Clutch & Gear BoxClutch The clutch is housed between the engine and transmission where it provides a mechanical coupling between the engine's flywheel and the transmission input shaft. The clutch is operated by a linkage that extends from the passenger compartment to the clutch housing. The purpose of the clutch is to disconnect the engine from the driven wheels when a vehicle is changing gears or being started from rest. Disengaging the clutch separates the flywheel, the clutch plate and the pressure plate from each other. The flywheel is bolted to the end of the crankshaft and rotates with it. The clutch plate is splined to the gearbox in order for both to rotate together and the pressure plate clamps the clutch plate to the flywheel. When the pressure is released by depressing the clutch pedal, the crankshaft and gearbox input shaft rotate independently. When the foot is taken off they rotate as one. Requirements Of ClutchTorque 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 theoccurrence 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 with in the 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 inertia. Otherwise, when theclutch 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.

Basic Principle Of The Friction Type Clutch To understand the working principle of clutch, lets take two sanding discs, first one driven by a power drill corresponds to the flywheel of a car, driven by the engine. If a second sanding disc is brought into contact with the first, friction makes it revolve too but more slowly. But when the second disc pressed against the first disc which is connect to the power drill, as the pressure increases the two discs revolve as one. This is how a friction clutch works.

Types of Friction Materials1. Millboard type.2. Moulded type.3. Woven type;(a) Solid woven variety.(b) Laminated varietyMillboard 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.Common Clutch Facing MaterialsOrganic 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 canwithstand higher temperatures. However, its disadvantage:' is that it tends to weld itself to themating 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.

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.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.AdvantageThe 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.DisadvantagesThis 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.

Multi Coil Spring Single Plate Clutch CONSTRUCTION A typical clutch actuated by a number of coil springs on a pitch circle nears the periphery is shown. The driven shaft which normally is a forward extension of gearbox primary shaft is supported at its front end in ball bearing in a hole in the centre of flywheel web, which is spigot and bolted on to a flange at the rear end of the crankshaft.In this clutch, the coil springs force the pressure plate forwards to clamp the driven plate between it and the rear face of the flywheel. Three lugs extend rearwards from periphery of pressure plate both to rotate the pressure plate and to cause it to rotate with the rest of the assembly. The driven plate of course is splined onto the shaft. There are three release levers pressing the coil springs at the outer end. The inner ends of the levers can be forced forward by means of thrust bearing made of graphite and slide along the clutch shaft when clutch pedal is depressed. The driven plate mounted between flywheel and pressure plate makes the clutch shaft to rotate to transmit power. It has the clutch facing made of friction materials around the periphery of disc. WORKING When the clutch is engaged, the clutch plate is gripped between the flywheel and pressure plate. The friction linings are on both sides of clutch plate. Due to friction between flywheel, clutch plate and pressure plate, the clutch plate revolves with the flywheel. As clutch plate revolves the clutch shaft also revolves. Thus, engine power is transmitted to the clutch shaft. When the clutch pedal is pressed the pressure plate moves back against the spring force and clutch plate becomes free between flywheel and pressure plate. Thus flywheel remains rotating as long as the clutch pedal is pressed, the clutch is said to be disengaged and clutch shaft speed reduces slowly and finally it stops rotating.

Diaphragm Spring Single Plate Clutch Diaphragm spring pressure plate assemblies are widely used in most modern cars. The diaphragm spring is a single thin sheet of metal which yields when pressure is applied to it. When pressure is removed the metal springs back to its original shape. The centre portion of the diaphragm spring is slit into numerous fingers that act as release levers. During disengagement of the clutch the fingers are moved forward by the release bearing. The spring pivots over the fulcrum ring and its outer rim moves away from the flywheel. The retracting spring pulls the pressure plate away from the clutch plate thus disengaging the clutch. When engaged the release bearing and the fingers of the diaphragm spring move towards the transmission. As the diaphragm pivots over the pivot ring its outer rim forces the pressure plate against the clutch disc so that the clutch plate is engaged to the flywheel.

