Maximum tension

[Delwin D Fandrich]

Beg to differ...

Using the published specifications of a modern music wire of a diameter that
would be typically used in the highest 4 to 6 treble unisons, such as Mapes
'International Gold,' #13 wire has a diameter of 0.785 mm, or 0.031" --
using the U.S. wire gage -- and a breaking point of approximately 135 kgf
(or approx. 295 lbs) of tension.  This specification is an average derived
by stretching a number of samples to their breaking point and averaging the
results together.  Various individual string samples will break at tensions
just a bit lower than this, others somewhat higher.

The commonly accepted maximum upper tension limit for this wire would be 70%
of this average figure, or about 94.5 kgf (206 lbs).  This is considerably
higher than the figures commonly derived by calculation using the various
formulas cited for this purpose.

I should point out that designing a scale to 70% of the breaking strength
would leave very little tolerance for error.  If the bridge is off by just a
bit, or if a tuner came along who believed in lots of stretch, the wire will
probably be over-stretched and there will probably be a premature failure.

As to what pitch this would be depends entirely on the speaking length of
the wire.  At 54.0 mm, C-88 will have a tension of 79.5 kgf (175 lbs),
assuming no stretch to the tuning.  By my calculation this works out to
about 59% of the average breaking strength -- a figure I consider to be
safe.


Del
Delwin D Fandrich
Piano Designer & Builder
Hoquiam, Washington  USA
E.mail:  pianobuilders@olynet.com
Web Site:  http://pianobuilders.olynet.com/

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----- Original Message -----
From: Newton Hunt <nhunt@jagat.com>
To: <pianotech@ptg.org>
Sent: May 21, 2000 12:59 PM
Subject: Re: Maximum tension


> Kristinn,
>
> At what pitch a string will break is a function of hit's
> speaking length.  A top C note of 51 mm speaking length, 13
> gauge wire, 156 pounds tension and has a breaking per cent
> tension of 64%.  Change the speaking length to 53 mm, 13
> gauge, 168 pounds and 70% breaking tension.  70% is outside
> acceptable range because any additional friction and a bit
> of age and the string will break.  It does not matter what
> size string you install it will break at the same pitch,
> approximately.
>
> 49 to 51 mm speaking length is the acceptable range for a
> piano.
>
> In order to determine at which tension such a string will
> break I would have to put on my thinking cap and math hat
> and right now I am too tired for such arduous work.
>
> Newton
>
>