Chapter.No-6GEARS &GEAR BOX

Functions of TransmissionsThe 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 byconsidering:(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 thevehicle speed.(ii) Resistance due to gradient-This remains constant at all speeds. This is thecomponent of the vehicle weight parallel to the plane of the road.(iii) Miscellaneous-Apart from the above two types, various other factors also contribute towards the vehicles resistance. These are: type of the road, tire friction, etc. Thismay 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 inFig. 4.1.

The total resistance for same type of road with different gradients may then be represented bycurves shown in Fig. 4.2. The higher curve represents steeper gradients.Tractive EffortThe curves 1, 2 and 3 respectively in Fig. 4.3 represent the tractive effort in first, second andtop gears respectively.

Transmission NecessityBy now we understand the variation of total resistance to the vehicle motion and the tractiveeffort of the vehicle with speed. It is obvious that whenever the tractive effort exceeds the totalresistance, the vehicle will accelerate to a speed where tractive effort becomes equal to the totalresistance.For further clarification, consider Fig. 4.4. This is obtained by superimposing Fig. 4.2 on Fig4.3. Let the vehicle be in the top gear and suppose the vehicle is travelling on a gradient whichgives total resistance curve I. Then from Fig. 4.4, it is seen that OA is the stabilizing speed. Ifthe speed at any instant is less, say, OB, the excess of tractive effort will accelerate it to speedOA. 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 thestabilizing- speed has decreased. Next consider further the curve Ill. At this gradient, we see thatnowhere does the curve 3 cross curve III. Therefore the vehicle will not be able to go at thisgradient 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 shallhave to shift to first gear.

Again at start more acceleration is needed to gain speed quickly. This can best be done in firstgear 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 thevehicle speed has to be simply maintained and no acceleration is required.Necessity of Gearbox:In addition to many advantages of internal combustion engine , such as high power to weightratio, relatively good efficiency and relatively compact energy storage it has 3 fundamentaldisadvantages1. Unlike steam engines or electic motors the combustion engine is incapable of producingtorque 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 theoperating point in the engines performance map.

SLIDING MESH TYPE GEAR BOX

This is the simplest type of gear box. The figure gives a simplified view if the gear box. The power comes from the engine to the clutch shaft and hence 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.

CONSTANT MESH GEAR BOXConstant mesh gearbox is a type of Transmission in which all or most of the gears are always in mesh with one another, as opposed to a sliding-gear transmission, in which engagement is obtained by sliding some of the gears along a shaft into mesh. In a constant-mesh manual gearbox, Gear ratios are selected by small Clutches that connect the various gear sets to their shafts so that power is transmitted through them.

1. I speed gear 2. II speed gear 3. Main shaft 4. III speed gear 5. top and III speed engaging dogs 6. top gear 7. Primary shaft or main drive gear 8. counter shaft/cluster gear

DOUBLE DECLUTCHINGIn 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.

AdvantagesCompared 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.

SYNCHROMESH GEAR BOX

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 gears on the lay shaft are 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.

Figure shows the construction and working of a synchromesh gear box. In most of the cars, however the synchromesh devices are mot fitted to all the gears as is shown in this figure. They are fitted 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 the above figure is the engine shaft, Gears B, C, D, E are free on the main shaft and are always in mesh with corresponding gears in 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. Menders 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. T1and T2 are the ball supported by springs. These tend to prevent the sliding of members G1 (G2) on F1

(F2).however, when the force applied in G1 (G2) through fork S1 (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, N1, N2, P1, P2, R1, R2 are the frictional surfaces.

The working of the gear box is as follows .for direct gear, member G1 and hence member F1 (through spring –loaded balls) is slid towards left till comes M1 and M2 rub and friction makes their speed equal. Further pushing the member G1 to left causes it to override the balls and get engaged with dogs K1. Now the drive to the main shaft is direct from B via F1 and the splines. We have to give sufficient time for synchronization of speeds, otherwise clash may result.

For the second gear the members F1 and G1 are slid to the right so that finally the internal teeth on G1 are engaged with L1.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 right. In this case the drive will be from B via U1, 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, U1, U4, U5, D, F2 are splines to the main shaft.

Semi-automatic Transmission A semi-automatic transmission is a very advanced system, which still uses a clutch to perform the gear shift instead of a torque converter. Unlike the manual transmission, the computer does all of the clutch disengaging, gear shifting, and clutch engaging. This not only makes the gear shifting faster than manual transmission, but also prevents the vehicle from stalling when the car is stationary. Like a tiptronic transmission, a semi-automatic transmission can also be switched to manual mode to perform gear shifting at the drivers‟ wish. The two most

common semi-auto-matic transmissions are direct shift transmission (aka dual-clutch transmission) and electrohy-draulic manual transmission (aka sequential transmission).

