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Default Hydraulic solenoid valve issue

I'm having a weird issue with some solenoid valves in a simple hydraulic circuit, let me try and describe the situation as best as I could.

Coming out of a 2 station manifold, I have 2 4-way spring return center drain valves. 120VAC, controlled through relay outputs by a PLC. Each valve controls a set of cylinders, A position down, B position up. Wired up such that 1 leg of each coil jumped together to my 120VAC control, the other 4 legs to their corresponding PLC output.

Initially, I had valves with a center position, A&B drain to T, P blocked. Everything worked correctly from a controls standpoint, only problem was when both valves were "off", I was holding at full pressure (~1700 psi)and building up a lot of heat.

So I swapped out the valves for center position, A to B blocked, P drain to T. Wired up exactly the same as before. Now when I actuate, let's say, valve 1, side A, nothing happens...until I actuate valve 2 side A, then cylinder on valve 1 actuates, as well as the cylinder on valve 2. Electrically everything is right when I check with a meter, program is running as it should as well. So that leads me to believe that something's going on hydraulically.

Anyone have any idea what's going on? It seems like when I have one valve, P going to A and the other "off" (P to T) that I might have to much pressure going to tank, preventing B to T thus keeping my cylinder from moving?

Let's assume that's the issue, how can I modify my design to remedy the problem? I'm thinking, change from one, 2 station manifold to 2, independent station blocks and add check valves to the T outputs to prevent pressure from backing into the T line and preventing the oil from draining from the exhaust of the cylinder.

Solved: A "dump" or relief valve was added to the circuit, right before the solenoid manifold, plumbed into the tank line, right before the radiator. In the instances when the pump is running, but the machine/solenoid valves are not operating, the dump valve is opened, draining all pressure to tank through radiator.

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  • $\begingroup$ Can you show a rough diagram or something? I know a bit about high powered fluids (and have some pretty good reference books beside me); but I find them complicated enough with an actual diagram. I'm sure it would help with the ability for people to help. $\endgroup$
    – JMac
    Commented May 15, 2017 at 19:54
  • $\begingroup$ certainly @JMac, I added my schematic to the post. If anything looks super odd, just let me know. This is my first go at hydraulic system design. $\endgroup$
    – Corey
    Commented May 15, 2017 at 20:03
  • $\begingroup$ That looks like a pretty standard circuit diagram. I'll take a look in a little bit (though there is a good chance I don't end up figuring anything out). $\endgroup$
    – JMac
    Commented May 15, 2017 at 20:05
  • $\begingroup$ Could it be that there is so little resistance through the T port that there's insufficient pressure in the system? If so, you might need a pressure reducer on the T port so the system pressure stays higher. $\endgroup$
    – Mark
    Commented May 16, 2017 at 0:19
  • $\begingroup$ @Corey did the answer help? If so, could you accept it so the question does not get bounced by the system. $\endgroup$
    – Solar Mike
    Commented May 8, 2019 at 6:06

1 Answer 1

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Yes. You are losing your pump pressure through the center (off) position of the the other valve. Once you do some work through the other valve, so as not to drain your pressure to the tank, then both cylinders get some pressure to operate.

Here are two approaches.

1) Connect the T output from one valve to the P input of the other valve. The fluid can return to the tank easily when both valves are in the center position, but both valves will receive pressure from the pump. The pressure and volume going to the second valve will not be constant when the valve that is first in line is being switched, but both will still work.

2) I have seen this approach, but I don't know the exact circuit. Go back to your earlier approach, and add a pressure sensing pilot controlled relief valve that opens the return path to the tank whenever neither of your control valves is drawing fluid. So your full 1700 psi drops to some much lower pressure to reduce heat when the pressure is not being drawn by either valve.

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  • $\begingroup$ Thanks @Entrepreneur My valves are on a manifold block so I cant run them in series as you say in approach 1. I was thinking about approach 2, only adding in an accumulator to deal with the pressure spikes/lulls as Il be switching between valves and off quite frequently ( ~2 times per second) Do you have any experience with accumulators? I read that they should be charged roughly half of your needed oil pressure but im not sure $\endgroup$
    – Corey
    Commented May 18, 2017 at 13:37
  • $\begingroup$ Oops, I didn't pay close attention to the manifold block. I don't have hands-on experience with accumulators. I have an upcoming project in which I will use them extensively, so I have researched them a little, but as of now I am still pretty green. $\endgroup$
    – Entrepreneur
    Commented May 18, 2017 at 21:34

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