ADVANTAGES OF DIAPHRAGM SPRING CLUTCH 1. It is more compact than other designs. 2. It is easier to balance rotationally and is less subjected to unwanted effects due to centrifugal force at high rotational speeds. 3. It gives uniformly distributed pressure on pressure plate. 4. It needs no release levers. 5. Minimum effort is sufficient to disengage the clutch. 6. It provides minimum number of moving components and hence minimum internal friction is experienced. 7. This is very commonly used in cars, light Lorries and mini trucks but is not much used in heavy vehiclesMultiplate Clutch

The multi-plate clutch is an extension of single plate type where the number of frictional and the metal plates are increased. The increase in the number of friction surfaces obviously increase capacity of the clutch to transmit torque, the size remaining fixed. Alternatively, the overall diameter of the clutch is reduced for the same torque transmission as a single plate clutch. This type of clutch is, used in some heavy transport vehicles, in epicyclic gearboxes and racing cars where high torque is to be transmitted. Besides, this finds applications in case of scooters and motorcycles, where space available is limited. Extension of flywheel is a drum; which on its inner circumference is splined to carry a number of thin metal plates. These must consequently revolve with drum but are able to slide axially. Interleaved with these outer plates are a number of inner plates that are splined to an inner drum which is coupled rotationally to the gearbox shaft.This drum is supported on a spigot extension of crankshaft. Between the web of inner drum and sleeve in inner drum is a strong coil spring. The inner drum is thus pressed to left being provided with a flange it squeezes the inner and outer plates together so that friction between them transmits driving torque from outer to inner drum. The clutch is disengaged by pulling inner drum right against spring force. The plates of multi-plate clutch were at one time made alternately of steel and phosphor bronze but now are all of steel or one set may be lined with a friction material. With metal contact lubrication is essential and so clutch is made oil-tight and partly filled with oil. The oil tends to make the plates drag when clutch is disengaged and so some mean should be provided to avoid this drag.Centrifugal Clutch In this type of clutches the springs are eliminated altogether and only the centrifugal force is used to apply the required pressure for keeping the clutch in engagement position. The advantage of the centrifugal clutch is that no separate clutch pedal is required. The clutch is operated automatically depending upon the engine speed. This means that car can be stopped in gear without stalling the engine. Similarly while starting, the driver can first select the gear, put the car into the gear and simply press the accelerator pedal. This makes the driving operation very easy.

Figure shows a schematic diagram of a centrifugal clutch. As the speed increases, the weight A fly off, thereby operating the bell crank lever B that 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 engages 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 figure. 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 that net force on the plate D for various engine speeds. At the upper end the curve is made f lat by means of stop H.TORQUE CONVERTERThetorque converteris modified form of fluid flywheel. Fluid flywheel is used for the transmission of power, whereas torque converters are used to transmit the power with varied torque as per the requirement. In addition the driving member (impeller / pump) and driven member (turbine), it consists of a reaction member also (stator). In its simplest form it consists of an impeller connected to the crankshaft, turbine connected to output shaft and stator mounted on overrunning clutch on stationary component impeller is normally an integral part of the converter housing. (It is generally welded to the cover half during the manufacturing).

