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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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22.322 Mechanical Design II

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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

• 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 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 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 9

Gear Tooth Nomenclature

### 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 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 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

Example

Example