Official Answer
Answer
Over-travel occurs when a spring is compressed beyond its intended working height, potentially reaching its solid height. This causes the stress to exceed the yield point $S_y$, leading to 'permanent set'. If the spring is used in a cyclic application, over-travel drastically reduces fatigue life because the stress range $\Delta \sigma = \sigma_{max} - \sigma_{min}$ becomes too large. Mitigation includes adding a 'mechanical stop' (a shoulder in the housing) or designing the spring with 'infinite life' parameters where $\sigma_{solid} < S_{endurance}$. Using a 'Nested' spring can also help by providing higher loads at smaller deflections, reducing the likelihood of hitting the solid height.
