How does 'Cyclic Axial Loading' lead to fatigue failure in a spiral retaining ring?

While retaining rings are often considered static components, they can experience fatigue if the retained part applies a cyclic axial load (e.g., in a piston assembly). The fatigue crack typically initiates at the inner edge of the ring due to the bending moment created as the ring 'dishes' within the groove. The stress range $Δ\sigma$ is determined by the play between the ring and the groove. If the groove is too wide, the ring can 'hammer' against the wall, increasing the effective stress. SEM analysis usually shows 'striations' characteristic of fatigue. To prevent this, the assembly should be designed with minimal axial end-play, and the ring material should be upgraded to 17-7PH for its superior fatigue limit.