Automotive Engineering _ Gear Box_Steering PDF

Automotive Engineering Course after 1st Term K Vivek Chawla  Automotive Engineering Topics to be covered for Gear Description of working of Sliding mesh and Constant mesh gear boxes, synchro-mesh Gear Box Semi Automatic transmission Final drive and differentials, Re...

Automotive Engineering Course after 1st Term K Vivek Chawla  Automotive Engineering Topics to be covered for Gear Description of working of Sliding mesh and Constant mesh gear boxes, synchro-mesh Gear Box Semi Automatic transmission Final drive and differentials, Rear Axles, Overdrive K Vivek Chawla  Automotive Engineering Introduction to Gear Box (Transmission) Gearbox is a speed and torque changing device between the engine and the driving wheels. serves as follow in a transmission system : 1. It exchanges engine power for greater torque and thus provides a mechanical advantage to drive the vehicle under different conditions. 2. It exchanges forward motion for reverse motion. 3. It provides a neutral position to disallow power flow to the rest of power train. K Vivek Chawla  Automotive Engineering Introduction to Gear Box (Transmission) Transmission (gear box) acts in accordance with the running conditions. When driving power is required, it reduces the engine speed and transmits stronger torque to the driving wheels. When high running speed is desired, it transmits high speed low-torque to the wheels. Transmission serves to reverse the vehicle by meshing gears in such a manner to allow running the vehicle in the reverse direction. K Vivek Chawla  Automotive Engineering Introduction to Gear Box (Transmission) The gears used in transmissions are helical gears. In these gears, the teeth are set at angle to the gear centreline. The teeth have a wiping action which improves their contact and lubrication. Helical gears  K Vivek Chawla  Automotive Engineering Introduction to Gear Box (Transmission) The relative speed of the two meshing gears (gear ratio ) is determined by the number of teeth of the two gears. If one gear has 15 teeth and other has 30 teeth the smaller gear will rotate twice for every revolution of the larger gear. When the smaller gear is driving the larger gear, this is a two-to-one gear ratio (written 2:1). If the 15-tooth gear is driving the 45-tooth gear, the gear ratio would be 3:1. K Vivek Chawla  Automotive Engineering Introduction to Gear Box (Transmission) Typical gear box ratio in a small car with a four-speed gear box is 3.5 : 1 in first, 2 : 1 in second, 1.4: 1 in third and 1 : 1 in top. All these are multiplied by the axle ratio, which is taken as 4 : 1 to give the corresponding ratios between the engine speed and the road wheel speed. Power available from the engine is directly related to the engine torque T and the gear box ratio G. K Vivek Chawla  Automotive Engineering Introduction to Gear Box (Transmission) Power available from the engine is directly related to the engine torque T and the gear box ratio G. Assuming no loss in transmission, Torque available at the wheel , Tw = T x G and produces the driving force along the road. This driving force is known as tractive effort The tractive effort also varies with the vehicle speed as the engine torque varies with engine speed. K Vivek Chawla  Automotive Engineering Introduction to Gear Box (Transmission) Torque and gear ratio. When the smaller gear is driving the larger gear, the gear ratio is 2 : 1. but, the torque ratio is 1 : 2. The larger gear turns at half the speed of the smaller gear. As a result, the larger gear will have twice the torque of the smaller gear. In gear systems, the speed reduction means torque increase. K Vivek Chawla  Automotive Engineering Types of Gear Boxes The following types of gear boxes are used in automobiles : 1. Selective type : (i) Sliding mesh (ii) Constant mesh (iii) Synchromesh. 2. Progressive type 3. Epicyclic or planetary type. K Vivek Chawla  Automotive Engineering Selective Type Gear Boxes It is that 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 position. Advantages of selective type gear boxes : (i) Simple in construction(ii) Relatively free from troubles. (iii) Light and small. (iv) Low production costs. Disadvantages : (i) Gear ratios not being continuous but being in steps (3 to 5 steps), making it necessary to shift gears each time when vehicle running conditions change. (ii) Noisy in operation. K Vivek Chawla  Automotive Engineering Sliding mesh gear box :. K Vivek Chawla  Automotive Engineering Sliding mesh gear box : Gears on the splined main shaft are moved right or left for meshing them with appropriate gears on the lay shaft for obtaining different speeds. Gears are meshed by sliding or crashing one on to the other. This gear box is also known as crash-type gear box. K Vivek Chawla Sliding mesh gear box : Gear box consists of a clutch shaft, counter or lay shaft and main shaft. The clutch shaft has one gear. The counter or lay shaft has four gears. All four gears form an integral part of the counter shaft. The main shaft has two gears. These two gears can slide horizontally along the splines at the main shaft. K Vivek Chawla Sliding mesh gear box : At the same time the gears rotate with the main shaft. There is no such spline arrangement in the countershaft, as such the gears cannot move along it K Vivek Chawla Sliding mesh gear box : First gear. Refer Fig When the gear shift lever is applied such that gear '6' meshes with gear '5', gear '4' does not mesh with gear '3'. When the clutch shaft is rotating, the drive is transmitted from gear ‘1' to gear '2'. Thus the counter shaft is also rotated. The direction of rotation of counter shaft is opposite to that of the clutch shaft. K Vivek Chawla Sliding mesh gear box : First gear. Refer to Fig When the counter shaft is rotating, gear '5' rotates gear '6'. Therefore, gear '6' rotates the main shaft. Thus the drive is transmitted along gears ‘1', '2', '5' and '6'. Gear ‘1' is smaller than gear '2', and gear '5' is smaller than gear '6'. Consequently, the speed of the main shaft is reduced considerably. K Vivek Chawla Sliding mesh gear box : Second gear. Refer to Fig. Gear '4' is shown in mesh with gear '3'. When the clutch shaft is rotating, the transmission takes place between gears ' 1 ' and '2', and '3' and '4'; the gear '4' rotates the main shaft. The second gear is developed on the basis of the sizes of the gears. K Vivek Chawla Sliding mesh gear box : Third gear. Refer to Fig In this gear, the dog of gear '4' is directly meshed with gear ‘1'. The main shaft is in direct contact with the clutch shaft. Therefore, the main shaft acquires the same speed of the clutch shaft. This is the position for this gear. K Vivek Chawla Sliding mesh gear box : Reverse gear. Refer to Fig In the reverse gear the transmission takes place form gears '1' to '2', '7' to '8' and then to '6'. The small intermediate gear '8' (idler) causes the reverse position of the gear box. In this arrangement, the speed reduction is usually same as that in the first gear. K Vivek Chawla Sliding mesh gear box : Neutral gear. Refer to Fig In this arrangement the clutch shaft is in connection with the counter or lay shaft. As the lay shaft is not connected to the main shaft, there is no power transmission to the wheels. K Vivek Chawla Constant mesh gear box :Refer Fig It is that gear box in which all the gears are in constant mesh with each other (hence the name