Answer
Permanent set is identified when the free height $H_0$ of the spring after loading is significantly less than the original $H_0$ as manufactured. This is a result of the material being stressed beyond its proportional limit. The most common cause is 'over-compression'—taking the spring too close to its solid height. The stress at solid height $\sigma_s$ should never exceed the yield strength $\sigma_y$. If a design requires the spring to be compressed near solid, a 'preset' operation (compressing the spring to solid during manufacturing) can be performed to 'remove' the initial set and induce beneficial residual stresses. Another cause is 'thermal relaxation' where the operating temperature exceeds the material's limits, causing the microstructure to rearrange and lose its elastic energy.
