Table of Contents

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:

Addendum:The radial distance between the Pitch Circle and the top of the teeth.
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.
Arc of Approach: Is the arc of the Pitch Circle between the first point of contact of the gear
teeth and the Pitch Point.
Arc of Recession: That arc of the Pitch Circle between the Pitch Point and the last point of
contact of the gear teeth.
Backlash: Play between mating teeth.
Base Circle: The circle from which is generated the involute curve upon which the tooth profile is based.
Center Distance: The distance between centers of two gears.
Chordal Addendum: The distance between a chord, passing through the points where the Pitch Circle crosses the tooth profile, and the tooth top.
Chordal Thickness: The thickness of the tooth measured along a chord passing through the points where the Pitch Circle crosses the tooth profile.
Circular Pitch: Millimeter of Pitch Circle circumference per tooth.
Circular Thickness: The thickness of the tooth measured along an arc following the Pitch Circle.
Clearance: The distance between the top of a tooth and the bottom of the space into which it fits on the meshing gear.
Contact Ratio: The ratio of the length of the Arc of Action to the Circular Pitch.
Dedendum: The radial distance between the bottom of the tooth to pitch circle.
Diametral Pitch: Teeth per mm of diameter.
Face: The working surface of a gear tooth, located between the pitch diameter and the top of the tooth.
Face Width: The width of the tooth measured parallel to the gear axis.
Flank: The working surface of a gear tooth, located between the pitch diameter and the bottom of the teeth
Wheel:Larger of the two meshing gears is called wheel.
Pinion: The smaller of the two meshing gears is called pinion.
Land: The top surface of the tooth.
Line of Action: That line along which the point of contact between gear teeth travels, between the first point of contact and the last.
Module: Ratio of Pitch Diameter to the number of teeth.
Pitch Circle: The circle, the radius of which is equal to the distance from the center of the gear to the pitch point.
Diametral pitch: Ratio of the number of teeth to the of pitch circle diameter.
Pitch Point: The point of tangency of the pitch circles of two meshing gears, where the Line of Centers crosses the pitch circles.
Pressure Angle: Angle between the Line of Action and a line perpendicular to the Line of Centers.
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.
Ratio: Ratio of the numbers of teeth on mating gears.
Root Circle: The circle that passes through the bottom of the tooth spaces.
Root Diameter: The diameter of the Root Circle.
Working Depth: The depth to which a tooth extends into the space between teeth on the mating gear.