Soundboard recrowning and rib re-arching.

[gordon stelter]

Yeah yeah Ok sure. But none of this changes the fact
that if you stressed the board into an exaggerated
crown, then permeated the ribs ( and board, maybe )
with something that rersists compression, you ought to
end up with better crown when you "knock the chocks
out".
     T

P.S. Still, that rib matter has to go somewhere when
the board loses crown. Either the rib is compressed
longitudinally, over the decades, shrinks, or the rim
expands. Probably a combination of all three. This is
simpleton stuff.

--- David Ilvedson <ilvey at sbcglobal.net> wrote:

> Wouldn't the top cells be stretching outwards and
> the bottom cells would be compressing.
> 
> David Ilvedson, RPT
> Pacifica, CA  94044
> 
> 
> ----- Original message
> ----------------------------------------
> From: "Ric Brekne" <ricbrek at broadpark.no>
> To: pianotech at ptg.org
> Received: 8/18/2006 3:11:06 PM
> Subject: Soundboard recrowning and rib re-arching.
> 
> 
> 
> >Thumpy writes:
> 
> >    Hey, Terry, yore a  real smart guy, so I know,
> that you know that
> >    when a  board is bent, 1 side goes into
> compression, the other into
> >    tension.
> 
> >Everything is relative there Thumpy. You have to
> remember that we are 
> >not talking about a plain ol bent board.  Its a
> board which is far more 
> >compressed then it would be if it was just plain ol
> bent.  In a bent 
> >board the part of the board that is neither
> compressed or tension is 
> >along the exact center line thickness wise.  But if
> the board is 
> >subjected to some degree of compression independent
> of plain ol bending 
> >compression... then this has to be added into the
> situation as it were.  
> >If you add enough of this independently caused
> compression then the 
> >whole panel will be under compression no matter how
> much you bend it.  
> >Now the top half of such a compressed panel would
> be less compressed as 
> >it were then the bottom half to be sure.  Thats
> where the relativity bit 
> >comes it... you could look at things like a number
> line where you move 0 
> >around to fit the middle point.  In that case you
> could say the top half 
> >is in tension relative to the bottom half... but
> you cant forget to take 
> >into consideration the absolute degree of
> compression there is (or isnt) 
> >if you are going to speak in absolute terms at all.
> 
> 
> >Just how much compression is created in a
> compression soundboard and 
> >whether that is enough to put the entire panel in a
> state of compression 
> >despite the relative state of affairs when the bend
> is taken into 
> >account I dont know.  But no doubt the center line
> is no longer halfway 
> >between the bottom and top sides... its been moved
> up towards the top 
> >and most certainly more then half of the panel is
> under compression.
> 
> 
> >    If it is then glued to a panel, and said panel
> is smashed flatter
> >    over the years, this rib must also go
> intocompression, somewhat, or
> >    no crown would be lost.
> 
> >I'm not at all sure I buy this.  I'll be the first
> to argue that a rib 
> >is tensioned somewhat as part of the process of the
> panel taking on 
> >humidity and compression... I'll even go so far as
> to suggest that 
> >perhaps the rib tensions a bit more then it would
> if it was just plain 
> >bent... i.e. its center line drops just a hair
> leaving more then half 
> >its mass under tension and less then half under
> compression....  but 
> >when a compression panel flattens out to the degree
> that it would stay 
> >flat in the abscence of any bearing... well in that
> case the ribs are 
> >simply going to relax me thinks.
> 
> >Cheers
> >RicB
> 


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