Here is photo of the machine....................
The whole head hinges at the back. The cam and follower are about a foot in front of the pivot point, so there is a fair amount of weight or twisting force on that cam. Not a lot of speed or impact, but it is constant load/unload. There are 2 smallish set screws holding that cam to the shaft. I like GM's suggestion to replace the screws with bolts. There is a 1/4-20 over the keyway and a 5/16 at 90 degrees of that. Thinking about dropping a couple of brass flat washers in the holes to give the bolts a little more traction on the shaft and lock them in with a lock nut.
The spline shaft idea would totally kill the problem. If were going to build machines like this, that would be the way to go.
Thanks again for the input.