constant mesh gear box) all the time and this gives a silent or quiet operation. Helical gears are used to make gear changing easier. The gears on the main shaft which is splined, are free. K Vivek Chawla Constant mesh gear box : Refer Fig. The gears on the counter or lay shaft are, however, fixed. Two dog clutches are provided on the main shaft-one between the clutch gear and the second gear and the other between the low/first gear and reverse gear. Dog clutch can slide on the main shaft and rotates with it. K Vivek Chawla Constant mesh gear box : Refer Fig. When the left-hand dog clutch is made to slide to the left by means of the gearshift lever, it meshes with the clutch gear and the top speed gear is obtained. K Vivek Chawla Constant mesh gear box : Refer Fig. When the dog clutch meshes with the second gear the second speed gear is obtained. Similarly by sliding the right-hand dog clutch to the left and right, the first speed gear and reverse gear are obtained respectively. High skill driver is required while changing gears to have clash free engagement. K Vivek Chawla Synchro mesh gear box : Gear box in which sliding synchronizing units are provided in place of sliding dog clutches as in case of constant mesh gear box. With the help of synchronizing unit, the speed of both the driving and driven shafts is synchronized before they are clutched together through train of gears. The arrangement of power flow for the various gears remains the same as in the constant mesh gear box. K Vivek Chawla Synchro mesh gear box : Synchromesh gear devices work on the principle that two gears to be engaged are first brought into frictional contact which equalises their speed after which they are engaged readily and smoothly. Following types of such devices are mostly used in vehicles : (i) Pin type ; (ii) Synchronizer ring type. A synchronising system is used for smooth meshing. A synchromesh works like a friction clutch. K Vivek Chawla Synchro mesh gear box : Refer Fig. (i) shows two conical surfaces ; cone-1 is the part of the collar and cone-2 is part of the gear wheel. Cones 1, 2 are revolving at different speeds. While cone- 2 is revolving, cone-1 gradually slides into it. Friction slows or speeds up the gear wheel. Finally both cones revolve at the same speed. They revolve one as shown in Fig.(ii ). K Vivek Chawla Synchro mesh gear box : Refer to Fig. (i), shown collar (1) and gear wheel (2) as separate and revolving at different speeds. In Fig. (ii), the internal cone of the collar comes in contact with the outer cone of the gear wheel. Here again, the friction slows or speeds up the gear wheel. K Vivek Chawla Synchro mesh gear box : Refer to Fig.(i ) the collar and gear wheel rotate at the same speed. In Fig.(ii) the spring-loaded outer ring of the collar is pushed forward. The dogs slide smoothly into mesh without clashing. The collar and gear wheel lock and revolve at the same speed. This is the principle of synchromesh. K Vivek Chawla Synchro mesh gear box :Refer to Fig The cone of gear wheel and internal cone of the collar are in contact. The collar acquires the same speed as of the gear wheel. In Fig.(ii ), a further movement of the collar makes the toothed Outer ring slide into engagement. K Vivek Chawla Synchro mesh gear box : In most of the cars, the synchromesh devices are not fitted to all the gears. They are fitted only on the top gears. Reverse gear, and in some cases the first gear, do not have synchromesh device, since they are intended to be engaged when the vehicle is stationary. Advantage. The synchromesh type of transmission has the big advantage of allowing smooth and quick shifting of gears without danger of damaging the gears. The synchromesh gear box is considered the most advanced and has been adopted in most cars. K Vivek Chawla K Vivek Chawla Epicyclic or Planetary Type Gear Box: Epicyclic gears (or planetary gears) are used in one form or the other in overdrives and automatic transmissions. In overdrive, they are used as over gear while in automatic transmission as reduction gear. The name, planetary gear system, is derived from its similarity to our solar system. The planet gears (pinions) turn on their own axes while revolving at the same time around the central gear (sun) in a manner similar to the earth and other planets rotating and revolving around the sun. K Vivek Chawla Epicyclic or Planetary Type Gear Box: It consists of mainly three components. 1. Sun gear A, which is larger of the two gears. 2. Planet pinion B, and 3. Arm which connects the two gears through their shafts. The simplest epicyclic gear train yields three different gear ratios when (a) arm is locked, (b) larger gear A is locked, and (c) smaller pinion B is locked. K Vivek Chawla Epicyclic or Planetary Type Gear Box: When the arm is locked and gear A is imparted a rotation, then gear B rotates in the direction The epicyclic gear set, in this configuration, works as a simple gear train and provides a gear ratio K Vivek Chawla Epicyclic or Planetary Type Gear Box: If gears A and B are locked respectively and the arm is given a rotation. Consequently gear A and B rotate as shown, and yield the following gear ratios. K Vivek Chawla Epicyclic or Planetary Type Gear Box: An epicyclic gearbox consists of two or more epicyclic gear sets. A simple planetary gearbox (gear train) is shown in Fig. It has a sun gear S, a planet gear P, an internal gear A and an arm. The internal gear is also called annular gear or annulus, and the arm is also known as link. K Vivek Chawla Epicyclic or Planetary Type Gear Box: The planetary gear set is always in mesh. Each of the three parts of the planetary gear set can be a driver or a driven member, depending upon the gear ratio needed. All automatic transmissions use two or more planetary gear sets, with either a common or separate sun gear, to obtain the three or four speeds ,to move the vehicle. K Vivek Chawla Epicyclic or Planetary Type Gear Box: Gear reduction To obtain gear reduction, the sun gear is held stationary while the ring gear is driven, which causes the planetary gears and carrier to rotate or walk around the sun gear in the same direction as the ring gear turns, but not as fast. This causes the engine torque to be multiplied since the output shaft is not turning at the same RPM as the input shaft. K Vivek Chawla Epicyclic or Planetary Type Gear Box: Direct Drive To obtain direct drive, the entire planetary gear set must rotate as a unit by having both the sun gear and ring gear locked together and being driven from the same torque input through the planetary carrier. K Vivek Chawla Epicyclic or Planetary Type Gear Box: Reverse To change the input power flow to a reverse rotation at the output shaft, the pinion carrier is held stationary while the sun gear is driven. It causes the planetary pinions to turn in the opposite direction, which causes the ring gear also to be driven in the opposite direction as well, thereby providing a reverse gear to the transmission K Vivek Chawla Epicyclic or Planetary Type Gear Box: Neutral Obtaining the neutral position where a torque input is directed into the transmission, but no torque output is developed at the output shaft, results from having none of the driving members coupled to the planetary gear set. Overdrive  For overdrive gear ratio hold the sun gear and drive the pinion carrier, which causes the pinion gears to move around the stationary