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momentarily increases to
the "2A" valve setting, the
spool will shift open and
port the pressurized fluid
through the directional
valve and back to the
tank. When the pressure
drops below the "2A"
setting, the valve spool
will shift closed and the
motor will start rotating.
When the directional
valve solenoid is de-ener-
gized to stop the motor, the
valve spool will shift to the
closed center position
(Figure 5). The motor will
tend to continue rotating due to the inertia of the moving load
and momentarily turn into a hydraulic pump, delivering oil to its
outlet port. The pressure will build until the setting of the "2B"
crossport relief valve is reached. The "2B" valve will then shift
open and direct the oil flow back to the motor's inlet port. The
setting of the "2B" spring will determine how fast the motor will
come to a stop.
If you're experiencing shock and leakage issues with hydraulic
motor circuits, first verify that the crossport relief valves are
located in the system. I have seen some systems where they have
been omitted, allowing the shock to be taken out in the lines,
hoses and fittings, which results in leakage. Secondly, make sure
the crossport relief valves are properly set. When there is a
problem in a hydraulic system, usually the first course of action is
to turn up the pressure. Thirdly, the crossport relief valves should
be located as close as possible to the hydraulic motor.
A plywood plant in North Carolina was having a problem
shearing the motor shaft off its rotary log-kicker hydraulic motor. As
the logs came down the conveyor, the motor rotated and kicked the
log off the conveyor and onto the infeed conveyor to the lathe. Upon
inspection, the crossport relief valves were found in a block under-
neath the directional valve, which was mounted 30 feet away from
the motor. An additional set of crossport relief valves was installed
near the motor, which eliminated the shearing of the motor shafts.
Likewise, by using these three remedies, you can greatly reduce
the hydraulic shock in your systems and help to eliminate oil
leakage at your plant.
About the Author
Al Smiley is the president of GPM Hydraulic Consulting Inc.,
located in Monroe, Georgia. Since 1994, GPM has provided hydraulic
training, consulting and reliability assessments to companies in the
United States, Canada, the United Kingdom and South America.
Contact Al at gpm@gpmhydraulic.com.
ML
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