pic: Stress plots related to broken output shaft

[AdamHeard]

By properly sized, I assume you mean cut all the way down into the hex? On hex shaft, you can get away with doing the next size larger snap ring, it just won't handle as much load. Somewhat off topic.

Say the groove did go all the way around, it would have similar effects. Stress will concentrate at the sharp cornered junction; the decrease in diameter is much less "damaging" than the sharp corner of the change. Varying diameters and radii (of that diameter change) will all create different reactions. It's really a simple guy to calculate if you know all the physical properties.

I found this book on google that has one of the tables you would use (this for torsion, for other types of loading a different chart is used).