Official Answer
Answer
Permanent set is diagnosed by measuring the free height ($L_f$) before and after service. If $L_f$ has decreased, the spring was stressed beyond its proportional limit. Possible causes include: 1) The assembly was 'over-deflected' to its solid height during installation; 2) The operating temperature exceeded the material's relaxation limit; 3) The actual load was higher than the design load due to dynamic surging. Troubleshooting involves checking the 'solid height' stress. If $S_{solid} > 80\%$ of $S_y$, the design is 'set-prone'. A 'preset' operation (compressing the spring to solid during manufacturing) can help stabilize $L_f$ by inducing beneficial residual stresses.
