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Worm gears are usually used when large speed reductions are needed. The reduction ratio is determined by the number of starts of the worm and number of tooth on the worm gear. But worm gears possess sliding contact which is noiseless but will produce heat and have relatively low transmission effectiveness.
For the materials for production, in general, worm is made from hard metal while the worm gear is manufactured out of relatively soft steel such as aluminum bronze. That is because the number of teeth on the worm gear is relatively high compared to worm using its number of starts being usually 1 to 4, by reducing the worm equipment hardness, the friction on the worm tooth is reduced. Another feature of worm manufacturing is the need of specialized machine for gear trimming and tooth grinding of worms. The worm gear, however, may be made with the hobbing machine used for spur gears. But due to the various tooth shape, it is not possible to cut a number of gears simultaneously by stacking the apparatus blanks as can be carried out with spur gears.
The applications for worm gears include gear boxes, fishing pole reels, guitar worm wheel string tuning pegs, and where a delicate velocity adjustment by utilizing a big speed reduction is necessary. While you can rotate the worm gear by worm, it is generally not possible to rotate worm by using the worm gear. This is called the self locking feature. The self locking feature cannot always be assured and a separate method is preferred for true positive reverse prevention.