All the important gear terminologies are shown in Figure 15.14.
Pitch Point: The point of contact between pitch circles of two gears is known as pitch point.
Pitch Circle: The circle passing through point of contacts of two gears is known as pitch circle.
Pitch Diameter, D: Diameter of pitch circle is known as pitch diameter.
Circular Pitch, Pc: It is the distance measured along the circumference of the pitch circle from a point on one tooth of the corresponding point on the adjacent tooth.
Figure 15.14 Nomenclature of Gear
Diametral Pitch, Pd: It is the number of teeth per unit length of the pitch circle diameter.
Module, m: It is ratio of pitch diameter to the number of teeth.
Gear Ratio: It is the ratio of number of teeth on gear and pinion.
Velocity Ratio: It is ratio of angular velocity of the driving gear to driven gear.
Here, subscripts 1 and 2 are used for driving and driven gears, respectively.
Addendum Circle: It is a circle passing through the tips of the teeth.
Addendum: It is the radial height of tooth above the pitch circle. Its standard value is one module.
Dedendum Circle: It is a circle passing through roots of the teeth.
Dedendum: It is a radial depth of a tooth below the pitch circle. Its standard value is 1.157 m.
Full Depth of Teeth: It is the total depth of the tooth space, i.e., Full depth = Addendum + Dedendum = (1 + 1.157) × Module = 2.157 × Module.
Working Depth of Teeth: The maximum depth at which a tooth penetrates into tooth space of the mating gear is known as working depth of teeth.
Space Width: It is the width of the space between two consecutive teeth on pitch circle.
Tooth Thickness: It is thickness of the tooth measured along the pitch circle.
Backlash: It is difference between the space width and the tooth thickness along the pitch circle.
Face Width: It is length of tooth parallel to the gear axis.
Top Land: It is the surface of the top of the tooth.
Bottom Land: The surface of the bottom of the tooth between the adjacent fillets.
Face: It is the tooth surface between the pitch circle and the top land.
Flank: It is the curved portion of the tooth flank at the root circle.
Pressure Angle, φ: The angle between the pressure line and the common tangent at the pitch point is known as the pressure angle or angle of obliquity.
Path of Contact or Contact Length: Locus of the point of contact of teeth of two mating gears from beginning of engagement to end of engagement is known as path of contact or the contact length.
Path of Approach: Portion of the path of contact from the beginning of engagement to the pitch point is known as path of approach.
Path of Recess: Portion of the path of contact from the pitch point to the end of engagement is known as path of recess.
Arc of Contact: Locus of points on the pitch circle from the beginning of engagement to the end of engagement of two mating gears is known as arc of contact.
Arc of Approach: It is the portion of arc of contact from the beginning of engagement to the pitch point of two mating gears is known as arc of contact.
Arc of Recess: It is the portion of arc of contact from the pitch point to the end of engagement of two mating gears is known as arc of recess.
Contact Ratio: It is the ratio of the length of arc of contact to circular pitch.
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