[EricH]

You have a CAD of the file. Most CAD programs have a stress analysis component to them. In this case, you'd estimate the maximum force exerted by the pounding part when it jerks upwards and figure out the torque exerted by your motor. Fix the shaft in the appropriate places and run the program.

Oh, and make sure your material properties are set properly.

That should tell you how much stress the shaft takes. If it's more than the breaking stress of the material, you have a problem. If it's less than 1/2 the breaking stress, you should be good. (Note, in this case you probably want shear stress instead of normal stress.)

Off the top of my head, it *should* be OK, depending on the forces... but there are a number of teams that use axles somewhere in the neighborhood of 3/8" for drive, though admittedly