1. It is more compact means of storing

energy. Thus compact design results in

smaller clutch housing.

2. As the diaphragm spring iscomparatively less affected by the

centrifugal forces, it can withstand higher

rotational speed. On the other hand, thecoil springs have tendency to distort in the

transverse direction at higher speeds.

3. In case of coil springs, load-deflectioncurve is linear. Therefore with the wear of

the clutch facing the springs have less

deflection due to which they would apply

less force against the clutch plate. On the

other hand, in case of diaphragm spring

the load-deflection curve is not linear (Fig

1.11).

Fig 1.11 Load- Deflection curve

Therefore in this case as the clutch facing

wears, force on the plate gradually

increases, which means that even in wornout condition, the spring force is not less

than its value in case of new clutch.

Further it is also seen from Fig 1.12 thatthe load-deflection curve depends upon

the ratios h/t, where h is the free dishheight and tis the thickness of the spring.

Therefore in this case with suitable design,the load-deflection curve can be improved

to give lower release loads.



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Fig 1.12 Effect ofh/ton Load- Deflection curve

4. The diaphragm acts as both clampingspring and release levers. Therefore many

extra parts like struts, eye bolts, levers,

etc. are eliminated in the diaphragm springbecause of which the loss of efficiency

due to friction wear of these parts also

does not occur, which results inelimination of squeaks and rattles.

Fig 1.13 Cut away section of clutch assembly of single

plate clutch (Diaphragm spring type)

Fig 1.13 shows cut away section of clutch

assembly and Fig 1.14 shows explodedphotographic view of Diaphragm spring type

single plate clutch

Fig 1.14 Exploded photographic view of single plate clutch

(Diaphragm spring type)

1.8 Design of Single plate Clutch:

Consider two friction surfaces maintained in

contact by an axial thrust W as shown in Fig1.15(a). Let,

T = Toque Transmitted by clutch p = Intensity of pressure with which the

contact surfaces are held togetherr2 = ri = Inner radius of the friction surface

r1 = ro= Inner radius of the friction

surface

= Coefficient of frictionConsider an elementary ring of radius r and

thickness dr as shown in Fig 1.15(b).

We know that,

Area of the contact surface or friction surface,

dA = 2r.dr



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Fig 1.15 Forces on a single plate clutch

Normal or axial force on the ring,

dW = Pressure x Area = p . 2r.dr (i)

The friction force acting on the ring tangentiallyat radius r,Fr= . dW = . p . 2r.dr

Hence Friction torque acting on the ring,

Tr= r. Fr = . p . 2r2.dr (1)

Friction torque of a clutch is usually calculated on

the basis of two assumptions. Each assumption

leads to a different value of torque. In one case itis assumed that the intensity of pressure on the

contact surface or friction surface is constant

whereas in the second case, it is uniform wearing

of the contact surface or friction surface.

i) Considering uniform pressure:

Under this assumption, pressure is assumed to beuniform over the surface area and the intensity of

pressure is given by,

( )(2)

areasectional-Cross

ForceAxial

22

io rr

Wp

pressure

=

=

Equation (2) can be obtained by integrating

equation (i) within the limits from ri to ro.The total friction torque can be found by

integrating equation (1) within the limits from ri to

ro.

The total friction torque acting on the frictionsurface or on the clutch,

Substituting expression for p from equation (2),

( )

=3

.233

22

io

io

rr

rr

WT

)3(3

222

33

RWrr

rrWT

io

io =

=

SurfacFrictionofRadiusMean3

222

33

=

=

io

io

rr

rrR

Where

ii) Considering uniform axial wear:

For uniform wear over an area, the intensity ofpressure should vary inversely proportional to the

elementary areas, i.e. it should decrease with

increase in the elementary area and vice-versa.This can be illustrated by drawing a line with a

chalk. In doing so a little quantity of chalk is worn

from the stick. Now if it is desired that the chalk

is worn by the same amount, but the length of theline is doubled, the pressure on the chalk has to be

reduced to half that in the previous case.

