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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

= 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= WR

Thus we see that the torque transmitted by a friction clutch depends upon three factors i.e., , 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, 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

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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 :

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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:

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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

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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.

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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.

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Fluid coupling

FLUID Flywheel:

Construction

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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

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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)*(2RN1)^2, (m/2)*(2RN1)^2), (m/2)*(2RN2)^2), (m/2)*(2RN2)^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

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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.

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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:

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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

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Final Ratio:

Selecting the intermediate gears:

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Sliding Mesh Type of gearbox

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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.

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Synchromesh Gearbox

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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

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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).

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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 drivers cabin.

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Compound Transmission:

(To be taught during automatic transmissions class)

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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

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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.

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Problems:

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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.

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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