Answer
Spiral retaining rings are typically 2-turn or 3-turn. The radial stress during installation is caused by the expansion (for external rings) or contraction (for internal rings) required to pass over the shaft or into the bore. The maximum stress $\sigma_{inst}$ is given by $\sigma_{inst} = \frac{E t (D_f - D_i)}{D_m^2}$, where $D_f$ is the free diameter and $D_i$ is the installation diameter. Because a 2-turn ring is thinner than a single-turn stamped ring of the same strength, it can undergo greater elastic deformation without yielding. However, if the ring is 'over-expanded' during installation, it will take a permanent set and will not seat tightly in the groove, leading to a loose fit and reduced thrust capacity. Multi-turn rings distribute the installation stress more uniformly across the coils compared to a single-turn constant section ring.