Therefore for uniform wear, product of thepressure applied and the distance traveled must be

constant. For uniform wear of the surface, let

)4(CConstant

r2r2

rradiiatarearradiiatarea

width)(equalrandrradiiatsurfacetheofwidth

ratsurfacesobetween twpressureNormal

ratsurfacesobetween twpressureNormal

oi

oi

oi

o

i

==

==

==

==

pr

rprp

bpbp

pp

b

p

p

ooii

oi

oi

o

i

Thus in case of uniform wear of the two surfaces,

product of the normal pressure and thecorresponding radius must be constant. This

means the pressure is less where the radius is

more and vice-versa. Pressure on an elemental

area at radius r can be found as given below.


=

=.=

3.2

3.22p..

33

32

io

r

r

r

r

rrpT

rpdrrT

o

i

o

i


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Fig 1.16 Multi-plate Clutch

Fig 1.17 Multi-plate Clutch (Inner and outer plates)

Design of a multiplate clutch:

The total friction torque acting on the friction

surface or on the clutch is given by,

surfacesfrictionofpairsofNumberWhere

)8(

==

n

RWnT

1. If N is the total number of friction plates inthe multiplate clutch, then 1= Nn

2. If 1n is the number of plates on the

driving shaft and 2n is the number of

plates on the driven shaft, then

121 += nnn

1.10 Cone Clutch:

Fig 1.18 Cone clutch

Fig 1.18 shows simplified diagram of the cone

clutch. It was extensively used in automobiles, butnow-a-days it has been completely replaced by

the plate clutch. It consists of only one pair of

friction surface which is in the form of cones. In

the engaged position the male cone is fully insidethe female cone so that the friction surfaces are in

complete contact. This is done by means of

springs which keep the male cone pressed all thetime.

Advantages:

The only advantage of cone clutch is that the

normal force acting on the contact surfaces is

larger than the axial force as compared to thesingle plate clutch in which the normal force

acting on the contact surfaces is equal to the axial

force.

Disadvantages:

The cone clutch is practically obsolete because of

certain inherent disadvantages.1. If the angle of cone is made

smaller that 200 the male cone tends to

bind or join in the female cone and itbecomes difficult to disengage the clutch.

2. A small amount of wear on the cone surface

results in considerable amount of axialmovement of the male cone for which it is

difficult to allow.



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1.11 Design of Cone Clutch:

Consider a pair of friction surfaces of a coneclutch maintained in contact by an axial thrust W

as shown in Fig 1.19(2). Let

T = Toque Transmitted by clutch

p = Intensity of pressure with whichthe

conical contact surfaces are held togetherri = Inner radius of the friction

surface

ro= Inner radius of the friction surface = Coefficient of friction

= Semi-angle of the cone or the

angle of the friction surface with the axis of

the clutchb = Width of the friction surface

(alsoknown as face width or cone face)

Consider an elementary ring of radius r and

thickness dr as shown in Fig 1.19(1).

Let dlis the length of ring of the friction surface,

such that

sin

cosdr

ecdrdl ==

We know that,

Area of the contact surface or friction surface, dA

= 2r. dl=

sin

r2 dr

Fig 1.19 Forces on Friction surface of Cone Clutch

Normal force on the ring,

dP = Pressure x Area = p . dA=

sin

r2dr

p

Axial force on the ring,

)9(2

sin

sin

2sin

drprdW

drprdPdW

=

==

The friction force acting on the ring tangentiallyat radius r,

Fr = . dP = .

sin

r2dr

p

Hence Friction torque acting on the ring,

Tr = r. Fr= . )10(sin

r2 2

drp

i) Considering uniform pressure:The total axial force can be found by integrating

equation (9) within the limits from ri to ro.

( ) 11(2

22

force,axialTotal

222

io

r

r

r

r

rrpr

pdrprW

o

i

o

i

=

=.=



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Equation (11) shows that the total axial force is

independent of the cone angle.

The total friction torque can be found by

integrating equation (10) within the limits from rito ro.


