Compression Question

[Delwin D Fandrich]

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  ----- Original Message -----=20
  From: Richard Brekne=20
  To: Pianotech=20
  Sent: August 29, 2003 1:57 PM
  Subject: Re: Compression Question


    A few days back you said=20
    "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 ?=20
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

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