Figure 13-11.-Steering colunn transmission control lever and linkage.
synchromesh transmission that uses constant-mesh
truck, the auxiliary transmission doubles the mechanical
helical gears for the three forward speeds and a sliding
spur gear for reverse.
Some transmissions are controlled by a steering
column control lever (fig. 13-11). The positions for the
various speeds are the same as those for the vertical
control lever except that the lever is horizontal. The
shifter fork is pivoted on bell cranks that are turned by
a steering column control lever through the linkage
shown. The poppets shown in figure 13-10 engage
notches at the inner end of each bell crank. Other types
of synchromesh transmissions controlled by steering
column levers have shifter shafts and forks moved by a
linkage similar to those used with a vertical control
lever.
AUXILIARY TRANSMISSION
The auxiliary transmission allows a rather small
truck engine to move heavy loads by increasing the
engine-to-axle gear ratios. The auxiliary transmission
provides a link in the power trains of construction
vehicles. This link diverts engine power to drive four
and six wheels and to operate accessory equipment
through transfer cases and power takeoff units. (See
fig. 13-12).
Trucks require a greater engine-to-axle gear ratio
than passenger cars, particularly when manufacturers
put the same engine in both types of equipment. In a
advantage. It connects to the rear of the main
transmission by a short propeller shaft and universal
joint. Its weight is supported on a frame crossmember
as shown in figure 13-12. The illustration also shows
how the shifting lever would extend into the driver’s
compartment near the lever operating the main
transmission.
In appearance and in operation, auxiliary
transmissions are similar to main transmissions, except
that some may have two and some three speeds (low,
direct, and overdrive).
TRANSFER CASES
Transfer cases are put in the power trains of vehicles
driven by all wheels. Their purpose is to provide the
necessary offsets for additional propeller shaft
connections to drive the wheels.
Transfer cases in heavier vehicles have two speed
positions and a declutching device for disconnecting the
front driving wheels. Two speed transfer cases, such as
the one shown in figure 13-13, serve also as auxiliary
transmissions.
Some transfer cases are complicated. When they
have speed-changing gears, declutching devices, and
attachments for three or more propeller shafts, they are
even larger than the main transmission. A cross section
13-10
