---------------------- multipart/alternative attachment Terry THANK YOU !!... You saw exactly where I was going with this, and I appreciate the effort in thinking through somebody elses perspective to see whats being said and asked. A couple comments below. Well done tho :):) Farrell wrote: > Okay Richard, a light bulb went off in my head while reading this post > and I think I finally understand what you are asking. Let me state my > understanding of your question in my words and let me know if I am on > the right track. And if I am right, I also see that it could raise the > questions you ask. Here goes: CC soundboard. Panel dried way down, > glued to ribs, then exposed to normal room environmental conditions. > Panel hydrates and tries to expand, but because it is glued to > relatively unstretchable ribs, it causes the ribs to bend and we get > our crowned soundboard. Let's examine the stresses at this crowned but > unloaded point. There is compression in the panel. There is tension in > the top area of the rib (panel is trying to expand the rib top, but > has only very limited success). There is compression in the lower area > of the rib. And as Ron N. stated, there will be a line/plane going > through the rib where these is no stress. If somehow you were able to > instantly de-bond one of the ribs on this soundboard from the panel, > it would pop right back to its original straight configuration, i.e. > all the ribs in this soundboard are constantly trying to go flat. Now > comes the tricky part - and as I am writing this it is becoming clear > to me that I am not clear on this either. (Below is my third attempt > carrying on with this thought.) Now we add downbearing. What happens > to stresses? Compression in panel increases. Now I understand you to > suggest that tension in the upper part of the ribs will increase - for > the same reason the tension increased in the top portion of the rib as > the panel took up moisture after being taken out of the hotbox. Yes... more or less. Except one needs to make the distinction between tensioning the ribs top half by simple bending, and tensioning by overall stretching... the reverse of what we see in the sound board... it gets bent too... really.. but it also gets compressed...top and bottom... The net amount of compression at any point is then more or less its over all compression added to the amount of compression / tension that would occur from just bending the thing to the same shape... at least thats one way of figuring things. So the net amount of compression / tension at any spot in the rib is the amount of pure tension that occurs from the panel attempting to stretch it, combined with the stresses that occur from simple bending...and I aggree with your reasoning there is probably very little of the former, and very much of the later. > But does it? This is where about three different theories pop in and > out of my head and I'm really not sure what makes sense to me. To tell > you the truth, I'm not sure where tensions/compressions > increase/decrease in the rib upon loading. I suspect tension decreases > in the upper portion and compression decreases in the lower portion. I would suspect the net result would be as you describe... but just how much is what has me pondering. If the rib is simply being bent and unbent... then I agree with the << no support >> idea. But if the rib is being stretched at all by the panel, then that stretching has to be accounted for.... when ever it occurs... before downbearing is applied or after. If that comes into play... then it resists downbearing, even tho the straightening of the rib works the other way. > Let's try this thought on for size. Upon taking the dried flat > soundboard out of the hotbox, the panel hydrates and forms crown. The > ribs bend. The ribs are tensioned a bit on the top side and compressed > a bit on the bottom side. Is the tension on the top side simply from > the rib bending - whether a panel is glued to it or you are just > holding on a table and bending it with your hand (applying pressure to > the middle of it)? I suspect, and I don't know for sure, that almost > all the tension in the upper portions of the soundboard ribs are from > the rib simply being bent (convex side is being stretched along a > larger arc than the concave side) and very little tension in the rib > is actually related to the rib-panel interface. Thats the question allright. And I aggree with you... probably little if any pure tension from stretching the rib... probably almost all from a simple bending point of view... but how much is another thing,,, and at what point it becomes at all significant is another after that. > If what I stated above holds any water, now when we load the > soundboard (as in adding downbearing), the panel compresses, panel > compression - via the panel-rib interface - does not significantly > affect stresses in the top of the rib, tension in the upper part of > the rib decreases and compression in the lower part of the rib > decreases. As more load is applied to the board, those trends will > continue until the board is flat, at which point the ribs will be > essentially unstressed and back to where they were trying to go ever > since getting out of the hotbox. Yes... and all depends on just how much the ribs are being elongogated from the tension stress coming from the panel. If there is some at all... then the rib will be flat.. but slightly tensed... and the degree of that tension determines the degree of its support against downbearing. If its none... or none to speak of... then the ribs offering no support at all, if there is some tension.. then its supporting...if even slightly. > But now we have all this gobs of compression in the board (it has > tried to expand with moisture intake - compressing the panel - plus we > have squished this crowned structure down into a pancake - thus > increasing panel compression even more). Surely all this panel > compression has to affect the rib in some way. I don't know. Seems > like a reasonable question. Now that the panel is flat, is the rib > unstressed? If the panel has all this compression in it, there has to > be something out there in this universe countering the internal > compression in the panel. Is it the rib somehow? Could the rib be > stressed by the compressed panel in such a way that the net effect is > neutral (doesn't seem likely if the panel is glued to one side of the > rib). If the rib is flat, happy, relaxed and unstressed, is the > counter force somehow the downbearing? Is that kinda where your > questions were going Richard? I thought it was a reasonable question as well :) You pretty much hit the nail right where it does the most good there Terry. Grin... you are really something there partner ! Glad to have you around. > Can anyone out there shed light on this? I think I was correct a post > or two ago saying that vector analyses of the stresses and strains > here can adequately describe what is going on. But it is also now > clear to me that I don't have much of a clue as to how to conduct such > an analyses - it appears much more complex than I had originally > thought. Terry Farrell Thanks again Terry. I look forward to hearing other responses now that this quandry should be made clear to one and all. Cheers RicB -- Richard Brekne RPT, N.P.T.F. UiB, Bergen, Norway mailto:rbrekne@broadpark.no http://home.broadpark.no/~rbrekne/ricmain.html http://www.hf.uib.no/grieg/personer/cv_RB.html ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... URL: https://www.moypiano.com/ptg/pianotech.php/attachments/bc/db/81/0e/attachment.htm ---------------------- multipart/alternative attachment--
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