Official Answer
Answer
Dishing occurs when the thrust load causes the ring to bend into a conical shape. This is usually due to the groove's edge margin being too small or the groove wall deforming. As the ring dishes, it begins to 'walk' out of the groove. The relationship for the moment leading to dishing involves the load $P$ and the lever arm between the load application point and the groove support. If the groove radius is too large ($R > 0.1 \cdot T$), the ring will dish prematurely. The safety factor must be derated by a factor of $K \approx 1 - (R/T)$ to account for this geometric instability.
