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About two and a half weeks ago I replaced an unwound string and wanted it to get stretched out as soon as practical. So I brought the pitch up to the next semi-tone for 5-10 minutes and then left it 5-10 cents sharp. When I retuned it yesterday it was about 10 cents flat. The pianist apparently had not noticed the drop (the unreplaced string of the trichord was muted) so I was pleased.

I understand that the typical tension on piano strings is beyond the elastic limit, that once a string is brought to pitch, it will never go back to its original length, but will normally take three years to stretch out and have a stable pitch. So I am thinking that there must be a certain tension above normal that a string could be subjected to for a certain amount of time to cause the deformation necessary for a stable string.

Anybody have ideas about what this might be? I know that it may not be best practice to do this to strings due to creating bends where they should not be, but there are times when it would be appropriate. I understand that the breaking point is about 300 cents above normal pitch. Maybe half of that for half an hour?


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300 cents above normal pitch? Don't you mean 30?


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No, I understand it to be 300 cents. It certainly was not 30 cents on the string that I raised a semi-tone!


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The basic tension at pitch is not beyond the elastic limit and the term "stretching" that is often used to describe what is happening to the string (slowely dropping in pitch) after being pulled to tension is actually incorrect. What is happening is called material "creep".

"Creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material."
(Wikipedia)

There are different types of creep with some dependant on elevated temperatures and others on elevated stress over time. The amount of normalizing in piano strings after manufucturing also effects the rate and amount the strings creep afterwards. With piano strings at tension, the amount of creep decreases over time.

You do not ever want to go to or over the strings critical point or elastic limit since a permanent deformation will occur that has nothing to do with the creep (creep will continue even after letting back the string in tension).

I had a scientist explain to me years ago that the massaging that we do to strings by rubbing or rolling after stringing does very little to effect string creep and it just helps seat the string better. Pulling to a higher tension does speed the process of creep slightly since it is both a temperature and stress related process, but eventually the string stabilizes enough to hold a tuning for a reasonable time. It does however never stop the process of creep wich continues for the life of the string while under tension.



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Quote
You do not ever want to go to or over the strings critical point or elastic limit since a permanent deformation will occur that has nothing to do with the creep (creep will continue even after letting back the string in tension).


Moral: Never let a creep tune your piano. laugh


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I still say 300 cents is waaay excessive. Would you pull that far over on a pitch raise?


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Originally Posted by Loren D
I still say 300 cents is waaay excessive. Would you pull that far over on a pitch raise?

I agree that 300 cents is excessive. I understand that is the breaking point. For pitch raises I will go up to about 20 cents over pitch to start, but I think that I may be closer to 50 cents in the high treble when I give it extra. After the temperament I raise all the F's, then all the F#'s and so on. I give the first ones some extra raise. But then I am not trying to aggressively stretch a new string.


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In Piano Tone Building, it is stated that one cause of dead strings in new pianos was traced to installers overpulling strings during assembly.


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I have had pretty good success when replacing a string to pull it a half step sharp at the beginning of the tuning. I then use a hammer shank to massage the string and get it hot. And I mean HOT - it will burn you if you touch it. Of course you can't do this on wound strings! The combination of pulling sharp and using heat seems to get the string to settle down a little more quickly.

James Arledge has advice about how sharp to pull a set of bass strings to get them to settle down quickly. I can't remember what it was, but I'm sure if you contacted him he could tell you.


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The percentage of the breaking strength that strings are at varies from note to note as well as from piano to piano. You cannot come up with a formula that will work in all cases.

Trying to speed up the settling process by pulling strings too high is less effective than making sure coils and loops are snug. New strings just have to be adjusted for a while.


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Thanks, BDB:

I guess mostly I am looking for rules of thumb. Be interesting to know what Arledge suggests as mentioned by Ryan.


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I replaced a bass wire on a Kawai RX-6 today. I did exactly as BDB stated. Before, the string was brought up to tension, I made sure the becket was shoved in so as not to have a bend in that area. Then as I raised it up the rest of the way to pitch, I made sure the coils were snug together and even. Then, I raised it sharp by, I don't know, it was a lot less than 1/4 tone. I then firmly but yet, gently, pulled on the wire in the center of it to stretch it. You have to be careful when doing this because you can break it and it will raise the wire up making it unlevel with the other one. This process brought it flat of pitch. I raised it up again, to about the same as before repeating this process. The 3rd time, I raised it sharp by less. I did this about 4 times and then seated the bridge and hitch pins and then I leveled the string and muted it. Doing this helps to remove some of the stretching but in no way will eliminate it.

