Official Answer
Answer
Under impact loading, a spiral retaining ring may 'dish' or become conical. The dishing angle $\theta$ can be approximated by $\theta = \frac{6 \cdot P \cdot (D_o - D_i)}{E \cdot T^3 \cdot \ln(D_o/D_i)}$. Impact loads multiply the effective force $P$ by an impact factor $I_f$, which can range from 2 to 5 depending on the kinetic energy of the retained part. If $\theta$ exceeds the angle that would cause the ring to slip out of the groove (typically 10-15 degrees), the ring will fail. Multi-turn rings (2-turn or 3-turn) provide greater resistance to dishing compared to single-turn snap rings because the turns 'nest' and support each other against the bending moment.
