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Constant-mesh four-speed gearbox

Constant-mesh four-speed gearbox The input shaft takes the drive from the engine (when the clutch is engaged) into the gearbox. The selector rods and forks move the synchromesh units back and forth, according to which gear is selected. Some gearboxes have only one selector rod, others three; but the principle is the same. A spring-loaded ball at the base of the gear lever aligns with a recess in the selector rod holding the lever in position until the driver changes gears. When the reverse idler gear is selected, it is interposed in the gear train, reversing the normal direction of the mainshaft. The layshaft transmits drive from the input shaft to the mainshaft when a constantly meshed gear is locked to the mainshaft by means of a splined mesh. The mainshaft, or output shaft, transmits drive from the layshaft to the propellor shaft, when a gear is selected.

The gears are selected by a system of rods and levers operated by the gear lever. Drive is transmitted through the input shaft to the layshaft and then to the mainshaft, except in direct drive - top gear - when the input shaft and the mainshaft are locked together.

Explained in

    How manual gearboxes work

    Internal-combustion engines run at high speeds, so a reduction in gearing is necessary to transmit power to the drive wheels, which turn much more slowly. Read more

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