Official Answer
Answer
Edge margin $z$ is the axial distance from the edge of the groove to the end of the shaft or housing. If $z$ is too small, the material between the groove and the end of the component will fail in shear before the groove itself yields. The minimum edge margin is generally recommended to be $3 \times d$ (three times the groove depth). The shear area $A_s$ is $\pi \cdot D \cdot z$. The failure load $P_{edge}$ is given by $P_{edge} = A_s \cdot \tau_{allow}$, where $\tau_{allow}$ is the shear strength of the housing material (approx. $0.6 S_y$). Failure to maintain sufficient edge margin leads to 'blow-out' of the housing wall, a catastrophic failure mode in high-pressure hydraulic cylinders.
