More on soundboard crown

[Delwin D Fandrich]

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  ----- Original Message -----=20
  From: Richard Brekne=20
  To: Pianotech=20
  Sent: August 18, 2003 3:02 PM
  Subject: Re: More on soundboard crown


   =20
  Now correct me if I am wrong.. but I understand this to mean that a CC =
board will be stiffer, both unstrung... and increasingly so when =
downbearing is applied.  Assuming that is correct,

    Which it is not.

    =20

  Please refer again to the statement made by Ron=20

    "Given a panel crowned assembly with a severely dried  panel, and =
one rib crowned with a non dried panel, both to identical crowns  at =
room EMC, the one with the higher panel compression level will have the =
steeper spring rate gradient, and will be stiffer."
  Under the qualifiers that we are talking about boards with same =
amounts of crown at the same RH. If then I've misunderstood the above =
quote... then it would be nice to hear what was really meant here.=20
I don't know the context in which the above was written, but....
A rib-crowned soundboard system can be made with any amount of crown and =
any amount of stiffness simply by altering the crown machined into the =
ribs and by altering the height and width of the ribs. In other words, =
there is no direct relationship between a compression-crowned soundboard =
with an 18 m crown radius and a rib-crowned soundboard system with an 18 =
m crown radius. The fact that both have an 18 m radius is meaningless. =
Yes, it is possible to design the rib-crowned soundboard system to have =
at least approximately the same amount of stiffness. But it is equally =
possible to design it to have twice as much stiffness. Or half as much. =
All with the same crown radius.


    No. Compression set continues until some neutrality is reached. =
I.e., the point at which there is so little compression left within the =
panel that in practical terms the wood fibers no longer deform. At this =
point compression and crown and stiffness will be at their minimum =
point.
  AH !!... Thankyou... That was what I was after. So.... stiffness then =
is in a sense a function of how much compression there is in the board, =
but if the degree of compression is beyond what the wood can handle it =
will loose both? =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.

    ....I meant that it is the ribs constraint upon the panel that cause =
the panel to either compress or become tensioned depending on what the =
MC was when the ribs were glued on and what it is at any other given =
time. If RH brings MC below that starting point the board becomes =
tensioned right ?? And when RH causes MC to rise above that starting =
point there will be some compression.=20
Well, not really. Whether the panel is under compression or tension =
would depend on its MC when it was glued into the rim. Whether or not =
there were any ribs attached is not really relavent. Without the ribs, =
of course, the panel would be some floppy and distorted so it surely =
wouldn't develop the same amount of compression. But it would still =
swell up if its MC goes above what it was when it was glued to the rim. =
Without the ribs there to hold it in a plane it would buckle quite a =
lot. And if the MC is taken below its original MC level the panel will =
still be under some tension. Again, regardless of whether the ribs are =
there or not.


  So assuming that Climatic conditions can get over stress even an RC =
panel... I just was musing if there was some way of contiving a rib that =
gave crown support, but was a bit less constraining to the panels =
tendancy to expand and contract across the grain.... sort of sliding =
ribs if you get my meaning...Probably a silly question I know... but I =
was just curious.=20
Well, sure. I expect you could somehow slip-joint the panel to a set of =
crowned ribs much like a solid wood table top is attached to its frame. =
Theoretically, at least. I expect there might be the random buzz or two =
in real practice.

Del
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