Rib Support

[Richard Brekne]

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Cy Shuster wrote:

>      OK, I think I get it.  Let me keep my dumb questions going
>      (apologies to the rest of you).  Without the ribs, the SB
>      would stay flat, but increase its diameter, as it takes on
>      humidity (more across the grain than with it?  Or the other
>      way round?).  But gluing the ribs on first is like attaching
>      your cable: it prevents the SB from getting dimensionally
>      bigger on one side, therefore it has no choice but to bow
>      up.  So just thinking about this one effect, the ribs are
>      trying to be stretched along their long axis, like your
>      cable: "tension" rather than "compression" -- right?  (There
>      wouldn't seem to be any compression here ; where the ribs
>      touch the board, it's the same dimension, more or less, as
>      at low humidity.  If you tightened your cable to get crown,
>      then I'd see compression.)
>

No dumb questions here in my book, Cy.... and by all means understand I
am just as likely to be totally confused about this as anything else...
but you seem to have basically understood the view from where I'm
standing at the moment.  tho I'm not quite sure of what you at in the
parenthesis.

Thing is there are two types of compression (and tension) going on in
both the panel and ribs. There is the kind that comes directly from the
two pieces having pulling at each other in opposite directions, and
there is the type that comes from bending. Adding them all up would be a
fun task, but thats not the point here.

The claim has been made that ribs do not provide any support of any kind
for crown against downbearing... that  the only thing they do is try to
straighten out.. ie defeat crown..  All that is needed to show that is
an inadequate description is to show that the ribs do function in a way
that includes at least some sigificant resistance against
downbearing.... whatever else they do is besides the point. And for the
life of me, I cant see how we can just ignore the ribs resistance to
panel expansion just because it is downbearing that is the source of
that expanding force instead of an increase in humidity.

>
>      So if the ribs are being stretched along their long axis by
>      the expanded SB, what happens when a downbearing force is
>      applied?  Especially if it's just sitting on a flat surface,
>      no rim, and the ends of the ribs aren't even touching the
>      flat surface?  I dunno...
>
>

Well... if you press flat then something got to give... somehow... if
you dont stress the glue and parts past their limits, then you will just
be straining one of more of those components. If you leave it there long
enough, then whatever was physically strained will not be able to
recover. Yes ?

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


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