Answer
In pneumatic actuators, the piston often strikes the retaining ring with high kinetic energy $E_k = 0.5 \cdot m \cdot v^2$. Unlike static loads, impact loads create dynamic stress waves that can exceed the material's yield strength for a fraction of a millisecond. This can cause the spiral ring to 'dish' or even fracture due to the high strain rate sensitivity of some steels. To analyze this, the 'dynamic load factor' $L_d$ is used, where $P_{dynamic} = P_{static} \cdot L_d$. For sudden impacts, $L_d$ can be as high as 2.0. If failure occurs, the solution is to incorporate a 'buffer' or 'shock absorber' (like a polyurethane washer) between the piston and the ring, or to increase the ring's thickness $t$ to increase its stiffness and energy absorption capacity.
