evaluating sdbd. crown & bridge downbearings in a new piano

[Frank Weston]

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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=20


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