Elastic limits,

[Jim]

Hi Ed:

In your post you asked about the percentage of break strength of piano
scales. Most piano designers try to stay around 60% to 70% of b.s. In
my post which you referenced, I mentioned that normal friction at the
agraffe may add 10 to 20 lbs tension to the tuning pin segment. When there
is corrosion, electrolytic bonding, etc, the resistance may increase
drastically.

The large majority of string breakage during pitch raising occurs at the
bend at the tuning pin.  When strings break at the V-bar, it is usually
due to very heavy playing (or heavy tuning?). As I believe Ron pointed
out, severe angles at the Vbar can contribute to string breakage in that
area, but it occurs much less frequently than at the T pin during
pitch raising. It will however cause a greater differntial in the tensions
of the two or three segments of the wire.

On a vertical piano there is resistance at the V-bar and then there is
added resistance at the pressure bar causing there to be greater tension
in the segment next to the tuning pin than in the second section. Until
the string renders thru the friction points the speaking length has the
least tension during pull up. The amount of frictional resistance
determines what the differential in tension is between the three segments.
The pressure bar usually has a much great radius than the V-bar or the
tuning pin, so strings seldom break at the pressure bar or in the case of
a grand at the counterbearing bar. (in fact, I have never seen it happen).

Now to your main question about scales near 250 lbs. If a plate is badly
located so that the speaking length of note 88 is over 2.25 inches, there
is great likelihood of encountering string breakage in just normal tuning.
Of course, poor bridge location may do the same thing. There was a
case within the past year where my son Phil encountered a length of 2:315
in a piano at note 88.  He could not get a new string to go on without
breaking when he tried to sock in the tuning. Using the Parson PSCALE
program (an excellent investment)  tension is 207 lbs. Add to that the
added tension of the hammer shock, and the need to pull the string
above before settling, you can see the danger of breakage. It is my belief
at this time that there is more work hardening at the T pin since the
tension is greater there during pullups than at the V-bar except for real
heavy repeated pounding.

Have you noticed that when a string breaks, right next to the rupture the
string has started to neck down? this deformation begins at the yield
point for that particular wire size.

One time while stringing the tenor section of a 6'4 Steinway A, I was
using strings made by a substitute string maker that did not use the same
wire specs. As I would pull up the string toward pitch, about 1/2 step
below, the tone began to go dead. and then BANG before it could arrive at
pitch. The yield point was reached at the time the string began going dead.
After breaking 4 or 5 of the 10 strings in that section, I called
Steinway to order replacements. That piano is still going good today and
that restring job was 40 years ago.

Jim Coleman, Sr.

On Sat, 26 Apr 1997 A440A@aol.com wrote:

> Greetings all,
>
> I read in Jim Coleman's (<snipped>) response;
>
>  > using strain gauges to mark what
> >happens when a string breaks. When tension is gradually added to a string
> >without frictional resistances (such as agraffes) a string begins to
> >yield a bit before it breaks.  This is where time enters the picture.
> >
> >Here are some examples of tests taken at the CGConn engineering labs while
> >I was there.
> >
> >wire size    yield point    break point
> >
> >13           258 lbs        290
> >13.5         267            303
> >14           317            355
> >15
> >16           347            398
> >18           431            487
> >
> >
> >From all of this you can see where time enters into the picture. There is
> >a measureable period of time between when the string comes to the yield
> >point and the point where it breaks.  My impact method obviously
> >allows me to live somewhere between the yield point and the break point,
> >because I definitely do have less string breakage now than I used to have.
>
> <snip>
> >There can be as much as 10 to 20 pounds greater tension in the segment
> >between the tuning pin and agraffe as there is in the speaking length
> >while pulling pitch up.
>
>       I have to ask,  if these numbers are giving the yield and breaking
> tensions of music wire,  and the usual string tension in a piano is a
> percentage of the yield, ( 80%, approx?),  then how are these higher  numbers
> applicable to our tuning at the much lower tensions pianos use?  Anybody got
> scales that use music wire at  250 lbs for size 13??  It seems not likely.
>   Is the capo, or agraffe friction able to generate the difference in tension
> between speaking length and tuning pin?
>
> >the Chickering (which I did this morning).  The string approaches the
> >tuning pin at such a high angle from the V-bar Agraffe pin that it tends
> >to climb over the previous coil on the pin.  A brand new string can break
> >easily at that point of greater bend. Before raising the pitch of this
> >piano, I took the precaution of changing the angle of the coils on the
> >pin and tapping the string back down under the previous coil.
>
>      I would suggest,  that breakage is usually caused by the elastic limit
> being  well surpassed on the outside of the curve of the wire, where it goes
> around the agraffe, or V-bar.  This outer circumference  is the place where
> breaks begin.  If the string is put near it's break limit, and then forced to
> bend,  the wire will micro-fracture on the outside bend.  If these fractures
> continue, they effectively reduce the diameter of the string at this site,
> and the breaking tension for this site is reduced.  Eventually,  that limit
> will decrease to the tension used in play, or tuning, and the string will
> break at it's weakest section.   I believe that this is the reason for the
> propensity of steeply angled strings to break, rather than the excessive
> friction generated by the sharpness of the turn.
>      But perhaps I am totally missing something here,   ( I have found a
> forest before by walking into a tree......)
>
> Regards,
> Ed Foote
> Precision Piano Works
> Nashville, Tn.
>
>
>
>
>