Official Answer
Answer
If a groove wall is not square ($>0.5^{\circ}$ out of perpendicularity), the thrust load $P$ is not applied uniformly. This creates a component of force $P \cdot sin(\theta)$ acting radially, which encourages the ring to expand and exit the groove. In troubleshooting failures where the ring appears to have 'slipped' out, the groove geometry should be measured using a CMM (Coordinate Measuring Machine). A non-square groove significantly reduces the effective thrust capacity, often by as much as $50\%$, because the ring only contacts the 'high point' of the groove, leading to localized yielding and subsequent dishing.
