AUTOMOTIVE ENGINEERING

AUTOMOTIVE ENGINEERING
UNIT 2 TRANSMISSION AND DRIVE LINE

TRANSMISSION & DRIVELINE
TRANSMISSION SYSTEM Chief function of the device is to receive power at one torque and angular velocity and to deliver it at another torque and the corresponding angular velocity.

REQUIREMENTS OF TRANSMISSION SYSTEM
1. To provide for disconnecting the engine from the driving wheels. 2. When the engine is running, or more, according to the relative size of engine and weight of vehicle to enable the connection to the driving wheels to be made smoothly and without shock. 3. To enable the leverage between the engine and driving wheels to be varied. 4. It must reduce the drive-line speed from that of the engine to that of the driving wheels in a ratio of somewhere between about 3:1 and 10:1 5. Turn the drive, if necessary, through 90° or perhaps otherwise re-align it. 6. Enable the driving wheels to rotate at different speeds. Provide for relative movement between the engine and driving wheels. CLUTCH 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. Leverage:power or ability to act. Spline:Any of a series of projections on a shaft that fit into slots on a corresponding shaft, enabling both to rotate together. A hinge is a type of bearing that connects two solid objects, typically allowing only a limited angle of rotation between them.

When the foot is taken off they rotate as one.
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 A CLUTCH The clutch must 1. Pick up its load smoothly without grab or clatter. 2. Have a driven disc of low moment of inertia to permit easy shifting. 3. Damp out any vibration of the crankshaft to prevent gear clatter. 4. Require little pedal pressure to operate it. 5. Be easy to adjust and service. 6. Be cheap to manufacture. Clatter:A continuous rattling sound as of hard objects falling or striking each other. Moment of inertia is a property of a body that defines its resistance to a change in angular velocity about an axis of rotation.

BASIC PRINCIPLE OF THE FRICTION TYPE CLUTCH
To understand the working principle of clutch, let’s 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 connected to the power drill, as the pressure increases the two discs revolve as one. This is how a friction clutch works.

TYPES OF CLUTCHES MULTI COIL SPRING SINGLE PLATE CLUTCH DIAPHRAGM SPRING SINGLE PLATE CLUTCH MULTIPLATE CLUTCH CENTRIFUGAL CLUTCH SEMI - CENTRIFUGAL CLUTCH CONSTRUCTION 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.

MULTIPLATE 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.

MULTIPLATE CLUTCH 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. 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 rpm. The stop H limits the amount of centrifugal force.

SEMI - CENTRIFUGAL CLUTCH
It uses both centrifugal and spring force for keeping it in an engaged position. The springs are designed to transmit torque at normal speed, while centrifugal force assists in torque transmission at high speed. This clutch consists of three hinged and weighted levers and three clutch springs alternately arranged at equal spaces on the pressure plate. At low speeds the springs keep the clutch engaged and the weighted levers do not have any pressure on pressure plate. At high speeds when power transmission is high, weights fly off and the levers also exert pressure on plate, keeping the clutch firmly engaged. When the speed decreases the weights do not exert any pressure on the pressure plate. Only spring pressure is exerted on pressure plate which keeps the clutch engaged. An adjusting screw is provided at the end of the lever by means of which the centrifugal force on pressure plate can be adjusted. At low speeds pressure on the spring is sufficient to transmit the torque required. At high speeds, the centrifugal force due to weight moves about the fulcrum thereby pressing the pressure plate. The centrifugal force is proportional to the square of speed so that adequate pressure level is attained.

GEAR BOX Gear box: Also referred to as transmission
Whole of the mechanism that transmits the power from the engine crankshaft to the rear wheels. Functions of transmission: Main purpose of the transmission is to provide a means to vary torque ratio between the engine and road wheels as required. Means to back the car by reversing the direction of rotation of the drive.

TYPES OF GEAR BOX Progressive type gear box Epicyclic or planetary type gear box Selective type gear box This gear boxes are used in motor cycles. In this gear boxes the gears pass through the intervening speeds while shifting from one speed to another. There is a neutral position between two positions. These gear boxes are a combination of sliding and constant mesh gear boxes. The various gear speeds are obtained by sliding the dog clutch or gear to the required position. The epicyclic or planetary type transmission uses no sliding dogs or gears to engage but different gear speeds are obtained by merely tightening brake-bands on the gear drums, which simplify gear changing. A planetary gear set consists of ring gear or annular wheel, sun gear and planet gears with carrier. In order to obtain different speeds any one of these three units can be held from rotation by means of brake bands.

SELECTIVE TYPE GEAR BOX
It is the transmission in which any speed may be selected from the neutral position. In this type of transmission neutral position has to be obtained before selecting any forward or reverse gear. Some selective type gear boxes are, 1. Constant mesh gear box with positive dog clutch. 2. Constant mesh gear box with synchromesh device. 3. Sliding mesh gear box.

Sliding Mesh Gear Box: Oldest type of gearbox Mechanical efficiency low and noise level high. The clutch gear is rigidly fixed to the clutch shaft. The clutch gear always remains connected to the drive gear of countershaft. The other lay shaft gears are also rigidly fixed with it. Two gears are mounted on the main shaft and can be sliding by shifter yoke when shifter is operated. One gear is the second speed gear and the other is the first and reverse speed gears. All gears used are spur gears. A reverse idler gear is mounted on another shaft and always remains connected to reverse gear of counter shaft.

FIRST GEAR By operating gearshift lever, the larger gear on main shaft is made to slide and mesh with first gear of countershaft. The main shaft turns in the same direction as clutch shaft in the ratio of 3:1. SECOND GEAR By operating gear shaft lever, the smaller gear on the main shaft is made to slide and mesh with second gear of counter shaft. A gear reduction of approximately 2:1 is obtained. TOP GEAR By operating gearshift lever, the combined second speed gear and top speed gear is forced axially against clutch shaft gear. External teeth on clutch gear mesh with internal teeth on top gear and the gear ratio is 1:1. REVERSE GEAR By operating gearshift lever, the larger gear of main shaft is meshed with reverse idler gear. The reverse idler gear is always on the mesh with counter shaft reverse gear. Interposing the idler gear, between reverse and main shaft gear, the main shaft turns in a direction opposite to clutch shaft.

NEUTRAL GEAR When engine is running and the clutch is engaged, clutch shaft gear drives the drive gear of the lay shaft and thus lay shaft also rotates. But the main shaft remains stationary as no gears in main shaft are engaged with lay shaft gears. Constant Mesh Gear box In this type of gearbox, all the gears of the main shaft are in constant mesh with corresponding gears of the countershaft. The gears on the main shaft which are bushed are free to rotate. The dog clutches are provided on main shaft. The gears on the lay shaft are, however, fixed. 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. Usually the helical gears are used in constant mesh gearbox for smooth and noiseless operation.

CONSTANT MESH GEAR BOX

Double Declutching: In the constant mesh gear box, for the smooth engagement of the dog clutches it is necessary that the speed of the mainshaft gear and the sliding dog must be equal. Therefore to obtain lower gear, the speed of the clutch shaft, lay shaft and main shaft must be increased. This is done by double declutching. Procedure for double declutching: The clutch is disengaged and the gear is brought to neutral. Then the clutch is engaged and the accelerator pedal pressed to increase the speed of the main shaft gears. After this the clutch is again disengaged and the gear is moved to the required lower gear and the clutch is again engaged. As the clutch is disengaged twice in this process, it is called declutching. For changing to higher gear, however reverse effect is desired, driver has to wait with the gear in neutral till the main shaft speed is decreased sufficiently for a smooth engagement of the gear.

Synchromesh Gear Box This type of gearbox is similar to the constant mesh type gearbox. Instead of using dog clutches here synchronizers are used. The modern cars use helical gears and synchromesh devices in gearboxes, that synchronize the rotation of gears that are about to be meshed. SYNCHRONIZERS This type of gearbox 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 that ultimately are to be engaged are first brought into frictional contact, which equalizes their speed, after which these may be engaged smoothly.

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.

FLUID COUPLING Fluid coupling is a device used to transmit torque from engine to gear box with fluid as working medium. Construction: The function of the FC is to act as an automatic clutch between engine and gearbox. It allows the engine to idle when the car is stationary but takes up the drive smoothly and progressively when the driver speeds up the engine by depressing the accelerator pedal. There are two main rotating parts; an impeller driven by the engine and a turbine which drives a gearbox. Each is bowl shaped and contains a number of partitions called vanes. The two bowls are placed face to face in a casing filled with oil, and they are separated by a small clearance so that no rubbing contact between them.

