This shaft collar is compatible with the DUO Build System and is used to prevent gears, sprockets, wheels, and other parts from sliding out of place on a 5mm hex shaft. Shaft collars can also be used to prevent the shaft itself from sliding out of its mounts. Once in place, the set screw is tightened on one of the flat sides of the hex shaft. 1.5mm Allen Wrench for use with set screws sold separately.