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<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 27, 2003 =
2:23
AM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: Rib Support</DIV>
<DIV> <BR><B><FONT color=#000099>Would you feel the same =
if the SB
and ribs were out of the piano, just sitting on a flat surface, crown
up? If you take the rim out of the picture, then it seems to me =
the ribs
don't support crown, unless they have a strong "spring" action in =
trying to
straighten themselves. Would you agree?</FONT></B></DIV>
<P><FONT size=2></FONT><BR>This is that other analogy that just isnt =
quite
right. To begin with, the ribs are not primarilly trying to straighten =
themselves. Primarilly they are resisting panel expansion. The =
"straighten
themselves" perspective ignores the fact that pressing downwards on =
the panel
increases the panels tendancy to expand... which is exactly the thing =
the ribs
primarilly resist. It is this resistance to the panels expansion that =
causes
crown to begin with. (opposite and equal way of saying its the panels
compression that causes crown). This resistance just doesnt reverse =
itself and
turn into a "flattening aid" simply because panel expansion is caused =
by a
different source then taking on humidity.
<P><FONT color=#0000ff>OK, I think I get it. Let me keep my =
dumb
questions going (apologies to the rest of you). Without the =
ribs, the SB
would stay flat, but increase its diameter, as it takes on humidity =
(more
across the grain than with it? Or the other way round?). =
But
gluing the ribs on first is like attaching your cable: it =
prevents the SB
from getting dimensionally bigger on one side, therefore it has no =
choice but
to bow up. So just thinking about this one effect, the ribs are =
trying
to be stretched along their long axis, like your cable: "tension" =
rather than
"compression" -- right? (There wouldn't seem to be any =
compression here;
where the ribs touch the board, it's the same dimension, more or less, =
as at
low humidity. If you tightened your cable to get crown, then I'd =
see
compression.)</FONT>
<P><FONT color=#0000ff>So if the ribs are being stretched along =
their long
axis by the expanded SB, what happens when a downbearing force is
applied? Especially if it's just sitting on a flat surface, no =
rim, and
the ends of the ribs aren't even touching the flat surface? I =
dunno...
</FONT>
<P><FONT color=#0000ff>--Cy--</FONT></P></BLOCKQUOTE></BODY></HTML>