Worm gears are special gear that resemble screws and can be used to drive spur gears or helical gears. Worm gears like helical gears allows to non – intersecting “Skew” shafts to mesh normally the two shafts are at right angle to each other. A worm gear is equivalent to a ‘V’ type skew thread and another way of looking at a worm gear is that it is helical gear with a very high helix angle. Worm gears are normally used when a high gear ratio is desired or again when the shafts are perpendicular to each other. One important feature of worm gear meshes that is often of use is their irreversibility. When worm gear is turned the meshing spur gear will turn, but turning the spur gear will not turn the worm gear. The resulting mesh is “Self – locking” and it is useful in ratcheting mechanisms