Why is the parallelism of the groove walls critical for the thrust capacity of a spiral retaining ring?

If the groove walls are not parallel (i.e., 'wedged'), the retaining ring will not seat squarely. This causes the axial load to be applied eccentrically, creating a twisting moment on the ring. This 'dishing' effect reduces the effective contact area and can lead to the ring 'popping' out of the groove at loads far below the theoretical shear strength. For precision gearboxes, the groove wall parallelism should be maintained within 0.02 mm. A 'hook-type' groove, where the wall is slightly undercut, can be used in extreme cases to pull the ring deeper into the groove as the load increases.