evaluating sdbd. crown & bridge downbearings in a new piano

[Richard Brekne]

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Frank Weston wrote:

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

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

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

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