In this chapter I will explain the most important principle of operation which allows the player piano mechanism to operate. And while I fully admit that it is not the most important piece of information you will need to comprehend the player piano mechanism, it is singularly the most essential operating characteristic of the mechanism and must be completely understood.
How do notes on a piano play when holes in a piece of paper pass over a set of holes in a metal or wooden bar full of tiny holes? First we should review a few basic facts of physical science. Our atmosphere exerts approximately 14.7 pounds per square inch of pressure on everything that exists at sea level. At higher altitudes the atmosphere exerts less pressure because the air is less dense. The effect of this pressure can be easily demonstrated by the action of inflating a rubber balloon. When air is blown into the balloon, the air pressure on the inside becomes greater than the air pressure on the outside. And, since the material the balloon is made of is flexible, it (the flexible material) has a natural tendency to move in the direction of least resistance, or lower air pressure, and the balloon inflates. When the opening at the top of the balloon is opened, the air inside the balloon, being at a higher pressure, escapes until the pressure inside the balloon is equalized. The point is: if a piece of flexible material is placed between two different atmospheric pressures, the material will try to move in the direction of least resistance.
At one time or another almost everyone has sucked the air out of a bottle, put their tongue over the hole and felt their tongue being sucked into the bottle. That "sucking in" feeling is most commonly referred to as a vacuum. A vacuum is described by Webster's as "a space from which most, or all, of the air has been removed". If an uninflated balloon is placed inside of a bottle and then sealed around the top (see Fig.1) and a small hole is cut in the bottom of the bottle so that the air inside of the bottle can be sucked out, the balloon will inflate inside of that bottle as the air is sucked out. (see Fig 2). This happens because the air pressure inside the bottle is lower than the air pressure inside the balloon. This is exactly the opposite of what happens when you blow a balloon "up". When you blow a balloon "up", the pressure inside of the balloon is greater than the pressure on the utside of the balloon. In the player piano, it is the movement of a flexible membrane responding to differences in air pressure that make it work. And while there are many ways to use the principle that was just described, it is the manner in which this principle is utilized to turn a valve "on" and "off" that is most important to the operation of the player piano.
There are many types of valves in every player piano. There are sliding valves, flap valves, pallet valves and poppet valves just to name a few, but it is the poppet valve and its associated parts that best demonstrates the above principle and how that principle is used to make notes play in a player piano.
(1) An "air tight" chamber about the size of a book of matches (1.5 inches by 1.5 inches by 0.75 inches) is created. Then a hole about the size of a quarter is cut in the top and a flexible membrane is glued over that hole, like the bottle and the balloon. (see Fig 3). Then a second "air tight" chamber (like the second floor of a house), the same size as the first, is placed over the top of the first chamber so that the top of the first chamber is the bottom of the second chamber (see Fig 4). Next, if you drill a hole in the side of the lower chamber (a, Fig.3) and "suck out" the air, the membrane will move downward in the direction of the lower pressure or in the direction of least resistance. If the air is " sucked out" of the upper chamber the membrane will move upward in the direction of least resistance. Also, if an equal amount of air is "sucked out" of both the upper and lower chambers there would be no difference in pressure and the membrane would not move at all.
(2) A very small hole about the size of a pin, called the bleed (b, Fig 3), is drilled between the two chambers so that when air is " sucked out" of the upper chamber an equal amount will come out of the lower chamber. Then, because the vacuum, or negative air pressure, on both sides of the membrane is "equal", the membrane will not move at all. Next, the hole in the lower chamber (a, Fig 3) is cut so that it is six times larger than the bleed hole between the two chambers. It is important to note that it is the difference in the sizes of the bleed and tracker holes that set the stage for creating the differences in air pressure that are critical to the operation of the player piano
(3) If the larger hole (a, Fig 3), or tracker hole as it is properly called, is closed and a partial vacuum is applied to the upper chamber, the vacuum on both sides of the membrane is equal and the membrane does not move. However, when the partial vacuum exists in the upper and lower chambers in equal amounts (due to the bleed) and the "tracker hole" is opened to the outside air, the air, being higher in pressure, rushes in and tries to "fill up" , or balance, the vacuum in the lower chamber. And due to the fact that the bleed between the upper and lower chambers is six times smaller than the tracker hole, more air rushes into the lower chamber ( through the tracker hole) than can be replaced by the bleed vacuum from the upper chamber which maintains the imbalance for as long as the tracker hole remains open. Furthermore, the flexible membrane reacts to the imbalance and "puffs out" in the direction of the lower pressure, or least resistance, towards the upper chamber. When the tracker hole is once again closed, the vacuum from the upper chamber reenters the lower chamber through the bleed which balances the pressure and the membrane returns to a relaxed state. This movement of the membrane, or pouch as it is properly called, is like a switch that can be turned on and off by opening and closing the tracker hole. Also note (in Fig.5) that the lowest chamber (the pouch chamber) has been reduced in thickness. In actuality the size of the pouch well is just deep enough to accommodate a "dished" pouch, so named because when it is glued in, there is a slight 1/16" clearance between the bottom of the well and the "bottom" of the pouch, and it looks like a very small dish. This reduction in the size of the pouch well and dishing allows the pouch to react quicker and more positively because the signal from the tracker hole doesn't have to "overcome" the "reservoir" of negative air pressure in a larger chamber. The pouch would still react in a larger chamber but it's action would be slower and less forceful.
