These universal mounting hubs are designed to work with most 4 mm diameter shafts, including round shafts, and “D” shafts. Each of the two included hubs has four mounting holes for M3 screws (not included), letting you mount custom wheels or mechanisms to your motors. The two included M3 hex set screws (one for each hub) allow secure coupling of shaft to hub, and a 1.5 mm hex wrench is included for use with the set screws.
The picture shows a hub with Pololu’s 25:1 metal gearmotor 20Dx44L mm and a Pololu 20D mm metal gearmotor bracket. This hub will work with any Pololu 20D mm metal gearmotor.