Rib Support

[Cy Shuster]

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  ----- Original Message -----=20
  From: Richard Brekne=20
  To: Pianotech=20
  Sent: Monday, October 27, 2003 2:23 AM
  Subject: Re: Rib Support
   =20
  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?

  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.=20

  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.)

  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...=20

  --Cy--

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