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

[CNC Dan]

So then typical tractor we have been calling open center is really tandem center?

Open center refers to the _input_ of the valve body. In the center position the pump flow is open to the tank return.

You can have an open center valve where the _outputs_ are blocked in the center position, or are open to the tank return.

The latter are called motor spools.
They allow a hydraulic motor to freewheel when not being powered.
You could use a standard valve like you would on a cylinder, but the hydraulic motor would be locked when not being powered. This could cause problems with a large rotating mass. This can be fixed by adding a pressure relief between the lines of the hydraulic motor to limit how high the pressure can get.
And you would need two presure relief valves, one for each direction.

 

[xtn]

Okay I think I'm starting to get it. I'd never heard of the "tandem" valve before.

So you're saying that an open center valve doesn't need a specific "motor spool" in it because all of them already connect all ports back to reservoir when in the neutral position. And that although most of us have compact tractors with open center systems, our loader valves are actually tandem center valves.

So... for an open center system, we need to choose either a tandem valve or an open valve depending on cylinder or motor operation respectively.
And for a closed center system, we need closed center valves either with or without motor spools.

Really appreciate you suffering through our dumb questions,
xtn

 

[TomSeller]

I think what they are saying:

There is an open center Tandem Spool that passes fluid out the PB port when in neutral while it CLOSES the cylinder connections.

There is an open center Motor Spool that passes fluid out the PB port when in neutral while it OPENS the motor connections to the Tank port.

 

[CNC Dan]

Okay I think I'm starting to get it. I'd never heard of the "tandem" valve before.

So you're saying that an open center valve doesn't need a specific "motor spool" in it because all of them already connect all ports back to reservoir when in the neutral position.

NO. You need a motor spool OR dual pressure reliefs on the output side.

So... for an open center system, we need to choose either a tandem valve or an open valve depending on cylinder or motor operation respectively.
And for a closed center system, we need closed center valves either with or without motor spools.

Really appreciate you suffering through our dumb questions,
xtn

I'm not sure what a "tandom" valve is. Might be an open center motor spool? That should work.

Think of a valve that has Foward, Float, Reverse. That's what you need. Open center or closed center depending on your tractors hyd. system.

 

[xtn]

Dan,

That's what I thought before this thread. But J_J said all you need to coast down a hydraulic motor is an open center valve. That started this whole confusing string of posts.

xtn

 

[xtn]

TomSeller,

Now THAT makes sense of everything if you're right. Their are either closed center or open center valves, and there are either tandem or motor spools for them. J_J is this what you're trying to say?

xtn

 

[J_J]

Read post # 20 again.

Everything I said is correct.

 

[CNC Dan]

Dan,

That's what I thought before this thread. But J_J said all you need to coast down a hydraulic motor is an open center valve. That started this whole confusing string of posts.

xtn

Open center usualy means open on the input side of the valve. IE: pump port to tank port.
But the output ports are both blocked in the center postion. That's what holds a cylinder in position.

If the outputs are also open, then thats a motor spool. If you were to use that kind of valve with a cylinder, it would drop as soon as you let the valve return to center.

I found a site that has some good diagrams. Look for the vavle symbols that have three boxes that represents the three positions of the valve, with the different flow paths drawn in each box.

http://hydraulicspneumatics.com/other-technologies/chapter-10-directional-control-valves-part-2

 

[wdchyd]

Here's something to confuse everybody more......

http://www.phtruck.com/download/hvd/HVD-Service-Literature/Gresen-Spare-Parts/V20SpareParts.pdf

There's a distinct difference between tandem, parallel and series control valves.....all can be open center but different on how they control the flow after the spool is shifted.....look at page 3

then check out further down (IIRC page 27) and it shows several different drawings and schematics between tandem and parallel circuits....make no mistake there is a difference

 

[xtn]

Open center usualy means open on the input side of the valve. IE: pump port to tank port.
But the output ports are both blocked in the center postion. That's what holds a cylinder in position.

If the outputs are also open, then thats a motor spool.

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.

Foster Manufacturing has some interesting language here: http://www.fostermfgcorp.com/page/circuits/circuits.htm that would suggest J_J is wrong. Their language seems to imply that "tandem" is simply the type of open center valves designed to be connected in blocks, as most of us are familiar with. And that such tandem valves can have circuit arrangements for the supplied fluid that are either parallel, series, or combo parallel/series, which simply determines the functionality of the block with regards to whether multiple functions or implements can operate at once or not. But the word tandem, by itself, doesn't suggest anything about whether or not the implement-side ports are blocked in neutral or not. It simply denotes that multiple single-spool, open center valves can be physically put together into a block of valves. Whether or not said valves have the implement-side ports blocked in the neutral position is purely a function of the spools that are installed into the valves. A block of tandem, open center valves could have some with motor spools and some with regular spools.

But the glossary of terms I found here: http://www.acshydraulics.com/glossary.html agrees with J_J. In fact it looks like maybe he took his definitions from this glossary.

And other language I read describes "tandem" as one of the available circuit types along with parallel and series.

xtn