<!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 6.00.2800.1226" name=GENERATOR></HEAD> <BODY bgColor=#ffffff> <DIV><FONT face=Arial color=#008080>Comments below - I need some = clarification to address these issues.</FONT></DIV> <DIV><FONT face=Arial color=#008080></FONT> </DIV> <DIV><FONT face=Arial color=#008080>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">Pianotech</A> </DIV> <DIV style="FONT: 10pt arial"><B>Sent:</B> Monday, October 06, 2003 = 9:42 AM</DIV> <DIV style="FONT: 10pt arial"><B>Subject:</B> Re: RC vs CC = again</DIV> <DIV><BR></DIV>Terry... et al <P>I wanted to get back to you on this with a better explaintion of = what bothers me about this position you take below. <P>Consider the following. <P>When ever a board is bent the, convex side will be stretched = relative to the concave side. <P><FONT face=Arial color=#008080>Correct - I'm assuming we are = talking about one solid board here - not a soundboard - but rather, let's say, a = one-inch by one-inch by three-foot solid piece of wood.</FONT> <P> This is the same regardless of the over all compression level = found in the board. <P><FONT face=Arial color=#008080>I think so, not quite sure what = you mean by the "over all compression level found in the board."</FONT> <P>  Pressing downwards on this curved board will tend to have = the opposite effect... ie stretch the concave side and compress the convex = side. <P><FONT face=Arial color=#008080>If the board is curved and it is = the same board - presumably had originally been straight - then some force is = holding it in its curved configuration. What is that force? Is it trapped at = the ends? Is it glued to a panel that has expanded with increased moisture = content? But yes, if you have this curved board the net change in tension or = compression will be to stretch the concave side (decrease compression - however, = being that it was straight to begin with, it was under some amount of = compression in its curved state, so when you press down that state of compression = lessened - but it is still compressed relative to its state when straight). Same = deal with the convex side.</FONT> <P>Ribs glued to the concave side will fight any tendency of the panel = to stretch on its concave side. <P><FONT face=Arial color=#008080>I'm not sure exactly what is = being described here. "Ribs glued to the concave side" of what? What kind of = ribs?</FONT> <P> Thats what causes them to bend in the process of crown = formation in  the first place. Downbearing applied to the convex side of = the panel will tend to compress the convex side, and stretch the concave side... = but the ribs resist stretching of the concave side. <P><FONT face=Arial color=#008080>I don't think it is correct to = say that the "ribs resist stretching of the concave side." If I am understanding = you correctly you are describing a CC crowned soundboard above. In that = case, the concave side of the bent rib is in a state of compression and is = trying to expand on its own to straighten back out. So as stated above, when you = apply downbearing, yes, the concave side of the rib stretches, but only with = reference to its curved compressed state - it doesn't stretch relative = to when it was straight. I think it is more appropriate and more clear to = state that when downbearing is applied, the concave side of the rib experiences a = reduction of its compressed state.</FONT> <P>So how does that equate to <BLOCKQUOTE> <DIV><FONT face=Arial>"As the crown lessens under increasing load, = the ribs will pull downward a bit less, but only because they are being bent = upwards less by the panel."</FONT>? </DIV> <DIV> </DIV> <DIV><FONT face=Arial color=#008080>See = above.</FONT></DIV></BLOCKQUOTE> <P>Seems to me the ribs will be even more stressed by down = bearing...erg they support crown against applied downbearing. <P>Yes ??? <P><FONT face=Arial color=#008080>To the best of my knowledge, and = I think I am thinking correct physics here, no. In their bent state, the = originally straight ribs are always trying to straighten out, i.e. pull the panel = down, but the panel's physical properties are able to resist some = compression and not allow the rib to flatten. When you apply downbearing, the rib = becomes less stressed because it is closer to a relaxed, natural, state. The panel experiences increased compression with increased downbearing.</FONT> <P>RicB <P>Farrell wrote: <BLOCKQUOTE TYPE="CITE"> <STYLE></STYLE> <FONT face=Arial>Richard wrote:</FONT> <FONT = face=Arial>"This assertion that ribs don't support the bearing load also is bothersome. = Why</FONT> <BR><FONT face=Arial>wouldn't the ribs in a CC board support both = crown and bearing... just because the</FONT> <BR><FONT face=Arial>panels = compression forces the ribs to bend?"</FONT>  <FONT face=Arial>When = flat ribs are glued to a very dry flat panel, no force vectors exist in the = system. As the panel takes on moisture and the ribs bend, the ribs will = <I>always</I> be trying to pull the panel flat. Period. If that assembly has = crown, the ribs will <I>always</I> try to go straight by pulling down on the = panel. The compression forces in the panel are the only thing that has provided = the force necessary to bend the rib into a crown. Whether or not you = ever load that soundboard on top, the ribs will always be fighting the panel compression to go flat. When you do apply a load to the top of the soundboard, the compression on the panel will increase, but the ribs = will do nothing other than still try to go flat. As the crown lessens under increasing load, the ribs will pull downward a bit less, but only = because they are being bent upwards less by the panel.</FONT> <FONT face=Arial></FONT>  <FONT face=Arial>Terry Farrell</FONT> </BLOCKQUOTE> <P><BR>-- <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>