[CAUT] Wire Stretch

[Wigent, Donald E]

Don Wigent here:  
I agree with you I am not concerned about string stretch as much as
board movement,  give me RH control any time and I will be a lot happier
with a lot less work turning pins.

-----Original Message-----
From: caut-bounces at ptg.org [mailto:caut-bounces at ptg.org] On Behalf Of
Mark Cramer
Sent: Thursday, May 03, 2007 1:24 PM
To: College and University Technicians
Subject: Re: [CAUT] Wire Stretch

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



-----Original Message-----
From: caut-bounces at ptg.org [mailto:caut-bounces at ptg.org]On Behalf Of
RicB
Sent: Thursday, May 03, 2007 12:00 PM
To: caut at ptg.org
Subject: [CAUT] Wire Stretch


Hi Fred.

I have never seen a piano behave like this in 30 + years of working on
them.  There is always a major shift at each break in the scale. Most
times one can even see a slight jump even when the scale is
broken/widened at the front termination and not at the bridge.  I've
watched pianos very closely for a long time relative to climatic
changes, and what I see is a similar type of movement in each section.
If the lowest part of the tenor is sharp relative to the rest of the
tenor section... then you see the same thing in the tenor/treble
break... and the treble/diskant break (up to the last highest octave)
and in the bass section.   Same thing in reverse if the low tenor has
become low with respect to the rest of its section.  The pattern is
nowhere near as severe in the bass or highest treble (diskant)... but
its there.  I dont think I have ever seen an exception to this <<rule>>.

The only part of the piano that doesnt seem to fit into this is the
highest octave...especially from F-G7 upwards.  Seems to do what the low
tenor is doing and seems to have a very large reaction.

There is another problem with the idea that bridge/soundboard growth or
shrinkage can be so very much involved.  The amount of downbearing
change required to account for any significant pitch change is just too
high to account for more then a small amount of vertical deflection
change. If you start off with a say half degree string deflection in the
tenor and graduate to 1.5 degrees in the treble as a starting point...
and start figuring out how much vertical change is needed to account for
pitch changes and then look at resultant downbearings.... it gets scary
real quick.  Even with the change in length across the bridge surface
that would happen as a result of a change in string offset angle through
the bridge pins when the string  climbs up/down the pins.

I posted a small calculator that does shows rough results for a single
string a while back.  If anyone wants to take a look at what one has to
accept as <<doable>> for a soundboard in terms of vertical deflections
for pitch changes... drop me a line.

Cheers
RicB


     > What is problematic about
     > this is that any such change in string position height to the
    bridge pin
     > is governed by the response of the surface of the bridge to
climatic
     > changes, and hence quite uniform... which in turn dictates a
pitch
     > change that is directly related to the length of the string.
Shorter
     > strings will be more effected then longer strings

        Yes, and that does happen on some pianos (response to humidity
        change very
        large in the top octave or two). Steinway and Baldwin products
        come to mind.
        Not so on other pianos, as Yamahas for instance. I haven't a
        clue why there
        should be that difference between makes, but I do always know to
        anticipate
        that enormous pitch change in octaves 6 - 7 on Hamiltons and
        1098s (and
        Everett studios), and to a lesser extent on the grands of those
        makes. We're
        talking within 20 cents in the rest of the piano, and 30 - 50 in
        that top
        octave and a half.


    ... hence a very
     > graduated and even curve of change in pitch.... which clearly
    does not
     > happen. We see big breaks where the bridge is doglegged which
    need to be
     > accounted for amoung other things.

        Yep, lots of strange things happen, and lots of factors are
        involved. I have
        become more and more convinced that bridge growth and shrinkage
        in all
        dimensions (height, width _and_ length) are quite significant,
        sometimes
        more than soundboard. This could explain why some pianos just
        don't seem to
        want to stabilize as much as others with a Dampp-chaser (excuse
        me, ahem,
        Piano Lifesaver) system installed. A thought, anyway.
            So many factors, including lamination/solid (bridge and
board),
        compression/rib crown (and does it still have crown?
        Downbearing?), rib
        cross section, age of wood (how hygroscopically active is it?),
        orientation
        of grain, fineness of grain . . .  And then you add the wire
        factors.
            We will never really get a handle on the inter-relations of
        all these
        factors. I'm not saying it can't be done, but nobody is going to
        go to that
        much trouble. We might tease out a few things, though. Ron's
        epoxy laminated
        bridge, for instance, and how much of a difference it makes.
        Regards,
        Fred Sturm
        University of New Mexico