Can someone explain to me how a Direct Acting Counterbalance Valve (Parker BC6PFV) works?  Does it need pressure on both sides in order to open?

It might be easier to answer the question to know what kind of circuit you are using it in.  But to clarify your nomenclature, if I might, it's not really Direct Acting but Direct (Pilot) Operated as opposed to the optional Remote (Pilot) Operated.
See http://www.parker.com/Products/EPD/EPDResults.asp?body=pdf%2Easp%3FPSID%3D985&rnd=10%2F19%2F2005+9%3A34%3A50+AM
To make an example how this might work, suppose you were trying to safely lower a load while retracting a cylinder.  You would plumb from the blind side of the cylinder to the valve "pressure inlet".  Since the line mounted model has 2 optional inlets, you WILL have to plug the other one.  Then the "pressure outlet" would go to your directional valve.  While holding the load, you will be inducing pressure from the load to the valve so the spring setting has to be sufficiently higher to hold the load.  As you begin to retract, the pressure going to the rod side will have to induce a pressure sufficiently high on the piston side to force the valve/spring open.  Think of it as really just a direct-piloted relief with a built-in bypass check.

Most of my experience has been to use the remote-piloted version where the remote pilot goes to the rod-side.  This usually allows the valve to open with less induced pressure on the piston side.  If the load tries to run away, by definition the pilot pressure drops and the valve closes.  But all this happens smoothly without the chatter you might get using pilot operated check valves.

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2011-02-18

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