clutch gear box-final

7/23/2019 Clutch Gear Box-Final

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Clutch & Gear Box

Prof Pradeep Kumar Shetty, Dept of Mech engg., MIT,Manipal 1

CHAPTER 5

CLUTCH

5.1 Funct ion of a Clutch

The torque developed by the engine at starting speed is very low. Therefore, it is not

possible to start the engine under load. This requires that the transmission system

should provide a means of connecting and disconnecting the engine from rest of the

transmission system. Such an operation must be smooth and without shock to the

occupants of the vehicle.

Thus the two main functions of a clutch are:

1. To allow the engine to be separated from rest of the transmission system.

This is required when:

a) starting and running the engine at a sufficiently high speed to generate

sufficient power necessary for moving the vehicle from rest;

b) shifting the gears so that damage to gear teeth can be avoided; and

c) Stopping the vehicle after applying brakes.

2. The second function of the clutch is to allow the engine to take up the

driving load of the vehicle gradually and without shock.

Frict ion Surface

The following are the requirements of the friction mating surfaces used in clutches.

1. It must have good mechanical strength and thermal expansion to

withstand the torque and thermal stresses over a wide temperature range

without distortion, bell-mouthing or excessive expansion.

2. Its heat soak capacity and thermal conductivity should be sufficient.

3. Its structure and finish must provide a consistent mating surface under all

conditions of operation.



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4. It must be hard enough to give a long wear life and resist scoring and

abrasion.

5. It must be cheap.

5.2 The main requirements of a c lutch :

1. It should be able to transmit maximum torque of the engine under all

conditions.(Torque transmission)

2. It should engage gradually to avoid sudden jerks.( Gradual Engagement)

3. It should be able to dissipate large amount of heat generated during clutch

operation.(Heat dissipation)

4. It should be dynamically balanced, particularly in the case of high speed

engine clutches.(Dynamic Balancing)

5. It should have suitable mechanism to damp vibrations and to eliminate noise

produced during power transmission.(Vibration Damping)

6. It should be as small as possible so that it will occupy minimum space.(Size)

7. It should be easy to operate requiring as little exertion as possible on the part

of the driver.(Ease of operation)

8. It should be made as light as possible so that it will continue to rotate for any

length of time after the clutch has been disengaged.

5.3 Method o f op erat ion of a Single Plate Clutch:

The movable parts of the clutch are pressed together by the thrust springs sothat the driven plate is trapped between the flywheel and the pressure plate. The

three components thus clamped together connect the engine to the primary shaft of

the gearbox. When the driver of the vehicle operates the clutch pedal the release

levers draw back the pressure plate, compressing the springs, thus separating the

flywheel, driven plate, and pressure plate hence disconnecting the drive.

As the pedal is released the pressure plate under the influence of the

expanding spring pressure, pushes the driven plate along the splines on the primary



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shaft. As the clutch faces press closer together the slip between these faces is

gradually reduced, until the speed of the gearbox primary shaft corresponds to the

engine crankshaft speed .

Fig.5.1 Single Plate clutch i) Engaged ii) Disengaged



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Fig.5.2 Single Plate Clutch

5.4 Semi-centr i fugal clutc h:

A semi-centrifugal clutch is used to transmit power from high-powered engines

where clutch disengagement requires appreciable and tiresome driver’s effort. The

transmission of power in such clutches is partly by the clutch springs and the rest by

centrifugal action of an extra weight provided in the system. The clutch springs

serve to transmit the torque up to normal speeds, while the centrifugal force assists

at speeds higher than the normal.

Construction: Construction of a semi-centrifugal clutch is shown in figure ‘a’.

Besides having a clutch plate, a pressure plate, and a splined shaft, it mainly

consists of (i) Compression spring (3 numbers), and (ii) Weighted levers (3

numbers). In this figure, only one compression spring and weighted lever are

shown.

The three weighted levers are hinged, and spaced at equal intervals on the periphery

of the clutch assembly. One suc h weighted lever hinged at B is shown in figure ‘b’.

This lever is hinged to pressure plate also through the needle bearing. The bearingcontains several needle rollers. The upper side A of the lever is weighted.



