Lesson 9. Introduction to transmission systems in four wheeled tractors

The transmission system is basically the drive line of any automobile or a tractor in our case. This system consists of components that are used to transmit the torque developed by the prime-mover or the engine to the driving wheels and to vary the torque and direction of rotation of the ground wheels. The main drive line units are shown schematically in the Fig. 9.1.



A power transmission system usually consists of the following parts:

  1. Clutch – The device that connects or disconnects two torque transmitting devices.

  2. Transmission – A device for transmitting power at a multiplicity of speed and torque.

  3. P.T.O. Drive – The parts that transmit torque from the engine to the PTO spline on the rear of the tractor.

  4. Differential – The device, usually in the axle housing, that allows the two wheels on an axle to rotate at different speeds.

  5. Brake – The device, usually in the axle housing, that stops the motion of the tractor.

  6. Axle – The shaft and connecting parts that transmits torque from the differential or final gear reduction, to the wheels.

Engine – Crankshaft – flywheel – clutch – transmission box – differential – final drives – axle – drive wheels.

A power transmission system for a tractor has two functions:

  1. To disconnect the engine from the road wheels when desired.

  2. To transmit the torque in a smooth manner without shocks and jerks.

  3. To reduce the engine speed as desired based on tyre size and forward speed required.

  4. To change the axis of rotation of power to align it as per the orientation of the drive wheels.

  5. To change engine torque and speed into the torque and speed required  by the wheel for different task required of a tractor.

  6. To provide for auxillary power outlet in the form of Power Take Off for powering the implements and also for stationary machinery.


Te Ne (eff) = Tw Nw = Constant


Te = torque at engine

Ne = rpm of engine

Tw= torque at drive wheels

Nw= rpm of drive wheels

Eff= efficiency of the power transmission system.

A classification of tractor transmissions can be made as follows:

  1. Selective-gear, fixed-ratio

  2. Selective-gear, fixed ratio plus one planetary

  3. Planetary gears in series

  4. Hydrodynamic

  5. Hydro static

Variations of these classifications exist. All transmissions convert the engine torque and speed into a more useful combination of torque and speed at the drive wheels.

Because the maximum speed of a tractor is regulated by a governor, the potential maximum drawbar power of the tractor is nearly constant regardless of the forward speed, except for the lowest speeds when the maximum power is limited by traction.

The greatest difference between transmission for farm tractors and those for highway vehicles is that in the tractor most of the gears may be used continuously under full load. Automobile transmissions would fail if they were run in low gear at full power for any length of time.

Last modified: Monday, 3 March 2014, 4:45 AM