[CAUT] Wire Stretch

[John Ross]

A few years ago, I was called to tune a piano, and it was down, I think 
about 2 tones. I should have written the specifics down, but didn't.
I said, that it had been a while since it had been tuned. Naturally, I 
checked for a separated pinblock, or some reason for this extremely low 
pitch. I asked him when it had been tuned. He remembered the year, as 1929, 
he had been a child when it was delivered.
I believe the reason was string stretch.
I brought it up, a semi-tone at a time, and then a fine tune.
It must still be ok, as he hasn't called me back.
Then again, he might believe that every 75 years is good enough. :-)
John M. Ross
Windsor, Nova Scotia, Canada
jrpiano at win.eastlink.ca
----- Original Message ----- 
From: "John Delacour" <JD at Pianomaker.co.uk>
To: "College and University Technicians" <caut at ptg.org>
Sent: Friday, April 27, 2007 3:47 PM
Subject: Re: [CAUT] Wire Stretch


> At 11:09 am -0400 27/4/07, Jeff Tanner wrote:
>
>>On Apr 26, 2007, at 7:17 PM, Fred Sturm wrote:
>>
>>>     Wrapped strings are much closer to breaking point/higher tension 
>>> than the neighbor plain wires. The lower tension wires take much less 
>>> change in any factor (length, deflection, tension) to produce a given 
>>> pitch change than the higher tension wires. Think about how much turn on 
>>> the tuning pin it takes to raise that lowest plain wire 25 cents, 
>>> compared to what it takes to raise the top wrapped string the same 
>>> amount. Think about the same comparison when you are first chipping to 
>>> pitch. Crank and crank on those wrapped strings, where a single yank 
>>> gets the plain ones to pitch and past.
>>>
>>
>>But which one stabilizes first?
>
> I think the point Fred makes, which I agree with, is that the plain
> wire strings at low tension do not stabilize as regards pitch.  The
> most succinct description I know of of this phenomenon comes from
> Wolfenden, writing in 1916.
>
> _____________________________
>
>         It is now known from experience that practical equality of tension
>      throughout the instrument tends to prevent changes in the tuning due
>      to variations in the temperature.  When the tension is equal the
>      temperature movements are equal.
>         In former days pianofortes were exceedingly sensitive to changes
>      of temperature, mainly because the fact noted above was unrecognized
>      or disregarded.
>         A thermometric movement of a few degrees often sufficed to render
>      (in a very short time) an instrument unusable, and had the tuner
>      recently paid one of his visits, the discredit of the change was
>      charged to him.
>         The notes in which the greatest changes occurred, were the lower
>      ones which were strung with plain steel wire.
>         I have known numerous instances in which the changes in these 
> notes
>      were equal to nearly a semitone between midsummer and midwinter,
>      while the other parts were relatively stable.
>         This was due to the customary very low tension of these notes.
>         There seems to be a point in an ascending scale of tension, at
>      which the elasticity of the wire is almost suddenly developed (*) to
>      an extent we could not anticipate, so that a difference, so that a
>      difference, which would be very serious at a general low tension,
>      will become more tolerable.
>         To make this intelligible, let us suppose an instrument in the
>      tension of pitch C is 130 lbs., and that of C two octaves lower is
>      only 100 lbs.  This piano will be extremely sensitive.
>         But let us now suppose that we can lift the tension so that pitch 
> C
>      stands at 200 lbs. and the other at 154 lbs.  While there still
>      remains a liability to change, it is much reduced, although the ratio
>      of the difference in unaltered.  Covered bass strings, which are
>      usually at a rather higher tension, seem immune from this disease.
> _____________________________
>
> I have marked with an asterisk the observation that I find most 
> interesting in this, namely: -
> "There seems to be a point in an ascending scale of tension, at which the 
> elasticity of the wire is almost suddenly developed to an extent we could 
> not anticipate" -- and I think we would search in vain on the WWW for a 
> simple explanation, or any explanation at all, of this phenomenon so 
> familiar to piano makers.
>
>
>>In other words, the plain wire is more "elastic"?
>
> Are you sure you do mean elastic?
>
>>I'm no physicist.  But it just seems like plain wire keeps on and keeps on 
>>stretching over the years.
>
> I'm no physicist either but the answer is fundamentally yes, in many 
> cases, but there are many factors in play and the mathematics and physics 
> of it are highly involved.  Besides that I am not aware that any serious 
> specific study has ever been carried out to quantify or explain what does 
> happen to patented steel wire (piano wire) under the strains to which it 
> is subjected in a piano.  It is worth noting too that patented wire of 
> different manufacturers behaves differently.
>
> As we all know, new strings fall in pitch quite dramatically and become 
> more and more stable with subsequent tunings.  It is said that piano wire 
> is elastic, in other words that any deformation it undergoes when strained 
> within its "elastic limit" will be reversed when the strain is removed and 
> the wire return to exactly its former state.  This is a gross 
> simplification of what actually happens, as experience shows.
>
> A few years ago I asked the brother of a friend of mine, an 
> extraordinarily brilliant Swiss scientist, about this business of initial 
> pitch drop, and he was able to explain it immediately and in more detail 
> than I could actually absorb, and his explanation is that when the stain 
> is applied the molecules of the steel (which incidentally in the case of 
> patented wire is by no means homogeneous) very slowly through minute 
> vibrations and displacements, realign themselves to achieve for themselves 
> the position of least stress for the given new strain, and this leads to a 
> slight lengthening of the wire.  When the wire is next re-stretched to 
> pitch, the difference is far less between the previous "comfort alignment" 
> and the the alignment required for maximum comfort under the slightly 
> higher new strain.  Less internal repositioning is thus needed to achieve 
> the equilibrium and the drop in pitch is consequently less.
>
> I think the common term for this phenomenon is "creep".
>
> There are then to be considered a) the "true elastic limit" or "limit of 
> perfect elasticity" of the wire, and b) the "yield point".  The "breaking 
> strain" or "ultimate tensile strength" of the wire is not a useful figure 
> for the practical piano maker.
>
> The "true elastic limit" is quite a low figure.  I can't quantify it, but 
> it seems probably that most strings on any piano will exceed this limit. 
> Possibly it is beyond this limit that "creep" begins.  A.E.H. Love gives 
> for Bessemer steel an elastic limit of 1780 atmospheres and a yield point 
> of 2650 atmospheres.  That is to say the elastic limit for this material 
> (_not_ patented steel wire, which is very different) is 67% of the yield 
> point.
>
> Love (A Treatise of the Mathematical Theory of Elasticity - 1926. From 
> Dover) cites a number of previous researchers and notes an experiment by 
> Vicat:- " He found that wires held stretched, with a tension equal to one 
> quarter of the breaking stress, retained the length to which this tension 
> brought them throughout the whole time of his experiments (33 months) , 
> while similar wires stretched with a tension equal to half the breaking 
> stress, exhibited a notable gradual increase of extension."
>
> There is much other tantalizing stuff in this treatise.  I wish I were 
> qualified to read it all, but the reason I have it is that the text is 
> highly readable and at moments comprehensible in part even to my untutored 
> mind.
>
> JD
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