# Basic Of Gears -Gear Terminology/ Gear nomenclature

## Gears:

### Introduction:

The slip and creep in the belt or rope drives is a common phenomenon, in the transmission of motion or power between two shafts. The effect of slip is to reduce the velocity ratio of the drive. In precision machine, in which a definite velocity ratio is importance (as in watch mechanism, special purpose machines..etc), the only positive drive is by means of gears or toothed wheels.

## Gear Terminology:

1. Addendum:The radial distance between the Pitch Circle and the top of the teeth.
2. Arc of Action: Is the arc of the Pitch Circle between the beginning and the end of the
engagement of a given pair of teeth.
3. Arc of Approach: Is the arc of the Pitch Circle between the first point of contact of the gear
teeth and the Pitch Point.
4. Arc of Recession: That arc of the Pitch Circle between the Pitch Point and the last point of
contact of the gear teeth.
5. Backlash: Play between mating teeth.
6. Base Circle: The circle from which is generated the involute curve upon which the tooth profile is based.
7. Center Distance: The distance between centers of two gears.
8. Chordal Addendum: The distance between a chord, passing through the points where the Pitch Circle crosses the tooth profile, and the tooth top.
9. Chordal Thickness: The thickness of the tooth measured along a chord passing through the points where the Pitch Circle crosses the tooth profile.
10. Circular Pitch: Millimeter of Pitch Circle circumference per tooth.
11. Circular Thickness: The thickness of the tooth measured along an arc following the Pitch Circle.
12. Clearance: The distance between the top of a tooth and the bottom of the space into which it fits on the meshing gear.
13. Contact Ratio: The ratio of the length of the Arc of Action to the Circular Pitch.
14. Dedendum: The radial distance between the bottom of the tooth to pitch circle.
15. Diametral Pitch: Teeth per mm of diameter.
16. Face: The working surface of a gear tooth, located between the pitch diameter and the top of the tooth.
17. Face Width: The width of the tooth measured parallel to the gear axis.
18. Flank: The working surface of a gear tooth, located between the pitch diameter and the bottom of the teeth
19. Wheel:Larger of the two meshing gears is called wheel.
20. Pinion: The smaller of the two meshing gears is called pinion.
21. Land: The top surface of the tooth.
22. Line of Action: That line along which the point of contact between gear teeth travels, between the first point of contact and the last.
23. Module: Ratio of Pitch Diameter to the number of teeth.
24. Pitch Circle: The circle, the radius of which is equal to the distance from the center of the gear to the pitch point.
25. Diametral pitch: Ratio of the number of teeth to the of pitch circle diameter.
26. Pitch Point: The point of tangency of the pitch circles of two meshing gears, where the Line of Centers crosses the pitch circles.
27. Pressure Angle: Angle between the Line of Action and a line perpendicular to the Line of Centers.
28. Profile Shift: An increase in the Outer Diameter and Root Diameter of a gear, introduced to lower the practical tooth number or acheive a non-standard Center Distance.
29. Ratio: Ratio of the numbers of teeth on mating gears.
30. Root Circle: The circle that passes through the bottom of the tooth spaces.
31. Root Diameter: The diameter of the Root Circle.
32. Working Depth: The depth to which a tooth extends into the space between teeth on the mating gear.