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Working: In its operation at low engine speeds, the compression springs assist in

preventing the clutch slipping as the centrifugal effect upon the weighted upper side

is very small. But when the engine accelerates, the upper end tends to move

outwards under centrifugal action. It thus introduces a torque about hinge B which

causes a normal force over the pressure plate at the needle bearings (3 numbers).

Effect of normal force is to increase the pressure on the pressure plate which is

sufficient to prevent slip at full engine load since the centrifugal force Fc increases as

square of the speed N. Thus Fc is proportional to N 2 which is depicted in figure b

and c below.

Fig.5.3 Variation of force on Pressure Plate in Semi-Centrifugal Clutch



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Fig.5.4 Semi-Centrifu gal Clutc h

5.5 Centr i fugal clu tches

Fig.5.6 Characteristics of Centrifugal clutch

Fig .5.5 a) Single Plate b) Multiplate Clutch



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Fig.5.5 (b) Centrifugal clutch

The principle of these is shown in the simple arrangement in Fig. ( a ) above where a

single-plate clutch, of ordinary construction has its presser plate A actuated by the

‘centrifugal’ forces acting on masses B formed on the ends of bell -crank levers

pivoted on pins in the cover plate C. This arrangement has two principal drawbacks.

First, there would be some force acting on the presser plate whenever the clutch was

rotating and thus the clutch would never be completely disengaged. Secondly, if the

force P due to the centrifugal force CF were sufficient to engage the clutch fully at

say 1000 rev/min then it would become nine times as great at 3000 rev/min and it

would require nine times the force necessary with an ordinary clutch to produce

disengagement at that higher speed by pulling the presser plate back in the ordinary

way, and it is desirable to be able to disengage the clutch in that way.

The first drawback can be overcome by putting in springs D (shown dotted) which

apply a force Q opposing the force P. The centrifugal forces will then not give rise to

any pressure on the driven plate until they have increased sufficiently to overcome

the force Q and until then the clutch will be completely disengaged. By choosing the

magnitude of Q suitably, the commencement of the engagement can be made to

occur at any desired speed. Usually in motor car clutches at speed of about 500rev/min is chosen.

The second drawback can be overcome by modifying the construction as shown at

( b ) where the bell-crank levers press on a floating plate E between which and the

presser plate are placed springs F. These springs transmit the force P from the

floating plate to the presser plate. A stop G limits the outward motion of the masses

B and thus limits the amount the springs F can be compressed. The force that must

be applied to the presser plate in order to pull it back so as to disengage the clutch is



Clutch & Gear Box


now limited to the difference between the force Q and the force exerted by the

springs F when the masses having come against the stops G) have compressed

them fully. This difference can be made to have any desired value.

5.6 The Fluid Flywh eel

The fluid flywheel shown in figure consists of a split housing which is rotated by the

engine. Inside this housing is a turbine, or driven rotor, which is connected by a

shaft to the gearbox.

Fluid Flywheel Characteristic.

Fig.5.6 Fluid Flywheel

The flywheel housing is divided up into a number of cells by means of radial vanes,

and these cells correspond to similar openings in the turbine. As the driving member

rotates, the fluid flows outwards under the action of centrifugal force, and circulates

from the flywheel to the turbine cells. Because the fluid is also being carried round

by the driving member, the fluid tends to rotate the turbine. The constantly

circulating fluid thus gains energy from the driving member and imparts this energy

to the turbine.



Clutch & Gear Box


At maximum efficiency the amount of slip between the two rotating parts is only

about 2%, the slip being greater at lower speeds. Complete disconnection of the

drive is not possible with a fluid coupling, and it is not suitable for use with an

ordinary gearbox. It is generally used in conjunction with epicyclic gears to provide a

semi- or fully-automatic gearbox.

Advantages:

- Fluid flywheels require less attention than friction clutches and need no

adjustment.

- The drive is taken up smoothly,

- Torsional vibration of the crankshaft and the transmission are damped out,

- The fluid absorbs transmission shocks when braking or coasting down a hill

and

- The clutch pedal is eliminated.

5.7 GEAR BOX

Torque avai lable and torqu e requ ired by an automo bi le:

Fig.5.7 Torqu e Required and Avai lable



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5.8 Transm ission Requirements

The primary duty of the transmission system is to provide a drive from the engine to

the rear wheel. The nature of the transmission system is greatly affected by the

characteristics of the power plant installed for propelling the vehicle.

