[pianotech] Breaking bass string

[David Love]

Yes it has been gone over and I wish it were more clear to me the basis of
decision making including the particular tonal aspects and blending
disparate parts of the scale.  Thus, I try and avoid any real work in
scaling other than an examination for my own interest.  Can you direct me to
where to find published tables of breaking strains of piano wire.  I'm not
able to locate them.  

The formula I use I believe is derived from the tensile strength rather than
breaking strain and I do understand that bass string requirements would be
somewhat lower than treble string requirements presumably because of the
elongation that may take play in the course of playing.  

That being said, it would appear by your standards that the maximum speaking
length at #88 would be something like 46 or 47 mm for #13 gauge and I see
this exceeded all the time both by accident and design without adverse
consequences.  While this is outside the scope of the discussion on bass
string scaling I'd be interested in your comments.  Typically frictional
factors are said to potentially add about 15 - 20 lbs of tension during the
tuning process, still well within the safety margin even at 52 mm it would
seem.  

David Love
www.davidlovepianos.com


-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of John Delacour
Sent: Sunday, April 10, 2011 10:37 AM
To: pianotech at ptg.org
Subject: Re: [pianotech] Breaking bass string

At 09:10 -0700 10/04/2011, David Love wrote:


>The maximum safe tension for a given string that I have used is T = 
>.557d^1.667 representing 60% of the breaking strength,  d given in 
>mils so for your #13 gauge I would come up with .557*31^1.667 which 
>equals about 170 lbs.  That's quite a bit different from your 137 
>lbs even if you figure in the differences for metric  versus English 
>diameters.  I realize there is some disagreement in how one figures 
>the maximum allowable tension but on the surface it appears that 
>your numbers are a bit low or I'm not understanding it correctly. 
>It's certainly not unusual for a piano to have a 52 mm note #88 with 
>#13 gauge wire which pushes the tension up to about 163 lbs without 
>a breakage problem.  So please correct me if I am misinterpreting 
>your chart or better please provide the formula that you are using 
>so that I can have some means of comparison.

At 08:25 +0100 10/04/2011, John Delacour wrote:
>For bass strings the green (or orange) column in the table below 
>gives the maximum tension to which the string should be subjected 
>when at pitch to eliminate the risk of breaking.

All this has been discussed before, David.  These are the figures I 
use for bass strings and they are figures that work in the long term. 
If these figures are greatly exceeded then sooner or later the 
strings will break.  At least half the odd strings I get for 
replacement have broken because they did exceed these figures.  It 
may take a month, a year or 50 years but they will break.  I base 
these figures on published "breaking strains" and not on some 
catch-all formula.  It it well known that the thinner gauges have 
higher tensile strength.  You can just about get away with 52mm and a 
#13 on more 88 (which I emphasize is not a bass string!) but how many 
50 year-old pianos so designed do you come across without breakages 
in the top treble?  Besides that, tuners are generally careful not to 
take these high strings much above pitch.

You have given me no actual figures for "breaking strain".  From 
previous discussions on this list I gather there is some American 
table of breaking strains and that other people on the list use 50% 
of this value and not 60%, which would mean their figures come much 
close to mine and Paulello's, but it's a free country and the more 
people continue to design strings with excessive tension the better 
it is for us stringmakers.

I stick with the figures I have given and the  majority of makers of 
quality pianos have always used such figures.  Luckily quite a few of 
the Chinese makers exceed them and that is great news for future 
stringmakers.

JD