Steel wire in tension

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    Member greybeard's Avatar
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    Default Steel wire in tension

    I'm familiar with Hooke's law and Young's modulus at school level, but I need a little more info please, so a quick lesson covering the following situation would be a good start to the New Year.

    "Strings" of musical instruments (in my case stainless steel wire of unknown parentage) when first mounted under tension are stretched until they vibrate at the desired pitch.
    24 hours later they need tightening a little more as they have "relaxed"(?) a little. Possibly the frame of the instrument has compressed a little as well, but that's another issue. Or is it ?
    This process continues, and may be weeks, even months, before the pitch and tone/timbre of the string has settled. Again the changes to the frame will have an input.

    Firstly , I suspect that there may be some changes occurring at the crystal level within the metal, so some guidance here would be good.
    Secondly, if it were possible to stretch a long length of wire to some appropriate tension, leave it for a period of say a month, then cut the required lengths from that pre-stretched wire, would this process shorten, or even remove the need for the above "ageing" process ?

    Happy New Year, everyone,

    John

    PS via EDIT
    This change in the tension of the string is occurring without any degree of playing, immediately after the manufacture of the instrument is completed, and leads to a "settling in" period that I should like to reduce/avoid if possible, before offering it for sale.
    All input welcome, particularly on my two points above.

    Similar Threads:
    Last edited by greybeard; 12-29-2011 at 08:26 AM. Reason: Afterthoughts
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    Hi John

    Hook's law and Young's modulus are only ever guides and approximations. For example the extension of a spring depending on the weight applied to it is never truly linear as Hook's law denotes. As the spring stretches it characteristics are altered in a very subtle complex way.

    I worked for a company many years ago who sold weighing machine that used a vibrating wire, similar to a guitar string that had a magnetic pick up below it. The frequency then could be related to the weight being carried on the scale. The electronics always had to be tweaked to compensate for the non linear characteristics of weight to frequency that where observed.

    With regard to having to constantly re-tension the guitar string for a time until it, somehow, settles down is not so surprising. The manufacturing process is establishing all sorts of stresses with the structure of the wire and over time these will want to normalise themselves. In some manufacturing process this can be speeded up by heat treatment, other use time to do this. Years ago manufactures of machine castings used to age their casting out doors for a few years to let them 'rest', this gave a much more stable casting and hence a consistently accurate machine.

    I would suggest some experimentation and pre tension the strings you are using until they are ready to be used.

    I hope this may help Brian

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    Thank you Brian.
    "Normalisation" - another key word, I think.
    I have some distant memory of crystal domains changing when under stress. Not just an elastic deformation, but a change of size/shape/lattice structure.
    Whatever the change, I can see that a long period of rest, or a heat treatment possibly reversing the process.
    What I'm looking for, I suppose is hints along the lines of " over-stress by 200%, hold for three weeks, then cut and fit".
    Regards
    John

    It's like doing jigsaw puzzles in the dark.
    Enjoy today's problems, for tomorrow's may be worse.


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    I suspect much of the 'relaxation' is taking place at the anchor points at both ends of the string and the bridge and fret the string passes over at each end. (Am I using the correct terms, bridge and fret, I am not a musician.)

    During drawing music wire is deformed and stretched, and very likely subjected to more stress than it ever sees in use, that I doubt whether it changes much during use. Heat treating or normalizing it would probably reduce its yield strength to the point that it would plastically deform during tuning to the extent it would be un-tunable.

    An open mind is a virtue...so long as all the common sense has not leaked out.


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    Hi Geof,
    Good to read you again, and Happy New Year.
    My reference to heat treatment was only in response to the previous post, and part of my acquisition of extra knowledge, rather than a contemplation of heating the strings.
    An interesting point about the effects at the points of anchorage and the bridges, rather than in the length of the string.
    However, in my case, I've noted a drop in pitch of about a semitone overnight on a steel string about 9" long.
    I'll have to do some head scratching to work out what that represents in term of % change in tension, but might be interesting.
    John

    Last edited by greybeard; 12-29-2011 at 10:50 AM. Reason: typo
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    Quote Originally Posted by Geof View Post
    Heat treating or normalizing it would probably reduce its yield strength to the point that it would plastically deform during tuning to the extent it would be un-tunable.
    Both normalizing and stress relieving can be done at relatively low temperatures with only minimal affect to mechanical properties.

    I doubt this is done on instrument strings though. I have experienced this in the past with steel parts which involve raising the temperature to a couple of hundred degrees C and down to room temperature, then repeating this cycle several time. Is was a bit of 'cookbook' engineering though.

    Brian

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    Quote Originally Posted by bwprice100 View Post
    ....... a bit of 'cookbook' engineering though.
    Brian
    By the same author as my "Bucket Chemistry"




    John

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    I'm no expert on this but I have a few guitars & know how to setup them etc. And I know how to build guitars.
    (note, I'm guessing here, I don't have facts)

    I would not worry about strings "stretching". If they do it's very minor IMO.
    Because:
    - When you tighten the strings, they will start pulling the neck which is usually made of wood. Wood takes time to settle... Inside the neck is a metal truss rod to counteract the pull of the strings and to adjust how much the neck bows.
    When putting & tightening strings (or adjusting truss rod) it can take a long time for the wooden neck to settle. For this reason, it's good practice to never remove all strings; but to replace one at a time.

