FORWARD AND REVERSE HYDRAULIC
CLUTCH OPERATION.The forward and reverse
hydraulic clutches actually have two clutches on a
common shaft with a common apply force piston
between them. The clutches allow the simple transfer
of oil from the disengaging clutch into a cavity created
by the engaging clutch. This allows a low volume of
main pressure to actuate the clutch for high-speed
The heart of the clutch is contained in two
pistonsthe accelerator piston and the force piston.
Pump oil volume is not needed to fill the applying
clutch cavity, and only relatively low volume is needed
to pressurize the clutch. In neutral, all accelerator and
force piston cavities are filled with oil at lube pressure
(10 to 25 psi). A selector valve, located on the top of the
transmission case, directs the oil to the accelerator
piston cavity and, in turn, to the force piston cavity.
Once the pistons are filled with oil, they remain full
under lube pressure. Other small cross-drilled
passages furnish a constant supply of lube oil to the
drive gear bushing, the drum assemblies, and the
clutch hubs for distribution through the clutch plates.
In neutral, neither clutch is engaged, the drive gear and
drum assemblies are free. and no torque is transmitted
through the clutch, as shown in figure 6-6.
Upon application of the clutch, main oil pressure
(approximately 200 to 300 psi) is directed through the
clutch shaft for the specific side of the clutch desired.
The oil enters the force piston cavity causing the clutch
to engage (fig. 6-7). When engaged, the clutch holds
the gear stationary in relation to the shaft. Power then
flows from the shaft, via the clutch, to the gear.
When the transmission is returned to neutral, an
immediate pressure drop occurs within the
disengaging accelerator piston cavity and the
compressed piston centering springs return the
common apply force piston to its centered position or
GEAR SHIFTER MECHANISM.On many
older models, the gearshift lever is connected through
Figure 6-6.Flow of oil through the clutch in NEUTRAL position.