Official Answer
Answer
Friction in multi-turn wave springs occurs at the contact points between waves (crests) and at the interface between the spring and the bore or shaft. This leads to a hysteresis loop in the load-deflection curve where the loading force $P_{load} = P_{theoretical} + F_{friction}$ and the unloading force $P_{unload} = P_{theoretical} - F_{friction}$. The magnitude of $F_{friction}$ is a function of the coefficient of friction $\mu$ and the normal force at the contact points. In high-frequency applications, this friction generates heat, which can lead to localized thermal expansion and a reduction in the fatigue life. For precision applications, lubrication or PEEK coating is applied to reduce $\mu$ and flatten the hysteresis loop.