Turbine and stator are enclosed within the welded housing. The stator incorporates a one-way clutch and mounts on a stationary support shaft that is grounded to the transmission case directly or indirectly through the transmission pump assembly. The impeller and turbine blades are designed with special features. The curved shape of the impeller blades in a backward direction gives added acceleration and energy to the oil as it leaves the impeller. The curved shape of the turbine vanes is designed to absorb as much energy as possible from the moving oil as it passes through the turbine. The vane curvature has two functions that give the turbine excellent torque absorbing capacity. It reduces shock losses due to the sudden change in oil direction between the impeller and turbine. It also takes advantage of the hydraulic principle that the more the direction of a moving fluid is diverted, the greater the fore that fluid exerts on the diverting surface. The fluid impact is absorbed along the full length of the vane surface as the fluid reverses itself. The stator is the third bladed member of the converter. During the torque phase, its function is to redirect the fluid flow as it leaves the turbine and reenters the impeller. This assists the impeller rotation and gives a thrust boost to the fluid discharge. Converter starts operating when the impeller starts rotating, with the engine providing the required input. The impeller creates a centrifugal pumping head or vortex flow. At the same time, the fluid must follow the rotational inertia or the effort of the impeller. These two fluid forces combine to produce a resultant force in the form of an accelerated jetstreamagainst the turbine vanes. The impeller and turbine attempts to act as an effective fluid coupling by featuring curved impeller and turbine vanes rather than astraightradial design. The turbine vanes reverse the fluid direction. The curved turbine vanes provideefficient energytransfer, but the reentry of the remaining fluid thrust back to the impeller, works against the impeller and crankshaft direction. Hence, it is necessary to introduce the stator element to make the converter work. The stator is employed between the turbine, outflow and impeller inflow to reverse the direction of the fluid and make it flow in the same direction as that of the impeller. Instead of the fluid opposing the impeller, the fluid energy now assists the impeller and crankshaft rotation. This results in boosting the rpm of the impeller. This allows the impeller to accelerate more and recycle the fluid with a greater thrust against the turbine vanes. The purpose of using the remaining fluid energy to drive the impeller is referred as regeneration gain. The stator is mounted on a one-way clutch. During the torque phase, the stator remains locked and at coupling speed it overruns.

The torque multiplication occurs when the turbine is turning at a slower speed than the impeller and the stator is stationary / reactionary. This is called torque phase, slip phase or stall phase. This sequence generates a boost in output torque. Recycling of the fluid permits more of the impeller input to be used in increasing the jetstreamvelocity and turning effort of the turbine. By helping the impeller to accelerate the fluid thrust against the turbine, the stator provides the basis for torque multiplication The curved turbine blades absorb the energy from the impeller discharge until the force of fluid is great enough to overcome the turbine resistance to motion. The converter torque is equal to the product of effective fluid force and working radius of the turbine (torque= force x lever arm radius). It is similar to the torque multiplication by gear reduction. The maximum throttle occurs with the engine at wide open throttle (WOT) and zero turbine speed. This is commonly referred as torque rating or stall torque of the converter. For best efficiency, engineering design of the three-element converter keeps the maximum torque ratings within a range of 2:1 to 2 5:1.During the torque phase, vortex flow is the predominant force in the fluid. Therefore the fluid cycles like a continuous chain from the impeller to the turbine and back to the impeller through the locked stator. This action is continuous until the turbine speed is at nine-tenths of the impeller speed, at which the converter has achieved a speed ratio more than 90%. After a moment at stall the turbine and vehicle starts moving. Once the turbine starts, it becomes easier and easier for the fluid force to drive the turbine and vehicle. The turbine rpm actually starts to gain and approach impeller speed. As the turbine gains the speed, the turbine lever arm absorbs less and less of the fluid force and converter torque output gradually drops.

The fluid thrust under vortex influence is trying to hit a moving target that is moving away from it faster and faster. Finally when the converter has reached a speed ratio of more than 90%, the converter enters the coupling phase of the operation. The stator is no longer needed and must freewheel with the rotary flow. The vortex effect on the fluid has dropped significantly and the rotary flow is now the main force. The rotating inertia of the fluid mass, impeller and the turbine form a hydraulic lock or bond. The converter is now in coupling phase and the torque ratio is 1:1. When the speed difference between the impeller and turbine is at its minimum, it is referred as coupling phase. It occurs when the torque converter is operating at its greatest hydraulic efficiency. At this point the stator freewheels and there is no torque multiplication.FLUID COUPLINGFluid coupling is the simplest form of the hydrodynamic drive consisting of two similar members withstraightradial vanes referred as impeller (pump) and turbine. It is used to transmit the power from the engine to the remaining parts of the transmission. Since the fluid coupling is always a major part of the engine flywheel assembly, it is also called fluid flywheel.

