automotive transmission -lecture notes

1

LECTURE

NOTES

Adarsha Hiriyannaiah]

[For 6th semester BE Mechanical

PES Institute of Technology]

LECTURE NOTES

AUTOMOTIVE TRANSMISSIONS

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CLUTCHES

Clutch is a mechanism which enables the rotary motion of one shaft to be transmitted,

when desired, to a second shaft the axis of which is coincident with that of the first.

REQUIREMENTS OF CLUTCH

Torque transmission: The clutch should be able to transmit the maximum torque of the

engine under all condition. It is usually designed to transmit 125 to 150 per cent of the maximum

engine torque. As the clutch slips during engagement, the clutch facing is heated. Clutch

temperature is the major factor limiting the clutch capacity. This requires that the clutch facing

must maintain a reasonable coefficient of friction with the mating surfaces under all working

conditions. Moreover the friction material should not crush at high temperatures and clamping

loads.

Gradual engagement: The clutch should positively take the drive gradually without the

occurrence of sudden jerks.

Heat dissipation: During clutch application, large amounts of heat are generated. The

rubbing surfaces should have sufficient area and mass to absorb the heat generated. The proper

design of the clutch should ensure proper ventilation or cooling for adequate dissipation of the

heat.

Dynamic balancing: This is necessary particularly in the high speed clutches.

Vibration damping : Suitable mechanism should be incorporated witfiinthe clutch, to

eliminate noise produced in the transmission.

Size: The size of the clutch must be smallest possible so that it should occupy minimum

amount of space.

Inertia : The clutch rotating parts should have minimum intertia. Otherwise, when the

clutch is released for gear changing, the clutch plate will keep on spinning, causing hard shifting

and gear clashing in spite of synchronizer.

Clutch free pedal play: To reduce effective damping load on the carbon thrust

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

clutch.

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Ease of operation: For higher torque transmissions the operation of disengaging

the clutch must not be tiresome to the driver.

TYPES OF CLUTCHES

The following are the main types of clutches:

1. Friction clutches

2. Fluid flywheel

The friction clutches work on the fact that friction is caused when two rotatmg discs

come into contact with each other. On the other hand th fluid flywheel 'Works on the transfer of

energy from one rotor to the other by means of s~e fluid.)

Friction clutches may be dry or the wet type. In an overwhelming majority of vehicles,

the dry type of clutch is used because of mainly the lower coefficient of friction in the wet type.

However, the wet type of clutches have also some definite advantage over the dry type because

of which they are being again increasingly put to use in modem vehicles.

All these types will now be described in detail.

PRINCIPLE OF FRICTION CLUTCHES

The principle of a friction clutch may be explained by means of Fig. 3.1 Let shaft A and

disc C be revolving at some speed, say N r.p.m. Shaft Band the disc D keyed to it are stationary,

initially when the clutch is not engaged [Fig. 3.1(a»). 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 and consequently the disc D will also start revolving. 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 correspondingly till the

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

fully engaged [Fig. 3.1(b)].

Let W = axial load applied

µ = coefficient of friction

T = torque transmitted

R = effective mean radius of friction surface. The expressions for the same for different

types of clutches have been derived at appropriate places in this chapter.

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Then T= µWR

Thus we see that the torque transmitted by a friction clutch depends upon three factors

i.e., µ, Wand R. This means that increasing any or all of the above factors would increase the

amount of torque which a clutch can transmit. However, there are upper limits in each of these

cases.

(a) Coefficient of friction, µ

This depends upon the materials compnsmg friction surfaces. The coefficient of friction

values for commonly used materials for friction clutch are given in Table 3.1.

TABLE 3.1. COEFFICIENTS OF FRICTION FOR CLUTCH FACING

MATERIALS

S.No. Material Coefficient of friction I. Leather 0.27 2. Cork 0.37 3. Cotton Fabric 0.4-0.5 4. Asbestos-base materials 0.35-0.4

Most of the clutch friction materials have different coefficients of friction under static and

dynamic conditions; the dynamic coefficient being slightly less than the static coefficient. The

friction coefficient for a given material also varies with operating conditions, such as

temperature, pressure and rubbing velocity. These variations are usually furnished by the

materi,al manufacturers and are helpful in designing a clutch to operate under specified

conditions. As such, the values of the friction coefficients given above are only representative

values.

(b) Axial Pressure, W

The maximUlp value of W is limited to that which a driver can exert without undue strain.

This is found to be about 100--120 N. The other limitation is the type of material for friction

surfaces, e.g. for leather clutches maximum allowable pressure is 50 \cPa and for Ferodo lined

clutches 130 to 200 kPa. Where good cooling of the plates is possible a pressure of 300 kPa

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could also be attained in case of asbestos i.e., Ferodo clutches.

(c) Effective Mean Radius of contact surfaces, R

The value of R cannot be increased beyond a certain maximum which depends npon the

space available in the particular type of vehicle.

Dry Friction clutches

The following types of dry friction clutches will be described here:

1.Cone clutch

2.Single plate clutch.

3.Multiplate clutch .

4.Semi-centrifugal clutch

5.Centrifugal clutch

Cone Clutch

Fig. 3.2 shows a simplified diagram of the cone clutch.

In this type the contact surfaces are in the form of cones as shown in the figure. In the engaged

position, the male cone is fully inside the 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 the

time.

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When the clutch is engaged, the torque is transmitted from the engine via the fly wheel

and the male cone to the splined gear box shaft. For disengaging the clutch the male cone is

pulled out by means of the lever system operated through the clutch pedal thereby separating the

contact surfaces.

Advantage

The only advantage of the cone clutch is that the normal force acting on the contact surfaces in

this case is larger than the axial force, as compared to the simple single plate clutch in which the

normal force acting on the contact surfaces is equal to the axial force.

Disadvantages

This type of clutch is practically obsolete because of certain inherent disadvantages:

If the angle of cone is made smaller than about 20° the male cone tends to bind or join in the

female cone and it becomes difficult to disengage the clutch.

A small amount of wear on the cone surface results.in a considerable amount of the axial

movement of the male cone for which it will be difficult to allow.

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Design detials:

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Single Plate Clutch

A simplified sketch of a single plate clutch is given in Fig. 3.Friction plate is held between

the flywheel and the pressure plate. There are springs (the number may vary. depending upon

design) arranged circumferentially. which provide axial force to keep the clutch in engaged

position The friction plate is mounted on a hub which is splined from inside and is us to slide

over the gear box shaft. Friction facing is

attached to the friction plate on both

sides to provide two annular friction

surfaces for the transmission of power :A

pedal is provided to pull the pressure

plate against the spring force

whenever it is required to be

disengaged. Ordinarily it

remains in engaged position as is shown

in Fig. 3.4.

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Multiplate Clutch

The multi plate clutch is an extension of single plate type where the number of frictional

and the metal plates is increased. The increase in the number of friction surfaces obviously

increases capacity of the clutch to transmit torque, the size remaining fixe. Alternatively, the

overall diameter of the clutch is reduced for the same torque transmission as a single plate clutch.

This type of clutch is, therefore, used in some heavy transport vehicles and ing cars where high

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torque is to be transmitted. Besides, this finds application in case of scooters and motor cycles,

where space available is limited.

A simplified diagram of multi plate clutch is given below (Fig. 3.19). The construction is

similar to that of single plate type except that all the friction plates in this case are in two sets,

i.e., one set of plates slides in grooves on the flywheel and the other one slides on splines on the

pressure plate hub. Alternative plates belong to each set (Fig. 3.20).

Fig. 3.20. Friction plates of a multiplatc clutch. (a) Plates with outer tccth. (b) Plates with inner

tecth.

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.

Semi-centrifugal Clutch

For small torque transmission the clutch springs may be designed so that they have sufficient

strength for applying the required amount of force and at the same time are not so stiff as to

cause any strain to the driver while disengaging. However. for high powered engines. the clutch

spring pressures required may be considerable and thus the action of disengaging the clutch

becomes fatiguing to the driver.

To obviate this trouble, the help is taken of the centrifugal force. The clutch springs are

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designed to transmit the torque at normal speeds, while for higher speeds, centrifugal force

assists in torque transmission. Such type of clutches are called semicentrifugal clutches.

Fig. 3.21 shows a semicentrifugal clutch. Three hinged and weighted levers are arranged at

equal intervals. One of these is shown in Fig. 3.22 on enlarged scale. This lever is having

fulcrum at A and is hinged to pressure plate at B. The upper end of the lever is weighted at C. 0

is the adjusting screw, by means of which the maximum centrifugal force on the pressure plate

can be adjusted. To reduce friction, the levers are mounted on needle roller bearings on the

pressure plate. At moderate speeds the pressure of the springs is sufficient to transmit the

required torque. However at higher speeds, the weight C, due to the centrifugal force moves

about A as fulcrum thereby pressing the pressure plate. The centrifugal force is proportional to

the square of the speed so that adequate pressure level is attained. Fig. 3.23. shows the variation

of force on the pressure plate at various speeds.

Centrifugal Clutch

In the fully centrifugal type of clutches, the springs are eliminated altoget er and only the

centrifugal force is used to apply the required presure for keeping the clutch in engaged position.

The advantage of the centrifugal clutch is that no separate clutch pedal is r quire The clutch is

operated automatically de ending upon the engine sQ.~e(h Th' s means that car can be stopped in

gear without stalling the engine. Similarly while starting, the driver can first select the gear, put

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fhe car into the gear and simply press the accelerator pedal. This makes the driving operation

very easy.

Fig. 3.24. Principle of Centrifugal Clutch

Fig. 3.24 shows a schematic diagram of a centrifugal clutch. As the speed incr'eases, the

weight A flies, thereby operating the bell crank lever B which presses the plate C. This force is

transmitted to the plate D by means of springs E. The plate D containing friction lining is thus

pressed against the flywheel F thereby engaging the clutch.

Spring G serves to keep the clutch disengaged at low speed say 500 rpm.

The stop H limits the amount of centrifugal force.

The operating characteristics of this type of clutch will be then as shown in Fig. 3.25.

Force P is proportional to the centrifugal force at a particular speed, while force Q exerted by

spring G is constant at all speeds. The firm line in the figure shows the net force on the plate D

for various engine spee~s. At the upper end the curve is made flat by means of stop H.

CLUTCH OPERATION:

Generally, the clutches are operated mechanically through

a linkage. However, other means of operation viz . electrical.

hydraulic or even vacuum, have also been used. An these will

be described in the following briefly.

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Fig. 3.27. Clutch free pedal play.

Mechanical operation

The clutch linkage for this purpose iS,shown in Fig. 3.26. On pressing the clutch pedal, the

shaft A turns, which moves the fork lever and then through shaft B, actuates the release fork to

press the thrust bearing. This movement is further conveyed to clutch levers to disengage the

clutch. Generally, mechanical leverage from 10 : I to 12 : I is employed that would require a

pedal force of about 100-120 N when using travel of 75 mm.

When the clutch pedal is pressed, the thrust

bearing is not pressed immediately. Rather a part of

the pedal movement is purposely kept idle (Fig.

3.27).1bis is done to avoid a rapid wear of the thrust

bearing and the clutch plates and is called clutch

free pedal play. Usually this is kept about 25 mm at

the pedal.

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Electromagnetic operation

This type of clutch has been employed on some Renault cars. The construction and working

of this clutch may be understood by means of simplified Fig. 3.28. A is the engine flywheel

incorporating the winding B. Clutch plate C is lined with friction surfaces and is free to slide on

splines on the clutch shaft. D is the pressure plate. The winding B is supplied with current from

battery dynamo.

When the winding B is energized, it attracts the pressure plate D, thereby engaging the

clutch. When supply to winding B is cut off, the clutch is disengaged.

There is a clutch release switch in the gear lever. This switch is operated as soon as the driver

holds the gear lever to change the gear, cutting off current to the winding and thus causing clutch

disengagement.

Ordinarily the winding is connected to engine dynamo. At lower engine speeds, dynamo

output is also low which makes the force in winding very small. Three springs are also provided

in the clutch (not shown) to balance this reduced electromagnetic force at low speeds, thus

disengaging .the clutch.

During normal operation, the electromagnetic force of the winding is regulated by means of

an electrical resistance, which itself is controlled by means of accelerator pedal. As the

acceleration pedal is pressed the resistance is gradually cut, thus increasing the electromagnetic

force.

The electromagnetic type of clutch is best suited where remote operation is desired since no

linkages are required to control its engagement. A major limitation of tillS type is that of heat

capacirj since the clutch-operating temperature is limited by the temperature rating of the

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insulation of the magnetic coil. Another disadvantage is its higher initial cost.

Hydraulic operation

In heavy-duty mechanically operated clutches with high clutch- spring pressure, the force

required by the driver to release the clutch becomes excessive This can be remedied by the use of

hydraulic operation. This type of operation is also suitable for vehicles in which the clutch pedal

and the clutch have to be located too far away from each other. Hydraulically operated clutch

may be either single plate type or the more modern multiplate type. Both are described

below

Hydraulic single plate clutch

Fig. 3.29 shows a hydraulically operated clutch. When the clutch pedal is pressed the fluid

under pressure from the master cylinder reaches the slave cylinder which is mounted on the

clutch itself. The fluid under pressure actuates slave cylinder push rod which further operates the

clutch to relese fork to disengage the clutch. In India, this type of clutch is being used in

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Standard 20, SwaraJ Mazda and Eicher Mitsubishi's 'Canter' vehicles.

