6-16 4203780 First Edition
HYDRAULICS
6
Lower Circuit Schematic and Theory
of Operation
System Conditions:
• Engine running
• Right, left, and/or front lift/lower lever(s) in LOWER
position
Oil Supply to Lift Valves
Hydraulic oil is drawn from the hydraulic oil tank by
section 1 of the gear pump assembly. Before entering the
gear pump assembly, the inlet oil is filtered by a
100-mesh screen to remove any debris.
Operating-pressure oil from section 1 of the gear pump is
routed to port “P” of the control valve and flows through
the 4.5-gpm priority valve. The four-section gear pump
will need to produce a flow rate of 4.5 gpm before the
priority valve opens to supply oil to the lift valve.
The priority valve will supply oil flow to the steering valve
before allowing oil flow to the lift valve, to ensure
maximum steering control at any engine rpm range.
Lower Circuit
Operating-pressure oil enters the lift valve body at the
“IN” port, and flows to the relief valve (1500 psi) and to
each of the spool valves. With the lift/lower levers in the
LOWER position, operating-pressure oil flows through
the spools. Oil is directed to the detent load check spools.
The spools are shifted, and open the ports to allow
trapped oil in the cylinder circuit to flow to the lift spools.
Gravity pulls the cutting units down, pulling against the lift
cylinders. Oil from the piston end of the lift cylinders flows
into the lift valve body at port “B,” through the detent
valves and lift valves. The oil exits the lift valve body
through the “OUT” port, through the weight transfer valve
to the hydraulic tank.
Circuit Relief
The lift system is protected by the 1500-psi relief valve. If
the pressure on the pilot line of the relief valve reaches
1500 psi, the valve will open, dumping
operating-pressure oil through the “OUT” port of the lift
valve body, through the weight transfer valve to the
hydraulic tank.
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