SECTION 4 - HYDRAULICS
3121642 – 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 cylin-
der 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 pis-
ton 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.
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 retraction
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 activated
and the control valve solenoid energizes, the spool is shifted
and the corresponding work port opens to permit oil flow to
the component in the selected circuit, with the opposite work
port opening to reservoir. Once the circuit is deactivated (con-
trol 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, prohibiting any flow
through the circuit.
Relief Valves
Main relief valves are installed at various points within the
hydraulic system to protect associated systems and compo-
nents 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 circuit 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 pressure
of the component is reached.
Crossover Relief Valves
Crossover relief valves are used in circuits where the actuator
requires an operating pressure lower than that supplied to the
system. When the circuit is activated and the required pressure
at the actuator is developed, the crossover relief diverts excess
pump flow to the reservoir. Individual, integral relief’s are pro-
vided for each side of the circuit.
4.3 CYLINDER CHECKING PROCEDURE
NOTE: Cylinder check must be performed anytime a system com-
ponent 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 cyl-
inder. If leakage continues at a rate of 6-8 drops per min-
ute 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 minute,
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 min-
ute or more, cylinder repairs must be made.
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