The direct shift transmission was de-signed and developed by the Audi and Volkswagen auto groups. Figure 24 shows the structure of the direct shift transmission. Like a conventional manual transmission system, it uses a collar, syn-chronizer, and gear set to perform gear shift. The clutch set is like that inside the automatic planetary gear transmission system, which controls the gear ratio change. Unlike the conventional manual transmission system, there are two differ-ent gear/collar sets, with each connected to two different input/output shafts. One set contains odd (1st, 3rd, 5th…) gears, while the other contains even (2nd, 4th, 6th…) gears. It is just like two conven-tional manual transmission gear boxes in one. To automatically shift from 1st gear to 2nd gear, first the computer detects that the spinning speed of the input shaft is too high, and engages the 2nd gear‟s collar to the 2nd gear. The clutch then disengages from 1st gear‟s input shaft, and engages the 2nd gear‟s input shaft. Controlled by computer, the gear shift becomes extremely fast compared with a conven-tional manual transmission. Using direct contact of the clutch instead of fluid coupling also im-proves power transmission efficiency. Another advanced technology used for direct shift trans-mission allows it to perform “double clutching” by shifting the gear to neutral first, adjusting the spinning speed of the input shaft, and then shifting to the next gear. This makes gear shifting very smooth.

AUTOMATIC TRANSMISSION

An automatic transmission uses a fluid-coupling torque converter to replace the clutch to avoid engaging/disengaging clutch during gear change. A completed gear set, called planetary gears, is used to perform gear ratio change instead of selecting gear manually. With the invention of the automatic transmission, a driver no longer needs to worry about gear selection during driving. It makes driving a car much easier, especially for a disabled or new driver. However, the indirect gear contact of the torque converter causes power loss during power transmission, and the complicated planetary gear structure makes the transmission heavy and easily broken.

OVER DRIVES:

Overdrive is a device to step up the gear ratio in the car.It is fitted in between transmission and the propeller

shaft. It enables a high cruising speed to be attained with a comparatively low engine speed on long journey.

This results in less wear of the engine parts and decreases vibration and noise. As the friction losses at lower

speed are less,there is saving of fuel also with overdrive.

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 parts1. A set of planetary gear2. 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.

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.

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.

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

overdrive.

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.

EXERCISES:

1. In a gear box the clutch shaft pinion has 14 teeth and low gear main shaft pinion 32 teeth. The

corresponding layshaft(counter shaft) pinion have 36 and 18 teeeth. The rear axle ratio is 3.7:1 and the

effective radius of rear tyre (wheel) is 35.5 cm. Calculate the car speed if the speed of the engine is 25

rpm. (Ans-19. 8 km/h)

2. A sliding mesh type of gear box with forward speeds only is to be designed. The gear box should have

the following gear ratios available approximately: 1.0, 1.5, 2.5 and 3.9.The centre distance between

the lay shaft(counter shaft) and the main shaft is 78 mm and the smallest gear is to have at least 16

teeth with a diametral pitch of 3.25 mm. Calculate the number of teeth of the various gears and the

exact gear ratios thus available. (Ans-

No of teeth on clutch shaft-A

Corresponding gear on

counter shaft-8

First GearNo of teeth on counter

shaft

First GearNo of teeth on

main shaft

Second GearNo of teeth on counter

shaft

Second GearNo of teeth

on main shaft

Third GearNo of teeth on counter

shaft

Third GearNo of teeth

on main shaft

TA=16 TB=32 TC=16 TD=32 TE=21 TF=27 TG=27 TH=21

Gear ratios 4.0:1 2.57:1 1.55:1

3. An automotive gear box gives three-forward speeds and one reverse speed with a top gear ratio of

unity and bottom gear ratio of approximately 3.3:1.The centre distance between the shafts is to be 110

mm approximately. Gear teeth of module 3.25 mm are to be employed. Find the number of teeth for

various gear wheels and the exact gear ratios. (Ans

No of teeth on clutch shaft-A

Corresponding gear on

counter shaft-8

First GearNo of teeth on counter

shaft

First GearNo of teeth on

main shaft

Second GearNo of teeth on counter

shaft

Second GearNo of teeth

on main shaft

Reverse Gear

No of teeth on counter

shaft

Reverse Gear

No of teeth on main

shaft

TA=24 TB=44 TC=24 TD=44 TE=34 TF=34 TI=22 TJ=40Gear ratios 3.36:1 1.835:1 3.33:1

4. The input shaft of an epicyclic type of gear box has two sun wheels each with 25 tteth splined to the

shaft.Their corresponding annuli have 100 teeth each.The output shaft has a sun running free on that

shaft with 40teeth,while the corresponding annulus has 80 teeth.Calculate the first,second and reverse

gear ratios. (Ans-First gear ratio-5:1;second gear ratio-1.25:1;Reverse gear ratio-2:1)