At 01:07 PM 9/21/99 -0400, you wrote: >OK engineers: > >The soundboard assembly must withstand a substantial amount of downbearing >force, no matter how it is constructed. A so called "compression" crowned >soundboard in an unloaded state will have the fibers in the top half of the >board subjected to tension and those in the bottom subjected to compression >(it's the rule, you got to have both). * Ok engineer. A spruce panel that is 24" wide at 4% EMC will grow to 24.14" at 8% EMC. Glued to an infinitely flexible rib 1" tall, it would bow to approximately a 101" radius curve. With a real world spruce rib, it will bow to a (roughly) 600 - 700" radius curve. That's in an unloaded condition. The resistance of the ribs to bending is enough to compress the entire panel, top included, before string load is applied. If not, where did the extra panel width go? > As downbearing load is applied, the >tension in the top half of the board will be decreased, as will the >compression in the bottom half. This decrease in internal stress in the >board assembly is what provides the resistance to downbearing. * As load is applied, the compression throughout the panel will be increased, more at the top than at the bottom. The leverage from this compression is what provides the resistance to downbearing. >A "rib" crowned soundboard assembly will have little to no internal stress >in the unloaded state. It is essentially built to shape. As a "rib" >crowned soundboard assembly is subjected to downbearing forces, the top half >of the board will be subject to a compression load, and the bottom half will >be subjected to a tension load. The absolute magnitude of these loads (all >other things being equal) will be exactly the same as those removed from a >"compression" crowned soundboard when it is subjected to the same >downbearing. * The panel in a rib crowned board is also entirely under compression, only somewhat less. The rib is under similar stress to the compression crowned board (countering the panel's bending leverage), only less so. The forces are less because the panel is typically not dried down as much in a rib crowned assembly. >In theory, if a "compression" crowned soundboard is loaded to a perfectly >flat shape, it will have no internal stress. * The rib will be (cumulatively) at rest, but the panel will be under extreme compression. >On the other hand, a "rib" >crowned soundboard under downbearing will have all of the internal stress >the "compression" crowned board did have before it was loaded. * The rib will be bent to force the top surface flat, and the panel will be under extreme, though not as much as the CC system, compression. > So, the >"rib" crowned soundboard, in its working configuration will be under much >more stress than the "compression" crowned soundboard. Maybe those >traditional builders who use compression crowning really know what they're >doing after all. * The total force required to support string bearing load is the same with either crowning system. The point in a rib crowned system is to have the ribs help hold up the string load, instead of adding their own load to the already over stressed panel. It's a more equitable distribution of abuse. >What causes "compression" crowned soundboards to fail is poor selection of >materials, improper control of humidity during assembly, poor assembly >techniques and, finally, cyclic loads over time. There is nothing wrong >with the concept that can be improved upon by "rib" or even "hybrid" >crowning. I know I'm pissing into the wind here, but there are some pretty >successful manufacturers who agree with me. > >Frank Weston * Regardless of the wood selection process, or the care taken in the assembly, the fact is that the design of a compression crowned soundboard puts a compressive force on the panel that is very near, or exceeds the fiber stress proportional limit of the spruce. Try to compute the expansive force per linear inch across the top of a 1"x1"x48" and see what kind of psi load it would put on a 3/8" panel to both bend the rib and lift another twenty pounds or so. If your arrived at figure is anywhere near 580 psi, the panel is in trouble. Early failure is designed into a compression crowned board. The surprise is that some last as long, and perform as well as they do. Ron N
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