JLG 150HAX Service Manual - Manual (Page 97)

SECTION 5 - HYDRAULICS
3120817 – JLG Lift – 5-1
SECTION 5. HYDRAULICS
5.1 VALVES - THEORY OF OPERATION
Solenoid Control Valves (Bang-Bang)
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 consists 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, pro-
hibiting any flow through the circuit.
Proportional Control Valves - Vickers
The Vickers proportional 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 a
sequence valve. The sequence valve senses the differ-
ence between the load and pump outlet pressure, and
varies the pump displacement to keep the difference con-
stant. This differential pressure is applied across the
valve’s 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 cavi-
tation.
Load sensing lines connect through shuttle valves to feed
the highest load signal back to the sequence valve. Inte-
gral actuator port relief valves, anti-cavitation check
valves, and load check valves are standard.
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.
5.2 CYLINDERS - THEORY OF OPERATION
Cylinders are of the double acting type. Systems incorpo-
rating double acting cylinders are as follows: Tower Lift,
Tower Telescope, Main Lift, Main Telescope, Front and
Rear Steer, Lower Master, Upper Master, Platform Slave
Level, Front and Rear Frame Lift and Front and Rear Axle
Extend. A double acting cylinder is one that requires oil
flow to operate the cylinder rod in both directions. Direct-
ing oil (by actuating the corresponding 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 the rod will stop. By directing oil to the rod
side of the cylinder, the piston will be forced in the oppo-
site direction and the cylinder rod will retract.
Holding valves are used in the Tower Lift, Tower Tele-
scope, Main Lift, Main Telescope and Platform Slave Level
circuits to prevent retraction of the cylinder rod should a
hydraulic line rupture or a leak develop between the cylin-
der and its related control valve.