JLG 80HX_HX+6_HXER Service Manual - Manual (Page 26)

SECTION 2 - PROCEDURES
2-4 – JLG Lift – 3120863
2.4 CYLINDERS - THEORY OF OPERATION
Double Acting Cylinders
Cylinders are of the double-acting type. Systems incorpo-
rating double-acting cylinders are as follows: Lift, Tele-
scope, Platform Leveling, Steer and Lockout. A double
acting cylinder is one that requires oil flow to operate the
cylinder rod in both directions. Directing 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 rod will
stop. By directing oil to the rod side of the cylinder, the
piston will be forced in the opposite direction and the cyl-
inder rod will retract.
2.5 VALVES - THEORY OF OPERATION
Holding Valves
Holding Valves are used in the Lift, Telescope, Slave Level
and lockout 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.
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 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.
Proportional Control Valve - Vickers (See
Figure 2-1)
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.
Main 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.