Change-speed and four-speed gearboxes

A change-speed gearbox usually comprises the driving shaft end, the lay-shaft, and the driven shaft, which are installed (parallel to the longitudinal axis of the vehicle) in the gear-case.

A gear wheel is rigidly mounted on the driving shaft end which protrudes into the gear-case. This gear wheel is driven directly by the engine, through the clutch, and therefore rotates at the speed of the engine.

It drives a second, somewhat larger gear wheel which is mounted on the lay-shaft, so that this shaft rotates at a lower speed. Rigidly mounted on the lay-shaft are the transmission gear wheels for the low speeds (1st, 2nd and 3rd gear in a four-speed, 1st and 2nd in a three-speed gearbox).

The driven shaft - i.e., the shaft which transmits the desired speed to the final drive of the vehicle - is mounted in line with the driving shaft and carries the longitudinally movable driven gear wheels corresponding to the various speeds.

The lay-shaft is rotating all the time. When the vehicle is in ‘1st gear’, a small gear wheel on the lay-shaft drives a large gear wheel on the driven shaft; in ‘2nd gear’ a larger gear wheel on the lay-shaft drives an only slightly larger gear wheel on the driven shaft, so that the speed of rotation transmitted to the road wheels is somewhat higher than in ‘1st gear’.

In a four-speed gearbox a third pair of gear wheels on the lay-shaft and driven shaft drives the vehicle in ‘3rd gear’. In ‘top gear’ (direct drive) the engine speed is transmitted unreduced through the gearbox. For ‘reverse gear’ the direction of rotation of the driven shaft is reversed by the interposition of a second gear wheel.

Many gearboxes are equipped with what is known as an overdrive or cruising gear: a large gear wheel on the layshaft drives a smaller gear wheel on the driven shaft, thereby causing the latter to rotate faster than the engine.

This means that the engine need only run at a relatively low speed even when the vehicle is travelling fast.