Spur Gear Terms
&
Calculations


Three spur gear tooth forms are generally used with pressure angles of 14½, 20, and 25 degrees. The 14½ degree tooth form is being replaced and made obsolete by the 20 and 25 degree forms. Figure 1. illustrates these three pressure angles as applied to a gear rack with all teeth being the same depth.
The larger pressure angle makes teeth with a much larger base, which  makes these teeth much stronger
and also allows the production of gears with fewer teeth. Any two gears in mesh with each other must be
of the same pressure angle.
When gear tooth measurements are to be made with gear tooth calipers, the chordal tooth thickness and
the chordal addendum must be calculated.
 
 




Spur Gear Term Definitions

 
Addendum. The radial distance from the pitch circle to the outside diameter.
Dedendum. The radial distance between the pitch circle and the root diameter.
Circular thickness. The distance of the arc along the pitch circle from one side of a gear tooth to the other.
Circular pitch. The length of the arc of the pitch circle from one point on a tooth to the same point on the adjacent tooth.
Pitch diameter. The diameter of the pitch circle.
Outside diameter. The major diameter of the gear.
Root diameter. The diameter of the root circle measured from the bottom of the tooth spaces.
Chordal addendum. The distance from the top of the tooth to the chord connecting the circular thickness arc.
Chordal thickness. The thickness of a tooth on a straight line or chord on the pitch circle.
Whole depth. The total depth of a tooth space equal to the sum of the addendum and dedendum.
Working depth. The depth of engagement of two mating gears.
Clearance. The amount by which the tooth space is cut deeper than the working depth.
Backlash. The amount by which the width of a tooth space exceeds the thickness of the engaging tooth on the pitch circles.
Diametral pitch. The number of gear teeth to each inch of pitch diameter.
Pressure angle. The angle between a tooth profile and a radial line at the pitch circle.
Center distance. The distance between the centers of the pitch circles.


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