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TM 94000
22-2. Provide Gear Ratio Selection. Another major
22-1. Change Vehicle Direction. One major purpose of
function of the transmission is to provide the operator
the transmission is to provide the operator with the option
with a selection of gear ratios between engine and
of maneuvering the vehicle in either the forward or
wheels so that the vehicle can operate at best efficiency
reverse direction. This is a basic requirement of all
under a variety of driving conditions and loads.
automotive vehicles. Almost all vehicles have multiple
forward gear ratios, but in most cases, only one ratio is
provided for reverse.
spur gears for easy engagement; consequently, the
22-3. Sliding Gear. The sliding-gear type Is known as
transmission usually Is noisy when operating in the
the conventional transmission. There are two types of
Intermediate speeds.
sliding-gear transmissions. One Is the progressive, in
which the arrangement is such that it is necessary to
22-4. Constant Mesh. The conventional sliding-gear
pass one gear through another In definite order. Thus, In
a three-speed progressive transmission, it is Impossible
transmission has been superseded, particularly on
to shift from low to high without going through second.
passenger vehicles, by systems In which the gears
The us of this system Is limited almost entirely to
always are In mesh with their mates, and selection is
motorcycles. The other sliding-gear type Is known as
made by sliding components In and out of connection.
selective. In this system, the operator can select any ratio
Two of the most common of these systems are the
without going through intermediate stages. Sliding-gear
constant-mesh and the synchromesh, which have
transmissions use
additional features to prevent clashing of gears.
mediate stages. Sliding-gear transmissions use mesh
22-5. Construction. Conventional transmissions have
with the external teeth on the rear of the main drive gear
the following fundamental components: the case, which
when the gear Is shifted forward into the direct-drive
houses the gears and shafts; the control cover, which
position. The  first-and-reverse speed main shaft gear
houses the shifter mechanism; and the various shafts
can be shifted forward to mesh with the countershaft
and gears. Three-speed selective transmissions have
first-speed gear or rearward to mesh with the reverse
three shafts. They are, In the order of the flow of power,
idler gear. The countershaft reverse gear usually is In
the Input shaft, countershaft, and main shaft. The
constant mesh with the reverse Idler gear. In some
function of the three shafts, together with the gears that
transmissions, the reverse Idler gear is shifted to mesh
connect them, Is discussed in detail below.
with the countershaft reverse gear at the same time that
the first-and-reverse speed main shaft gear Is shifted to
a. Gears.  The  transmission second-and- third and
mesh with the reverse idler gear.
first-and-reverse speed main shaft gears have grooved
hub extensions, Into which the shift forks are fitted that
b. Shafts and Bearings. The Input shaft has an Integral
slide them back and forth on the main shaft splines.
main drive gear and rotates with the clutch driven plate or
Thus, the second-and-third speed main shaft gear can
disks; that is, the shaft rotates all the time the clutch is
be shifted rearward to mesh with the countershaft
engaged and the engine Is running. The main drive gear
second-speed gear. The second-and-third speed main
Is In constant mesh with the countershaft drive gear.
shaft gear also has internal teeth that
Because all the gears in the countershaft cluster


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