Fluid coupling Fluid coupling is the simplest form of the hydrodynamic drive consisting of two similar members with straight radial 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.

Fluid coupling 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 air stream causes 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 figure shows 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.

Fluid coupling The impact of the fluid jet stream against 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 fluid members (it is at right angles to the rotary flow) and is caused by the centrifugal pumping action of the rotating impeller.

Fluid coupling 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, flow gradually 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.

IMPELLER/ PUMP TURBINE

DRAG TORQUE The torque transmitted when the engine is in idling condition is known as drag torque. Even when % slip is 100% there is a drag on gear box shafts, which renders gear changing with ordinary type of gearbox very difficult and vehicle will tend to move when it is parked. To reduce the drag torque, following methods are used, Using anti drag torque baffle Using fluid reservoir Using combination of both. Fluid coupling is used on combination with epicyclic gear box. Fluid coupling with conventional synchromesh gearbox

Drag Torque reduction Anti Drag baffle plate Fluid reservoir

TORQUE CONVERTER TC has an engine driven impeller and a turbine connected to gearbox input shaft. It is also able to deliver a higher torque than that engine produces, because it is also able to deliver a higher torque and small vane wheel known as reactor(stator). A one way clutch (ORC) lock reactor to gear box casing at lower engine speed. In a fluid flywheel, oil returning from turbine tends to curb the speed of impeller. But in TC, the vanes of locked reactor direct oil along a torque favorable path back to the centre of impeller enabling it to give extra thrust to turbine blades.

TORQUE CONVERTER At pull away speeds, TC double the effort produced by engine. As engine picks up speed, this 2:1 increase in torque is reduced until at cruising speed, there is no torque increase at all. The reactor is spun around by oil at same rate as turbine. Tc now acts like a fluid flywheel with reactor free wheeling and no torque increasing effort.

IMPELLER/ PUMP TURBINE

CONVERTER COUPLING The converter coupling is a next type of fluid drive to consider. This type represents an attempt to combine the best part of converter performance with those of fluid coupling. The converter is made to operate as coupling by releasing the reactors from external constraint at the time when the applied torque on this member reverses direction. The reactor then rotates slowly to give the least resistance to the fluid flow. As a result any torque applied to the pump alters flow conditions in a way that induces an equal torque on turbine member. Various methods of controlling the reactor have been utilized but most convenient is the use of free wheel or over running clutch unit into reactor so that it is free to rotate in same direction as the turbine but is locked against rotation in opposite direction.

PERFORMANCE CHARACTERISTICS
Conditions of equilibrium ensures that torque on reactor changes direction when o/p torque has fallen to value of i/p, so that introduction of free wheel unit provides an automatic method of changing from converter to coupling operation at correct time. This change over point is normally referred to as coupling point. There is a change in efficiency curve around coupling point. To remove this change a higher speed ratio of operation was selected. This resulted in improved performance at high speed at the expense of performance at lower speed ratio. Reason for sluggish performance is because of curved blade of turbine – useful for converter operation but not for coupling operation.

MULTISTAGE TORQUE CONVERTERS
The M.S converter is one in which the circulating fluid impinges two or more turbine members separated by reactors. M.S converter is best for highest torque ratio. First stage of conversion is reached when the oil has traveled through the impeller, turbine and reactors and extra stages are sometimes added to obtain a particular type of performance. Provision of an additional turbine is referred to as an extra stage and thus conforms to steam turbine practice. Additional turbine member must be separated by a reactor from a previous turbine to create an extra stage.

The reason for using an increased number of stages is usually to increase torque conversion ratio but certain other advantages are obtainable. A multistage converter having a turbine immediately preceding impeller has advantage that as the vehicle accelerates, fluid can be delivered at greater velocity head to turbine which is enabled to rotate at faster speed. The increased number of stages may increase fluid friction on account of longer circulation path and efficiency of multistage converter tends to fall of rather sharply and racing usually occurs at lower speed ratios than for single stage converter.

Although increased torque ratios are obtained with multistage converter, it is noticed that all the forms of converter discussed so far exhibit a similar o/p characteristic which is roughly parabolic in shape. Variations in blade design or number of stages has the effect of moving the peak of curve towards low ratio (stall) end of o/p speed range or towards the higher ratios (racing). A typical multistage converter will develop an o/p torque equal to about 2½ times the value of i/p torque at its maximum efficiency point. But as the turbine accelerates, efficiency falls rapidly and o/p torque soon falls to zero. This type of converters thus would give a good initial acceleration of vehicle from rest nut would be inefficient for normal cruising which is mainly carried out at unity torque ratio (direct drive).

POLY PHASE CONVERTER COUPLING
The next type of fluid drive represents an attempt to combine the best operating characteristics of two or more different designs of converters into a single converter. These ideas are usually incorporated into converter couplings, but it is converter operation it takes place in several phases. A poly phase converter coupling is a variation of a normal 3-element machine in which at least one of the 3 basic members is divided into further elements. The reactor has to deal with fluid flow from widely changing entrance direction and this member can be divided into a number of elements which adjusts them selves to the changing flow. The practice of dividing a rotating member into several elements is widely used at the present time and it is usual to have 2&3 phases, each of which is represented by a bladed ring element which is also rotate freely when the fluid flow has changed the direction by a given amount. In this way, the elements of reactor enables the operation of redirecting the flow to be carried out in a number of distinct phases, giving rise to the use of the term poly phase converter, or poly phase converter coupling.

POLY PHASE CONVERTER COUPLING

This method is not confined to reaction member, and impeller or turbine member may also be divided into a number of these elements which may rotate at different speed by the introduction of free-wheel units. Such free-wheeling blade elements detach themselves from parent member and rotate at speeds that least resistance to fluid flow occurs. At this point, the detached members can be considered as turbines at raising speed. From figure, it is seen that efficiency curve combines the most useful parts of curves of three different converter designs as the fluid drive effects a three phase transition from converter to coupling.

CHARACTERISTICS OF TORQUE CONVERTER
The graph (a) shows the manner in which torque-increases and efficiency vary when rotor speed varies from zero to maximum value (2700 rpm) the impeller speed being constant at 300 rpm. When the rotor speed is zero (because the resistance opposing its motion is large enough to hold it fixed), the torque tending to rotate will be yearly 6 ½ times the torque developed by engine at its speed of 3000 rpm.

If the resistance to the motion of rotor now decreases, so that rotor starts to rotate, then as it gathers speed so the driving torque driving action on it falls off . At a rotor speed of 1000, for e.g.: the driving torque would have fallen off to about 3 times the engine torque, at 1900 rpm. The driving torque would be just equal to engine torque, while at 2700 rpm, then driving torque would have fallen to zero The efficiency on the other hand starts at zero, when the rotor speed is zero, because although torque acting on the rotor is then larger, no rotation occurs and torque does not work, thus no work is being got out of converter but a lot of work being put in by the engine and so the efficiency is zero, as the rotor speed increases, so does the efficiency and a peak efficiency of % is reached at a rotor speed of about 1000 rpm. As the rotor speed continuous to increase, the efficiency falls off again and at 2700 rpm becomes zero one more, this time because although rotor is revolving rapidly, the driving torque on it is zero, and no work can be out of it. The falling off of the efficiency at low speed end of range can be treated because these speeds are normally used only for short period when starting and climbing several hill, but the fall off at higher speeds cannot be tolerated and must be circumvented. There are two principle ways in which this can be done a) By substituting a direct drive for TC at high speeds. b) By making the TC function as a fluid flywheel at the higher speeds.

Automatic transmission
Planetary automatic transmission shift gears smoothly. Regulated by transmission control unit. Best gear always selected from operating conditions. Disadvantages Parasitic losses due to hydraulic pump required to operate clutches. Slip in the torque converter generating heat. Latest types are CVT’s (Continuously Variable Transmission) and Automated Manual Transmissions (AMT’s). The term parasitic loss is often applied to devices that take energy from the engine in order to enhance the engine's ability to create more energy. In the internal combustion engine, almost everything, including the drive line, causes parasitic loss. Bearings, oil pumps, piston rings, valve springs, flywheels, transmissions, driveshafts, and differentials also rob the system of power. An oil pump, being used to lubricate the engine, is a necessary parasite that consumes power from the engine (its host).

