---------------------- multipart/alternative attachment > >I wasn't really referring to a compression crowned board. I think >the configuration that I have in mind would be uncrowned or reverse >rib crowned, if that makes sense. I wasn't thinking of a CC board >that had collapsed. I was thinking more of a board that was made >deliberately made to have a reverse crown, such as some of the >pianos that Dale has seen (assuming they were intended to have >reverse crown). So, take an undried panel, glue on some ribs, and >load it up. It will reverse crown and the board will be in tension >I think. Or machine crown some ribs to give the board a reverse >crown when they are glued on. Load it up and once again I think the >board will be in tension. Now if this board is subjected to a drier >environment that that at which is was glued up what will happen? I >was assuming that the board would take on additional tension. >Enough to overload it? I don't know. But if not, then when this >board is subjected to more humid environments than that at which it >was glued up it would probably be less likely to sustain compression >damage than a CC board, or even a rib crowned board with positive >crown. > >Yes, I see what you mean. I don't see how, in anything remotely >resembling average climate conditions, you would generate enough >tension to cause the thing to crack. Assuming the panel was at some >MC in the 7% to 9% range when it was ribbed. > >I still think, however, if the board were taken down to >typical compression-crowning MC levels before ribbing the effect of >the developing stress interface would work to force the board into a >positive crown. I agree with you. So I don't think you could achieve this reverse crowned board with CC methods, unless you put the ribs on top of the board. > >The propensity of any soundboard to develop cracks depends on its MC >when glued to the ribs. If a panel is glued up at 3.8% to 4.0% MC >even in a reverse crown configuration it is going to develop >significant compression as it takes on moisture. How could it avoid >doing so? It is the extremely low initial MC and the act of gluing >the thing to those perpendicular-to-grain ribs that creates the >problem. > >It will be interesting to read about the results of your experiment, >however. When will you be finished? I'll start this right after the experiments to convince myself that there is no arch effect in a soundboard. That should put me into year six on my list of things to do. I'll get back to you by 2010. > > >I was also speculating about what would happen to this board if it >was subjected to a drier or more humid environment. My thought was >that in a drier environment the board would want to move up. My >thought was that the board is describing an arc. As it dried this >arc would have to get shorter, so the board would want to flatten >out, which in this case would mean moving up. You seem to think it >would move down. Why do you think that? > >Good question. You'll have two forces fighting each other, but >they'll not be equal forces. I think the stress interface will win. >At least until the panel self-destructs. Do you think this panel will be any more likely to self destruct than a rib crowned soundboard with positive crown? > >I think the shrinking panel will crack before it creates enough >lifting force via tension to accomplish anything and the stress >interface will pull it down. As well, I think an expanding panel >will work against itself but the stress interface will create the >enough force to push the ribs up no matter how those ribs are >initially crowned. If the ribs are machined with a reverse crown and >glued to a panel with very low MC, the expanding panel will still >want to force the assembly positive. It may not make it but it will >sure try. > >Will this be part of your experiment? As long as I'm going to all the effort I might as well include it. > > > >With a conventional board having crown and with a downbearing load I >think the board is always in compression.... > >>I am uncertain what is meant by "a conventional board." Assuming >>this refers to a compression-crowned soundboard system, the key >>words in the above are then "having crown." However, a strong >>argument can be made that this is no longer--if it ever was--the >>conventional soundboard system.... >> > >I wasn't intending to comment on CC vs RC boards here. By >conventional, I meant a board that was intended to to have a >positive crown, which includes every piano I've ever seen (but not >every piano that Dale's ever seen). I therefore felt safe in >calling this 'conventional'. > >Ah, but that doesn't correlate to the first statement above which >states, "With a conventional board having crown and with a >downbearing load I think the board is always in compression." This >is the comment I was responding to and wondering just what was meant >by "conventional." Over the decades I suspect far more pianos have >been built with either hybrid systems (probably the most common) or >rib-crowned system than have been built using compression-crowned >systems. And both the hybrid and the pure, or nearly pure, rib >crowned board will have crown even without any compression in the >panel. Our panels are ribbed at 6.5% MC. When atmospheric conditions >are such that the panel is at 6.5% MC there is no internal >compression within the panel. Yet there is design crown. Over the >years if these boards are subjected to very high levels of humidity >(and the corresponding high MC) they will also undergo some amount >of compression set. Still, there will be crown when they return to >equilibrium at whatever MC that may end up being. I see the source of the confusion. Poor choice of words. The reason for with crown and downbearing was to distinguish from the case where the board flattened enough to eliminate downbearing, in which case it's not clear to me that there would be compression in the panel, and the case where the board has gone into reverse crown with downbearing, in which case it's also not clear to me that the panel would be in compression. I was making the assumption that any board (whether RC or CC) with positive crown and having downbearing would have compression in the panel. But I can see that this gets back into the arch behavior territory again. I presume your position would be that with positive crown and downbearing on a RC soundboard there would be no compression in the panel because the panel can't support it and all the load is being resisted by rib bending. > > > > I was pondering a what if scenario. What if you built a board that >was intended to have a reverse crown. How would it behave? Would >it have any advantages over a 'conventional' board that is intended >to have positive crown? > > >>As well, it is quite possible to have a string downbearing load >>without having positive crown. >> > > >Yes, I know. But in this case it would be a deliberate design >feature rather than an unintended consequence. > >Well, there is always the Rippen--the only modern piano I know of >built deliberately to have reverse crown. Actually, the (laminated) >boards were ribbed flat and, when loaded with string bearing, were >forced to a reverse crown. They worked. At least within the >limitations of their less than desirable scaling. As to whether they >might have worked better with some form of positive crown, I have no >idea. Nor do I have any idea how they would have worked had their >panels been made of solid stock. > >Del Perhaps I can explore that in my experiment as well. Phil Ford ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... URL: https://www.moypiano.com/ptg/pianotech.php/attachments/11/d1/fe/e7/attachment.htm ---------------------- multipart/alternative attachment--
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