Figure 6-7.Flow of oil through the clutch in ENGAGED position.
linkage to the range selector valve assembly on top of
the transmission case. Movement of the gearshift lever
positions the selector valve to allow main oil pressure
to engage the desired clutch assembly.
In modern power shift transmissions, the gearshift
lever is connected to a range selector valve by
hydraulic means. A spool valve (pilot control valve),
actuated by the gearshift lever, directs main oil
pressure to the range selector valve and causes it to
direct main oil pressure to the desired clutch assembly.
The hi-lo-shifting lever (on the transmission front
cover) is held in position by a poppet lock in the hi-lo
shifting housing. To shift from one range to another,
the engine must be running and the gearshift lever must
be in NEUTRAL position. This allows main oil
pressure from the pump to pass through a drilled hole
in the pilot valve and through an oil line to the shifter
housing. Here it releases the poppet lock to enable
Some power shift transmissions use planetary
gearsets to perform the same functions as the
transmission just described. A planetary gearset (fig.
6-8) consist of three memberssun gear, ring gear,
and a planetary carrier that holds the planetary gears in
proper relation with the sun and ring gear. The
planetary gears are free to "walk" around the sun gear
or inside the ring gear.
To cause a reduction or increase in torque, six
different methods of connecting this gearset to the
power train are possible (fig. 6-9). Direct drive is
achieved by locking any two members together. and
neutral is obtained by allowing all the gears to turn
Figure 6-8.Planetary gearset.