The detailed construction of

clutch master cylinder has been

shown in Fig. 3.30. In engaged

condition when the clutch pedal

is in the. released position, the push

rod rests against its stop due to the

pedal return spring. Also the pressure of master cylinder spring keeps the plunger in its back

position. The flange at the end of the valve shank contacts the spring retainer. As the plunger has

moved to its rear position, the valve shank has the valve seal lifted from its seal and seal spring

compressed. Hydraulic fluid can then flow past 1he three distance pieces and valve seal in either

direction. This means the pressure in the slave cylinder then is atmospheric and the clutch

remains in its engaged position.

However. when the clutch pedal is pressed to disengage the clutch , the initial movement of

the push rod and-plunger permit the real spring to press the valve shan; and seal against its seat.

his disconnects the cylinder from the reservoir. further movement of the plunger displaces fluid

through the pipelines to the slave cylinder and disengages the clutch. The construction of the

slave cylinder is made clear by-me ns 0 Ig. 3.31. The return spring in the slave cylinder

maintains some pressure on the release fork so that the thrust bearing is always in contact with

the release levers. Moreover, in case of wear of clutch facing, the return spring and the piston

move out automatically to take up the tilt of the release fork lever.

Unlike cables. hydraulic operation does not involve frictional wear, especially when

subjected to large forces. Due to this reason hydraulic operation is particularly suitable for heavy

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duty application, i.e., on large

vehicles.

Hydraulic multiple disc

clutch

This is a modern clutch and it is

increasingly being used in

heavy duty applications. e.g .. trucks. It may be in the form of a single or double clutch package.

Fig. 3.32 shows a double hydraulic clutch

incorporating hydraulic balance, internal

oil transfer and internal pressure modulation.

With the oil transfer system which enables oil to be transferred from one force chamber to

the other without passing through the hydraulic pump or external oil supply system, a much

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smaller hydraulic pump is needed and the clutch engagement is also faster. Starting with both

clutches released the following sequence of operations takes place when one clutch is engaged:

(i) Oil under pressure enters the accelerator piston cavity A, which causes the corresponding

accelerator piston to move towards the separator plate. This further results in the sealing of the

disc valve assembly near cavity B against the separator and opening of the disc valve cavity C.

(ii) The main force piston, then, moves towards right, into the engaged position.

Simultaneously, oil is also being forced from chamber C through the pressure plate opening, into

chamber B by opening one-way valve adjacent to chamber B. The oil pressure in chamber C

being higher than that in chamber B, because of the movement of the force piston, causes the oil

transfer to take place.

(iii) With the force piston in the engaged position, the engagement is completed by

pressurising chamber B from chamber A at a controlled rate through an orifice in the accelerator

piston.

The clutch engagement rate can be controlled by controlling the pressure build-up in the

force cavity of the clutch, which can be done either externally or internally. Internal pressure

modulation is found to be better than the external system because there the modulation is

controlled by metering a much smaller quantity of oil. The hydraulic clutch shown in Fig. 3.32

here

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utiliLes internal pressure modulation. This is achieved with the use of an orifice between the

accelerator cavity and the force cavity passing through the accelerator piston. With the clutch in

the engaged position, the oil under pressure enters the accelerator position cavity closing the disc

valve and moving the piston into engaged position. Since the displacement of the accelerator

piston is small, the pressure drop in the accelerator cavity is only instantaneous, thereby

generating a portion of the clamping force in an extremely short time. The remaining clamping

force is then generated by a controlled pressure build-up in the major force cavity created by

metering the small amount of oil required through the orifice in the accelerator system.

Fig. 3.33 illustrates the effect of internai pressure modulation as compared to the

unmodulated clutch. It is seen that the 1Jlodulated clutch begins the engagement much quicker

than the unmodulated clutch. This is due to oil transfer system. The engagement increases

steadily and smoothly till the lock-up occurs. The pressure in the clutch then continues to rise till

maximum torque capacity of the clutch has been reached. On the other hand, in case of

unmodulated clutch, the time required to get the piston in the engaged position is considerably

longer. At the point of piston engagement the clutch torque rises suddenly from zero to

maximum, resulting in very harsh engagement and very high rate of heat generation.

Vacuum Operation

The partial vacuum existing in the engine manifold is put to use for operating the clutch (Fig.

3.34). A reservoir is connected to the engine manifold through a non-return valve. The reservoir

is further connected to a vacuum cylinder through a solenoid-operated valve. The solenoid itself

is operated from the battery and the circuit incorporates a switch which is placed in the gear lever

and is operated when the driver holds the lever to change gears. Vacuum cylinder contains a

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piston wllicb is exposed to atmospheric pressure on one side. TIle piston is further connected

through linkage to the clutch. The movement of the piston thus operates the clutch.

In the part throttle position there is sufficient vacuum in the engine inlet manifold. When the

throttle is opened wide, the pressure in the manifold increases, hut due to this increase of

pressure the non-return valve closes, isolating the reservoir from the manifold. Thus a vacuum

exists in the reservoir all the time.

In the normal operation the switch in the gear lever remains open and the solenoid-operated

valve remains in its bottom position. In this position, the atmospheric pressure acts on both sides

of the piston in the vacuum cylinder. However, when the driver is to change gears, he holds the

gear lever. This action of the driver closes the switch, energizing the solenoid which pulls the

valve up, connecting the vacuum cylinder to the vacuum in the reservoir. Thus the piston is

subjected to useful pressures on two sides, which causes it to move. This movement is

transmitted by linkage to disengage the clutch. The clutch used in this case is an ordinary friction

clutch, which remains engaged due to the force of the springs provided in the clutch itself. The

gear lever switch is opened as soon as the driver releases the lever after changing the gear and

the clutch is again engaged.

Types of Friction Materials

1. Millboard type.

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2. Moulded type.

3. Woven type;

(a) Solid woven variety.

(b) Laminated variety

Millboard type - This is only asbestos sheet treated with certain impregnants.

From this sheet are then the facing discs cut according to different size

requirements. This is the cheapest available type but is quite satisfactory.

Moulded type - This is made by mixing asbestos fibres with a suitable binding material, heating

to a certain well defined temperature and then moulding in dies under pressure. Metallic wires

are also sometimes inserted to improve wearing qualities.

This type of facing is more dense and capable of taking heavier working loads. However

there is one disadvantage that each clutch facing has to be moulded separately.

Woven type - This type consists of a cloth impregnated with certain binders. The cloth may

either be woven like ordinary cloth with wrap and weft or by winding the fibres in

circumferential

direction only.

Fig. 3.44. Ferodo 2129F-moulded asbestos based clutch facing material suitaole for all types of

vehicles. (Courtesy-Ferodo Ud., England)

In the solid woven variety, the cloth is woven just to the required thickness. In the case

of laminated variety, the layers of cloth one upon the other are held together by a binder.

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Stitches are provided in addition to the _binder.

Common Clutch Facing Materials

Organic friction materials are the most common types of clutch facing

materials. Examples are :

1. Leather: Dry leather on iron has coefficient in friction of 0.27.

2. Cork: Cork on dry steel or iron has coefficient of friction of 0.32.

3.Fabric: Good quality fabric materials have coefficient of friction of about 0.4. But they

cannot be used at high temperatures.

4. Asbestos : Asbestos facing have coefficient of friction of about 0.2.

However it has got anti-heat characteristics.

5.Reybestos and Ferodo. These have a coefficient of friction of about 0.35 and are most

suitable as friction facings. They are almost universally used for clutch facings.

For more severe applications sintered metal friction material is sometimes used because it can

withstand higher temperatures. However, its disadvantage:' is that it tends to weld itself to the

mating pressure plate and flywheel surfaces at high temperatures. For very heavy vehicles

operating under extreme conditions, combined metal-ceramic friction material can be used.

However these materials are satisfactory only when operating under very high temperatures

rather than under light duty and low temperature when they tend to have an abrasive action on

the mating plate surfaces.

OTHER CLUTH COMPONENTS

11.1. Pressure Plate

High tensile grey iron is the most commonly used material for pressure plate, which must

be sufficiently rigid so as not to distort under the pressure of the clutch springs. Adequate

rigidity is also needed to provide uniform pressure to clutch plate. The pressure plate should

also have sufficient mass and thermal conductivity to absorb and conduct away the heat

generated during engagement.

On the back of the pressure plate are cast a number of lugs to locate and support the release

lever and strut assemblies (Fig. 3.7).

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Release levers

The pressure plate in case of coil spring type clutch has a number of release levers, usually

three or four (Fig. 3.8). especially spaced around the pressure plate.

Cover

This is a steel pressing bolted onto the flywheel and houses the pressure plate assembly

(Fig. 3.7). It provides pivot for the release levers and takes the reaction of the springs, due to

which reason it must be sufficiently rigid. It should also have holes for the dissipation of heat.

Straps

A number of steel straps, usually four (Fig. 3.45) are arranged around the pressure plate.

These straps hold together the cover and the pressure plate THease straps hold together the cover

and the pressure plate while the other end is connected to the cover. When the engine is running

and the clutch is engaged, these straps deflect without affecting adversely the concentricity of

the cover and the pressure plate and thereby transmit the drive from the cover to the pressure

plate without any friction between them.

Springs:

Normal duty clutch springs are made from oil tempered spring steel wire However, for

severe conditions they arc .nade from silica-chrome steel t< prevent heat set. Insulating washers

are also sometimes used under extreml conditions to reduce heat conduction from the pressure

plate to the springs.

The stiffness of the clutch springs should be the maximum possible SI that sufficient spring

force is left after their extension due to wear of the c1utcl facings. If the spring stiffness is

excessively high, either excessive rdeas pressure will be required when the clutch plate is new

or else insufficien spring pressure will be available when the clutch facing has worn CUI

Usually, a 10-15% pressure variation is acceptable between the new and worn positions of the

clutch facings.

Throwout Bearing:

It is used to transfer force at the pedal from the stationery linkage to the rotating.

c1utch. This is either a thrust ball bearing which is packed with grease for lubrication, or else a

graphite impregnated one fitted in a steel carrier. TI later type, obviously, does not require any

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lubrication.

PRELIMINARY INSPECTION OF CLUTCH

I. Start the engine and with the clutch released try to shift various gear If the shifting is

smooth, the adjustment is correct. However, if the gear shifting is not smooth, it indicates the

need for readjustment.

2. Check the free pedal play. The exact amount of the permissible pI: may b~ ~oun.d out

from the manual, but in general a minimum of 12 mm pI:

IS specifIed m majority of vehicles.

Fig. 3.46. Adjustment of clutch free pedal play. I. Lock nut, 2. Split pin, 3. Push rod (Courtesy-

Tata Engineering & Locomotive Co., India)

The only adjustment required in a clutch is of

the free pedal play, which is necessitated on account of

wear of the friction lining due to continuous use, or with

the wear of the throwout bearing carbon ring due to

the habit of the driver to rest his foot always on the clutch

pedal. The: wear of the friction lining decreases the free pedal play, whereas the wear of the

carbon ring causes the same to increase. If the free pedal play is less, the clutch cannot engage

fully, whereas excessive free pedal play restricts the complete disengagement of the clutch.

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The procedure of adjustment and the amount of free pedal play varies, depending upon the make

for which the concerned manual can be referred. For example, in case of Hindustan Ambassador

car, an adjusting nut is provided at the lower end of the clutch lever. You have to slacken the

lock nut first, make the desired adjustment with the adjusting nut and retighten the lock nut. The

free pedal play in this position should be 31 mm. Fig. 3.46 shows the method of freeplay

adjustment in a Tata 1210E vehicle. This is done by changing the length of linkage between the

clutch pedal and the clutch release fork. Lock nut is loosened, the split pin is removed and the

yoke is disconnected. Then it is rotated as much as is necessary so that the desired free play is

obtained, after which the lock nut is tightened and the split pin is reset. In Tata vehicles, the free

play is between 30 and 35 mm.

CLUTCH OVERHAUL

A general procedure for clutch overhaul has been explained in the following paragraphs. The

main steps for this are:

1. Removing the clutch. 2. Disassembling.

3. Inspection and service. 4. Assembly.

5. Refitting the clutch.

These have been discussed further in detail.

Removing the Clutch

The exact procedure to be followed for removing the clutch depends upon the particular

make of the car and the instruction manual for the same must be consulted. However the general

procedure may be outlined as follows:

1. Remove the transmission (gear box) from the chassis including various clutch and

transmission linkages

2. Loosen the bolts securing clutch to the flywheel. This must be done diagonal wise and

loosening gradually till the entire spring pressure is completely removed.

3. Remove the securing bolts. Now the cover assembly and the clutch friction plate may be

lifted separately.

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Disassembling:

l. Before starting dismantling the clutch cover assembly, it is very important to mark the

relative positions of various components so that they can be reassembled easily. Mark the

pressure plate, the cover and the release levers. Remove the release levers alongwith the plate.

2. Place the cover assembly under a press, with wooden block suitably placed above and

below it (Fig. 3.47).

3. Apply the pressure on the cover assembly and in this position loosen the adjusting units.

Remove the pressure gradually till the clutch springs are completely free.

4. Lift off the cover to inspect various parts inside.

5.If it is required to remove the other components, mark their positions first and remove

them according to the procedure given in the manual.

