Notes on 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 circle and the addendum circle.

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.

Various Terminology of 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 the diametral pitch.
m = 1/p

Circular Pitch :

Circular pitch is the space in the pitch circle used by each tooth.

Gear Ratio :

The gear ratio is the number of teeth of the larger gear to the smaller gear.

Pressure Line :

The 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 the common tangent to the pitch circles. It is also called the angle of obliquity. High-pressure angle requires a broader base and stronger teeth.

Pitch Angle :

Pitch angle is the angle captured by a tooth.
Pitch angle = 360/T

Contact Ratio :

Contact ratio is the angle of action and pitch angle.

Path of Approach :

The 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 :

The 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.