Worm gears are usually used when large velocity reductions are needed. The decrease ratio is determined by the number of starts of the worm and number of tooth on the worm gear. But worm gears have sliding get in touch with which is silent but will produce heat and also have relatively low transmission effectiveness.
As for the materials for creation, in general, worm is made from hard metal while the worm gear is manufactured out of relatively soft steel such as aluminum bronze. This is because the number of the teeth on the worm gear is relatively high in comparison to worm with its number of starts being usually 1 to 4, by reducing the worm gear hardness, the friction on the worm teeth is reduced. Another characteristic of worm manufacturing may be the need of specialized machine for gear trimming and tooth grinding of worms. The worm gear, however, may be made out of the hobbing machine used for spur gears. But because of the various tooth shape, it is not possible to cut many gears at once by stacking the gear blanks as can be carried out with spur gears.
The applications for worm gears include equipment boxes, angling pole reels, guitar string tuning pegs, and where a delicate rate adjustment by utilizing 
a big speed worm wheel china reduction is necessary. While you can rotate the worm gear by worm, it is normally not possible to rotate worm by using the worm gear. That is called the personal locking feature. The self locking feature cannot continually be assured and another method is preferred for accurate positive reverse prevention.