Official Answer
Answer
Radial clearance is the gap between the ring's inner diameter (for internal rings) or outer diameter (for external rings) and the bottom of the groove. For external rings, if there is excessive clearance, the ring has more room to expand under centrifugal force before it is constrained. This lowers the effective 'lift-off' RPM. To maximize centrifugal capacity, the ring should be designed with an 'interference fit' on the groove bottom. The formula for the force required to expand the ring is $F = (4 \cdot \pi \cdot E \cdot I \cdot Δr) / R^3$. By ensuring $Δr$ is negative (interference), a significant portion of the centrifugal force is consumed just to bring the ring to a neutral state, thereby extending the safe operating speed.
