JLG 40H Service Manual - Manual (Page 24)

SECTION 2 - PROCEDURES
2-4
– JLG Lift – 3120240
Proportional Control Valve - Vickers.
CMX series valves provide a power output matching that
required by the load. A small line connected to a load-
sensing port feeds load pressure back to the pump. The
pump senses the difference between the load and pump
outlet pressures, and varies the pump displacement to
keep the difference constant. This differential pressure is
applied across the valves meter-in spool, with the effect
that pump flow is determined by the degree of spool
opening, independent of load pressure. Return lines are
connected together simplifying routing of return flow and
to help reduce cavitation. Load sensing lines connect
through shuttle valves to feed the highest load signal back
to the pump. Integral actuator port relief valves, anti cavi-
tation check valves, and load check valves are standard.
The load drop check prevents any drop of a suspended
load before upward movement.
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.
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
circuits.
2.6 BOOM CHAINS. (SEE FIGURE 2-1)
Adjusting Procedures.
ENSURE MACHINE IS ON A FIRM AND LEVEL SURFACE.
1. Fully retract boom in the horizontal position.
2. Torque fly boom retract chains, adjust to
28 ft. lbs. (38 NM).
3. Torque fly boom extend chains, adjust to
28 ft. lbs. (38 NM).
4. Cycle boom (extend at least three feet and
return to the fully retracted position).
5. Recheck fly boom retract chains
(28 ft. lbs. (38 NM) required).
6. Recheck fly boom extend chains
(28 ft. lbs. (38 NM) required).
7. Repeat steps #2, #3 and #4 if necessary.
8. Check for proper operation of boom.
JLG Industries, Inc. requires a complete boom disassem-
bly, per instructions outlined in the 2-11 boom disassem-
b ly, ever y tw o year s. A ll bo om ch ains an d r ela te d
components (i.e., sheaves, pins, sprockets, wear pads,
etc.) must also be inspected and replaced (as necessary)
during this disassembly.
A more frequent disassembly of the boom assembly and
inspection of the boom chains and related components is
required if machine is exposed to hostile environments or
conditions (i.e. extreme cold, dust, sand, blasting grit, salt,
chemicals, etc.), which could adversely affect boom oper-
ation. Such a disassembly is required if either debris has
accumulated inside the boom assembly or an inspection
of the boom chain and related components, in accor-
dance with the INSPECTION PROCEDURES in this sec-
tion, reveals any discrepancies to the boom chain or
related components.
An immediate disassembly of the boom assembly and
inspection of the boom chains and related components is
required if any of the following conditions occur:
1. Erratic boom operation or unusual noise exists, due
to discrepancies listed in the INSPECTION PROCE-
DURES in this section, to the boom chains or related
components. See troubleshooting section in Service
Manual for probable causes.
2. Chain adjustment is required more often than speci-
fied in Service Manual or links need to be removed
(chain shortened) to make adjustment.
3. Machine is idle for an extended period (6 months or
longer.)
4. Boom is overloaded or sustained a shock load.
FAILURE TO DISASSEMBLE THE BOOM ASSEMBLY AND PROP-
ERLY INSPECT AND/OR REPLACE THE BOOM CHAINS AND
RELATED COMPONENTS (I.E., SHEAVES, PINS, SPROCKETS,
WEAR PADS, ETC.) COULD RESULT IN THE DAMAGE AND/OR
BREAKAGE OF THE BOOM CHAINS AND/OR RELATED COMPO-