Gear Box Types PDF

 We know that, the horsepower delivered to the rear wheels depends upon the turning effort & the speed of rotation. Mathematically, it is represented as, 2  x Torque (kg-m) x rpm Power = 4500 If t...

 We know that, the horsepower delivered to the rear wheels depends upon the turning effort & the speed of rotation. Mathematically, it is represented as, 2  x Torque (kg-m) x rpm Power = 4500 If the engine horsepower is constant, from the above relation it is obvious that for higher torque at wheels, low speed is required & vice-versa. Hence, the gearbox is incorporated in-between the engine & rear wheels which is a solution for the variable torque & speed.  Necessity Of Gear Box:  The tractive effort required on rear wheels varies with the magnitude of the load being pulled & soil characteristics.  If the tractor engine power is delivered to the rear wheels directly the tractor will run at constant speed, which obviously, will be very high & a fixed tractive effort will be available at the rear wheels.  In tractors, the driving wheels require power supply at low rpm and high torque, whereas the engine runs at high rpm and low torque.  Hence, some means have to be introduced between engine & rear wheels where high speed of engine could be reduced for torque multiplication to achieve desired tractive effort.   Thus, a gear box serves the purpose of reduced wheel speed & variable tractive effort.  Principle of Gearing : Any combination of gear wheels by means of which motion is transmitted from one shaft to another shaft is called a gear train. Consider a single train in which the driving gear A has 10 teeth & driven gear B has 20 teeth. If gear A is rotating with a speed of 100 rpm, the speed of gear B is given by. Speed of driving gear (NA) No. of teeth on driven gear (TB) -------------------------------------- = ------------------------------------------------ Speed of driven gear (NB) No. of teeth on driving gear (TA) 100 x 10 or Speed of driven gear = ------------ = 50 rpm 20 This shows that if the no. of teeth on driven gear is doubled, the speed becomes half of the driving gear. However, the torque of gear B (TB) will be twice, that of gear A (TA) as shown below considering no loss of power between the gears. Power = 2  NA TA = 2  NB TB Putting the values of NA TA & NB  TB = [100 x TA] / 50  TB = 2 TA The direction of rotation of gear B will be opposite to that of gear A. However, if an additional gear is mounted or introduced or a compound gear the direction of final gear will be the same as that of the driving gear. The speed of the final driven gear will depend upon the product of the gear ratio of each pair. NA TB NC TD ------ = ------ & ----- = ----- NB TA ND TC  NA = NA x Nc = TB x TD  ND NB ND TA TC Types of Gear Box 1. Sliding mesh, 2. Constant mesh & 3. Synchromesh  1st speed  Most of the indigenous tractors are fitted with 3rd speed 2nd speed a sliding mesh or sliding mesh type gear box.   Usually, the main & counter shafts are Main shaft parallel to the clutch shaft, in addition to the two shafts, a third shaft carries reverse gear. Counter shaft  The upper shaft is known as the main shaft & the lower one is known as counter shaft or lay shaft. The gears on lay shaft are fixed, whereas, those on the main shaft slide to mesh with a suitable gear on the countershaft. Gearshift rails & forks are used to shift gears from one position to another. In order to allow only one pair of gears to engage at one time, the shift rails are kept in position with the help of spring loaded detent balls or by another device. This can provide 3-4 forward and one reverse speed.  Advantages:  It has highest transmission efficiency.  It is simple in design.  It is cheaper in cost  Limitations:  It is limited to small size tractors only.  Large gears are hard to move.  Larger teeth cause interference in engagement and the teeth can be damaged during shifting.  Tractor needs to be stopped before gear shifting to avoid teeth damage.  Constant Gear Box  The constant mesh transmission is similar to sliding mesh type gear box except the gears on main shaft and counter shaft are in constant mesh.  The gears on the main shaft are mounted on bushings & are free to rotate along with the counter shaft gear without affecting the main shaft when in neutral.  The engagement and disengagement is achieved by splined sleeve couplings operated by gear lever.  When the gear lever is engaged, the coupling devices move from one side to another to lock a free gear on the main shaft.  Thus the main shaft gets drive through the driven gear & coupling unit.  This type of gear transmission is slightly less efficient than sliding mesh type due to friction between rotating gears and shafts.  Tractor needs to be stopped before gear shifting to avoid teeth damage.  Advantages:  All the gears are in constant mesh, wear and tear of gears and any possible damage of gears do not occur in engaging and disengaging gears.  Helical gears are used to reduce shifting noise and shifting efforts.  Limitations: Tractor needs to be stopped before gear shifting to avoid teeth damage.  Synchromesh Gear Box :  The synchromesh transmission is basically a constant mesh transmission with an extra device called a synchronizer to equalize the speed of the mating parts before they engage.  This system is very common on automobiles & is very rarely used on tractor transmission.  This type of transmission is suitable for changing the speeds when tractor is moving.  The synchronizing device synchronizes the speed of two gears and shafts before changing the appropriate gear.  Small friction clutches usually cone type are used to synchronize the speed of collars and gears.  Advantages  Gear shift can be made without damaging, even when tractor is moving  Popular in tractors used for highway transport purpose   Disadvantages  It has low power transmission efficiency than sliding mesh gear box  This system is more costly.