[pianotech] New string broke twice

[Noah Frere]

Okay, confused.
I read through the calculations page, did your calculation examples therein
to make sure I was using the calculator correctly, and plugged in my values.
However, my results differ significantly. I assume I am doing something
wrong. Here is what I got (*in italics)*:

*T=(fld)sqrd/K

Using .116" diameter:
T=(78*80cm*.295cm)sqrd/19400
T=174.67

Now, using 70% of this T, calculate diameter:
70% of 174.67# = 122.27#

*d=sqrt(K*t)/f*l
*d=sqrt(19400*122)/78*80
d=.247 cm = 2.47 mm = .097"*

Assuming that 308# tension is needed to bring the .118" string to pitch
(though I don't know how), using 70% of 300#:

d=sqrt(19400*210)/78*80
d=2018.4/6240
d=.323cm = 3.2 mm = .127"

Interestingly, the example on the Calculation webpage of yours is very near
my case, only it is a half-step higher and 10 cm longer. The resulting
diameter in your example is 2.63 mm, or .104", which is about half-way
between your recommended diameter of .093" and my current diameter of .116".


Also, using a lower tension of 190# (like in your example) yeilds a diameter
of 3.08 mm, or .121", only slightly lower than the above diameter.

Note that in either calculation, the resulting d is above what I was going
to order (.116").

Questions: why did our calculations differ?
How do we know what T to shoot for?

I apologize if any of this is less than spectacular. I haven't done math
like this in many years and apparently that part of my brain has atrophied,


On Tue, Jul 12, 2011 at 5:25 PM, John Delacour <JD at pianomaker.co.uk> wrote:

> At 16:19 -0400 12/07/2011, Noah Frere wrote:
>
>  ...I'm thinking of ordering from a different company, being super careful
>> while replacing, and seeing what happens. If it works normally, I could call
>> Schaff and ask for a refund. If they refuse, no big deal.
>>
>
> Did you tell Schaff which note the string came from?  If not then they had
> no alternative but to make the string to pattern, since they lacked the data
> required to calculate whether the tension was too high.
>
>
>  FYI the strings that broke - and yes I mic'd them before installing - were
>> .040 core; .118 winding; hitch to winding 5 1/8"; winding length 30 3/4".
>> The new string will be the same lengths but .039 core and .116 winding.
>>
>
> So you have a core of 17.5 or 18 with the length of winding 77.1 cm. Let's
> add 3 cm to get the speaking length (which is the only relevant measurement)
> and we get 80 cm.
>
> O.116" (your lower figure) is 2.95 mm.  The tension you require to bring
> this string to pitch is 308 lbs. which is miles too high and the string is
> bound to break.
>
> Use a value of 19400 for K in the formula you find here:
>
> <http://pianomaker.co.uk/**technical/string_formulae/<http://pianomaker.co.uk/technical/string_formulae/>
> >
>
> and work out what diameter you need the string to be, considering that for
> mwg. 17.5 you should not exceed 70% or 300, ie. 210 lbs., 220 lbs. for mwg.
> 18.
>
> You will find that the proper overall diameter for the string is 0.093" or
> less if you use a #17.5 core.
>
> Do this every time you order an odd string and you will get no breakages.
>  Otherwise you have nobody to blame but yourself.
>
> JD
>
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