Transmission requirements of a vehicle using conventional power plants:

The torque developed by the engine at the starting speed is very low.

Therefore, it is not possible to start the engine under load. This requires that

the transmission system should provide a means of connecting and

disconnecting the engine from rest of the transmission system. Such an

operation must be smooth and without shock to the occupants of the vehicle.

The engine should be able to start and propel the fully laden car up to a

gradient of at least 1 in 4. This roughly requires that the overall transmission

torque ratio should be about 10:1 for sports cars and 20:1 for small vehicles.

Since the final drive ratio varies from 3.5:1 to 5:1, it means that an additional

torque ratio of 3:1 to 4:1 is required to give an overall ratio to cope with themaximum gradients.

The transmission should allow the engine to run in the region of its maximum

power speed when the car is traveling at maximum speed. This means

matching of the maximum engine to the maximum vehicle speed.

It must allow transmission of maximum engine power continuously over as

large a speed range as possible to permit the fastest possible acceleration.

For obtaining good fuel economy the transmission system should give

automatically the highest possible torque ratio at steady running.

Thus an optimum transmission system would give a smooth start from the rest

and bridge the gap between the maximum speed and starting speed by suitable

gears so as to provide good acceleration and hill climbing consistent with fuel

economy over a wide speed range.



Clutch & Gear Box


In addition to these, other transmission system requirements are:

- It should allow the power transmission at an angle of 90 degrees.

- It should allow the driven wheels to be driven at different speeds.

- It should permit relative movement between the frame mounted engine and

the spring supported axle.

5.9 Vehicle Resistances

Tract ive effort is defined as the force exerted by the driving wheel in a direction

parallel to the ground against the road resistance.

A moving vehicle has to overcome the following road resistances:

1. Rolling resistance,

2. Air resistance, and

3. Gradient resistance

1. Rolling Resistance:

Rolling resistance is defined as the resistance to rolling motion offered by

road over which the vehicle is moving. It is mainly the losses occurring due to

deformation of road and tyre and losses occurring due to dissipation of energy in

tyre. The main cause of the rolling resistance is the hysterics losses, the greater part

of which are converted into heat. Then factors which affect the rolling resistance are

speed and inflation pressure of the tyre, tyre design, etc. The effect of speed,

however, is quite small and the rolling resistance can for practical purposes, be

considered as independent of speed.

2. Air Resistance:

The vehicle while moving experiences aerodynamic drag the magnitude of which

depends upon the frontal area of the vehicle, shape and the speed of the vehicle.

This aerodynamic drag is called the air resistance and is given by

Air resistance = 212

d C AV



Clutch & Gear Box


Where C d is the coefficient of drag, A the frontal area, is the density of air and V

the vehicle speed. Since the air resistance is proportional to the square of the

vehicle speed, it becomes quite significant for high speed vehicles and requires

careful study of the frontal area, its profile, and the surface finish, etc.

3. Gradient Resistance:

When a vehicle moves up a gradient, a part of its weight tries to pull it down. That

means a force equal to this pull must be supplied by the engine to overcome this

resistance which is referred to as gradient resistance.

The gradient resistance depends upon the weight of the vehicle and the steepness

or t he grade of the road. It is independent of the speed of the vehicle.

4. Total Resistanc e

The total resistance to the motion of the vehicle is thus given by

R t = R r + R a + R g

5.10 TYPES OF GEAR BOX

a. Sliding mesh type gear box,

b. Constant –mesh Type gear box:

- Dog clutch type,

- Synchromesh type.

- Automatic gear box,

- Overdrive,

c. Torque converter.



Clutch & Gear Box


Figure 5.8 shows the method of engagement of gears in a sliding mesh and a

constant mesh gear box.

Figure 5.8.

Figure 5.9 Sliding mesh gear box with 4 forward and one reverse speed

.



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Figure 5.10. A typical 3 forward and one reverse speed type sliding mesh type gear

box

a. Sl id ing m esh gear box: In a sliding mesh gear box the movement of the fork will

make the gears on the main shaft to slide on the splines provided on the shaft. This

will lead to one of the gears on the main shaft to engage with the corresponding



Clutch & Gear Box


matching gear on the counter shaft (lay shaft) and thus receive the required power

and speed.

