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

Professor Emeritus
The University of Michigan
Ann Arbor, Michigan

9.1 DEFINITIONS

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.

The pitch...

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Bevel and Miter Gears

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.

Spur Gears

Spur gears are the simplest type of gear. Transmitting power between parallel axes, the teeth project radially on the disc.

Hypoid Gears

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.

Worms and Worm Gears

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.

Metric Gears
Metric gears are defined by module, which designates the center-to-center distance between successive teeth. This criteria of gear selection is used in countries which have adopted the metric system and is intended to standardize gear selection.

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GEARING Nomenclature and Comparative Sizes of Gear Teeth American National Standard and Former American Standard Gear Tooth Forms ANSI B6.1-1968,R1974 and ASAB6.1-1932 Basic Rack of the 20-Degree...

Gear tooth geometry is determined primarily by pitch, depth, and pressure angle. Standard pitches are usually whole numbers when measured as diametral pitch , the ratio of the number of teeth to the...

Module System Gear Design Gear Design Based upon Module System. The module of a gear is equal to the pitch diameter divided by the number of teeth, whereas diametral pitch is equal to the number of...

Overview Q3-1 With spur gears having twisted teeth (cycloidal), what is the form of the line of contact along the tooth surface? What is the form of pitch surface in spur gears, bevel gears, hypoid...