----- Original Message ----- From: "Richard Brekne" <Richard.Brekne@grieg.uib.no> To: "Pianotech" <pianotech@ptg.org> Sent: October 09, 2003 12:43 AM Subject: Re: RC vs CC again > > > Delwin D Fandrich wrote: > > > > > Will the rib stretch? Sure it will. At least the top of it will. To about > > the same degree as Ron N's aircraft carrier will slow down as it impacts > > that kamikaze mosquito. Spruce is rather strong in tension, it doesn't like > > to stretch at all. As well, the bottom will compress a bit and there will > > be a neutral line somewhere towards the center. The net result is that the > > thing wants to straighten itself out again. > > > > No arguement there.... heck the soundboard wants to straighten itself out again > for that matter... I dont suppose it likes being all squeezed and bent anymore > the the rib likes whats happening to it. > > I just dont see where the justifcation for writting off the ribs overall > tension is covered in the literature. Perhaps you can point me to some section > or table some where ? And I don't see the justification for claiming that some mysterious "overall tension" is in any way supporting crown. I don't see it as any factor at all let alone a significant factor. I doubt many others do either. Which probably explains why you don't see many charts on the effect. It does suggest an avenue for research, however. When can we expect to see numbers and charts explaining the Brekne Effect? As glued up the top of the rib is neutral relative to the soundboard panel. If the panel and the ribs are kept at a stable moisture content there is no stress differential between them and, obviously, there is no crown. It is not until the panel begins to absorb moisture and the wood fibers begin to swell that the assembly begins to take on crown. The rib is now forced to bend against its will and it will always be trying to return to equilibrium -- i.e., to being the straight chunk of wood it was cut out to be. It's whole heart and soul go into that desire until such time as its time under load is sufficient to permanently deform the poor thing through creep. The exception to the above, of course, is the compression-crowned soundboard assembly which still uses flat ribs but the assembly is glued up at a somewhat higher moisture content using curved cauls to come up with essentially the same amount of crown. In this case the top of the rib is under some tension at the time of gluing. How much depends on the amount of curve found in the caul. The end effect on the soundboard panel, though, is essentially the same -- for a given amount of crown the level of internal compression will be the same. And just one more point, while I'm on the subject. Most of us are aware of the approximate perpendicular-to-grain compression strength of the various spruces. (The test numbers are readily available in the 'USDA Wood Handbook.') What is generally left out of the discussion is the effect of time under load. For a wood structure expected to survive any length of time -- say, ten years -- these test figures have to be reduced by about 40%. De.
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