evaluating sdbd. crown & bridge downbearings in a new piano

[Delwin D Fandrich]

OK.  But, you'll have to get your hands dirty.

Go out to your shop and cut a piece of spruce 25 mm by 25 mm by 1,000 mm
long.  Just one will do.  This is your rib.

Now edge glue together enough 8 mm thick by 200 mm long vertical grain
spruce boards of random width to make a panel that is a little over 1,000 mm
wide.  About 1,025 mm should do it.  Place this panel in your oven and cook
it at 130º to 140º IF (55º to 60º C) for a few hours.  Measure it from time
to time and when it stops shrinking and stabilizes for a while it is ready.
It should now be fairly close to 4.0% moisture content.  Give or take a bit.
Quickly saw off enough to make the panel exactly 1 meter wide (just to make
calculating easier).  Compare this to the length of your rib and you should
find that they are exactly the same.

Now, set both pieces of wood on your kitchen table for a day or two.  If it
is a warm and humid summer day you will find that the panel has expanded.
Now instead of being 1,000 mm wide it will be about 1,020 mm wide.  It has
expanded because the moisture that has been absorbed by the cell walls
within the wood panel has caused them to expand.  Since there is nothing to
restrain it, the panel has expanded across grain.  Also, measure the length
of your rib.  It should be 1,000 mm long.

So.  Put the panel back in the oven -- go ahead and stick the rib in as
well -- and bake them both for a few hours.  The same amount of time as it
took before should do it.  Now measure your panel.  You will find that it is
back to 1,000 mm.  Measure the rib as well.  It will still be 1,000 mm long,
or very close to it.  (Observation:  Wood expands and contracts very little
along the grain.)

Now, very quickly take the rib and glue it down the center of your panel.
At this point they will both be flat.  There will be no curve -- crown -- in
either piece.  And they will both be 1,000 mm long/wide.  However, as you
observe the glued-up assembly sitting on your kitchen table you will notice
that they will begin to bend or warp.  The wood fibers in the panel will
again be absorbing moisture and will be trying to expand.  But the rib will
be preventing the panel from getting any wider as it was able to do earlier.
Eventually, there will be a substantial crown in the soundboard assembly.

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.

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.

Now, don't take my word for this.  Actually try it.  Get the wood.  Cut it,
glue it and cook it.  Watch it work.  As I said in another post, this is
really basic wood technology.  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.

Del




----- Original Message -----
From: Richard Brekne <richardb@c2i.net>
To: <pianotech@ptg.org>
Sent: Wednesday, September 22, 1999 1:34 PM
Subject: Re: evaluating sdbd. crown & bridge downbearings in a new piano


>
>
> List:
>
> After reading Dels last post on the issue I have become convinced that
> the term "compression" is at the root of the "dissagreement" or at least
> my own problem grasping all this. For my own part I believe the whole
> thing could be cleared up if one of you would explain just how a board
> curved by force or by use of humidity can be "Compressed" on the upper
> surface. I can easily see how compression applies to the bottom side,
> but I cant get past this idea that the convex side is "stretched" rather
> then "squeezed".
>
> >From what I was able to gather from Dels last, it seems like this has
> something to do with cell structure and how it reacts to differing
> stress. In any case.. I would really really appreciate somebody clearing
> this up.
>
> Richard Brekne
> I.C.P.T.G.  N.P.T.F.
> Bergen, Norway
>
>