A contractor was awarded a contract to build a hydraulic system that needed 60 gpm flow at up to 1000 psi, then a controlled flow from 20 down to 3 gpm in 12 sec, then up again to 2200 psi. It then had to return at full speed with a maximum required return pressure of 600 psi. They were told to build the entire system with a total maximum height of 24 in. The width and length of the complete system needed to be as small as possible. The cylinder was not specified or ordered from the contractor.
The circuit was designed and built per the accompanying schematic. The system was tested and inspected by the Dept. of Defense and shipped overseas.
About two months after shipment, the contractor received a call about a problem during start-up. They could not exceed 300 psi from the pump, and the return line filter element kept coming loose and leaking. They also told the contractor that they were able to set the load-sensing adjustment to a maximum of 500 psi and could cycle the cylinder somewhat. They wanted to know what could be causing the two problems.
Any idea what they should do next?
Robert J. Sheaf Jr., is founder and president of CFC Industrial Training, a Div. of CFC Solar, which provides technical training, consulting, and field services to any industry using fluid power technology. Visit www.cfc-solar.com for more information.
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Solution to November's barge cable system overheating
The overheating coal barge problem was caused simply from the lack of documentation and instructions on how to set the different pressures.
The pump compensator was set to provide enough force to pull the loaded barge through the un-loader system, and the safety relief was set properly at about 150 psi higher to protect the pump and system from excessive pressure spikes. These changes helped lower the heating problem.
When the feed-out valve was energized to allow the barge to drift down stream, the pulling cable hydraulic motor became a pump. The cable tension valve’s pressure should be set low enough to allow the barge to drift downstream without allowing it to run away. It operated somewhat like a counterbalance valve would on a cylinder.
The drifting barge pulling on the cable caused the hydraulic motor to act as a pump. Pumps need a suction line, and providing a small supercharge of 100 to 200 psi will prevent cavitation to the pump-motor unit, extending its operating life. Another benefit is that noise is greatly reduced by having a charged suction line.
However, the supercharge flow control was set way above the amount of oil lost to leakage. The excess flow was relieving over the supercharge pressure control, generating unwanted heat. To add to this problem, the supercharge relief was set above 400 psi.
After lowering the supercharge relief to 100 psi, the supercharge flow control was set as low as possible, just above the oil leakage rate. Lowering the valve to 100 psi also caused the differential pressure across the cable tension valve to increase by 300 psi. An adjustment was made to lower its setting by this amount. The system now operates normally.