Official Answer
Answer
The edge margin is the distance from the groove to the end of the shaft or bore. If this margin is too small, the groove wall may shear off under load. The required margin $z$ is calculated using $z = \frac{3 F}{D \pi \sigma_y}$, where $F$ is the thrust load. As a rule of thumb, for steel housings, $z$ should be at least $3 \cdot d$ (three times the groove depth). For lighter alloys, this should be increased to $5 \cdot d$. Insufficient edge margin leads to 'Groove Wall Blowout', a catastrophic failure mode in hydraulic and pneumatic cylinders.
