A large forming press had a problem with a small, low-pressure return line hose. The press was used for forming a 1-in. thick flat stainless plate into a convex shape with a lip around the edges. The part is called a head and is welded to the end of a round tank that used on tanker trucks and storage systems.
A large directional valve and pre-fill valve fills the top of a vertical 30-in. bore cylinder with a 400-in. stroke. The press uses a torque limiting pressure compensated pump to transmit the force needed to bend the plate over a fixture. Before retracting the piston rod, a ¼-in. decompression valve is opened to allow the fluid to decompress from 4500 psi to 2500 psi before the large directional and pre-fill valves open.
The operators complained that fluid would blow a hole in the side of the ¼-in. hose on the outlet side of the decompression valve every two to three weeks. They installed a 1000-psi rated hose, and it lasted six to seven weeks before it started to leak oil. Then they installed a 2000 psi hose that lasted a little less than eight weeks.
This line went directly to tank without any restrictions. The stand pipe in the tank was clear, free of any contamination, and 2 to 3 in. above the bottom of the tank. The hose was attached to the horizontal valve with a 37° fitting, bent downward along a corner of the press, and over to the tank. The bend radius on the hose where it was attached to the decompression valve was slightly larger than the minimum required and did not have any twists or kinks to it.
Any idea what the problem is?
Robert J. Sheaf, Jr. is the founder of Certified Fluid Consultants (CFC) and President of CFC-Solar Inc. CFC-Solar 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 June’s circuit design problems
Designing circuits requires a good understanding of hydraulic principles, and, many times, past experience teaches us what to do and what not to do. The filter element blowing off was due to the flow rating used for sizing. A 25 gpm rating sounds like a good size until you calculate the flow rate returning to tank when the cylinder is retracting. A 15 gpm pump flow rate is magnified by the cylinder area ratio of 2.29. (Area ratio is the cap area divided by the rod-end area). This would push 34.3 gpm through the filter, not counting the accumulator’s effect. Filter flow is undersized.
The chattering dump valve in the ISO-3 size might handle 10 gpm. However, without an orifice to slow down the accumulator discharge rate, the flow rating of the valve is greatly exceeded. An orifice needs to be installed before or after the valve.
The meter-in and counterbalance design is a much better way to control over-running loads by eliminating flow intensification problems. His only mistake was to pilot the counterbalance valve before the meter-in flow control. Instead of the pilot sensing the piston pressure, it was sensing the supply pressure before the flow control. He needed to move the pilot line downstream of the flow control, not upstream.