MACHINERY LUBRICATION- INDIA NOVEMBER-DECEMBER 2019 | Page 24

HYDRAULICS
schematics of the system. She emailed me
a file with the schematic. The design was
about what I expected. Downstream of
the power supply and four-way directional
valve, a flow divider and two cylinders were
shown, similar to the illustration at the top
of page 24.
Crossports added to the original ferry
ramp installation
be driven aboard. Once the ferry had been
loaded, the ramp was lifted, and the ferry
proceeded to the island.
After only a couple of months, signifi-
cant vibration was observed. Before long,
the ramp had to be shut down for fear
the mechanical infrastructure would be
damaged because it was twisting toward
one side. Plans were made to replace one
of the million-dollar cylinders after only a
few months of operation.
The representative from the Department
of Transportation was now asking me
whether I had an idea of anything else that
could be causing the vibration? I agreed to
help and asked if she could provide some
22 | November - December 2019 |
What I did not see on the schematic was
a set of crossport relief valves at the flow
divider. Often, crossport relief valves,
while in place, may not be drawn on the
schematic, so I asked her to verify their
presence. It turned out the crossports had
been omitted from the system, and the
small difference in cylinder bypass had
rendered them out of synch.
I recommended a set of crossports be
installed so when the first cylinder reached
the end of its stroke, its associated crossport
would open and allow the second cylinder
to complete its stroke before stopping the
movement of the ramp, as illustrated on
page 24. Once the crossport relief valves
were added, the cylinders synched up
and all vibration ceased. A million-dollar
cylinder replacement was avoided by
installing a $200 set of crossports.
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Cautions and
Recommendations
If crossports are built into the flow divider,
they likely will come factory-adjusted. This
will work fine for most applications, but
they may not be as responsive as you would
like. Actuator damage is also possible
at high pressures because flow dividers
have a tendency to amplify pressure to
one side. Typically, when the flow divider
and crossports are part of the original
design, their recommended setting will be
provided. In the absence of designer speci-
fications, I recommend the crossports be set
approximately 200-400 pounds per square
inch higher than the pressure required to
move the heaviest load at each output of the
flow divider. ML
About the Author
Jack Weeks is a hydraulic instructor and
consultant for GPM Hydraulic Consulting.
Since 1997 he has trained thousands of
electricians and mechanics in hydraulic trou-
bleshooting methods. Jack has also taught
radio-wave propagation for the U.S. Air Force
and telecommunications equipment opera-
tion and repair for the Central Intelligence
Agency at American embassies overseas.