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Chapter 2: The Bellow

While there is no argument about the fact that it is vacuum which makes the player piano work, it is the bellows which uses and creates that vacuum. The average player piano contains one hundred bellows of various sizes and each bellows has its own job. In order to operate correctly, every bellows must be air-tight. Also, every bellows works together with a valve of one sort or another.

Since Chapter 1 dealt with the note, or poppet, valve it seems appropriate to continue with the note bellows, or striker pneumatic. In the drawings below, the bellows is mounted on a "wind chest" which acts as a channel between the valve assembly and the bellow. Typically, the bellows would be mounted on the underside of the wind chest but for purposes of explanation and ease of understanding I have placed it on top.

Referring to Figure 8, when the pouch rises, it pushes the valve (in red) up and closes off the exhaust port. This the entire pneumatic assembly with all it's partsThis allows the vacuum, from the source, to pass through the valve guide into the upper half of the valve assembly and from there into the bellow. As the vacuum sucks the air out of the bellows it collapses and activates a note on the piano. When the pouch relaxes the valve drops back to its starting point and seals off the port through which the vacuum passed and in doing so simultaneously opens the exhaust port allowing outside air to rush in and "fill up" the previously collapsed bellows (see Figure 9). At this point it should be noted that the configuration of the wind chest is a bit more complicated than the drawings indicate. This the entire pneumatic assembly with all it's partsTo begin with, as stated earlier, the bellows is normally mounted on the bottom tier of the wind chest so that when it is activated, or closed, the moveable board will travel in an upward direction pushing on the underside of a key or a tab protruding from the action of the piano. And since the weight of the piano key or the piano action is resting on the moveable board there is a natural tendency for the bellows to return to an open position once the valve is deactivated.

Now that the basics have been explained, lets look at how this really happens. In the below graphic of a Pratt-Read system, we see all of the parts mentioned previously plus the striker finger and the pushrod, which pushes on the piano action to make the note Play. Also, you see the channel that leads to the trackerbar. To see the valve in action, click here

Pratt-Reed - Valve in Off position

Pratt-Reed - Valve in On position

In the above graphic, which shows the bellows in the closed position, note the 'reduced atmosphere' area. This area is marked as 'reduced' because of the 'vacuum' which passes through the 'bleed'. While the function of the bleed was discussed in Chapter 1, a few more details should be presented. First, let's consider what would happen if the bleed was to become clogged or closed off althogether. Would the valve change state when the hole in the trackerbar 'opened'? Yes, it would! In fact, it would change state very quickly! That's because the full force of the atmosphere would enter into the pouch well and the pouch would 'puff up' almost immediately. However, what would happen when the hole in the trackerbar 'closed'? Now, there would be little-to-no vacuum to equalize the pressure in the pouch well and the valve would stay 'on' until the pouch relaxed. (This is a relatively common problem in players that are not well maintained. More information about keeping the bleed clean is presented here.) Now let's consider what would happen if the bleed was to big. In reality, this can't happen unless someone physically changes the bleed cup. However, there is another way for vacuum to get into the pouch well, and that's 'through' the leather pouch. This is a relatively common problem in both older player pianos and players where the pouches have been replaced during a rebuild. In older players, the leather loses it's air-tight quality as it dries out and/or becomes worn where it flexes. In rebuilt players, the leather is not as air-tight to begin with because the animals that are raised for their leather and the tanning process used to prepare the leather are not specialized like they were during the heyday of player pianos. The result, in both cases, is that the leather 'leaks', and that leakage is exactly the same as a 'bleed'. But how does all of this relate to performance? Simply put, if there's too much 'bleed', the valve will be slow to come 'on' when the hole in the trackerbar is 'open' and fast to turn 'off' when the hole in the trackerbar is 'closed'. Conversely, if the bleed is too little, the valve will be fast to come 'on' when the hole in the trackerbar is 'open' and slow to turn 'off' when the hole in the trackerbar is 'closed'. So, as you can see, the bleed has a lot to do with the optimum performance of the valve.

With regards to the striker bellows, there are three critical measurements; the length, the width, and the span. All three were engineered when the system was created to work in coincidence with the valve. So, it would be unwise to change any of the parameters when rebuilding the bellows. Information about removing, cleaning, and recovering the striker pneumatics can be found at Player-Care on these pages: Removing, Cleaning, and Recovering

To Be Continued

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This page was last revised February 8, 2013 by John A. Tuttle, who Assumes No Liability
For The Accuracy or Validity of the Statements and/or Opinions
Expressed within the Pages of the Player-Care Domain.
Cartoon Graphics by E7 Style Graphics (Eric Styles)

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