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Gearing Systems : Gear Tooth Nomenclature

Do to the complexity of a gear tooth a nomenclature has been developed to describe the different parts of the tooth. Refer to the image below .

Gear tooth Nomenclature

From the image above you can see that the tooth height is dependent on the addendum and the dedendum. The dedendum and addendum are separated by the nominal pitch circle. The gear thickness is determined at this point also. The reason why the pitch circle is used is because this is point where the pinion and gear would mesh causing the gears to have contact at this point. The dedendum is normally slightly larger than the addendum to include clearance for the mating gear tooth. The arc length along the pitch circle measured from identical points on two gear teeth will determine the circular pitch of the gear. This can also be calculated using the equation below. You must know the pitch diameter (d) and the number of teeth (N) to use this equation.

pitch circle calculation (1)

The base pitch can be determined by using the equation below.

base pitch calculation (2)

Finally, the diametral pitch can be determined by using equation 3.

diametral pitch calculation (3)

The diametral pitch must be the same for both the pinion and gear for the two to mate correctly. Refer to the table below to determine the dimensional values of a gear tooth based off of the AGMA (American Gear Manufacturers Association) specifications.

dimensional values of a gear tooth

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