Official Answer
Answer
Edge margin is the distance $z$ from the groove to the end of the shaft. If $z$ is too small, the material between the groove and the shaft end can fail in 'double shear' or 'shear-out.' The failure looks like a small 'plug' of material being pushed off the end of the shaft. The required margin is typically $z \ge 3 \cdot d$ (where $d$ is groove depth). If a failure occurs even with $z > 3 \cdot d$, it may be due to 'moment-induced bending' of the shaft end. This is common in brittle materials like cast iron. Engineers should perform a finite element analysis (FEA) to ensure that the combined stress (axial shear + bending) at the corner of the groove does not exceed the material's shear strength.
