Open Center versus Tandem valve

[TexasDave]

Can't believe I can't find this question answered somewhere on the net!!

Going to install a third-function circuit on the front of my MX4700 to operate a root grapple. Being frugal (cheap?), I am trying to economize by buying the individual components versus a kit.

Here's my understanding:

Tapping into the power-beyond circuit, you need an open-center valve. Or a tandem valve. Both flow freely from P to T in the center position (three-position valve). But the OC valve connects A to B when centered, allowing the grapple to be moved, which wouldn't hold the load unless the switch was held in the "close" position. The tandem valve does NOT connect A to B in center position, so the grapple would hold the load if the switch was released.

I'm looking at a three-position tandem valve such as a Northman 22305-2306 or similar.

Do I have this right?

Thanks in advance!
Dave

 

[Kennyd4110]

Dave, welcome to OTT.

Technically, you need a "Tandem Center" valve for everything but a motor. The term "Open Center" has gotten used, abused and misconstrued on most tractor forums and at the Surplus Center as well.

A Tandem Center valve will have the work ports blocked in neutral, thereby not allowing the cylinder connected to them to move.

A Open Center valve will have the work ports open to the T (tank) port in neutral, thus allowing a motor to coast to a stop or freewheel.

Clear as mud yet?

 

[ShaunRH]

I thought all modern 'open center' valves with at least 2 work ports were 'tandem center' and thus the term got used. Only single work port units (lift and release types) were the old 'open center' definition.

Isn't the modern definition of 'open center' that incoming pressure flows to the tank until the spool is shifted and the spool will hold both work ports where they are (tandem center)?

The other definition is 'closed center' which has to have a pressure control system back at the pump to work right, right?

I haven't seen the neutral to tank work ports back to 'open center' in a long, long time... at least on 2 or more work ports. You're right though, I do see it on motor control valves all the time!

 

[Kennyd4110]

Here is the issue/confusion, these valves work the same way-both are meant to operate DA (double acting) cylinders, but one (at NT) is described as Tandem Center:

http://www.northerntool.com/shop/tools/product_200329586_200329586


While this one (at the SC) is described as Open Center, but "work ports blocked in neutral" as well:

http://www.surpluscenter.com/Hydrau...20-VAC-10-GPM-OC-DA-SOLENOID-VALVE-9-7235.axd

 

[aeblank]

There is a guy on Ebay selling kits for $400. He says $45 more than the sum of the parts. Anyone have any experience? I want to get it for my dad for Christmas.......

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