    Just my best guess

    :) Current build: http://www.cnczone.com/forums/diy-cnc-router-table-machines/264838-new-machine-desing-quot-cnm13-quot.html


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    Quote Originally Posted by greybeard View Post
    ......I've noted a drop in pitch of about a semitone overnight on a steel string about 9" long.
    I'll have to do some head scratching to work out what that represents in term of % change in tension, but might be interesting.
    John
    I suppose I could dig the equations out of one of my physics text books; but I am not going to try. Thirty seven years ago this was the type of problem I used to give my students as a homework challenge.

    If you keep retuning does the string eventually stabilise and stay in tune?

    Even if it does not stabilise does the pitch change more slowly with time; i.e. more than overnight for a semitone?

    If you tune and retune a string for several days then relax it briefly, and then retune it do you replicate the original tuning sequence?

    If you tune and retune a string for several days then relax it for several days, and then retune it do you replicate the original tuning sequence?

    If the change in tune is due to changes in the string it should not matter whether the relaxing is for a brief time or a long time because steel has virtually no hysteresis.

    If the change in tune is due to changes in the anchor points you could expect to see a difference between brief or extended relaxation because wood does show hysteresis.

    An open mind is a virtue...so long as all the common sense has not leaked out.


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    Quote Originally Posted by greybeard View Post
    By the same author as my "Bucket Chemistry"




    John
    in a job interview situation use the word 'empirical'

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    Quote Originally Posted by hub View Post
    ......................
    I would not worry about strings "stretching". If they do it's very minor IMO..........................
    OK, I'll come clean. I'm not building guitars, but hammered dulcimers. Five so far, and still learning.

    Geof
    I suppose I could dig the equations out of one of my physics text books; but I am not going to try.
    Damn, I though you'd take the bait if I left it hanging
    But I like your analysis and set of experiments.

    John

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    Enjoy today's problems, for tomorrow's may be worse.


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    Quote Originally Posted by greybeard View Post
    OK, I'll come clean. I'm not building guitars, but hammered dulcimers. Five so far, and still learning.
    Wow! Nice
    It's made of wood too. I would still guess the wood is the reason for the "stretching".
    Dulcimer reminded me, if you are interested, a similar Finnish traditional instrument "Kantele":
    Kantele - Wikipedia, the free encyclopedia
    You can play it with fingers or a bow (like playing a violin).
    A properly made one sounds great.

    :) Current build: http://www.cnczone.com/forums/diy-cnc-router-table-machines/264838-new-machine-desing-quot-cnm13-quot.html


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    Väinämöinen makes the first kantele from the jawbone of a giant pike
    OK hub, when are you going to build one ?
    Getting the giant pike to lay on the bed of your cnc might be a problem, but that's what makes life interesting.
    John

    It's like doing jigsaw puzzles in the dark.
    Enjoy today's problems, for tomorrow's may be worse.


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    Quote Originally Posted by greybeard View Post
    OK hub, when are you going to build one ?
    Getting the giant pike to lay on the bed of your cnc might be a problem, but that's what makes life interesting.
    John
    I hope some day I will build a few (one working after the few failed prototypes as usual).

    Even more challenging than fixing a giant pike to my CNC is to find a giant pike.. I've fished all my life, no giant pikes so far...

    Also, CNC would only be of little help.. Mostly manual work by hand.

    Hub

    :) Current build: http://www.cnczone.com/forums/diy-cnc-router-table-machines/264838-new-machine-desing-quot-cnm13-quot.html


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    But if you had one of these.....
    strung with the hair of a willing maiden
    John

    It's like doing jigsaw puzzles in the dark.
    Enjoy today's problems, for tomorrow's may be worse.


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    Go modern: Make the frame from carbon composite, much higher E, particularly specific E, and less hysteresis than wood, also not as much creep.

    An open mind is a virtue...so long as all the common sense has not leaked out.


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    Quote Originally Posted by greybeard View Post
    But if you had one of these.....


    John


    :) Current build: http://www.cnczone.com/forums/diy-cnc-router-table-machines/264838-new-machine-desing-quot-cnm13-quot.html


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    Just a thought on stress relieving. Easy to try.

    Put strings in a small amount of liquid nitrogen in some styrofoam.
    The liquid will boil.
    Cover and allow to slowly return to room temperature.
    You could dry ice to similar effect.

    I am no expert, and this may do nothing, but it might do something.

    This is where I am leading...
    Liquid Nitrogen

    Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. THREE ways to fix things: The RIGHT way, the OTHER way, and maybe YOUR way, which is possibly a FASTER WRONG WAY!


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    Geof - thoughts are in that direction but lots to learn first.
    John

    It's like doing jigsaw puzzles in the dark.
    Enjoy today's problems, for tomorrow's may be worse.


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    Quote Originally Posted by Geof View Post
    Go modern: Make the frame from carbon composite, much higher E, particularly specific E, and less hysteresis than wood, also not as much creep.
    Would fix a few problems but I'm not sure the sound would be that great..

    :) Current build: http://www.cnczone.com/forums/diy-cnc-router-table-machines/264838-new-machine-desing-quot-cnm13-quot.html


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