Fig.5.11 Sliding mesh type gear box

b.Constant –Mesh gear box : In the constant-mesh type gearbox all the gears are

all the time in constant mesh. It has three shafts, (like that in the sliding mesh gearbox) namely, primary shaft, main-shaft and the layshaft. The primary shaft which

carries the clutch is splined and carries a gear that meshes with the first gear on the

layshaft. The main shaft has a number of gears (in descending sizes) that mesh with

the gears on the layshaft. However, these gears, being on bushes or ball or roller

bearings, are free to move on the main shaft but they do not transmit torque in this

situation. The large layshaft gear is engaged with the primary shaft gear and other

layshaft gears are engaged with main shaft gears. A dog clutch splined to the

mainshaft provides means of locking the freely rotating main shaft gears to the main

shaft to allow a gear ratio change.

Fig.5.12 Constant mesh gear box



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c. Synchrom esh Mechanisms

The purpose of the synchromesh mechanism is to bring to the same speed, the two

coupling members just before the actual coupling so that the gear teeth do not clash

and a smooth and silent coupling is obtained.



Clutch & Gear Box


Figure 5.13 above shows the synchro-dog clutch principle.



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Synchromesh Mechanisms

Figure5.14 below gives the exploded view of the synchro unit



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When the synchromesh is disengaged, gears are running free on the main shaft and

the two gears to be engaged are running at different speeds. When the selector

lever is moved, the sliding sleeve and the sliding gear slide together because of the

pressure of the spring-loaded balls until the cones on the gears contact. Both gears

have now reached the same speed. As the selector lever is moved further, the

sliding gear cone is held against the high speed gear cone as shown in the extreme

right part of figure and the sliding sleeve presses the spring loaded balls and slides

over on to the high-speed gear, thereby, locking the sliding gear to it. Since at the

time of final engagement of the gears, both the gears are running at the same speed,

the meshing is quiet and clash-free.

5.11 TORQUE CONVERTER

Performance of a Torque Converter

Fig.5.15 Torque converter



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Torque converters are similar in construction to the fluid flywheel, the main

difference being that the former has a reaction member or stator. This stator

produces a change of torque between the input and output shafts of the converter,

enabling an infinite number of speed and torque changes to be obtained to suit

varying conditions. If this conversion is multiplied by the ratio of an epicyclic gearbox,

the torque demands of a large-engine car are covered for all practical purpose.

A single stage three-element torque converter is shown in a simple form in

figure. The three elements are the impeller, stator, and turbine; other designs have

more elements and additional stages to improve the efficiency over a wide range,

and may have variable-angle stator-vanes.

Basically, the action of the torque converter is as follows. The stator, or

stationary member, redirects the flow of oil leaving the turbine outlets and, with the

stator as a fulcrum, boosts the action of the impeller. To improve the performance,

the blades of the turbine are curved and are fewer in number than the impeller to

overcome the effects of vane interference.

As the turbine speed increases the stator vanes would tend to impede the

return of oil to the impeller, and to prevent this, the stator is mounted on a one-wayclutch. Thus the stator can rotate in the same direction as the engine, together with

the turbine and impeller, but is held in the opposite direction and possesses similar

characteristics to the fluid flywheel, the speed ratio between the engine and output

shaft approaching 1:1.

Figure.5.16.Flow of fluid in torque converter (a) under low speed condition the stator deflects the fluid

flow back to the impeller (b) under high speed condition the stator free wheels and fluid flow is not

deflected.



Clutch & Gear Box


4.12 Epicycl ic Gearbo x

Figure shows a simple epi-cyclic gearbox. In its simplest form it consists of three

elements: a sun gear, an

annulus (or ring gear) and a

carrier, the latter supporting

two or more planet wheels

which mesh with both sun

gear and annulus. The teeth

are constantly in mesh and

changes in ratio are made

by holding one of the three

components of the system and causing the others to rotate around it.

Action Arm ‘a’ S (Ts) P (Tp) A (T A)

Fix the arm and give +1

rev. to S0 1

S

P

T

T S S P

P A A

T T T

T T T

Fix the arm and give +x

rev. to S0 +x

S

P

T x

T S

A

T x

T

A

S

P

arm aCarrier

S

p

a

A



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Give +y rev. to arm and

+x rev. to Sy Y + x

S

P

T y x

T

S

A

T y x

T

Sl.