Anyway, that's my method.


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With unimportant variations, I use Jerry's method. Getting the becket and coils tight seems to have the most effect.


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It seems you can stretch more than 300 cents (at least on the example below) before the string breaks.
I'll take the example Del put in a prior thread

Originally Posted by Del
For a given speaking length = 400 mm (15.7") and a string diameter = 0.038" (0.965 mm):
At A-49; Tension - 160 lbf.
ddf


For a typical steel string of that diameter the break load is ~ 410lbf or 1824N
The string frequency in Herzt can be computed as
f=(T/L^2*d^2*pi*Ro)^0.5
being
T= string tension in N
L= string length in m
d= string diameter in m
R0= steel density ~7800kg/m^3

If you replace on the formula you'll get that the break frequency is ~707Hz

Converting to cents
Cents=(1200/ln2)*ln(707/442)=~815 Cents

It is interesting to note that for that string length the break frequency is independent of the wire diameter for the same steel since T/d^2= stress on the first formula, and the rupture stress is a material constant.

Originally Posted by Emmery
What is happening is called material "creep".
With piano strings at tension, the amount of creep decreases over time.


Technically what is happening is not defined as creep but as its cousin "stress relaxation". Under creep conditions you are holding the applied load constant. Under stress relaxation conditions you are holding the displacement constant which is the case of piano strings.
The internal material deformation mechanisms are the same but the macroscopic deformation vs. time behavior is different.

Originally Posted by Emmery

With piano strings at tension, the amount of creep decreases over time.


I would rather say that under stress relaxation the amount of deformation always increases over time, nevertheless the deformation rate decreases in an exponential form. That along with string geometry setting are the reasons why you need tune a string more often when installed.

I have not clear why the string becomes tubby if taken above its elastic limit. There is a region of stresses between the elastic limit and the break in which the string can still deform plastically uniformly along its lenght prior to for "necking". I can see why necking could change the tone being a geometric anomoly, but I have not clear why the sound changes prior to it.
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I have read either here or training material that strings are at 60% of their breaking point when tuned to 440. On pitch raises of 1/2 tone low, I do middle section around 17- 20 cents, treble up 30-33 cents, and bass 8-12 cents. I too raise replacement string 1/4 to 1/2 sharp and massage it down 2-3 times.


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I'l have to play around with some of these with my next project piano. I am not sure why pulling on the center of the string would have a different effect than just bringing the pitch up.


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Originally Posted by UnrightTooner
I am not sure why pulling on the center of the string would have a different effect than just bringing the pitch up.


My understanding is that when pulling the pitch up, the tension is highest between the upper/front termination and the tuning pin (because of rendering friction) - i.e. higher than the speaking portion. The section closest to the tuning pin could therefore be subjected to excessive tension - and could possibly be weakened prematurely?

With Jerry's method, I think that would not happen.

So, while the effect on the speaking portion is perhaps the same, could it be that Jerry's method is less destructive on the front/top duplex?

Just my two cents, from an amateur's perspective.


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Patience. You've just installed a brand new string; the last thing you want to do is stress/deform/fatigue it!

I mean jeez, we're talking about pianos here...why do we want to do aggressive anything?


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Originally Posted by Loren D
Patience. You've just installed a brand new string; the last thing you want to do is stress/deform/fatigue it!

I mean jeez, we're talking about pianos here...why do we want to do aggressive anything?


Because the piano was to be used for daily choral practice after being tuned, the director is very picky pitch-wise (which is nice to hear), I would not be attending the piano again for a few weeks, and it had enough other problems that a little deformation would be an "undetectable fudge." There's a time and a place for every technique.


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Because, Loren, we want to eliminate the stretching effects ASAP on any piano but, especially on concert type instruments. If we can give the string/s some friendly encouragement to stretch itself out sooner, we do. I return within a day or two on most of these types on instruments repeating my process over again for about a week or so. Believe it or not but, it really does eliminate a lot of stretching. If NO stretching were done at all, just brought up to pitch, strings and coils seated etc., then the wire would stretch immediately before we left. I've tried that. By the time I'm done tuning the piano, the wire is already many waa waa's flat. By stretching it, it stays put for the duration of my tuning. Before I leave, I pull it sharp (forgot to mention that part) by 1/8 or so and leave it there before muting the wire.
Unless that is, it is a single bass wire. Then, obviously, I do not mute it. smile

Last edited by Jerry Groot RPT; 12/10/10 09:58 AM.

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