----- Original Message ----- From: Frank Weston <klavier@annap.infi.net> To: <pianotech@ptg.org> Sent: Wednesday, September 22, 1999 5:45 AM Subject: Re: evaluating sdbd. crown & bridge downbearings in a new piano > .... For a "compression" crowned board, > the top portion of a panel starts out in tension, No. It does not. If the panel is at equalibrium at 3.8% (winter) to 4.0% (summer) moisture content (MC), then once the panel is ribbed and exposed to ambient temperature and humidity conditions, the entire panel is under compression. It is this compression that provides the energy to force crown into the assembly. The ribs do not distort into a curved condition all by themselves! In the compression-crowned soundboard assembly, the ribs actually resist the formation of crown. They would like to remain straight. Indeed, throughout the life of the soundboard assembly they will be trying to straighten themselves out again. And, when the compression stress within the soundboard panel that originally forced crown into the assembly is finally dissipated (through the inevitable wood cell damage, creep, etc.), and the soundboard assembly has lost all crown, they will achieve this goal. Where do you think the energy comes from that forces the ribs into a warped shape and keeps them there? It takes some amount of energy to do that. If the soundboard panel is not under compression across grain, where else does it come from? For a compression-crowned soundboard to have any crown at all, the entire panel must be under extreme compression. Without this compression, there is no crown. And this compression must not only warp the rib set, it must also be enough to resist the downforce of the string plane that is also attempting to force the assembly flat. The only place you will find any tension in one of these assemblies is along the top of the ribs. And that does decrease as the soundboard panel is forced flat. When the soundboard assembly has been pressed fully flat, only the rib will be at equalibrium. > as it flattens out, the > ultimate compression load will be less than if the top of the panel is > unstressed or in compression to begin with. As a panel bends toward flat, > the original compression load in the bottom half will also be diminished at > a rate somewhat less than compression is being increased on the entire > panel. Not so. Since the entire panel is under compression, as the panel bends toward flat the amount of compression in the soundboard panel will increase. The tension along the top of the ribs will decrease and the compression along the bottom of the rib will decrease. Indeed, if the soundboard is forced back to a fully flat condition, the only component of the assembly that will be without stress is the rib. At this point compression throughout the soundboard panel will be at a maximum. > For a soundboard constructed by any other method, the extra panel width > still has to be accomodated. Ultimate compression in the top half of the > panel will be greater than for a "compression" crowned board, while ultimate > compression in the bottom will be less by the same amount. Apples and oranges. The rib-crowned soundboard is designed and built differently. There is no "extra width." > Given that a "compression" crowned soundboard assembly is at zero stress due > to deformation when it is flat, Sorry, but this is not a given. At least not in the real world or in the normal atmosphere in which real pianos exist. It is true only in the hot box, and then only while the soundboard panel is held at a steady 3.8% or 4.0% MC. At that point, it does not matter if the panel is ribbed or not. Even with the ribs glued on, as long as the soundboard panel is at a steady 3.8% to 4.0% MC there will be compression in the panel and no crown in the assembly. Compression and crown don't enter the picture until the assembly is exposed to the ambient atmospheric conditions of the factory or shop and the wood cells begin to take on moisture. As the wood cells absorb moisture they will attempt to swell, or expand, thereby creating the compression that forces crown into the assembly. > and a "rib" crowned soundboard assembly is > at zero stress due to deformation when it is crowned, Well, not quite. In the rib crowned soundboard assembly the panel will have a very slight amount tension along the top surface immediatly after ribbing. There will also be a very slight amount of compression along the bottem surface. Once this assembly is in the piano and there is string downforce on the bridge, the crown of soundboard assembly is reduced and there will be some small amount of compression throughout the soundboard panel. The amount of compression in this soundboard panel is considerably less than is found in the compression crowned soundboard assembly. The actual amount of compression varies with atmospheric conditions, but is generally much less than 0.5%. This is compared to the approximately 2.0% found in the compression-crowned soundboard panel under similar conditions. > then it follows that > as download is applied to a "compression" crowned soundboard and it is > displaced to a flatter shape, stress due to deformation will be decreased > toward zero. For a "rib" crowned board, this stress will be increased. > There will be an additional stress placed upon each board as a result of > resisting downbearing loads. This stress will be the same, no matter how > the board is constructed. So, with a "compression" crowned board, total > stress, which is the sum of stress due to deformation and stress due to > compression required to resist downbearing, will be less than total stress > in a "rib" crowned board. This conclusion is incorrect since the basic premise is incorrect. > .... As I have argued, the ultimate stress on a "rib" crowned soundboard is even > greater than on one "compression" crowned. Could it be that there are just > not as many old "rib" crowned boards around to observe in failure? Ditto, the above. The basic premise is incorrect. The ultimate stress -- in this case compression stress -- on the rib-crowned soundboard panel is considerably less than is found in the compression-crowned soundboard panel. > There is another whole issue that we can argue. I say that because of the > differences in stress on "compression" crowned boards (almost zero internal > stress due to deformation) they are more lively and more responsive. I have > no figures to prove this. Nor will you find any. Again, because it simply isn't true. This whole issue should have been -- indeed, has been -- laid to rest years ago. I often wonder if techno/mythology has as strong a hold on technicians in other industries/professions as it does in the piano world. This is all fairly basic wood technology. None of this is really open to speculation or to argument. The basic principles are easily proven if you wish to take the time to do so. Del
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