Automobile Gearbox: Definition, Diagram, Types, Parts, Pdf

What is a Gearbox?

  • An automobile gearbox Is an assemblage of gears and shafts, which is used to control the output speed & torque of a machine/vehicle.
  • The engine provides a fixed speed for a certain amount of power & If it is directly coupled to the output shaft then it will give the fixed velocity of the vehicle.
  • In the case of a vehicle running on a straight road with a higher speed, It doesn’t require a higher amount of torque because of the momentum of the vehicle. Hence it can run with less amount of torque.
  • In Another case, when the vehicle is climbing the heel, It requires a higher amount of torque.
  • Therefore the power transmission depends upon the Running condition of the vehicle.
  • The vehicle also requires neutral condition even the engine is started
  • Therefore gearbox is needed for a vehicle to control output speed & torque.
  • It is located between the clutch & propeller shaft
  • As per the need of the vehicle, different speeds or torque we can achieve with the help of a gearbox.
  • For Example – In the case of the tractor which requires higher torque & lower speed hence reduction ratio is higher.
  • While in the case of Racing vehicles which require higher speeds with less torque. In this case reduction ratio is lower.
  • It has N number of speed steps & one reverse step.

Gearbox diagram:

automobile gearbox

Working principle:

The gearbox works on the following principle:-

  • When Smaller gear meshes with Larger gear, low speed & high torque will obtain at output shaft.
  • When larger gear meshes with Smaller gear, high speed & low torque will obtain at output shaft.


The gearbox consists of an engine shaft, Lay shaft, Output shaft (Externally Splined), Clutch gear, Gears (1, 2, 3, 4, A, B, C), Idler gear etc.

Necessity/Significance of gearbox:

  1. To Control Output Torque.
  2. To Control Output Speed.
  3. To Achieve Neutral Position.
  4. To Achieve Reverse Gear transmission.

Types of automobile gearbox:

A) Manual Transmission:-

  1. Sliding Mesh gearbox
  2. Constant Mesh Gearbox
  3. Synchromesh Gearbox

B) Automatic Transmission:-

  1. Torque Converter
  2. Epicyclic Gear Transmission  

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Pratik is a Graduated Mechanical engineer. He enjoys sharing the engineering knowledge learned by him with people.

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