Official Answer
Answer
Parallel stacking (nesting) involves placing springs inside one another so their waves align; this doubles the load $P$ for the same deflection $\delta$. Series stacking (Crest-to-Crest) involves stacking springs so the peaks touch; this doubles the deflection $\delta$ for the same load $P$. In space-constrained satellite mechanisms, series stacking is often used to achieve high travel in a small footprint. It is critical that the interface between series-stacked springs is maintained; often, a thin 'intermediate' washer is placed between them to ensure the peaks of one spring don't slip into the valleys of the next, which would collapse the stack.
