JLG 740AJ Service Manual - Manual (Page 289)

SECTION 5 - HYDRAULICS
3121160 – 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 CYLINDERS - THEORY OF OPERATION
Systems Incorporating Double Acting
Cylinders
Cylinders are of the double acting type. Systems incorpo-
rating double acting cylinders are as follows: - Lower Lift,
Tower Telescope, Slave Level/Main Level, Upper Lift,
Upper Telescope, Master Level/Upright Level, Articulating
Jib Boom Lift, Steer and Axle lockout. A double acting cyl-
inder is one that requires oil flow to operate the cylinder
rod in both directions. Directing oil (by actuating the corre-
sponding control valve to the piston side of the cylinder)
forces the piston to travel toward the rod end of the barrel,
extending the cylinder rod (piston attached to rod). When
the oil flow is stopped, movement of rod will stop. By
directing oil to the rod side of the cylinder, the piston will
be forced in the opposite direction and the cylinder rod
will retract.
Systems Incorporating Holding Valves
Holding valves are used in the - Lower Lift, Tower Tele-
scope, Upright Level, Lockout, Articulating Jib Boom Lift,
Upper Lift/Slave Level and Upper Telescope circuits to
prevent retraction of the cylinder rod should a hydraulic
line rupture or a leak develop between the cylinder and its
related control valve.
5.4 CYLINDER CHECKING PROCEDURE
NOTE: Cylinder check must be performed anytime a system
component is replaced or when improper system
operation is suspected.
Cylinders Without Counterbalance Valves -
Master Cylinder and Steer Cylinders
1. Using all applicable safety precautions, activate
engine and fully extend cylinder to be checked. Shut
down engine.
2. Carefully disconnect hydraulic hoses from retract
port of cylinder. There will be some initial weeping of
hydraulic fluid which can be caught in a suitable
container. After the initial discharge, there should be
no further drainage from the retract port.
3. Activate engine and extend cylinder.
4. If cylinder retract port leakage is less than 6-8 drops
per minute, carefully reconnect hose to port and
retract cylinder. If leakage continues at a rate of 6-8
drops per minute or more, cylinder repair must be
made.