Answer
External spiral retaining rings are subject to centrifugal forces that tend to expand the ring radially. At a certain rotational speed $\omega$, the ring will lose its grip on the groove bottom, known as lift-off. The critical speed $V$ (in RPM) can be approximated by $V = \sqrt{\frac{4.48 \times 10^{12} E t p^2}{D_g^3 D_r \rho}}$, where $E$ is the Modulus, $t$ is the material thickness, $p$ is the radial wall, $D_g$ is the groove diameter, $D_r$ is the ring free diameter, and $\rho$ is the density. To prevent lift-off in high-speed shafts (e.g., turbochargers), engineers can specify 'Self-Locking' rings, which feature a tab-and-slot mechanism that mechanically prevents the ring from expanding beyond a certain point, allowing the assembly to operate at significantly higher RPMs.
