[pianotech] Reversing Crown

[David Love]

I think what's confusing is the idea that there can be negative bearing
throughout the piano.  Unless the plate height was set up that way negative
bearing can only occur in some section(s) of the piano, not the entire
piano.  Negative bearing probably occurs mostly because the crown in those
sections has collapsed or because the bearing in adjacent sections has been
set too high (or both).  There is likely net positive pressure on the
assembly pushing everything downward but in those sections where there was
inadequate crown the board has been pushed past the point where the bearing
will remain positive.  Recall that the bridge itself connects all these
sections together so that if you press down on the bridge in one section
that pressure is transferred to adjacent sections by virtue of the stiffness
of the bridge itself.  On these pianos, then, the panel is going through
quite a distortion of being pushed down in one area and pulled up in
another, a sort of soundboard scoliosis.  It's no wonder that these types of
instruments often have some distortion and a dramatically changing tonal
character as you go through the scale.   It also illustrates why it's so
important to assess the crown and bearing of the entire assembly when
determining whether you can keep the board or reset the bearing in a way
that will give a positive result through the scale.   

 

David Love

www.davidlovepianos.com

 

From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Noah Frere
Sent: Thursday, December 31, 2009 7:22 AM
To: pianotech at ptg.org
Subject: Re: [pianotech] Reversing Crown

 

I find this confusing because I would have assumed that as soon as the
soundboard starts pulling away from the strings, and negative bearing sets
in, then the strings would be pushing the soundboard down further, rather
than pulling up. I imagine it this way: imagine a tunpperware container with
a lid that's slightly convex - you push the sides together and it forces the
lid up higher. (the equivalent of putting on strings). Now take the pressure
off the container and push the lid down just enough to make it concave,
reapply pressure, and the lid will continue its downward path. 
   Visually this description reminds us of the soundboard, but i mean to
apply it to downbearing. That is, no matter which direction the crown is
facing, the tupperware lid represents the board/bridges as a whole.  
   However, being neither a physicist nor a rebuilder, i must have it wrong,

On Thu, Dec 31, 2009 at 1:06 AM, David Love <davidlovepianos at comcast.net>
wrote:

That's right.  Anytime you have negative bearing, the strings are pulling
the bridge up just as with positive bearing they are pushing the bridge
down.  If you were to detach the strings from the bridge pins where there
was negative bearing they would hover above the bridge.  You would have to
push the string down to reattach it to the bridge thus the force from the
string would be upward.  

 

David Love

www.davidlovepianos.com

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