JLG 4394RT Service Manual - Manual (Page 167)

SECTION 4 - HYDRAULICS
3121249 – JLG Lift – 4-1
SECTION 4. HYDRAULICS
4.1 CYLINDERS - THEORY OF OPERATION
Cylinders are of the double acting type. The steer system
incorporates a double acting cylinder. 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 the 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.
NOTE: The lift cylinder is a single acting cylinder which
takes hydraulic pressure to extend and gravity to
retract.
A holding valve is used in the lift circuit to prevent retrac-
tion of the cylinder rod should a hydraulic line rupture or a
leak develop between the cylinder and its related control
valve.
4.2 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.
Relief Valves
Main 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.
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 relief’s are provided for each side of the
circuit.
4.3 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
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.
5. With cylinder fully retracted, shut down engine and
carefully disconnect hydraulic hose from cylinder
extend port.
6. Activate engine and retract cylinder. Check extend
port for leakage.
7. If extend port leakage is less than 6-8 drops per min-
ute, carefully reconnect hose to extend port, than
activate cylinder through one complete cycle and
check for leaks. If leakage continues at a rate of 6-8
drops per minute or more, cylinder repairs must be
made.