RE: Air Flow in Valves
By Craig Brougher
Ron Yost asked about valve settings for optimum air flow.
I have never set valves with a gauge because there are so many variables among the dozens of different designs one will encounter. Air flow is dependent upon the valve size and overlap, true. But it is also dependent on leather type and roughness, the clearances around the poppet and stem, the degree of directness and smoothness of the channel into the pneumatic, and in general, the degree of turbulence created by everything.
Reproducing PIANOS (sorry, Robin, I didn't really mean to be talking about phonographs prior to this) are sensitive to the effective valve travel or gap.
Some of these otherwise well-restored instruments could never achieve full power as they were designed to have, simply because the rebuilder had been told to set all valves to a maximum of 1/32". What we find the original leather set to today is NOT optimum once a rebuild has been effected.
If you want to know what to set your valves at, set up several test valves and try them out-- until a further increase in valve gap does not make a further increase in poppet impulse force. If you are going to be a perfectionist, then you will set them all to the threshold of their optimum power. If you want to set a few and guess at the rest, then you can build up your gauge to set the gap at an average travel of the test valves. [I suggest the first way].
Simple valve seat area measurements won't tell you much. You can't guess at the optimum travel by taking a measurement of the seating ratio. Besides, some seats are flat with sharp edges. Some are curved. Others are rolled and tightly rounded, and others are relatively rough wood seats. Those seats sit into holes that are deep, shallow, large, small and tight, rough , and smooth. The air flow is relatively slow, low pressure stuff. Turbulence creates the greatest amount of resistance of any common factor in a valve, anyway, and the leather chosen will usually be the roughest surface that air comes in contact with. So why guess and gauge? Test it first! Then you'll know what you're doing after awhile and pretty soon you'll be able to look at a valve and know what you're going to get.
One other factor you need to take into account is this: NO matter what you attempt to do, everything is a tradeoff, to one degree or another. So when you start throwing pouch leather, lifters, stems, pouch clearances, bleed sizes, pouch tube length, and whatever else affects a valve into the mix, you may have to concern yourself with sensitivity as well. Optimum travel and sensitivity are counter to each other. Test for that, too.
Craig Brougher
craig_brougher@msn.com |
(Message sent Mon 25 Mar 1996, 13:50:36 GMT, from time zone GMT.) |
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