[Richard Wallace]

Sorry, I had to go out for a couple of hours. Thanks for the picture and for clarifying how you're using the hub.

I agree with Martijn, hex bore hubs and hex bar shafts distribute the load much better than keys, and they make it easier to get a tight fit.

Also, dynamic loading on arms can be much higher than what the same hub would see in a drivetrain, because wheel loading is limited by the robot's weight and the wheel's CoF on carpet.

Have you calculated the static loading on the key slot of your shaft? The dynamic loading will be a multiple of that, due to bouncing and rocking when your arm handles the ball, strikes things, etc. High dynamic loads could have loosened the fit of the key by expanding the key slots in the hub and/or the shaft.

If you are correct about the hub rotating 5 degrees relative to the shaft, that's quite a bit. A little trig shows that a 5 degree shift on a half-inch circle is about 0.022" displacement. That amount of slop is ten times the "looseness" that I would typically allow on the fit of a key. Does the keyslot in the hub look deformed? Was the key filed too much? I'm guessing the key is steel and I know the hub is aluminum -- is the shaft steel or aluminum?