Textron JACOBSEN GP400, Technical/Repair Manual

Page: 214 / 354

6-22 4252490-Rev A
HYDRAULICS
6
Lower Circuit Schematic and Theory
of Operation
System Conditions:
• Engine running
• Mower raise/lower switch in the lower position
Oil Supply to Lift Valve
Hydraulic oil is drawn from the hydraulic oil tank by the
gear pump. Before entering the gear pump, the inlet oil is
filtered by a 125 micron suction strainer to remove any
debris. Operating pressure oil from the gear pump is
routed to port “P” of the reel valve unit, through the 3020
psi (207 bar) mowing relief valve, through port “T” of the
reel valve unit, and to port “P” of the lift valve.
Lower Circuit
Operating pressure oil supplied from port “T” of the reel
valve unit enters the lift valve at port “P.” The oil flows
against the 1250 psi (80 bar) relief valve, through the
solenoid valve, to the pilot of the check valve, opening
the check valve, and then exits the lift valve at port “B.”
Oil from port “B” of the lift valve is routed to port “B” of the
bottom sequence delay valve and to port “A” of the soft
drop valve.
Oil entering port “B” of the bottom sequence delay valve
flows unrestricted through the check valve, exits port “A”
of the top sequence delay valve, and flows to the rod side
of the center lift cylinder. Oil flow to the rod side of the
center lift cylinder lowers the center cutting unit.
Oil entering port “A” of the soft drop valve flows against
the check valve, through the orifice, and exits port “B” of
the soft drop valve. Oil exiting port “B” of the drop valve
flows to the rod side of the right and left lift cylinders. Oil
flow to the rod side of the right and left lift cylinders
lowers the right and left cutting units. The orifice restricts
the flow of oil, dampening the reaction of the right and left
cylinders’ retraction.
Oil from the piston end of the right and left lift cylinders
flows through the lift valve, back to the oil cooler, and
then to the hydraulic tank.
Oil from the piston end of the center lift cylinder flows
through port “A” of the top sequence delay valve, against
the check valve, through the orifice, and exits port “B” of
the top sequence delay valve. The orifice restricts the
flow of oil, dampening the reaction of the center cylinder
retraction. Oil exiting port “B” of the top sequence delay
valve is routed through the lift valve, back to the oil
cooler, and then to the hydraulic tank.
Circuit Relief
The lift system is protected by the 1250 psi (86 bar) relief
valve in the lift valve. If the pressure on the pilot line of
the relief valve reaches 1250 ± 10% psi (86 ± 10% bar),
the valve will open, dumping operating pressure oil
through port “T” of the lift valve to the oil cooler, and then
the hydraulic tank.