From Standard Handbook of Machine Design, 3rd Edition
- Chapter 9: SPUR GEARS
- Chapter 10: HELICAL GEARS
- Chapter 11: BEVEL AND HYPOID GEARS
- Chapter 12: WORM GEARING
- Chapter 13: POWER SCREWS
Joseph E. Shigley
The University of Michigan
Ann Arbor, Michigan
Spur gears are used to transmit rotary motion between parallel shafts. They are cylindrical, and the teeth are straight and parallel to the axis of rotation.
The pinion is the smaller of two mating gears; the larger is called the gear or the wheel.
The pitch circle, B in Fig. 9.1, is a theoretical circle upon which all calculations are based. The operating pitch circles of a pair of gears in mesh are tangent to each other.
Figure 9.1: Terminology of gear teeth. A, addendum circle; B, pitch circle; C, clearance circle; D, dedendum circle; E, bottom land; F, top land; G, flank; H, face; a = addendum distance; b = dedendum distance; c = clearance distance; p = circular pitch; t = tooth thickness; u = undercut distance.
The circular pitch, p in Fig. 9.1, is the distance, measured on the theoretical pitch circle, from a point on one tooth to a corresponding point on an adjacent tooth. The circular pitch is measured in inches or in millimeters. Note, in Fig. 9.1, that the circular pitch is the sum of the tooth thickness t and the width of space.
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Spur gears are the simplest type of gear. Transmitting power between parallel axes, the teeth project radially on the disc.
Bevel gears are gears designed to transmit motion between intersecting axes. Perpendicular arrangements are most common, but bevel gears can be manufactured for nearly any angle. The teeth bearing surface of the gear, or surface pitch, is conically shaped or tapered. Miter gears are bevel gears manufactured in a 1:1 ratio, with the same number of teeth on mating gears and with perpendicular axes.
A hypoid gear is a style of spiral bevel gear whose main variance is that the mating gears' axes do not intersect. The hypoid gear is offset from the gear center, allowing unique configurations and a large diameter shaft. The teeth on a hypoid gear are helical, and the pitch surface is best described as a hyperboloid. A hypoid gear can be considered a cross between a bevel gear and a worm drive.
Herringbone gears, also called double helical gears, are gear sets designed to transmit power through parallel or, less commonly, perpendicular axes. The unique tooth structure of a herringbone gear consists of two adjoining, opposite helixes that appear in the shape of the letter 'V'.
Worms and worm gears are gear sets that offer high gear reduction and torque multiplication with a small footprint. A worm drive is a cylindrical gear with a shallow spiral thread that engages the worm gear in a non-intersecting, perpendicular axes configuration.
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