AUTOMATIC TRANSMISSION
It is the transmission which automatically provides varying gear ratios to suit operating conditions. Main components of automatic transmission: Torque converter Gearbox of planetary type with friction brake bands and multiple clutch operated by hydraulic system. Hydraulic control system. This system has a source of hydraulic pressure servo units and control valves.

BLOCK DIAGRAM OF AUTOMATIC TRANSMISSION

EPICYCLIC GEAR TRAIN Sun gear (S) = 30 teeth (A)
Annulus or Ring gear (A) = 75 teeth (B)

SIMPLE EPICYCLIC GEAR TRAIN

ALL SPUR TYPE GEAR

PRINCIPLE OF THE ALL SPUR TYPE
It comprises three adjacent independent gears A,B,D concentric with the driving and driven shafts S and S’ which are telescoped and three corresponding pinions A’, B’, D’ forming a rigid planetary unit. There are usually three such units, spaced uniformly around the gears mounted on the driving and driven shafts. Gear A is the driving and gear D the driven member. Low forward speed is obtained in a simple manner by holding the planet carrier from rotation, by means of a brake drum provided on it for purpose. Power then enters at gear A and is transmitted through the pairs of back gears A’, D’ to gear D, hence no planetary motion is involved in the low speed forward drive. For high forward speed the driving shafts is coupled to the driven shaft by means of a friction clutch incorporated in the assembly, which gives a direct drive. For the reverse motion gear B, which is free on the driving shaft, is held from rotation.

FORD T-MODEL TRANSMISSION

FORD T - MODEL The flywheel rim A serves as planet carrier and driving member, having lateral studs secured into it which carry triple planetary pinions. Gear B is the driven member, being keyed to the hub of clutch drum C, which in turn is secured to driven shaft D. By applying a brake band to drum E, gear F is held stationary, pinion G rolls on it, and the smaller pinion H causes gear B to turn slowly in the same direction as pinion carrier A. By applying a brake band to drum I, gear J is held stationary ; pinion H turns gear B slowly in the reverse direction. For the high gear or direct drive, the friction clutch locks clutch drum C to the engine crankshaft and the gear rotates as a unit.

The three pedals control the transmission and brakes.
When the left pedal is push down all the way, the car is low gear. To remain in low gear, you must continue pushing on the left pedal. When the left pedal is completely released, the car is in high gear. If the car is in neutral, the middle pedal can be pushed to engage reverse gear. The right pedal is the brake that acts on the transmission when pushed.

WILSON GEAR BOX

COTAL GEARBOX The wheel A is integral with the engine shaft and meshes with pinions carried by a spider A which is free to slide along the outside of the engine shaft. When the spider B is slid to the left, its teeth E mesh with teeth F of an annulus which is fixed to the gearbox casing. The pins of the spider form fixed bearings for the pinions and so the annulus C with which the latter mesh is driven in opposite direction to the wheel A. this gives the reverse drive. When the spider B is slid to the right its teeth E engage the teeth of the annulus C and then the wheel A, pinions, spider B and annulus C revolve solid. This gives the forward drives.

The four forward trains are obtained by means of two epicyclic trains arranged in tandem.
One consists of the sun D (fixed to the annulus C), compound planets P1, P2 (carried on the pins L of the arm which is integral with the annulus H of the second train) and the annulus G which can be held fixed. When this is done the annulus H is driven in the same direction as the sun D but at a lower speed. The second train consists of the annulus H, the sun K and the arm J which is fixed on the output shaft. The sun K can be held at rest so that the train gives a reduction between the annulus H and arm J and it can be locked to the output shaft so that the train must revolve solid. The annulus G can be locked to the sun D so that the first train must revolve solid.

WORKING The fixing and locking of the members is done by electromagnets whose windings S1, S2, S3 and S4 are energized as may be required. For first gear S2 and S3 are energized and both epicyclic trains provide a reduction since both annulus G and sun K are fixed. For second gear S2 and S4 are energized and the second train revolves solid, the only reduction being in the first train. For third gear, S1 and S3 are energized, the first train is locked solid and the only reduction occurs in the second train. For fourth gear S1 and S4 are energized and both trains revolves solid so that a direct drive is obtained. The windings S1 and S4 are carried by parts that sometimes rotate and so these windings are connected to slip rings on which brushes bear. The current for energizing the windings is supplied by the battery or generator of the car and is between 2 and 3 amperes The control is extremely simple consisting merely of a switch which connects the appropriate winding to the battery. This switch is mounted at the center of the steering wheel.

An automatic transmission is an automobile gear box that can change gear ratios automatically as the automobile moves under varying conditions, thus freeing the driver from having to shift gears manually. The main components of an automatic transmission are the converter housing case, oil pan and the extension housing. The converter housing encloses the torque converter and may be integral with the case or separately bolted to the case. The case contains the epicylic gear train while the extension housing encloses the output shaft. The oil pan is bolted to the case. The entire transmission unit is attached to the engine block by means of bolts through holes in the converter housing flange.

COMPARISON WITH MANUAL TRANSMISSION
MERITS More precise control of shifting results in better drivability No more skill required for operating. DEMERITS Higher fuel consumption due to lower mechanical efficiency. Higher initial cost. More complex construction which results in expensive repairs.

CONTINUOUSLY VARIABLE TRANSMISSIONS
CVT is an automatic transmission that can select any desired drive ratio within its operating range. Unlike a conventional four or five - speed transmission, CVT is an ‘infinite speed ratio’ transmission. Initially its use was limited and that too in small cars, yet the numerous advantages which a modern CVT offers, have made it quite popular in the modern cars. Honda Multimatic used in the Honda hybrid ‘Insight’ and Audi Multitronic fitted in their A6 model are recent examples.

PRINCIPLE OF WORKING OF CVT
A simple CVT has three main components: A variable – input driving pulley An output (driven) pulley A metal belt Apart from the above basic components, there are also various microprocessors and sensors and even epicyclic gearing and clutch. Both the driving as well as driven pulleys are of variable – diameter type. Each pulley is made of two 20° cones facing each other. A belt is there in the groove between the two cones. Earlier when rubber belt was used, they were of V – shaped section. The modern metal belts are specially constructed to provide for the required flexibility.

PRINCIPLE OF WORKING OF CVT
The driving pulley is connected to the engine crankshaft while the driven pulley transfers motion to the drive shaft. When the two cones of the pulley are close together, the belt rides higher in the groove and pulley diameter apparently increases, whereas the belt rides lower in the groove, making the effective diameter decrease, when the two cones of the pulley are far apart. The two cones are moved closer or far apart using hydraulic pressure, centrifugal force or spring tension (all through electronic control). When the diameter of driving pulley increases, the diameter of the driven pulley decreases and vice versa. This is done to keep the belt always tight. As the two pulleys change their diameter relative to each other, infinite number of gear ratios are obtained. For example, when diameter is small on the driving pulley and large on the driven pulley, the speed of the driven pulley is decreased resulting in low gear. Similarly high gear is obtained when diameter on the driving pulley is larger than the diameter of the driven pulley. A conventional CVT uses the information from engine management ECU to determine the torque being delivered by the engine and accordingly the pressure exerted on the belt to avoid slipping, usually exerting excessive pressure.

In Audi Multitronic, the torque is measured only when it is entering the CVT so that the pressure exerted is just enough for the torque transmission and no more thus improving its efficiency. Further the drive is transmitted separately to forward gears and to reverse through two multiplate clutches. There are also two small hydraulic chambers for moving the variator pulleys instead of one large chamber in case of conventional CVT’s, this reduces the power consumed. ADVANTAGES Constant stepless acceleration from start to high speed, eliminating ‘shift shock’ to provide a smoother ride. It keeps the car in optimum power range under all conditions, thus decreasing fuel consumption. Responds better to changing conditions e.g. throttle and speed changes, which eliminates gear hunting while moving up an incline. Less emissions due to better engine control under all conditions. DISADVANTAGES Torque handling capability of a CVT is limited by the strength of the chain or belt inside it, limiting its use in small cars.

