Soundboard tension & compression

[Delwin D Fandrich]

----- Original Message -----
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: "Pianotech" <pianotech@ptg.org>
Sent: November 26, 2002 3:15 PM
Subject: Re: Soundboard tension & compression


> Delwin D Fandrich wrote:
>
> >
> > >
> > > WHAT IF ??... you took a panel at 12 % MC and secured its
> > > cross grain edges so they couldnt budge... and then dried it
> > > out to 4% MC.... and proceed back down that bunny trail ?...
> > > just for edification mind you :)
> > >
> > > Cheers
> > >
> > > RicB
> > >
> >
> > You figure it out. It will assist in the edification process and the
numbers
> > are readily available. The amount of wood fiber shrinkage that would
take
> > place, the relative weakness of spruce/pine under tension cross-grain,
etc.
> > Wood Technology 101.
> >
> > Del
> >
>
> Well, assuming wood reacts basically the same (but opposite) when tensed >
thus, I
> would imagine you'd put the panel under about 1.5 to 2 % internal tension.

Nope. You didn't read the manuals did you? Check out what the Wood Handbook
has to say about wood under tension. Especially under tension across-grain.


> And I
> would suppose that this would be just as much over the safety line as it
> would
> be for similiar compression levels.

As I said, you didn't read the manuals, did you?


> That being said you'd only leave it this
> way
> for just a short while so'st you can get the ribs clued on. Ok so you glue
flat
> ribs on with the panel in this state, let cure and pop out of the device
> restaining the cross grain edges and what happens... ? I'm not really
sure...  I
> would imagine the panel would try to de-tense, and the ribs would fight
> that,
> and there would be a slight tendency towards reverse crown. But then just
>how
> much I dont know because the panel has been "stretch dried" as it were,
and
> as
> long as the humidity is the same then.... ??  Allowing it to take on
humidity
> back up to 12 % MC... the panel would crown as usual and seemingly end up
in a
> state of neither compression or tension...

Assuming the panel was glued to cross-grain ribs in the normal manner you
would have multiple cracks and splits well before you got to 4% MC. But you
would also have some reverse crown.

Del