Advice - torsion spring using piano wire?

I need to replace a broken light duty torsion spring and need some advice.
It's broken right at the bend, I may be able to buy a new one but it
will have to come from the UK to Australia and I need it ASAP.
https://dl.dropboxusercontent.com/u/92240200/Spring%20 (1).JPG
https://dl.dropboxusercontent.com/u/92240200/Spring%20 (2).JPG
The spring returns an aluminum flap through and rotates through an angle
The wire diameter is 4.5mm and the overall length of the spring is around 24".
I can get some 3/16" diameter "piano wire" (4.76mm) or 5/32" (4mm) from my local hobby shop. Will piano wire work OK as a spring in this sort of application?
Will I have trouble cold bending the ends?
I was thinking of drilling a 5mm hole through a heavy piece of metal and using pipe, then a hammer, applying the bend. I would try to leave a more generous radius than the on the original.
Thoughts?
Thanks guys.
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wrote:


I've made a number of springs from music wire and they seem to work pretty well. and yes, a bit more generous radius on the bends will probably last longer.
I would think that you could bend 3/16" by hand, or perhaps a couple of pairs of pliers, so a fancy jig shouldn't be necessary, and I wouldn't hammer on the bends as it might work harden then even more and could cause them to fail.
--
cheers,

John B.
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3/16" steel welding rod can be bent by hand, but the work hardening at the bend stiffens it there and causes the whole piece to curve rather than allowing a tight localized radius. When I need a tight U bend with straight legs I have to rubber-hammer the rod around a mandrel.
AFAIK music wire is hardened by cold drawing and doesn't respond to heat treating as well as you might like.
-jsw
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On Fri, 19 Jun 2015 08:01:44 -0400, "Jim Wilkins"


Will, firstly, the guy was asking about 90 degree bends in 3/16" rod which if you cannot make with your bare hands and maybe a couple of pairs of pliers sort of implies that there is something wrong with you.
Out of curiosity I just went out to the shop and bent a piece of 5.5 mm (0.216") rod and had no problem what so ever bending a nice 90 degree bend with about a 1/4" radius.
--
cheers,

John B.
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I have a Z bend for those piano wire wires we used in RC flying. It was used cold and pressed a z with square corners. So rounds especially using a mandrel would be nicely done. Martin
On 6/19/2015 6:31 AM, John B. wrote:


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On Fri, 19 Jun 2015 20:59:45 -0500, Martin Eastburn

I used to build "U" control planes and as I remember it I bent nearly all of the wire - landing gars, push-pull rods, etc., by hand or perhaps with a pair of pliers. If you use two pair of pliers the distance between then tends to govern the radius of the curve.


--
cheers,

John B.
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wrote:


Piano wire will be fine, and you can cold bend it as long as you don't try for too sharp a radius and avoid nicking or dinging the wire. I'd avoid hammering, if possible. Stress relieving the bends will improve the life of the spring. See the "Finishing" section of this document:
http://educypedia.karadimov.info/library/springs.pdf
Keep in mind that the stiffness of your spring will vary with the 4th power of the diameter. The 5/32 wire will be 1/2 the stiffness of the original, and the 3/16 25% stiffer.
--
Ned Simmons

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On 6/19/2015 8:52 AM, Ned Simmons wrote: ... See the "Finishing" section of this document:

That doesn't load for me. But I have a new Firefox version. Anyone else have a problem?
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On Fri, 19 Jun 2015 09:13:44 -0400, Bob Engelhardt

No problem here. I'm using the latest Firefox, too.
But the problem may be your Acrobat reader, which Firefox links to for downloading PDFs. Try just downloading the file and opening it with your Acrobat reader.
BTW, I'm using the full version of Acrobat Pro DC, but the free reader should handle it as well.
It's worth the effort. It's a great reference.
--
Ed Huntress

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Bob Engelhardt wrote:

Loaded fine for me using FF . Lots of good info there Ned ! I bookmarked it in my metalworking folder ...
--
Snag



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It loads for me in XP + IE8 and Firefox 28, both using Foxit 6 to view the .pdf.
I don't have up-to-date antivirus on a Windows 7 + Acrobat computer to try it.
-jsw
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It downloaded for me. It came up in my other terminal. Might have to add an app.
I have the new version and it blew my passwords and Usenet files away. My password was a large file. Didn't like large.
Martin
On 6/19/2015 8:13 AM, Bob Engelhardt wrote:

