A car requires high torque (a force that causes rotation) when going over hills and when starting, even though they are performed at low speeds. On the other hand, when moving at high speeds at level roads, high torque isn’t required because of the natural momentum that is already present. Gearboxes were developed to change the vehicle’s torque and its speed according to road or surface condition (including load condition).

What is the function of Gearboxes?

A gearbox or transmission box converts the rotational energy of the engine to a rotational speed appropriate for the wheels of a vehicle. Mechanical gearboxes uses some simple gears to achieve this and automatic gearboxes use more complicated planetary gear sets.


A car’s engine cannot be attached directly to the wheels. Vehicle engines can only operate in a relatively small Revolutions Per Minute (RPM) band, and the maximum speed that they could achieve in this band is fairly low. Gearboxes change the ratio between the engine’s rotational speed and the rotational speed of the wheels. Cars depend on small gears at low speeds and larger gears are required at higher speeds.

On manual gearboxes, the driver is required to select manually which gear he needs to use and automatic gearboxes on the other hand, change to other gears automatically. When the engine’s speed drops in an automatic car, the gearing is being changed. When they were first invented, manual gearboxes were more efficient than automatic gearboxes, but that has changed with new technology, modern automatic gearboxes are far more efficient.

Main Components of a Gearbox

A gearbox requires 4 components to fulfil its function.

  • Counter Shaft – a counter shaft connects with the clutch shaft directly as well as the main shaft. It may run some runs at the engine speed or at lower than engine speed according to gear ratio.
  • Main Shaft – it is the shaft that runs at the vehicle speed. It carries power from the counter shaft by use of gears and according to the gear ratio.
  • Gears – gears are used to transmit the power from one shaft to another. A very important component. A small car gearbox contains four speed gear ratio and one reverse gear ratio.
  • Bearings – bearings are required to support the revolving part and reduce friction.