[Yahya (OP)]

how many cycles can steel spring stores before it reaches fatigue ? giving steel spring energy density 300 J/Kg , what if I stored 300 Joules in say 10 Kg instead of 1 Kg how many times will the number of cycles increase ?

[chris]

This article on the mechanism of metal fatigue might help to get the ball rolling.

[Colin2B]

The best place to start is by finding the Stress Life Cycle curve for the particular steel you are working with.
These usually show the the applied stress S as a proportion of the steel’s ultimate tensile stress Su, against the number of stress cycles to failure. The scale is usually log.
Obviously the applied stresses are in Pa so are dependant on csa rather than mass, but you could easily do the conversions. Also bear in mind that these curves assume no stress concentration points or stress raisers, nor any surface treatments. Although you are probably not too worried about it at the moment, remember that compressive surface stresses increase fatigue load carrying capacity, so compression is better than tension.

[Yahya (OP)]

typical springs have 10 million cycles until they reach fatigue so a traditional clock would work hundreds of years before its spring lose efficiency

[Colin2B]

Depends on the quality of the spring. Many postwar alarm clocks failed after <20yrs and hairspring failure was even more common, although that might have been through people throwing them across the room!