A transmission or gearbox is the gear and/or hydraulic system that transmits mechanical power from a prime mover (which can be an engine or electric motor), to some form of useful output device. Transmission is a speed reducing mechanism, equipped with several gears. The simplest transmissions, often called gearboxes, provide gear reduction sometimes in conjunction with a right-angle change in direction of the shaft. It may be called a sequence of gears and shafts, through, which the engine power is transmitted to the tractor wheels. These are often used on PTO-powered agricultural equipments The complete path of power from the engine to the wheels is called power train.

Function of transmission system:

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 be 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:

(a) Clutch

(b) Transmission gears

(d) Final drive

(e) Rear axle

(f) Rear wheels