Answer
A spiral retaining ring relies on square-corner contact with the groove wall to maximize its thrust capacity. If the groove has a large radius at the bottom, or if the mating part has a large chamfer, the point of contact shifts, creating a moment arm that tries to 'dish' the ring (turn it inside out). The reduction in thrust capacity can be modeled by a factor $C_f = \frac{d - (r + c)}{d}$, where $d$ is the groove depth, $r$ is the groove radius, and $c$ is the chamfer of the retained part. If $C_f$ is significantly less than 1, the ring will fail prematurely by being pushed out of the groove. In aerospace gearboxes, 'sharp-cornered' grooves are often specified, and shim rings are used between the chamfered bearing race and the retaining ring to ensure the load is applied as close to the groove wall as possible.
