Answer
Spring relaxation (or taking a 'set') is the permanent reduction in free height and corresponding load loss that occurs when a wave spring is held at high stress levels for extended periods, accelerated by high temperature.
Manufacturing Mitigation (Presetting):
1. Over-coiling and Pressing: The wave spring is initially coiled to a free height ($H_0$) higher than the target design specification.
2. Presetting / Coining: The spring is compressed completely to its solid height (solid pressing) multiple times, or held at solid height for a specified duration at a elevated temperature. This intentionally induces localized plastic deformation (yielding) at the highly stressed wave crests.
3. Residual Stress Generation: This localized yielding introduces beneficial compressive residual stresses on the outer surfaces of the wave bends. When the spring is subsequently loaded in operation, these residual compressive stresses oppose the active tensile stresses, increasing fatigue life and eliminating further height loss during service.
