[Brandon Holley]

There's a lot more to the question you've asked besides just what material to use. Size, stress risers, fatigue, etc. are all factors you should consider when choosing a material.

That being said, you can often look at similar applications success/failure to determine what may be a good place to start. Dustin's advice is a good start. To add to it, we will generally use 4140 steel for our shafts. We've never had a failure, it's a relatively easy to work with material (not that the aluminum alloys aren't either), and it's quite cheap and easy to find.

Obviously the big advantage to the Aluminum shafts is the weight savings. However, to put it in perspective... A 3/8" hex shaft of 4" length weighs ~0.137lbs when made in steel, and ~0.048lbs when made of aluminum. A total weight savings of ~0.089lbs. Like I said it is measurable, however even replacing 6 of these steel shafts with aluminum gives us a total weight savings of 1/2 a pound.

We've had plenty of success with aluminum shafts as well as the steel ones. Weight has generally not been a huge issue for us, so we take the extra security and go with the steel. The design priorities for your team most certainly are different so you should be sure to make your own evaluation based on our advice.

-Brando