How efficient in energy storage is a high tensile steel spring? An example might be a watch spring.?

Question

I am wondering if a coiled ribbon type spring has equal the storage potential, or greater, then let's say a rod coil spring, such as what is used on automobile suspensions.

Answer 1

Efficiency is usually defined as the ratio of work out to work in. In both cases the efficiency is very high and the same in both. Specific potential energy capacity, however, is the maximum energy stored per unit mass of spring before any part of the spring permanently deforms. The rod spring is better in this regard since the (shear) strain in the rod is more uniformly distributed than the (tensile) strain in the ribbon. In both cases strain drops to zero in the center, but the cylindrical geometry of the rod puts more energy in the outer regions, where strain is greatest. You could say, then, that the rod coil has a greater efficiency of mass utilization. A tube coiled into a helical spring would be more efficient still, by this definition, since the core is basically just dead weight anyway.