JLG 660SJ Service Manual - Manual (Page 175)

SECTION 5 - HYDRAULICS
3120840 – JLG Lift – 5-43
5.7 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.
5.8 PRESSURE SETTING PROCEDURES (S
AND A MODELS)
COLD TEMPERATURES HAVE A SIGNIFICANT IMPACT ON PRES-
SURE READINGS. JLG INDUSTRIES, INC. RECOMMENDS OPER-
ATING THE MACHINE UNTIL THE HYDRAULIC SYSTEM HAS
WARMED TO NORMAL OPERATING TEMPERATURES PRIOR TO
CHECKING PRESSURES. JLG ALSO RECOMMENDS USING A
CALIBRATED GAUGE. PRESSURE READINGS ARE ACCEPTABLE
IF WITHIN +/- 5% OF SPECIFIED PRESSURES.
Main Relief, Steer, Swing and Lift Down
1. Install pressure gauge at quick disconnect on port
MP on main valve.
2. With the aid of an assistant, activate telescope in.
3. While monitoring pressure gauge, adjust main relief
to 207 Bar.
4. With the aid of an assistant, activate steer left.
5. While monitoring pressure gauge, adjust steer left
relief to 124 Bar.
6. With the aid of an assistant, activate steer right.
7. While monitoring pressure gauge, adjust steer right
relief to 124 Bar.
8. With the aid of an assistant, activate swing left or
right.
9. While monitoring pressure gauge, adjust swing relief
to 117 Bar.
10. With the aid of an assistant, activate lift down.
11. While monitoring pressure gauge, adjust lift down
relief to 103 Bar - S Models 83 Bar - A Models.