Organ Reeds and Air Pressure
By Howard Wyman
After reading about pressure and organ reeds I felt prompted to add my two cents worth. I believe the original question was about converting a reed organ so that it could play rolls. I have an Aeolian player reed organ which I rebuilt about 5 years ago. It is a model 1500 which was the forerunner of the Aeolian Grand and was manufactured in 1895. I tried to measure the suction with a dial type gauge. The suction is actually too low for an accurate measurement, plus I don't really trust the gauge at low readings. To get a more accurate measurement one would have to use a water type gauge. The needle barely moved, so it seems that the organ operates on a suction of 2 or 3 inches of water at the most. The player mechanism is designed to operate on this low suction. BTW, the player section is actually separate from the manual. The only thing they have in common is the bellows and reservoir. Each has it own banks of reeds and valves. When a hole in the (wooden) tracker bar is uncovered, it inflates a tiny "pillow" pouch pneumatic. This pneumatic is 3/8" wide by 1 11/16" long with a span of 1/2", and uses pouch leather as a covering. Everything is designed to be narrow, so that it takes up no more width than an ordinary key on the keyboard. When that pouch expands it presses open a pallet valve which is designed exactly as are most reed organ pallet valves. The pallet valve is inside the suction chamber. When it is pushed open it pulls a small outside valve closed to close off the atmospheric pressure going to the "action" pneumatic. The valve clearance is .030". The action pneumatic is 9/16" by 6" with a span of 7/8". The original covering was something like a thin chrome pneumatic leather making them very flexible. Even though the span is 7/8", these pneumatics do not need to close in order to sound a note. As the pneumatic closes it pushes a pitman rod which goes through into the suction chamber and pushes open the pallet valve which allows air to pass through the reed. Remember that the force of the pneumatic does not have to move the mass that it would have to in a piano action. It only needs to open a pallet valve enough to permit air flow. That pallet valve is inside the suction chamber and therefore the atmospheric pressure is already trying to push it open. The only thing that is holding it closed is a spring. It is important that these springs be just strong enough to hold the valve closed with the most vigorous pumping. Therefore, it does not take very much force for the "action" pneumatic to push the pitman which opens the pallet valve.
It does take more vigorous pumping to play a roll than it does to play the organ manually but this is probably because of the extra air required to turn the wind motor.
Howard Wyman (hwyman@digital.net)
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(Message sent Wed 6 Dec 1995, 03:09:52 GMT, from time zone GMT-0500.) |
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