Official Answer
Answer
The edge margin ($E_m$) is the distance from the edge of the groove to the end of the shaft or bore. If $E_m$ is too small, the material between the groove and the face of the part may shear or 'blow out' under axial load. The required $E_m$ is roughly $3 imes d$ (three times the groove depth). The stress in this region is modeled as a shear tear-out: $\tau = \frac{P}{\pi imes D imes E_m}$. In aerospace applications with lightweight aluminum housings, the edge margin must be carefully calculated and often verified by FEA to prevent catastrophic failure of the retention shoulder under impact loading.
