<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META content="text/html; charset=windows-1252" = http-equiv=Content-Type><!DOCTYPE HTML PUBLIC "-//W3C//DTD W3 = HTML//EN"><!doctype html public "-//w3c//dtd html 4.0 transitional//en"> <META content="MSHTML 5.00.2314.1000" name=GENERATOR></HEAD> <BODY aLink=#ff0000 bgColor=#ffffff link=#0000ee style="BACKGROUND-COLOR: #ffffff; COLOR: #000000; FONT-FAMILY: Times = New Roman; FONT-SIZE: x-small" text=#000000 vLink=#551a8b> <DIV><FONT face=Arial>David,</FONT></DIV> <DIV><FONT face=Arial></FONT> </DIV> <DIV><FONT face=Arial>Bravo!  Well said!  Yeah for = simplicity!  Ron, Brian T., Delwin, didn't he summed it up in laymen's terms?  </FONT></DIV> <DIV><FONT face=Arial></FONT> </DIV> <DIV><FONT face=Arial>Frank, I think David nailed it.  Unlike = your yardstick analogy, no one is grabbing the ends of the soundboard and artificially "forcing" an arch.  In a compression crown, the arch = is created by internal forces (increased water content) pressing outward = against the cell walls (COMPRESSION).  The rib is simply limiting the = GROWTH of the underside at a greater rate than the growth on the topside.  Even = though ALL of the cells are GROWING, the arch is created only because the top = of the soundboard is GROWING with less resistance because the rib is on the bottom.</FONT></DIV> <DIV> </DIV> <DIV><FONT face=Arial>Did I get it right?</FONT></DIV> <DIV> </DIV> <DIV><FONT face=Arial>As to rib-crowning.  Don't think of it as = the rib "forcing" the board to arch as in Frank's yardstick analogy.  Rib = crowned boards are still dried before ribs are glued on right?  Just not as = much.  If this is the case, once humidity is re-introduced to the = cells, then think of  tapered rib as "following" the board, and not = "fighting" it like a straight rib does.</FONT></DIV> <DIV> </DIV> <DIV><FONT face=Arial>I'm not an expert either.  To quote you, = David, "I'm learning, not teaching."  Hopefully though, I've figured this = out finally.</FONT></DIV> <DIV> </DIV> <DIV><FONT face=Arial>Cheers,</FONT></DIV> <DIV><FONT face=Arial>Brian Henselman,RPT</FONT></DIV> <BLOCKQUOTE style="BORDER-LEFT: #000000 2px solid; MARGIN-LEFT: 5px; MARGIN-RIGHT: = 0px; PADDING-LEFT: 5px"> <DIV><FONT face=Arial size=2><B>-----Original = Message-----</B><BR><B>From: </B>David M. Porritt <<A = href="mailto:dporritt@post.cis.smu.edu">dporritt@post.cis.smu.edu</A>&g= t;<BR><B>To: </B><A href="mailto:pianotech@ptg.org">pianotech@ptg.org</A> <<A = href="mailto:pianotech@ptg.org">pianotech@ptg.org</A>><BR><B>Date: </B>Monday, September 27, 1999 11:33 AM<BR><B>Subject: </B>Re: = evaluating sdbd. crown & bridge downbearings in a new = piano<BR><BR></DIV></FONT> <DIV><FONT color=#000000><FONT size=3>Frank:</FONT></FONT></DIV> <DIV><FONT color=#000000><FONT size=3></FONT></FONT><FONT size=3></FONT> </DIV> <DIV><FONT size=3>I have not participated in this discussion simply = because it is over my head.  I'm learning, not teaching.  </FONT></DIV> <DIV><FONT size=3></FONT> </DIV> <DIV><FONT size=3>However, if you think if compression as the state = where something is smaller than it wants to be, and tension as something = being larger than it wants to be, that can simplify the engineering definitions.  If a traditionally crowned board is held smaller = than it wants to be, it is under compression.  On this board that is = constrained by the rib, the bottom is smaller than it wants to be because of the rib.  