Answer
The centrifugal force acting on an external spiral retaining ring can cause it to expand and lift out of its groove. The maximum allowable RPM $V$ is calculated using $V = \sqrt{(4.48 \cdot 10^{12} \cdot E \cdot I \cdot g) / (w \cdot \rho \cdot R^3 \cdot (1 + \nu))}$, where $E$ is the modulus, $I$ is the moment of inertia, $g$ is gravity, $w$ is the material weight per unit length, $\rho$ is the density, $R$ is the groove radius, and $\nu$ is Poisson's ratio. This limit is reached when the ring's expansion due to centripetal acceleration exceeds the interference fit within the groove. In high-speed aerospace turbines, self-locking features (tabs and slots) are added to the spiral ring to mechanically prevent this expansion, effectively negating the RPM limit imposed by the centrifugal calculation.
