Ron Nossaman wrote: > > Of course it's not true, and I said no such thing. A compression crowned > assembly's panel is already supporting whatever load is required to bend > the ribs from straight to crowned before a gram of string bearing load > is ever applied. Therefore a panel in a panel crowned board under string > bearing load is supplying over 100% of the spring resistance necessary > to provide what we'd call stiffness in the assembly. That's not a > theory, that's a fact. The bent rib is a built in pre-load, supplying > negative lift - it's trying to pull the panel flat - but the resulting > stiffness comes entirely from the panel compression. The rib provides no > positive spring resistance (to string load) of it's own until the board > goes concave - often soon after stringing. If the ribs provides no "positive spring resistance" then the panel receives no support from the ribs and it would be no stiffer by having the ribs attached. Yet this contradicts hands on experience. A panel crowned soundboard is stiffer than just the panel itself. There is added stiffness so it comes from somewhere. That somewhere has to be the ribs. > >> Using your theory we can draw all sorts of faulty conclusions. > > > Apparently. Why don't you address the stiffness of laminated ribs? A panel crowned sound board is essentially a cold formed glued curved laminate. The only difference is that one of the laminates is cross grain. Otherwise a panel crowned soundboard works on the same principles as any other cold formed glued laminate. These structures show the same physical properties as wood structures made from solid wood. By this I mean they both have the same resistance to load or stiffness. >> I am going to go out on limb here and predict that from a stiffness >> point of view it doesn't much matter whether the crown is formed by >> the shape of the ribs or whether the crown is formed by an expanding >> panel. The main factor when it comes to stiffness is the cross section >> and length of the ribs. > > > You know better than that. Take that 5"x30" piece of panel, and two more > just like it. Dry two down to 4%MC, and leave the other at equilibrium > in the shop. Glue a flat 20mm wide x 20mm tall rib on the back of the > dried down strip, and a 20mm wide x 20mm tall crowned strip on the strip > at shop EMC, and another crowned rib just like it to the last panel > dried to 4%MC. Let them all reach the shop EMC. All three assemblies > have equal length ribs of identical cross sections. Do you really > believe they'll be of identical, or even remotely similar stiffness? > Note that, as with a soundboard assembly in a piano, we're talking about > stiffness in only one direction. Of the three, the rib crowned assembly > with the non-compressed panel will be the least stiff, the compression > crowned with straight rib will be stiffer, and the combination > compression and rib crowned assembly will be stiffest. > O.K. I will say it again. If two or more soundboards have the same dimensioned ribs and the same panel thickness and THE SAME CROWN at the same EMC. Then the two Soundbaords assemblies will have the same stiffness even if the method of crowning is not the same. >> There are many advantages to rib crowned soundboards (God knows we've >> heard plenty) > > > There sure are, and I think I recall hearing some from you. Don't you > redesign Steinway soundboards with crowned ribs when you replace them? And your point being what? >> but the kind of things reported above just muddies the water with >> absurd claims. You make it sound like a panel crowned soundboard will >> be as flimsy as an un-ribbed panel. > > > The claim is an observation, and is quite valid, though it may be > interpreted unreasonably. Maybe valid to your experience, not to mine. Are you calling me unreasonable. You got to do better than that. John Hartman RPT John Hartman Pianos [link redacted at request of site owner - Jul 25, 2015] Rebuilding Steinway and Mason & Hamlin Grand Pianos Since 1979 Piano Technicians Journal Journal Illustrator/Contributing Editor [link redacted at request of site owner - Jul 25, 2015]
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