Adding a valve, how important is GPM matching pump output.

[CNC Dan]

But J_J contradicts this. He defines an open center valve in post #20 as one that, by definition, has a motor spool. His definitions don't allow an open center valve for use to control cylinders in use cases such as a front end loader; such valves as are needed for a loader are, according to J_J's definitions, called tandem valves (even though they are open center as regards the input side).

I thought "open center" referred to the input side only, and then the spool choice was made depending on whether you're controlling a cylinder or a motor. This seems like a logical convention. But some conventions aren't logical and J_J's definitions throw a wrench into the mix. Either he is wrong, we are wrong, or there is some weird twisted black hole of communication going on in this thread.

xtn

I agree.
Prehaps the problem is the simmilar terms:
"open center hyd. system"
and
"open center valve"

???

 

[J_J]

I did not write that data, that excerpt came directly from the hydraulic glossary.

But what is the problem. Is the statement not true.

Your loader valves are, guess what, tandem center.

http://www.acshydraulics.com/glossary.html

 

[wdchyd]

Understanding hydraulics can be complicated but these reference links are making more trouble.....the difference between series, tandem and parallel has always perplexed me on how they are described

My best guess on why there are multiple ways of describing "Tandem center" ect is beyond me

I would say the difference between the three would be how the spool configuration handles the exhaust flow while it is shifted......let's give an example on a control valve (all open center flow back to tank and standard blocked a & b ports when in the neutral position

For the sake of argument say a manual control valve with two spools is shifted max at the same time....with two double acting circuits (ie: FEL)

Scenario #1....(tandem or priority flow)............ pressure/ flow only goes to cyl #1 (flow is blocked from spool #2), exhaust flow goes back direct to tank and the circuit #2 doesn't move

Scenario #2.....(parallel)............both spools share the same inlet port and move at the same time (assuming they are both under the same load, otherwise the cyl with the least resistance moves first, then the second), return flow back to tank

Scenario #3......(series)...........pressure /flow goes in cyl #1 to move, then the exhaust flow goes back in to the spool #2 to move cyl #2, then exhaust from cyl #2 goes back to tank

Can you see the three differences?

My Kubota has FEL controls like the scenario #3, I can shift both spools on the joystick and the loader will raise and tilt at the same time until one of the cyls sets get to end of stroke, then they both stop.....the remaining circuit can be moved further if I just shift the one spool

My understanding of "Tandem" is not the open vs closed condition of the spool when not shifted, but rather what/where the oil does in the exhaust mode (parallel, tandem, series)

 

[xtn]

J_J, I don't know what is "true." It seems that different manufacturers have different understandings. The explanation that makes the most sense to me - due simply to the logical implication of the terminology - and that seems to be understood by multiple manufacturers is this:

1. Two main categories of valves: open center and closed center.

2. A tandem center valve is just another name for an open center valve. This has nothing to do with whether said valve is suitable for driving a cylinder or a motor. That's still determined by the spool choice installed in the valve.

4. Whether or not any one open/tandem center valve is best for driving a cylinder or a motor is only dependent upon the spool installed into the valve. An open/tandem center valve could have a motor spool in it. Or an open/tandem center valve could have a regular spool. Or it could have a float spool that does the same thing that a motor spool does except at one end of it's travel range instead of at neutral.

4. With open/tandem center valves we also need to decide if we want parallel or series fluid circuitry, which determines whether multiple valves in a ganged block of valves (or multiple spools in a monoblock body) will work at the same time or if those more upstream will take priority.

4. We don't have to worry about these questions with closed center valves, because they will each be supplied with fluid directly from a manifold or from a parallel system of hoses. Well, I guess you could plumb them in series, but that seems silly. Either way, they may be said to each have their own branch supply from the pump and each have their own branch return to the tank. They will not be ganged together. Well, I suppose there are "monoblocks" of them, such that many spools are built into one body, just for convenience of layout. But they will not be internally connected to each other. Actually, I suppose there could be a monoblock setup of closed center valves with an integral/internal parallel manifold. Probably are. But fluid will never pass from the output of one valve to the input of the next. Each spool will be independent of the others.

