Describe the 'Dishing' failure mode of a spiral retaining ring and its root causes.

'Dishing' is a deformation where the retaining ring becomes cone-shaped under axial load. This occurs when the axial force $P$ creates a torque that exceeds the ring's torsional rigidity. Root causes include: 1) Excessive axial load beyond the rated capacity; 2) Groove wall yielding (soft housing material); 3) Excessive mating part chamfer. As the ring dishes, it loses its contact area with the groove wall and eventually 'cams' out of the groove. Dishing is often a precursor to catastrophic dislodgement. Prevention involves using a 'Heavy Duty' series ring (thicker $t$) or hardening the groove walls to prevent the initial tilt.