Fig. 3.47. Clutch placed in the press for disassembling

Inspection and Service:

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After disassembling the clutch, various components are inspected and serviced according

to requirements.

1. Clutch facing. Inspect the clutch facing for wear. In case it is worn out upto the rivet heads

(of course, with a service limit), the same has to be replaced. In Suzuki (Maruti) 800 car, for

example, facing has to be replaced when the same has worn to 0.5 mm above the rivet heads.

The facing may even be loose, in which case again a new facing will have to be fitted. The

procedure for refitting of facings has been explained in Art. 15.

There may be grease or oil on the facings. This may have come from excess grease in the

throwout bearing. Even too high a level of oil in the transmission may force the oil into the

clutch facings through the input sh~ft.

With use, the clutch facings acquire a shining surface, which is not bad. This shining polish on

the surface is transparent through which grains of the material are visible. But in case oil in small

quantities has reached the clutch facing, it will bum there and darken the facing colour. Yet the

performance of clutch is not affected. However, in case large quantity of oil reaches the facings

and bums there, the colour of the facings gets almost black. This causes clutch slip or judder and

the facings have to be replaced. Apart from replacement, it is very necessary that source of this

leakage should also be rectified.

2. Clutch plate springs. Inspect the cushioning and the torsional springs on the clutch plate. In

case they are found to be cracked or weak, complete plate has to be replaced.

3. Pressure springs. Check the pressure springs for stiffness. If variation in case of a particular

spring from the original value is more than the allowable, the same should be replaced.

4. Throwout bearing. Clean and grease the throwout bearing. Now hold the inner race and try to

rotate the outer race keeping it under pressure. If the rotation is not uniform, the bearing needs

replacement.

5. Pressure Plate. It should have a smooth plane surface. In case it is distorted by more than 0.3

mm, or is badly scored, replace it.

Assembly :

53

Grease various clutch components requiring lubrication before reassembling. Place the

pressure plate on the blocks placed over the press bed and place pressure springs on it at suitable

places. Fit also the release levers and place the cover over the assembled parts, ensuring that all

the parts which were marked before disassembling are placed in their correct positions. Apply

pressure gradually taking care that the bolts are guided properly through the holes in the cover.

Tighten the nuts in proper order and with the correct maximum torque. Remove the pressure by

releasing the press.

Refitting the clutch:

Attach the clutch cover assembly to the flywheel by means of bolts, placing the clutch plate in

between the flywheel and the 'cover assembly. Make sure that the clutch plate is centralized. This

may be done by using a clutch alignment bar.

Place the throwout bearing on the release levers and refit the gear box at the proper place on the

vehicle chassis.

Refit the clutch operating linkages and check for the pedal movement. In case of any excess or

lesser pedal play, readjust the same as already explained.

CLUTCH REFACING:

Refacing of clutch plates demands caution. The facing of suitable material only should be

used and the rivets for facing must be the tubular ones.

Use suitable drill to remove the rivets from the worn out facing. Do not punch the rivets

out. This may damage the friction plate. First attach on one side using a blunt centre punch.

Similarly attach the facing on the other hand.

It is very important that the plate after refacing must be perfectly flat. The tolerance for.

the run-out should generally be less than about 0.5 mm. This can be checked by mounting the

plate on a mandrel between centres and using a dial indicator as much near the edges as possible.

In case the run-out is more than the prescribed limit, dress it up after locating the high spots.

CLUTCH TROUBLE SHOOTING:

54

It is not within the scope of this book to deal with very exhaustively the trouble shooting

of the automobile clutch. However, common troubles experienced alongwith their causes, are

explained below briefly. The remedies have also been suggested.

1. Clutch Slip

It is sometimes experienced that the clutch slips while in engagement. In this condition it

fails to transmit completely the engine torque. Moreover, because of slipping, a large amount of

heat is generated due to which clutch facings wear out rapidly and even burn out. The flywheel

face also wears out, there is rapid wear of pressure plate and the stiffness of the springs is also

decreased: This may be caused by any or more of the following reasons :

(a) Incorrect linkage adjustment which causes insufficient 'free pedal play' . Adjustment of the

linkage will remedy this defect.

(b) Oil or grease on friction facings due to leakage from the engine crankcase or the gear box or

to excessive lubrication of the slutcn shaft and its support bearing. This causes glazing of the

friction surfaces leading to slipping. The remedy in this case is simply to clean the components

and replace the clutch facing.

(c) Weak or broken clutch springs. The springs may be overheated, which will be revealed by

their blue colour. Overheating reduces the spring stiffness and makes them weak. In this case the

only alternative is to replace the springs.

(d) Worn out facings, which should be replaced.

Clutch drag or spin :

Sometimes when the clutch is to be disengaged, it is not disengaged completely and it

causes difficulty in changing the gears. This defect is called clutch drag. Reasons for the

presence of this defect may be :

(a) Excessive "free pedal play." This may have been caused by the driver 'riding' the clutch

pedal. i.e., when he is in the habit of keeping his foot on the clutch pedal while driving. When the

clutch drags, the first thing to be done is to check the 'free pedal play' . If found incorrect, it

55

should be adjusted. If this play is already correct, then the trouble may be due to other reasons

and to locate them the clutch has to be opened.

(b) Oil or grease on friction facings. The remedy is to clean the facings or if excessively

damaged, to replace them

(c) Pressure plate warped or damaged is needs replacement.

(d) Clutch plate cracked or buckled. The only alternative to remedy this is the replacement of the

complete plate.

(e) Clutch plate may be seized on clutch shaft splines. This may be remedied by cleaning up the

splines on the shaft and lubricating them.

Clutch Judder :

Sometimes as the clutch is engaged, a vibration or judder is produced instead of smooth

gradual engagement and the vehicle suddenly jumps forward. The possible causes are :

(a) Loose or worn out clutch facings, which must be replaced. (b) Loose rivets. The whole facing

should be replaced.

(c) Distorted clutch plate may also be one of the reasons to cause clutch judder. The same has to

be replaced.

(d) Misalignment of the pressure plate with the flywheel. This has to be corrected. This requires,

however, special equipment.

(e) Flywheel may be loose on the crankshaft flange, which may be tightened to remove the

defect.

(j) Bent splined clutch shaft. If the defect is not much, it may be possible to straighten the shaft,

otherwise this has to be replaced.

(g) Oil, grease or dirt on the friction surfaces causing uneven engagement. The friction surfaces

on the flywheel and the pressure plate should be cleaned and the clutch facing replaced.

56

Clutch Rattle :

Apart from the defects explained earlier in the engagement of clutches, some peculiar

noises may be noticeable when the engine is idling. Clutch rattle is the prominent noise

observed.

To locate the cause, press the clutch pedal to take up only the free movement. If the rattle

disappears, it may be due to worn out or loose throwout bearing or it may be that pedal return

spring is disconnected and is loose. In the former case, the bearing has to be replaced, while in

the latter case, the spring is simply to be replaced.

If, however, the rattle continues, it may be due to damaged clutch plate ir. which case it

has to be replaced. The bent splined shaft may also be a source of rattle.

Knock :

This is observed clearly when the engine is idling and the clutch is engaged. This may be due to

worn out splines of the clutch plate hub or the clutch shaft. Such a situation would require

replacement of the defective part i.e. either the clutch plate or the clutch shaft or both. The

wearing out of the spigot bearing in the flywheel may also be a cause of knock in the clutch. The

bearing will have to be replaced in this case.

Pulsation of the clutch pedal :

This may be caused by the misalignment of the engine and the transmission. Due to

misalignment, the clutch disc moves to and fro on the clutch shaft in each revolution and this

movement is transmitted back to the pedal. This results in rapid wear of all clutch parts. To

remedy this, the proper realignment has to be done. The pedal pulsations may also be caused by

a wobbling flywheel, mostly due to its improper mounting on 'the engine crank shaft, which may

be redone properly. If the flywheel is otherwise unbalanced, the same may be either balanced or

replaced.

57

Fluid coupling FLUID Flywheel:

Construction

58

The fluid flywheel or the hydraulic coupling as it is frequently called has been used in cars employing automatic transmissions.

It consists of two members, the diving and the driven as shown in Fig. 3.48. The driving member is attached to the engine flywheel and the driven member, to the transmission

shaft. The two members do not have any direct contact with each other. The driven member is free to slide on splines on the transmission shaft. The two rotors are always filled with fluid of suitable viscosity. These are provided with radial ribs to form a number of passages, which avoid formation of eddies and also guide the fluid to flow in the desired direction.

Principle of Fluid Flywheel.

Torque Transmission

A simplified diagram representing the fluid flywheel (Fig. 3.50) makes it easier to

59

understand the process of transmission of torque. At the start, tube, X is rotating, say, at N1 rpm

and tube Y is stationary. With the movement of fluid in X and Y, Y also starts rotating though at

lower speed. This speed goes on increasing till it becomes equal to the speed of X. Then the

coupling IS fully engaged.

To understand how all this happens, consider a particle A, which after small intervals of

time takes successively the positions B, C and D. If m is the mass of the particle. The kinetic

energy values at A, B, C and D respectively will be (m/2)*(2¶RN1)^2, (m/2)*(2¶RN1)^2),

(m/2)*(2¶RN2)^2), (m/2)*(2¶RN2)^2). Thus we see that particle A gains K.E. as it moves from A

to B in tube X; and then when it passes to tube Y, it gives the same to it, thereby increasing its

speed.

Characteristics:

Fig 3.51 shows the variation of percentage slip with speed. The percentage slip is defined as

((N1-N2)/N1) where N1 and N2 are the speed of driving and driven members respectively. It is

seen that for engine speeds below about 500 r.p.m (fixed by the designer), percentage slip is 100

which means clutch is fully disengaged. As the engine speed increases further to about 1000

r.p.m., the percentage slip falls rapidly to about 10, beyond which the slip decreases gradually to

a small value of about 2 per cent at about 3000 r.p.m. As percentage slip represents definite loss

of energy and consequently increased fuel consumption, the engine should not be allowed to run

at a speed between approximately 500 and 1000 r.p.m. This condition is similar to a slipping

clutch in case of ordinary friction clutches.

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Advantages

1. No wear on moving parts.

2. No adjustment to be made.

3. No maintenance necessary except oil level.

4. Simple design.

5.No jerk on transmission when the gear engages. It damps all shocks and strains incident

with connecting a revolving engine to transmission.

6. No skill required for operating it.

7.Car can stop in gear and move off also by pressing accelerator pedal only.

Disadvantages

The only disadvantage of the fluid flywheel is that there is a drag on the gear box-shaft even

when the percentage slip is 100. This makes the gear changing difficult with the ordinary

crash type gear box. Hence the fluid flywheel is generally used with epicyclic gear box

which avoids this difficulty.

FLUID FLYWHEEL TROUBLE SHOOTING

The faults experienced in the case of fluid flywheel are not many. In the absence of many

mechanical components, the maintenance job for fluid flywheel is much easier as compared with

ordinary friction clutches. The major faults that occur in flywheel are:

1. Large Slip

As is clear from the characteristics of a fluid flywheel, some slip always exists. But

sometimes it may become excessive due to either the shortage of fluid or the fluid in the

flywheel not being of proper viscosity.

2. Drag

If appreciable drag is experienced in the flywheel when the engine is idling it may be only

due to wrong grade of fluid.

3. Vibration

The vibration in the fluid flywheel may be caused due to upsetting of the balance of the

rotors. The unbalance may be due to reasons such as nut being changed on the bolts, oil filler

plug being changed over, etc.

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A fluid coupling is a hydrodynamic device used to transmit rotating mechanical power. It has been used in automobile transmissions as an alternative to a mechanical clutch. It also has widespread application in marine and industrial machine drives, where variable speed operation and/or controlled start-up without shock loading of the power transmission system is essential.

A fluid coupling consists of three components, plus the hydraulic fluid:

• The housing, also known as the shell (which must have an oil tight seal around the drive shafts), contains the fluid and turbines.

• Two turbines (fan like components): o One connected to the input shaft; known as the pump or impellor, primary wheel

input turbine o The other connected to the output shaft, known as the turbine, output turbine,

secondary wheel or runner

The driving turbine, known as the 'pump', (or driving torus)is rotated by the prime mover, which is typically an internal combustion engine or electric motor. The impellor's motion imparts both outwards linear and rotational motion to the fluid.

The hydraulic fluid is directed by the 'pump' whose shape forces the flow in the direction of the 'output turbine' (or driven torus). Here, any difference in the angular velocities of ‘input stage’ and ‘output stage’ results in a net force on the 'output turbine' causing a torque; thus causing it to rotate in the same direction as the pump.

The motion of the fluid is effectively toroidal - travelling in one direction on paths that can be visualized as being on the surface of a torus:

• If there is a difference between input and output angular velocities the motion has a component which is circular (i.e. round the rings formed by sections of the torus)

• If the input and output stages have identical angular velocities there is no net centripetal force - and the motion of the fluid is circular and co-axial with the axis of rotation (i.e. round the edges of a torus), there is no flow of fluid from one turbine to the other.

Stall speed

An important characteristic of a fluid coupling is its stall speed. The stall speed is defined as the highest speed at which the pump can turn when the output turbine is locked and maximum input power is applied. Under stall conditions all of the engine's power would be dissipated in the fluid coupling as heat, possibly leading to damage.