sun gear and drive the ring gear at a faster speed than the input shaft K Vivek Chawla Compound Planetary Gear Assembly: The planetary gear set only provides one reduction and one direct drive in the same direction of rotation. It is necessary to use two planetary units connected together in series to obtain the three and four forward speeds. K Vivek Chawla Compound Planetary Gear Assembly: The front and rear planetary gear assemblies, which comprise the compound planetary unit, are similar in gear arrangement, but normally differ in size to obtain a different percentage of the reduction. K Vivek Chawla Compound Planetary Gear Assembly: Gear assembly is normally interconnected by a common sun gear or by long and short pinion gears, intermeshed to each other and both capable of being ·either a driving or a driven member. Each pinion grouping is meshed to a sun gear (the primary and secondary) which are of different sizes to allow power flow through different stages of gear reduction A change in gear ratio is affected when bands hold one of the three members of the gear train. When none of the members are held, the unit is in neutral K Vivek Chawla Epicyclic or Planetary Type Gear Box : This type of gear box/transmission uses no sliding dogs or gears to engage but different gear speeds are obtained by merely tightening brake-bands on the gear drums. K Vivek Chawla Epicyclic or Planetary Type Gear Box: In order to obtain different speeds any one of these units can be held from rotation by means of brake bands. The ring gear contains teeth on it inner circumference and is surrounded by a brake band. The brake band is operated by a gear stick or lever to grip the ring gear and hold its movement. K Vivek Chawla Epicyclic or Planetary Type Gear Box: Sun Gear is rotated by the driving shaft from the engine and thus moves along with the movement of the engine crankshaft. Planet gears are in constant mesh with both the sun gear and ring gear or annular wheel and are free to rotate on their axes carried by the carrier frame which in turn is connected to the driven shaft. K Vivek Chawla Epicyclic or Planetary Type Gear Box:Working:- When the ring gear is locked by the brake band , the rotating sun gear causes the planet gears to rotate. As the ring gear cannot move, the planet gears are forced to climb over it and Ring gear acts as track for the planet gears to move over. The driven shaft which is connected to the planet gear carrier is thus rotated. K Vivek Chawla Epicyclic or Planetary Type Gear Box:Working:- When the ring gear is released, it is free to move in consequence to the rotation of planet gears which rotate around their axis. During this position, there is no movement of planet carriers and hence the driven shaft remains stationary Gear Box have various units to achieve speed reductions and a idler gear reverses direction of motion K Vivek Chawla Epicyclic or Planetary Type Gear Box:Advantages:- 1. It provides a more compact and smaller unit operating about a common central axis, because the planetary gear operate within a ring gear. 2. The planetary gears are in constant mesh and hence dog clutches or sliding gears are not used. 3. Load is not taken by one gear but is distributed over several gears K Vivek Chawla Pre-Selective Gearbox / Wilson Gear Box:- Gearbox is mounted with a special mechanism that enables the driver to select the desired gear set before actual engagement is executed. For that a lever is fitted on the steering column that moves in an arch sector to the corresponding positions of various gear ratios marked on it. After pre-selecting the next gear ratio : higher or lower, the driver has to press the gear change pedal only when he actually wants to engage the desired gear. Pre-selection does not require any effort and can be carried out at any convenient moment. K Vivek Chawla Pre-Selective Gearbox / Wilson Gear Box:- Wilson gearbox is a popular example of this kind. It has been employed on Daimler and Siddley cars. Wilson gearbox is basically a sun and planet type compound gearbox. The number of sun and planet gears to be used are equal to the number of gear ratios required. Equal number of brake bands are also provided in the construction and each of them is applied in turn to lock a particular gear set K Vivek Chawla Overdrive Overdrive is a device interposed between the transmission (gear box) and propeller shaft to permit the propeller shaft to turn faster than the transmission main shaft. It is so called because it provides a speed ratio over that of the high speed ratio. The overdrive permits the engine to operate only about 70 percent of the propeller shaft speed, when the vehicle is operating in the high speed ranges. The overdrive is suited to high powered cars employing three-speed gear boxes. K Vivek Chawla Overdrive An overdrive provides a super-top gear. The ratio of engine crankshaft speed to road wheel speed is unity (= 1) in top gear but in super top gear it is less than 1. Overdrive is usually employed to supplement conventional transmission. It is bolted to the rear of the transmission between the transmission and propeller shaft. K Vivek Chawla Overdrive Vehicle attached with an overdrive implies that the speed of its output (gearbox) shaft is higher than that of the input shaft. Basically in top gear the speeds of input and output shafts are equal i.e. gear ratio is 1:1. In overdrive, the gear ratio. is I : λ where λ < I. Thus if the input gearbox shaft rotates at 3000 rpm, then the output shaft will rotate at a speed greater than 3000 rpm. K Vivek Chawla Overdrive Overdrive is a mechanism used to obtain a gear ratio that increases the speed of the auto vehicle above top gear. It is generally fitted on luxury and racing cars. It can be fitted in the gearbox, outside the gearbox, or between the gearbox and propeller shaft. In sports cars they may be fitted on gear pairs other than the top gear. For example, an overdrive may be fitted on each of the second gear, third gear, and top gear in a 4-forward speed gearbox. It, thus, makes a total of seven forward speeds available in this case. K Vivek Chawla Advantages of Overdrive : Modern practice is towards increasing use of overdrives on auto vehicles. 1. Ability to attain the cruising speed at highways and expressways at comparatively low engine speeds. 2. Reduced wear of engine parts due to low engine speeds. 3. Decreased vibration and noise in the engine and the vehicle due to low engine speeds. 4. Lesser frictional losses in the engine due to its lower speeds. 