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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> August 18, 2003 3:02 =
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: More on soundboard =
crown</DIV>
<DIV><BR></DIV>
<P>Now correct me if I am wrong.. but I understand this to mean that a =
CC
board will be stiffer, both unstrung... and increasingly so when =
downbearing
is applied. Assuming that is correct,
<BLOCKQUOTE TYPE="CITE">
<DIV dir=ltr><FONT face="Comic Sans MS"><FONT =
color=#000080>Which it is
not.</FONT></FONT></DIV><BR> </BLOCKQUOTE>
<P><FONT face="Comic Sans MS" color=#000080></FONT><BR>Please =
refer again to
the statement made by Ron
<BLOCKQUOTE>"Given a panel crowned assembly with a severely =
dried
panel, and one rib crowned with a non dried panel, both to identical =
crowns at room EMC, the one with the higher panel compression =
level
will have the steeper spring rate gradient, and will be
stiffer."</BLOCKQUOTE>Under the qualifiers that we are talking about =
boards
with same amounts of crown at the same RH. If then I've misunderstood =
the
above quote... then it would be nice to hear what was really meant =
here.
</BLOCKQUOTE>
<DIV dir=ltr><FONT face="Comic Sans MS" color=#000080>I don't know =
the context
in which the above was written, but....</FONT></DIV>
<DIV dir=ltr><FONT face="Comic Sans MS" color=#000080>A =
rib-crowned soundboard
system can be made with any amount of crown and any amount of stiffness =
simply
by altering the crown machined into the ribs and by altering the height =
and
width of the ribs. In other words, there is no direct relationship =
between a
compression-crowned soundboard with an 18 m crown radius and a =
rib-crowned
soundboard system with an 18 m crown radius. The fact that both have an =
18 m
radius is meaningless. Yes, it is possible to design the rib-crowned =
soundboard
system to have at least approximately the same amount of stiffness. But =
it is
equally possible to design it to have twice as much stiffness. Or half =
as much.
All with the same crown radius.</FONT></DIV>
<DIV dir=ltr><FONT face="Comic Sans MS" =
color=#000080></FONT> </DIV>
<DIV dir=ltr><FONT face="Comic Sans MS" =
color=#000080></FONT> </DIV>
<BLOCKQUOTE dir=ltr
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000080 2px solid; MARGIN-RIGHT: 0px">
<BLOCKQUOTE>
<DIV dir=ltr>No. Compression set continues until some neutrality =
is reached.
I.e., the point at which there is so little compression left within =
the
panel that in practical terms the wood fibers no longer deform. At =
this
point compression and crown and stiffness will be at their minimum
point.</DIV></BLOCKQUOTE>
<DIV dir=ltr>AH !!... Thankyou... That was what I was after. So.... =
stiffness
then is in a sense a function of how much compression there is in the =
board,
but if the degree of compression is beyond what the wood can handle it =
will
loose both? </DIV></BLOCKQUOTE>
<DIV dir=ltr><FONT face="Comic Sans MS" color=#000080>If the =
degree of
compression is beyond what the wood can handle--i.e., it is stressed =
beyond its
proportional limit--the wood fibers will crush immediately. If it is =
compression
stressed below its proportional limit compression-set will work to =
reduce that
compression stress slowly over some period of time. In both cases there =
will be
forces at work to reduce compression, in the first situation it simply =
happens
faster.</FONT></DIV>
<DIV dir=ltr><FONT face="Comic Sans MS" =
color=#000080></FONT> </DIV>
<BLOCKQUOTE dir=ltr
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000080 2px solid; MARGIN-RIGHT: 0px">
<BLOCKQUOTE><FONT face="Comic Sans MS"><FONT color=#000000>....I =
meant that
it is the ribs constraint upon the panel that cause the panel to =
either
compress or become tensioned depending on what the MC was when the =
ribs were
glued on and what it is at any other given time. If RH brings MC =
below that
starting point the board becomes tensioned right ?? And when RH =
causes MC to
rise above that starting point there will be some compression.
</FONT></FONT></BLOCKQUOTE></BLOCKQUOTE>
<DIV dir=ltr><FONT face="Comic Sans MS" color=#000080>Well, not =
really. Whether
the panel is under compression or tension would depend on its MC when it =
was
glued into the rim. Whether or not there were any ribs attached is not =
really
relavent. Without the ribs, of course, the panel would be some floppy =
and
distorted so it surely wouldn't develop the same amount of =
compression. But
it would still swell up if its MC goes above what it was when it was =
glued to
the rim. Without the ribs there to hold it in a plane it would buckle =
quite a
lot. And if the MC is taken below its original MC level the panel will =
still be
under some tension. Again, regardless of whether the ribs are there or
not.</FONT></DIV>
<DIV dir=ltr><FONT face="Comic Sans MS" =
color=#000080></FONT> </DIV>
<DIV dir=ltr><FONT face="Comic Sans MS"></FONT><FONT face="Comic =
Sans MS"><FONT
color=#000000></FONT></FONT> </DIV>
<BLOCKQUOTE dir=ltr
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000080 2px solid; MARGIN-RIGHT: 0px">
<DIV><FONT face="Comic Sans MS"><FONT color=#000000>So assuming =
that Climatic
conditions can get over stress even an RC panel... I just was musing =
if there
was some way of contiving a rib that gave crown support, but was a bit =
less
constraining to the panels tendancy to expand and contract across the
grain.... sort of sliding ribs if you get my meaning...Probably a =
silly
question I know... but I was just curious.</FONT></FONT> =
</DIV></BLOCKQUOTE>
<DIV dir=ltr><FONT face="Comic Sans MS" color=#000080>Well, sure. =
I expect you
could somehow slip-joint the panel to a set of crowned ribs much like a =
solid
wood table top is attached to its frame. Theoretically, at least. I =
expect there
might be the random buzz or two in real practice.</FONT></DIV>
<DIV dir=ltr><FONT face="Comic Sans MS" color=#000080></FONT><FONT =
face="Comic Sans MS" color=#000080></FONT> </DIV>
<DIV dir=ltr><FONT face="Comic Sans MS"
color=#000080>Del</FONT></DIV></BODY></HTML>