[pianotech] Tension and compression in soundboard panels

[David Love]

So it's the end of a long day and I was thinking about this and couldn't
quite answer it--maybe a glass of wine will help.

If you simply bend a panel around a set of curved ribs then the bottom of
the panel (the concave side) is under compression and the top part of the
panel (the convex side) is under tension.  

But if you dry the panel down to some low EMC and then bend it around curved
ribs and allow it take on moisture thus adding compression, there must be a
point at which the top of the panel come under compression, i.e. when is it
no longer under tension.  It first occurred to me, well no, as the panel
expands the radius becomes tighter and so there is an increase in
compression at the bottom part of the panel but also an increase in tension
at the top of the panel.  But then I thought about the formation of pressure
ridges (which form at the top of the panel as well as at the bottom) and I
wondered if that meant the top of the panel *was* under compression or was
it that the pressure ridge was forming from deeper in the panel and only
manifesting itself at the top.  So then it occurred to me that the top part
of the panel would come under compression at the point at which the rate of
expansion of the panel exceeded the panel's ability to compensate (read
bend) by forming a tighter radius (i.e. lengthening the arc).  That would
happen as a function of the resistance of the ribs to bending as well as (in
a real piano situation) the downward pressure of the strings. 

In general, the target of this mental exercise was to determine where the
panel starts out in terms of putting the top of the panel under compression
at some EMC and what kind of change must take place (how much does the width
of the panel when measured across the grain need to change as a function of
compression set) in order to put the top part of the panel under tension at
that same EMC.  It's leading to other questions as well but I must stop here
for now.  

Thanks.    


David Love
www.davidlovepianos.com