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Slip

A fluid coupling cannot develop output torque when the input and output angular velocities are identical. Hence a fluid coupling cannot achieve 100 percent power transmission efficiency. Due to slippage that will occur in any fluid coupling under load, some power will always be lost in fluid friction and turbulence, and dissipated as heat.

The very best efficiency a fluid coupling can achieve is 94%, that is for every 100 revolutions input, there will be 94 revolutions output. Like other fluid dynamical devices, its efficiency tends to increase gradually with increasing scale, as measured by the Reynolds number.

Hydraulic fluid

As a fluid coupling operates kinetically, low viscosity fluids are preferred Generally speaking, multi-grade motor oils or automatic transmission fluids are used. Increasing density of the fluid increases the amount of torque that can be transmitted at a given input speed.

One-Way Clutch A one-way clutch (also known as a "sprag" clutch) is a device that will allow a component such

as ring gear to turn freely in one direction but not in the other. This effect is just like that of a

bicycle, where the pedals will turn the wheel when pedaling forward, but will spin free when

pedaling backward.

A common place where a one-way clutch is used is in first gear when the shifter is in the drive

position. When you begin to accelerate from a stop, the transmission starts out in first gear. But

have you ever noticed what happens if you release the gas while it is still in first gear? The

vehicle continues to coast as if you were in neutral. Now, shift into Low gear instead of Drive.

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When you let go of the gas in this case, you will feel the engine slow you down just like a

standard shift car. The reason for this is that in Drive, a one-way clutch is used whereas in Low,

a clutch pack or a band is used.

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Functions of Transmissions

The main functions which are performed by the transmissions are:

1. The torque produced by engine varies with speed only with narrow limiits. But under practical considerations running of automobile demands a large variation of torque available at the road wheels. Hence the main purpose of the transmission is to provide a means to vary the torque ratio b/w the engine and the road wheels as required.

2. The transmission also provides a neutral position so that the engine and the road wheels are disconnected even when the clutch is in engaged position.

3. A means to back the car by reversing the direction of rotation of the drive is also provided by transmission.

Necessity of Transmission:

The question as to how far is the transmission necessary in a vehicle may be answered by

considering:

(a) Variation of resistance to the vehicle motion at various speeds.

(b) Variation of tractive effort of the vehicle available at various speeds.

Total Resistance to the vehicle motion It consists of :

(i) Resistance due to wind-This is taken to be proportional to the square of the

vehicle speed.

(ii) Resistance due to gradient-This remains constant at all speeds. This is the

component of the vehicle weight parallel to the plane of the road.

(iii) Miscellaneous-Apart from the above two types, various other factors also contribute

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towards the vehicles resistance. These are: type of the road, tire friction, etc. This

may also be taken approximately to remain constant with the speed.

The total resistance is for a particular type of road, therefore, may be represented as shown in

Fig. 4.1.

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The total resistance for same type of road with different gradients may then be represented by

curves shown in Fig. 4.2. The higher curve represents steeper gradients.

Tractive Effort

The curves 1, 2 and 3 respectively in Fig. 4.3 represent the tractive effort in first, second and

top gears respectively.

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Transmission Necessity

By now we understand the variation of total resistance to the vehicle motion and the tractive

effort of the vehicle with speed. It is obvious that whenever the tractive effort exceeds the total

resistance, the vehicle will accelerate to a speed where tractive effort becomes equal to the total

resistance.

For further clarification, consider Fig. 4.4. This is obtained by superimposing Fig. 4.2 on Fig

4.3. Let the vehicle be in the top gear and suppose the vehicle is travelling on a gradient which

gives total resistance curve I. Then from Fig. 4.4, it is seen that OA is the stabilizing speed. If

the speed at any instant is less, say, OB, the excess of tractive effort will accelerate it to speed

OA. Similarly if the speed at any instant is OC, the excess of resistance will decelerate it to OA.

Now let the vehicle, go on next gradient of curve II. In this case it is noticed that the

stabilizing- speed has decreased. Next consider further the curve Ill. At this gradient, we see that

nowhere does the curve 3 cross curve III. Therefore the vehicle will not be able to go at this

gradient in the top gear. However, if we pass on to second gear, we get a stabilizing speed OD.

Similarly in second gear also the vehicle will not be running on gradient IV for which we shall

have to shift to first gear.

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Again at start more acceleration is needed to gain speed quickly. This can best be done in first

gear because in this gear the maximum tractive effort is available for acceleration. However,

when the necessary speed has been obtained, we may shift into higher gears, because then the

vehicle speed has to be simply maintained and no acceleration is required.

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Necessity of Gearbox:

In addition to many advantages of internal combustion engine , such as high power to weight ratio, relatively good efficiency and relatively compact energy storage it has 3 fundamental disadvantages

1. Unlike steam engines or electic motors the combustion engine is incapable of producing torque from the rest.

2. An IC engine can produce maximum power at a certain engine speed.

3. The efficiency of the engine, ie . its fuel consumption is verymuch dependent on the operating point in the engines performance map.

With the maximum available engine power Pmax and road speed v the ideal traction hyperbola Fz,Aid and the effective traction hyperbola Fz,Ae can be calculated as follows

75

The whole shaded area in the figure cannot be used without ana dditional output converter. The output converter must convert the charecteristic of the IC engine in such a way that it approximates as closely as possoble to the ideal traction hyperbola.

76

Calculation of gear ratios for vehicles:

The powertrain has to offer ratios between engine speed and road wheel speed enabling the vehicle to

1. Move off under difficult conditions

2. Reach the rerquired maximum speed

3. Operate in the fuel efficient ranges of the engine performance map.

Overall Gear ratio;

The overall gear ratio of the transmission , often reffered to as the range of ratios is the ratio

between the largest and the smallest ratio.

i G, tot = i G , max = i1 with gears n=1 upto z

i G ,min iz

The overall gear ratio depends on:

77

• The specific power output of the vehicle

• The overall gear ratio of the engine

• The intended use.

Selecting the largest powertrain gear ratio

when a=0m/s2

78

Final Ratio:

Selecting the intermediate gears:

79

Sliding Mesh Type of gearbox

80

This is the simplest type of gear box. Fig 4.6 gives a simplified view of the gear box. The

power comes from the engine to the clutch shaft and thence to the clutch gear which is always in

mesh with a gear on the lay shaft. All the gears on the lay shaft are fixed to it and as such they

are all the time rotating when the engine is running and the clutch is engaged. Three direct and

one reverse speeds are attained on suitably moving the gear on the main shaft by means of

selector mechanism. These various positions are shown in Fig. 4.7

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Constant Mesh Gearbox

In this type of gear box, all the gears are in constant mesh with the corresponding gears on

the lay shaft. The gears on the main shaft which is splined are free (Fig. 4.9). The dog clutches

are provided which are free to slide on the main shaft. The gears on the lay shaft are, however,

fixed.

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When the left dog clutch is slid to the left by means of the selector mechanism, its teeth are

engaged with those on the clutch gear and we get (the direct gear. The same dog clutch,

however, when slid to right makes contact with the second gear and second gear is obtained.

Similarly movement of the right dog clutch to the left results in low gear and towards right in

reverse gear.

Double Declutching

In the constant mesh box, for the smooth engagement of the dog clutches it is necessary that

the speed of main shaft gear and the sliding dog must be equal. Therefore to obtain lower gear,

the speed of the clutch shaft, lay shaft and main shaft gear must be increased. This is done by

double declutching. The procedure for double declutching is as given below:

The clutch is disengaged and the gear is brought to neutral. Then the clutch is engaged and

accelerator pedal pressed to increase the speed of the main shaft gears. After this the clutch is

again disengaged and the gear moved to the required lower gear and the clutch is again engaged.

As the clutch is disengaged twice in this process, it is called double declutching.

For changing to higher gear, however, reverse effect is desired i.e., the driver has to wait with

the gear in neutral till the main shaft speed is decreased sufficiently for a smooth engagement of

the gear.

Advantages

Compared to the sliding mesh type, the constant mesh gear box has the following

advantages:

I. As the gears have to remain always in mesh, it is no longer necessary to use straight spur

gears. Instead, helical gears are used which are quieter running.

2. Wear of dog teeth on account of engaging and disengaging is reduced because here all the

teeth of the dog clutches are involved compared to only two or three teeth in the case of sliding

gears.

83

Synchromesh Gearbox

85

This type of gear box is similar to the constant mesh type in that all the gears on the main

shaft are in constant mesh with the corresponding gears on the lay shaft. The gear on the layshaft

is fixed to it while those on the main shaft are free to rotate on the same. Its working is also

similar to the constant mesh type, but in the former there is one definite improvement over the

latter. This is the provision of synchromesh device which avoids the necessity of double

declutching. The parts which ultimately are to be engaged are first brought into frictional contact

which equalizes their speed, after which these may be engaged smoothly.

Fig. 4.10 shows the construction and working of a synchromesh gear box. In most of the cars,

however, the synchromesh devices are not fitted to all the gears as is shown in this figure. They

are filled only on the high gears and on the low and reverse gears ordinary dog clutches are only

provided. This is done to reduce the cost.

In Fig. 4.10 A is the engine shaft, Gears B, C, D, E are free on the main shaft and are always

in mesh with corresponding gears on the lay shaft. Thus all the gears on main shaft as well as on

lay shaft continue to rotate so long as shaft A is rotating. Members F1 and F2 are free to slide on

splines on the main shaft. G1 and G2 are ring shaped members having internal teeth fit onto the

external teeth members F1 and F2 respectively. K1 and K2 are dog teeth on B and D respectively

and these also fit onto the teeth of G1 and G2. S1and S2 are the forks. T. and T2 are the balls

supported by springs. These tend to prevent the sliding of members G1(G2) on F1(F2). However,

when the force applied on G1(G2) through fork SI(S2) exceeds a certain value, the balls are

overcome and member G1(G2) slides over F1(F2). There are usually six of these balls

symmetrically placed circumferentially in one synchromesh device. M1, M2, NJ, Nz, PI, P2, Rt. R2

are the frictional surfaces.

To understand the working of this gear pox, consider Fig. 4.11 which shows in steps how the

gears are engaged. For direct gear, member G1 and hence member F1 (through spring-loaded

balls) is slid towards left till cones M1 and M2 rub and friction makes their speed equal [Fig. 4.

ll(a)]. Further pushing the member G1 to left causes it to override the balls and get engaged with

dogs Kl [Fig. 4.11(b)]. Now the drive to the main shaft is direct from B via F1 and the splines.

However, if member G. is pushed too quickly so that there is not sufficient time for

86

synchronization of speeds, a clash may result. Likewise defect will arise in case springs

supporting the balls T F have become weak.

Similarly for second gear the members F1 and G1 are slid to the right so that finally the

internal teeth on G1are engaged with L\. Then the drive to main shaft will be from B via U1. U2,

C, F1 and splines.

For first gear. G2 and F2 are moved towards left. The drive will be from B via UJ, U3, D. F2

and splines to the main shaft.

For reverse. G2 and F2 are slid towards right. In this case the drive will be from B via. UJ,

U4. Us. E. F2 and splines to the main shaft.

In this type of gear box it is very necessary for the smooth operation that sufficient time is

allowed for the equalization of the speeds before the gears are finally brought into mesh. To help

in this special modifications have been employed in many gear boxes. One such modified

synchromesh device is shown in Fig. 4.12. A synchronizer ring is provided between the dog

teeth K1 and member F1. To push this synchronizer ring in the desired direction. Three guide

bars equally spaced along the circumference are provided. These are retained in place by means

of circlips. The synchronizer ring has dog teeth at its outer circumference and is cut at three

places to provide space for the guide bars. The width of each cut is equal to the width of the

guide bar plus half the pitch of the teeth on the synchronizer ring.

When the gear is to be engaged. fork S1 slides F1 to left, pushing synchronizer ring also

along till the inclined friction surface on the inside of the ring Comes into contact with the

corresponding friction surface of the gear. Till the speeds of the two mating surfaces have not

equalized, the guide bars would be contacting one side of the corresponding cuts in the

synchronizer ring as shown in Fig. 4.12 (a). In this position GI cannot move further. However.

as the speeds are equalized. the guides bars became cenQ'a1 in the cuts and the member G\ can

be pushed further, overriding the spring-loaded balls as explained earlier so as to engage the

gear. This position has been shown in Fig. 4.12 (b).

87

Auxiliary Transmissions:

Auxiliary transmissions are mounted on the rear of the regular transmission to provide more gear ratios. Most auxiliary transmissions have only a L-low and a H-high (direct) range in a transfer assembly. The low range provides an extremely low gear ratio for hard pulls. At all other times, the high range should be used. Gears are shifted by a separate gearshift lever in the driver’s cabin.

88

Compound Transmission:

(To be taught during automatic transmissions class)

89

Transfer box:

This is also called 'transfer case' and is suspended from the chassis cross members behind the

transmission (gear box), in four wheel drive vehicles. The simplified construction and working

of the transfer box has been made clear by means of Fig. 4.24. The transfer box shown therein

enables the driver to (i) drive in two wheel drive on highways or shift to four-wheel drive for

cross-country operation (ii) to drive in high gear or low gear as required. Obviously low gear is

90

used for cross-country driving.

The input shaft is connected to the gear pox and carries on it a member having axial teeth.

