How does the use of a pilot rod or housing guide affect the alignment and life of a Crest-to-Crest wave spring?

Multi-turn wave springs are inherently unstable and can buckle if not properly guided. Guidance can be provided by a central pilot rod or an outer housing bore. The clearance between the spring and the guide should be roughly $10\%$ of the radial wall $b$. If the spring is unguided, the load $P$ creates a moment that causes the turns to shift laterally, leading to non-uniform stress $\sigma = \frac{M y}{I} + \frac{P}{A}$. This lateral shifting causes friction against the guide, which can lead to 'fretting' and premature crack initiation, particularly in high-frequency applications like engine valves.