[CAUT] Wire Stretch

[RicB]

Hi Mark.

Yes.. I'd say that was a lot of negative deflection.  I'd also have less 
trouble understanding how that was possible, at least on the surface of 
it, then very much more positive deflection then a couple mm on average. 

At some point... tho I will be the first to say I am unsure where that 
point is,  a soundboard assembly will not be able to over come the 
bearing tension placed on it by the string plane... that much I think we 
can all be in agreement on.

The whole point of the exercise of figuring how much bearing strain is 
caused for a given change in deflection is simply to shed light on how 
much vertical force is required of the soundboard for that change.  
Those of you who know just how much a soundboard can push (or pull) can 
perhaps chime in on where things become less then realistic :)

For my part... it seems to me like there must be more twisting and 
turning going on that effectively changes string segment lengths then we 
generally think. If I get some time later on tonite... I throw out some 
bearing numbers that result from a 5 mm change in deflection from an 
assumed 0 deflection starting point for three or four different string 
lengths.

Cheers and thanks for the interesting post... I agree that there has to 
be significant vertical movement going on... its just that it seems like 
there has to be a limit to how much the soundboard can move vertically, 
and it looks to me like the pitch changes we see require quite a bit 
more then what that limit allows for.

RicB



    Ric, and all

    I have a Baldwin L awaiting stringing, where the bridges have dropped so
    far, that a bearing string stretched from hitch-pin to agraffe will not
    touch the top of the bridge-pins, never mind the bridges! We're talking
    5mm - 6mm negative.... how's that for deflection?

    Last summer, when the piano was torn down for re-string prep, bridge
    re-pinning, notching, etc. the bearing was positive. And this is not the
    only piano I've witnessed this on.

    Last November, on another instrument, several of us noticed the
    low-tenor
    strings resting on the plate, on their way "down" to reach the tenor
    bridge.
    This bridge was "positive" in August.

    Our shop falls into single-digit RH in winter, nonetheless, it's not
    uncommon to see this kind of movement on instruments w/o Dampp-Chaser
    systems, to the point one feels the bridges are actually and barely
    being
    held "up" by the strings.

    BTW, none of these boards have cracks, and they seem to return to
    positive
    bearing, once moisture content is restored.

    So, in my experience anyhow, there is "plenty" of deflection going on.

    I have to work with "simple ideas" :>), so I'm still stuck on soundboard
    movement in response to changing RH,  governed by the varying
    elasticity of
    strings across the scale, as the most obvious cause of pitch change.

    In winter, I witness a common curve beginning with anywhere from
    30cts flat
    at the lowest plain wire note, dropping incrementally up the scale, and
    leveling-off at about 5cts flat by A4, or so.

    In the summer I see a vrtual reverse, again, in a very smooth curve,
    dropping typically from about 30cts sharp, a cent or two at a time,
    towards
    the middle. And yes, there is usually a "spike" at the breaks, in some
    pianos more than others.

    So, as enticing as the study of wire stretch, bridge-compression,
    etc. are,
    I have a hard time "imagining" these factors reversing with such an
    equal
    and opposite reaction as my seasonal pitch readings indicate.

    A soundboard you can spring up or down, and it will spring back, there's
    lots of elasticity there. And as witnessed in our shop, those things
    really
    do move up and down with the seasons.

    best regards,
    Mark Cramer,
    Brandon University


-------------- next part --------------
An HTML attachment was scrubbed...
URL: https://www.moypiano.com/ptg/caut.php/attachments/20070503/93cceabf/attachment.html