Well... here is a simple solution; If you have big sprocket on there it is most likely parallel to the arm at most points, so as close to the outer edge of the sprocket put a large (3/8"-1/2") bolt from the sprocket to the arm.
This single bolt at 3-4" from the point of rotation can take more more torque than a 6 bolt pattern of #10's at 15/16" (That's AM's pattern).
Here is a good example; zoom in on the sprocket on beachbot's arm and you'll see.
There is really no reason to depend on the shaft for torque transmission, and most likely no reason why a bolt like that can't be used.