HYDRAULICS
displacement pump is used. e
best way to check this pump is
through the system relief valve.
Install a flow meter in the relief
valve tank line. Sometimes, this
is not possible due to the machine
configuration. Perhaps the relief
valve is attached directly to the
reservoir or mounted in a mani-
fold that is directly attached to the
reservoir. In this case, install the
flow meter in the pressure line of
the pump.
If the hand valve is closed,
deadheading the system, you know
that any flow from the pump has
only the flow path through the
relief valve back to the tank. Turn
the relief valve adjustment coun-
terclockwise to a very low pressure.
Some relief valves do not have a
stop on their adjustment, so it may
back all the way out. Adjust the
valve counterclockwise until no
spring resistance is felt.
When the system is turned
on, the pump flow should dump
across the relief valve at a very low
pressure. Since there is no resis-
tance to the pump flow, it will
deliver all or nearly all its flow.
Gradually increase the pressure
setting of the relief. If the pump
can maintain flow with the relief
valve set at the normal system
pressure, the pump is good. If,
however, the flow drops as the
pressure is increased, the pump
should be replaced.
Suppose pressure built in the
system when the hand valve was
closed to isolate the power supply.
en you know the problem is
downstream. Bypassing across
the directional valve or across the
cylinder is causing the pressure
loss. In most systems, the direc-
tional valve would be the easier
component to check first. Are
the solenoids firing? Notice the
tandem center position. ere will
be no pressure in the system unless
one of the solenoids is energized.
Check for a magnetic field with a
metal ruler or small screwdriver
while each solenoid is energized.
A schematic with a flow meter installed in the cylinder's rod-side line.
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