Two input shaft gears are free to rotate on the shaft. Each of these gears have bosses on the side

which have axial teeth of the same pitch as the central member on the input shaft. Depending

upon the movement of the transfer box gear lever, the central member and thereby the input

shaft may be connected either to the small gear or to the big gear. There are two output shafts,

one going to the front axle and the second going to the rear axle. The front output shaft is

smaller in diameter and is supported inside the rear output shaft which is directly connected to

the output gear. The front output shaft has fitted on it a shifter mechanism and also has splines

over a small length of it, which when engage with the corresponding internal splines on the rear

output shaft, connect the two shafts rotationally with each other.

When the shifter mechanism A is at the centre so that no gear is connected to the input shaft,

the drive is in neutral as shown Fig. 4.24 (a). Fig 4.24 (b) shows a position when the shifter

mechanism A connects the input shaft with the big input gear, but the shifter mechanism B

disconnects the front output shaft from the rear output shaft. In this position, two-wheel drive

with the high gear is obtained. In the same way Fig. 4.24 (c) depicts the situation with four

wheel drive in low gear.

Obviously, four-wheel drive with low gear should be used invariably with the low gears on

the main transmission. Also, the transfer box gears should be engaged with the vehicle stationary

since these are not provided with synchromesh devices.

91

Problems:

102

Epicyclic Transmission Unit 4

Principle of operation

An Epicyclic gear box consists of two, three or even four Epicyclic or planetary gear sets. A

simple gear set has a sun gear, about which planets turn round. These planet gears are carried by

a carrier and a shaft and are also in mesh internally with a ring gear, which is also called annulus

or internal gear sometimes.

Different torque ratios i.e. speed ratios are obtained by making anyone of the parts, viz. the sun

gear, the planets and the annulus stationary. Similarly by locking two parts with each other, a

solid drive i.e. direct gear is obtained.

103

Types of Planetary Transmission

Simple Epicyclic Gear Train

An epicyclic single-stage gear train consists of an internally toothed annular (ring) A with a

band brake encircling it. In the centre of this gear is sun gear S, which forms part of the

input shaft. The sun gear and the annular gear are connected by a number of planet (pinion)

gears P which are mounted on a carrier C and is integral with the output shaft. For

transmission of torque, either the sun gear, the carrier, or the annular gear must be held

stationary.

The situation is considered when only the annular gear is stationary. When the input sun

gear shaft is driven keeping the annular gear band brake fixed, the planet gears

simultaneously rotate around their axes and revolve around the input sun gear axis along

the inner circumference of the annular gear. Consequently, the carrier and the output shaft,

which support the planet-gear axes, also rotate, but slower than the input shaft.

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Let, TA = number of teeth on annular, internal or ring gear,

Ts = number of teeth on sun or centre gear,

Tp = number of teeth on planet gear,

and Tc = number of effective teeth on arm or planet carrier.

Also TA = Ts + 2 Tp and Tc = Ts + TA.

First Gear Ratio: The annular gear is held stationary and the planet carrier is driven by

the power supplied to the sun gear.

Gear Ratio= Speed of the driving shaft = Teeth on the driven gear

Speed of the driven shaft = Teeth on the driving gear

= Teeth on planet carrier = Tc= Ts + TA = 1 + TA

Teeth on sun gear Ts Ts Ts

Second Gear Ratio. The sun gear is held stationary. The planet carrier is driven

member and the annular gear is the driving member.

Gear Ratio= Teeth on the driven gear =Teeth on planet carrier.

Teeth of the driving gears= Teeth on the planet carriers

Tc = Ts + TA = 1 + Ts

TA TA TA

Reverse Gear ratio:

Here the planet carrier is held stationary the annular gear is driven by the sun gear to

which the power is applied.

Reverse gear ratio = Teeth on the driven gear= Teeth on the annular gear =TA

Teeth on the driving gear= Teeth on the sun gear = TS

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Wilson Gearbox;

This type of gear box consists of a number of simple epicyclic gear sets compounded together. A

four forward and one reverse· speed epicyclic gearing used in Wilson gear box is shown in Fig. A

is the input shaft connected directly to the engine crankshaft, while R is the output shaft

Coupled with the propeller shaft through universal joints. C is the multi plate clutch. There are

four epicyclic gear trains I, 2, 3 and 4, interconnected as shown. Various gear ratios are obtained

as follows.

Direct gear. This is obtained by locking Sl to A by applying the clutch C. In this position we get

a 'solid' drive and direct gear is obtained.

Third gear. For third gear, S1 is held stationary by means of brake B1. In this position, arm A1 is

coupled to ring R2 and arm A2 is coupled to ring R1.

Second gear. To obtain

second gear, brake B2 is

applied to keep the ring R2

stationary. The sun gear S2

is already fixed to the engine

shaft A. Arm A2 is also coupled to

the ring R1.

First gear. Brake B3 is applied to

obtain the low gear.

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Reverse gear. For reverse gear, the brake B4 is applied which holds the ring R4 stationary.

This gear box may be of the preselected type i.e. fitted with a special mechanism which enables

the driver to select the suitable gear beforehand. A separate lever is there on steering column,

which moves in a sector in a plane parallel to the plane of the wheel. On the wheel are marked

the corresponding positions of various gears.

Whenever we have to shift into the next higher or next lower gear we can preselect it. This can

be done by bringing the lever to the desired position. After that when we actually want to engage

the desired gear, the only thing to do is to press the gear change pedal and the desired gear will

be engaged.

Compound Epicyclic Gear Train;

A simple epicyclic gear train presented above cannot provide adequate velocity ratios.

Therefore, a compound epicyclic gear train is used in a gearbox to give higher velocity

ratios and to allow several ratios to be obtained. A compound epicyclic gear train is

obtained by joining together all the arms of simple gear train; of course the compounding

can be made by different methods. In these trains the members, which become fixed when

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the trains are in use, are arranged to be free. The brakes are provided to bring any of these

members to rest as and when required. The train to which that member belongs then come

into operation and if that member is released the train becomes non-operational. Generally

some of the wheels are common to all the epicyclic trains.

For only small degrees of overdrive (under-gearing), for example 0.82:1 (22%), the

simple epicyclic gearing requires a relatively large diameter annulus ring gear; about 175

mm, to provide sufficiently large gear teeth for adequate strength. To reduce the diameter

of the annulus ring gear for a similar degree of overdrive, a compound epicyclic gear train

can be used, which incorporates double pinion gears on each carrier pin. This reduces the

annulus diameter to about 100 mm and the number of annulus teeth to 60 only as compared

to the 96 annulus teeth in the simple epicyclic gear train.

1.

2.

3. (Large pinion (Small pinion (annulus ring gear)

gear) gear)

1. Algebraic Method:

Algebraic method of determining velocity ratios is most suitable in the case of

simple compound epicyclic gear trains. To apply this method to the simple gear

train it is convenient to consider the velocity of gear relative to that of the arm or

planet carrier as the arm ca imagined fixed.

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Refer Fig.

Thus the speed of the sun relative to arm =NS-NC

The speed of the planet wheel relative to arm =NP-NC

The speed of the annular or internal gear relative to arm =NA-NC

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Over drives:

Overdrive is a device interposed between the transmission and propeller shaft to permit the propeller shaft to turn faster than, or overdrive, the transmission ratio shaft. It is so called because it provides a speed ratio over that of the high speed radio. The overdrive permits the engine to operate only about 70% of the propeller shaft speed. When the vehicle is operating in the high speed ranges, which in turn extends the engine life, improve the fuel consumption and reduces vibration and noises. The overdrive is essentially 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 desired. Generally an overdrive is fitted to the top gear only, but some sports cars have an overdrive on second, third and top gear, giving seven forward speeds. Overdrive is usually employed to supplement conventional transmission. It is bolted to the rear of the transmission between the transmission and the propeller shaft a slightly higher rear-axle gear ratio is employed with an overdrive than without one.

The overdrive includes two essential devices, a freewheeling mechanism and a planetary gear set these are also explained in the following articles.

Overdrive Construction:

It consists of the following parts

1. A set of planetary gear

2. A solenoid and planetary gear arrangement for locking the sun gear

3. A rail and fork assembly linked to dash control knob for disconnecting the overdrive when not in use.

4. A free wheel assembly or over running clutch that drives the main shaft below the cut in speed.

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The planetary gears are used to increase speed by arranging to have the ring gear driven by the planet-pinion cage when the sun gear is locked. Because the increase in speed of the main shaft decreases the power available to drive the wheels, the overdrive ratio can be used only when the engine is running fast enough to develop enough torque to offset this handicap. The maximum speed at which the engine can do this is called cut in speed. Below this speed, the drive is made direct by unlocking the sun gear. The ring gear is splined to the outer case of the freewheel assembly, which is a part of the overdrive main shaft. When the pawl is not engaged in the gear plate, the sun gear is unlocked and the planetary gears cannot transmit power. Then the unit is in direct drive. In this case, the power flows from transmission main shaft to the freewheel assembly and then to the overdrive main shaft. Overdrive operation:

If the driver wants to go into overdrive, when the car is travelling above a pre-determined cut-in speed (usually 35 to 55 km/h), momentarily releases the accelerator pedal. If the driver wants to come out of the overdrive, he merely pushes the accelerator pedal past the full throttle position. If the driver wants to lock out of the overdrive, he pulls a control knob on the car dash. .

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Overdrive Electric

Controls:

Fig. Shows a wiring circuit of electric control system used with the overdrive. When the driver wants to go into overdrive, he pushes in the control knob on the dash. When the car reaches out-in speed, the governor closes its contacts to connect the overdrive relay winding to the battery. The overdrive relay in turn, closes its contacts to connect solenoid to the battery. Now the overdrive is ready to go into action. When the driver momentarily releases the accelerator pedal, the solenoid sends the pawl into a notch in the run gear control plate. This puts the transmission into overdrive. When the driver wants to come out of overdrive, he pushes the accelerator pedal past the full throttle position. It causes the upper contacts of the kick down switch to open and the lower contacts to close. The opening of the upper contacts causes to open the overdrive relay circuit The overdrive relay, therefore, opens its' contacts to open the solenoid circuit. Also, closing the lower contacts in the kick-down switch causes to ground the ignition. With, this interruption of ignition system action, the engine stops delivering power and begins to slow down. With this action, the thrust on the solenoid pawl is relieved, and the spring pressure pulls the pawl out of the notch in the sun gear control plate. It causes to underground the ignition coil and thereby permit the ignition system to function again. This series of actions takes place so quickly that no appreciable lag is noticeable in power delivery.

The overdrive electric control serves the following purposes: 1. It energizes the solenoid as the car reaches cut in speed. 2. It disconnects the ignition circuit momentarily.

3. It opens the solenoid circuit when the-Kick:-down switch is close as the driver wants to come out of overdrive.

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Freewheel Unit: Freewheel units is also known as overrunning clutch, sprang clutch or one-way clutch. It is an essential part of every overdrive. It transmits power in one direction 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.

Construction. A flywheel unit consists of a hub and an outer race. The hub has internal splines to connect it to the transmission main shaft. The outer 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.

Working. When the hub is driven in the clockwise direction, as shown in Fig. 24.4, the rollers ride up the cams, and by their wedging action, they force the outer race of follow the hub. Thus the outer race moves in the same direction and at the same speed as the hub. When the hub speed slows down, and the outer race is still moving faster than the hub, the rollers move down the cams, releasing the outer race from the hub. Thus the outer race moves independent of the hub and the unit acts like a roller bearing.

The transmission main shaft is connected to the hub and the output shaft is connected to the outer race. Thus the freewheel unit can transmit power only from the main shaft to the output shaft. PLANETARY GEAR SYSTEM

The planetary gear set is another essential part of the overdrive. It is also used in automatic transmission. A planetary gear system consists of three types of gears:

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1. An outer ring gear having inside teeth. It is sometimes called internal gear. 2. Three planet pinions held on pinion shafts in a cage or carrier. 3. A sun gear at the centre of the three planet pinions.

The planet pinions mesh with the ring gear internally and with the sun gear externally. The planet gear system gets its name from the fact that the pinion revolve around the sun gear and rotate at the same time, just as the planets in the solar system rotate and revolve around the sun.

The planetary system, when used in the overdrive, works as follows. The ring gear is attached to the output shaft. The three planet pinions are assembled into a cage that is splined to the transmission main shaft. The sun gear has an arrangement where by it may be permitted to turn, or it may be locked in a stationary position. When it is locked, the ring gear and hence the output shaft is forced to turn faster than the transmission main shaft. That is, the output shaft overdrives the transmission main shaft.

A planetary gear system can be used for various functions by holding one of the three members (ring gears, sun gear and planet pinion cage) stationary and turning another member. The various combinations are as follows:

1. Speed increase. Hold the sun gear stationary and turn the planet pinion cage, the ring gear will rotate faster than the planet pinion cage. The ratio between the planet pinion cage and the ring gear depends upon the sizes of the different gears.

2. Speed increase. Another combination to get increased speed is to hold the ring gear stationary and turn the planet pinion cage. The sun gear will rotate faster than the cage.

3. Speed reduction. Hold the sun gear stationary and turn the ring gear. The planet gear case will turn more slowly than the ring gear.

4. Speed reduction. Hold the ring gear stationary and turn the sun gear. The planet pinion cage will rotate at a speed less than the sun gear speed.

