Rib Support

[Cy Shuster]

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  ----- Original Message -----=20
  From: Richard Brekne=20
  To: Pianotech=20
  Sent: Sunday, October 26, 2003 1:34 PM
  Subject: Re: Rib Support


   =20
  Cy Shuster wrote:=20

     The situation seems more like a stronger force overcoming a weaker =
one.  The weaker one subtracts from the stronger one, and then the =
situation is in equilibrium.  I'm thinking of a small leaf spring, like =
for pedals, about 6" long, say curved in an arc of one foot radius.  It =
resists being straightened.  If you take a stronger spring, of thicker =
metal, same length, but curved to a 6" radius, and nest the two so the =
stronger's on the outside, the thinner spring will resist a bit, but the =
vector of its force is overwhelmed by the stronger spring.

  Hmmmm....  Not sure where you are at here... if you mean nesting like =
my third drawing... then I would suppose their affects become additive.=20

  >>Yes, I mean like the third drawing.  Yes, the forces would add, but =
like vectors: the weak spring resists bending by "x" amount; the strong =
spring bends by "10x"; so the result is a bend equal to "9x".

  I dont see that the ribs are exerting a stronger force then the =
soundboard... if that's what you mean... I dont even see one as having a =
separate force from the other.  Or at least.. I dont see that as being =
what yields and supports crown. I see that the ribs are holding, and the =
soundboard are pushing against a common force.=20

  >>I'm thinking of the case as you've drawn it, where it's just the =
ribs and SB, out of the piano, not attached to a rim.  In that case, =
there's no "common force".  The SB holds the ribs down.
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    Same with the soundboard and ribs.  If the force of the SB absorbing =
moisture is so strong as to bend the ribs, then they're irrelevant after =
that point, it seems.

  I keep hearing this one... but turn the reasoning around a bit... if =
the SB absorbing moisture is so strong as to bend the panel then its =
irrelevant, or if the Ribs are so strong as to force the panel to crown =
then they are irrelevant.... I could go on. Its this irrelevancy that I =
am questioning. Who says the ribs are irrelevant... ?  and what =
reasoning can back that up...? and what set of physics principles for =
that matter ? And how does any of this explain the basic <<cable holding =
the panel bent>> that was my first example ?=20

  Are you suggesting that the cable in that example doesn't support the =
<<crown>> against a downward pressure ? How could that be ? and if =
not... why would the wooden ribs bending, in anyway relieve it of that =
same duty ? How could that be ?=20
   =20
   >> I don't think the cable's a good analogy, because as soon as the =
panel bends, it will go slack. =20

    =20
    The material used seems to matter.  If the ribs were spring steel, =
and were initially flat, then of course they'd continue to want to =
straighten out, forever.  What does wood do?

  Hmmmm... if you used spring steel straight out like the cable =
example... does it not support crown ? The only difference I see between =
that and attaching it to the whole width of the panel is that you are =
more or less stringing together a continuos set of very short panel =
widths and rib lengths.=20

  I can't see that we can escape from the fact that pushing down on the =
soundboard is synonymous with exerting an outwards pressure on whatever =
is holding against that outward pressure. And if something is holding =
against that pressure... well that's spells support as far as I can see. =



    >> 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?

  --Cy--

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