What causes 'Wave Nesting' or 'Out-of-Phase' conditions in multi-turn springs?

Wave nesting occurs when the waves of adjacent turns in a multi-turn spring align and 'stack' inside one another rather than remaining crest-to-crest. This effectively reduces the number of active turns $N$, causing a massive increase in the spring rate $k$ and potentially causing the spring to go solid prematurely. This is usually caused by excessive radial clearance in the assembly, allowing the spring to twist or distort. It can also be a manufacturing defect where the 'pitch' of the waves is inconsistent. To prevent this, designers should specify a tighter fit on the pilot and ensure the spring ends are properly squared or use shim ends which provide more stability to the coil stack.