Official Answer
Answer
The edge margin is the distance from the edge of the groove to the end of the shaft or housing. If this margin is too small, the thrust load $P$ can cause 'break-out' failure, where the housing material shears off. For steel housings, the minimum edge margin is typically $3 \times$ the groove depth $d$. For aluminum, it should be $4 \times$ or $5 \times$. During installation, a small edge margin also risks deformation of the housing lip when the ring is wound into place, especially if a high-force installation tool is used. This is a common failure mode in lightweight aerospace gearboxes.
