>All, > >Back in 1900, if the patents-office examiner had been paying attention, >Richard Gertz never would have got his patent. The sound board of a real >world piano does not push outward as downbearing is applied. While the >sound board shown in the piano diagram at Mason and Hamlin's website >(http://www.masonhamlin.com/crown/crown.html) might push outwards, real >pianos do not have the sound board ribs offset higher than the cord-line >as shown in their image (a jpeg conversion of the M&H website image is >shown below). Ron, Would you explain what you mean by the cord line? >... >So yes, a substantial back beam configuration is a good thing, but the >Richard Gertz 'crown retention system' will be of virtually zero assistance. > >Ron O. Unless it works in both directions, in which case it's essentially doing the same job as the back beams. It's not clear that it does from the diagrams on the M-H site (which diagrams are hard to read, BTW, since they chose to use red lettering on a black background). I don't know if those long bolts are a press fit in the holes in the rim or not. There certainly seems to be a gap between the bolt head and the outer rim which would seem at first glance to allow the bolt to move outward, offering no resistance to the inward movement of the rim. On older M-H pianos the rods were attached to tapered plugs in the inner rim which were captive by the outer rim. You can see drawings of this on the patent, No. 783,781. I believe that this assembly would form quite a rigid frame. Those rods may look insubstantial next to the large softwood beams beneath them, but because they are steel, each of them provides the stiffness of a large softwood beam. Assuming a 20 mm diameter rod and a modulus for softwood of 10 GPa and steel of 200 GPa, then each rod is equivalent to a softwood beam of 63 cm^2. Bump the rod up to 25 mm and it's equivalent to a softwood beam of 98 cm^2 (15.2 in^2 - roughly a 2.5 in. x 6 in. beam), which I would consider a fairly massive beam. It's my feeling that if this device were properly designed, then the wooden frame members would be superfluous. A property of the steel is that it is consistent in strength and stiffness, which is not the case with the softwood (for those that value consistency from piano to piano). A downside may be that this type of frame would be more expensive and heavier than the softwood frame, but I wouldn't think significantly so. Also, the centerline or line of action of the 'spider' could be closer to the centerline of the soundboard, since the spider needs less physical space. On the other hand, because it has less vertical depth, it's probably not very good at resisting twisting of the rim, the way a 15 cm deep beam would be. Looking at the M-H page that you mentioned I also noticed a couple of other things that I hadn't noticed before. They're expecting not only the rim to act against the soundboard panel to resist board flattening, but the rim acting on the end grain of the ribs to resist flattening of the board. They also make a point of mentioning that the grain of the ribs is perpendicular to the grain of the rim. I'm not sure why they think that's important, and I'm not sure how that can be the case unless the ribs were at 90 degrees to the rim at the points where they intersect the rim, which is almost never the case. They also seem to think that white spruce is stronger than Sitka spruce. Phil Ford Phillip Ford Piano Service and Restoration San Francisco, CA
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