5. Fuel saving as the fuel consumption decreases at comparatively lower engine speeds. K Vivek Chawla Working Principle of an Overdrive : lt comprises of the following main components. (i) Epicyclic gear train consisting of a sun gear, three planet gears and a planet carrier. (ii) A freewheel clutch fitted on splines of the input shaft and connected to the output shaft through casing. (iii) Output shaft A of a manual gearbox which acts as input shaft for the overdrive. (iv) Output shaft B which transmits power to the propeller shaft. This shaft is integral with the casing. K Vivek Chawla Working Principle of an Overdrive : lt comprises of the following main components. K Vivek Chawla Overdrive The sun gear is free to rotate on the input shaft while the carrier can slide on splines of the input shaft. The planet gears mesh with the internal (ring) gear provided in the extended and enlarged end of the output shaft. K Vivek Chawla Overdrive Working. To accomplish change of gears from normal to overdrive mode, the driver has to lift his foot-off from the accelerator pedal. Then the locking of sun gear in vivid manners through a solenoid and governor arrangement yields different gear ratios. K Vivek Chawla Overdrive Working. (i) When sun gear is locked with the casing, the overdrive is engaged. It is because the ring gear is now driven by the planet carrier. Due to increase in speed of the output shaft, the power available to drive the wheels decreases. That is why an overdrive is used only when the engine is running fast to produce the required torque. K Vivek Chawla Overdrive Working. (ii) When sun gear is locked with the planet carrier, the normal direct drive is obtained. (iii) When sun gear rotates freely on the input shaft A, a direct drive results in through the freewheel. However when engine is idling, the vehicle freewheels and the output shaft does not override the input shaft. K Vivek Chawla Final drive A final drive is used to transfer power from the propeller shaft end (away from the gearbox) to the differential gear assembly. Basically it consists of a combination of large sized crown wheel and a bevel pinion. K Vivek Chawla Final drive Final drive is employed to provide a permanent reduction in speed ratio i.e. a non-variable torque hike between the propeller-Shaft and differential gears through a large sized crown gear. K Vivek Chawla Final drive The final drive is employed to serve the following functions. 1. To provide a permanent (fixed) speed reduction from the gearbox to the rear axles. 2. To transform the direction of drive at right angles (through 90°) from engine to the rear wheel axles. The permanent speed reduction provided by the final drive is other than the variable reduction obtained from the gearbox. It is normally 3 to 4 : 1 in cars, 6 to 7 : 1 in trucks and other heavy vehicles. K Vivek Chawla Final drive Overall reduction ratio achieved is equal to the multiplication of gearbox ratio (GR) and the final drive ratio (FR) Its overall ratio can be found as follows. K Vivek Chawla Arrangement of Final Drive : The permanent reduction by final drive may be accomplished in single stage or in two stages. If reduction up to 7 : 1 is desired, a single stage arrangement is preferred, but a two-stage reduction is required for higher reductions. Different arrangements of final drives are employed in automobiles are as 1. Crown and bevel pinion drive (More popular & used) 2. Worm and worm wheel drive. K Vivek Chawla Arrangement of Final Drive : The worm and worm wheel drive is used on heavier vehicles where a large increase in torque and a greater reduction in speed is desired. worm and worm wheel drive is expensive, outdated yet used on certain Peugeot car models. K Vivek Chawla Arrangement of Final Drive : The crown and bevel pinion drives basically differ in their construction. Different types of crown wheels and bevel pinion constructions are of following types. Straight teeth type. Spiral teeth type. Hypoid teeth type. K Vivek Chawla Arrangement of Final Drive : A straight teeth arrangement (Fig a) is simple and easy to manufacture but is not strong enough. The spiral teeth drive (Fig.b) is stronger than the straight teeth drive and possesses more favorable features. The hypoid drive is the best (Fig.c ). They are widely used as final drive in modern vehicles. K Vivek Chawla Arrangement of Final Drive : The term 'hypoid' relates to a surface called 'hyperboloid of revolution' which is obtained when a geometrical curve 'hyperbola' is rotated about an offset axis. Spiral-bevel drive has been used on Ashok Leyland's Viking. Comet; while the hypoid drive on Eicher,Mitsubishi, Premier Padmini etc Off-Centre hypoid-bevel type.  K Vivek Chawla Arrangement of Final Drive : straight teeth was an early design and is no longer used. Spiral bevel is an improvement on it. Due to curved teeth profile, the load is transferred smoothly and quietly from one tooth to the adjoining tooth. In both the types, axis of the pinion passes through the centre of crown wheel. K Vivek Chawla Arrangement of Final Drive : hypoid gearing engages above or below the centre line. Engagement above the centre line helps in designing a vehicle body having a normal ground clearance with a flat floor. If the engagement is below the centre line, it permits lowering of the line of propeller shaft so as to achieve less ground clearance. It, thus, improves the road holding ability of vehicle by lowering its centre of gravity. Off-Centre hypoid-bevel type.  K Vivek Chawla Arrangement of Final Drive : K Vivek Chawla Differential : A differential assembly is a mechanism of epic-yclic gear train which is located in between the final drive and rear axles (or half-shafts). It serves the following functions. 1. Avoids skidding of rear wheels on a road turning. 2. Reduces the speed of inner wheels and increases the speed of outer wheels, while negotiating a curve. 3. Keeps equal speeds of all the wheels while moving on a straight stretch. K Vivek Chawla 4. Eliminates a single rigid rear axle, and instead provides a coupling between two rear axles. Differential : Necessity of Differential : The rear wheels of a vehicle roll-down the road at equal speeds when in straight motion. while rolling on curve, or turning otherwise, the outer wheels have to cover a longer distance than the inner wheels. referring Fig. which shows the vehicle negotiating a rightward turn. K Vivek Chawla Differential : Necessity of Differential : The outer wheels (left side wheels in this case) are turning at a radius of curvature of Ro while the inner wheels at Ri If they turn by an angle θ, then the distance covered by outer wheels will be AB = Ro x θ whereas by the inner wheels it will be CD = Ri x θ. Thus additional distance covered by outer wheels is AB - CD = (Ro x θ ) - (Ri x θ) = (Ro - Ri) x θ K Vivek Chawla Differential : Necessity of Differential : Consequence of such motion will be double fold. (i) the outer wheels will exhibit tendency of skidding, (ii) the inner wheels will be pressing hard on the road surface Tyres will also be badly effected and its life will reduce considerably. The above happenings will occur only when the vehicle has a single rigid axle. Since such occurrences are undesired, a single rigid axle is K Vivek Chawla not used at all. Differential : Necessity of Differential : Hence a two-piece rear axle (also known as half-shaft) is universally employed in conjunction with a mechanism called differential assembly. K Vivek Chawla  Construction and Working of a Differential Assembly: It consist of , i) bevel pinion and the crown wheel (gear), ii) an assembly consisting of a cage, two sun gears S, two planet pinions P, and a cross-pin or spider. iii) cage is attached to the crown wheel and carries a cross-pin K Vivek Chawla Construction and Working of a Differential Assembly: K Vivek Chawla  Construction and Working of a Differential Assembly: The sun gears are always in mesh with the planet pinions. The half shaft (rear axle) is splined to allow small movement in sun gear. Outer ends of half shaft are connected to the wheel hub Crown wheel rotates freely on the bush mounted over one of the half-shafts. K Vivek Chawla  Construction and Working of a Differential Assembly: Vehicle going straight. In this situation, the cage