with the employment of double- and treble-threaded worms. For ordinary
service, cast gears with single-threaded worms are still used, and are less
costly than those produced by hobbing.
Worm gears have the helix for their basis, though this is somewhat dis-
guised in the case of the wheel. The worm is a continuous thread of ex-
Fig- 55-—Development of Bevel Gears
tremely short axial pitch or lead. The wheel which it drives has a number
of short helical teeth of extremely long axial pitch. The axial pitch of the
worm, if single-threaded, measures the same as the circular pitch of the
wheel teeth. The wheel may contain any number of teeth. The worm
diameter is usually from twice to three times the pitch. The curvature
therefore being small, the teeth of the wheel should form envelopes of the
worm thread to ensure durability and smooth movement.
The section of the worm thread is that of the involute rack. ' A worm