Compression Question

[John Hartman]

Richard Brekne wrote:
> Hi folks.
> 
> Been scratching my head a bit pondering the last bit of input Del was
> kind enough to provide and wanted to make sure I understood one
> particular point clearly. Dont know how else to put it so I will ask it
> in the following manner..
> 
> If you take an unribbed panel, and dry it out to the extremes of 4% MC,
> and then constrain its dimensions at that so that it is not allowed to
> expand either outwards, or upwards or in any fashion and bring the MC up
> to 13 % the panel will get pretty severely compressed... yes ?... ok..
> If you allow this to sit over enough time that if in releasing the
> constraints the panel simply retains the dimensions it had under
> constrainment.... then what happens to the size of this panel  if you
> dry it down to 4 % again ?

Richard,

You can find information on this in any good book on wood technology. 
There are simple ways to estimate the compression set. Using 4.3% as the 
radial shrinkage of spruce I estimate the panel would expand if not 
constrained about 1.38%. If the panel is 48" wide it will expand .66". 
When the panel is constrained it can expand 1% and still recover but any 
more than this will crush the wood. To get the compression set subtract 
.48" from .66". The panel will shrink (compress).18" after it returns to 
4% EMC about.

As an assignment Richard, run these formulas with tangential (flat sawn) 
spruce and see what happens.

BTW you can use some of the same formulas to find the change in EMC that 
will start to cause compression set if the wood is confined. For quarter 
sawn spruce I get 6.5%.  Using the figure you can come up with a range 
of relative humidity that a soundboard can safely live in. If you rib 
your board at 4.5% the safe range is 20% to 60% disregarding the 
compression due to ribbing and down bearing. If you rib your board at 
6.5% the range is from 30% to 70% RH.

I would say you need to narrow theses figures about 10% due to other 
stresses on the board. I consider the safe range is from 30% to 60% RH 
for a board ribbed at 6.5% EMC.


John Hartman RPT

John Hartman Pianos
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