over center valve question about closed center option

[MRSSPOCK]

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

 

[MikeHalloran]

The two types of valves
{ open center | closed center }
don't interchange in any useful way.

Mike Halloran
Pembroke Pines, FL, USA

 

[MRSSPOCK]

Mike, it isn't so much I'm thinking of them being interchangeable in a useful way.

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

 

[MikeHalloran]

It's always safer to spec the correct valve.

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

 

[hydroman247]

Do you mean the OCV is put on a service function in a circuit that uses an open centre control valve? If so, that is fine. It makes no different. We actually do this all the time.

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.

 

[hydtools]

The definition of open- and closed-center applies to the circuit power directional control valve, not the down-stream control valves. The closed-center valve blocks pump flow when the valve is centered or 'off'. The open-center valve bypasses pump flow to return when the valve I centered or 'off'. The exception I can think of is a two-port flow control that restricts flow to less than fixed-displacement pump flow capacity forcing excess flow over the relief valve.

Ted

 

[MRSSPOCK]

Sorry, I should have said I had a hand pump in mind.

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

 

[hydtools]

Well you asked ocv and closed center.
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

 

[MRSSPOCK]

Thanks again.

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.php?file=3.10.DYN53917f0781989.pdf

 

[HPost]

It just means that the valve side of the OCV spool has a drain in to allow trapped oil to drain away. Without the drain the OCV can take longer to fully close. resulting in cylinder over travel or creep.

Hpost CEng MIMechE
Hydraulic System Specialist
Caterpillar Inc

 

[MRSSPOCK]

Thanks HPost.

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?

 

[hydtools]

Mrsspock, yes the Parker valve vent is only an air vent connected to the dry spring cavity.

Ted

 

[hydtools]

I believe that the Oleodinamica valve you also reference is incorrectly identified as being suitable for closed-center circuits. It should be identified as for use with blocked work port cylinder valves, in my opinion.

Ted

 

[MRSSPOCK]

So, my question now is, what happens when this venting stroke reaches its limit? (See attachment).

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?

 

[hydtools]

Recognize that all the green colored pieces make a single valve piece. If the movement you describe happens the valve spool opens to connect the valve port to the work port.

Ted

 

[MRSSPOCK]

hydtools, I still can't see where the excess fluid goes.

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.

 

[hydtools]

That is the problem with using the Parker-referenced valve or any other brand like it with a cylinder spool, blocked outlet ports, directional valve. There ultimately in no where for the expanding fluid to go and if pressure goes high enough something will break/burst. The vent on the Parker valve has nothing to do with venting fluid. It would not make sense to vent fluid to atmosphere. The only case where fluid would exit that spring cavity vent is if the internal seals leaked fluid into the spring cavity. Hydraulic systems still carry a stigma for being leaking without purposely creaking a leak and perpetuating that misconception. A solution is to use a directional valve with a 'motor spool' which when centered the spool connects both outlet ports to the tank port.

Ted

 

[MRSSPOCK]

Thanks for taking the time to explain.

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.

 

[hydroman247]

See type D.