5. But closed center valves, just like open/tandem center valves need to drive either a cylinder or a motor, and just like open center valves need an appropriate spool installed.

The glossary you got your definitions from is the only one I can find that agrees with you. I'm looking at the ASC Hydraulics website and I don't see that they actually manufacture valves at all.

 

[wdchyd]

The glossary you got your definitions from is the only one I can find that agrees with you. I'm looking at the ASC Hydraulics website and I don't see that they actually manufacture valves at all.

IMHO, ACS is wrong in their definitions

 

[wdchyd]

I think it's because if you have something big and heavy spinning on the motor, it wants to coast down smoothly. If you suddenly shut off the fluid flow by closing a valve, then WHAM! you're shock loading everything severely. You either need a double check valve configured in such a way as to let the fluid keep moving around through the motor even after you've closed the control valve, or you need a control valve designed for motors which will have a "coast down" feature by connecting the supply and return ports connected when not active. With such a valve, when you cut off the flow, you've made two loops: One loop letting the fluid from the pump go back to the tank, and another downstream loop letting all the trapped fluid between the valve and the motor keep going around. So the motor can coast down to a stop gently.

I may have some details wrong in the above, but I think that's the idea. Any of you experts please confirm or correct?

xtn

You are correct if you want the hyd motor to hold the load after it stops (ie: wheel motor to prevent coasting down hill or conveyor kept from sliding back down)).......but that's only if the hyd valve has "Work Port Reliefs" or external cushion valve.....if the hyd valve only has an inlet relief valve then the motor will act like a pump and WHAM, shock the system and break shafts ect ( assuming you might have blocked ports a & b in neutral)

 

[J_J]

You all can argue and purchase what you want to power your hyd.

How many illustrations and definitions do you need?

They are from different sources.

Control Valves
The ways you can harness the power of a hydraulic system is through the use of control valves. The three basic types of control valves are the tandem center type, the open center (motoring spool) type, and the closed center type. Both the tandem center and the closed center types are available in a three way or four way valve. BY opening or closing valves, you can control how much or in what direction a hydraulic piston moves. Valves can also control many pistons working with each other at the same time.

Open Center Valves
Open center valves are the same as the tandem center, except that in the neutral position all lines are connected back to the reservoir. The primary use of this system is to prevent "shock" loading when the valve is placed in neutral. This takes pressure off the motor. This system is used in situations where the operating device needs to be moved by hand.

Tandem Center Valves
Using tandem center valves when in a neutral position bypasses the flow of hydraulic oil to the return line. This is used to hold the cylinder/piston in position with no load on the pumps. When this system is the pump is running constantly to keep a ready supply of hydraulic oil, but as long as the cylinder/piston is not in operation the pump is working under no pressure or load. This system keeps wear and tear on the pump down to a minimum.

If you don't like these definitions, make your own.

I am sticking to what I said, like it or not.

 

[TomSeller]

You all can argue and purchase what yo want to power your hyd.
How many illustrations and definitions do you need?
.

J_J,
First I would like to thank you for all the answers to hydraulic questions, you are absolutely the expert.

I believe some of the questions stem from one sentence which makes it sound to me like tandem valves are not open centers valves.

INSTEAD OF
The three basic types of control valves are the tandem center type, the open center (motoring spool) type, and the closed center type.

THIS WORDING MAY BE MORE CORRECT?
The three basic types of control valves are the open center (tandem spool) type, the open center (motor spool) type, and the closed center type.

 

[xtn]

J_J,

I certainly don't mean to be argumentative. It's just that there is much confusing information to absorb. I really appreciate you contributing your expertise.

xtn

PS - found ANOTHER source using the word "tandem" synonymously with the word "series," when referring to the internal circuitry configuration of open center valve bodies. This does make a lot of sense, linguistically speaking. I'm surprised such an old industry (compared to the flatscreen television industry or similar anyway) hasn't settled in with consistent terminology.

 

[CNC Dan]

That does seem to be the whole problem we are having.

Darn semantics.