evaluating sdbd. crown & bridge downbearings in a new piano

[Brian Henselman]

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David,

Bravo!  Well said!  Yeah for simplicity!  Ron, Brian T., Delwin, didn't =
he summed it up in laymen's terms? =20

Frank, I think David nailed it.  Unlike your yardstick analogy, no one =
is grabbing the ends of the soundboard and artificially "forcing" an =
arch.  In a compression crown, the arch is created by internal forces =
(increased water content) pressing outward against the cell walls =
(COMPRESSION).  The rib is simply limiting the GROWTH of the underside =
at a greater rate than the growth on the topside.  Even though ALL of =
the cells are GROWING, the arch is created only because the top of the =
soundboard is GROWING with less resistance because the rib is on the =
bottom.

Did I get it right?

As to rib-crowning.  Don't think of it as the rib "forcing" the board to =
arch as in Frank's yardstick analogy.  Rib crowned boards are still =
dried before ribs are glued on right?  Just not as much.  If this is the =
case, once humidity is re-introduced to the cells, then think of  =
tapered rib as "following" the board, and not "fighting" it like a =
straight rib does.

I'm not an expert either.  To quote you, David, "I'm learning, not =
teaching."  Hopefully though, I've figured this out finally.

Cheers,
Brian Henselman,RPT
  -----Original Message-----
  From: David M. Porritt <dporritt@post.cis.smu.edu>
  To: pianotech@ptg.org <pianotech@ptg.org>
  Date: Monday, September 27, 1999 11:33 AM
  Subject: Re: evaluating sdbd. crown & bridge downbearings in a new =
piano


  Frank:
  =20
  I have not participated in this discussion simply because it is over =
my head.  I'm learning, not teaching. =20
  =20
  However, if you think if compression as the state where something is =
smaller than it wants to be, and tension as something being larger than =
it wants to be, that can simplify the engineering definitions.  If a =
traditionally crowned board is held smaller than it wants to be, it is =
under compression.  On this board that is constrained by the rib, the =
bottom is smaller than it wants to be because of the rib.  The top is =
smaller than it wants to be because of the bottom - and the fact that it =
is one piece, this panel.  If the top, although it is larger than the =
bottom, it's smaller than it wants to be, so it is under compression.
  =20
  Does this make sense???
  =20
  dave
  *********** REPLY SEPARATOR ***********

  On 9/27/99 at 12:01 PM Frank Weston wrote:
    Richard,
    =20
    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.
    =20
    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.
    =20
    I hope this is simple enough for you.
    =20
    Frank Weston =20
    =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

_____________________________
David M. Porritt
dporritt@swbell.net
Meadows School of the Arts
Southern Methodist University
Dallas, TX 75275
_____________________________


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