The working principle of a fluid coupling can be understood easily with the help of two fans facing each other. When one fan is turned on and the airstreamcauses the second fan to turn even though it is not switched on. The first fan is the driving member or the impeller and the second fan is the driven member or the runner. This is the simple fluid coupling with air serving the function of fluid.The figureshows the simple construction of a fluid flywheel. It consists of a two half dough nut shaped shells equipped with interior fins that radiates from the hubs. One shell is mounted on the crankshaft and is called impeller or driving member. The other shell is mounted on the driven shaft and is called runner or driven member. The two shells are very close with their ends facing each other and enclosed in housing, so that they can be turned without touching each other. The housing is filled with liquid / fluid. When the engine drives the impeller it sets up the fluid mass into motion, creating a fluid force. The path of the fluid force strikes on a solid object, the turbine.

The impact of the fluid jetstreamagainst the turbine blades sets the turbine in motion. With this energy cycle has been completed: mechanical to fluid and back to mechanical. When the impeller spins up, two separate forces are generated in the fluid. One is rotary flow, which is the rotational effort or the inertia of the impeller rotation. The other is vortex flow which circulates the fluidmembers (it is at right angles to the rotary flow) and is caused by the centrifugal pumping action of the rotating impeller. The lag of the runner behind the impeller is known as slip, and depends upon the engine speed and load. The slip is maximum with the vehicle at rest (turbine stationary) and the throttle open to cause the impeller to start circulating oil. As explained earlier the oil is having both rotary and vertex flow at this time. The oil flies out against the curved interior due to the centrifugal force. The rotary flow starts the movement of the runner. As the turbine begins to rotate and catch up impeller speed, flowgradually decreases because of the counter pumping action of the turbine. This permits the rotary action to become greater influence on the fluid and the resultant thrust becomes more effective in propelling the turbine. Finally, at greater speed ratios over 90% the rotary inertia or momentum of the fluid and coupling members forms a hydraulic lock or bond, and the coupling members turns as a single unit. This is referred as coupling point. In an ideal liquid coupling, the runner would attain the same speed as the impeller, so as to receive all the power imparted by the engine. In commercial designs the runner speed becomes almost equal to that of the impeller only under the best operating conditions, when the efficiency of the coupling is highest.

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 underpractical considerations running of automobile demands a large variation of torqueavailable at the road wheels. Hence the main purpose of the transmission is to provide ameans 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 wheelsare 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 alsoprovided 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 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 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 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 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 Fig4.3. 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.

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.SLIDING MESH GEARBOX

Sliding mesh gearbox is the oldest and simplest form of gear box. In order to mesh gears on the splined main shaft with appropriate gears on the layshaft for obtaining different speeds, they are moved to the right or left. It derives its name from the fact that the meshing of the gears takes place by sliding of gears on each other.A three speed sliding mesh gear-box is shown in Figure. Splines are provided on the main shaft. For meshing the pinions with the matching gears on the layshaft, the pinions are slided along the spline. When the main shaft is driven from the layshaft the gear reduction is provided by the first pair of gears which are always in mesh. They are usually known as constant mesh gears. For changing gear the clutch is depressed and the gear lever is moved till the selector pinion on the main shaft engages with its mating gear on the layshaft. The drive from the engine will be again transmitted through the gear-box when the clutch is released. To obtain three forward speeds, reverse and neutral, the relative position of the gears will be as below:

First gear:The largest gear on the main shaft is driven by the smallest gear or pinion on the layshaft. With corresponding increase torque, the speed reduction is quite high. When climbing and moving off steep hill, starting the vehicle from rest this gear is usually used.Second gear:In this gear, there is less speed reduction and smaller torque increase.Third or top gear:In order to revolve primary or main shaft at the same speed without any charge in the torque the main shaft is driven through a dog clutch in this gear.Reverse:In this gear, the peed reduction is usually same as that in the first gear. But the direction of rotation of the main shaft will be reversed by introducing an idler in it. It is due to this change in the direction of rotation of the drivingwheelsprovided by the idler that the motor vehicle moves in reverse direction.

