99903514:TELESCOPIC CRANE: 3-12 SECTION 3: REPAIR
HYDRAULICS-VALVEBANK
RELIEF VALVE
The relief valve limits the maximum system
pressure. Pressure limits the amount of torque
or force an actuator will see. This pressure is
preset to 3000 psi at 10 gpm. If the relief valve
should fail, it would likely stick open. This would
prevent system pressure from developing and
cause a lack of torque/force at the actuator.
The relief valve can be changed easily by
screwing it out and replacing with a new one.
PROPORTIONAL VALVE
The proportional valve varies the oil flow to the
individual crane functions. Doing so dictates the
speed of the crane functions. As the electrical
current (0.2 to 2.2 Amps) increases to the valve,
by using the trigger on the control handle, more
oil is ported downstream to the crane function.
If the valve coil burns out, the operator would be
unable to vary the flow to the crane functions. If
the valve spool becomes stuck, the operator
would be unable to vary the downstream flow. If
speed control is the problem, it is likely an
indication of a proportional valve problem. It is
necessary to verify that current is flowing to the
coil correctly, and that it is not an electrical
problem.The proportional valve can also be
operated manually for test purposes. The valve
stem can be screwed in manually to port oil
downstream. Doing so will manually position
the valve spool and hold it in the manually
commanded position.
Figure C-13: Valvebank Functions and Features
DIRECTIONAL VALVES
The directional valves (4) control the direction of
the crane functions. When one of the solenoids
is energized, it shifts the valve spool. This
allows oil to flow out one of the valve ports. If a
function does not work, a directional valve may
be to blame.These valves have a standard
manual override. You may manually shift the
valve by pushing the pin, located in the middle of
the solenoid.
CAUTION
MANUALLY OVERRIDING A DIRECTIONAL
VALVE WILL PORT OIL IMMEDIATELY TO
THE VALVE FUNCTION. THIS WILL
CAUSE A SUDDEN MOVEMENT OF THE
ACTUATOR. OPERATORS AND MAINTE-
NANCE PERSONNEL MUST KEEP THE
WORK AREA CLEAR OF OTHER PER-
SONNEL WHEN OVERRIDING A DIREC-
TIONAL VALVE.
If the valve shifts using manual overrides, the
problem is of an electrical nature. Valve coils
are interchangeable and may be changed by
removing the coil nut. This allows maintenance
personnel to isolate individual coil failures. If the
valve cannot be actuated manually or electri-
cally, it is necessary to replace the section.
20030115
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