Re: Force Versus Distance of a Pneumatic
By Douglas K. Rhodes
Status: R
In the Digest for 96.03.21 Brother Robbie Rhodes asks for pertinent information about a fundamental aspect of our common interest:
> Has anybody any numerical data on how the force of a simple "board"
> pneumatic varies with the opening?
I believe that Wm. Braid White's old treatise on Piano Playing Mechanisms had some of that stuff.
I have no data, but I do have one observation: The pneumatics on spool (or *wind*) motors are sized such that the individual pneumatics never open to _anywhere near_ full extension. I think this is because the force vs. (constant) vacuum curve is quite non-linear - the greatest pallet movement occurs at the beginning of the cycle as the cloth walls are first sucked in. The manufacturers didn't want small variations in response between the pneumatics to be amplified as uneveness in the spool movement, particularly at slow tempos, so they made the pneumatics extra baggy (as compared to a striker, for instance), but only operated them through the somewhat more linear range from more than one-third closed to almost fully closed.
Rob also comments:
> When the hammer rest
> rail is lifted it cuts the hammer travel distance to one-half.
> If the pneumatic force were constant that would result in 3
> decibels of sound volume reduction. But my ear and microphone
> indicates its actually more, and the reason is because the
> available pneumatic force is much less as the device closes.
I think the wippen and/or key travel are also critical issues here. I can't give you numbers or a curve on a graph, but I do know that the movement of the hammer is not in direct proportion to the movement of the key (and wippen) throughout the hammer blow distance. The ratio of hammer to key movement diminishes significantly toward the end of the cycle as the balancer hits the drop screw and the jack hits the jack tender. Relative to the key (and the striker pneumatic push rod) the hammer is hardly being moved at all during the last ten or fifteen percent of the key motion. This percentage will depend of course on how the action has been regulated with regard to let-off distance, aftertouch and so forth. The *soft pedal* pneumatic on the reproducing pianos may indeed cut hammer travel in half, but the movement given the hammer through that last half is significantly less than that imparted during the first half of the hammer travel, other things being equal.
The striker pneumatic in that case is also, as Rob points out, operating at the weak end of its cycle, for the same reasons as cited with regard to the spool motor pneumatics. Those combined factors would certainly diminish the sound by more than 3 db. Otherwise, it would hardly have been worth the effort to install that extra pneumatic system. I have seen a pneumatically operated Una Corda shifter, but I can't recall which company used it. I _do_ remember that it sucked a whole lot of power from the pump momentarily. Of course, that made the piano softer, too...
Cheers
Doug Rhodes
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(Message sent Sat 23 Mar 1996, 05:50:00 GMT, from time zone GMT-0800.) |
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