(4) A third chamber is placed on top of the other two chambers (see Fig 5). In the top of the third chamber, a hole, a bit smaller than a dime, is cut in the center. This hole is called the exhaust port, but as you will see later, it is actually where air will enter into the valve assembly. Now another hole, slightly smaller than the hole in the top, is cut into the bottom of the third chamber (which is also the top of the second chamber) and is called the valve guide because it permits the valve stem to travel only up and down.
(5) Another device is needed to cover and uncover the exhaust port and valve guide or intake port. The device, known as a valve (see Fig 6), is constructed of a valve stem and a valve face. In modern units, like the one being explained, the valve facing is made of a "sponge-neoprene" material. The valve stem is made of a very light material, like wood or plastic, fashioned so it will just fit through the valve guide and is cut one sixteenth of an inch longer than the full height of the second chamber. Looking down at the top of the valve stem, it looks like a large plus sign (+). Valve stems can be made in shapes from round to square but they are all fashioned so that the valve will travel in only two directions and allow vacuum to pass by the intake valve face. Mounted on top of the valve stem is the valve face. Since the job of the valve face is to keep outside air "out" until needed and to keep vacuum "in" until needed, it is cut about 30% bigger than the exhaust port.
The material the valve face is made of has to be able to create an "air- tight" seal around the holes in the tops of the second and third chambers. For many years, leather was the most popular and longest lasting material for making valve faces. Today the most common material is neoprene, which has thousands of tiny bubbles and is cut into disks about the size of a thick nickel. The exact thickness of the valve face is determined by the inside space between the exhaust port and the intake port minus 0.028" - 0.035" (slightly thicker than a paper match). This " extra space" is called the valve clearance and is the distance the valve moves from the "on" to the "off" position.
The valve is installed in the valve assembly so that the bottom of the valve stem passes through the valve guide. Since the valve face is connected to the top of the valve stem, and is at least 30% bigger than the valve guide, the bottom of the stem comes to rest one sixteenths of an inch above the pouch when the pouch is relaxed, and completely closes the intake port, or valve guide hole.
(6) When the tracker bar hole is "opened", the atmosphere, which is positive in comparison to the vacuum inside the chamber, rushes in to the underside of the pouch in an effort to " fill up" the vacuum. And since the bleed is six times smaller than the tracker bar hole, the atmosphere overcomes the vacuum and the pouch puffs up and pushes on the bottom of the valve stem causing it to move upwards thereby opening the intake port and closing the exhaust port. When this happens, the vacuum, which was only in the pouch chamber and the middle chamber, is allowed to enter the top chamber.
(7) Finally, one last hole is cut into the top chamber (see Fig 7). This opening is connected directly to a bellow. Since there is a channel leading from the upper chamber to the bellow, the vacuum enters the bellow and it closes. (The mechanical movement of the bellow is like the mechanical movement of your finger. And, as it is the movement of your finger that causes a note to play on the piano, the movement of the bellow causes a note to play.) When the tracker hole is closed, the pouch relaxes, the valve drops, and outside air flows back into the bellow through the exhaust port and the bellow "opens" back up. The physical connections and design of the bellow will be explained more fully in the chapter on bellows.
The Simple Explanation
For those interested in reading a more concise treatise of the principles of operation, read the series of articles by Wilberton Gould, which were written in 1927-1928 for the Tuners' Journal - click here.
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