The Test for Cross and Round Valves
By Craig Brougher
Much has been written and claimed about cross valve and round valve configurations. I built a tester once with a cross valve plate on one side and a round valve on the other. Instead of just calculating and modeling the performance in theory (which I also did), I tested the actual performance in a situation which would allow me to have identical valve travel, since it was the same poppet, captured betwen the two different valve plates under test (No sense guessing, right?). The answer was so demonstrable that I demonstrated the tester at the home of Mr. and Mrs. Paul Stelzner who at that time lived in Washington, Mo. and were hosting an AMICA meeting at their home. It was inarguable. Round valve plates also have better volumetric eficiency and are quieter at the same time. That is because a cross valve has a larger circumferential length for the amount of area it uncorks, and creates a much greater turbulence.
The article referenced here was one which I wrote for the AMICA Bulletin some years ago called, "The Cross Valve, Was It a Plus?"
Regarding "special considerations in valve settings" when cross valves are changed out, however, I must demure. Let me suggest a test to anyone who really wants to know.
Don't take anybody's word for it! Find out for yourself.
Take a plastic film can, insert a 3/8" nipple through the bottom and seal around it with a hot glue gun. Bore a 1-1/4" hole in a strip of wood so you can clamp the board to your bench and rest the assembly in the hole, upside-down (The poppet will be underneath and your supply nipple pointing up, everything resting on the valve plate). Set a cross valve plate down on the open end of the can, sealing around it perfectly with whatever you want to use, then taping the hole in the valve plate temporarily and testing the can nipple with your tongue to make sure it's cartridge tight and can't leak around the valve plate's top. Remove the tape.
Tape a wire hook to the valve poppet stem threads that you intend to set upon the cross valve plate. Rig up a guide so the stem cannot be tipped or wrenched off the plate. It has one direction only-- up and down. Your test fixture is now supported upside-down by the strip of wood with the hole, sitting on the valve plate itself. Turn on the vacuum, snap the poppet up to its hole in the home-made stem guides, measure vacuum accurately. Now take a dixie cup, make a string bail for it, hang it on the poppet stem hook, and start gently filling the dixie cup with shot through a straw. When the poppet breaks free, weigh the cup on an accurate postage meter scale. Repeat the experiment 5 or more times for reliability. Then do the same experiment using the round hole plate.
To calculate, P x .036 = lbs/sq. in. (where P is inches of water vacuum in the can). You will then be able to accurately estimate the effective sq. in. area (not a theoretical one) as long as your trials are within about 5% of each other, or closer. Both tests must use the same poppet, of course. And what do you know-- the difference is barely noticeable. Your assignment, should you choose to accept, is to gap Duo-Art valves such that you have an optimum travel to allow the piano to achieve its full power without having too large a transit loss or slow repetition. Duo-Art pouch size is plenty adequate to handle the small difference in opening pressure (if any), as clearly demonstrated by other pianos having roughly the same size valve footprint and pouch diameter.
Part of the reason that there isn't hardly any noticeable difference between valve plates is (my opinion, now) because the cross valve plate allows the leather to "wrap itself" or conform to the sharp edges around the sealing surface of a cross valve, caused by grinding it after being punched. And what do we see after 70 years? A deep imprint in the cross valve leather, shaped like a cross. Conversely, you do not find round imprints of round valves, deeply imbossed, in any player piano! I have dozens of old, used valves and cannot find even one of them with an imbossed imprint! Isn't that interesting? And isn't it also interesting that no other valve plate in the business was ever ground with sharp edges like that one, after having been stamped out? Do we suppose there may be a connection? I do.
As you might also guess, this can vary quite a bit according to the leather you use. But you have to add the extra sealing area for each little bit of leather nap that is attracted to the plate. The force is dependent upon the total area which the leather poppet contacts, not just the area of the hole. This also varies greatly by the depth of leather nap and the cushion or softness and thickness of new garment leather. You will probably also discover that if anything, the cross valve requires more-- not less- force to remove itself from the valve plate at any given amount of pressure.
I don't want anyone to believe that we have a secret way of calculating things that they cannot do for themselves. The real secret to player rebuilding has always been patience, consistency, and the determination to tear something down and do it again as many times as it is not quite right, and to not begrudge yourself that time, as long as you can do it better. That's about the only thing we can't teach, but it's also the hardest of all to learn. So just hang in there and remember, things aren't as mysterious as they may sound.
Craig B.
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(Message sent Thu 12 Sep 1996, 16:57:22 GMT, from time zone GMT.) |
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