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Gear: Gear is the larger of two mating gears.
Pinion: A pinion is a smaller of the two mating gears.
Pitch surface: An imaginary surface passing through the teeth of one cogwheel, pinion, rack etc., so as to touch the corresponding surface in another, without sliding, when the two components are geared together.
Pitch circle: It is the imaginary circle that the mating gears roll without slipping. It is a right section of the gear's pitch surface.
Addendum circle: The addendum circle is an imaginary circle that bounding the tops of the teeth, in a right section of gear.
Root (or dedendum)
circle: The Dedendum circle is an imaginary circle that bounding the bottom of spaces between the teeth, in a right section of gear.
Addendum: The Addendum is radial distance between the pitch circle and addendum circle. It gives the height of the gear teeth above the pitch circle.
Dedendum: The Dedendum is radial distance between the pitch circle and Dedendum circle. It gives the depth of the gear teeth below the pitch circle.
Clearance: The clearance is the amount of difference between the dedendum of one gear and the addendum of the mating gear.
Face of tooth: The working surface of gear tooth, between the pitch circle and addendum circle.
Face width: The width of the tooth parallel to the gear axis.
Flank of tooth: The surface of gear tooth, between the pitch circle and dedendum circle.
Tooth thickness (also called the circular thickness): The thickness of tooth measured along the length of an arc following pitch circle. It is not the length of the straight line, it is the length of an arc.
Pitch: It is the distance between a point on one tooth and the corresponding point on an adjacent tooth. It is a dimension measured along a line or curve in the transverse, normal, or axial direction.
Pitch Point: It is a point on the line of centers of two gears, at which the point of tangency of two mating gears occurs.
Backlash: The clearance between the circle thickness of one gear and the tooth space of mating gear.
Backlash = Space width  thickness of tooth
Circular pitch (p): Arc distance along a specified pitch circle corresponding to the width of tooth and space of mating gears.
Diametral pitch (P_{d}): It is the ratio of number of teeth of a gear per inch of its pitch diameter.
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Module (m): It is the ratio of pitch diameter to the number of teeth.
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Velocity ratio: The ratio of RPM of driving gear to the rpm of driven gear.
Image Courtesy : Google

Image Courtesy : Google

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