9X20 Lathe Spur Gear replacement.

[Frank S]

I really like Cap's upgrade.
I find that plastic gears when used in machinery are done so for several reasons Economics probably nearing the top of the list in most cases. Another reason might be for smoother quieter running especially where spur gears are concerned, engineered obsolescence would also be a huge factor to some manufactures. But being designed to fail in event of crashing would probably be rated very low on any manufactures list. At least a reputable manufacture, since such a failure would usually cause more damage to other gears requiring the replacement of several gears. installing soft keys shear pins and torque limiters at time of manufacture are both simple and economically sound procedures.
But that is just my opinion.