RC vs CC again

[Farrell]

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Comments below - I need some clarification to address these issues.

Terry Farrell
  ----- Original Message -----=20
  From: Richard Brekne=20
  To: Pianotech=20
  Sent: Monday, October 06, 2003 9:42 AM
  Subject: Re: RC vs CC again


  Terry... et al=20
  I wanted to get back to you on this with a better explaintion of what =
bothers me about this position you take below.=20

  Consider the following.=20

  When ever a board is bent the, convex side will be stretched relative =
to the concave side.

  Correct - I'm assuming we are talking about one solid board here - not =
a soundboard - but rather, let's say, a one-inch by one-inch by =
three-foot solid piece of wood.

   This is the same regardless of the over all compression level found =
in the board.

  I think so, not quite sure what you mean by the "over all compression =
level found in the board."

    Pressing downwards on this curved board will tend to have the =
opposite effect... ie stretch the concave side and compress the convex =
side.=20

  If the board is curved and it is the same board - presumably had =
originally been straight - then some force is holding it in its curved =
configuration. What is that force? Is it trapped at the ends? Is it =
glued to a panel that has expanded with increased moisture content? But =
yes, if you have this curved board the net change in tension or =
compression will be to stretch the concave side (decrease compression - =
however, being that it was straight to begin with, it was under some =
amount of compression in its curved state, so when you press down that =
state of compression lessened - but it is still compressed relative to =
its state when straight). Same deal with the convex side.

  Ribs glued to the concave side will fight any tendency of the panel to =
stretch on its concave side.

  I'm not sure exactly what is being described here. "Ribs glued to the =
concave side" of what? What kind of ribs?

   Thats what causes them to bend in the process of crown formation in  =
the first place. Downbearing applied to the convex side of the panel =
will tend to compress the convex side, and stretch the concave side... =
but the ribs resist stretching of the concave side.=20

  I don't think it is correct to say that the "ribs resist stretching of =
the concave side." If I am understanding you correctly you are =
describing a CC crowned soundboard above. In that case, the concave side =
of the bent rib is in a state of compression and is trying to expand on =
its own to straighten back out. So as stated above, when you apply =
downbearing, yes, the concave side of the rib stretches, but only with =
reference to its curved compressed state - it doesn't stretch relative =
to when it was straight. I think it is more appropriate and more clear =
to state that when downbearing is applied, the concave side of the rib =
experiences a reduction of its compressed state.

  So how does that equate to=20

    "As the crown lessens under increasing load, the ribs will pull =
downward a bit less, but only because they are being bent upwards less =
by the panel."?=20

    See above.
  Seems to me the ribs will be even more stressed by down bearing...erg =
they support crown against applied downbearing.=20

  Yes ???=20

  To the best of my knowledge, and I think I am thinking correct physics =
here, no. In their bent state, the originally straight ribs are always =
trying to straighten out, i.e. pull the panel down, but the panel's =
physical properties are able to resist some compression and not allow =
the rib to flatten. When you apply downbearing, the rib becomes less =
stressed because it is closer to a relaxed, natural, state. The panel =
experiences increased compression with increased downbearing.

  RicB=20

  Farrell wrote:=20

    Richard wrote: "This assertion that ribs don't support the bearing =
load also is bothersome. Why=20
    wouldn't the ribs in a CC board support both crown and bearing... =
just because the=20
    panels compression forces the ribs to bend?"  When flat ribs are =
glued to a very dry flat panel, no force vectors exist in the system. As =
the panel takes on moisture and the ribs bend, the ribs will always be =
trying to pull the panel flat. Period. If that assembly has crown, the =
ribs will always try to go straight by pulling down on the panel. The =
compression forces in the panel are the only thing that has provided the =
force necessary to bend the rib into a crown. Whether or not you ever =
load that soundboard on top, the ribs will always be fighting the panel =
compression to go flat. When you do apply a load to the top of the =
soundboard, the compression on the panel will increase, but the ribs =
will do nothing other than still try to go flat. As the crown lessens =
under increasing load, the ribs will pull downward a bit less, but only =
because they are being bent upwards less by the panel.   Terry Farrell=20

  --=20
  Richard Brekne=20
  RPT, N.P.T.F.=20
  UiB, Bergen, Norway=20
  mailto:rbrekne@broadpark.no=20
  http://home.broadpark.no/~rbrekne/ricmain.html=20
  http://www.hf.uib.no/grieg/personer/cv_RB.html=20
   =20

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