Answer
The groove yield strength is the axial load at which the groove material will plastically deform. For 6061-T6 Aluminum, the yield strength $\sigma_y$ is approximately $240-270$ MPa, which is much lower than the steel of the ring. The formula is $P_g = (D \cdot d \cdot \pi \cdot \sigma_y) / S$. Because aluminum is ductile, the groove edge will tend to 'smear' or 'roll' under high loads. To maximize the capacity, the groove should be as deep as possible without compromising the shaft's structural integrity. Additionally, using a 'Multi-Turn' spiral ring helps by distributing the load over more surface area, but the fundamental limit remains the compressive yield of the aluminum. A safety factor $S$ of at least $3.0$ is recommended for soft alloys.
