soundboardinstal again

[Farrell]

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
> 
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