and the epicyclic gears rotate as a single unit, and the differential unit helps the two half-shafts to revolve at equal speeds. Actually the driving torque of propeller shaft and the pinion is increased due to speed reduction at final drive and hence the differentials become inoperative. K Vivek Chawla  Construction and Working of a Differential Assembly: Vehicle taking a turn. In this case, the speed of outer wheels has to be speeded-up while those of the inner wheels must be slowed-down. This tendency causes a decrease in resistance on sun gear of the outer wheel, but an increase in reistance on the sun gear of inner wheel. K Vivek Chawla  Vehicle taking a turn…Cont The spider spindle still turns end-to-end at the crown wheel speed. As the speed of sun gear connected to the inner wheel slows-down, the planet gears are forced to rotate on the spider spindle, about the inner sun gear. In doing so the speed of outer sun gear and the outer road wheel increases by the same proportion as the speed of inner sun gear reduces. K Vivek Chawla Action of differential during cornering of the vehicles If T and N are the torque and speed of the propeller shaft respectively, and the gear reduction at final drive is λ, then following torques and speeds at the road wheels connected to the rear axles. On outer wheels, Torque O = λ T and Speed O = N + Δ N λ On inner wheels, Torque i = λ T and Speed i = N - Δ N λ Value of λ generally lies between 3.5 to 7. Δ N is the differential speed between crown wheel and sun gears. K Vivek Chawla Numerical In the differential gear assembly the propeller shaft transfers 120 Nm torque at 2000 rpm. The final drive pinion has 12 teeth and maintains a permanent gear reduction of 5 with the crown wheel. Detemine (a) speed and direction of road wheels when the vehicle is going straight, (b) speed of left side road wheel if the right side road wheel rotates at 420 rpm while negotiating a left turn, and (c) torques at the rear axle. K Vivek Chawla  Steering System: Functions, requirements and geometry. Steering gears, steering ratio, Camber, King-pin inclination, Caster, Toe-in, Toe-out; Steering Mechanisms: Ackerman Steering, Power steering. K Vivek Chawla Steering  The steering system is an assembly of linkages that are used to provide directional control to the vehicle. In controlling the vehicle, an input effort is given to a steering wheel by the driver whose output comes in the form of lateral movement (or swiveling) of the road wheels. An efficient directional control can be achieved only when the wheels execute pure rolling motion. K Vivek Chawla. The design of linkages is done accordingly although it is not possible to meet the requirement of pure rolling under all conditions of steering. Depending upon the wheels to be steered, a steering system may be of two types. 1. Front wheels steering system, and 2. All-wheels steering system K Vivek Chawla Front Wheel Steering System Only front wheels are steered by turning them on the road and the rear wheels follow them Front wheels steering system is popular and is almost universally used. The steering to the front wheels is imparted by either manual effort or by power. A power steering is more recent advancement and is being increasingly used on all sorts of vehicles. K Vivek Chawla All Wheel Steering System All the wheels (front and rear) are steered. This is a more recent development and has been employed on several vehicles such as Honda Siel car etc The all-wheel steering system, invariably, employs power control. K Vivek Chawla Following requirements are desired in a good steering system. 1. The vehicle should get steered with a minimum of effort so that the driver does not feel driving fatigue. 2. The steering mechanism should work accurately and should provide pure rolling as far as possible. 3. The steering system should not be affected by the side thrusts, cornering forces and wind effects. 4. The mechanism should have a tendency to regain the straight ahead configuration after steering need is over K Vivek Chawla  Front Axle Merely hold the front wheels and allow wheels to rotate Obey the command of the steering linkages for swiveling the road wheels. Two kinds of front axle arrangements are employed on automobiles l. Stub axle with rigid axle beam type. 2. Stub-axle without rigid axle beam type K Vivek Chawla  Front Axle Rigid Axle Beam : The rigid axle beam is a stationary structural member used to sustain bending and torsional loads, and to connect the stub axles. The bending is induced in its central region due to the vehicle's weight while the torsional loads are caused near its ends due to the wheel's braking. As I section resists bending and the circular section can resists torsion more efficiently, therefore the axle beam consists of a central region made of I -section and the ends of circular section. K Vivek Chawla  Front Axle Rigid Axle Beam : To keep the un sprung weight of the vehicle as less as possible the vehicle is supported on the springs for which the provision of spring seats is made on this axle beam. The two ends of the rigid axle have the provision of connecting stub axles by means of king pins. The front wheels are mounted on these stub axles. K Vivek Chawla  Front Axle Rigid Axle Beam : The front rigid axle is also called dead axle since it does not rotate. The Rear axles are called 'live axles' as they rotate. It is made of medium carbon steel (0.4 to 0.5% carbon) or nickel alloy steel (1.2 to 1.4% Ni) by drop forging process Used in trucks and heavy vehicles K Vivek Chawla  Front Axle Stub Axle : A stub axle is an intermediatory small shaft between the road wheel and the front (dead) axle. On one end it remains connected to the axle beam by means of a king pin or a ball joint, while on the other end the front wheel is mounted on it. The stub axles are available in different shapes, and they are connected to the axle beam in different styles (a) Elliot type, (b) reverse elliot type, K Vivek Chawla Front Axle Stub Axle : K Vivek Chawla  Front Axle Stub Axle : Amongst king pin connected stub axles, the reversed elliot type is commonly employed. It is because of its simplicity and greater strength. c) lamoine type d) reverse lamoine type. K Vivek Chawla  Front Axle Stub Axle : The ball joint connected stub axles are more recent and find use on modem cars. stub axle is connected to the suspension members by means of ball joints. Thus the need of a king-pin is eliminated K Vivek Chawla  Front Wheel and Stub Axle Assembly The connection of wheel with the stub axle is through the two bearings. On the inner side, the stub axle is attached on the rigid axle beam by means of a king-pin. The king-pin is placed within a bush It can be seen that the king-pin is located outside the wheel and also the king-pin is inclined inwards while the wheel is canted outwards. K Vivek Chawla Front Wheel and Stub Axle Assembly K Vivek Chawla Front Wheel and Stub Axle Assembly The king-pin inclination is 3° to 9°. The outward canting of the wheel at its top is termed as 'positive camber'. An important point in this assembly is the tilt of king-pin and the wheel is in opposite directions. It is done so as to achieve 'centre-point steering' K Vivek Chawla  Centre Point Steering. see at the king-pin axis and the centre line of the tyre in Fig. The two lines meet the road surface at points A and B. If the inclination of king-pin is such that its axis