Worm gears are usually used when large velocity reductions are needed. The reduction ratio depends upon the number of starts of the worm and number of tooth on the worm equipment. But worm gears possess sliding contact which is tranquil but tends to produce heat and have relatively low transmission performance.
As for the materials for production, in general, worm is constructed of hard metal while the worm gear is manufactured out of relatively soft steel such as for example aluminum bronze. This is since the number of tooth on the worm gear is relatively high in comparison to worm with its number of starts being generally 1 to 4, by reducing the worm gear hardness, the friction on the worm tooth is reduced. Another characteristic of worm manufacturing is the need of specific machine for gear trimming and tooth grinding of worms. The worm equipment, however, may be made out of the hobbing machine utilized for spur gears. But because of the various tooth shape, it is not possible to cut many gears simultaneously by stacking the gear blanks as can be done with spur gears.
The applications for worm gears include equipment boxes, fishing pole reels, guitar string tuning pegs, and in which a delicate 
speed adjustment by utilizing a huge speed reduction is needed. While you can rotate the worm gear by worm, it is normally not possible to rotate worm by using the worm gear. This is called the self worm wheel china locking feature. The self locking feature cannot continually be assured and another method is preferred for accurate positive reverse prevention.