The following are the important dimensions and geometries concerned with toothed gear:

#### Pitch Circle :

*Pitch circle* is the apparent circle that two gears can be taken like smooth cylinders rolling without friction.

#### Addendum Circle :

*Addendum circle* is the outer most profile circle of a gear. Addendum is the radial distance between the pitch circle and the addendum circle.

#### Dedendum Circle :

*Dedendum circle* is the inner most profile circle. Dedendum is the radial distance between the pitch circle and the dedendum circle.

#### Clearance :

*Clearance* is the radial distance from top of the tooth to the bottom of the tooth space in the mating gear.

#### Backlash :

*Backlash* is the tangential space between teeth of mating gears at pitch circles.

#### Full Depth :

*Full depth* is sum of the addendum and the dedendum.

#### Face Width :

*Face width* is length of tooth parallel to axes.

#### Diametral Pitch :

*Diametral pitch* (p) is the number of teeth per unit volume.

p = (Number of Teeth) / (Diameter of Pitch circle)

#### Module :

*Module (m)*is the inverse of diametral pitch.

#### Circular Pitch :

*Circular pitch* is the space in pitch circle used by each teeth.

#### Gear Ratio :

*Gear ratio* is numbers of teeth of larger gear to smaller gear.

#### Pressure Line :

*Pressure line* is the common normal at the point of contact of mating gears along which the driving tooth exerts force on the driven tooth.

#### Pressure Angle :

*Pressure angle* is the angle between the pressure line and common tangent to pitch circles. It is also called angle of obliquity. high pressure angle requires wider base and stronger teeth.

#### Pitch Angle :

*Pitch angle* is the angle captured by a tooth.

Pitch angle = 360/T

#### Contact Ratio :

*Contact ratio* is angle of angle of action and pitch angle.

#### Path of Approach :

*Path of approach* is the distance along the pressure line traveled by the contact point from the point of engagement to the pitch point.

#### Path of Recess :

*Path of recess* is the distance traveled along the pressure line by the contact point from the pitch point to the path of disengagement.

#### Path of Contact :

*Patch of contact* is the sum of path of approach and path of recess.

#### Arc of Approach :

*Arc of approach* is the distance traveled by a point on either pitch circle of the two wheels from the point of engagement to the pitch.

#### Arc of Recess :

*Arc of recess* is the distance traveled by a point on either pitch circle of the two wheels from the point to the point of disengagement.

#### Arc of Contact :

*Arc of contact* is the distance traveled by a point on either pitch circle of the two wheels during the period of contact of a pair of teeth.

#### Angle of Action :

*Angle of action* is the angle turned by a gear during arc of contact.

## All Comments

excellent

excellent

What does the term identification groove on spur gear imply.

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infomation is good but, i have some confusion , external diameter is pitch diameter plus two addendum althouh upper portion of pitch diameter is addendum