[pianotech] Reversing Crown

[Noah Frere]

got it

On Thu, Dec 31, 2009 at 10:44 AM, David Love <davidlovepianos at comcast.net>wrote:

>  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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