No.

Reaction member

(member is fixed

to frame)

Driving member -

speed

Driven member -

speed

Type of output

1. Sun gear S

(x +y) = 0 or x =

-y

Speed of A = S

A

T y x

T

or S A

A

T T y

T

Speed of arm =

y

Speed

Reduction

2. Annulus A

S

A

T y x

T = 0 or

A

S

T x y

T

Speed of S = x + y

or S A

S

T T y

T

Speed of arm =

y

Speed

Reduction

3. Sun gear S

(x +y) = 0 or x =-y

Speed of arm = y

Speed of A =

S

A

T

y xT

or S A

A

T T y

T

Speedincrease

(Overdrive)

4. Annulus A

S

A

T

y xT

= 0 or

Speed of S = x + Speed

increase



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A

S

T x y

T

Speed of arm = y y

or S A

S

T T y

T

(Overdrive)

5. Carrier arm a y =

0

Speed of Sun S = x +

y =x as y = 0

Speed of

Annulus A =

S

A

T y x

T

= S

A

T x

T

Reverse gear

speed

reduction

6. Carrier arm a y =

0

Speed of Annulus A =

S

A

T y x

T = S

A

T x

T

Speed of Sun S

= x +y = x as y =

0

Reverse gear

overdrive

A lay-shaft gear arrangement with a given set of parts can provide only two ratios buta given epicyclic train can provide six ratios depending upon which member is held

stationary for reaction and which is used for input. Brake bands are used to hold the

member of epicyclic gear train. Out of these six ratios two are reducing ratios, by

driving firstly from sun wheel to career and secondly from annulus to carrier. Another

two gear ratios are speed increasing ratios (overdrives) by driving firstly from carrier

to sun wheel and secondly from planet carrier to annulus. The rest of the two gear

ratios are reversing ratios-one giving speed reduction by driving from sun wheel to

annulus and the other giving a speed increase by driving from annulus to sun wheel.

If any two members are locked together the entire epicyclic gear train acts as a solid

shaft because locking two members locks up the whole system.



Clutch & Gear Box


5.13 THE OVERDRIVE MECHANISM

The overdrive permits the propeller to rotate faster in the top gear than the

primary shaft. Engine revolutions can be reduced by 20-25% so permitting high

cruising at lower engine revolutions. This in turn extends the life of the engine,

improves the fuel consumption, and reduces vibration and noise. When fitted to

intermediate gears it increases the ratios available, and ratios of the normal

reduction gears are altered to take advantages of the additional ratios.

Fig.5.16 Over drive mechanism



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Fig.5.16 Over drive mechanism



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Generally an overdrive is fitted to top or third and top gear only. Over drives are

especially suited to high powered cars employing three speed gear boxes since in

order to produce flexible top gear performance a low gear final drive may be

necessary, resulting in the engine running faster at high speeds than is desirable.

The over drive may be operated manually or automatically; in the latter case it

comes to operation at a particular speed.

Typical over drive unit is shown in the figure. Epicyclic gears are normally used in the

over drive. During normal drive, the cone clutch locks the sun wheel to the annulus

and the entire assembly rotates, giving a direct drive through the one way clutch. To

bring the over drive into operation, the clone clutch is moved until it contacts the

stationary brake lining. Now the planet wheels and their carrier are driven round the



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stationary sun wheel, the one way clutch is over run by the annulus and the output

shaft speed is increased.

Adv antages of epicyc l ic gear tra ins:

1. The epicyclic gear trains are very popular for automatic transmissions

because change from one ratio to another can be made without loosing any

traction as there is no axial engagement or disengagement of gear teeth or

dogs.

2. It is very compact compared to the layshaft gear train.

3. The loads coming to the gear teeth are lower because of being shared by at

least two (more usually three) tooth contact points.

4. The epicyclic gear box is inheritently quieter in operation.

5. By using several planet gears the loads on the bearings can be balanced; the

shaft bearings in a layshaft gear train are comparatively heavily loaded.

