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.