JLG 800A_AJ ANSI Service Manual - Manual (Page 283)

SECTION 5 - HYDRAULICS
3120740 – JLG Lift – 5-3
5.2 VALVES - THEORY OF OPERATION
Solenoid Control Valve - Rexroth
Control valves used are four-way three-position solenoid
valves of the sliding spool design. When a circuit is acti-
vated and the control valve solenoid energizes, the spool
is shifted and the corresponding work port opens to per-
mit oil flow to the component in the selected circuit with
the opposite work port opening to reservoir. Once the cir-
cuit is deactivated (control returned to neutral) the valve
spool returns to neutral (center) and oil flow is then
directed through the valve body and returns to reservoir. A
typical control valve consist of the valve body, sliding
spool, and two solenoid assemblies. The spool is
machine fitted in the bore of the valve body. Lands on the
spool divide the bore into various chambers, which, when
the spool is shifted, align with corresponding ports in the
valve body open to common flow. At the same time other
ports would be blocked to flow. The spool is spring loaded
to center position, therefore when the control is released,
the spool automatically returns to neutral, prohibiting any
flow through the circuit.
Relief Valves
Relief valves are installed at various points within the
hydraulic system to protect associated systems and com-
ponents against excessive pressure. Excessive pressure
can be developed when a cylinder reaches its limit of
travel and the flow of pressurized fluid continues from the
system control. The relief valve provides an alternate path
for the continuing flow from the pump, thus preventing
rupture of the cylinder, hydraulic line or fitting. Complete
failure of the system pump is also avoided by relieving cir-
cuit pressure. The relief valve is installed in the circuit
between the pump outlet (pressure line) and the cylinder
of the circuit, generally as an integral part of the system
valve bank. Relief pressures are set slightly higher than
the load requirement, with the valve diverting excess
pump delivery back to the reservoir when operating pres-
sure of the component is reached.
5.3 HOLDING VALVE CHECKS
1. Start the machine and warm the hydraulic system to
operating temperature.
PERFORM ALL HOLDING VALVE CHECKS FROM THE GROUND
CONTROL STATION WITH AN EMPTY PLATFORM.
2. Check the Upright level cylinder rod side holding
valve as follows:
a. Fully retract and fully lower the main boom and
tower boom assemblies.
b. Power the main boom lift down function into the
turntable boom rest by holding the function
switch down between 10 and 20 seconds.
c. Verify the upright remains perpendicular to the
turntable and that the Upright Monitoring Sys-
tem alarms have not been activated.
3. Check the Upright level cylinder barrel side holding
valve function as follows:
a. Fully retract and fully lower the main boom and
tower boom assemblies. Raise the tower boom
between 2 feet and 5 feet (0.6 m and 1.5 m)
b. Pull and hold the re-leveling knob between 20
and 30 seconds.
c. Verify the upright remains perpendicular to the
turntable and that the Upright Monitoring Sys-
tem alarms have not been activated.
4. Check the Tower lift cylinder barrel side holding
valve function as follows:
a. Fully raise and fully retract the tower boom.
Fully raise and fully extend the main boom.
b. Using auxiliary power, fully lower the tower
boom.
c. Verify the upright remains perpendicular to the
turntable and that the Upright Monitoring Sys-
tem alarms have not been activated.