CVTs not been successfully used for rear wheel drive vehicles due to packaging, CVT needs large centre distance hence successfully used in front wheel drive vehicles in small and medium vehicles. Life of a CVT automatic in a light duty use has been found to be same as conventional automatic transmission but when subjected to heavy stop and go driving as in case of delivery vehicles CVT is found to have much shorter life. General Information Simple CVT does not have a neutral position, which is provided by some form of clutch arrangement. Reversing is done by use of a single epicyclic gear train, which would reverse the output from the variator pulley. Next gen CVT combines CVT with an auxiliary gear box and significantly increased gear ratio.

TOROIDAL CVTs These transmissions consists of two sets of planetary type steerable rollers between two toroidal shaped discs, one driving and the other driven. By tilting the steerable rollers, the relative diameters of the input and the output discs can be varied to get a desired speed ratio. Torque is transferred at the point of contact between the rollers and the discs under a high shear stiffness traction fluid under very heavy pressures which causes the fluid to become glass like exhibiting elasto hydraulic behavior which prevents metal to metal contact between the rollers and discs. Toroidal CVTs can transmit very high torques at high efficiencies. Due to their larger mass and higher manufacturing cost, same have not found wide acceptance.

AUTOMATED MANUAL TRANSMISSIONS
These are basically the manual transmissions operated automatically, hydraulically or electrically. They do not require clutch actuation and gear shifting manually by the driver. Thus there is no clutch pedal and no mechanical connection between the transmission and the selector lever. The transmission is controlled electronically through shift by wire. AMTs can be broadly divided in to the following types: Single sided clutch transmission Double sided clutch transmission Dual clutch transmission Single sided clutch transmission: This type incorporates actuators with an existing production manual transmission for operating the clutch and shifting the gears. Since this type can use a conventional manual transmission without any modification to its existing shafts and gears, it is the cheapest of all the AMT types. However it has the disadvantage of the inherent power – interruption during the gear shifting which is present in a manual transmission.

This type is still better than the manual type because it operates the clutch and moves the shift lever much faster than a human being. It also adjusts the engine speed faster and more accurately to achieve a synchronous shifting. Double – sided clutch transmission (DSCT) In this type, the double sided clutch allows power from the engine to be transferred between either of two power paths in the transmission itself. Due to this the total clutch travel time is drastically reduced, consequently improving the shift quality. Dual clutch transmission (DCT) Also called Direct Shift Gear Box, it consists of two , linked layshaft transmissions with two power paths; one for even and the other for odd number gears. There is a layshaft as in an ordinary manual gearbox, but in this case, the layshaft consists of two concentric shafts, one inside the other. There are two output shafts that mesh to a third shaft which goes to the differential. At the input end each layshaft has a clutch to allow shifting. The shift quality is better and it can shift gears in 8 milliseconds which is 50 times quicker than the blinking of an eye which takes 400 milliseconds.

6 SPEED DUAL CLUTCH TRANSMISSION SCHEMATIC

6 SPEED DCT ARRANGEMENT One clutch engages gear as the other one disengages, thus minimizing the time the gear box is not driven under power, which means least fluctuations in the torque output. Both in DSCT and DCT, since the shifting is achieved through synchronization of only the non – engaged partial transmission, it becomes impossible to skip two gears.

Advantages of DCT: Improved acceleration, fuel economy and emissions compared to conventional automatic transmissions. Gentle, jerk free gear changes as in an automatic transmission, combined with the efficiency of a manual transmission. Less complicated and lighter since automatic’s torque converter, fluid pump and multiple clutches are eliminated, reducing the weight by approximately 30%. Less costly. Disadvantage: Limited ability to skip gears which is an essential characteristic of an automatic transmission. A dry clutch CVT has 1% additional fuel economy benefit compared to a wet clutch DCT due to the elimination of the oil pump and its accompanying losses.

DRIVELINE It is the group of parts connecting the transmission with the driving wheels. It consists of the driveshaft (also called propeller shaft), universal joints/constant velocity joints and the half shafts. DRIVESHAFT Propeller shaft connects gearbox to the final drive gears of the vehicle through universal joint and serves as drive shaft. A universal joint allows the drive to be transmitted through a variable angle. The drive system is an arrangement for transmitting the driving thrust from the road wheels to the vehicle body. The final drive is the transmission system between propeller shaft and differential. The differential mechanism is built into the centre portion of the final drive. This permits the wheels to rotate at different speeds without interfering with the propulsion of the vehicle while taking a turn. In case of rear wheel drive, the rear axle is “live”, which in addition to support the weight of the vehicle contains a gear and shaft mechanism to drive the road wheels.

DRIVELINE

PROPELLER SHAFT AND DRIVE SHAFT
Propeller shaft, sometimes called a carden shaft, transmits power from the gearbox to the rear axle. Normally the shaft has a tubular section and is made in one- or two-piece construction. The two-piece arrangement is supported at the mid point by a rubber mounted bearing. Short drive shafts are incorporated for the transmission of power from the final drive assembly to the road wheels in both front and rear wheel drive layouts.

PROPELLER SHAFT This shaft must be strong to resist the twisting action of the driving torque and it should be resilient to absorb the torsional shocks. It must resist the natural tendency to sag under its own weight because vibration occurs when the centre of gravity does not coincide with the axis of the shaft. A tubular-section propeller shaft is normally used because it has low weight, provides large resistance to misalignment, especially sag, has good torsional strength, and provides low resistance (low inertia) to changes in angular speed, which arise when a Hookes type coupling is used to drive the shaft. Since a propeller shaft often rotates at high speed, specifically during the use of the overdrive gear, it must be manufactured, and repaired, meeting design specifications and good balance limits. Even after a perfect static alignment, shaft sags (i.e. forms a bow) at the centre due to its own weight. When this sagging becomes excessive, rotation of the shaft causes the bow to increase due to the centrifugal effect. This deformation, or whip of the shaft, sets up a vibration that becomes severe as it approaches the whirling speed.

PROPELLER SHAFT – WHIRLING SPEED
The critical speed at which this condition occurs depends on two vital dimensions i.e., the mean diameter of the tube and the length of the shaft. Since propeller shafts of road vehicles are sufficiently long and operate in general at high speed, whirling may occur at certain critical speed. This produces bending stresses in the material that are higher than the shearing stresses caused by transmitted torque. While the critical speed increases with decrease in the mass of the shaft, the moment of inertia of the section increases. The tendency for the propeller shaft to whirl should be reduced and to do so, it should be made tubular and should be perfectly balanced. Critical speed of the propeller shaft varies directly as the diameter of the tube and inversely as the square of the length. Therefore, diameters are selected as large as possible and lengths as short as possible to keep the critical speed frequency of the shaft above the driving speed range.

CRITICAL SPEED TORQUE RATING

Many vehicles with rear- and four-wheel drive require a long propeller shaft to span between the gearbox and final drive. In these situations the drive line is normally divided and a bearing is fitted to support the shaft at the point of division . This bearing is mounted in rubber to absorb any vibration that would otherwise be transmitted to the body. Although axle movement is restricted to the rear shaft and universal joints are fitted to accommodate this movement, extra joints are needed on the front shaft to allow for the slight flexing of the vehicle body. It is practically impossible to maintain the correct drive angles of Hooke-type couplings fitted to a two piece shaft layout, so one or more CV joints are used in many arrangements.

PROPELLER SHAFT The composite propeller shaft is an alternative to the divided arrangement. The tubular shaft is made of epoxy resin, which is strengthened by using glass and carbon fibres, and bonded to a steel spigot for connection to the universal joints. The advantages of the composite shaft over a conventional two piece steel shaft arrangements are : Weight reduction by about 50 percent. High internal shock absorption. Good noise, vibration, harshness (NVH) performance. (iv) Exceptional corrosion resistance.

DRIVE SHAFTS These shafts are comparatively short in length and where space is a limitation, they are made solid to provide clearance for movement of the suspension, and otherwise the lightweight tubular section is often used. The short distance between the road wheel and the final drive housing, combined with a large road wheel movement due to suspension deflection, causes the maximum drive angle of the universal joints and large variation of length of the shaft. A CV joint at each end of the drive shaft meets the angle requirement and a plunge CV joint accommodates the length change. Rear-wheel drive vehicles having independent rear suspension need a drive shaft to connect the road wheel to the fixed final drive assembly. On these vehicles normally a plunge-type CV joint is incorporated at each end of the drive shaft.

