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Hydraulic Circuit Design When You're New To The Game

As the economy started improving after the recent downturn, several hydraulic distributors were adding new people that sometimes got in over their head designing circuits for their customers. We received a call from a new salesman asking for our help in solving problems he was having with a system he designed and installed. He sent the circuit (shown below) to us for review and comment. The problems he was experiencing were as follows:

  • When debugging the system, the customer blew a 25 gpm spin-on filter element off its filter head. He bypassed it for the time being.

  • The safety ISO-3 “solenoid dump valve” for the accumulators chattered when energized. The customer also indicated the electric motor’s current draw fluctuated.

  • Another concern was how high the counterbalance valve adjustment had to set, and how the “extend flow control” adjustment was very sensitive.

What would you have advised him to do?

Problematic Circuit

Solution:

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.

Robert J. Sheaf, CFC Industrial Training

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