containing point A passes through point B (i.e. points A and B coincide) then the assembly will give 'true centre-point steering‘ (TCPS). K Vivek Chawla  Centre Point Steering. The true centre-point steering is undesirable owing to high sensitivity and lack of self-centering or caster action. Therefore instead of TCPS only CPS (centre-point steering) is preferred. In this arrangement, a side swinging wheel rises slightly and tends to lift the car's front also. The weight of the car then resists this action and assists in self-centering of the steering. K Vivek Chawla  Wheel Alignment 'wheel alignment' refers to such a setting of front wheels and the steering mechanism that provides an easier directional control to the vehicle, minimum tyre wear, stability to the vehicle while taking a curve, and parallel rolling of front wheels moving straight. wheel alignment also includes alignment of frame with respect to the front wheels, the correct positions of rear axle and the suspension system (spring and shock absorbers) on the vehicle. K Vivek Chawla  Wheel Alignment Excessive Toe in or Toe out cause more wear and make taking turn of vehicle difficult and also during straight ahead motion. The front-wheel alignment is influenced by the correct steering geometry which depends upon several factors. These are : Toe-in, Toe-out, Camber, Caster, King-Pin inclination etc K Vivek Chawla Wheel Alignment K Vivek Chawla  Toe-In and Toe-Out : In the initial setting of the front wheels, carried-out in the industry or the repairing garage, the front wheels are set closer at their front than at their rear in stationary state when viewed from the top. The difference in the amount of B and A is called toe-in i.e. B -A = toe-in. Intital setting of the front wheels shows (a) toe-in, and (b) toe-out as viewed from the top of a stationary vehicle. K Vivek Chawla  Toe-In and Toe-Out : When fronts of the front wheels are far-off than their rears. This is toe-out whose value is equal to the difference between A and B. Thus A - B =toe-out. Intital setting of the front wheels shows (a) toe-in, and (b) toe-out as viewed from the top of a stationary vehicle. K Vivek Chawla Purpose for Toe in and Toe Out The toe-in is provided on all kinds of vehicles except tractors whose front wheels are set for the toe-out. The purpose of providing toe-in is to offset the tendency of wheel rolling (i) on the curves due to the limitation of correct steering (ii) due to possible play in the steering linkages (iii) due to the camber effect The amount of toe-in varies from 0 to 6 mm on different vehicles. It is 1.5 mm on Standard 20 car, 2 mm on Honda (4 wd) car, 2 to 4 mm on Maruti 800 car, Up to 6 mm on Swaraj Mazda, but 0 mm on Ashok LeylandK Vivek Comet. Chawla Purpose for Toe in and Toe Out The toe-out has also been provided on some front- wheel driven cars The toe-out is provided to counter the tendency of inward rolling of the wheels (i) due to the soil condition on agricultural land. (ii) on account of side thrusts and cross-wind effects. K Vivek Chawla Camber Angle (or Wheel Rake) : The front wheels are usually mounted in such a way that they are tilted outwards at the top and inwards at the bottom, when viewed from the front of the vehicle. Due to such positioning, the centre line of the tyre forms an angle with its vertical. This angle is known as 'camber angle', or only camber. K Vivek Chawla Camber Angle (or Wheel Rake) : K Vivek Chawla Camber Angle (or Wheel Rake) : Some vehicles are provided with positive camber while the others have negative camber. A negative camber is just opposite of the positive camber. Mostly vehicles are provided with positive camber than negative camber. Negative camber has been provided on Fiat 1100 car, Honda 4-wheel drive car etc. Positive camber K Vivek Chawla Negative Camber Purpose of Camber Angle (or Wheel Rake) : Camber is provided to prevent inward tilting of top of the wheel caused due to (i) excessive load, (ii) play in the king-pins, and (iii) play in the wheel bearings. The camber on both the wheels must be compulsorily equal in amount otherwise the vehicle will roll-on in the direction of the wheel having a larger camber , which will also effect directional stability and life of tyres. K Vivek Chawla Purpose of Camber Angle (or Wheel Rake) : The amount of camber is generally kept between 0° to 1.5° If it is kept more than required, the contact ·between the tyre and the road will reduce, and also the tyre will wear eccentrically on the outer faces. In excessive negative camber one sided tyre wear will occur on the inner faces. K Vivek Chawla King-Pin Inclination and Steering Axis Inclination : A king-pin is mounted in such a way that it remains inclined inward with respect to the vertical axis. The angle thus formed between the centre line of king- pin and the vertical axis is called king - pin angle (or king-pin rake). If ball joints are used instead of a king-pin then term 'steering axis inclination' is referred instead of 'king-pin inclination'. Thus the 'steering axis inclination' is the angle made by the ball joints axis with the vertical. K Vivek Chawla Purpose king-pin or the centre line of ball joints act as pivot for movement of steering linkages. So the purpose of keeping a slanting inward king-pin or the ball joints is (i) to keep the front wheels pointing forward. (ii) to bring back the wheels in a straight position after a turn. K Vivek Chawla Purpose The king-pin angle is generally kept between 3° to 9°, and the steering-axis angles between 4 to 11°. If these angles are larger than the required values, steering the vehicle will become difficult. These must also be equal on both the wheels otherwise tendency of pull will develop on a side having greater angle. The king-pin angle on Maruti 800 car is 12° and on Ashok Leyland Comet is 3°. K Vivek Chawla K Vivek Chawla Caster : A king-pin has a two-plane inclination. The inclinations are from the vertical when viewed from front as well as when viewed from side of the vehicle. In the former case, the tilt forms 'king-pin angle' and in the later case it is called 'caster'. K Vivek Chawla Caster : K Vivek Chawla  Caster : caster angle formed between the vertical line and the king-pin inclination. Depending upon the manner in which a king-pin is tilted, the caster may be of two different natures 1. Positive caster and 2. Negative caster K Vivek Chawla  Caster : A negative caster angle is preferred on many modem automobiles. In this arrangement the top of the king-pin is tilted in the front direction and the bottom in the backward direction. Purpose. K Vivek Chawla  Purpose. The purpose of providing caster is to (i) produce directional stability, (ii) avoid or minimize the tendency of wheel wander', and (iii) avoid shimmy (i.e. oscillation of the front wheels). K Vivek Chawla Included Angle and Scrub Radius : The sum of camber angle and the king-pin angle is referred as 'included angle' or 'combined angle'. This angle is formed in the vertical plane and can be noticed on viewing the vehicle from the front Its value ranges between 5° to 12° on different vehicles. This variation in the included angle is due to varying inclinations of the king-pin. K Vivek Chawla PRINCIPLE OF CORRECT STEERING A correct steering means such a steering state in which all the wheels undergo only a pure rolling motion under all conditions of vehicle's motion whether straight negotiating a