The most common types of gears are illustrated in Figs. 5.16 to 5.25. Other available types are
generally modifications of the basic gears shown. Gear nomenclature and definitions can be
found in ANSI/AGMA 1012-F90, Gear Nomenclature, Definitions of Terms with Symbols.1
Spur gears A spur gear has a cylindrical pitch surface and teeth that are parallel to the axis.
Spur gears operate on parallel axes (Fig. 5.16).
Spur rack A spur rack has a plane pitch surface and straight teeth that are at right angles to
the direction of motion (Fig. 5.16).
Helical gears A helical gear has a cylindrical pitch surface and teeth that are helical. Parallel
helical gears operate on parallel axes. Mating external helical gears on parallel axes have
helices of opposite hands. If one of the mating members is an internal gear, the helices are of
the same hand (Fig. 5.17).
Single-helical gears Gears have teeth of only one hand on each gear (Fig. 5.18).
Double-helical gears Gears have both right-hand and left-hand teeth on each gear. The
teeth are separated by a gap between the helices (Fig. 5.19).Where there is no gap, they are
known as herringbone gears.
Wormgearing Includes worms and their mating gears. The axes are usually at right angles
(Fig. 5.20).
Wormgear (wormwheel) The gear that is the mate to a worm. A wormgear that is completely
conjugate to its worm has a line contact and is said to be enveloping. An involute spur
gear or helical gear used with a cylindrical worm has point contact only and is said to be
nonenveloping (Fig. 5.20).
Cylindrical worm A worm that has one or more teeth in the form of screw threads on a
cylinder.
Enveloping worm (hourglass) A worm that has one or more teeth and increases in diameter
from its middle portion toward both ends, conforming to the curvature of the gear (Fig. 5.20).
Double-enveloping wormgearing This is comprised of enveloping (hourglass) worms mated
with fully enveloping wormgears (Fig. 5.21).
Bevel gears These are gears that have conical pitch surfaces and operate on intersecting
axes that are usually at right angles (Fig. 5.22).
Miter gears These are mating bevel gears with equal numbers of teeth and with axes at right
angles (Fig. 5.23).
Straight bevel gears These have straight tooth elements which, if extended, would pass
through the point of intersection of their axes (Fig. 5.24).
Spiral bevel gears These have teeth that are curved and oblique (Fig. 5.24).
Hypoid gears Similar in general form to bevel gears, hypoid gears operate on nonintersecting
axes (Fig. 5.25).
Standard Handbook of Plant Engineering (3rd Edition)
By: Rosaler, Robert © 2002 McGraw-Hill
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