Answer
The rotational capacity of an external spiral retaining ring is the speed at which centrifugal force causes the ring to expand and lose contact with the groove bottom. The maximum allowable RPM $N$ is calculated as $N = \frac{1}{\pi} \sqrt{\frac{4.48 E I g}{w r^3 (1+K)}}$, where $E$ is the modulus, $I$ is the moment of inertia of the cross-section, $g$ is gravity, $w$ is the weight of the ring per inch, $r$ is the radius to the center of gravity, and $K$ is a factor related to the 'clinch' or interference fit of the ring in the groove. If the operating RPM exceeds this value, the ring may 'float' out of the groove, leading to a catastrophic failure of the assembly. For high-speed applications like turbochargers, 'self-locking' spiral rings are used, where a tab on the inner turn locks into a slot on the outer turn to mechanically prevent expansion.
