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22.322 Mechanical Design II. Spring 2013. Lecture 9. Gear Tooth Nomenclature. Be familiar enough to be able to label on Final Exam!. Lecture 9. Gear Tooth Nomenclature. Pitch circle – circle that represents the size of corresponding rolling cylinders that could replace the gear

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Lecture 9
Lecture 9

Gear Tooth Nomenclature

Be familiar enough to be able to label on Final Exam!


Lecture 91
Lecture 9

Gear Tooth Nomenclature

  • Pitch circle – circle that represents the size of corresponding rolling cylinders that could replace the gear

  • Pitch point – point of contact of two pitch circles

  • Circular pitch – distance measured along the pitch circle from a point on one tooth to the corresponding point on the adjacent tooth of the gear

  • Face width – length of the gear tooth parallel with the shaft axis

  • Addendum – radial distance from the pitch circle to the top of the gear tooth

  • Dedendum – radial distance from the pitch circle to the bottom of the gear tooth.

  • Clearance – amount the dedendum exceeds the addendum. This is the room between the top of the gear tooth and the bottom of the mating gear tooth.

  • Tooth Thickness – thickness of the tooth measured along the pitch circle


Lecture 92
Lecture 9

Gear Tooth Nomenclature

  • The circular pitch is the arc length along the pitch circle from one tooth to the next

  • Diametral pitch is the # of teeth divided by pitch diameter

  • For metric gears, the “module” is the reciprocal of diametral pitch (in mm)


Lecture 93
Lecture 9

Gear Tooth Nomenclature

  • The diametral pitch can theoretically be any size, however there are standards prescribed by the American Gear Manufacturers Association (AGMA).

  • The standards are defined based on standard gear cutting tools.

  • The diametral pitch of meshing gears must be the same!

Pressure angle influence on tooth shapes


Lecture 94
Lecture 9

Gear Tooth Nomenclature


Lecture 95
Lecture 9

Interference & Undercutting

  • In some cases the dedendum will extend below the base circle.

  • If so, the portion of the tooth below the base circle will not be involute and will interfere with the tip of the mating tooth.

  • The cutting tool will also interfere with the portion of the tooth below the base circle and will result in “undercutting”.

  • This problem can be prevented by avoiding gears with too few teeth.


Lecture 96
Lecture 9

Interference & Undercutting

  • Tables 9-4 and 9-5 show the minimum number of pinion teeth and the maximum number of gear teeth to avoid undercutting.

“Hob” = standard gear shaper


Lecture 97
Lecture 9

Contact Ratio

  • The contact ratio defines the average number of teeth in contact at any one time.

  • Should always be greater than or equal to 1


Lecture 98
Lecture 9

Example


Lecture 99
Lecture 9

Example


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