CONSTANT MESH GEAR BOX

In constant mesh gear box all the gears are always in mesh and the engagement between the gears which are freely rotating on thetransmissionmain shaft and thetransmissionmain shaft is effected by moving the dog clutches, as explained below.The engine gear box shaft is integral with a pinion. The pinion meshes with a wheel on the layshaft. The layshaft is therefore driven by the engine shaft. Three more wheels are fixed to the layshaft as in the sliding mesh gearbox. These gears rotate with the layshaft. Thetransmissionmain shaft is just above the layshaft and in line with the engine shaft. The three gears (first gear, second gear and reverse gear) on the main shaft are perfectly free to turn on the main shaft. These three gears are in constant mesh with the three wheels on the layshaft. One of these three gears meshes with a wheel on the layshaft through an idler wheel which is mounted and freely rotating on a pin fixed to the gearbox casing.The three main shaft gears are, therefore constantly driven by the engine shaft, but at different speeds. The first gear and the second gear rotate in the same direction as the engine shaft while the reverse gear rotates in the opposite direction to the engine shaft.

If anyone of the gears on/the mainshaft is coupled up to the main shaft, then there will be a driving connection between the main shaft and the engine shaft. The coupling is affected by the dog clutch units. The dog clutch members are carried on splined (or squared) portions of the mainshaft. They are free to slide on those squared portions, but have to revolve with the shaft.If one of the dog clutch members (l) is slid to the left it will couple the wheel (first gear) to the main shaft giving the first gear. The drive is then through the wheels and this dog clutch member. The other dog clutch is meanwhile in its neutral position.If, with the above dog clutch member in its neutral position, the other dog clutch member (2) is slid to the right, it will couple the wheel (second gear) to the mainshaft and give second gear. If this dog clutch member is slid to the left, it will couple the mainshaft directly to the pinion fixed to the engine shaft. This will give adirect drive, as in the sliding mesh gear box.The reverse gear is engaged by sliding the dog clutch member (which gives the first gear) to the right. Then it will couple the wheel (reverse gear) to the mainshaft. The drive is then through the wheels, the idler and the dog clutch member.In the constant mesh gear box, the gears on the mainshaft must be free to revolve. For this, they are either be bushed or be carried on ball or roller orneedle bearings.

The main advantages of the constant mesh gear box over the sliding mesh type are as follows:1. Helical or double helical gear teeth can be used for the gears instead ofspur gears. Then gearing is quieter.2. Synchronizing devices can be used for smooth engagement.3. Any damage that results from faulty manipulation occurs to the dog clutch teeth and not to the teeth of the gear wheels.4. Once the dog clutches are engaged, there is no motion between their teeth. But when gear teeth are engaged, the power is transmitted through the sliding action of the teeth of one wheel on those of the other. The teeth have to be suitably shaped to transmit the motion properly.5. If the teeth on the wheel are damaged, the motion will be imperfect and noise will result.6. Damage is less likely to occur to the teeth of the dog clutches, since all the teeth engage at once, whereas in sliding a pair of gears into mesh the engagement is between two or three teeth.

SYNCHROMESH GEAR BOX

Synchromesh gear box is an automatic means for matching the speeds of engaging dogs. Synchromesh gear box is a device which facilitates the coupling of two shafts rotating at different speeds. Synchromesh unit is used in most of modern gear boxes. In synchromesh gear box, sliding dog clutches are replaced by synchromesh device. The synchromesh devices are used to simplify the operation of changing gear. Synchromesh device helps unskilled drivers to change gears without the occurrence of clashes and damages.

By synchromesh device, the members which ultimately are to be engaged positively are first brought into frictional contact and then when the friction has equalized their speeds, the positive connection is made.

The basic requirements of synchromesh device are:(1) A braking device such as cone clutch.(2) To permit easy meshing means of releasing pressure on the clutch before engagement of gears.

The engine shaft carries a pinion which meshes with a wheel fixed to the layshaft, while the gear on the mainshaft is free to rotate and is permanently meshed with another wheel fixed to the layshaft. Both the pinion and the wheel on the mainshaft have integral dog tooth portions and conical portions. The synchronizing drum is free to slide on splines on the mainshaft. This drum has conical portions to correspond with the conical portions on the gearbox shaft pinion and on the wheel that rotates freely on the mainshaft. The synchronizing drum carries a sliding sleeve. In the neutral position, the sliding sleeve is held in place by the spring loaded balls which rest in the dents in the sliding sleeve (or ring gear). There are usually six of these balls.