The top is smaller than it wants to be because of the = bottom - and the fact that it is one piece, this panel.  If the top, although = it is larger than the bottom, it's smaller than it wants to be, so it is = under compression.</FONT></DIV> <DIV><FONT size=3></FONT> </DIV> <DIV><FONT size=3>Does this make sense???</FONT></DIV> <DIV><FONT size=3></FONT> </DIV> <DIV><FONT size=3>dave<BR></FONT><FONT face=Arial = size=2>*********** REPLY SEPARATOR ***********<BR><BR>On 9/27/99 at 12:01 PM Frank Weston wrote:</FONT></DIV> <BLOCKQUOTE style="BORDER-LEFT: #000000 2px solid; MARGIN-LEFT: 5px; = PADDING-LEFT: 5px"> <DIV><FONT color=#000000 size=2>Richard,</FONT></DIV> <DIV><FONT color=#000000 size=2></FONT> </DIV> <DIV><FONT size=2>Forget humidity.  Forget made up = numbers.  Do this.  Tape a flexible steel rule onto the end of one side of a = spruce stick  (a wooden yard stick will do for demo purposes).  = Mark the opposite end of the steel rule on the spruce stick.  Bend the = spruce stick.  Notice that as you bend it, the mark on the spruce = changes position relative to the steel rule.  If the rule is on the = outside of the bend, the spruce gets longer.  If it is on the inside of = the bend, the spruce gets shorter.</FONT></DIV> <DIV><FONT size=2></FONT> </DIV> <DIV><FONT size=2>If the spruce is elongated it is in tension, if = shortened, in compression.  Now, bend the spruce stick and glue another = piece on the bottom.  Nothing has changed.  When the glue dries, = the assembly will hold a curve.  The outside curve of the spruce is = still longer than it was originally, and it is still in = tension.</FONT></DIV> <DIV><FONT size=2></FONT> </DIV> <DIV><FONT size=2>I hope this is simple enough for = you.</FONT></DIV> <DIV><FONT size=2></FONT> </DIV> <DIV><FONT size=2>Frank Weston  </FONT></DIV> <DIV><FONT color=#000000 size=2></FONT> </DIV> <DIV> </DIV> <BLOCKQUOTE style="BORDER-LEFT: #000000 2px solid; MARGIN-LEFT: 5px; = PADDING-LEFT: 5px"> <P>Lets say we have a panel exactly 1 meter across grain, and a = rib that is 998 cm long in a room with 50% relative humidity. The panel is = put into the oven and dried to the point that it is also 998 mm long. Now = if we take the panel out and simply let it re-adjust to the room = humidity it will "grow" back to 1 meter in length. The top and the bottom of = the panel will both shrink the same, and grow back the same amounts... ok so = far ??  But if we quickly attach the ribs while the panel is at = 998 and then allow it to re-grow to what ever length the room humidity and = this constraint from the rib allows for, its length will be less the 1 = meter. (This following what I think I got from Dels description.) Ok.. = assuming this is a correct picture so far, stretching a measuring line = across the <I>top</I> of this  ribbed panel after its re-adjusted to = room humidity shows that it measures less then 1 meter. <P>If, (Frank) you accept that this is correct so far,  I = would appreciate it very much to know the reasoning behind why this top = half of the panel is not (if it is not) to be considered in compression. = As initially stated, please follow Dels kind "laymans language" = approach to demonstrating the reasoning. <P>Richard Brekne <BR>I.C.P.T.G. N.P.T.F. <BR>Bergen, Norway </P></BLOCKQUOTE><FONT size=2 = Arial></BLOCKQUOTE></FONT><PRE>_____________________________ David M. Porritt dporritt@swbell.net Meadows School of the Arts Southern Methodist University Dallas, TX 75275 _____________________________</PRE></BLOCKQUOTE></BODY></HTML>