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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