pre stretched strings

[Richard Moody]

----------
> From: KUANG <v137z2ng@ubvms.cc.buffalo.edu>
> To: pianotech@byu.edu
> Cc: V137Z2NG@ubvms.cc.buffalo.edu
> Subject: Re: pre stretched strings
> Date: Friday, April 11, 1997 6:56 PM
>
> Hi Mr. Moody and list:
> A piano string stretches because it's under a constant tension.

Umm, if it stretched, the tension wouldn't be constant. OK continuous
tension.  For it to be musical, the wire at some point would have to
maintain a constant tension.  This "constant tension" I think is what
the laws (theories?) of elasticity are based. (what ever they are).
But so what, music wire does maintain a constant tension enough to
satifsy the musical and tuning demands.

<The rate  at which it streches depends on the material, temperature,
tension and the
> relative length it already has streched (plus other factor(s) I
missed?).

The way I understand it, the makers of piano wire in the late 19th
and early 20th centurys made a steel wire that would remain elastic,
(which means stay constant under a certain tension) for a period  of
time to be determined not because of the continuous tension itself
but because physical changes would occur in the wire because of the
constant tension. This tension they predicted would weaken the wire
because it would cause molecular changes in its composition.
Something about crystalization.  I am sorry I cannot at the moment
quote sources. (This might be the reason the wires on the Golden Gate
bridge were changed) However wire from that period of time has not
shown failure due to their predictions.  When say 25% of the wire on
pianos from the 1890's and 1910's starts breaking by itself, we will
know their predictions are comming true.
	As far as piano wire stretching over the years, how do you account
for pianos that go out of tune sharp?   (the sound board swells
usually with humidity)

> Of course, the longer it has stretched, the slower it stretches
further.
> And when the rate at which it streches reaches about zero, it will
lose
> its elasticity and breaks (the elasticity decreases as the string
> stretches, does this make sense?).

	No, the string would not be elastic if it continued to stretch.
Music wire is know to have limits of elasticity.  If stretched over
this, it no longer is elastic.  The illustration given is that a wire
of a certain diameter is "stretched" to a certain tension.  The
diameter will not change.  If it is stretched over its elasticity
limit, it will at its weakest point become narrower, and will stay
narrow there when the tension is released.  It is this narrow point
that the string truly has been "stretched".

> Something is "elastic" if the material can be compressed and
stretched.

I might be wrong here, but compression is not elastic execpt if the
object is considered a spring."elastic" may be properties of
stretching or torsion, or bending, correct if wrong. In fact if the
elastic object is released from tension, I don't think you can say
compression occured.

> When a wire is bent, one side is stretched and other side is
compressed.

this is the subject advanced physicists have written about. But so
what, it doesn't change the way the strings on the piano  you are
playing behave. A lot we have to take as it comes, reasoning about it
may or may not be relevant. If it aids in understanding.

> I believe it's easier to stretch metals than to compress it, I
could be
> wrong (isn't that because metallic bonds are much shorter?).
> 	A string breaks at striking point because the rate of stretching
> is faster there due to frequent striking _and_ higher local
temperature
> (the temperature at that point of the string).

	Piano wire never breaks at the striking point.  The striking point
is where the hammer strikes the string.  From hard playing the string
tends to break at the upper bearing, ie the agraffe, capo bar, or it
upper bearing point.  It breaks at the tuning pin from too much
bending or too sudden bending. Exceptions to the above might be due
to corrosion.

Richard Moody
4-12-97