UNIVERSAL JOINTS Universal joints are capable of transmitting torque and rotational motion from one shaft to another when their axes are inclined to each other by some angle, which may constantly vary under working conditions. Universal joints are incorporated in the of vehicle’s transmission system to perform three basic applications : (a) Propeller shaft end joints between longitudinally front mounted gearbox and rear final drive axle. (b) Rear axle drive shaft end joints between the sprung final drive and the unsprung rear wheel stub axle. (c) Front axle drive shaft end joints between the sprung front mounted final drive and the unsprung front wheel steered stub axle. Universal joints have movement only in the vertical plane when they are used for longitudinally mounted propeller shafts and transverse rear mounted drive shafts. When these joints have been used for front outer drive shaft they have to move in both the vertical and horizontal plane to accommodate both vertical suspension deflection and the swivel pin angular movement to steer the front road wheels.

BASIC TYPES OF UNIVERSAL JOINTS
A) Cross – type B) Hooke joint RUBBER JOINTS (C) Moulton joint (D) Layrub joint (E) Doughnut joint

Cross-type Joint. This type of joint is also called a Hooke-type coupling as it was developed from the joint invented by Robert Hooke in the seventeenth century. This joint is commonly used today. They use two yokes set at 90 degrees to each other and a cross-shaped trunnion block joins these yokes. In more developed joints like Hardy Spicer type, contact between the two parts is made by needle roller bearings held in a hardened steel cup retained in each arm of the yoke. For the alignment of the trunnion, the bottom of the cup forms a contact with the end of the block.

A special viscous oil, similar to that used in a final drive, is used for bearing lubrication, which is contained in a reservoir formed by drilling out the centre of the trunnion arms. The oil is introduced by a lubrication hippie or is prefilled once for entire life. An oil seal, retained on each arm of the block, prevents the escape of the lubricant. The cups are held in the yoke either by circlips or staking. The replacement of worm parts in the joint becomes more difficult due to peening over the edge of the yokes to stake the cups. Therefore replacement of the complete shaft assembly is recommended when the joint is worn. These joints offer several advantages such as they (i) are compact, (ii) have high mechanical efficiency, (iii) have ability to drive through a large occasional ‘bump’ angle (maximum about 25 degrees), and (iv) due to accurate centring of shaft, are suitable for high speed operation. Disadvantage of the cross-type joint is its inadequate flexibility to absorb torsional shocks and drive-line vibrations, especially when a comparatively rigid transmission system is used.

Lubrication failure, especially when a grease nipple in the trunnion block is missed, causes the needle rollers to indent the bearing surfaces. This type of wear causes a slight angular movement and produces a noise, commonly described as a clonk, during the change over from drive to over-run and vice versa. If this fault is not rectified in time, the rate of wear accelerates leading to misalignment and severe vibration. RUBBER JOINTS A smoother and less harsh drive is obtained by incorporating one or more rubber joints in the transmission driveline. Three types of rubber joints in use include Moulton, Layrub and Doughnut. Moulton Joint This rubber trunnion type joint is based on a Hooke type coupling. It uses moulded rubber bushings for the transmission of drive between the trunnion and yokes. These synthetic rubber mouldings require no lubrication and due to high flexibility they damp the torsional shocks produced when the drive is transmitted through an angle.

Layrub Joint. This type joint originally made by the Laycock company, was constructed of a series of rubber bushings. The name layrub is used to describe this joint. It uses a number of moulded rubber blocks, with specially shaped cavities at the ends. These blocks are sandwiched between two steel pressings. Each shaft is connected by means of a fork to alternate rubber blocks. This arrangement permits the rubber blocks to deform making the drive possible for transmission through a small angle. Also the blocks accommodate small axial and angular movements for shaft length alteration and torsional damping. This coupling is relatively large in diameter. The layrub type joint offers several advantages, such as (i) it does not require lubrication, (ii) it is capable of driving through bump angles up to about 15 degrees, (iii) it allows for axial movement, requiring no splining of the shaft and (iv) its resilience damps shocks and insulates vehicle from transmission noise.

Doughnut Joint. Although large in size, the great flexibility of this joint provides soft cushioning. This absorbs the majority of torsional shocks generated by the action of other joints or by vibration from either the engine or road wheel. The synthetic rubber coupling is near-circular in shape and is moulded around cylindrical steel inserts, which are bolted alternatively to the three-arm forks fixed to the shafts. The merits of this coupling are similar to that of layrub joint. Speed Variation of a Hooke-type Joint due to Drive and Driven Shaft Inclination When a hooke-type coupling transmits a drive through an angle, the output shaft does not rotate through 360 degrees at a constant speed. Instead the speed varies every 90 degrees of rotation, and the rate of movement for one revolution is fast, slow, fast, slow . This cyclic speed variation, and its associated vibration, is insignificant when the drive angle is less than about 5 degrees, but becomes much more intense as the angle is increased.

Due to the above variation of the speed of the driven shaft for various positions of the driving shaft, a single Hooke’s joint becomes unsuitable for the power transmission in automobiles. But a constant velocity ratio can be obtained by the correct use of a double joint. To achieve a constant speed output from the propeller shaft two Hooke-type couplings can be mounted either back-to-back or positioned in a certain way at each end of the propeller shaft. From this diagram it can be seen that to obtain a constant speed, (i) yokes at each end of the propeller shaft must be placed in the same plane, and (ii) drive angle of each coupling must be equal.

CONSTANT VELOCITY JOINTS
A constant velocity (CV) joint imply that when two shafts are inclined to one another at some angle and are coupled together by some sort of joint, then a uniform input speed transmitted to the output shaft produces the same angular output speed throughout one revolution. There are no angular acceleration and deceleration as the shafts rotate. Various CV joints in use have a construction, which is based on either the twin hooke-type coupling arrangement or the angle bisects principle. The CV joints in use include : i.) Tracta ii.) Rzeppa iii.) Weiss iv.) Tripode

TRACTA CONSTANT VELOCITY JOINT /BENDIX TRACTA JOINT
Pictorial view. Exploded view. Front wheel drive hub swivel pins and axle incorporating a tracta joint. Plan view.

RZEPPA JOINT

WEISS JOINT

Tripode (Tripot) Type CV Joint

REAR AXLES FINAL – DRIVE
The rear axles final drive (i) transmits the drive through a angle of 90 degrees, and (ii) gears down the engine revolutions to provide a ‘direct top’ gearbox ratio. In the case of cars a final drive ratio of approximately 4 : 1 is used. Bevel or worm gears are employed to achieve the various functions of the final drive. Bevel Gears A bevel gear layout, which represents two friction cones ‘A’ forming the crown wheel and ‘B’ the pinion. For avoidance of slippage and wear, the apex of the pinion must coincide with the centre line of the crown wheel. The system with incorrectly positioned pinion causes unequal . peripheral speeds of the crown wheel and pinion. It is necessary to mount the gear in the correct position so that angle of the bevel is governed by the gear ratio.

TYPES OF BEVEL GEAR. Straight Bevel.
The main features of the bevel type of gear is illustrated in the figure. The tapered teeth, generated from the centre, are machined on the case-hardened steel gears and then ground together to form a ‘mated pair’. The position of the crown wheel relative to the pinion determines the direction of rotation of the axle shaft. If the crown wheel is fitted on the wrong side, which is possible on some vehicles, then this provides one forward and several reverse ratios. For correct meshing and for setting the clearance between the teeth (backlash), adjusters in the form of distance pieces, shims or screwed rings are used. When backlash is too small, expansion results due to heat and wear is caused by lack of lubrication. On the other hand excessive backlash produces slackness and noise. Each manufacturer recommends a suitable backlash, but it is generally in the region of 0.15 mm for cars and 0.25 mm for heavy vehicles.

Spiral Bevel. Although the straight bevel is cheaper and mechanically efficient, the meshing of the gears causes an unwanted noise, which has been reduced by introducing a helical form of tooth. It is impossible to generate a helix on a tapered pinion, so the gear is called as a spiral bevel. A number of teeth are generated from the centre of the crown wheel, and form a left-handed spiral in the case of the pinion. This direction provides a large outward thrust on the drive and a smaller inward thrust on the over-run so that wear of the pinion bearing increases the backlash instead of causing seizure of the gear.

