## How to calculate energy density of mainspring?

Hi,

How do I calculate the energy density of either a main-spring (like in a clock) or a motor-spring?
Can someone show me values put into the correct formula for something like steel?

I did find some material properties like this: http://www.engineeringtoolbox.com/yo...lus-d_417.html

The motor-spring I am interested in is a type like this: http://www.sdp-si.com/Gateway/D220-T183.htm

Secondly, I would like to experiment with various materials (composites especially). If I make a plank of this material, how can I measure its properties to be able to put it into the correct formula for energy density of such a spring (if made into a spring)? I would have to take into account the breaking-point of the material of course (i.e. I bend the plank and at some point it either snaps or buckles).

I did study mech. engineering for a coupple of years but it is 20 years ago and I am rusty ;-)
I have googled and read a lot but am still stuck. Most stuff only relates to compression-springs but I need to work out spring constant of basically a beam (?) and not sure of to work out the breaking point (of the clockspring) when I know that a certain material will snap when it is deflected a certain amount.

Thanks a lot.

Regards
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 A concrete example I would very much like to see is something like this: Take an acrylic ruler. it is 30 cm long, 3 cm wide and 2mm thick. It weighs 20grams (just estimated for this example). Acrylic has Youngs Modulus of 3.2 GPa and ultimate Tensile Strength of 70 Mpa (according to the page I posted ealier). If I took this material and used 500cm of it and turned it into a mainspring (or motor-spring), then: a) What is the energy density of the spring? b) If I didn't have Youngs Modulus and Ultimate Tensile Strength, how could I work these out (and any other values that I might need)? Thanks and regards
 Bump! Sorry I have cross-posted this in the coursework forum, many reads but no answers :-( Is no one able to help me with this? When I google, I only find articles related to compression springs. Why is it so difficult to find stuff on main-springs? Can someone recommend a good book on the subject? Not interrested at all in compression-springs, only main-springs and tensator springs and the like. Thanks.