5. Reverse. Hold the planet pinion cage stationary and turn the ring gear. In this case, the planet pinions act as idlers and they cause the sun gear to turn in the reverse direction to the ring gear rotation. Thus, the system functions as reverse rotation system. with the sun gear turning faster than the ring gear.

6. Reverse. Hold the planet pinion cage stationary and turn the sun gear. The ring gear will turn in a reverse direction, but slower than the sun gear.

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7. Direct drive. If any two of the three

members are locked

together, then the

entire planetary

gear system is locked out, and the input shaft and output shaft must turn at the same speeds. On the other hands if no member is held stationary and no two members are locked together then the system will not transmit power at all. The input shaft may turn, but the output shaft does not.

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A Short Course on Automatic Transmissions

The modern automatic transmission is by far,

the most complicated mechanical component

in today's automobile. Automatic transmissions

contain mechanical systems, hydraulic

systems, electrical systems and computer

controls, all working together in perfect

harmony which goes virtually unnoticed until

there is a problem. This article will help you

understand the concepts behind what goes on

inside these technological marvels and what

goes into repairing them when they fail.

This article is broken down into five sections:

• What is a transmission breaks down in the simplest terms what the purpose of a transmission is.

• Transmission Components describes the general principals behind each system in simple terms to help you understand

how an automatic transmission works.

• Spotting problems before they get worse shows what to look for to prevent a minor problem from becoming major.

• Maintenance talks about preventative maintenance that everyone should know about.

• Transmission repairs describes the types of repairs that are typically performed on transmissions from minor

adjustments to complete overhauls.

What is a transmission?

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The transmission is a device that is connected to the back of the engine and sends the power from the engine to the drive wheels.

An automobile engine runs at its best at a certain RPM (Revolutions Per Minute) range and it is the transmission's job to make sure

that the power is delivered to the wheels while keeping the engine within that range. It does this through various gear combinations.

In first gear, the engine turns much faster in relation to the drive wheels, while in high gear the engine is loafing even though the car

may be going in excess of 70 MPH. In addition to the various forward gears, a transmission also has a neutral position which

disconnects the engine from the drive wheels, and reverse, which causes the drive wheels to turn in the opposite direction allowing

you to back up. Finally, there is the Park position. In this position, a latch mechanism (not unlike a deadbolt lock on a door) is

inserted into a slot in the output shaft to lock the drive wheels and keep them from turning, thereby preventing the vehicle from

rolling.

There are two basic types of automatic transmissions based on whether the vehicle is rear wheel drive or front wheel drive.

On a rear wheel drive car, the transmission is

usually mounted to the back of the engine and

is located under the hump in the center of the

floorboard alongside the gas pedal position. A

drive shaft connects the rear of the

transmission to the final drive which is located

in the rear axle and is used to send power to

the rear wheels. Power flow on this system is

simple and straight forward going from the

engine, through the torque converter, then through the transmission and drive shaft until it reaches the final drive where it is split and

sent to the two rear wheels.

On a front wheel drive car, the transmission

is usually combined with the final drive to form

what is called a transaxle. The engine on a

front wheel drive car is usually mounted

sideways in the car with the transaxle tucked

under it on the side of the engine facing the

rear of the car. Front axles are connected

directly to the transaxle and provide power to

the front wheels. In this example, power flows

from the engine, through the torque converter to a large chain that sends the power through a 180 degree turn to the transmission

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that is along side the engine. From there, the power is routed through the transmission to the final drive where it is split and sent to

the two front wheels through the drive axles.

There are a number of other arrangements including front drive vehicles where the engine is mounted front to back instead of

sideways and there are other systems that drive all four wheels but the two systems described here are by far the most popular. A

much less popular rear drive arrangement has the transmission mounted directly to the final drive at the rear and is connected by a

drive shaft to the torque converter which is still mounted on the engine. This system is found on the new Corvette and is used in

order to balance the weight evenly between the front and rear wheels for improved performance and handling. Another rear drive

system mounts everything, the engine, transmission and final drive in the rear. This rear engine arrangement is popular on the

Porsche.

Transmission Components

The modern automatic transmission consists of many components and systems that are designed to work together

in a symphony of clever mechanical, hydraulic and electrical technology that has evolved over the years into what many

mechanically inclined individuals consider to be an art form. We try to use simple, generic explanations where possible to describe

these systems but, due to the complexity of some of these components, you may have to use some mental gymnastics to visualize

their operation.

The main components that make up an automatic transmission include:

• Planetary Gear Sets which are the mechanical systems that provide the various forward gear ratios as well as reverse.

• The Hydraulic System which uses a special transmission fluid sent under pressure by an Oil Pump through the Valve

Body to control the Clutches and the Bands in order to control the planetary gear sets.

• Seals and Gaskets are used to keep the oil where it is supposed to be and prevent it from leaking out.

• The Torque Converter which acts like a clutch to allow the vehicle to come to a stop in gear while the engine is still

running.

• The Governor and the Modulator or Throttle Cable that monitor speed and throttle position in order to determine when

to shift.

• On newer vehicles, shift points are controlled by Computer which directs electrical solenoids to shift oil flow to the

appropriate component at the right instant.

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Planetary Gear Sets

Automatic transmissions contain many gears in various combinations. In a manual

transmission, gears slide along shafts as you move the shift lever from one position

to another, engaging various sized gears as required in order to provide the correct

gear ratio. In an automatic transmission, however, the gears are never physically

moved and are always engaged to the same gears. This is accomplished through

the use of planetary gear sets.

The basic planetary gear set consists of a sun gear, a ring gear and two or more

planet gears, all remaining in constant mesh. The planet gears are connected to

each other through a common carrier which allows the gears to spin on shafts

called "pinions" which are attached to the carrier .

One example of a way that this system can be used is by connecting the ring gear to the input shaft coming from the engine,

connecting the planet carrier to the output shaft, and locking the sun gear so that it can't move. In this scenario, when we turn the

ring gear, the planets will "walk" along the sun gear (which is held stationary) causing the planet carrier to turn the output shaft in the

same direction as the input shaft but at a slower speed causing gear reduction (similar to a car in first gear).

If we unlock the sun gear and lock any two elements together, this will cause all three elements to turn at the same speed so that

the output shaft will turn at the same rate of speed as the input shaft. This is like a car that is in third or high gear. Another way that

we can use a Planetary gear set is by locking the planet carrier from moving, then applying power to the ring gear which will cause

the sun gear to turn in the opposite direction giving us reverse gear.

The illustration on the right shows how the simple system described

above would look in an actual transmission. The input shaft is

connected to the ring gear (Blue), The Output shaft is connected to

the planet carrier (Green) which is also connected to a "Multi-disk"

clutch pack. The sun gear is connected to a drum (yellow) which is

also connected to the other half of the clutch pack. Surrounding the

outside of the drum is a band (red) that can be tightened around the

drum when required to prevent the drum with the attached sun gear

from turning.

The clutch pack is used, in this instance, to lock the planet carrier with the sun gear forcing both to turn at the same speed. If both

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the clutch pack and the band were released, the system would be in neutral. Turning the input shaft would turn the planet gears

against the sun gear, but since nothing is holding the sun gear, it will just spin free and have no effect on the output shaft. To place

the unit in first gear, the band is applied to hold the sun gear from moving. To shift from first to high gear, the band is released and

the clutch is applied causing the output shaft to turn at the same speed as the input shaft.

Many more combinations are possible using

two or more planetary sets connected in

various ways to provide the different forward

speeds and reverse that are found in modern

automatic transmissions.

Some of the clever gear arrangements found in

four and now, five, six and even seven and

eight-speed automatics are complex enough to

make a technically astute lay person's head

spin trying to understand the flow of power

through the transmission as it shifts from first

gear through top gear while the vehicle

accelerates to highway speed. On modern

vehicles (mid '80s to the present), the vehicle's computer monitors and controls these shifts so that they are almost imperceptible.

Clutch Packs

A clutch pack consists of alternating disks that fit inside a clutch

drum. Half of the disks are steel and have splines that fit into groves

on the inside of the drum. The other half have a friction material

bonded to their surface and have splines on the inside edge that fit

groves on the outer surface of the adjoining hub. There is a piston

inside the drum that is activated by oil pressure at the appropriate

time to squeeze the clutch pack together so that the two

components become locked and turn as one.

One-Way Clutch

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A one-way clutch (also known as a "sprag" clutch) is a device that will allow a component such as ring gear to turn freely in one

direction but not in the other. This effect is just like that of a bicycle, where the pedals will turn the wheel when pedaling forward, but

will spin free when pedaling backward.

A common place where a one-way clutch is used is in first gear when the shifter is in the drive position. When you begin to

accelerate from a stop, the transmission starts out in first gear. But have you ever noticed what happens if you release the gas while

it is still in first gear? The vehicle continues to coast as if you were in neutral. Now, shift into Low gear instead of Drive. When you

let go of the gas in this case, you will feel the engine slow you down just like a standard shift car. The reason for this is that in Drive,

a one-way clutch is used whereas in Low, a clutch pack or a band is used.

Bands

A band is a steel strap with friction material bonded to the inside surface. One end of the

band is anchored against the transmission case while the other end is connected to a

servo. At the appropriate time hydraulic oil is sent to the servo under pressure to tighten

the band around the drum to stop the drum from turning.

Torque Converter

On automatic transmissions, the torque converter takes the place of the clutch

found on standard shift vehicles. It is there to allow the engine to continue

running when the vehicle comes to a stop. The principle behind a torque

converter is like taking a fan that is plugged into the wall and blowing air into

another fan which is unplugged. If you grab the blade on the unplugged fan,

you are able to hold it from turning but as soon as you let go, it will begin to

speed up until it comes close to the speed of the powered fan. The difference

with a torque converter is that instead of using air, it uses oil or transmission fluid, to be more precise.

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A torque converter is a large doughnut shaped device (10" to 15" in diameter) that is mounted between the engine and the

transmission. It consists of three internal elements that work together to transmit power to the transmission. The three elements of

the torque converter are the Pump, the Turbine, and the Stator. The pump is mounted directly to the

converter housing which in turn is bolted directly to the engine's crankshaft and turns at engine speed. The

turbine is inside the housing and is

connected directly to the input shaft of the

transmission providing power to move the

vehicle. The stator is mounted to a one-

way clutch so that it can spin freely in one direction but not in the

other. Each of the three elements have fins mounted in them to

precisely direct the flow of oil through the converter

With the engine running, transmission fluid is pulled into the pump

section and is pushed outward by centrifugal force until it reaches

the turbine section which starts it turning. The fluid continues in a

circular motion back towards the center of the turbine where it enters

the stator. If the turbine is moving considerably slower than the pump, the fluid will make contact with the front of the stator fins

which push the stator into the one way clutch and prevent it from turning. With the stator stopped, the fluid is directed by the stator

fins to re-enter the pump at a "helping" angle providing a torque increase. As the speed of the turbine catches up with the pump,

the fluid starts hitting the stator blades on the back-side causing the

stator to turn in the same direction as the pump and turbine. As the

speed increases, all three elements begin to turn at approximately

the same speed.

Since the '80s, in order to improve fuel economy, torque converters

have been equipped with a lockup clutch (not shown) which locks

the turbine to the pump as the vehicle speed reaches approximately

45 - 50 MPH. This lockup is controlled by computer and usually

won't engage unless the transmission is in 3rd or 4th gear.

Hydraulic System

The Hydraulic system is a complex maze of passages and tubes

that sends transmission fluid under pressure to all parts of the

transmission and torque converter. The diagram at left is a simple

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one from a 3-speed automatic from the '60s. The newer systems are much more complex and are combined with computerized

electrical components. Transmission fluid serves a number of purposes including: shift control, general lubrication and transmission

cooling. Unlike the engine, which uses oil primarily for lubrication, every aspect of a transmission's functions are dependant on a

constant supply of fluid under pressure. This is not unlike the human circulatory system (the fluid is even red) where even a few

minutes of operation when there is a lack of pressure can be harmful or even fatal to the life of the transmission. In order to keep

the transmission at normal operating temperature, a portion of the fluid is sent through one of two steel tubes to a special chamber

that is submerged in anti-freeze in the radiator. Fluid passing through this chamber is cooled and then returned to the transmission

through the other steel tube. A typical transmission has an average of ten quarts of fluid between the transmission, torque

converter, and cooler tank. In fact, most of the components of a transmission are constantly submerged in fluid including the clutch

packs and bands. The friction surfaces on these parts are designed to operate properly only when they are submerged in oil.

Oil Pump

The transmission oil pump (not to be confused with the pump element inside the torque converter) is responsible for producing all the

oil pressure that is required in the transmission. The oil pump is mounted to the front of the transmission case and is directly

connected to a flange on the torque converter housing. Since the torque converter housing is directly connected to the engine

crankshaft, the pump will produce pressure whenever the engine is running as long as there is a sufficient amount of transmission

fluid available. The oil enters the pump through a filter that is located at the bottom of the transmission oil pan and travels up a

pickup tube directly to the oil pump. The oil is then sent, under pressure to the pressure regulator, the valve body and the rest of the

components, as required.

Valve Body

The valve body is the control center

of the automatic transmission. It

contains a maze of channels and

passages that direct hydraulic fluid

to the numerous valves which then

activate the appropriate clutch pack

or band servo to smoothly shift to

the appropriate gear for each driving

situation. Each of the many valves

in the valve body has a specific

purpose and is named for that

function. For example the 2-3 shift

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valve activates the 2nd gear to 3rd gear up-shift or the 3-2 shift timing valve which determines when a downshift should occur.

