evaluating sdbd. crown & bridge downbearings in a new piano

[Frank Weston]

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Richard,

You have served as a wonderful goad to this discussion, but ultimately =
there comes a time when you must commit to an opinion of your own.  What =
will it be?

In a nutshell, my argument is that a traditionally crowned soundboard =
can be constructed such that the top of the board is in tension.  I am =
not disputing that with drying and manipulation of humidity that the top =
can be made to be under less tension, or even compression.  What I am =
arguing is that the final stress distribution DEPENDS on starting and =
ending humidity of all parts.  Further, if the top of a board starts in =
tension and the bottom in compression, as it is loaded by downbearing, =
the tension/compression load will be decreased.  Finally, it is my =
opinion, that because of the way it responds to load, a traditionally =
crowned soundboard will just sound better (this point is pure opinion, =
and I won't argue further).  Others have made these same arguments in =
the past and have tired of butting their heads against the wall of =
pedagogy and sycophancy.  I now begin to understand their fatigue.

I will not address a situation in which parts are assembled under =
differing and controlled humidity.  There are too many variables, and =
too many correct answers in such a situation.  Stress in the board will =
vary, and it will depend.

Here is an experiment I conducted in my shop yesterday.  I selected a =
room dried strip of sitka spruce, made pencil marks on it 18 inches from =
the end.  I bent the spruce to an approximate 3' radius and glued a rib =
to the bottom.  When I measured the top of the spruce, I found that the =
pencil mark position indicated the top had stretched approximately 1/16 =
inch, and the bottom had compressed about as much.  This model indicates =
to me that the top is in tension and the bottom in compression. =20

I then placed the model between constraints at each end and loaded the =
top.  As the load increased, the line on the spruce showed me that =
tension in the top was decreasing.  I loaded the model to failure.  The =
rib failed first, and the failure was the most beautiful textbook =
indication of a failure in tension on the top and compression on the =
bottom (if you don't know what this kind of failure looks like, buy a =
box of chalk and break a few pieces in different ways - the most =
interesting is when you break it by twisting).  Because of one of =
Newton's laws, we might deduce that the soundboard was experiencing =
equal and opposite loads when the rib failed.

If you care to construct such a model, I'm sure your results would be =
similar.  If you would like to see pictures, I will consider putting =
them up on a photo site.  If the actual physical demonstration of the =
main point I am arguing does not convince, then I fear we must promote =
this discussion to the level of religion where issues are resolved on =
the basis of faith and personality.

Frank Weston












    -----Original Message-----
    From: Richard Brekne <richardb@c2i.net>
    To: pianotech@ptg.org <pianotech@ptg.org>
    Date: Monday, September 27, 1999 1:59 PM
    Subject: Re: evaluating sdbd. crown & bridge downbearings in a new =
piano
   =20
   =20
     =20
    Frank Weston wrote:=20

    Frank... I would ask you again to consider the example below and =
simply answer the question. Its a simple enough example. I ask this =
because it will clear up a specific principle my own mind wants cleared =
up. The paragraph you write below doesnt do that. That much (your =
paragraph below) is obvious enough in any case. Please be so kind as to =
re-read the example below, taking it for what it is and answer my query. =


    Thanks so much=20

    Richard Brekne=20
    I.C.P.T.G.  N.P.T.F.=20
    Bergen, Norway=20

         Richard, Forget humidity.  Forget made up numbers.  Do this.  =
Tape a flexible steel rule onto the end of one side of a spruce stick  =
(a wooden yard stick will do for demo purposes).  Mark the opposite end =
of the steel rule on the spruce stick.  Bend the spruce stick.  Notice =
that as you bend it, the mark on the spruce changes position relative to =
the steel rule.  If the rule is on the outside of the bend, the spruce =
gets longer.  If it is on the inside of the bend, the spruce gets =
shorter. If the spruce is elongated it is in tension, if shortened, in =
compression.  Now, bend the spruce stick and glue another piece on the =
bottom.  Nothing has changed.  When the glue dries, the assembly will =
hold a curve.  The outside curve of the spruce is still longer than it =
was originally, and it is still in tension. I hope this is simple enough =
for you. Frank Weston  =20
            Lets say we have a panel exactly 1 meter across grain, and a =
rib that is 998 cm long in a room with 50% relative humidity. The panel =
is put into the oven and dried to the point that it is also 998 mm long. =
Now if we take the panel out and simply let it re-adjust to the room =
humidity it will "grow" back to 1 meter in length. The top and the =
bottom of the panel will both shrink the same, and grow back the same =
amounts... ok so far ??  But if we quickly attach the ribs while the =
panel is at 998 and then allow it to re-grow to what ever length the =
room humidity and this constraint from the rib allows for, its length =
will be less the 1 meter. (This following what I think I got from Dels =
description.) Ok.. assuming this is a correct picture so far, stretching =
a measuring line across the top of this  ribbed panel after its =
re-adjusted to room humidity shows that it measures less then 1 meter.=20
            If, (Frank) you accept that this is correct so far,  I would =
appreciate it very much to know the reasoning behind why this top half =
of the panel is not (if it is not) to be considered in compression. As =
initially stated, please follow Dels kind "laymans language" approach to =
demonstrating the reasoning.=20

            Richard Brekne=20
            I.C.P.T.G. N.P.T.F.=20
            Bergen, Norway


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