To connect a 4 mm motor shaft to a 30 mm steel rod, options include using a machine shop for custom machining or employing a belt and pulley system. The coupling method depends on the support and balance of the 30 mm bar, as well as the motor's mounting and torque output. Key considerations include the maximum RPM of both components and whether the coupling needs to be flexible. If low torque is sufficient, a spring-mounted motor with a rubber wheel against a friction band can be effective. Alternatively, machining the bar down to a compatible size or using a universal joint for connection are viable solutions.