leftward or rightward curve When a vehicle moving straight ahead. It has a wheelbase of length l and track (or track width) of w. A and C are the stub axle ends, CE and AF are the steering arms , and EF is the track rod. In this condition the instantaneous centre lies at infinity. K Vivek Chawla PRINCIPLE OF CORRECT STEERING Pure rolling without slipping is possible only when the axes of the stub axles of the two front wheels intersect at a point O lying on common axis of the rear wheels This point is called instantaneous centre of rotation of the chassis and the wheels. b) vehicle negotiating a right turn. K Vivek Chawla PRINCIPLE OF CORRECT STEERING If it is desired to steer the vehicle rightward, therefore the linkage CEFA takes on a new configuration Consequently the road wheels also turn to right but by different angular values. The inner front wheel turns by a larger angle θ than the outer front wheel which turns by an angle Φ only. Angular turning of the two front wheels by different amounts is essentially required for correct steering so that all the four wheels rotate about an instantaneous centre O. b) vehicle negotiating a right turn. K Vivek Chawla PRINCIPLE OF CORRECT STEERING On considering the geometry of two triangles Δ AOB and Δ COD, we find  Equation..1 This represents equation for correct steering of the vehicle. It can be seen that the values of θ and Φ differ for different vehicles depending upon their dimensions l and w. K Vivek Chawla PRINCIPLE OF CORRECT STEERING conclusions A vehicle can have only three correct steering positions (i) While moving straight. (ii) While turning right at a particular value of θ and Φ corresponding to the dimensions l and w which satisfies Eq.1 (previous slide). (iii) While turning left in a similar position as described in above. K Vivek Chawla Ackermann 's Steering Mechanism From the analysis of correct steering it is known that front wheels have to be turned by different values. The inner wheel has to be turned more than the outer wheel. In right hand turning, the right wheel is the inner wheel while in left hand turning of the vehicle, the left wheel will be the inner wheel. K Vivek Chawla Ackermann 's Steering Mechanism The Ackermann steering mechanism is shown in Fig.a. It consists of a four-bar mechanism ACEF whose link AC is kept fixed. The joints A, C, E and F are the turning pairs. Links AF and CE are of equal lengths but links AC and EF are unequal. K Vivek Chawla Ackermann 's Steering Mechanism The inclination of links AF and CE are equal with respect to the longitudinal axis of the vehicle. The configuration of the mechanism changes to as shown in Fig.b when the vehicle is steered. K Vivek Chawla Ackermann 's Steering Mechanism dimensions of links and the angle α are kept such that the Eq.1 is satisfied at least for three particular conditions discussed earlier. Generally the following values are adopted. K Vivek Chawla LAYOUT OF A STEERING SYSTEM Simple layout of a steering system is shown. It consists of the following main parts. 1. Steering wheel 2. Steering column 3. Steering gear 4. Drop arm (or Pitman arm) 5. Drag link (or link rod) 6. Tie rod (or track rod) 7. Steering arm (or track rod arm) 8. Track rod adjuster 9. Rigid axle beam 10. Front wheels 11. Tie rod end joints 12. King-pin K Vivek Chawla LAYOUT OF A STEERING SYSTEM K Vivek Chawla LAYOUT OF A STEERING SYSTEM The steering wheel is made circular so that the driver may apply the effort conveniently. The full or part rotation of this wheel causes rotation in the steering shaft (rod) which is encased within the steering column. From steering column the rotation is transmitted to the drop arm through the steering gear. The steering gear arrangement is such that a large reduction in the rotation of steering rod takes place here. K Vivek Chawla LAYOUT OF A STEERING SYSTEM This reduction is termed as steering reduction ratio or simply the 'steering ratio', and its value normally lies between 15 to 35 on different categories of vehicles. 'Tata Sierra' whose steering ratio is 18.2 : 1. It means that 18.2 complete rotations of steering wheel will cause only 1 rotation in the output of steering gear that connects the drop arm K Vivek Chawla LAYOUT OF A STEERING SYSTEM The rotation of drop arm is transferred to the tie rod via drag link. Since the tie rod is attached on the stub axle through steering arms, its motion causes the road wheels to steer (or turn). An adjuster is fitted on the track rod to adjust the play etc of it. K Vivek Chawla LAYOUT OF A STEERING SYSTEM The tie rod and the steering arms are connected by end joints. The end joints can be of several types but the ball joints are most common. The track rod may or may not be symmetrical to both the wheels. For example, it is symmetrical in Premier Padmini car but non-symmetric in Ambassador car K Vivek Chawla Types Of Steering System. K Vivek Chawla Steering Gear A driver steers the road wheels of a vehicle weighing 1 tonne or more using only a little manual effort of about 40-50 kgf ( 400-500 N). To accomplish the turning of road wheels by few degrees, he rotates the steering wheel many times. All these are possible only by use of a steering gear. K Vivek Chawla Steering Gear A steering gear serves two main functions. 1. It transforms rotary motion of the steering wheel into the reciprocating motion of the drag link. 2. It assists in multiplying a small effort applied at the steering wheel into a much greater force on the drag link. K Vivek Chawla Types of steering gears 1. Worm and worm-wheel type steering gear. 2. Worm and worm-sector type steering gear as in earlier Austin small cars 3. Worm and nut type steering gear as in Swaraj Mazda. 4. Worm and roller type steering gear as in Premier Padmini S 1 and Diesel cars, CJ series jeeps 5. Recirculating steel balls type steering gear as in Tata 1210, Dodge/Fargo 89 M4, Omni, Eicher 10. 70, Tata 407, Tempo Excel-4 pick up truck. In most of the above types a worm is the basic component which is used in conjunction with either a wheel, wheel sector, or nut etc. K Vivek Chawla Worm. The worm is a large screw consisting of several square, trapezoidal, or helical profiled threads. Its upper end remains attached to the steering column while the lower end is generally supported by the end cover. The worm meshes with the wheel, sector, nut, roller, or the peg on its side. The worm along with its meshing partner is encased within a housing which is referred as steering gearbox. K Vivek Chawla Worm. A worm may have a single or a double start threads. A double start thread gives greater axial movement to the worm. If we rotate the steering wheel by one revolution, this will rotate the worm also by one revolution. If the worm has single start thread, it will move the worm-wheel by one teeth. However in case of a double start thread, the worm wheel will move by two teeth. K Vivek Chawla Worm and Worm-Wheel Type Steering Gear: A worm and worm-wheel steering arrangement is shown in Fig The upper end of the worm is attached to the steering column while the lower end meshes with a toothed worm-wheel. The toothed wheel is mounted on a common shaft along with the drop arm. The complete steering gear is enclosed within a cast iron or aluminium alloy casing. The casing is filled with the gear oil and is bolted on the chassis K Vivek Chawla Worm and Worm-Wheel Type Steering Gear: When a vehicle is to be steered, the