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    No problem here, with an older Firefox running on a Sun Solaris system. If that can do it, pretty much anything should be able to do it.
    But -- an attempt to get it with "wget" came back with "Forbidden". :-)
    Good Luck,         DoN.
--
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The prior version used a GUI tool to show protected stuff and parameters... and passwords.
Now passwords is a small part of a web page that is looking at the data. The new version didn't like my 48k size and trashed it. I had printouts to help on most stuff. I have another computer in the shop that has most of it - only new computer lost the new stuff. 38.05 is the one that wiped passwords and news accounts, not part of Firefox. They take an odd area - many are reporting and they were giving some help and then killed the help section altogether. Don't like getting bad news with little ability to help.
What bugs me is where is the import / export tool. Get from and save for another. The form of the data base is different, but apparently it converts but only a set number...??
Martin
On 6/19/2015 10:25 PM, DoN. Nichols wrote:

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wrote:


As Jim and John said, leave as large a radius as you can. As strong as it is, music wire will take a surprisingly sharp bend without breaking, but it's still a very hard material and it has its limits.
It's also hard to predict. The standards for music wire are that it has a maximum diameter of 1/8 inch, to begin with. Your thicker wire is the same material, most likely, but its strength will not be as high as the smaller diameters. Still, it's strong material. In very small diameters, its ultimate tensile strength ranges well over 200,000 psi -- even close to 300,000 psi. (I'll leave it to you to convert it to GPa). In larger diameters, that falls off to just over 100,000 psi. Hard-drawing has a greater effect with smaller diameters.
Just for reference, genuine music wire is hard-drawn, plain-carbon wire. But it's not a standard grade. The carbon content may vary between 0.95% and 1.25%. It's off the high end of the scale, compared to regular, plain-carbon steel.
Because of the variations, and because its original purpose does not involve making sharp bend that require strength, model aircraft builders and others who use it have to experiment a bit to make sure it will work in their applications. You'll have no problem using it for a torsion spring, but, if you can, make some test bends and see how they break, by working them back a forth a bit.
Good luck!
--
Ed Huntress

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On Fri, 19 Jun 2015 18:15:00 +0800, Techman wrote:


What everyone else has said, except that to bend it I'd put it in a vice, pointing straight up, with a couple of bits of 1/16" thick soft aluminum on the bend side to act as a sacrificial one-time wire bending tool. Bending it down onto the aluminum (instead of across the sharp jaw of the vice) should give you a reasonable radius.
--

Tim Wescott
Wescott Design Services
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If I didn't have conical mandrels I'd turn a groove of the desired inside radius in a piece of scrap from the cutoff bin, clamp it upright in the vise with the groove just above the jaws, and use pipe (or padded Vise-Grips) as handles to bend the rod as far as it will stay tight in the groove. When the back side begins to pull out I'd rotate it 45 degrees in the vise to catch the back side under the jaw, and continue.
Take it out before it's done and check the bend radius. You may want to turn the groove smaller by the amount of springback.
-jsw
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On 19-Jun-15 6:15 PM, Techman wrote:


Many thanks for all the useful replies and ideas - as usual RCM helped me out of a bind!
I ended up buying some 4mm and 4.76mm piano wire from the hobby shop. The original spring was 4.5mm. For a one off purchase it was cheap at $5 for 36" length.
I filed a curve on my aluminium soft jaws and bent it in the vice using a length of tube slipped over the rod for leverage. It was pretty tough stuff. I had to fine tune the last few degrees of bend with a rubber mallet. I ended up with a inside bend radius of about 1/8", nice and smooth unlike the original harsh 90 bend.
To stress relieve it I put it in the oven for an hour at 240 degrees C as per the spring document Ned linked to. That is a GREAT document - thanks Ned!
http://educypedia.karadimov.info/library/springs.pdf
I made a few and ended up using the 4mm one, as suggested the spring strength was quite noticeably down on the 4.5mm original but still plenty for the application.
I put it through about 50 cycles by hand and it didn't break which was a good sign. I'll have to see how it goes. I might grab another length of the 4mm stuff and make up a replacement for next time as I think the 4.75mm stuff is going to put too much load on other things.
Thanks again guys, much appreciated.
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