Gear TerminologyA few of the most common terms are defined here to provide a working knowledge of gear terminology. Generally the basic terminology for spur gears applies to all other gear types as well. More complex gear types call only for additional terminology.

Addendum: External-Height of the tooth above pitch circle Internal- Height of the tooth from inside diameter to pitch circle

Backlash: Play between the teeth of gears in mesh

Center Distance: Distance between centers of gears in mesh

Circular Pitch Circular distance measured between corresponding points on adjacent teeth along the pitch circle Circular pitch.AddendumExtAddendumIntBacklashExt.BacklashInt.Centerdistance

Circular Tooth Thickness: Thickness of tooth measured as an arc along the pitch circleCircular Tooth Space Width: Space width of internal tooth measured as an arc along the pitch circle Root Clearance Radial distance between outer dia of one gear to root dia of other gear in meshed condition Dedundum: Radial distance from pitch circle diameter to root diameter Module: Pitch circle diameter in mm divided by number of teeth. ModuleIntCircular tooth thicknessCircular space widthRoot clearanceExtIntDedundumExtDedundum

Face Width: Width of tooth in axial planeFace widthFillet Radius: Radius of fillet at root diameterGear Ratio: The ratio of number of teeth in large gear to that of small gear in a meshed gear pair.(Always +ve)Large gearSmall gearOutside Diameter: Outer circle diameter. Pitch Circle: circle defined by number of teeth and module Pitch circlePinion gearRackFace widthIntExtFillet radiusexternal gearFillet radiusInternal gearOutside diameter

Angle between the normal line drawn to a profile at a point on the profile, and the tangent drawn at the same point to a circle generated from the center which passes through the same point on the profile. Angle between the line joining a point on profile to center and, a line joining the center to the tangent point on base circle Root Diameter: Diameter of root circle Tip Relief: Modification of tooth profile by removing a small amount of material from the tip of the tooth. Undercut: While generating tooth, material removed from the involute profile near the root dia, is called as undercut Pressure Angle:Pressure angleNormalPressure angleBase circlePitch circleRoot diameterExternalInternalTrue involuteModified involuteUndercutBase circle

Profile Shift: Correction given to tooth profile to modify tooth thickness / tooth space width. +ve correction-ve correctionNo correction Land At Addendum: Distance between two edges of tooth at addendum. Base Circle: The circle from which the involute is generated.Circular distance between corresponding points on adjacent profile measured on base circle. OrStraight distance between corresponding points on adjacent profile measured on base tangent.Base pitch:Land at ODExternalInternalLand at IDBase circleInternalExternalBase circleBase tangentBase pitch

Involute FunctionAngle formed by a line joining a point on involute profile to center of gear, and a line joining involute starting point on base circle to center of gear (Angle POA) is called Involute Function and this is the function of pressure angle (Angle POB).

If point P is on pitch circle, it is a involute function of the pressure angle at pitch point and, if point P is not on pitch circle, then it is a involute function of pres.angle at that point.

This is the most important function and used frequently in gear calculations BO ABAB PB (Involute property) TanPBBO Tan All angles are in radians