Official Answer
Answer
Permanent set is identified when the free height $H_{free}$ of the spring after use is significantly less than its original manufactured height. This indicates that the operating stress $S$ exceeded the yield strength $S_y$ of the material. This often happens if the spring was compressed to 'solid height' during an over-travel event. In terms of design, it implies that the chosen material thickness $t$ or number of waves $N$ is inappropriate for the required deflection. The solution is to either increase the number of turns $n$ (to reduce stress per turn) or switch to a material with a higher elastic limit like 17-7PH in the CH900 condition.