Spiral bevel Since the crown wheel teeth are inclined to the pinion, the tooth pressures are much higher. The gear oil with no additives, and high-viscosity, suitable for the straight bevel type, is not satisfactory when used in spiral bevel units. The oil film brakes down under the high loads, causing rapid wear and scoring. For spiral bevel gears special lubricants, known as EP (extreme pressure) lubricants, have been developed. They contain additives like sulphur, chlorine and phosphorus compounds, which chemically react at high temperatures with the metal surface to form a compound of low fractional resistance. Hypoid Gear This type of gear is the commonly used now a days. The pinion axis of this gear is offset to the centre line of the crown wheel. Although the gear can be placed above or below the centre, but in cars it is always placed below to allow for a lower propeller shaft so that a reduction in the tunnel height is possible. Pinion offset can vary with the application, but an offset of one-fifth the wheel diameter is commonly used. If the axis is lowered, the tooth pitch of the pinion increases, so that for a given ratio, the pinion diameter can be larger (30 percent for normal offset). This enables the use of a stronger gear specifically on commercial vehicles.

A hypoid is considered to be halfway between a normal bevel and a worm drive.
In the former case a rolling action occurs, whereas the latter case is totally sliding. An increase in the sliding motion in the hypoid gear reduces meshing noise, but the high temperature and pressure of the oil film puts a strain on the lubricant. To overcome this problem, a special extreme pressure oil is used which contains expendable EP agents to resist scuffing and wear at high temperature. It also contains a fatty acid to improved boundary lubrication at low temperature.

Worm and Wheel Drive Since this drive is expensive, it is rarely used nowadays as a final drive on light vehicles, but is still used on heavy vehicles. However, this type of gear has a number of other applications on motor vehicles. Various arrangements, illustrated in figure, can be employed to provide a very quiet and long-lasting gear, but efficiency is less than the bevel (94 percent against 98 percent). This type of gear provides a large reduction in a small space. The worm may be mounted below (under-slung) or above (overhead) the wheel. An hour-glass or Hindley worm embraces more teeth than the straight worm but adjustment becomes more critical.

The sliding action of the worm causes friction, which is reduced by using a worm wheel of phosphor-bronze and a worm of case-hardened steel, but still the unit becomes hot. A large, well-cooled sump is incorporated to reduce oxidation of the oil at high temperature, which otherwise causes the oil to thicken. To improve the boundary lubrication, vegetable-based oil is sometimes used as an alternative to straight bevel gear oil.

REAR WHEEL DRIVE ARRANGEMENTS
The statement “every action has an equal and opposite reaction”, means that every component that produces or changes a torque also exerts an equal and opposite torque tending to turn the casing. To understand the torque reaction consider a tractor with its rear driving wheels locked in a ditch. In this situation torque reaction is likely to lift the front of the tractor rather than turn the rear wheels. When the above principle is applied to rear axles, some arrangement must be provided to prevent the axle casing turning in the opposite direction to the driving wheels. A torque (t) applied to the wheel, which may be considered as a lever produces a tractive effort (Te) at the road surface, and an equal and opposite forward force at the axle shaft. This driving thrust must be transferred from the axle casing to the frame in order to propel the vehicle. The maximum tractive effort is limited by the adhesive force (P) of the type on the road. This force depends on the coefficient of friction (μ) and the load (W) on the wheel.

TYPES OF DRIVE A drive system is an arrangement, which transmits the driving thrust from the road wheels to the vehicle body and also incorporates a means to resist the movement of the main components due to torque reaction. In the early days only leaf springs were used for the rear suspension of a vehicle, and hence these springs were often utilized to provide the drive thrust and torque reaction functions of the drive system. Since 1950, three notable drive systems are in use on motor cars and these basic systems have undergone several modifications to meet modern requirements. The drive systems include, Hotchkiss open-type Four-link (semi-Hotchkiss) Torque tube de-Dion. Although these systems are not used today in their original form, the objective of describing them is to make the reader familiar with the arrangement used to resist the various forces associated with the propulsion of a vehicle. However, this knowledge may help in the diagnosis of many of the faults associated with drive systems.

Hotchkiss Open-type Drive
This type of drive is commonly used on passenger cars and heavy commercial vehicles. This arrangement uses two rear leaf springs, which are longitudinally mounted, and are connected to the frame by a ‘fixed’ pivot at the front, and swinging shackles at the rear. A universal joint is mounted at each end of the exposed or ‘open’ type propeller shaft, with provision for accommodating change in shaft length due to the deflection of the springs. This drive, therefore, incorporates an open propeller shaft with two universal joints and a slip joint.

To resist torque reaction the axle is clamped to the springs using ‘U’ bolts. Under heavy driving conditions the springs deflect up at the front and down at the rear and vice versa during braking. This movement helps to damp driving shocks and improves transmission flexibility. A universal joint is installed at rear to accommodate continuous up and down motion of the axle. Driving thrust is transferred from casing to the spring by the friction between the two surfaces, and then transmitted through the front section of the springs to the vehicle frame. If the ‘U’ bolts become loose, the spring centre bolt (axle location bolt) has to take the full driving thrust, so that early failure of the bolt takes place due to the high shearing force.

In the Hotchkiss drive, rear axle torque and the propelling and retarding forces are taken up by the rear springs. The rear springs, which are generally half-elliptic type, are shackled to spring seats on the axle housing at one end and are pinned to the chassis frame at their forward end. The shackles may be vertical or may be arranged at some angle (a) as shown in figure. Thus the springs act as both torque and thrust members in this case.

Four-link (Semi-Hotchkiss) Drive
When helical springs are used in conjunction with a live rear axle, these springs cannot take driving and braking thrust, torque reaction or give lateral support to the rear axle. Therefore additional arrangements must be incorporated to meet these requirements. It may appear that the helical spring provides a reduction in the unsprung weight, but in practice when the weight of the additional locating arms and rods fitted to support this arrangement is added, the unsprung weight difference becomes very small. The rear axles is positioned by upper and lower trailing suspension arms in the four-link drive system layout. These arms transmit driving thrust and prevent rotation of the axle casing.

Four-link (Semi-Hotchkiss) Drive
A transverse stabilizer, called a Panhard rod, connects the rear axle to the vehicle body and thereby controls sideways movement of the axle. Rubber mountings are fitted at each connection point to reduce noise transmission and to eliminate the need for lubrication. Also these mountings provide slight flexibility to allow for drive movement and geometric variations during spring deflection.

Torque-tube Drive This drive system is generally used in passenger cars and light commercial vehicles. Whereas the Hochkiss drive uses stiff springs to resist torque reaction and driving thrust, the torque tube drive permits the use of either ‘softer’ springs or another form of spring, like helical to perform their only intended duty so that a ‘softer’ ride is possible. A layout using laminated springs, which are connected to the frame by a swinging shackle at each end. A tubular member called torque-tube, encloses the propeller shaft and is bolted rigidly to the axle casing. The torque-tube is positioned at the front by a ball and socket joint, which is located at the rear of the gearbox or cross-member of the frame. Bracing rods are introduced between the axle casing and the torque tube to strengthen the arrangement. A small-diameter propeller shaft is installed inside the torque tube and splined to the final-drive pinion. A universal joint is installed in the centre of the ball joint to allow for angular deflections of the drive. In this design the torque reaction and driving thrust are taken up by the torque tube.

Since the forward thrust from the ball is taken on the rear housing of the gearbox, arrangements must be incorporated to transfer this force through the gearbox mountings to the frame. When helical or torsion bar springs are used as alternatives to laminated springs, side movement of the axle must be controlled by providing some form of transverse stabilizer, such as a Panhard rod, between the frame and the axle. This system uses one universal joint and a slip joint. Since the torque tube is fitted rigidly to the rear-axle centre housing, this eliminates the use of a universal joint at the rear end of the propeller shaft.

FORCES ACTING ON TORQUE TUBE DRIVE

FREE BODY DIAGRAM OF TORQUE TUBE DRIVE

de-Dion Drive The de-Dion axle is often considered as the halfway stage between the normal axle and independent suspension. This layout provides many of the advantages of the independent suspension, but the system is not classed as independent, as the rear wheels are still linked by an axle tube. In the basic arrangement laminated springs are mounted on the frame by a ‘fixed’ pivot at the front and a swinging shackle at the rear. To support the wheel on a stub axle shaft, each spring is equipped with a hub mounting, which is rigidly connected to a tubular axle beam. The final-drive unit, which is bolted to a cross-member of the frame, transfers the drive to road wheels through two universally jointed shafts. The main propeller shaft is fitted with a universal joint at each end to allow for flexing of the frame.

