This 27:1 ratio worm gear set provides a compact way to create a drastic reduction in the output speed of a gear motor. The 6mm bore on one side of the worm is intended to slide over the shaft of a motor, leaving the other side available to insert a 1/4" D-shaft into to provide additional axial support in demanding applications. The mating worm gear is cut to perfectly mesh with the worm with a center spacing of 0.75"; the same spacing as you'll find on the Actobotics Channel. The worm gear is mounted with the provided Steel 1/4" D-Bore Barrel Hub for Worm Gear. By the nature of the way the gears are cut, it's impossible to backdrive this setup which makes it an excellent solution for applications that have a moment arm that always acts on the motor which moves the arm.