Transmission - Gearbox

Transmission, Gearbox and Servicing

Purpose of Gearbox
A typical petrol or diesel engine is only efficient over a limited rotational speed range. Typically engines are used at 1,000 to 7,000 RPM. The driving force or torque will also vary depending upon the rotational speed.

This means that driving speed would be directly linked to engine speed giving very poor acceleration from rest, no ability to drive slowly in heavy traffic and no reverse.

In order to help the engine perform essential driving functions; such as:

  • Running Efficiently
  • Conserving Fuel
  • Accelerating Smoothly
  • Hill Climbing
  • Reversing
  • Maintaining Acceptable Speed

The engine is connected to the drive wheels via a gearbox so that the engine can maintain an efficient forward rotating speed while performing all the functions required by the driver. Without a gearbox travelling in the opposite direction (reversing) would force the engine to stop and then turn in the opposite direction!

Manual Gearbox and Automatic Gearboxes

Control of gear selection determines if a gearbox is "manual" or "automatic". With a manual gearbox the driver has full control over the gear selection while automatic gearboxes select forward gears automatically with a manual reverse gear.

Manual Gearbox

A typical manual gearbox has 5 forward gears and reverse,although some vehicles have 4 gears and 6 gears are becoming more common.

Manual gearboxes are available in three main types:

  • Sliding Mesh
  • Constant Mesh
  • Synchromesh

The synchromesh gearbox is most commonly used and contains aspects of the earlier sliding mesh and constant mesh gearboxes.

Automatic Gearbox - Torque Converter

Most automatic gearboxes use an epicyclic gearing system and a fluid clutch or torque converter. The torque converter monitors engine rotaion speed and disengages the engine from the drive train when engine speed drops below about 1000rpm. In addition, the torque converter can provide a smooth and variable torque to speed ratio to maintain optimal conditions between the built-in epicyclic gear ratios.

Rear Wheel Drive

The Engine-Gearbox is connected to the rear wheels via a propeller shaft.

Front Wheel Drive

Popular with most small and medium sized cars; the front wheel drive. To fit all of the drive components within the front engine compartments the engine is mounted tranversely (Transverse Engine Layout).

Differential (The Diff)
Transverse engine mounting allows the output drive shaft from the engine to be in a similar plane, the gearbox and the front dive wheels to be in a similar plane. The final (reduction) gear component incorporates a differential that is connected to the road wheels via short drive shafts.

Constant Velocity Joints (CV Joints)

Each drive shaft is fitted is fitted with an inner and outer universal joint The outer joint must also allow for the steering action of the front wheels and must maintain an even speed as the wheels are steered left and right hence the term Constant velocity Joint..

Four Wheel Drive

As its name implies; four wheel drive (4x4) can split the drive power between al four road wheels to maintain greater control and traction of the vehicle under adverse driving conditions.