Lift Circuit
The hydrostatic transmission charge pump supplies
charge oil for the closed loop traction circuit in addition
to supplying flow for the steering circuit and the cutting
deck (or implement) lift circuit. The charge pump takes
its suction through a filter from the reservoir (front axle).
Pump output flows to the steering control valve before
reaching the lift valve so the steering circuit has priority.
Liftcircuitpressureislimitedfrom700to1000PSIbythe
implement relief valve located in the hydrostat.
Lower Cutting Deck (Or Implement) (Fig. 10)
When the cutting deck (or implement) is to be lowered,
the control valve spool is moved forward causing the pi-
lot valve to shift. This shifted valve allows a return path
from the lift cylinders to the reservoir (front axle) (Fig.
10).
The weight of the cutting deck (or implement)
causes the cylinders to retract thus lowering the cutting
deck(orimplement).Asthecylindersretract,thepistons
draw hydraulic fluid from the reservoir (front axle) into
the lower portion of the lift cylinders.
During conditions of not steering or raising/lowering the
cutting deck (or implement), flow from the charge pump
goes to the lift control valve and is by--passed through
the oil cooler and then to the reservoir (front axle).
When the control valve lever is released, the lift control
spool returns to the center position and causes the pilot
valve to shift. Oil flow by--passes to the reservoir (front
axle) and lift cylinder movement is stopped.
Raise Cutting Deck (Or Implement) (Fig. 9)
Whenthecutting deck(orimplement)istoberaised,the
control valve spool is positioned back and flow is di-
rected out the B port of the control valve to the cylinder
(upper) end of the lift cylinders. Hydraulic pressure
against the cylinder pistons extends the lift cylinders. At
the same time, the pistons push the hydraulic fluid in the
lower portion of the lift cylinders out and to the reservoir
(front axle). The extending lift cylinders cause the cut-
ting deck (or implement) to raise.
Counterbalance
An adjustable counterbalance valve allows cutting unit
weight transfer to the machine to improve traction. On
early production machines, the counterbalance valve is
incorporated in the lift control valve. Later production
machines have a separate counterbalance manifold.
Changes in traction conditions (e.g. moving up an in-
cline) will cause a valve shift to slightly lift or lower the
cutting deck to alter weight transfer. Counterbalance
pressure settings are dependant on the cutting deck
used on a machine. Refer to Traction Unit Operator’s
Manual for counterbalance pressure recommenda-
tions.
When the control valve lever is released, spring action
returns the lift control spool to the center position and
by--passesflowbacktothereservoir (frontaxle).Liftcyl-
inder movement is stopped.
LIFT CIRCUIT (LOWER)
Figure 10
Page 5 -- 11 Rev. A
Groundsmaster 3280--D/3320
Hydraulic System
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