This is a rare to find but highly useful plastic worm gear.
Worm gears have a lot of advantages over conventional gears. First of all, they can change the angle of rotation by 90 degrees, which has a number of applications and uses. Secondly, turning the secondary gear does not rotate the worm gear and thus worm gears act as brakes for your robots!
This worm gear has an outer diameter of 13 mm and a central hole of diameter 4mm. The central hole comes with a set screw enabling it to be attached to motors and axles of various diameters. It perfectly fits all our 4mm axles.