Answer
The thrust capacity of a spiral retaining ring assembly is the lesser of the ring shear and the groove material shear. The ring shear capacity $P_r$ is $P_r = (A_r \cdot \tau_{ring}) / S_f$, where $A_r$ is the shear area of the ring and $\tau_{ring}$ is the shear strength (approx. 0.6 times the tensile strength). The groove shear capacity $P_g$ is calculated as $P_g = (D \cdot d \cdot \pi \cdot \sigma_y) / S_f$, where $D$ is the shaft diameter, $d$ is the groove depth, and $\sigma_y$ is the yield strength of the groove material. It is a common mistake to ignore the edge margin; the distance from the groove to the end of the shaft must be at least $3 \cdot d$ to prevent the groove wall from shearing out. A safety factor $S_f$ of 2 or 3 is typically applied.
