[Knufire]

Based on this, it sounds like you are driving your arm rotation by powering a 3/4" hex shaft off the motor, and having the arm locked to the hex shaft with some sort of hub with the appropriately broached 3/4" hex hole.

For extremely high torque applications such as this, I would advise mounting the arm on a dead axle vs a live axle. A common way to power high torque arms is to directly bolt a 60T #35 chain sprocket to the arm structure, and have the arm supported by bushings or bearings on a rigid shaft attached to the robot frame.