5.14 Free wheel mech anism :

The free-wheel assembly, or overrunning clutch, sprag clutch, or one-way clutch, is

an essential part of every overdrive. It transmits power only from the transmission

main shaft to the output shaft when the sun gear is unlocked, and releases the main

shaft from driving the output shaft when the planetary gears are in overdrive.

Figure A shows the construction and principles of operation of a typical free-wheelunit. One part consists of a hub with internal splines to connect it to the transmission



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shaft. The outside surface of the hub contains twelve cams so designed to hold

twelve rollers in a cage between them and the outer race. The outer race is splined

to the overdrive output shaft. When the hub is driven in the direction shown in figure,

(clockwise) the rollers ride up the cams and by their wedging action force the outer

race to follow the hub. This is the method of driving the output shaft in direct drive.

Figure B shows what happens when the outer race becomes the driving member.

The rollers move down the cams, to release the outer race from the hub, thus

allowing it to move independent of the hub, and so make the entire assembly act like

a roller bearing. In this way the unit runs free whenever power is being transmitted

through the planetary gears or whenever the engine slows down and the output shaft

starts to drive the transmission main shaft.



Clutch & Gear Box


Prob lems o n clu tch

Single -plate clutch:

5.1 A motor car engine develops 5.9 b.KW at 2100 r.p.m. Find the suitable size

of clutch plate having friction linings riveted on both sides, to transmit the

power, under the following conditions:

a.) intensity of the pressure on the surface not to exceed 6.87 x 104 Pa,

b.) Slip torque and losses due to wear etc. is 35% of engine torque.

c.) Co-efficient of friction on contact surface is 0.3.

d.) Inside diameter of the friction plate is 0.55 times the outside diameter.

Solu t ion :

60000 60000 5.9

2 2 2100

w P

T N

= 26.84 N-m.

Taking account of the losses, the total torque is

T = 26.84 x 1.35 = 36.23 N-m.

We have, T = πµC (r 22 – r 1

2) 2

Or 36.23 = πx 0.3 x 6.87 x 104 x r 1

2

211

20.55

r r

= πx 4.122 x 104 3

1

11

0.303

r

or 3 3

1 4 4

36.23 0.303 1.22

4.122 10 0.697 10r m

or r 1 = 49.5 mm

and r 2 = 90 mm

Hence inside diameter = 99 mm



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And outside diameter = 180 mm

5.2: In a gear box the clutch shaft pinion has 14 teeth and low gear main shaft

pinion 32 teeth. The corresponding layshaft pinions have 36 and 18 teeth. The

rear axle ratio is 3.7:1 and the effective radius of the rear tyre is 0.355 m.

calculate the car speed in the above arrangement at an engine speed of 2500

r.p.m.

Solu t ion :

Gear ratio =

Speed of clutch shaft

speed of main shaft

Teeth of layshaft pinion Teeth of mainshaft pinion

Teeth of clutch shaft pinion Teeth of lay shaft pinion

=36 32

14 18 = 4.57:1

The rear axle ratio is 3.7:1.

Hence overall gear ratio,

G = 4.57 x 3.7:1 = 16.92:1

Speed of the car ,2 2500 0.3552

/16.92 1000

Nr V km hr

G

= 19.8 Km/hr

********************************************************************************************

Exercise 5

1. What are the functions of Clutch?

2. What is the requirement of a clutch?

3. Explain with a neat sketch working of a Semi-centrifugal clutch.

4. With a neat sketch explain the working of Fluid flywheel.

5. Write a short note on i) Centrifugal clutch ii) Single plate clutch.

6. What are the requirements of Transmission? Explain

7. Define the terms i) Rolling resistance, ii) Air resistance, and iii) Gradient

resistance



Clutch & Gear Box

8. Explain with a neat sketch of a Sliding Mesh gear box and show the diagram

of engaging gear 1st and 4th gear.

9. Explain with a neat sketch of a Constant Mesh gear box and show the

diagram of engaging gear 1st and Reverse gear gear.

10. Explain with a neat sketch of a Synchromesh Mesh gear box and show the

diagram of engaging gear 1st and 3rd gear.

11. Write a short note on i) Torque converter ii) Epicyclic gear box with a neat

sketch.

12. Explain with a neat sketch working of a Overdrive.

13. What are the advantages of epicyclic gear box?

clutch gear box-final

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