SECTION 4 - HYDRAULICS
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4-1
SECTION 4. HYDRAULICS
4.1
CYLINDERS - THEORY OF OPERATION
Cylinders are of the double acting type. The Lift and
Steer systems incorporate double acting cylinders. 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 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
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 sole
-
noid 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 cir
-
cuit, with the opposite work port opening to reservoir.
Once the circuit is deactivated (control returned to neu
-
tral), 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 assem
-
blies. 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 auto
-
matically 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
components against excessive pressure. Excessive pres
-
sure 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 (pres
-
sure 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 reser
-
voir. Individual, integral relief’s are provided for each
side of the circuit.
Proportional Valve
Flow is proportional to the amount of voltage supplied
to the valve coil. Voltage is gained by the machine con
-
troller and determined by the position of the joystick.
Manual Descent Valve
The manual descent valve is located on top of the hold
-
ing valve on the lift cylinder. The holding valve is a nor
-
mally closed solenoid valve, and holds the platform in
place when raised. When activated, the valve opens to
permit lift down. The holding valve is connected to the
manual descent valve, which is connected to a cable
which, when pulled, manually opens the lift down port
of the valve and allows the platform to be lowered in the
event hydraulic power is lost.
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