In changing gear, the gear lever is brought to the neutral position in the ordinary way, but is immediately pressed in the direction it has to go to engage the required gear. When a shift starts, the spring loaded balls cause the synchronizing drum and sliding sleeve, as an assembly to move toward the selected gear. The first contact is between the synchronizing cones on the selected gear and the drum. This contact brings the two into synchronization. Both rotate at the same speed. When the speeds of the two have become equal, a slightly greater pressure on the gear lever overcomes the resistance of the balls. Further movement of the shift fork forces the sliding sleeve on toward the selected gear. The internal splines on the sliding sleeve i.e. the dog portion, match the external splines on the selected gear the dog teeth are locked up, or engaged, and thus positive connection is established. The gear shift is completed.

SynchronizersManual transmissions in modern passenger cars usesynchronizersto eliminate the need for double-clutching. A synchro's purpose is to allow the collar and the gear to make frictional contact before the dog teeth make contact. This lets the collar and the gear synchronize their speeds before the teeth need to engage, like this:

The cone on the blue gear fits into the cone-shaped area in the collar, and friction between the cone and the collar synchronize the collar and the gear. The outer portion of the collar then slides so that the dog teeth can engage the gear.

Five Speed Gear Box

OVERDRIVE

Overdrive is a device to step up the gear ratio in the car. It is fitted in betweentransmissionand the propeller shaft. It enables a high cruising speed to be attained with a comparatively low engine speed (upto 20 25%) on long journeys. This results in less wear of the engine and decreases vibration and noise. As the friction lows at lower speeds is less, there is a saving of fuel also with the overdrive. Overdrive is generally fitted on top gear only. But in some sport cars, over drives are also fitted on gears other than the top gear which increases the torque ratios available. For examples, when overdrive is fated on top, third and second gear, seven forward speeds or torque ratios are available. The overdrive may be operated either manually or automatically at a predetermined speed.

To understand the working of an overdrive, consider the above figure. It consists of an epicyclic gear train in which the sun gear is free to rotate on the input shaft, while the carrier can move on splines, on the input shaft. A freewheel clutch is also fitted on the input shaft splines. The output shaft is connected to the ring. When the sun gear is locked with the casing i.e., it becomes stationary, of the output shaft is increased i.e., overdrive is engaged.When the sun gear is locked to the carrier or to the ring, solid drive through n is obtained. Thus depending on the locking of the sun gear with ring gear or with carrier the overdrive or the normaldirect drive is obtained. There is another possible control of the mechanism i.e., when the sun wheel is kept free to rotate on the input shaft. In this case there isdirect drivethrough the freewheel clutch when the engine develops power. However when the accelerator is brought to zero position and the engine is simply idling, the output shaft tends to override the input shaft. The roller of the freewbeel clutch in this case no longer remains wedged and the car free wheels. Thus for gear changing one simply has to lift his foot off the accelerator pedal, the clutch need not be operated.

Overrunning ClutchA special form of aratchetis theoverrunning clutch. Have you ever thought about what kind of mechanism drives the rear axle of bicycle? It is a free-wheel mechanism which is an overrunning clutch. Figure illustrates a simplified model. As the driver delivers torque to the driven member, the rollers or balls are wedged into the tapered recesses. This is what gives the positive drive. Should the driven member attempt to drive the driver in the directions shown, the rollers or balls become free and no torque is transmitted.

