A transmission is a machine in a power transmission system, which provides a controlled application of power. Often the term 5-speed transmission refers simply to the gearbox, that uses gears and gear trains to provide speed and torque conversions from a rotating power source to another device.

Transmission is a speed varying mechanism, equipped with several gears. It may be called a sequence of gears and shafts, through which the engine power is transmitted to the tractor wheels. The system consists of various devices, which cause forward and backward movement of the tractor to suit different field condition. The complete path of power from the engine to the wheels is called Power train.

The main functions of the power transmission system are:

  • To transmit power from the engine to the rear wheels of the tractor.
  • To make reduced speed available, to rear wheels of the tractor.
  • To alter the ratio of wheel speed and engine speed in order to suit the field conditions.
  • To transmit power through right angle drive, because the crankshaft and rear axle are normally at right angles to each other.
  • The power transmission system consists of the clutch, transmission gears, differential, final drive, rear axle and rear wheels


Clutch is a device, used to connect and disconnect the tractor engine from the transmission gears and drive wheels. Clutch transmits power by means of friction between driving members and driven members.

Why clutch is essential?

  • The engine needs cranking by any suitable device. For easy cranking, the engine is disconnected from the rest of the transmission unit by a suitable clutch. After starting the engine, the clutch is engaged to transmit power from the engine to the gearbox.
  • In order to change the gears, the gearbox must be kept free from the engine power, otherwise, the gear teeth will be damaged and engagement of gear will not be perfect. This work is done by a clutch.
  • When the belt pulley of the tractor works in the field it needs to be stopped without stopping the engine. This is done by a clutch.


Final drive is a gear reduction unit in the power trains between the Differential and the Drive wheels. Final drive transmits the power finally to the rear axle and the wheels. The tractor rear wheels are not directly attached to the half shafts but the drive is taken through a pair of spur gears. Each half shaft terminates in a small gear which meshes with a large gear called Bull gear. The Bull gear is mounted on the shaft carrying the tractor rear wheel. The device for final speed reduction, suitable for tractor rear wheel is known as the Final drive mechanism.

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