Part Name: Pololu Universal Aluminum Mounting Hub for 3mm Shaft, #4-40 Holes
Part Description: The motor shaft hub in your kit is used to mechanically couple the output shaft of your DC gearmotor to the structure you are actuating. The motor shaft hub has a through-hole for the shaft that secures the shaft in place via a set screw. In order to access the set screw, you will need to use the accompanying hex wrench. You can design your structure to interface with the motor shaft hub by using the four radial #4-40 fastener holes.