JLG 120HX ANSI Service Manual - Manual (Page 24)

SECTION 2 - PROCEDURES
2-4 – JLG Lift – 3120686
Manual Hydraulic Control Valves
The manual hydraulic control valves consist of four-way,
multi-position valve sections, each section incorporating a
sliding spool, spring-loaded to neutral or off. Each spool is
attached to a control lever which provides for proportional
control of the selected system function. This proportional
control enables metering of oil flow in accordance with
spool position, affording variable and smooth speed con-
trol capability. Spool movement causes work ports within
the valve to align in a predetermined way, permitting flow
to the selected function, with the opposing work ports
positioned to allow return flow to the hydraulic reservoir.
Relief Valves
Main relief valves are installed at various points with 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.
Crossover Relief Valves
Crossover relief valves are used in circuits where the actu-
ator requires an operating pressure lower than that sup-
plied to the system. When the circuit is activated and the
required pressure at the actuator is developed, the cross-
over relief diverts excess pump flow to the reservoir. Indi-
vidual, integral reliefs are provided for each side of the
circuit.
2.6 BOOM CHAINS
Adjusting Procedures
ENSURE MACHINE IS ON A FIRM AND LEVEL SURFACE.
1. Position boom fully retracted at +5 degrees horizon-
tal, no load in platform.
2. Torque outer mid section extend chain adjuster to 59
ft. lb. (80 Nm).
3. Torque outer mid section retract chain adjuster to 59
ft. lb. (80 Nm).
4. Torque fly section extend chain adjuster to 59 ft. lb.
(80 Nm).
5. Torque fly section retract chain adjuster to 59 ft. lb.
(80 Nm).
6. Cycle boom (extend at least 6 feet (2 meters), then
retract fully).
7. Recheck outer mid section extend chain.
8. Recheck outer mid section retract chain.
9. Recheck fly section extend chain.
10. Recheck fly section retract chain.
11. Repeat steps (2) thru (10) if necessary.
12. Check for proper operation of boom.
JLG Industries, Inc. requires a complete boom disassem-
bly, per the instructions outlined in paragraph 2-11, Boom
Maintenance, every two years. All boom chains and
related components (i.e., sheaves, pins, sprockets, wear
pads, etc.) must also be inspected and replaced, as nec-
essary, during this disassembly.