The most important valve, and the one that you have direct control over is the manual valve. The manual valve is directly connected

to the gear shift handle and covers and uncovers various passages depending on what position the gear shift is placed in. When

you place the gear shift in Drive, for instance, the manual valve directs fluid to the clutch pack(s) that activates 1st gear. it also sets

up to monitor vehicle speed and throttle position so that it can determine the optimal time and the force for the 1 - 2 shift. On

computer controlled transmissions, you will also have electrical solenoids that are mounted in the valve body to direct fluid to the

appropriate clutch packs or bands under computer control to more precisely control shift points.

Computer Controls

The computer uses sensors on the engine and

transmission to detect such things as throttle position,

vehicle speed, engine speed, engine load, brake pedal

position, etc. to control exact shift points as well as how

soft or firm the shift should be. Once the computer

receives this information, it then sends signals to a

solenoid pack inside the transmission. The solenoid

pack contains several electrically controlled solenoids

that redirect the fluid to the appropriate clutch pack or

servo in order to control shifting. Computerized

transmissions even learn your driving style and

constantly adapt to it so that every shift is timed

precisely when you would need it.

Because of computer controls, sports models are coming out with the ability to take manual control of the transmission as though it

were a stick shift, allowing the driver to select gears manually. This is accomplished on some cars by passing the shift lever through

a special gate, then tapping it in one direction or the other in order to up-shift or down-shift at will. The computer monitors this

activity to make sure that the driver does not select a gear that could over speed the engine and damage it.

Another advantage to these "smart" transmissions is that they have a self diagnostic mode which can detect a problem early on and

warn you with an indicator light on the dash. A technician can then plug test equipment in and retrieve a list of trouble codes that

will help pinpoint where the problem is.

Governor, Vacuum Modulator, Throttle Cable

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These three components are important in the non-computerized transmissions. They provide the inputs that tell the transmission

when to shift. The Governor is connected to the output shaft and regulates hydraulic pressure based on vehicle speed. It

accomplishes this using centrifugal force to spin a pair of hinged weights against pull-back springs. As the weights pull further out

against the springs, more oil pressure is allowed past the governor to act on the shift valves that are in the valve body which then

signal the appropriate shifts.

Of course, vehicle speed is not the only thing that controls when a transmission should shift, the load that the engine is under is also

important. The more load you place on the engine, the longer the transmission will hold a gear before shifting to the next one.

There are two types of devices that serve the purpose of monitoring the engine load: the Throttle Cable and the Vacuum

Modulator. A transmission will use one or the other but generally not both of these devices. Each works in a different way to

monitor engine load.

The Throttle Cable simply monitors the position of the gas pedal through a cable that runs from the gas pedal to the throttle valve in

the valve body.

The Vacuum Modulator monitors engine vacuum by a rubber vacuum hose which is connected to the engine. Engine vacuum

reacts very accurately to engine load with high vacuum produced when the engine is under light load and diminishing down to zero

vacuum when the engine is under a heavy load. The modulator is attached to the outside of the transmission case and has a shaft

which passes through the case and attaches to the throttle valve in the valve body. When an engine is under a light load or no load,

high vacuum acts on the modulator which moves the throttle valve in one direction to allow the transmission to shift early and soft.

As the engine load increases, vacuum is diminished which moves the valve in the other direction causing the transmission to shift

later and more firmly.

Seals and Gaskets

An automatic transmission has many seals and gaskets to control the flow of hydraulic fluid and to keep it from leaking out. There

are two main external seals: the front seal and the rear seal. The front seal seals the point where the torque converter mounts to the

transmission case. This seal allows fluid to freely move from the converter to the transmission but keeps the fluid from leaking out.

The rear seal keeps fluid from leaking past the output shaft.

A seal is usually made of rubber (similar to the rubber in a windshield wiper blade) and is used to keep oil from leaking past a

moving part such as a spinning shaft. In some cases, the rubber is assisted by a spring that holds the rubber in close contact with

the spinning shaft.

A gasket is a type of seal used to seal two stationary parts that are fastened together. Some common gasket materials are: paper,

cork, rubber, silicone and soft metal.

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Aside from the main seals, there are also a number of other seals and gaskets that vary from transmission to transmission. A

common example is the rubber O-ring that seals the shaft for the shift control lever. This is the shaft that you move when you

manipulate the gear shifter. Another example that is common to most transmissions is the oil pan gasket. In fact, seals are required

anywhere that a device needs to pass through the transmission case with each one being a potential source for leaks.

Spotting problems before they get worse

• Watch for leaks or stains under the car

If there is a persistent red oil leak that you are sure is coming from your car, you should have your shop check to see if it

is coming from your transmission or possibly from your power steering system (most power steering systems also use

transmission fluid and leaks can appear on the ground in roughly the same areas as transmission leaks.) If all you see is

a few drops on the ground, you may be able to postpone repairs as long as you check your fluid level often (but check

with your technician to be sure.) If transmission fluid levels go down below minimum levels serious transmission damage

can occur (the same advice goes for power steering leaks as well.)

• Check fluid for color and odor

Most manufacturers require that you check transmission fluid levels when the vehicle is running and on level ground. Pull

the transmission dipstick out and check the fluid for color and odor. Transmission fluid is a transparent red oil that looks

something like cherry cough syrup. If the fluid is cloudy or muddy, or it has a burned odor, you should have it checked by

your technician who will most likely advise you to have a transmission drain and refill or transmission tune-up. See the

Maintenance section below for details on this service.

• be sensitive to new noises, vibrations and shift behavior

A modern transmission should shift smoothly and quietly under light acceleration. Heavier acceleration should produce

firmer shifts at higher speeds. If shift points are erratic or you hear noises when shifting, you should have it checked out

immediately. Whining noises coming from the floorboard are also a cause for concern. If caught early, many problems

can be resolved without costly transmission overhauls. Even if you feel that you can't afford repairs at this time, you

should at least have it checked. The technician may be able to give you some hints on what to do and not do to prolong

the transmission life until you can afford the repair.

Maintenance

Transmission fluid should be changed periodically. Your owner's manual should give you the recommended intervals which could

be anywhere from 15,000 miles to 100,000 miles. Most transmission experts recommend changing the fluid and filter every 25,000

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miles.

Few transmissions have drain plugs to drain the old fluid. In order to get the fluid out, the technician removes the transmission oil

pan. This is quite a messy job and generally not recommended for the casual do-it-yourselfer. Even if the transmission has a drain

plug, the only way to also change the transmission filter is to remove the pan. When the pan is down, the technician can check for

metal shavings and other debris which are indicators of impending transmission problems.

In most cases during these transmission services, only about half the oil is able to be removed from the unit. This is because much

of the oil is in the torque converter and cooler lines and cannot be drained without major disassembly. The fluid change intervals

are based on the fact that some old fluid remains in the system.

When the transmission is serviced, make sure that the correct fluid is used to re-fill it. Each transmission manufacturer has their

own recommendation for the proper fluid to use and the internal components are designed for that specific formula. GM usually uses

Dexron, Fords prior to 1983 use Type F while later models use Mercon. Late model Chrysler products use ATF +3 +4 (Not using

the correct fluid for Chrysler transmissions is the most common reason for their transmission problems.) Toyota sometimes uses

Type T which is only available through Toyota and Lexus Parts departments. Honda also specs out their own formula which is

available from Honda or Acura parts departments. A transmission will not work properly or may even slip or shudder with the

incorrect fluid, so make sure that you double check. Your owner's manual will tell you which fluid is required. Naturally, the owner's

manual will try to convince you to only use the manufacturer's branded fluid, but they will also provide you with the specs for the oil.

If the aftermarket product indicates on its container that they meet or exceed the specs for a particular type of transmission fluid, it is

generally ok to use that product.

Transmission repairs.

• Adjustments and In-Car Repairs

There are several problems that can be resolved with an adjustment (A simple adjustment is one that can be made

without removing the transmission from the vehicle.) or minor repair.

If a late model transmission (computer-controlled transmissions started becoming popular in the early '90s) is not shifting

properly, it is often the result of a computer sending incorrect signals due to a faulty sensor, or the transmission is not

reacting to the computer command because of a bad connection or defective solenoid pack. These problems can be

corrected while the transmission is in the car for considerably less money then a complete overhaul.

If a non computer-controlled transmission is shifting too early or too late, it may require an adjustment to the throttle cable.

Since throttle cables rarely go out of adjustment on their own or due to wear and tear, these mis-adjustments are usually

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due to other repair work or damage from an accident. If the vehicle has a vacuum modulator instead of a throttle cable,

there is an adjustment that can be made using an adjustment screw in some modulator designs. In vehicles with

modulators, however, it is very important that there are no vacuum leaks and the engine is running at peak efficiency.

Engine vacuum is very sensitive to how well the engine is running. In fact, many technicians use a vacuum gauge to

diagnose performance problems and state-of-tune. Many problems that seem to be transmission problems disappear after

a tune-up or engine performance related repair was completed.

In some older transmissions, bands can be adjusted to resolve "slipping" conditions. Slipping is when an engine races

briefly when the transmission shifts from one gear to the next. There are no adjustments for clutch packs however.

• Reseal job

A transmission is resealed in order to repair external transmission fluid leaks. If you see spots of red oil on the ground

under the car, your transmission may be a candidate for a reseal job. In order to check a transmission for leaks, a

technician will put the car on a lift and examine the unit for signs of oil leaks. If a leak is spotted at any of the external

seals or gaskets and the transmission otherwise performs well, the technician will most likely recommend that the

transmission be resealed.

Most of the external seals can be replaced while the transmission is still in the car but, if the front seal must be replaced,

the transmission must first be removed from the vehicle in order to gain access to it, making it a much costlier job.

• Replace accessible parts

There are a number of parts that are accessible without requiring the removal of the complete transmission. many of the

control parts including most of the electrical parts are serviceable by simply removing the oil pan. The parts that are

accessible, however, vary from transmission to transmission and most transmission repair facilities would hesitate to

provide meaningful warrantees on external repairs for the simple reason that they cannot see if there are any additional

internal problems in the components that are only accessible by transmission removal.

• Complete Overhaul

In a complete overhaul (also known as rebuilding a transmission), the transmission is removed from the vehicle and

completely disassembled with the parts laid out on a workbench. Each part is inspected for wear and damage and then

either cleaned in a special cleaning solution, or replaced with another part depending on its condition. Parts that have

friction surfaces, such as bands and clutches are replaced as are all seals and gaskets. The torque converter is also

replaced, usually with a remanufactured one. Technical service bulletins are checked to see if the auto manufacturer

recommends any modifications to correct design defects that were discovered after the transmission was built.

Automobile manufacturers often make upgrade kits available to transmission shops to resolve these design defects.

• Replacement unit vs. overhaul existing unit

When a transmission requires an overhaul, there are generally two options that you may have. The first is to remove your

existing transmission and overhaul it, then put the same, newly rebuilt unit back in your car. The second option is to

replace your existing unit with another unit that has already been rebuilt or remanufactured.

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The second option will get you out of the shop and on your way much faster but may cause you problems down the road.

The reason for this is that, in some but not all cases, a particular transmission model can have dozens of variations

depending on which model car, which engine, which axle ratio, even which tire size. The problems you could experience

could be as simple as a speedometer that reads too high or too low (the speedometer is usually connected by cable to a

gear in the transmission output shaft.) You may also experience incorrect shift points or even complete transmission

failure because your engine may be more powerful then the one the replacement unit was originally designed for. This is

not the case with all transmission models so voice your concerns with your technician. Most shops will rebuild your

existing unit if you request it as long as they can afford to have a lift tied up with your car while the transmission is being

rebuilt. Of course this is only important if you are sure that the transmission you have is the original one and has never

previously been replaced.

Drive Axle and Differential Operation INTRODUCTION Automakers use a precisely engineered assembly, called a rear-wheel drive (RWD) axle, as shown in Figure to deliver the power received from the propeller shaft to the rear wheels. The rear axle includes the following items: final drive, differential, drive axles, and housing.

The final drive multiplies and transfers the torque from the propeller shaft, delivering it to two axles through a 90-degree angle. The differential allows the drive wheels to rotate at different speeds when the vehicle turns and rides over road surface irregularities, preventing excessive noise, tire wear, and loss of driver control. The drive axles transmit the power from the differen-tial side gears to the wheel hub.

THE FINAL DRIVE

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Understanding rear axles

begins with knowing how torque flows through them. First the propeller shaft delivers torque to the first of two parts of the final drive, the pinion gear. It in turn drives the ring gear, pressed onto the differential case as the ring gear and case rotate, the differential gears inside the

case drive the axle shafts, which transfer torque to the rear drive wheels. To better

understand the rear axle and its final drive, this section discusses the torque multiplication factor and types of gear sets.

The Torque Multiplication Factor

As previously stated, the final drive gear ratio multiplies torque in addition to the driver selected transmission ratio, discussed later in the chapter titled, "Gearbox Operating Principles." Both ratios combined determine the overall torque multiplication factor (TMF). As with any other system of gear ratios in series with each other, calculate the TMF by multiply-ing the transmission ratio by the final drive ratio. Then, by multiplying the TMF by the crankshaft torque, you can calculate the amount of torque the driveline delivers to the drive wheels.

