Figure 4-15.Reverse idler shaft and gear assembly-exploded view.
Figure 4-16.Transmission main shaft assembly-exploded view.
the countershaft gears, the countershaft gears turns the
main shaft gears, and, when engaged, the reverse idler
In low gear, a small gear on the countershaft drives a
larger gear on the main shaft, providing for a high gear
ratio for accelerating. Then, in high gear, a larger
countershaft gear turns a small main shaft gear or a gear
of equal size, resulting in a low gear ratio, allowing the
vehicle to move faster. When reverse is engaged, power
flows from the countershaft gear, to the reverse idler
gear, and to the engaged main shaft gear. This action
reverses main shaft rotation.
The synchronizer is a drum or sleeve that slides back
and forth on the splined main shaft by means of the
shifting fork. Generally, it has a bronze cone on each side
that engages with a tapered mating cone on the second-
and high-speed gears. A transmission synchronizer (fig.
4-17) has two functions, which are as follows:
1. Lock the main shaft gear to the main shaft.
2. Prevent the gear from clashing or grinding
When the synchronizer is moved along the main
shaft, the cones act as a clutch. Upon touching the
gear that is to be engaged, the main shaft is acceler-
ated or slowed down until the speeds of the main
shaft and gear are synchronized. This action occurs
during partial movement of the shift lever. Com-
pletion of lever movement then slides the sleeve and
gear into complete engagement. This action can be
readily understood by remembering that the hub of
the sleeve slides on the splines of the main shaft to
engage the cones; then the sleeve slides on the hub to
engage the gears. As the synchronizer is slid against
a gear, the gear is locked to the synchronizer and to
the main shaft. Power can then be sent out of the
transmission to the wheels.
Shift forks fit around the synchronizer sleeves to
transfer movement to the sleeves from the shift