3. Pitch point: It is a common point of contact between two pitch circles.
4. Pitch surface: It is the surface of the rolling discs which the meshing gears have replaced at the pitch circle.
5. Pressure angle or angle of obliquity: It is the angle between the common normal to two gear teeth at the point of contact and the common tangent at the pitch point. It is usually denoted by φ. The standard pressure angles are 14.5 ° and 20°.
6. Addendum: It is the radial distance of a tooth from the pitch circle to the top of the tooth.
7. Dedendum: It is the radial distance of a tooth from the pitch circle to the bottom of the tooth.
8. Addendum circle: It is the circle drawn through the top of the teeth and is concentric with the pitch circle.
9. Dedendum circle: It is the circle drawn through the bottom of the teeth. It is also called root circle.
Note : Root circle diameter = Pitch circle diameter × cos φ, where φ is the pressure angle.
10. Circular pitch: It is the distance measured on the circumference of the pitch circle from a point of one tooth to the corresponding point on the next tooth. It is usually denoted by Pc.
It is denoted by Pd . Mathematically,
14. Total depth: It is the radial distance between the addendum and the dedendum circles of a gear. It is equal to the sum of the addendum and dedendum.
16. Tooth thickness: It is the width of the tooth measured along the pitch circle.
17. Tooth space: It is the width of space between the two adjacent teeth measured along the pitch circle.
18. Backlash: It is the difference between the tooth space and the tooth thickness, as measured along the pitch circle. Theoretically, the backlash should be zero, but in actual practice some backlash must be allowed to prevent jamming of the teeth due to tooth errors and thermal expansion.
19. Face of tooth: It is the surface of the gear tooth above the pitch surface.
20. Flank of tooth: It is the surface of the gear tooth below the pitch surface.
21. Top land: It is the surface of the top of the tooth.
22. Face width: It is the width of the gear tooth measured parallel to its axis.
23. Profile: It is the curve formed by the face and flank of the tooth.
24. Fillet radius: It is the radius that connects the root circle to the profile of the tooth.
25. Path of contact: It is the path traced by the point of contact of two teeth from the beginning to the end of engagement.
27. Arc of contact: It is the path traced by a point on the pitch circle from the beginning to the end of engagement of a given pair of teeth. The arc of contact consists of two parts, i.e.
(a) Arc of approach: It is the portion of the path of contact from the beginning of the engagement to the pitch point.
(b) Arc of recess: It is the portion of the path of contact from the pitch point to the end of the engagement of a pair of teeth.