---------------------- multipart/alternative attachment Ron Nossaman wrote: > Jim, Richard, Frank, and all the ships at sea, > > One more time. > > For the sake of illustration, I have two ribs, both 24" long and 1" high, > one flat, one machine crowned to a 60' radius. I also have two "panels", > both 0.375 thick, one dried to 4% EMC, and one dried to 6% EMC, and both > exactly 24 inches wide. We will assume that the modulus of elasticity is > such that we will end up with similar assembled crown heights with both > ribs, though that's not necessarily the case in real life. Since this is an > illustration of a basic principal, I'll try to keep it as simple, and with > as few variables, as possible. > > I now glue the 4% EMC panel to the flat rib (compression crowned (CC)), and > the 6% EMC panel to the crowned rib (rib crowned (RC)). At this point, the > CC panel and rib are at rest, and neither is under any tension, compression, > stress, or general discomfort. The rib of the RC assembly is of a similar > condition, but the panel has been bent, and is under slight compression at > the bottom, and slight tension at the top. Question here... In the first case (CC ) since the rib is flat and is glued onto a curved board doesnt that create to some degree the same kind of stress on the rib as you describe is created on the panel in the second case (RC) ?? I mean,, in both cases we have a curved surface glued together with a flat surface. Could you explain this further ? > > > The glue has dried, and we rehydrate both assemblies to 8% EMC, and both > assemblies are now crowned at a 55' radius. How is this figure arrived at ? > > The panel of the CC assembly has grown from 24" to 24.147" with the 4% EMC > increase. The top surface of the rib is 24.018 long, which should make the > top of the panel 24.019". That means that the panel has been compressed at > the top by 0.128", and the bottom by 0.129". Note that the entire panel is > under compression, top to bottom, before any string load is applied. Not quite sure I follow here. The top of the rib started at 24inches, is now 0,018 longer which means its compressed by 0.128 ? where does the "extra" 0.1 inches come from ? (similiar for top of panel) And why is this "compressed" ? It seems like "longer" and "convex" would mean "Tense". Is there a definition of "compressed" we are missing here ?`Otherwise I am following along ok. > > The panel of the RC assembly has grown from 24" to 24.074" with the 2% EMC > increase. The top surface of the rib, I admit, I haven't figured out exactly > how long this would be, but it will be within a couple of thousandths of an > inch of the CC rib. Assuming a top length of even 24.025, there is still > 0.049" compression in the panel of the RC assembly. > Again.. I cant see where the 24.074 comes from.. and again I am wondering about your use of the word compression. Clearly there is something about your use of the word which is throwing me off. > > So both assemblies now have similar crown, and both panels are under > compression, that's *compression*, before they are installed in the piano, > and before any string load is applied. Aside from this word compression, I follow your reasoning so far. > > Under string load, the CC panel must, all by itself, support the string > load, as well as the additional load of the rib trying to straighten itself > out again. The rib is under negative load in the CC system, and doesn't > assume a positive load to help support string bearing load until the crown > is negative Ok... fair enough. Doesnt quite jive with how I read your first paragraph above when you said.. now glue the 4% EMC panel to the flat rib (compression crowned (CC)), and the 6% EMC panel to the crowned rib (rib crowned (RC)). At this point, the CC panel and rib are at rest, and neither is under any tension, compression, stress, or general discomfort. ...but it makes plenty of sense in itself. > In the RC system, under string load, the ribs are under > positive load from the beginning, supporting the majority of the load, and > the panel is under compression, though much less than in the CC system. The > benefits of the RC system are predictability of crown height on assembly, > predictability of deflection under load, predictability of impedance result, > and the likelihood of a longer dependable service life because of the better > distribution of load among the components. Here I assume the use of the word compression is taken in relation to the load coming from the strings ? This seems like a different context from previoius uses of "compression" and is much easier to relate to from an intuitive standpoint. If my reading of this difference is correct then I have no trouble following what you are saying here. That last statement tho would probably need qualifying and some kinda data to back it up. > > That's the basic outline. While I'm surprised that some CC boards have > lasted as long as they have, that is not an endorsement. I am also disgusted > that so many CC boards in brand new, or under five years old, pianos are > shot. The rib crowned assembly is the only rational way to build a > soundboard, in my opinion, and given the materials and methods we have to > work with. When we switch to synthetic materials, there will be surely be > different options and techniques. > You are a fellow with firm opinions... grin.. thats for sure. I liked this post a lot Ron. (for what my two cents are worth). But I think we need to get past this word "compression" and what is meant at any given time with it, if I (and perhaps the others) are going to be able to fully understand where you are coming from. If you get a chance, re-read my last post. I would like to know why the concave side of a curved piece of wood is "compressed" and not "Tensed". This would get me through your reasoning a lot better. > > Hope this makes sense, I'm late... no time to proof. > > Ron N Thanks for an excellent posting. Richard Brekne I.C.P.T.G. N.P.T.F. Bergen, Norway ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... URL: https://www.moypiano.com/ptg/pianotech.php/attachments/47/af/e6/50/attachment.htm ---------------------- multipart/alternative attachment--
This PTG archive page provided courtesy of Moy Piano Service, LLC