over center valve question about closed center option
over center valve question about closed center option
(OP)
Hi. Can anyone say why all over center valves aren't made to be "closed center" type.
Is it purely cost related?
I suppose a better way to ask the same question is, if a "closed center" over center valve is fitted to an open center control valve, would there be a problem?
I don't imagine there would be, but am I wrong to think that?
Thanks
Is it purely cost related?
I suppose a better way to ask the same question is, if a "closed center" over center valve is fitted to an open center control valve, would there be a problem?
I don't imagine there would be, but am I wrong to think that?
Thanks





RE: over center valve question about closed center option
{ open center | closed center }
don't interchange in any useful way.
Mike Halloran
Pembroke Pines, FL, USA
RE: over center valve question about closed center option
I'm just thinking that if a closed center, Over Center Valve (OCV) is fitted, (let's say even by mistake), into a circuit which has an open center control valve, as far as I can see there would be no detrimental effect.
Whereas, if an open center OCV was fitted by mistake, into a circuit which has a closed center control valve, then there might be potential for problems.
If that is true, (maybe it isn't?), wouldn't it always be safer to spec' a closed center OCV, since in that case it is more or less fail safe, even if the incorrect control valve was replaced.
So, in a nutshell, is there any detrimental effect of fitting a closed center OCV to an open center control valve, that I am unaware of?
Thanks
RE: over center valve question about closed center option
Example: In a common open center circuit, the pump is always running, at very low pressure, pumping fluid through the centers of the control valves. Pump pressure rises only when a valve shifts, blocking the center and directing the flow to the load. If you put a closed center valve into that circuit, the flow through the valve centers stops, and the pump flow goes through the relief valve that you hope is present, or somewhere that you don't want it to go.
Mike Halloran
Pembroke Pines, FL, USA
RE: over center valve question about closed center option
Maybe I am mis-interpreting your post. Anything service port side of the control valve shouldn't care if it is open or a closed centre control valve.
RE: over center valve question about closed center option
Ted
RE: over center valve question about closed center option
I suppose I should have included a sketch in my initial post, so I've attached a very crude illustration.
I'm getting a bit confused with the descriptions "over center valve" and "closed center valve", since I don't know if you refer to the operator control levered valve, or the actual motion control valve.
So with a handpump would I be right to say that any format of the OCV would work okay?
Thanks
RE: over center valve question about closed center option
The ocv is not a circuit specific valve. The directional valve in your system should be a float-center valve instead of the blocked outlet valve you show. What you show is a valve commonly used for cylinder control to hold the cylinder when the valve is centered. The blocked ports will allow overload or temperature rise to raise pressure in including the ocv pilot line. That condition could upset the operation of the ocv. The ocv will hold the cylinder and the blocked cylonder ports on the directional valve are not necessary.
Ted
RE: over center valve question about closed center option
The very reason I asked the question was because I came across the valve linked below.
It's listed as "for closed center", so I was trying to figure out why they aren't all this type, but I can see now with a non- hand pump application it might not be suitable if the pump would be under continual load with closed center.
I contacted the manufacturer directly who said this valve is fine to use with a closed center directional control valve, "as it’s not sensitive to back pressure."
http://www.oleodinamicamarchesini.com/downloader.p...
RE: over center valve question about closed center option
Hpost CEng MIMechE
Hydraulic System Specialist
Caterpillar Inc
RE: over center valve question about closed center option
I think I have grasped the general idea. In the attached diagram from a Parker tutorial, I presume the vent is only venting air. Is that correct?
RE: over center valve question about closed center option
Ted
RE: over center valve question about closed center option
Ted
RE: over center valve question about closed center option
It doesn't seem to accommodate a very large volume of oil.
Supposing I have a steel hydraulic pipe with an internal diameter of 0.95cm, and it is 400 cms to my directional valve, (in its closed center position), the volume in the pipe is 283cc.
If I multiply that by 8.2E -04, (expansion rate per °C),
I get an oil volume increase of 0.24cc, (i.e. for 1°C rise)
Multiply that 0.24cc for a temperature rise of 15°C = 3.65cc
That vent doesn't look like it cope with anywhere near that amount, so in such a case, where does the oil go, or what happens?
I realise this scenario might seem unrealistic, but assume for example the system was just started up early one cold morning, and never reached working temperature.
Imagine a hydraulic actuator is shifted once and the system shut down before any heat is generated.
The system could then be sitting at ambient temperature, so could easily see a 15°C temperature rise due to climatic changes as the day progressed.
Am I missing something fundamental?
RE: over center valve question about closed center option
Ted
RE: over center valve question about closed center option
Since the "center closed" directional control valve, in its center position, has completely isolated the total circuit,
I can't see where excess fluid can go, even when the green piston assembly moves to permit the work and valve ports to communicate.
RE: over center valve question about closed center option
Ted
RE: over center valve question about closed center option
I'm new to hydraulics so I'm just trying to grasp the basics, so that all helps a lot.
I didn't expect the vent was for fluid, so couldn't see any logic in their design, so it's good to know there is none
I can see exactly your solution is the best but I was just exploring these other things to better grasp what options are available and how they work, (or don't very well, in the case of the atmospheric vented valve). Thanks again.
RE: over center valve question about closed center option