RC vs CC again

[Delwin D Fandrich]

----- Original Message ----- 
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: "Pianotech" <pianotech@ptg.org>
Sent: October 09, 2003 9:05 AM
Subject: Re: RC vs CC again


> >
> > As glued up the top of the rib is neutral relative to the soundboard
panel.
> > If the panel and the ribs are kept at a stable moisture content there
is no
> > stress differential between them and, obviously,  there is no crown. It
is
> > not until the panel begins to absorb moisture and the wood fibers begin
to
> > swell that the assembly begins to take on crown. The rib is now forced
to
> > bend against its will and it will always be trying to return to
> > equilibrium -- i.e., to being the straight chunk of wood it was cut out
to
> > be. It's whole heart and soul go into that desire until such time as
its
> > time under load is sufficient to permanently deform the poor thing
through
> > creep.
> >
>
> Well, that puts us back at the starting point I guess, and I'm none the
wiser
> for it. I want to find out what happens to all the tension placed on the
rib,
> thats all. If you say it all goes into a simple bending of the rib then
ok..
> but I would like to know where thats covered in the literature.

It's probably not covered at all in "the literature." Who writes the
literature? Historically it's been the folks who get in there and design or
build the pianos. I expect then, like now, they pretty much dealt with the
more-or-less practical and the more-or-less reasonable. At least as it
seemed to them at the time.


>
> I do have an experiement in mind tho... that should tell me just how much
(if
> at all)I can expect the rib to be elongated. I'll take a few pictures and
post
> them when I get it together.

We're waiting on pins and needles....


>
> > The exception to the above, of course, is the compression-crowned
> > soundboard assembly which still uses flat ribs but the assembly is
glued up
> > at a somewhat higher moisture content using curved cauls to come up
with
> > essentially the same amount of crown. In this case the top of the rib
is
> > under some tension at the time of gluing. How much depends on the
amount of
> > curve found in the caul. The end effect on the soundboard panel,
though, is
> > essentially the same -- for a given amount of crown the level of
internal
> > compression will be the same.
> >
>
> See... what I dont get is why the ribs are simply being treated as if
they were
> just being bent, yet the soundboard is treated as being bent AND stressed
> horizontally.

I don't treat the soundboard as if it is bent and stressed horizontally.
Across-grain the soundboard panel has so little stiffness it can reasonably
be ignored as a practical force. It's not until it is shrunk down and glued
to the ribs and allowed to attempt to swell up again that the assembly
takes on the additional stiffness contributed by the resulting crown. The
components assembled together take on properties none of them have
individually.


>
> The only real difference is the direction of the grain.

This is a pretty significant difference!



> ...If one can declare the compresion in the soundboard is
> what supports crown, then why is any tension in the ribs not doing the
same
> thing ? Its just two sides of the same coin... or what ???

Because by themselves the ribs want to lie flat. Coupled with an expanding
soundboard panel they are physically bent out of their equilibrium
configuration and they want to get back to that configuration. They are
going to fight being bent every step of the way. The soundboard panel is
also trying to reach equilibrium -- that equilibrium determined by its
changing moisture content. It is being restrained by the ribs. It is only
the effort of the soundboard panel to reach equilibrium that is forcing in
the crown.

Del