Gear Terminology

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

Pitch Circle :

A pitch circle is an apparent circle in which two gears can be taken like smooth cylinders rolling without friction.

Addendum Circle :

The addendum circle is the outermost profile circle of a gear. The addendum is the radial distance between the pitch and addendum circles.

Dedendum Circle :

The dedendum circle is the innermost profile circle. The dedendum is the radial distance between the pitch and dedendum circles.

Clearance :

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

Backlash :

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

Full Depth :

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

Face Width :

Face width is the length of the tooth parallel to the 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.
m = 1/p

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 :

The patch of contact is the sum of the path of approach and the path of recess.

Arc of Approach :

The 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 :

The Arc of the 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 contact period of a pair of teeth.

Angle of Action :

The angle of action is the angle a gear turns during an arc of contact.