Answer
Dishing occurs when a retaining ring is subjected to thrust loads that cause the groove's edge to deform plastically. As the groove wall yields, it forms a ramp or 'radius', and the ring begins to twist or 'dish' under the moment $M = P \times (lever arm)$. The angle of dish $\phi$ is proportional to the applied load and inversely proportional to the ring's torsional stiffness. Once the dish angle exceeds a critical value (typically 10-15 degrees), the ring can no longer be contained by the groove and will fail by 'scalloping' or ejecting. To mitigate this, the corner radius at the bottom of the groove must be kept to a minimum (usually $< 0.1 \times d$), and the 'abutment' or mating component should have a sharp corner to minimize the lever arm of the applied thrust load.
