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 11:55 AM
Subject: Re: RC vs CC again


>
>
> Delwin D Fandrich wrote:
>
> > >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....
> >
>
> Grin.. I doubt that... but I am interested... and hey.. its you who
encourages
> us to experiements. I thought you'd be pleased.

I am. Go for it.


>
>  I meant by  "bent and stressed horizontally" horizontal stress from the
ribs
> restricting the panel in swelling leading to crown. In anycase... these
two
> things happen to the panel, yet for some reason the rib is just being
bent...
> just as I could bend it in my hands. What happened to the tension on the
rib
> ?... does it all go into simply bending the rib ??

As far as I can tell, yes. Either way the top of the rib is under tension
and the bottom is under compression. It's bent and it wants to straighten
out.


>
> I see this much... I just dont see why the reverse is not also true,
i.e. ----
> The panel wants to lie flat, coupled with a restricting rib it is
physically
> bent out of its equilibrium configuration and wants to return to that
> configuration. It is going to fight being bent every step of the way. The
ribs
> are also trying to remain at equilibrium ... that equilibrium being
determined
> by the changing moisture content of the panel. They are prevented in this
by
> the panel. It is only the resistance by ribs to the panels expansion that
> forces crown.... ... actually...I dont see why its not working both ways.

The soundboard panel is so very flexible (across-grain) relative to the
ribs. Consider this: if you shrink down a soundboard panel and place it on
a set of flat ribs between two rigid buttresses but don't glue the ribs to
the board, what will happen as the panel takes on moisture? It's going to
start out developing some internal compression, then it's going to pop up
and, probably, distort all over the place. It has very little stiffness
that way. Now, holding the ends of the ribs down tight against the table,
force a crown in the ribs -- just push them up from the bottom -- until
they just fit snug against the bottom of the panel. What are the stresses?
Is there any overall tension in the ribs? Now, glue the ribs to the
soundboard panel and remove the buttresses. The assembly will collaps some,
of course, but not all the way. Why not? What's left is crown and again,
what are the stresses?


>
> Sigh... no need to persue it further tho... obviously I just need to
think it
> through more to see the sense in what you are saying. I'm not really
trying to
> argue with you here... just attempting to illustrate where I am confused.

Sorry, I got this far before I saw this...Ah, well....

Del