Official Answer
Answer
Permanent set, or 'taking a set,' occurs when the actual stress exceeds the material's yield strength. If theoretical calculations suggest the spring is safe, the failure often stems from 'stress relaxation' or 'creep' due to operating temperatures higher than specified. Another cause is 'dynamic surging,' where high-frequency vibrations cause localized deflections beyond the design work height. This can be analyzed using the natural frequency formula $f_n = \frac{1}{2 \cdot \pi} \cdot \sqrt{\frac{k}{m}}$. If the operating frequency matches $f_n$, the resulting resonance can cause the spring to compress to its solid height, exceeding the yield point and causing permanent deformation.
