Hex vs. Dead axle

[Billfred]

So far, so good.

Basically, hex shafts (and round shafts with keyways) are both forms of live axles. With live axles, your bearings are held in pillow blocks to allow the shaft to rotate. This way, for example, a sprocket doesn't have to be physically connected to a wheel to transmit power. This can yield some advantages in maintenance if you design correctly, and live axles can be cantilevered, where a bearing is not the outermost element on a shaft. (Look at the drivetrains of 254/968/60/22/4/980, and you'll see what I mean).

The big advantage that hex shafts have over round shafts with a keyway is that the hex shaft is the key--power is transmitted in six places, as opposed to the usual one or two with a keyway. It also means you don't have to fish for a new key when your original breaks...but generally at a higher cost, if you've got to broach your own. (1/2" hex broaches are by no means cheap.)