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Correctly Adjusting Pneumatics (Bellows)
Hi All, I can tell by Larry Mayo's posting yesterday that he doesn't really understand what I said in my posting about the Baldwin 'Manualo' action. And while I believe that Bernt Damm and I said basically the same exact thing with regards to adjusting the "lugs", I would like to repeat some information that has been well-known by rebuilders for over 80 years.
There are a couple of basic premises that should always be observed when dealing with a bellow that is used to activate any device. They are like 'unwritten rules', but I'll write them out.
A bellow should never be adjusted such that it fully closes. Why? Because collapsing a bellows 100% will put a tight permanent crease in the cloth. This causes the bellows cloth to wear out much faster. (If you need an explanation, write to me; I'll be happy to explain it in detail.)
Preventing the bellow from closing 100% can be accomplished in many ways, but the easiest way is to put a piece of felt, 1/8" to 1/4" thick, inside of the bellow at a point that is just behind the space taken up by the inward folding cloth.
When at rest, the bellows cloth should never be stretched open in any way, shape or form. Why? Because stretching the cloth deteriorates the layer of material that makes the cloth air-tight. And just like anything that is constantly stressed, it won't last as long. To prevent a bellow from opening 100% (thus eliminating any stretching), something must be put in place that is stronger than the forces working to open the bellow.
In the case of regular striker pneumatics, there is normally a wooden surface under the movable board to which a piece of bumper felt is affixed, which prevents the bellow from opening all the way. The thickness of the felt used should be such that the bellow never opens 100%. Anything under 100% is acceptable but normally the bellow opens to about 90% of its total span. In some cases, the device itself is designed in such a way that it is impossible for the bellow to open 100%. Here, I'm primarily referring to air motors and tracking devices.
In the case of spring-loaded bellows or bellows upon which there is a constant load, it's wise to install a 'limiter strap' either internally or externally to prevent the cloth from being stretched. The strongest evidence I can offer with regards to the believability of the statement that stretching the cloth decreases its life span is seen in reservoir bellows, auto-sustain bellows, air-motor governor bellows, and bass and treble soft pneumatics. In virtually every player piano I've ever worked on, bellows cloth that was constantly 'stressed' either started leaking or got stiff faster than identical cloth (in the system) that wasn't under any stress. (Don't understand? Write to me.)
Naturally, you shouldn't restrict the movement of the bellow (when at rest) so far that it no longer has enough travel to do its intended job.
Summing everything up, a correctly adjusted bellow is one that is never opened or closed 100%. The "working region" between the two limits (Rule One and Two) must be great enough that the bellow can do its intended job with ease. What describes that limit, or the working area, is the overall span of the bellow, minus the upper and lower restrictions combined. Also bear in mind that a pneumatic develops its maximum power from Time Zero (T'0'), and that power decreases at a non-linear rate to minimum when the bellow is fully collapsed. So, the idea here is to adjust the bellow such that it is open as far as possible without 'forcing' it open.
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