The ½" OD will slide nicely into any ½" hole found in a multitude of parts withing the Actobotics build system. The flange will rest against the perimeter of the hole and limit the bearing from sliding on through. It's common to have a shaft-mounted component nearby such as a hub, coupler or gear that will limit bearing from walking out in the other direction. When the OD of the neighboring component is larger than ⅜", be sure to include a shim or shaft spacer to keep that part from rubbing on the outer race of the bearing.