Machine Design

Gear Tooth Form

Gear tooth geometry is determined primarily by pitch, depth, and pressure angle.

Pitch: Standard pitches are usually whole numbers when measured as diametral pitch P, the ratio of the number of teeth to the pitch diameter in inches. Coarse-pitch gearing has teeth larger than 20 diametral pitch -- usually 0.5 to 19.99. Fine-pitch gearing usually has teeth of diametral pitch 20 to 200.

Depth:Standardized in terms of pitch. Standard full-depth teeth have working depths of 2/P. If the teeth have equal addenda (as in standard interchangeable gears), the addendum is 1/P. Stub teeth have a working depth usually 20% less than full-depth teeth. Full-depth teeth have a larger contact ratio than stub teeth. Gears with small numbers of teeth may have an undercut so they do not interfere with one another during engagement. Undercutting reduces the active profile and weakens the tooth.

Mating gears with long and short addendum have larger load-carrying capacity than standard gears. The addendum of the smaller gear (pinion) is increased while that of the larger gear is decreased, leaving the whole depth the same. This form is known as recess-action gearing.

Pressure angle: Standard angles are 20 and 25°. Earlier standards included a 14° pressure angle that is still used. Pressure angle affects the force that tends to separate mating gears. High pressure angle decreases the contact ratio (ratio of the number of teeth in contact) but provides a tooth of higher capacity and allows gears to have fewer teeth without undercutting.

Backlash: Shortest distance between the noncontacting surfaces of the adjacent teeth.

Gears are commonly specified according to AGMA Class Number, a code which denotes important quality characteristics. Quality numbers denote tooth-elements tolerances. The higher the number, the tighter the tolerance. Gears are heat treated by case hardening, nitriding, precipitation hardening, or through hardening. In general, harder gears are stronger and last longer than soft ones. Thus, hardening is a device that cuts the weight and size of gears. Some processes, such as flame hardening, improve service life but do not necessarily improve strength.

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