Figure Overrunning clutch

The simplest freewheel device consists of two saw-toothed,spring-loaded discs pressing against each other with the toothed sides together, somewhat like aratchet. Rotating in one direction, the saw teeth of the drive disc lock with the teeth of the driven disc, making it rotate at the same speed. If the drive disc slows down or stops rotating, the teeth of the driven disc slip over the drive disc teeth and continue rotating, producing a characteristic clicking sound proportionate to the speed difference of the driven gear relative to that of the (slower) driving gear.A more sophisticated and rugged design has spring-loaded steelrollersinside a driven cylinder. Rotating in one direction, the rollers lock with the cylinder making it rotate in unison. Rotating slower, or in the other direction, the steel rollers just slip inside the cylinder.Most bicycle freewheels use an internally step-toothed drum with two or more spring-loaded, hardened steelpawlsto transmit the load. More pawls help spread the wear and give greater reliability although, unless the device is made to tolerances not normally found in bicycle components, simultaneous engagement of more than two pawls is rarely achieved.Advantages and disadvantages[edit]By its nature, a freewheel mechanism acts as an automaticclutch, making it possible to change gears in a manualgearbox, either up- or downshifting, without depressing the clutch pedal, limiting the use of the manual clutch to starting from standstill or stopping. The Saab freewheel can be engaged or disengaged by the driver by respectively pushing or pulling a lever. This will lock or unlock the main shaft with the freewheel hub.A freewheel also produces slightly betterfuel economyon carbureted engines (without fuel turn-off on engine brake) and less wear on the manual clutch, but leads to more wear on thebrakesas there is no longer any ability to performengine braking. This may make freewheel transmissions dangerous for use ontrucksandautomobilesdriven inmountainousregions, as prolonged and continuous application ofbrakesto limit vehicle speed soon leads to brake-system overheating followed shortly by total failure.

Planetary Gear Sets ( Epicyclic Gear BOX)Automatic transmissions contain many gears in various combinations. In a manual transmission, gears slide along shafts as you move the shift lever from one position to another, engaging various sized gears as required in order to provide the correct gear ratio. In an automatic transmission, however, the gears are never physically moved and are always engaged to the same gears. This is accomplished through the use of planetary gear sets.The basic planetary gear set consists of a sun gear, a ring gear and two or more planet gears, all remaining in constant mesh. The planet gears are connected to each other through a common carrier which allows the gears to spin on shafts called "pinions" which are attached to the carrier .One example of a way that this system can be used is by connecting the ring gear to the input shaft coming from the engine, connecting the planet carrier to the output shaft, and locking the sun gear so that it can't move. In this scenario, when we turn the ring gear, the planets will "walk" along the sun gear (which is held stationary) causing the planet carrier to turn the output shaft in the same direction as the input shaft but at a slower speed causing gear reduction (similar to a car in first gear).If we unlock the sun gear and lock any two elements together, this will cause all three elements to turn at the same speed so that the output shaft will turn at the same rate of speed as the input shaft. This is like a car that is in third or high gear. Another way that we can use a Planetary gear set is by locking the planet carrier from moving, then applying power to the ring gear which will cause the sun gear to turn in the opposite direction giving us reverse gear.The illustration on the right shows how the simple system described above would look in an actual transmission. The input shaft is connected to the ring gear (Blue), The Output shaft is connected to the planet carrier (Green) which is also connected to a "Multi-disk" clutch pack. The sun gear is connected to a drum (yellow) which is also connected to the other half of the clutch pack. Surrounding the outside of the drum is a band (red) that can be tightened around the drum when required to prevent the drum with the attached sun gear from turning.The clutch pack is used, in this instance, to lock the planet carrier with the sun gear forcing both to turn at the same speed. If both the clutch pack and the band were released, the system would be in neutral. Turning the input shaft would turn the planet gears against the sun gear, but since nothing is holding the sun gear, it will just spin free and have no effect on the output shaft. To place the unit in first gear, the band is applied to hold the sun gear from moving. To shift from first to high gear, the band is released and the clutch is applied causing the output shaft to turn at the same speed as the input shaft.Many more combinations are possible using two or more planetary sets connected in various ways to provide the different forward speeds and reverse that are found in modern automatic transmissions.Some of the clever gear arrangements found in four and now, five, six and even seven and eight-speed automatics are complex enough to make a technically astute lay person's head spin trying to understand the flow of power through the transmission as it shifts from first gear through top gear while the vehicle accelerates to highway speed. On modern vehicles (mid '80s to the present), the vehicle's computer monitors and controls these shifts so that they are almost imperceptible.