A worm drive is a gear arrangement in which a worm (which is a gear in the form of a screw) meshes with a worm wheel (which is similar in appearance to a spur gear). The two elements are also called the worm screw and worm gear. The terminology is often confused by imprecise use of the term worm gear to refer to the worm, the worm wheel, or the worm drive as a unit. The worm drive or "endless screw" was invented by either Archytas of Tarentum, Apollonius of Perga, or Archimedes, the last one being the most probable author. The worm drive later appeared in the Indian subcontinent, for use in roller cotton gins, during the Delhi Sultanate in the thirteenth or fourteenth centuries. Explanation A gearbox designed using a worm and worm wheel is considerably smaller than one made from plain spur gears, and has its drive axes at 90° to each other. With a single-start worm, for each 360° turn of the worm, the worm wheel advances by only one tooth. Therefore, regardless of the worm's size (sensible engineering limits notwithstanding), the gear ratio is the "size of the worm wheel - to - 1". Given a single-start worm, a 20-tooth worm wheel reduces the speed by the ratio of 20:1. With spur gears, a gear of 12 teeth must match with a 240-tooth gear to achieve the same 20:1 ratio. Therefore, if the diametrical pitch (DP) of each gear is the same, then, in terms of the physical size of the 240 tooth gear to that of the 20 tooth gear, the worm arrangement is considerably smaller in volume. Types There are three different types of gears that can be used in a worm drive. The first are non-throated worm drives. These don't have a throat, or groove, machined around the circumference of either the worm or worm wheel. The second are single-throated worm drives, in which the worm wheel is throated. The final type are double-throated worm drives, which have both gears throated. This type of gearing can support the highest loading. An enveloping (hourglass) worm has one or more teeth, and increases in diameter from its middle portion toward both ends. Double-enveloping worm gearing comprises enveloping worms mated with fully enveloping worm wheels. It is also known as globoidal worm gearing. Direction of transmission Unlike with ordinary gear trains, the direction of transmission