Official Answer
Answer
A spiral ring depends on a sharp, square corner on the retained part to distribute load correctly. If the shaft or the retained component has an excessively large radius $R$, the load $P$ is applied at a point further out from the groove. This creates a large lever arm and a radial component of force $P_{rad} = P \cdot \sin(\phi)$ that acts to expand the ring. This 'camming' action forces the ring out of the groove. To prevent this, the retained part must have a sharp corner or a shim must be placed between the radiused part and the retaining ring to provide a square load face.
