Bob, In one of the pianos I tune, in a certain church, had stress or tension cracks on the soundboard. I remembered something that Randy Potter said about using a complete Dampp-Chaser System in the piano would fix this problem. So, I ordered a DCS from Webb Phillips and put the system in the piano. One week later I went back to see the results and the stress cracks were gone. These Dampp Chaser Systems do work, and they not only help keep the proper humidity in the wood, but preserve other parts of the piano as well. Leonard Childs, RPT Childs Piano Service San Antonio, TX lenny@stic.net ---------- > From: rhohf@idcnet.com > To: pianotech@ptg.org > Subject: Re: Soundboards/stress > Date: Thursday, June 12, 1997 7:26 AM > > There have been a lot of interesting and thoughtful ideas in this thread. One > of the things that makes this business so fascinating is that we have been > making soundboards for centuries, but the fundamentals are still so poorly > understood: > > "the strings' tension is attached to the board's compression" > > "The swelling board puts pressure, not tension on the cells......In all of > this, we never see the word tension." > > "... the unloaded soundboard is "crowned" as a result of tension, then > controlled by the amount of downbearing." > > None of these statements are quite right since a soundboard with ribs attached > is a complex system containing tension _and_ compression. > > Let's try to look a little more closely at the stresses that are introduced > into a rib/soundboard system by conventional construction, and see if we can > clarify this a bit. First the wood basics: wood expands and contracts with > changes in moisture at least twice as much per unit dimension in the tangential > direction (soundboard thickness) as the radial (ribbing direction), changes in > length are negligible. A normal piece of spruce, allowed to expand and > contract in the air developes no stresses (discounting the cellular level). > > When a rib is glued across the grain (radially) to a dried and shrunken spruce > panel, and then the whole thing is allowed to absorb moisture, the panel tries > to expand in width and the rib doesn't change in length. This creates > _compression_ on the bottom surface of the panel, _tension_ on the top surface > of the rib, and huge shearing stress on the glue joint. It is the action of > this compression and tension which causes the system to arch (if it is free to > move) until these stresses are balanced by _compression_ on the bottom surface > of the rib. The shearing stress on the glue joint is what makes failure of the > joint so common. > > So what stress is on the top surface of the soundboard? Is it under > compression or tension? This is difficult to determine in a non-destructive > way in a glued-up board or an assembled piano. It seems to me that there are > devices that can scan the surface of metal beams and determine the stresses on > the beam. Perhaps one of the engineers out there could comment on whether this > can work on wood. > > However, pursuing that question, I have made quite a few "model" > soundboard/ribs in different configurations and evaluated the stresses on > every surface of the panel and ribs. How this is done would take too long to > describe here. But every configuration I have tried has resulted in _tension_ > on the top surface of the soundboard. > > There is another factor which sheds some light on the top surface of a > soundboard: cracking can reveal a lot about the stresses. Cracks mean tension. > Compressed wood _cannot_ crack. This is not to say that excess compression > can't set up a condition that results in tension/cracking. In any case, if > soundboards were "compressed" as many seem to believe, cracks would not form. > As Brent Fisher said, "Each board reacts differently". I believe the > question is not, "Do some soundboards have tension on the top surface?" but > rather, "Is it possible to make a crowned soundboard which is not tensioned on > the top surface?" This is an opinion based largely on my models, which are not > real-life soundboards, and would be difficult to verify conclusively in > assembled pianos. > > All of my comments refer to unloaded soundboards. So what does loading do to > the stresses? Applying downbearing does not simply put the board into > compression: if it did, boards would not crack. I thought I might go into > this here, but this message is already kind of long. Maybe another time. > > Bob Hohf > Wisconsin
This PTG archive page provided courtesy of Moy Piano Service, LLC