Last week I tuned a piano I initially saw in 1996. It needed a significant pitch raise. It was tuned 8-10 times over the next 7 years, 2003 being the last year I tuned it. When I saw it last week it was dead on pitch. Why, after 5 years, had the pitch not fallen at all? Elastic yielding = no permanent set. After load is released the metal will spring back to its original state (or close enough for government work) Plastic yielding = permanent set occurs. After load is release the metal will not spring back to its original state. When a metal is cold formed, as in the drawing of carbon steel into piano wire, a remarkable thing happens. Certainly we see plastic yielding, because the steel takes on a brand new shape (hopefully, something that looks like wire). But when the metal yields, it causes the crystalline structure of the steel molecules to change alignment and it actually becomes stronger. I know that its yield strength increases (i.e., the load at which appreciable plastic deformation occurs) but I'm not sure about ultimate strength. This is called work hardening, http://en.wikipedia.org/wiki/Work_hardening. The steel becomes harder and stiffer. It can be repeatedly work hardened up to a point, until it becomes so brittle that it breaks. With that as a back drop (subject to correction by those more learned than I) allow me to apply that to my own anecdotal evidence of tuning pianos for 25 years and encountering a hundreds of major pitch raises. Whenever I encounter a major pitch raise (100-200 cents flat) it is nearly always on a piano that has either never been tuned or only tuned once or twice early in its lifetime. Conversely, consider a second piano that has been regularly tuned, say 10-15 times in its first 10-15 years of life, that piano will often be reasonably close to pitch even if it's been 60 years since it was last tuned. If they are in need of a pitch raise usually it is in the 20-30 cent range. I haven't done the test but I would be willing to bet that the amount of pin movement (a measure of the total elongation of the string) necessary to pull up the 150 cent flat piano (and it's subsequent tunings to get it stable) would exceed the total movement of all the second piano's tunings (i.e., 10-15 tunings early in its life, 1 tuning after 60 years). Why? Consider that when initial pitch is put on the string, some plastic deformation takes place, the wire stretches and takes a permanent set. This causes work hardening. The pitch falls, we bring it back up to pitch, again some work hardening. Every time we do this we are essentially stiffening the wire and raising its yield strength so that the next time it won't relax as much. Eventually it reaches equilibrium and essentially no longer yields at the loads we are subjecting it to. The wire that never got the early tunings seems to me to require more pin movement to finally get it to the point of equilibrium because it was not brought back up to pitch during the first cycle of relaxing. At that point the wire was at a lower yield strength because it had not gone through the cycles of work hardening, therefore it elongated more. Now if I understand what Ron N is saying, he says that once the wire is initially tensioned, it has yielded and will not yield any more. There are other factors causing the pitch to drop. I presume these other factors would be bridge movement, soundboard settling, wire bending around bridge pins & beckets, etc. It seems to me the practice of changing a broken string would tend to discredit this theory. A new wire is placed on an already tensioned scale. It won't cause any bridge movement or soundboard movement, which isolates deformation of the wire as being the sole cause of pitch dropping. Now when I put a new string on, no matter how careful I relieve all the bends around the pins, that string will drop in pitch in relation to the older strings over the course of the next few months. It will require a couple of "stops in the neighborhood" to bring it back up to tension and introduce stability. Apart from wire elongation I don't know how else to explain this phenomenon. And my understanding of steel loading is consistent with this paradigm. Dean Dean May cell 812.239.3359 PianoRebuilders.com 812.235.5272 Terre Haute IN 47802 -----Original Message----- From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of John Delacour Sent: Sunday, April 13, 2008 4:13 PM To: Pianotech List Subject: Re: stretching wire -- a preliminary test At 21:00 -0500 10/4/08, William R. Monroe wrote: >I think the point for me is that there are other factors to consider >re: pitch drops in pianos than long-term elastic deformation of >wire, and they shouldn't be dismissed without due diligence. Certainly not, but they can be eliminated in tests. The test I am about to describe is very basic and not satisfactory to me, but if there had been no pitch drop from this test I would have waited longer before devising a more decisive test. A length of 17.5 (1 mm) Röslau polished wire was stretched between two 7mm wrestpins knocked very tightly into a solid mahogany tapered piano leg about 42cm apart so that the wire was only slightly above the surface of the wood. The coils were well knocked down and the wire pulled firmly sideways while it was tuned to G-392. This requires a tension of about 150 lbs, which is roughly half the "elasticity limit" of the wire --300 lbs (Röslau advertises 395 lbs. as the breaking strain of this size). Over the space of an hour the string was pulled and plucked repeatedly and made to hold its pitch exactly under repeated plucking. During the day of the set-up there was no significant change in the pitch of the note. Humidity and temperature remained practically constant. When plucked about 24 hours later a fall in the pitch of the note of 4 or 5 cents was noted. 48 hours later the note is 9 or 10 cents flat. We shall see. The main objection to this test is that the unlikely but not excluded possibility of movement in or of the wood is not excluded. The next tests will eliminate this weakness. JD
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