the gears are free running but, when shifted, they are
the synchronizing clutch mechanisms lock the gears
locked to their shafts by sliding collars.
The following is an example of the operation of a
constant mesh transmission: When the shift lever is
moved to THIRD, the THIRD and FOURTH shifter
fork moves the sliding collar toward the THIRD speed
gear. This engages the external teeth of the sliding
collar with the internal teeth of the THIRD speed gear.
Since the THIRD speed gear is meshed and rotating
with the countershaft, the sliding collar must also
rotate. The sliding collar is splined to the main shaft,
and therefore, the main shaft rotates with the sliding
collar. This principle is carried out when the shift lever
moves from one speed to the next.
The synchronizer accelerates or slows down the
rotation of the shaft and gear, until both are rotating at
the same speed and can be locked together without a
gear clash. Since the vehicle is normally standing still
when it is shifted into reverse gear, a synchronizer is
not ordinarily used on the reverse gear.
Constant mesh gears are seldom used for all
speeds. Common practice is to use such gears for the
higher gears, with sliding gears for FIRST and
REVERSE, or for REVERSE only.
Now that you understand the basic parts and
construction of a manual transmission, we will cover
the flow of power through a five-speed synchromesh
transmission. In this example neither first gear nor
reverse gear are synchronized.
The construction of the synchromesh transmission
is the same as that of the constant mesh transmission
with the exception that a synchronizer has been added
(fig. 4-19). The addition of synchronizers allows the
gears to be constant mesh when the cluster gears and
In passing from neutral to reverse, the first-reverse
main shaft gear is shifted rearward to mesh with the
reverse idler gear (fig. 4-20, view A). The sole function
of this gear is to make the main shaft rotate in the
opposite direction to the input shaft; it does not affect
Figure 4-19.Synchromesh transmission.