[Ben Piecuch]

I know that there's been a discussion about axle diameter before. Search and see what else you can find. Depending on your drive setup, your shaft requirements will vary considerably. If you use a cantilevered setup, you'll need a much stronger/stiffer axle vs. a double supported shaft.

For a double supported shaft, I would recommend the following:
Steel axles: 3/8"
Aluminum axles:1/2" minimum

For a cantilevered setup, you'll have to analyze your specific arrangement, as the longer the cantilever, the stronger/stiffer an axle you'll need. Also, if you look at the weight difference b/w a 1/2" and 3/4" aluminum axle, you'll find that there isn't too much of a difference, when comparing it to the 120lbs you have in total.

YMMV, good luck.

BEN