Chapter 4 Gear Box

Chapter 4Gear BoxIntroductionGear box is a speed and torque changing device between the engine and the driving wheels.Purposes of Gear box:It exchanges engine power for greater torque and thus provides a mechanical advantages to drive the vehicle under different conditions.It exchanges forward motion to reverse motion.It provides a neutral position to disallow power flow to the rest of power train.IntroductionThe vehicles require high torque when climbing the hills and when starting, even at low speeds.When running at high speeds on level roads, high torque is not required because of momentum.The torque, which the engine can produce is limited to such as amount that the engine by itself can not develop the torque necessary for starting and for climbing hills.Functions of Gear boxWhen driving power is required, it reduces the engine speed and transmit the stronger torque to the driving wheels.When high running speed is desired, it transmits high speed and low torque to the wheels.The gear box serves to reverse the vehicle.Gears and Gear RatiosSpur Gears:

Fig. Spur GearHelical Gears:Gears and Gear Ratios

Fig. Helical GearGears and Gear RatiosBevel Gears:

Fig. Bevel GearGears and Gear RatiosWorm Gears:

Fig. Worm GearGears and Gear RatiosPlanetary Gears:

Fig. Planetary GearGears and Gear RatiosWhen one meshing gear rotates, the teeth of that gear cause the teeth of the other gear to move so that the other gear also rotates.The relative speed of the two meshing gears( gear ratio) is determined by the number of teeth of the two gears.Gear ratio = No. of teeth on driven gear No. of teeth on driving gearGears and Gear RatiosGear ratio = Speed of driving shaft Speed of driven shaftGear ratio = Engine speed . Propeller shaft speedExample: Same no. of teeth, rotate at the same speed.One gear has 15 teeth and other has 30 teeth. When the smaller gear is driving the larger gear, Gear ratio is 2:1.

Gears and Gear RatiosThe power available from the engine is directly related to the engine torque(T) and the gear ratio(G).Torque available at the wheel, Tw = T * G, when no loss in transmission.This driving force is known as tractive effort and the maximum amount which can be limited by the coefficient of adhesion between the tyre and the road.The tractive effort also varies with the vehicle speed as the engine torque varies with engine speed.To vary the speed of the road wheels relative to the engine, the gear box provides a number of varying ratios-usually 3 to 4 gear ratios are sufficient for passenger cars.Types of Gear BoxesSelective Type (Manual) Gearbox:Sliding mesh,Constant mesh,Synchromesh.Progressive Type.Epicyclic or Planetary Type.Selective Type (Manual) GearboxIts 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.Selective Type (Manual) GearboxAdvantages:Simple in construction.Relatively free from troubles.Light and simple.Low production costs.Disadvantages:Noisy in operation.Gear ratios not being continuous but being in steps, we need to shift gears each time when vehicle is in running conditions.Sliding Mesh Gearbox4-speed gearbox:

Fig. Sliding Mesh GearboxSliding Mesh GearboxFirst or Low Gear:

Fig. First or Low Gear PositionGear 1-2-5-6.Gear ratio(first gear) = Speed of clutch shaft Speed of main shaftGear ratio(first gear) = (N1/N2) * (N5/N6) Gear ratio(first gear) = (T2/T1) * (T6/T5)Sliding Mesh GearboxSliding Mesh GearboxSecond Gear:

Fig. Second Gear PositionSliding Mesh GearboxGear 1-2-3-4.Gear ratio(2nd gear) = Speed of clutch shaft Speed of main shaftGear ratio(2nd gear) = (N1/N2) * (N4/N3) Gear ratio(2nd gear) = (T2/T1) * (T3/T4)Sliding Mesh GearboxThird (Top) Gear:

Fig. Top Gear PositionSliding Mesh GearboxGear 1-2.Gear ratio(3rd gear) = 1:1.Sliding Mesh GearboxReverse Gear:

Fig. Reverse Gear PositionSliding Mesh GearboxGear 1-2-7-8-6.Gear ratio(reverse gear) = Speed of clutch shaft Speed of main shaftGear ratio(reverse gear) = (N1/N2) * (N6/N7) Gear ratio(reverse gear) = (T2/T1) * (T7/T6)Constant Mesh GearboxFig. Constant Mesh Gearbox

Constant Mesh GearboxConstant Mesh GearboxDouble Declutching:For the smooth engagement of the dog clutches, its necessary that the speed of main shaft gear and the sliding dog must be equal.Therefore to obtain lower gear, the speed of the clutch shaft, lay shaft and main shaft gear must be increased.This is done by double declutching.

Constant Mesh GearboxAdvantages:As the gears have to remain always in mesh, its no longer necessary to use straight spur gears. Helical gears are used for quieter running.Wear of dog teeth on account of engaging and disengaging is reduced because here all the teeth of dog clutches are involved compared to only two or three teeth in the case of sliding gears.Synchromesh Gearbox

Fig. Synchromesh Mesh GearboxSynchromesh GearboxA = Engine shaft,Gears B, C, D, E = free gears on the main shaft,F1 & F2 = free to slide on splines on the main shaft,G1 & G2 = ring shaped members having internal teeth fit onto the external teeth members F1 & F2 resp.,K1 & K2 = dog teeth on B and D resp.,S1 & S2 = forks,T1 & T2 = balls supported by springs, prevent the sliding of members G1(G2) on F1(F2),M1, M2, N1, N2, P1, P2, R1, R2 are frictional surfaces. Synchromesh Gearbox

Fig. Synchromesh Mesh GearboxSynchromesh Gearbox

Fig. Synchromesh Mesh Gearbox

Transfer BoxIt uses for:Drive in two wheel drive on highways or shift to four wheel drive for cross country operation,To drive in high gear or low gear as required.

Transfer Box

Fig. Transfer BoxTransfer box

Fig. 2WD in high gearFig. 4WD in low gear