In this design, the torque reaction of the final-drive casing is absorbed by the frame, and the driving thrust is resisted by the springs. The major advantage of this arrangement is the reduction in unsprung weight. This reduces wheel spin by permitting the light driving wheels to follow closely the contour of the road surface. Wheel spin is also caused by the tendency of the normal axle to rotate around the pinion due to a high propeller shaft torque. This lifting tendency of the wheel is eliminated in this drive system, and weight is equally distributed.

REAR AXLE CONSTRUCTION
The vehicle with non-independent rear suspension uses either a dead axle or a live axle. The dead axle only supports the weight of the vehicle, but the live axle besides fulfilling this task, contains a gear and shaft mechanism to drive the road wheels. The arrangements for supporting the road-wheels on live axles and providing the driving traction use an axle-hub mounted on to the axle-casing and supported by ball or roller-bearing. The two main components installed inside the axle of a rear-wheel drive vehicle are the final drive and differential. Axle Casing The casing used now a days is either a banjo or carrier-type. In the past a split (trumpet) casing was occasionally used.

BANJO TYPE The tubular axle section of this casing is built up of steel pressings, which is welded together and suitably strengthened to withstand the bending load. The centre of this casing with the axle tube on one side resembles a banjo. The final drive assembly is mounted in detachable malleable iron housing and is secured by a ring of bolts to the axle casing. The axle shafts are slid into this assembly from the road wheel end of the casing. On some banjo axles a domed plate is bolted to the rear face of the casing. Removal of this plate provides excess to the final drive gears and in cases where the axle shaft is secured to the differential, this enables the axle shaft to be unlocked from the sun gear (side gear). A lubricant level plug is screwed into the domed cover or the final drive housing at a height about one third up the crown wheel, which is normally just below the axle tubes. This allows lubrication of the hub bearings by splash caused due to rotation of the crown wheel. Overfilling of the lubricating oil swamps the oil seals causing the oil to enter the brakes and hence this should be avoided. The final drive becomes hot during operation, hence some form of air vent is provided to release the pressure in the axle casing. Consequently the possibility of oil being forced past the seals is prevented.

Carrier Type. This type of casing is more rigid than a banjo type and is often employed to support a hypoid gear. The final drive assembly is installed in a rigid malleable cast iron carrier, into which the axle tubes are pressed and welded. For extra rigidity reinforcing ribs extend from the pinion nose to the main carrier casing. A domed plate is fitted at the rear of the casing to provide access to the final drive gear. Axle Shafts and Hub Arrangements The axle shaft transmits the drive from the differential sun wheel to the rear hub. The various types of shafts may be compared based on the stresses they resist. A simple automobile shaft has to withstand (i) torsional stress due to driving and braking torque. (ii) shear and bending stresses due to the weight of the vehicle. (iii) tensile and compressive stresses due to cornering forces.

Axle Shafts and Hub Arrangements
Axle shafts are divided into semi-floating, three-quarter floating and fully floating depending on the stresses to which the shaft is subjected. Axle half-shafts are situated on each side of the final drive and convey motion to the road-wheels. There are basically three different arrangements of supporting axle wheel hubs on the rear-axle casing. These include : (i) Semi-floating axle hub (commonly used on cars). (ii) Three quarter floating axle hub (rarely used today). (iii) Fully floating axle hub (commonly used on heavy vehicles). A tough, hard material is used for the axle shaft to withstand the various stresses, resist spline wear and provide good resistance to fatigue. Medium carbon alloy steel containing nickel, chromium and molybdenum is generally used to manufacture axle shafts.

SEMI FLOATING AXLE HUB The road-wheel is attached to the axle hub, which is an extension of the axle half-shaft. A single bearing inside the tubular axle-casing supports the outer end of the shaft. The inner end of the shaft is splined and supported by the final-drive unit, which itself is mounted on bearings within the axle casing. The semi-floating axle along with its overhanging hub is subjected to the driving torque as well as to both vertical and horizontal loads. The vertical load produces a shearing force, and the distance between the wheel and the suspension-spring seat on the axle causes a bending moment, the reaction of which is shared between the axle bearing and the final-drive-unit bearings.

The horizontal load due to tilting of the vehicle, cornering centrifugal force, or side wind gives rise to both side-thrust and a bending moment. This bending moment may add to the vertical bending moment or may oppose it, depending on the direction of application of the side-force. A semi floating axle, suitable for small and medium sized cars. The axle half shaft and flanged hub are forged from a single piece of nickel chrome steel. The hub end of the shaft is provided with a larger diameter than the rest of its length, which resists the vertical and horizontal loads. The outer face of the flanged hub is shouldered so that it centralizes accurately the brake drum. The flange is provided with evenly spaced holes around it for wheel studs.

A semi-floating axle uses a taper-roller bearing, which is suitable for larger and higher-performance cars because of its greater load-carrying capacity. A separate hub is wedged on to a keyed and tapered half-shaft and a castellated nut holds it is position. The taper-roller-bearing inner cone fits with a light force inside the mouth of the casing. The exact position of the bearing in the casing is provided by shims packed between the casing flange and the brake back-plate. Increasing the thickness of the shims on one side and decreasing it on the other shifts both half-shafts further to one side relative to the axle casing. On either road-wheel the outward thrust is absorbed by the adjacent hub bearing, while inward thrust is transmitted to the opposite bearing through the axle half-shafts and a slotted axle-shaft spacer . Therefore, each hub bearing takes thrust in one direction only.

SEMI FLOATING AXLE BEARING LOADS
Bearing loads due to side thrust on a wheel in semi-floating axle is shown Let F = lateral force at the rim of the wheel r = radius of the wheel L = distance between the centres of wheel bearings , R1 and R2 = radial reactions of the wheel bearing on the wheel hub P = the thrust reaction of the bearing In practice, the ratio r/L = 0.6. Considering the forces in the horizontal and vertical directions, P = F and R1 = R2 Taking moments R1*L = R2*L = Fr Hence R1 = R2 = (r/L)*F = 0.6F = (3/5)*F Therefore, for semi-floating type axles, P is equal to F, and R1 and R2 each approximately equal to three fifth of F. R1 adds to the normal static load on the bearing, whereas R2 opposes it.

Three-quarter-floating Axle-hub.
The road-wheel, in this case also, is bolted to the hub forming part of the axle-shaft. The outer end of the shaft and hub is supported by a bearing located over the axle-casing. The bearing in this case is positioned between the hub and the casing unlike between the axle and the casing as in the semi - floating layout. In the three-quarter-floating axle and hub arrangement, the driving torque is transmitted by the shaft, but the shear force and bending moment are absorbed by the tubular axle-casing through the hub bearing, only if the road-wheel and the hub bearing lie in the same vertical plane.

Practically, a slight offset of wheel and bearing centres exist so that the hub is tilted relative to the axle-casing. This is resisted by the bearing, but in case this offset is large, the half-shaft provides the additional resistance. Horizontal loads, which create end-thrust, are opposed by the hub bearing and casing. However, the side-forces create a bending moment, which tends to twist the wheel relative to the axle-casing. This tilting tendency is resisted mostly by the hub bearing and partly by the axle-shaft. A large tilting force therefore tends to overload the bearing if it is not adequately sized.

A three-quarter-floating axle was once very popular for cars and light commercial vehicles when semi-floating half-shafts frequently failed due to fracture, specifically in cold weather. However, due to availability of the compact, cheap and reliable semi-floating axle, the three-quarter-floating arrangement is rarely used today. The half-shaft uses an upset-forged flange at the outer end, which is clamped to the bearing hub by the wheel studs. Either a large-diameter single-row or a double-row ball-race bearing is used depending upon light- or heavy-duty applications. This bearing is located on the axle-casing and is secured in position by a large nut. The outer bearing track supports the hub. An oil-seal is placed at the back of the hub to prevent excess oil, coming from the final drive, to escape to the brakes from the hub.

Fully Floating Axle-hub.
This axle-hub arrangement incorporates a flanged sleeve, which is positioned over the axle casing. The flange is provided to accommodate the road-wheel or wheels. Two bearings widely spaced are installed between the hub and the casing to support the hub assembly. This provides am improvement on the first two types of hub support. The axle-shaft in this case takes only the turning-effort or torque. Both the vertical and horizontal load reactions are resisted by a pair of widely spaced taper-roller bearings installed on the axle-casing. The axle half-shaft, therefore, is free from all the loads except the torsional drive to the wheel.

