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

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/...ring%20(1).JPG

https://dl.dropboxusercontent.com/u/...ring%20(2).JPG


The spring returns an aluminum flap through and rotates through an angle
of around 90°.
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.

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