SECTION 4 - BASIC HYDRAULIC INFORMATION & SCHEMATICS
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4-1
SECTION 4. BASIC HYDRAULIC INFORMATION & SCHEMATICS
4.1
CYLINDERS - THEORY OF OPERATION
Cylinders are of the double acting type. The steer sys-
tem incorporates a double acting cylinder. A double act-
ing cylinder is one that requires oil flow to operate the
cylinder rod in both directions. Directing oil (by actuat-
ing 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, move-
ment 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.
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 con-
tainer. 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.
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