Soundboards/stress

[lenny]

Bob,
In one of the pianos I tune, in a certain church, had stress or tension
cracks  on the soundboard.  I remembered something that Randy Potter said
about using a complete Dampp-Chaser System in the piano would fix this
problem.  So, I ordered a DCS from Webb Phillips and put the system in the
piano.  One week later I went back to see the results and the stress cracks
were gone.  These Dampp Chaser Systems do work, and they not only help keep
the proper humidity in the wood, but preserve other parts of the piano as
well.

Leonard Childs, RPT
Childs Piano Service
San Antonio, TX
lenny@stic.net

----------
> From: rhohf@idcnet.com
> To: pianotech@ptg.org
> Subject: Re:  Soundboards/stress
> Date: Thursday, June 12, 1997 7:26 AM
> 
> There have been a lot of interesting and thoughtful ideas in this thread.
 One 
> of the things that makes this business so fascinating is that we have
been 
> making soundboards for centuries, but the fundamentals are still so
poorly 
> understood:
> 
> "the strings' tension is attached to the board's compression"
> 
> "The swelling board puts pressure, not tension on the cells......In all
of 
> this, we never see the word tension."
> 
> "... the unloaded soundboard is "crowned" as a result of tension, then 
> controlled by the amount of downbearing."
> 
> None of these statements are quite right since a soundboard with ribs
attached 
> is a complex system containing tension _and_ compression.
> 
> Let's try to look a little more closely at the stresses that are
introduced 
> into a rib/soundboard system by conventional construction, and see if we
can 
> clarify this a bit. First the wood basics:  wood expands and contracts
with 
> changes in moisture at least twice as much per unit dimension in the
tangential 
> direction (soundboard thickness) as the radial (ribbing direction),
changes in 
> length are negligible.  A normal piece of  spruce, allowed to expand and 
> contract in the air developes no stresses (discounting the cellular
level).
> 
> When a rib is glued across the grain (radially) to a dried and shrunken
spruce 
> panel, and then the whole thing is allowed to absorb moisture, the panel
tries 
> to expand in width and the rib doesn't change in length.  This creates 
> _compression_ on the bottom surface of the panel, _tension_ on the top
surface 
> of the rib, and huge shearing stress on the glue joint.  It is the action
of 
> this compression and tension which causes the system to arch (if it is
free to 
> move) until these stresses are balanced by _compression_ on the bottom
surface 
> of the rib.  The shearing stress on the glue joint is what makes failure
of the 
> joint so common.  
> 
> So what stress is on the top surface of the soundboard?  Is it under 
> compression or tension?  This is difficult to determine in a
non-destructive 
> way in a glued-up board or an assembled piano.  It seems to me that there
are 
> devices that can scan the surface of metal beams and determine the
stresses on 
> the beam.  Perhaps one of the engineers out there could comment on
whether this 
> can work on wood.
> 
> However, pursuing that question, I have made quite a few "model" 
> soundboard/ribs  in different configurations and  evaluated the stresses
on 
> every surface of the panel and ribs.  How this is done would take too
long to 
> describe here.  But every configuration I have tried has resulted in
_tension_ 
> on the top surface of the soundboard. 
> 
> There is another factor which sheds some light on the top surface of a 
> soundboard: cracking can reveal a lot about the stresses.  Cracks mean
tension. 
>  Compressed wood _cannot_ crack.  This is not to say that excess
compression 
> can't set up a condition that results in tension/cracking.  In any case,
if 
> soundboards were "compressed" as many seem to believe, cracks would not
form.  
> As Brent Fisher said, "Each board reacts differently".    I believe the 
> question is not, "Do some soundboards have tension on the top surface?"
but 
> rather, "Is it possible to make a crowned soundboard which is not
tensioned on 
> the top surface?"  This is an opinion based largely on my models, which
are not 
> real-life soundboards, and would be difficult to verify conclusively in 
> assembled pianos.
> 
> All of my comments refer to unloaded soundboards.  So what does loading
do to 
> the stresses?  Applying downbearing does not simply put the board into 
> compression:  if it did, boards would not crack.  I thought I might go
into 
> this here, but this message is already kind of long.  Maybe another time.
> 
> Bob Hohf
> Wisconsin