Sizing of Fluid Power Components Matters

Earlier, we made reference to the natural frequency of fluid power systems. For high-performance motion, the natural frequency of the system needs to be three to four times higher than the desired frequency of motion. In simple terms, the system needs to be designed to move faster than it will actually be required to move to compensate for changes in pressure.

With this in mind, machine builders should use cylinders with a diameter large enough to get the desired acceleration/deceleration rates. They should size the pump to the average oil plus 10%. (There's no need to size the pump for peak load when using an accumulator.) And they should use adequate pressure and return lines with a minimum number of 45- and 90-degree joints.

Valves should be sized large enough to allow the system to accelerate at the desired rate and the maximum desired speed. Remember that, at high flow, the pressure drop across the valve will be large, and the pressure drop across the piston will drop, reducing the ability to accelerate.

Accumulators should be sized at least 10 times larger than the change in the volume of oil per cycle (and should be placed as close to the valves as reasonably possible). Any oil in the accumulator that's not being used reduces its effectiveness. The accumulator should be pre-charged to 80% of system pressure. The nitrogen bubble should be as large as possible, as long as there is always some oil in the accumulator. Energy is stored in the compressed nitrogen, not the oil.

Peter Nachtwey is president of Delta Computer Systems Inc.

Peter Nachtwey has more than 30 years of experience developing hydraulic, pneumatic, electronic and vision systems for industrial applications. He graduated from Oregon State University in 1975 with a BSEE and served in the US Navy until 1980. He became president of Delta Computer Systems, Inc. in 1992. In addition to leading Delta's engineering and R&D programs, he has presented technical papers for IFPE, NFPA, FPDA and various technical conferences.

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