Ron Nossaman wrote: If a soundboard is made with crowned ribs > that by themselves aren't stiff enough to support string bearing and > still retain crown, and the panel wasn't dried down enough to supply > bearing support when it rehydrates, then not much of anything is > supporting crown against string bearing load. If you are going to have > crown remaining under bearing load, it has to come from either the ribs, > panel expansion, or both. If the math says the ribs aren't doing it, it > pretty much has to be coming from the panel. > > For instance, Yamahas are rib crowned, but an analysis of the bearing > load the ribs are carrying will show that the ribs can't support the > load imposed on them without the panel compression carrying a large > percentage of the load. Bending force in the panel cross grain is NEVER > significant to crown to my knowledge. That's why I make the distinction > between rib crowned, and rib crowned and supported. Ron, I see where you are going with this but here is a little problem for you to solve. When string bearing is applied to a soundboard a portion of the crown is lost supporting the load. This is usually about 2.5 to 3.5 mm. It would be very simple to figure out how much load the panel supports. Take a measurement of the cross grain distance along a rib before you load the board. Then apply the down bearing pressure and remeasure the panel. The difference is how much the panel is compressed. You then need to figure this as a proportion of the total length. If you do this you will see that the panel is supporting very little of the bearing even on a CC board. If you look at how changes in moisture content compress the panel you will see that this is a lot more of a stress on the panel that down bearing. You can use a similar method to find how much the ribs are bearing; just look at how much they bend under the load. I am sure you will find that the ribs are holding much more than the panel. You will be perplex to find that both together do not equal the total load of bearing. So what else is supporting the board? I think you will find that how the crown is created will not make a lot of difference to how much support the panel gives to the down bearing. 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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