evaluating sdbd. crown & bridge downbearings in a new piano

[Richard Brekne]

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Delwin D Fandrich wrote:

> Now, lay a tape measure along the joint between the rib and the soundboard
> panel.  You will find that the rib is still 1,000 mm long.  Lay the tape
> measure along the top of the soundboard panel.  You will find that it is
> somewhere between 1,000.25 mm and 1,000.5 mm (or there about -- the exact
> figure escapes me and I'm to tired to figure it out all over again.).  The
> same force that would have caused the panel to expand has now turned into
> compression stress inside the panel and has forced the rib into a curve.
> And, as long as the compression stress is present inside the wood panel it
> will keep forcing the rib into that curve.
>
> So.  Keeping in mind that, if it weren't for that pesky rib, the entire
> soundboard panel -- top, middle & bottom -- would really like to be about
> 1,020 mm wide at this point, please explain to me how there can be any
> tension anywhere in this panel.  Indeed, there may well be fractionally less
> compression on the top of the soundboard panel than there is right next to
> the rib, but it would be very difficult to measure.
>

Ahhhh... Haaaaa!!  And now this all makes perfect sense. Excellent indeed. The
perfect explanation for someone who hasnt had any technical insight as to the
nature of wood. It also makes perfectly clear (at least to me) the disparagy
between Franks postings and Dels and Rons. The later two of course are the
"correct" ones. This simply has to be the case. This is not to say that Franks
descriptions are wrong... they are actually (unless I have misread him entirely)
simply based on a different perspective (I am tempted to use the word "paradigm"
here). I will sit down tonite and try and defend that statement in a seperate
posting, but for now I would ask you (Ron and Del, and others ) to consider his
statement in his last post...


     > Once the board takes on moisture the entire board is now under
     >compression.

     Yes and no.  If you talk in absolute terms, yes.  In relative terms,
     no.


Now if you try and see where he is comeing from here... accept his "relativity"
perspective, it is easy to follow his reasoning, and actually can easily be
interpreted as a very different way of saying the exact same thing. But as I
said. I will attempt to explain what I mean by that in a seperate post.


>
> If, at this point, you could magically remove the soundboard panel from the
> rib without damaging it,  would instantly expand to 1,020 mm and there would
> again be no compression stress present.  And the rib would pop back to its
> straight condition.  Until that happens, however, there is compression
> stress within the entire panel.  And it is enough to keep that rib bent.
>
>

Thanks immensely muchly Delwin. As far as your experiment is concerned... Your
explanation was quite sufficient to make things clear. That being said I will
carry out the exercise anyways as it has obvious value in itself


      The more you know about it, the better you
     will understand the piano.  And sometimes the best way to really learn

     something is get your hands dirty.


Nicely put. Thanks again for taking the time to write all this out Del. This
list has opened a whole bunch of doors for me. I am indeed gratefull

Richard Brekne
I.C.P.T.G.  N.P.T.F.
Bergen, Norway

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