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38
Shown in Fig. III.4 is the fourth basic gear type, the worm and worm
gear. As shown, the worm resembles a screw. The direction of rotation of the
worm gear, also called the worm wheel, depends upon the direction of
rotation of the worm and upon whether the worm teeth are cut right-hand or
left-hand. Worm-gear sets are also made so that the teeth of one or both wrap
partly around the other. Such sets are called single-enveloping and double-
enveloping worm-gear sets. Worm-gear sets are mostly used when the speed
ratios of the two shafts are quite high, say, 3 or more.
2. Nomenclature
The terminology of spur-gear teeth is illustrated in Fig. III.5. The pitch
circle is a theoretical circle upon which all calculations are usually based; its
diameter is the pitch diameter. The pitch circles of a pair of mating gears are
tangent to each other. A pinion is the smaller of two mating gears. The larger
is often called the gear.
The circular pitch p is the distance, measured on the pitch circle, from a
point on one tooth to a corresponding point on an adjacent tooth. Thus the
circular pitch is equal to the sum of the tooth thickness and the width of
space.
The module m is the ratio of the pitch diameter to the number of teeth.
The customary unit of length used is the millimeter. The module is the index
of tooth size in SI.
The diametral pitch P is the ratio of the number of teeth on the gear to
the pitch diameter. Thus, it is the reciprocal of the module. Since diametral
pitch is used only with U.S. units, it is expressed as teeth per inch.
Figure III.4
Worm gearsets are used to
transmit rotary motion
between nonparallel and
nonintersecting shafts.
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