Example: Transmission first gear ratio = 2.73:1

Final drive ratio = 3.56:1 TMF = 2.73 x 3.56 = 9.719

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The TMF of the drive train in first gear is 9.719:1. By multiplying engine torque output by 9.719, you can calculate drive wheel torque in first gear. Final drive ratios, like transmission. ratios, affect the ability of a vehicle to accelerate and to reach a particular cruising speed. A higher final drive ratio provides greater gear reduction, or torque multiplication, than a lower final drive ratio. This means the numerically

higher the gear ratio, the better the vehicle can deliver quick acceleration; the lower the gear ratio is numerically, the better it can attain a high cruising speed.

The input pinion is a smaller gear than the ring gear and is the last gear reduction in the car. You may have heard terms like rear axle ratio or final drive ratio. These refer to the gear ratio in the differential. If the final drive ratio is 4.10, then the ring gear has 4.10 times as many teeth as the input pinion gear. When selecting the final drive ratio, engineers must balance the desire for quick acceleration with the need for a high cruising speed. Selecting a final drive gear ratio giving quick acceleration also tends to increase vehicle emissions and lower fuel economy.

Final Drive Gear Types Over the years, automakers have used several final drive gear set types. On late-model passenger cars and light-duty trucks, the hypoid gear set is most commonly used because of its strength and quiet operation. This gear set also allows design engineers to lower the propeller shaft tunnel height .

To understand how a hypoid gear set lowers the tunnel height, consider where the centerline of the drive pinion shaft lies in relation to the centerline of the ring gear . As seen, the pinion shaft centerline lies lower than the ring gear centerline, allowing the propeller Shaft connected to the drive pinion shaft to also be positioned lower. This decreases the height of the hump running the length of the vehicle inside the passenger compartment. Another final drive gear set, the spiral bevel gear set, is as strong and quiet as the hypoid but requires a higher propeller shaft tunnel height. The centerline of its drive pinion gear shaft runs through the ring gear centerline. Trucks and buses commonly use the spiral bevel gear set because the higher propeller shaft location does not interfere with the floor pan height.

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DIFFERENTIALS While a vehicle travels straight ahead, the speed of each driven wheel must be allowed to vary slightly as each wheel goes over bumps, potholes, railroad tracks, and other road surface irregularities. While cornering, the wheels must be able to turn at much greater differences in speed. Without some mechanism to allow for a difference in speed between the wheels, the left wheel would skid through the turn. To prevent skidding, tire wear, and drive train damage, manufacturers install differentials into automobiles. After explaining the standard or open differential, this section describes other categories of differentials; such as, automatic locking, ratcheting, and manual locking.

To understand how a differential allows one driven axle to turn faster than another, consider the simplest type, the standard differential, as shown in Figure 12-10. Technicians also call this type a "single-trac" or "open" differential. Currently, RWD passenger cars, RWD light duty trucks, and 4WD vehicles use open differentials. As shown in Figure this differential consists of several components: the differential case, pinion (spider) shaft and gears, and two sides(axle end) gears. After describing how these parts fit together, we explain the torque path flow.

Parts of a Standard Differential

The differential case consists of two halves. The first is a plate with a hole in its center and a ring gear around its outer circumference. The second half is a cup-like housing that holds the differential gears. This half of the differential case has openings on its sides and ends. Depending on the type of rear axle,· the case halves mayor may not be separable: .

Inside the differential gear housing, four to six bevel gears help drive the axles. In most rear axles, two of these bevel gears are smaller pinion gears mounted on a shaft. They drive two side gears splined with each inner axle end.

During operation, the gears exert considerable backward force against the inside of the case.

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To prevent wear, each gear has a thrust washer installed behind it to provide a bearing surface, as shown in Figure .

TORQUE FLOW THROUGH A STANDARD DIFFERENTIAL

Torque flow through the final drive to the axles is not difficult to understand. The process may be broken down into 4 steps.

1The drive pinion gear drives the ring gear

2. The ring gear bolted to the differential plate turns the case.

3. The case rotates the differential pinion gear shaft and the pinion gears.

4. The pinion gears drive the side gears, splined to the axle ends, which drive the axles and wheels.

What the differential does?

The differential has three functions: • To aim the engine power at the wheels. • To act as the final gear reduction in the vehicle, slowing the rotational speed of the

transmission one final time before it hits the wheels. • To transmit the power to the wheels while allowing them to rotate at different speeds, this is

where the differential gets its name.

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AUTOMATIC LOCKING

DIFFERENTIALS When a vehicle equipped with a standard differential spins a tire, the opposite wheel does not re-ceive enough torque to move the vehicle. To solve this problem, most manufacturers use differentials that direct more power to the side gear attached to the spinning axle. Many differentials do this by forcing the side gear against the revolving case. This bypasses differential action, allowing the case to drive the side gear directly.

The first category of differential capable of preventing wheel slip is the limited-slip differen-tial or LSD. It distributes torque to both wheels equally or unequally, allowing the wheels to turn at the same or at different speeds

The second automatic locking differential category includes the Detroit Locker, also called the

NoSPIN differential. While the vehicle travels straight ahead this differential drives both axles locked to the differential case. Only when one tire rotates faster than the case does that axle unlock.

Clutch- Type Limited-Slip Differentials The only means of having the standard differential apply different amounts of torque to each axle is to have the case drive the side gear directly, bypassing the pinion gears. One means of accomplishing this is to literally "push" the side gear out of mesh with the pinion gears against the rotating case.

Pre-loaded clutches use two mechanisms to accomplish this action. First, a coil, Belleville, or leaf spring cocks the two side gears. Second, a multi-disc clutch pack or cones lie behind one or both of the side gears. Automakers refer to these differentials using various brand names, such

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as Positive Traction, Sure-Grip, Anti-Spin, and Traction-Lok. The following sections describe the parts and the operation of the pre-loaded clutch.

The clutch-type LSD is probably the most common version of the limited slip differential. This type of LSD has all of the same components as an open differential, but it adds a spring pack and a set of clutches. Some of these have a cone clutch that is just like the synchronizers in a manual transmission.

The spring pack pushes the side gears against the clutches, which are attached to the cage. Both side gears spin with the cage when both wheels are moving at the same speed, and the clutches aren't really needed-the only time the clutches step in is when something happens to make one wheel spin faster than the other, as in a turn. The clutches fight this behavior, wanting both wheels to go the same speed. If one wheel wants to spin faster than the other, it must first overpower the clutch. The stiffness of the springs combined with the friction of the clutch deter-mines how much torque it takes to overpower it.

Getting back to the situation in which one drive wheel is on the ice and the other one has good traction: With this limited-slip differential, even though the wheel on the ice is not able to transmit much torque to the ground, the other wheel will still get the torque it needs to move. The torque supplied to the wheel not on the ice is equal to the amount of torque it takes to overpower the clutches. The result is that you can move forward, although still not with the full power of your car.

VISCOUS COUPLED LIMITED-SLIP UNITS

Differentials can be designed to work on the principle of a viscous coupling. This design uses a series of closely positioned plates, which do not physically touch one another. Half of the plates are splined to the case, and the other half are alternately splined to each side gear. The plates are housed in a sealed chamber, which is filled with a thick and viscous silicone-based fluid. The silicone allows normal speed differences between two shafts, resisting high-speed differences associated with wheel spin on one shaft.

To date, this type of viscous-coupled differential has only been used as an interaxle differential on some all-wheel-drive systems.

Viscous Coupling The viscous coupling is often found in all-wheel drive (AWD) vehicles. It is commonly used to link the back wheels to the front wheels so that when one set of wheels starts to slip, torque will be transferred to the other set.

The viscous coupling has two sets of plates inside a sealed housing that is filled with a thick fluid. One set of plates is connected to each output shaft. Under normal conditions, both sets of plates and the viscous fluid spin at the same speed. When one set of wheels tries to spin faster, perhaps because it is slipping, the set of plates

corresponding to those wheels spins faster than the other. The viscous fluid, stuck between the

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plates, tries to catch up with the faster disks, dragging the slower disks along. This transfers more torque to the slower moving wheels-the wheels that are not slipping.

When a car is turning, the difference in speed between the wheels is not as large as when one wheel is slipping. The faster the plates are spinning relative to each other, the more torque the viscous coupling transfers. The coupling does not interfere with turns because the amount of torque transferred during a turn is so small. However, this also highlights a disadvantage of the viscous coupling: No torque transfer will occur until a wheel actually starts slipping.

DIFFERENTIAL ELIMINATORS There have been a variety of aftermarket devices which eliminate differential action altogether. These are typically spool-type cases, which are splined directly to each axle, without the use of dif-ferential gears or other hardware.

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Spider or side gears without full tooth cuts, or both, can also be installed in a

production unit to effectively eliminate the action of the differential. This type of

procedure has only been used on competition drag-racing vehicles that do not have

to execute any turns. Differential eliminators attempt to avoid wheel-spin from

side-to-side weight shift as the engine accelerates from takeoff. With no

differential action possible, the vehicle reliably accelerates straight ahead; on

turns, however, the tire wear and noise may cause the driver to lose control of the

vehicle. For this reason, differential eliminators are not street-legal in most

jurisdictions.

DRIVE PINION TYPES The final drive pinion shaft may be supported by bearings using one of two methods. In the first, two opposed taper roller bearings, with a compressible spacer separating the two inner races, support an overhung pinion . This term means all of the bearings supporting the pinion shaft lie on one side of the pinion gear. The bearings are preloaded, a state that minimizes wobble and endwise movement while the drive pinion shaft rotates.

Two opposed taper roller bearings also support a

straddle-mounted pinion, but the distance between them is smaller than in the overhung design. Most noticeably, a third smaller bearing attaches to a stem-like machined pilot protruding from the gear end of the drive pinion shaft .This third bearing, usually a straight roller type, fits into a bore in the carrier. Unlike the overhung design, bearings supporting a straddle-mounted pinion lie on both sides of the pinion gear.

Like the overhung design, a straddle-mounted pinion shaft usually has a compressible preload

spacer positioned between the two larger differential case bearings. Straddle-mounted pinions usually are mounted in a pinion housing that can be removed from the carrier.

REAR AXLE DESIGNS Rear drive axle assemblies can be categorized according to certain design and construction

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details. These designs depend on the following conditions:

• How the assembly retains the axles. • How the differential carrier can be removed from the axle housings. • How the bearings support the drive pinion gear and shaft assembly.

While these variations normally do not affect operation, they can affect disassembly and

service. 1.Solid Rear Axle Assemblies

On vehicles with non-independent (solid) rear axle assemblies, the axles are supported on their outboard side by a roller bearing and by the side gear on the inboard side. The axle is kept from moving outward by a bearing retaining plate and collar on the outboard end or by a C-c1ip or lock positioned in a groove on the axle's inner end. On some assemblies, the axle bearing is pressed into the outer end of the axle housing. An axle seal is provided to prevent lubricant leakage past the end of the axle. Both axles must be removed to allow removal of the final drive/differential assembly

2.Semi floating Design. Design differences also can exist in the way the wheel bearings are placed in relation to the axle shaft and housing. Bearing placement will determine the types of forces to which the axle is subjected. The common bearing placement method for rear-wheel drive axles on passenger cars and light trucks is the semi-floating design .A semi-floating axle assembly places a single wheel bearing between the axle and the inside of the axle housing. This arrangement subjects the outer end of the axle to the forces generated by cornering, accelerating and decelerating, vehicle weight, wheel and tire runout, and twisting force as the axle drives the wheel. As the wheel rolls forward, the axle's forward motion is transferred through the wheel bearing and axle housing to the suspension and frame. If a semi-floating axle breaks, its wheel could detach from the vehicle. Axle replacement requires removal of the tire and wheel assembly 3.Full-Floating Design The drive axle bearing arrangement used on all commercial vehicles is the full-floating bearing design, which places two opposed taper roller bearings between the outside of the axle housing and the inside of the wheel hub . This arrangement isolates the axle shaft so that it only carries

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drive torque to the wheel. As the axle turns the wheel, the wheel's forward motion is transferred through the wheel bearings to the axle housing, suspension, and frame. The forces generated by cornering, accelerating and decelerating, vehicle weight, and wheel and tire runout are taken by the axle housing. If a full-floating axle breaks, it can be removed without demounting the wheel

and tire from the vehicle. The wheel and wheel bearings are unaffected and, if necessary, the vehicle can be towed without elevating the end with the broken axle. Lubrication and Bearing

The ring gear moves oil forward through passages to lubricate the drive pinion bearings. Oil is also pushed into the differential bearings by the rotating differential case. Because of the high pressure exerted by the teeth of the final drive gears on each other, special lubricants are required for drive axle assemblies. Usually these are in the form of thick liquid greases and have additives that make them resistant to penetration. This allows the final drive gears to operate without metal-to metal contact. Figure shows the fill plug and the vent for a rear axle. Limited-slip differentials use lubricants with additional friction modifying agents, which allow proper operation of the clutches. Limited-slip lubricant is sometimes packaged ready for installa-tion, and sometimes it must be added as a separate ingredient to conventional hypoid lubricant. Manufacturers' recommendations must be followed to insure proper operation of any drive axle assembly.

automotive transmission -lecture notes

Documents

clutches clutch

clutch facing

clutch application

clutch slips

clutch temperature

clutch capacity

clutch plate

dry type of clutch