Answer
The edge margin ($Y$) is the distance between the end of the shaft or housing and the groove. If the edge margin is too small, the groove wall will fail through shear or 'blowout' before the ring fails. The shear strength of the groove is calculated as $P_{shear} = \pi \cdot D \cdot Y \cdot \tau$, where $\tau$ is the shear strength of the groove material ($\approx 0.6 \cdot \sigma_y$). A standard rule of thumb for engineering is that the edge margin $Y$ should be at least $3$ times the groove depth $d$ to ensure that the ring fails in shear before the groove wall shears off. In aerospace applications, where weight is critical, finite element analysis (FEA) is often used to optimize $Y$ while maintaining a safety factor of $1.5$ against shear blowout.
