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 TM 9-8000
Section IV. CONSTANT MESH TRANSMISSION
prevent it from moving endwise relative to the gear until
22-8. Construction. The basic components of the
the latter has reached the end of its travel. The driven
constant mesh transmission are similar to the sliding
members are the main drive gear and second-speed
gear transmission discussed in paragraph 22-5.
main shaft gear, each of which has external cones and
However, major differences in designs are discussed in
external teeth machined on its sides to engage the
the following text.
internal cones of the sliding gear and the internal teeth of
the sliding sleeve. The synchromesh clutch is shown
a. Gears. In this type of transmission (fig.22-7),
disengaged and engaged in figure 22-9. The
certain countershaft gears are constantly in mesh with
synchromesh clutch operates as follows. When the
gears on the main shaft. The meshing gears on the
transmission control lever is moved by the driver to the
main shaft are fixed so they cannot move endwise, but
third-speed or direct-drive position, the shift-fork moves
they are supported on roller bearings so they can rotate
the sliding gear and sliding sleeve forward as a unit until
independently of the main shaft. For example, the main
the internal cone on the sliding gear engages the
shaft assembly of the transmission shown in figure 22-7
external cone on the main drive gear. This action brings
is illustrated in figure 22-8 in disassembled view. Note
the two gears to the same speed and stops endwise
that the main shaft third-speed gear (4) is supported on
travel of the sliding gear. The sliding sleeve then slides
the shaft by bearing rollers (5). Note also that this gear
over the balls and silently engages the external teeth on
has internal teeth that match external teeth on the main
the main drive gear, locking the main drive gear and
shaft third-and-fourth-speed clutch gear (1). Usually,
transmission main shaft together as shown in figure 22-
helical gears are used in this type of transmission. The
9. When the transmission control lever is shifted to the
first-reverse-and-second gear (10) is a sliding type and,
second-speed position, the sliding gear and sleeve move
therefore, is a spur gear.
rearward and the same action takes place, locking the
b. Synchronizers. The construction of a typical
transmission main shaft to the second-speed main shaft
synchromesh clutch is shown in figure 22-9. The
gear. The synchromesh clutch is not applied to first
driving member consists of a sliding gear splined to the
speed or to reverse. First speed is engaged by an
transmission main shaft with bronze internal cones on
ordinary dog clutch when constant mesh is employed, or
each side. It is surrounded by a sliding sleeve having
by a sliding gear; reverse always is engaged by means of
internal teeth that are meshed with the external teeth of
a sliding gear. Figure 22-10 shows a synchromesh
the sliding gear. The sliding sleeve is grooved around
transmission in cross section that uses constant-mesh
the outside to receive the shift fork. Six spring loaded
helical gears for the three forward speeds and a sliding
balls in radially drilled holes in the gear fit into an internal
spur gear for reverse.
groove in the sliding sleeve and
22-6
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