Compression Question

[Delwin D Fandrich]

----- Original Message ----- 
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: "PTG" <pianotech@ptg.org>
Sent: August 28, 2003 4:27 AM
Subject: Compression Question


>
> 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 ?

The conditions of your hypothetical situation do not quite correspond with
reality. Compression set will certainly occur in your panel, but it will
not quite reach equilibrium. At least not in our lifetimes. But, assuming
that it did, when taken back to 4% MC the panel will shrink by the amount
of compression set.

That is, if a unrestrained 1500 mm wide panel (at 4% MC) is taken to 13% it
will probably end up being somewhere between 1520 and 1530 mm wide (at 13%
MC). So, if you restrain this expansion to 1500 mm and let compression set
work its magic for a few years and then take it back to 4% MC it will end
up somewhere between 1470 and 1480 mm wide.

Compression set is just what it says it is. The wood fibers take on a new
set at some compressed (smaller) physical size.

Del