Official Answer
Answer
A groove bottom radius is often required to reduce stress concentrations in the shaft or housing, especially in high-fatigue applications. However, if the radius $R$ is too large, it prevents the rectangular cross-section of the spiral ring from seating at the maximum groove depth. This effectively reduces the support surface area. The standard rule is that the radius should not exceed $10\%$ of the material thickness $T$. If a larger radius is needed for structural integrity, the engineer must use a ring with a 'special' offset or a backup washer. Failure to account for this leads to a 'rolling' failure mode where the ring twists out of the groove under axial load.