Substituting expression forp from equation (11),

( )3 3

2 2

3 3

2 2

2 .sin 3

2(12)

3 sin

o i

o i

o i

o i

r rWTr r

r rWT W R

r r

=

= =

3 3

2 2

2Mean Radius of Friction Surface

3

o i

o i

Where

r rR

r r

= =

ii) Considering uniform axial wear:

We know that,

dr2r.dr2r

C=r.dr2.p=dW = C

[ ] ( )

( ))13(

2

222

force,axialTotal

io

io

r

r

r

r

rr

WC

rrCrCdrCW oi

o

i

=

==.=

We know that the friction torque acting on the

ring,

Tr = r. Fr =

sin

r2 2dr

p =

sin

r2dr

C


Substituting expression for C from equation (13),

( )

( )

( )SurfaceFrictionofRadiusMean

sin2

)14(sin2

22.

sin

2 22

=+

=

=+

=

=

io

io

io

io

rrR

Where

RWrrW

T

rr

rr

WT

Equation (14) gives the more conservative results

as compared to equation (12) and hence can besafely used for design of cone clutch.

1.12 Centrifugal Clutch:

The centrifugal clutches are usually incorporated

in motor pulleys. It consists of a number of shoeson the inside of pulley as shown in Fig 1.20

Fig 1.20 Centrifugal Clutch

The outer surfaces of the shoes are covered with a

friction material. These shoes, which can move

radially in guides, are held against the boss ofspider on the driving shaft by means of springs.

The springs exert a radially inward force which is

assumed constant. When rotating, under the

action of centrifugal force, the shoes are movedradially outwards. The magnitude of this

centrifugal force depends upon the speed at which

the shoe is rotating. When centrifugal force isequal to the spring force, the shoe is just floating.


=

=.=

2sin

.2

2sin

.2

sinC2

22

2

io

r

r

r

r

rrCT

rCdrrT

o

i

o

i

=

=.=

3sin

.2

3sin

.2

sin2p..

33

32

io

r

r

r

r

rrpT

rpdrrT

o

i

o

i


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Therefore net outward radial force (i.e. centrifugal

force) with which the shoe presses against the rimat the running speed,

rmrmrmPPP sco222

16

7

16

9 ===

The friction force acting tangentially on each

shoe,

)( scor PPPF ==

The friction torque acting on each shoe,

RPPRPRFT scorr === )(

And the total friction torque transmitted,

)15()( RPPnT

RPnRnFnTT

sc

orr

=

===

The mass of the shoe can be evaluated fromequation (15).

ii) Size of the shoes:

Let,

l= Contact length of the shoe

b = Width f the shoeR = Radius of the shoe when it

presses

against the pulley rim (same as theinside radius of the pulley)

= Angle subtended by the shoe

at thecentre of spider in rad

p = Intensity of pressure exerted

on theshoe (In order to ensure

reasonable

life, it may be taken as 0.1N/mm2)

Now, Rl=Area of contact o the shoe, lbA =The force with which the shoe presses against the

rim= p . plbA =

Since the force with which the shoe presses

against the rim at the running speed is

sco PPP = ,

Therefore

)16(sc PPplb =

The width of the shoe can be obtained fromequation (16).

iii) Dimensions of the springs:

We have discussed above that the load on thespring is given by,

17(16

9

4

3 22

2

1 rmrmrmPs =

==

By using the load value from eqn (17), the

dimensions of spring may be obtained as usual.

References:

1) Theory of Machines by S S Rattan, Tata

McGraw Hill Education Pvt ltd.2) Automobile Engineering Vol. 1 by Kirpal

Singh, Standard Publishers Distributors.

3) Theory of Machines by R S Khurmi, S

Chand Technical4) Design of Machine Elements by V B

Bhandari, Tata McGraw Hill Education

Pvt ltd.5) http://books.google.com

6) http://www.google.co.in/imghp?

hl=en&tab=wi

http://books.google.com/http://www.google.co.in/imghp?hl=en&tab=wihttp://www.google.co.in/imghp?hl=en&tab=wihttp://books.google.com/http://www.google.co.in/imghp?hl=en&tab=wihttp://www.google.co.in/imghp?hl=en&tab=wi

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