Worm gears pitch

Pitch. Axial pitch of worm = circular pitch of gear. Keep the fraction simple so that accurate factoring can be used to determine change gears. 

Gear-throat diameter = gear pitch diameter -i- 2 x gear addenda. Gear outside diameter = gear throat diameter -i-1 addendum of worm rounded off to the nearest fraction of an inch. 

Pitch diameter of worm is the diameter of a circle that is tangent to the pitch circle of the mating gear in its midplane. 

Pitch diameter of gear is the diameter of the pitch circle at the midplane of the gear that would roll upon the pitch line of the worm if the latter were used as a rack. 

Circular pitch is the distance from a point on one gear tooth to the same point of the succeeding tooth measured circumferentially on the midplane pitch circle. It is equal to the axial pitch of the worm, that is, the distance from any point on a thread of the worm to the corresponding point on the next thread, measured parallel to the axis. 

Lead of worm is the distance parallel to the axis of the worm from a point on a given thread to the corresponding point on the same thread after it has made one turn around the worm. If the worm has only one thread, this distance is equal to the circular pitch, but if the worm has multiple threads, it is equal to the circular pitch multiplied by the number of threads. It is the distance that a point on the pitch circle of the gear is advanced by one revolution of the worm. 

One revolution of the worm advances the gear by as many teeth as there are threads on the worm. Therefore, the ratio of transmission is equal to the number of teeth on the gear, divided by the number of threads on the worm, without regard to the pitch. 

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