Worm Gear

worm gear



4-start worm , wheel

worms resemble screws. worm meshed worm wheel, looks similar spur gear.

worm-and-gear sets simple , compact way achieve high torque, low speed gear ratio. example, helical gears limited gear ratios of less 10:1 while worm-and-gear sets vary 10:1 500:1. disadvantage potential considerable sliding action, leading low efficiency.

a worm gear species of helical gear, helix angle large (close 90 degrees) , body long in axial direction. these attributes give screw qualities. distinction between worm , helical gear @ least 1 tooth persists full rotation around helix. if occurs, worm ; if not, helical gear . worm may have few 1 tooth. if tooth persists several turns around helix, worm appears, superficially, have more 1 tooth, 1 in fact sees same tooth reappearing @ intervals along length of worm. usual screw nomenclature applies: one-toothed worm called single thread or single start; worm more 1 tooth called multiple thread or multiple start. helix angle of worm not specified. instead, lead angle, equal 90 degrees minus helix angle, given.

in worm-and-gear set, worm can drive gear. however, if gear attempts drive worm, may or may not succeed. particularly if lead angle small, gear s teeth may lock against worm s teeth, because force component circumferential worm not sufficient overcome friction. in traditional music boxes, however, gear drives worm, has large helix angle. mesh drives speed-limiter vanes mounted on worm shaft.

worm-and-gear sets lock called self locking, can used advantage, instance when desired set position of mechanism turning worm , have mechanism hold position. example machine head found on types of stringed instruments.

if gear in worm-and-gear set ordinary helical gear single point of contact achieved. if medium high power transmission desired, tooth shape of gear modified achieve more intimate contact making both gears partially envelop each other. done making both concave , joining them @ saddle point; called cone-drive or double enveloping .

worm gears can right or left-handed, following long-established practice screw threads.

3d animation of worm-gear set




^ vallance & doughtie 1964, p. 287.
^ vallance & doughtie 1964, pp. 280, 296.
^ cite error: named reference hypoidgears invoked never defined (see page).
^ vallance & doughtie 1964, p. 290.
^ mcgraw-hill 2007, p. 744
^ american gear manufacturers association; american national standards institute, gear nomenclature, definitions of terms symbols (ansi/agma 1012-g05 ed.), american gear manufacturers association