Respectfully, Ron, these are two separate points, are
they not?
Yes, a soundboard ( when built ) MAY support a
full load without a rim. And this, I am willing to
believe, can be amply demonstrated in the shop. But
to presume that a board with no rim holding it inward
would resist the downward pressure of strings, replete
with climate variations, OVER TIME is just that -- a
presumption.
Unless, of course, you have already built a
board, laid it on a table ( preferably with a
concavity beneath ) and observed its deterioration
over DECADES! And then compared it to an identicle
board mounted in a rim.
Respectfully,
Thump
--- Farrell <mfarrel2@tampabay.rr.com> wrote:
> Ron N wrote:
> > Once again, the rim has nearly nothing whatsoever
> > to do with the maintenance of crown, nor the
> retention of compression in
> > the panel. The ribs do that all by themselves.
>
> Some ribs. ;-)
>
> > as is easily demonstrated that a
> > soundboard assembly has crown, and will support a
> full load without being
> > anywhere near a rim.
>
> Excellent point. So simple, but I had never thought
> to present this concept in these terms - but that's
> it - you can load a board (positive crowned)
> un-attached to any rim and it will have all the
> support it will ever have. I've done this, just
> playing with all these new things I am building, and
> it's quite amazing!!!
>
> I think folks fall into this mental image of a 1
> meter radius domed structure attached at the edges
> to a rigid rim. In such a case I am sure the rim may
> well be some significant contributor to support of
> the dome. But with the 2 mm to 10 mm (or whatever)
> crown one may find in a soundboard, 99% of the crown
> support is going to come from the ribs on a rib
> crowned assembly or from the panel (at least for a
> while - hopefully until it is sold) on a
> compression-crowned soundboard assembly.
>
> Terry Farrell
>
> ----- Original Message -----
> From: "Ron Nossaman" <RNossaman@cox.net>
> To: <oleg-i@noos.fr>; "Pianotech"
> <pianotech@ptg.org>
> Sent: Monday, July 21, 2003 10:15 AM
> Subject: RE: soundboardinstal again
>
>
> >
> > >Ron,
> > >
> > >Me think (may be wrongly) that on the opposite,
> the fact that a so
> > >small dimension is involved show us the
> importance of a better joint
> > >(last news from the day !)
> >
> > It doesn't work that way.
>
> > >The inserting under tension of the soundboard
> seem the only way to
> > >have some tension in the panel, in the rib
> direction (across grain)
> > >the modulus of elasticity is said to be 3 Kg cm2
> vs. 100 Kg cm2 in
> > >direction of grain.
> > >So if we wish to have some tension in the panel
> that looks like a
> > >convenient method, more than compression
> crowning.
> >
> > Again, it doesn't work that way. It's not tension
> you're putting in the
> > panel, it's compression. Modulus of elasticity and
> compression resistance
> > are not the same thing, and MOE is meaningless in
> this context. Soundboard
> > crown is still not an end supported arch, so
> buttressing the edges will
> > have no significant effect on the crown. In the
> example rib I used, of 36"
> > (915mm) and 60' (22M) radius, the outward thrust
> of the "arch" will be 40
> > times the load it carries. This means that at
> 580psi fiber stress
> > proportional limit of cross grain compression of
> Sitka spruce, that rib can
> > only hold up to 14.5 pounds load before the wood
> is crushed. It gets much
> > worse with shorter ribs. Once again, the rim has
> nearly nothing whatsoever
> > to do with the maintenance of crown, nor the
> retention of compression in
> > the panel. The ribs do that all by themselves.
> >
> > >Stephen have forced back some boards with shims
> on the straight side
> > >with good results he say, when we see the
> dimensions involved, that
> > >looks like a possibility.
> >
> > Yes, I have also heard (or heard of) all sorts of
> folks saying all sorts of
> > things. The physical fact is that this doesn't
> work. The physical
> > structural limits of the materials just don't make
> it possible.
> >
> >
> > >But I seem to understand that in the actual
> conception in the USA the
> > >soundboard assembly is viewed as an auto
> supported device is not it ?
> >
> > Yes, but it's not a view. It's a fact, as is
> easily demonstrated that a
> > soundboard assembly has crown, and will support a
> full load without being
> > anywhere near a rim.
> >
> > Ron N
> >
> > _______________________________________________
> > pianotech list info:
> https://www.moypiano.com/resources/#archives
> _______________________________________________
> pianotech list info:
https://www.moypiano.com/resources/#archives
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