How and Why a Valve Activates a Striking Pneumatic
Part I - By Wilberton Gould, Member N. A. of P. T., New York City
Service, as defined by Webster: "An act of one who serves."
It is one of God's greatest gifts to mankind, for who is happy who does not serve? Hence it follows that service requires sacrifice. It is the foundation stone of every enterprise. Whatever it may be, its success or failure depends upon whole-hearted co-operation.
And may service and sacrifice continue to be the keynote of our progressive Association.—The Author.
(Continued from November 1927)
Before going into this subject, let us go back to the old parlor reed organ, which many of us have had to tune. Those were the balmy days, eh, boys ? But I wonder how many of us gave any thought to what was going on in the wind chest of that old organ when we started to work the treadles, pulled out a stop and pressed down the keys?
Frankly, I gave it very little thought, being content to tighten every screw that I could see, clean out the silent reeds, and let it go at that, until one day it occurred to me how and why that reed spoke when a key was pressed down. It took me about a week to get the answer, but it was very simple when the principle was understood.
I discovered that the function of the bellows in the lower part of the organ was to reduce the atmospheric pressure within the wind chest, and so long as no stops were pulled out or swells operated there was no response from the instrument; but the moment I pulled out a stop and pressed down a key this is what happened:
Pressing down the valve opened up the pallet valve and allowed outside air to enter the reed valve chamber and pass over and down through the reed tongue, causing the reed to vibrate and emit its musical sound. Here was the principle of the outside air being of greater pressure than the air within the chest. Hence its weight caused the reed to vibrate as above stated. The outside air had performed its duty with the assistance of the bellows.
In our modern player piano we have much the same condition, only it is accomplished in a different manner. In the player piano, instead of using the human finger to depress a key we use a mechanical finger, or the striking pneumatic, but in order to cause the pneumatic finger to act there must be a suitable coordinated mechanism. This is in the form of a valve, a pouch, a bleed and a suitable channel from the tracker bar, connecting the pouch chamber, a channel from the striking pneumatic to the main airway of the pneumatic stack, and direct connection from the pneumatic stack to the bellows, or pump.
When we place a music roll on the top chucks of the spool box, and connect the roll to the take-up spool, it covers all the tracker ports, so that no atmosphere can enter through the tracker, and, as in the case of the reed organ, no matter how hard we press on the pedals there is no response from the player. As we move the tempo lever from zero and the roll begins to move to a speaking port, which registers with the opening on the tracker bar, the action takes place which is described below:
Normally the valve is down, allowing air to enter over the top seat of the valve in the valve block to the striking pneumatic, causing the pneumatic to remain open. At the same time the air that is passing over the top seat of the valve causes the valve to remain seated against its bottom seat. The question arises, what causes the valve to remain seated in this manner? The answer is, through the action of the atmosphere whose weight, 14.7+ pounds a square inch, is always constant, and because of gravity, or the fact that any object heavier than air will sink until it comes to rest. As soon as a speaking perforation registers with the tracker port air enters through the tracker port and passes down a tube that is connected to the valve chamber, to the underneath side of the pouch, inflating the pouch, which in turn raises the valve to its upper seat and cuts off the outside air, allowing the striking pneumatic to be connected to the reduced air chamber of the pneumatic stack. This in turn allows the bottom leaf of the striking pneumatic to close, engaging the underneath side of the wippen of the piano action, raising the hammer and striking the blow which produces the musical tone as registered on the music roll.
In this operation two questions arise, based upon the statement that air has weight and volume. (1 ) How can the valve close when the air pressure is the same on the top of the valve as it is in the bottom seat, and would not the bleed affect this operation? (2) Does the reduced air pressure within the pneumatic stack cause the striking pneumatic to collapse when the valve is closed to outside air?
Taking the questions in turn: (1) the reason that the air entering the tracker port closes the valve is the fact that the area of the pouch in the pouch block is about three times the area of the opening of the top seat of the valve, hence about three times the volume of air enters under the pouch and inflates it, overcoming the resistance of the air passing over the top seat of the valve. As for the bleed, the tracker port is about six times the size of the bleed, hence a greater air pressure prevails. (2) The answer to this question is "No," for the reason that as soon as outside air is cut off by the valve being raised to its upper seat and the pneumatic being connected to the reduced airway of the pneumatic stack, the outside air pressure being constant, or 14.7+ pounds a square inch, causes the pneumatic's movable leaf to close; and so long as the perforation remains open on the music roll just so long will the pneumatic remain collapsed.
As soon as the perforation has passed over the tracker port, and a blank space covers the port, the process is reversed; that is, the outside air is constantly pressing down upon the top seat of the valve, and there being no further resistance from the greater volume of air that has been passing in by way of the open tracker port the valve is forced down, with the assistance of gravity, and drops back to its bottom seat, cutting off the channel from the pneumatic to the reduced airway of the pneumatic stack. The outside air now has a clear passage to the striking pneumatic and inflates it, bringing it back to its normal position, which allows the piano hammer to come to rest against the hammer rail. The atmosphere is now equal within the striking pneumatic and the exterior, and at the same time the surplus air that was in the tracker tube and underneath the pouch is permitted to pass out through the small bleed, or vent, allowing the pouch to deflate, the air passing out through and over the top seat of the valve.
This same operation takes place in each of the eighty-eight valves and pneumatics and the auxiliary units of the player action. It is to be noted that this description applies to the single valve pneumatic action only.
In the double valve action we have two sets of valves and pouches and one set of bleeds; namely, a primary valve and pouch board, and a secondary valve and pouch board. The bleed board is usually in the primary pouch board, as the surplus air from the secondary pouches is bled back through the connecting channels to the primary valve board. In either case the basic principle remains the same.
In the succeeding article "Valves and Their Ills and Remedies" will be treated, as well as the various auxiliary units necessary to operate a foot-impelled and electrically operated player.
(to be continued)
End Part I