driver rotates the steering wheel due to which the worm also rotates. Several complete revolutions of steering wheel make the same number of revolutions in the worm. This causes the worm-wheel to move in an arc of about 60° to 80°. Consequently, the drop arm also moves by the same amount. The movement of drop arm pulls or pushes the drag link which transmits motion to onward linkages. K Vivek Chawla Worm and Worm-Sector Type Steering Gear : The construction and working of this type is similar to the worm and worm-wheel type steering gear but for the use of a worm-sector instead of a worm-wheel. The worm-sector is a fractional part of the worm-wheel, Fig. and is often mounted above the worm. worm-sector is smaller than the worm-wheel, it is cheaper and easier to install and also occupies lesser space. K Vivek Chawla Steering Gear Ratio and Overall Steering Ratio : The steering gear ratio (SGR) is a reduction ratio which is obtained by the use of steering gear. It is defined as the ratio of "angle turned by the steering wheel to the corresponding angle turned by the steering gear cross-shaft". The SGR is generally low for small vehicles and high for large and heavy vehicles. A low SGR produces fast steering while the high SGR produces slow steering. As very quick steering is required in racing cars, therefore a still lower SGR is desired on them K Vivek Chawla Steering Gear Ratio and Overall Steering Ratio : value is lower in power steered vehicles than the manual steered vehicles by about 15% to 25%. The SGR for different categories of vehicles For small cars (without power steering) = 15:1to25:1 (17.5 : 1 an Maruti 800) (with power steering) = 12: 1 to 20: 1 (14.2 to 20. 1: 1 on Ford Escort) For light commercial vehicles = 15:1to25:1 (20:1 on Jeep CJ series) For heavy commercial vehicles = 20:1 to 40:1 (20.2:1onTata LPT 2416 , 24.7:1 on Ashok Leyland Comet, 30: 1 on Swaraj Mazda) Ford tractors = 30: 1 to 40: 1 (34.2: 1 on Tata LPS 1612/32 tractor) For racing cars = 10:1to18:1 (14:1 on Ford) K Vivek Chawla Steering Gear Ratio and Overall Steering Ratio : Overall steering ratio (OSR) is different from SGR. In SGR, only steering gear is involved, but in evaluating OSR, the linkages are also considered. Rotation of the cross-shaft is utilized to swivel the road wheels via linkages, therefore the linkages provide a mechanical advantage i.e. leverage too. OSR is defined as the ratio of "angle turned by the steering wheel to the corresponding angle turned by the front road wheels". K Vivek Chawla Steering Gear Ratio and Overall Steering Ratio : The value of OSR is about 20% higher than the SGR. In modem vehicles about one and a half complete turns (about 360° x 1.5 = 540°) of the steering wheel causes maximum possible swiveling of the front road wheels (i.e. complete steering lock of about 43° turning ) on either side of mid position of the wheels K Vivek Chawla POWER STEERING Almost all modem luxury cars are equipped with power steering system instead of manual steering. The power steering is increasingly becoming popular on light, medium and heavy vehicles also since it ensures Lesser steering effort Reduced driver fatigue Efficient absorption of shocks Better directional stability High performance Enhanced safety K Vivek Chawla POWER STEERING The power steering systems take assistance of hydraulic power in vivid manners for their operation. They become operative when the manual effort exceeds about 1 kgf (10 N). It means that the driver's effort is analogous to a touch of the steering wheel only. The system is designed such that if power system fails, it can be operated manually. If power steering does not works then driver's effort will be more (about 40-50 kgf) such as in manual steering. K Vivek Chawla POWER STEERING Types of Power Steering Systems: Several kinds of power steering are currently in use. These are 1. Integral type as used on Chrysler cars. (i) reaction control valve type (ii) rotary spool valve type 2. Semi-integral type as used on trucks and other heavy vehicles. 3. Linkage booster type as used on Calais car (rack and pinion type) 4. Speed responding type as used on Honda car. K Vivek Chawla Principles of Power Steering : Power steering is one type of hydraulic device for utilizing engine power as steering effort. Consequently, the engine is used to drive a pump to develop fluid pressure. This pressure acts on a piston within the power cylinder so that the piston assists the rack effort. K Vivek Chawla Principles of Power Steering : The amount of this assistance depends on the extent of pressure acting on the piston. Therefore, if more steering force is required, the pressure must be raised. The variation in the fluid pressure is accomplished by a control valve which is linked to the steering main shaft. K Vivek Chawla Neutral (Straight-Ahead) Position (Fig a). Fluid from the pump is sent to the control valve. If the control valve is in the neutral position, all the fluid will flow pass through the control valve into the relief port and back to the pump. At this time, hardly any pressure is created and because the pressure on the cylinder piston is equal on both sides, the piston will not move in either direction. K Vivek Chawla When Turning (Fig. b ). When the steering main shaft is turned in either direction the control valve also moves, closing one of the fluid passages. The other passage then opens wider, causing a change in fluid flow volume and at the same time, pressure is created. Consequently, a pressure difference occurs between both sides of the piston and the piston moves in the direction of lower pressure. Thus, the fluid in that cylinder is forced back to the pump through the control valve. K Vivek Chawla Simplified Construction ofa Linkage Booster Type Power Steering System : A simplified diagram of a hydraulic booster is shown in Fig. When the steering wheel is turned, worm 1 turns sector 2 of the worm wheel and arm 5, which turns the wheels by means of drag link 6. K Vivek Chawla Simplified Construction ofa Linkage Booster Type Power Steering System : If the resistance offered to the turn of the wheels is too high and the effort applied by the driver to the steering wheel is too weak; the worm will be displaced axially (like a screw in a nut) together with distributor slide valve 7 and will thus admit oil (or compressed air) into booster cylinder 3 through pipeline 8. K Vivek Chawla Simplified Construction of a Linkage Booster Type Power Steering System : If the resistance offered to the turn of the wheels is too high and the effort applied by the driver to the steering wheel is too weak; the worm will be displaced axially (like a screw in a nut) together with distributor slide valve 7 and will thus admit oil (or compressed air) into booster cylinder 3 through pipeline 8 The piston will move in cylinder 3 and will turn the wheels via gear rack 4, a toothed sector, arm 5 and drag link 6. K Vivek Chawla Simplified Construction of a Linkage Booster Type Power Steering System : The piston will move in cylinder 3 and will turn wheels via gear rack 4, a toothed sector, arm 5 and drag link 6. At the same time, the worm sector will act upon the worm and will shift it together with the distributor slide valve to its initial position and stop the piston travel. When the steering wheel is turned in the other direction, the wheels will be turned appropriately in the same sequence. K Vivek Chawla