The construction is such that the two hubs on their bearing rotate independently of the half-shaft.
Studs connecting the shaft to the hub transmit the drive and when the nuts on these studs are removed, the shaft may be removed without jacking up the vehicle and without interfering with the load-supporting role of the hub. This layout, therefore, allows the vehicle to be towed with a broken half shaft. This is a larger and more expensive construction than both the other layouts. This is specifically suitable for all truck and heavy-duty vehicles employing live axles and for trailers using dead axles where torque and axle loads are greater. Depending upon the application, single or twin road-wheels are used.

FULLY FLOATING AXLE BEARING LOADS
F = lateral force at the rim of the wheel r = radius of the wheel L = distance between the centres of wheel bearing R1 and R2 = radial reactions of the wheel bearing on the wheel hub P = the thrust reaction of the bearing In practice, the ratio r/L = 4. Considering the force in the horizontal and vertical directions, P = F and R1=R2 Taking moment, Fr = R1L and Fr = R2L Hence, R1=R2 = (r/L)F = 4F R1 adds to the normal static load on the bearing, whereas R2 opposes it. The thrust load on the bearing is equal to the shock load, F on the wheel, whereas radial shock loads, R1 and R2 on the bearing are each approximately four times F.

Differential When both rear wheels are connected to a common driving shaft rapid wear of rear tyre, and difficulty in steering from the straight-ahead position are soon experienced. It can be seen in that the outer wheel must travel a greater distance than the inner wheels during cornering of the vehicle. Hence, if the wheels are interconnected, the tyres have to ‘scrub’ over the road surface and tend to keep the vehicle moving straight ahead. These problems can be minimized by driving one wheel and allowing the other to run free. But this provides unbalanced driving thrust and unequal cornering speeds due to which the arrangement was not accepted. The problem was solved in 1827 by Pequeur of France who invented the differential. This mechanism rotates the wheels at different speeds, while maintaining a drive to both wheels.

DIFFRERENTIAL PRINCIPLE
Consider the two discs, illustrated in Fig. A, are joined by shafts to the wheels and interconnected with a lever. When a force, F, is put to C at the centre of the lever, each disc receives half the applied force. The movement of the discs depend on the resistances, R, opposing the motion of the shafts. If a larger resistance acts on disc ‘B’, the lever tilts, and pushes disc ‘A’ forward a greater amount. This condition is illustrated in plan view in Fig. B and the increase in distance travelled by A equals decrease in distance travelled by B, and increase in speed of A equals decrease in speed of B. Therefore, A + B = 2C.

The disc system is replaced by bevel gears in Fig
The disc system is replaced by bevel gears in Fig. C, called sun wheels (disc) and planets (levers). The drive, applied to the cross pin, pushes the planet gears forward and exerts an equal torque on each sun wheel irrespective of the speed. When the vehicle negotiates corner, the inner wheel slows down and the planets rotate on their own axis allowing the outer wheel to speed up. During straight-ahead movement of the vehicle the whole unit rotates at the same speed.

A differential unit is shown in Fig
A differential unit is shown in Fig. D in which a crown wheel is bolted on to a differential cage. This cage also carries the sun wheels on plain bearings and transmits the drive to the cross pin. Two planet gears are sufficient for light vehicles but four gears are necessary in heavier vehicles to reduce tooth pressures. The differential is lubricated by the final-drive oil, which splashes through holes in the differential cage.

DIFFERENTIAL LOCK In a two-wheel drive vehicle if one driving wheel loses adhesion, the propelling force is considerably reduced. This results in the immobilization of the vehicle so that the differential action becomes undesirable on the vehicles designed to operate over poor surfaces. This action can be prevented by locking together any two individual units of a differential. A sliding dog clutch member is splined to a differential sun wheel, which engages with dog teeth formed on the cage of the differential. The clutch is engaged by means of a fork, which can be moved by a lever fitted on the outside of the axle. When engaged, the sun wheel, and hence the rear wheel connected to this sun gear, is made to turn at the same speed as the cage. Locking one sun gear to the cage in this way ensures that the other sun gear also turns at the same speed.

Limited-Slip Differential.
A differential does not provide a high mechanical efficiency, which is desirable for the majority of mechanical components. Even if a ‘low friction’ differential with reduced traction over slippery surfaces is fitted, it limits acceleration of a high-power vehicle and causes excessive tyre wear. The torque reaction of the engine of such a vehicle, during acceleration, tends to lift the left-hand driving wheel off the ground. When accompanied by an uneven road surface, this causes excessive wheel spin. To minimize these drawbacks, the differential action is counteracted by artificially increasing the friction between the sun wheel and the differential cage. When this feature is included, it is called limited slip differentials.

Two basic types of limited slip differential are :
Mechanical limited-slip differential. Visco-differential. A multi-disc clutch pack mounted behind each sun wheel has the inner and outer plates splined to the sun and cage respectively. The bevel gears exert an axial thrust proportional to the torque applied by the crown wheel to the differential. Under low torque transmission, the differential functions in the normal way. When transmitted torque is increased, the clutch pack is loaded, which resists motion of the sun gear so that it rotates at a different speed than that of the cage.

To operate with further increase of the load on the clutch pack, the additional features are incorporated in many designs. These include the following: (i) The discs are provided with an initial load through a Belleville disc-spring washer installed between the cage and the clutch discs of each pack. (ii) Angled cam faces are installed between the cage and the cross pins. The cage exerts driving thrust on the pin, which in turn forces the planets against the side gear ring. In a four planets system, two separate pins are flexibly linked at the centre with opposite cam faces, so that they cause two planets to act on one clutch pack and the other two to exert force in the opposite direction. Viscous Differential This type combines a standard differential unit with a viscous coupling. The coupling is used in this system as a viscous control device to regulate the speed difference between the two driving wheels. As explained above the locking action on a mechanical limited-slip differential depends on input torque, whereas on the viscous type it depends on the speed difference of the driving wheels. Very little resistance is offered with a small speed difference and this increases progressively with the increase of the difference.

Compared with the mechanical limited-slip differential, the viscous type provides lower tyre wear, easier steering and lower stresses in driveline components. For high-viscosity fluid, the coupling can be designed to deliver a comparatively high torque, which progressively builds up with the increase of shear rate. The visco unit is very similar in basic construction to a multi-plate clutch. It uses a housing and hub, and a series of perforated metal plates submersed in silicon fluid are sandwiched between them. Also these plates are alternatively attached to the hub and housing. A fluorinated rubber heat resistant seal isolates the silicone fluid from the lubricating oil in the final drive assembly.

The torque and shear rate relationship depends on fluid viscosity, the gap between plates and plate perforation. These factors are varied while designing a coupling suiting to the application. Prolonged slippage of the coupling causes generation of heat due to which the fluid expands and occupies some of air space. In case spacers are not used to position the plates apart, the increase in air pressure due to fluid expansion and the reduction in fluid pressure in the gaps pushes the plates together. This causes metal-to-metal contact so that the coupling temporarily departs from its viscous mode operation. During this phase the torque output rises considerably even six times as great in some designs. Viscous couplings can be used to control front, centre and rear differentials. Each application can be considered as different in view of its position, speed of operation and type of vehicle to which it is fitted. The viscous control unit can be installed and connected in a differential in two ways shaft-to-shaft coupling shaft-to-cage couplings.

Shaft-to-shaft Viscous Coupling.
A bevel-type differential having a viscous control unit connected between the two axle shafts is shown. The hub holding and inner plates is joined to one shaft, and the housing with its outer plates is connected to the other shaft. The unit is installed at the centre of the crown wheel by moving the differential to one side. The control unit does not function during movement of the car in a straight path with no wheel slip. But, if the shaft speed becomes different, such as when one driving wheel loses adhesion or when wheel spin occurs during acceleration, the resistance offered by the unit maintains equal driving torque to both wheels.

Shaft-to-cage Viscous Coupling.
The housing of the viscous control unit is integral with the differential cage and the hub is connected to one axle shaft. This arrangement is comparatively cheaper. The speed difference between the cage and axle shaft is half the difference of the road wheels. Therefore, compared with the shaft-to-shaft layout, the shaft-to-cage arrangement exhibits a torque characteristic about three times greater to have the same locking effect.

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