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# 22.322 Mechanical Design II - PowerPoint PPT Presentation

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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### 22.322 Mechanical Design II

Spring 2013

Gear Tooth Nomenclature

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

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

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)

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

Gear Tooth Nomenclature

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.

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

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

Example

Example