JLG 450AJ Service Manual - Manual (Page 20)

SECTION 2 - PROCEDURES
2-4 – JLG Lift – 3120869
2.5 VALVES - THEORY OF OPERATION
Solenoid Control Valve
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.
2.6 BOOM MAINTENANCE
IF PERFORMING MAINTENANCE ON THE BOOM, DO NOT USE A
LIFTING DEVICE TO LIFT THE BOOMS UNLESS THE HOLDING
VALVES HAVE BEEN REMOVED FIRST. FAILURE TO DO SO WILL
RESULT IN SEVERE DAMAGE TO THE BOOM.
Removal of the Boom Assembly
1. Remove the platform and platform support as fol-
lows:
a. Disconnect electrical cable from control con-
sole.
b. Tag and disconnect the hydraulic lines running
to the rotate cylinders. Cap the hydraulic lines
and ports.
c. Using an overhead crane or suitable lifting
device, use nylon support straps to support the
platform/support.
NOTE: When removing the retaining pin from the rod end of
the level cylinder, make sure the cylinder is properly
supported.
d. Remove bolts and keeper pins that secures the
retaining pins. Using a suitable brass drift and
hammer, remove the retaining pins from the plat-
form support.
LIFT CYLINDER
PIVOT PIN
BOOM
PIVOT
LEVEL
LINK
PIVOT
Figure 2-1. Location of Components - Boom Removal