Ron Nossaman wrote: >> I think a direct approach will be more conclusive. Make two cross >> grain strips of soundboard panel the same length at the same EMC. dry >> them both down to 4.5%. Glue a rib on to one of them using a flat >> surface. Let both samples reach EMC with the environment, say 40% RH. >> Check the level of crown and compare the length of the strip with the >> rib to the other strip. The difference in length will give you the >> compression. > > > No, it won't. It will give you the difference in length, but that > doesn't equate to the compression PSI. Panel compression isn't linear, > as that same experiment will show you. Load the crowned assembly and > measure deflection with a dial gage. It takes increasingly more load per > unit of deflection as the crown is pushed down. It's a variable rate > spring, as you have noted yourself. O.K. Ron. So you are saying that a cross grain spruce reacts to stress in a non linear way. It resists compression more and more as it is compressed? As I see it, with my limited understanding, it should react to compression forces in a linear and predictable fashion within the elastic range. Check out a stress strain graph to see what I mean. Once above this range - in the plastic range - it should become less stiff and more yielding (with compacting this will have a lesser effect). Do I have this wrong, what you are saying? If you are right we need to rewrite some our text on elastic properties of materials. "Call MIT a piano technician have found that spruce becomes more rigid as more force is applied"! Bowing may what to bring back wooden air ships. It looks like you may be confusing how an object reacts to a force on its own with how the same object reacts if combined with another object. I have said that the soundboard once installed in the case seems to react to bearing force as it it were a stiffening spring. Once you get all these things (panel, ribs, bridge and rim) working together this can happen. But this does not mean that any of the individual components (the panel) have this property in themselves. Considering all of the above, I think that the amount of change in the cross dimension of the panel is a very good way to measure the compression. 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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