Compression Question

[Richard Brekne]

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Ok.. Thanks muchly for your patience once again. This has been very
instructive. I think I am pretty clear on what I was asking about. It
will be easier reading the two texts at this point, knowing better what
I am after.

Cheers
RicB

Delwin D Fandrich wrote:

>
>
>      ----- Original Message -----
>      From: Richard Brekne
>      To: Pianotech
>      Sent: August 29, 2003 1:57 PM
>      Subject: Re: Compression Question
>         A few days back you said
>
>           "If the degree of compression is beyond what the
>           wood can handle--i.e., it is stressed beyond its
>           proportional limit--the wood fibers will crush
>           immediately. If it is compression stressed below
>           its proportional limit compression-set will work
>           to reduce that compression stress slowly over some
>           period of time. In both cases there will be forces
>           at work to reduce compression, in the first
>           situation it simply happens faster."
>
>      So.. given what you say above, am I to understand that a rib
>      crowned board ribbed when the panel is about 6.5 - 7 % MC
>      will not suffer enough compression set during its
>      lifetime... say 30 - 50 years  , to cause any real
>      compression / compression related problems ?
>
> No. Some compression set will occur any time the MC of the panel goes
> substantially above the MC at which the panel was ribbed. It is a
> matter of how much compression set can reasonably be expected to
> occur. In a perfect world the amount of compression set will be
> reduced to a level that will not seriously impair the wood's tensile
> strength at the lowest MC that it will be reasonably expected to
> survive. In the real world the possibility still exists that the wood
> fibers might be damaged to some extent but the chances this happening
> to an extent that will result in visible damage to the panel have been
> reduced considerably. In addition, if the soundboard's rib system has
> been properly designed any cosmetic damage that does appear will be
> just that--cosmetic. Crown and stiffness are born by the rib system
> not compression within the panel. I could deliberately cut slots
> periodically across the surface of the panel (longitudinally with the
> grain) through to, but not into, the ribs and it would not affect the
> acoustic performance of the system. (Except, of course, for some
> acoustical coupling between the top and bottom surfaces.) And, indeed,
> this has been done. By Vose, I think. In other words the panel is now
> functioning almost completely as a radiating surface, not as a
> load-carrying structural member.
>
> Del

--
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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