> > So I will humbly disagree that the rim is not a supporting agent of the > crown for without it the strings would squash more of the crown out as the > rib ends and board edge are moving outward and lifting. Without the Rim the > rib ends and board edge would still woggle around and lift when depressed in > the middle. > The other isuue is that although the bearing is essentially a downward > force it does push out wards and into the rim at least a bit once the board > IS glued in. Since we have an arched system, vaguely similarto the concept of > a flying buttress used in cathedral construction,it is a resistive element. > How much is I think part of the debate here. > I'm not saying the rim does alot to hold a crown from diminishing I'm > just saying it's a supporting agent which aids the boards stiffness. > >>>>>...Just my two cents worth--------Dale Erwin>>>>>>>>>> Dale, And while you're reading up on the physical characteristics of spruce, consider this. The difference in the length of an arc segment described by a chord 800mm long, with a 5mm height (crown) is 0.083mm, or 0.0033". If the rim is pushed out 0.083mm, the crown would fall if it were a buttressed arch. Set up a dial indicator and see what it takes to spread a rim by 0.083mm. While we're at it, consider the leverage pressures. Picture an "arch" consisting of two 400mm long straight sticks each buttressed at one end and meeting each other in the middle (like a gable), so that the sticks are 5mm above the surface at their confluence. Now assume a load roughly corresponding to 4 unisons @ 0.75° bearing, or 25 lb. There will be 1000lb of pressure exerted against each buttress. At 4mm elevation with the same load, there will be 1250lb. At 3mm, 1666lb. At 2mm, 2500lb. In a piano, this would be carried by the side grain compression of a piece of spruce about 8mm thick, and 150mm long, or 1200sq mm (1.86 sg"), the width being from rim to rim. With a short term side grain yield point of 580psi, and not taking into account any panel diaphragmming that diminishes the cross section, or the necessity of having to force flat ribs into the crown as well adding to the string load, and never mind the long term cumulative compression set that takes place at a much lower stress level - how much of the crown would you suppose is being supported by the buttressed arch? Just a couple more cents to add some perspective while you're reading. Incidentally, "The Wood Book: Wood as an Engineering Material" is available on line. You don't even have to buy it anymore. At least not again, since it's a government produced publication and we've already paid for it once. I've apparently lost the URL, but maybe some helpful soul will post it for us again. Ron N
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