<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META http-equiv=Content-Type content="text/html; = charset=iso-8859-1"> <META content="MSHTML 6.00.2800.1264" name=GENERATOR> <STYLE></STYLE> </HEAD> <BODY bgColor=#ffffff> <DIV><FONT face=Arial color=#800000>Hello Richard. I know some have = express some disenchantment with your persistant theoretical explorations of = soundboard crown, but I am rather enjoying it. It's a good mental excercise. And I = also understand that even though you may well never build a soundboard, it is = still a great thing to try and understand how they might work. There are not = very many authoritative references available to find answers to these questions.</FONT></DIV> <DIV><FONT face=Arial color=#800000></FONT> </DIV> <DIV><FONT face=Arial color=#800000>So with that in mind, let's = proceed to do the best job we can leading each other into an advanced state of = total confusion............ </FONT></DIV> <DIV><FONT face=Arial color=#800000></FONT> </DIV> <DIV><FONT face=Arial color=#800000>Comments interspersed = below:</FONT></DIV> <DIV><FONT face=Arial color=#800000></FONT> </DIV> <DIV><FONT face=Arial color=#800000>Terry Farrell</FONT></DIV> <BLOCKQUOTE dir=ltr style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; = BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px"> <DIV style="FONT: 10pt arial">----- Original Message ----- </DIV> <DIV style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: = black"><B>From:</B> <A title=Richard.Brekne@grieg.uib.no href="mailto:Richard.Brekne@grieg.uib.no">Richard Brekne</A> </DIV> <DIV style="FONT: 10pt arial"><B>To:</B> <A = title=pianotech@ptg.org href="mailto:pianotech@ptg.org">Newtonburg</A> </DIV> <DIV style="FONT: 10pt arial"><B>Sent:</B> Sunday, October 26, 2003 = 5:47 AM</DIV> <DIV style="FONT: 10pt arial"><B>Subject:</B> Rib Support</DIV> <DIV><BR></DIV>Hi List <P>I wanted to try and better explain why I am having difficulty in = accepting that the ribs in a CC board do not support crown against down bearing. = Please follow the following reasoning and diagrams and you will see where I = am stumped. <P>First is a panel that is simply bent across the grain and held into = place with a cable appropriately attached on the ends of the panel. = Obviously, when down bearing is applied, the outward pressure on the cable is = increased. This entire situation is going to be the same if one first dries out the soundboard, then securing that same cable tightly, let the panel take = on enough MC to form the same crown. <P><IMG height=305 = src="cid:00dd01c39bcd$e83f9d70$b9b52341@youro0kwkw9jwc" width=482> <BR>  <P>Replacing the cable with a piece of wood doesn't really change a = whole lot conceptually.  The rib will  be under the same amount of = tension as the cable was, though the amount of physical strain may differ.  = Pressing down on the panel is definitely going to be resisted by both the panel = and the rib. If the assembly is only resting on supports at the edge, down = bearing might even result in some downwards bending of the rib.. but I will = have to try this to see. Still, the panels edge will <<bend>> in a = downwards direction, and there might be some reaction in similar = direction by the ribs. In any case, the assembly as a whole will resist any down = bearing for the same basic reasons as above. <P><IMG height=174 = src="cid:00de01c39bcd$e83f9d70$b9b52341@youro0kwkw9jwc" width=472> <BR>  <BR>  <P>Now if the rib is secured to the panel such that it bends a bit in = the same direction as the panel, (i.e. glue say about 10 inches in on each = side)  then the only thing that has really changed is that the rib bends = upwards a bit... as in the below drawing. Yet the same basic condition exists. = Downwards pressure in the panel increases the outwards pressure, yet this is = resisted just in the same way as in the previous two examples, so in response = both panel and rib will feel an increased stress and will strain against = that stress until one of the pieces fail, which of course will be the = panel. <P><FONT face=Arial color=#800000>Keep in mind here that stress = and stain, although related, are two very different things. Stress is a force = that is applied to an object - or an object's ability to resist deformation. = Stain is the deformation. With the soundboard, I suppose stress is delivered to = both panel and rib, but strain is negligible for the rib, and orders of = magnitude larger for the panel. I've searched for numbers that describe this relationship, but have found none.</FONT> <P>But until that point is reached, the entire system will resist down = bearing vigorously. No doubt the rib will straighten at some point, and the = relative tension/compression on the top and bottom sides will equalize, but it = will have to experience an increase in overall tension for this to happen, = which it will resist. The panel will also straight out at somepoint... but = nobody believes for one second that it wont be under stress when its flat.. = So why should the rib be any different ? Opposite side of the same coin. Both = will physically strain against the stress, the panel a good deal more then = the ribs, but the stress levels are the same on both. <P><IMG height=319 = src="cid:00df01c39bcd$e83f9d70$b9b52341@youro0kwkw9jwc" width=506> <P>The only thing that remains is to simply glue the entire rib to the = soundboard, but I cant see that that changes anything. If the above = three steps hold, then it seems in error to say that the only thing that = supports crown in a CC board is compression in the panel. The entire system, = panel and ribs alike will strain against down bearing. <P>Ok... so this seems to make perfect sense to me, and though ribs = are not acting as beams in their support, they can neither be said to be = purely fighting crown. Quite the opposite. The fact that the rib is bent in = the process, and of course resists this bending can be said about the = soundboard as well. But I dont see that simple bending of either describes their condition adequately. <BR>  <BR>  <P>Cheers <BR>RicB <P>-- <BR>Richard Brekne <BR>RPT, N.P.T.F. <BR>UiB, Bergen, Norway = <BR><A href="mailto:rbrekne@broadpark.no">mailto:rbrekne@broadpark.no</A> = <BR><A = href="http://home.broadpark.no/~rbrekne/ricmain.html">http://home.broad= park.no/~rbrekne/ricmain.html</A> <BR><A = href="http://www.hf.uib.no/grieg/personer/cv_RB.html">http://www.hf.uib= .no/grieg/personer/cv_RB.html</A> <BR>  </P></BLOCKQUOTE></BODY></HTML>