stretching wire

[David Ilvedson]

Long ago a tech here in the Bay Area, I believe Jack Roll, used to make and sell pre-stretched piano wire.  He made it by leaving it under tension for some time with weights.   It laid flat and the idea was it held it's tuning better for repairs.



David Ilvedson, RPT

Pacifica, CA 94044









Original message

From: "Dean May" 

To: "Pianotech List" 

Received: 4/10/2008 6:54:39 AM

Subject: RE: stretching wire





One more point that would seem to lend credence to the wire taking a set or creep well below the yield strength. When you pull the wire off the spool it has a curve to it. This is evidence of plastic deformation, it won’t go back unless sufficient stress is put on it in the opposite direction to introduce enough plastic deformation in that direction to counteract it. We’re not talking about much stress to do it. Now imagine what 150 lbs tension will do. 



Dean 

Dean May             cell 812.239.3359 

PianoRebuilders.com   812.235.5272 

Terre Haute IN  47802 









From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of Dean May

Sent: Thursday, April 10, 2008 3:25 AM

To: 'Pianotech List'

Subject: RE: stretching wire



I’ve done a little more research and found some numbers. 



>From Wikipedia: http://en.wikipedia.org/wiki/Tensile_strength#cite_note-0 

For most metals yield point is not sharply defined. Below the yield strength all deformation is recoverable, and the material will return to its initial shape when the load is removed. For stresses above the yield point the deformation is not recoverable, and the material will not return to its initial shape. This unrecoverable deformation is known as plastic deformation. For many applications plastic deformation is unacceptable, and the yield strength is used as the design limitation.



This substantially concurs with what Mike says below, permanent stretch occurs when you pull the tension above the yield point. It is, however, a little misleading when it says “all” deformation is recoverable. 



I haven’t yet been able to substantiate Mike’s claim that with modern piano wire the yield strength is closer to the ultimate. I have found sources that place it at 43-35%. The Wiki article above is one and here is another: http://www.mech.uwa.edu.au/DANotes/springs/fatigue/fatigue.html



This means if your design tension is above 45% of breaking strength you will exceed the Yield Strength, unless my sources are wrong. This is not really a problem as we are still a long ways from breaking strength. 



It is important to understand though that some plastic deformation does occur before Yield Strength, though not really appreciable. If you’re really bored check out this site: http://books.google.com/books?id=dVQOAAAAYAAJ&pg=RA7-PA319#PRA7-PA335,M1

Scroll down to page 336, for example. You will find a strain table for 17 gauge wire. At 100 lbs tension the wire elongated .0122 inches. When they took the 100 lbs off it had a permanent set (i.e., plastic deformation) of .0002 inches. When they got up to 300 lbs the permanent set was .0047 inches. At 422 lbs the wire sample broke (ultimate strength). 



Mike, do you have a reference we can look at that says differently? The reference above was printed in 1918 and perhaps “modern” piano wire would be different. 





Dean 

Dean May             cell 812.239.3359 

PianoRebuilders.com   812.235.5272 

Terre Haute   IN  47802 









From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of Mike Imbler

Sent: Wednesday, April 09, 2008 7:01 PM

To: pianotech at ptg.org

Subject: re: stretching wire



As a mechanical engineer, I concur with Ron.  John's cite of the high tension electrical wires makes it clear that they are being tensioned beyond yield strength, which is why the article suggests heat treating the wire (which increases yield strength).  Yield strength is the point where the wire would plastically deform (stretch) which is less than ultimate strength (breaking stress).  Therefore, you -could- stretch piano wire, but you would be pulling it above its yield strength.  This would be more likely with more ductile wire which has a yield strength appreciably lower than ultimate.  With a more modern piano wire, yield strength is closer to ultimate.

              Mike 
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