On 19-Jun-15 6:15 PM, 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.


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.