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Design & Engineering

Gear Terminology

Circular Pitch: the distance between corresponding points on adjacent teeth measured along the pitch circle.

Pitch Diameter: the diameter measured at the pitch circle.

Pressure Angle: the angle made by a line tangent to the gear tooth at the pitch diameter and a line drawn through this point of tangency.

Addendum: the length of the tooth from the pitch line to the outside diameter.

Dedendum: the length of the tooth from the pitch line to the base of the tooth.

Face Width: the working surface of the tooth above the pitch line.

Center Distance: the distance between the centers of two meshing gears.

M.O.W.: measurement over wires (a test for pitch diameter excluding backlash.)

Pressure Angle: Meshing gears must have the same pressure angle as well as the same diametral pitch. The 20 degree full depth tooth is recommended for molded gears. The strength and wear characteristics are improved over the 141/2 degree tooth and the 20 degree stub tooth.

Backlash: Gears require operating clearance between the engaging teeth on the pitch circle. This is usually provided by the extending of the center distance of meshing gears. This has the advantages of allowing more clearance between the outside diameter of one gear and the base diameter of the mating gear so that the gears can expand at high temperatures without radial interference.

Backlash must be provided for gear expansion, since plastic gear teeth deflect more that metal teeth. Backlash is not critical when considering gear errors. Suggested backlash when assembled at room temperature is as follows:

16 0.004 to 0.006
20 0.003 to 0.005
32 and finer 0.002 to 0.004

For higher temperature, loads or speeds, additional backlash should be provided to compensate for thermal expansion.

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