Ron, I know I shouldn't even try to unravel this but I will try. Ron Nossaman wrote: > >> If you cut a rib from a Panel crowned soundboard (parallel to the >> rib but half way between the adjacent ribs) you have a laminate made >> from a rib and a strip of cross grain spruce. The two pieces of wood >> form their crown just the same as the above laminated rib. Once formed >> this structure is going to behave just the same as if the crown were >> formed from a crowned rib. > > > This doesn't equate in the real world with actual wood. Wood is > anisotropic, and compresses considerably more across the grain than it > does along the grain. It takes considerably more dimensional change in a > cross grain spruce panel to generate the cross grain compression levels > it takes to both bend the rib (which, itself is not made of multiple > layers formed and glued, but is one solid piece that resists bending far > more than a stack of laminations of the same overall depth), and support > the bearing load. Yes I know about anisotropic properties of wood I don't need a lesson on this from you. I was trying to find an analogy that would be familiar. My point is that once the soundboard is assembled the ribs attached to a Panel Crowned (PC) board contribute to the stiffness of the structure. If this were not true then a PC board would not have any more resistance to deflection than a panel without ribs. I have handled many soundboards some of which were built with the PC method. When the ribs are attached the whole structure is noticeably stiffer. So were does this stiffness come from if not from the ribs? >> Furthermore, the idea that the small additional compression of the panel >> due to bearing will stress the panel of a PC soundboard more than a RC >> board is just not right. > > > Additional compression? You mean in addition to the compression > necessary to force crown in the ribs before the string bearing is even > applied. How then, are the ribs supporting most of the bearing load in a > CC board when the panel is already supporting the ribs? That's the > question I'm addressing. I am just going on the evidence I see. When you apply down bearing the board moves down about 3mm. You can compute how much more (in addition to the compression of crowning) compression the panel is holding by figuring how much this straightening of the crown compresses the panel. If you do this you will find that bearing does not compress the panel to any significant degree. This is true for PC and RC boards alike. On that 48" rib you mentioned, the cross grain > dimension of a panel that is 48" at 4.5%MC, will be approximately 48.5" > at 12%MC. It takes half an inch of compression in a 48" panel just to > form the crown in a flat rib. That's a bit over 1% of the total width. > Immediate and permanent compression set occurs at anything over 1%, > according to the literature. While I believe that any board made this way is at risk to long term deterioration the PC method as used at the current Steinway factory does not compress the panel to the point of damage (above the elastic limit). From my own experience with examining boards I have pulled from relatively new (under 20 years) Steinways you do see most of the crown intact (Steinway does not use a lot of crown). My crude attempts to find how much compression is involved indicates that about 1/3 the elastic limit is reached when a PC board is crowned and reaches 6.5% moisture content. This is about 200 psi to create the crown. Yes this is a concern and I would not build a board this way but the panel is not at the point were it can't hold the crown with even some leeway for increased moisture (it doesn't begin to get damaged until its above 11%EMC and even then in can still support the load). As you know cross grain spruce can withstand about 580 psi before it starts to fail. The 1/3 figure I found corresponds well to what we know about the moisture levels used to rib a board with the PC method in comparison to the moisture level used with the RC method. It is a difference of 2 percent which if viewed as the stress in a constrained soundboard panel comes out to be a little less than 200psi. If you have experiments or data to contrast with this I would like to see them. What does bearing add? Typically more than > what it takes for ribs of that dimension (no panel) to be deflected flat > if they had been machine crowned. So the bearing adds at least as much > load as it took to bend the ribs. No I don't think bearing adds much compression to the panel for the reasons I went into above. Little stress on the panel, and the > ribs carrying the majority of the bearing load? I think not. And when > you force that panel flat, so the rib is again straight, the rib is then > under no stress at all except at the top edge. What is it that is > pushing back on the force it takes to push it flat? It's the panel > compression. So please explain how the ribs are supporting most of the > bearing, with the panel supporting very little. The best way I can find to look at this is to examine the evidence. We have a PC board with ribs that are shallower than what is necessary to hold the bearing. We put the strings on and they push the board down. We find that the soundboard has not collapsed and holds the bearing load more than we predicted looking at the ribs. We look at the panel and find that it has not been significantly compressed from the bearing. We recheck the deflection of the ribs and find that they are not holding all of the load ether. Jumping to conclusions we think the panel must be holding it - this is sloppy thinking and flies in the face of the evidence. I think that the model that a few have been using to explain how the soundboard works is, well wrong. We have been fed this idea that the ribs are like floor joists and the panel is like the floor boards. The ribs hold all the weight and the panel just goes alonge for the ride. May I propose an alternate model? The ribs and the panel and the bridge form a single crowned unit that has pretty much the same stiffness along the ribs as it does along the grain of the panel. It is like a crowned diaphragm. When the soundboard is attached to the rim all of the components form one structure well suited to support bearing. If you want to have a complete picture of how bearing is supported you must consider the contribution of the rim. As the diaphragm is pushed down the rim holds it in and provides the added stiffness to resist bearing. This theory at least makes some attempt to show how crown can improve soundboard performance (again something we know from experience). With the floor joist model all that can be said for crown is that it compresses the panel. I say So What! compressing the panel can do nothing to improve tone. The soundbaord we inherited has evolved to have equal stiffness both along the ribs and along the grain of the panel. This makes sense because it assures that the vibration modes of the soundboard will be fairly circular in shape to best utilize the soundboard area. If you start to design the ribs to be more stiff than the panel the vibrating areas (modes) of the soundboard will begin to elongate in the direction of the ribs. At some point if you continue to design the ribs to support all the bearing load you will make a soundboard less able to project tone. The board will break up into smaller less efficient vibrating areas and this defeats the whole purpose of supporting bearing in the first place. 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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