Gear Terminology: Addendum, Arc of Action and more

• Addendum refers to the tooth height above the pitch circle, measuring the distance between the pitch circle and the top of the tooth.
• The Addendum Circle (or Outside Circle) defines the boundary of the outer edges of the teeth.
• Arc of Action refers to the arc of the pitch circle through which a tooth moves, spanning from initial contact with a mating tooth to when contact ends.
• Arc of Approach is the arc of the circle where a tooth travels from the point of contact with the mating tooth to the pitch point.
• Arc of Recess refers to the arc of the pitch circle where a tooth travels from its contact with the mating tooth at the pitch point to where the contact ceases.
• Axial Plane in a pair of gears contains the two axes, while for a single gear, it can be any plane that includes the axis and a given point.
• Backlash quantifies the amount by which a tooth space's width exceeds the thickness of the engaging tooth on the pitch circles.
• Base Circle refers to the circle from which an involute tooth is generated or developed.
• Base Helix Angle refers to the angle at the base cylinder in an involute gear, defining the angle between the tooth and the gear axis.
• Base Pitch in an involute gear is the pitch on the base circle or along the line of action.
• Normal Base Pitch refers to the base pitch measured in the normal plane.
• Axial Base Pitch refers to the base pitch measured in the axial plane.
• Center Distance is the measurement between the parallel axes of spur and parallel helical gears, or between the crossed axes of helical and worm gears; it also indicates the distance between pitch circle centers.
• Central Plane in a worm gear is perpendicular to the gear axis and contains the common perpendicular of the gear and worm axes.
• Chordal Addendum is the height from the top of the tooth to the chord that subtends the circular-thickness arc.
• Chordal Thickness measures the length of the chord subtended by the circular thickness arc, often gauged using a gear tooth caliper at the pitch circle.
• Circular Pitch defines the arc length along the pitch circle between corresponding points on adjacent teeth.
• Normal Circular Pitch is the circular pitch as it is in the normal plane.
• Circular Thickness refers to the arc length between the two sides of a gear tooth, as measured on the pitch circles, unless specified otherwise.
• Normal Circular Thickness refers to the circular thickness as it is in the normal plane.
• Clearance is the amount by which the dedendum exceeds the addendum of the mating tooth, and it represents the radial distance from a tooth's top to the bottom of the mating tooth space.
• Clearance = Dedendum - Mating Addendum
• Central Diameter represents the smallest diameter on a gear tooth where the mating gear makes contact.
• Contact Ratio is the ratio of the arc of action to the circular pitch, often seen as the average number of teeth in contact. For involute gears, it is derived from the length of action to the base pitch ratio.
• Cycloid is the curve traced by a point on a circle as it rolls along a straight line.

Epicycloid and Hypocycloid

• Epicycloid: The curve when the circle rolls along the outer side of another circle.
• Hypocycloid: The curve when the circle rolls along the inner side of another circle.
• Dedendum is the depth of the tooth space located below the pitch circle, or the radial dimension between the pitch circle and the bottom of tooth space.
• Diametral Pitch is the ratio of the number of teeth to the number of millimeters of pitch diameter.
• Normal Diametral Pitch is calculated in the normal plane and equals the diametral pitch divided by the cosine of the helix angle.
• Effective Face Width is the section of the face width actually in contact with mating teeth, especially when one gear has a greater face width.
• Efficiency is the actual torque ratio of a gear set divided by its gear ratio.
• External Gear has teeth on the outer cylindrical surface.
• Face of Tooth is the flat surface of the tooth that lies between the pitch circle and the top of the tooth.
• Face Width is the length of the teeth measured in the axial plane.
• Fillet Curve is the concave part of the tooth profile connecting to the bottom of the tooth space; its approximate radius is called the fillet radius, which is included in the flank.
• Flank of Tooth is the surface between the pitch circle and the bottom land, and includes the fillet.
• Helical Overlap refers to the effective face width of a helical gear divided by the gear axial pitch and is also called the Face Overlap.
• Helix Angle is the angle between a helical gear tooth and the gear axis.
• Highest Point of Single Tooth Contact is the largest diameter on a spur gear where one tooth is in contact with the mating gear.
• Internal Diameter is the diameter of a circle coinciding with the tops of the teeth on an internal gear.
• Internal Gear has teeth on the inner cylindrical surface.
• Involute is the curve formed by a point on a straight line (generatrix) as it rolls along a convex base curve and is commonly used as the profile for gear teeth.
• Land: The top land is the tooth's top surface, while the bottom land is the gear's surface between the fillets of adjacent teeth.
• Lead is the distance a helical gear or worm would advance along its axis in one revolution if free to move axially.
• Length of Action is the distance on an involute line of action where the contact point moves during tooth profile action.
• Line of Action is the path of contact in involute gears, represented by a straight line through the pitch point and tangent to the base circles.
• Lowest Point of Single Tooth Contact is the smallest diameter on a spur gear where only one tooth of a gear is in contact with its mating gear.
• Module is the ratio of pitch diameter in millimeters to the number of teeth. The English module is the ratio of pitch diameter in inches to the number of teeth.
• Normal Plane is a plane normal to the tooth surface at a contact point and perpendicular to the pitch plane.
• Outside Diameter is the diameter of the outside circle.
• Pitch is the distance between similar, equally spaced tooth surfaces along a curve or line.
• Pitch Circle is a circle with a radius equal to the distance from the gear axis to the pitch point.
• Pitch Diameter is the diameter of the pitch circle.
• Pitch Plane is the plane perpendicular to the axial plane and tangent to the pitch surfaces in a pair of gears.
• Pitch Point is the point of tangency between two pitch circles and lies on the line of centers.
• Pitch Surface is the surface of the rolling cylinder that can replace the gear conceptually.
• Plane Rotation is any plane perpendicular to a gear axis.
• Pressure Angle is the angle between the tooth profile and a radial line at its pitch point. In involute teeth, it is the angle between the line of action and the line tangent to the pitch circle.
• Principal Reference Planes include the pitch plane, axial plane, and transverse plane, intersecting perpendicularly at a point.
• Ratio of Gearing is the ratio of the number of teeth on mating gears.
• Roll Angle is the angle subtended at the center of a base circle, from the origin of an involute to the tangency point of the generatrix on the same involute.
• Root Circle is a circle that coincides with or is tangent to the bottoms of the tooth spaces.
• Root Diameter is the diameter of the root circle.
• Tangent Plane is a plane tangent to the tooth surfaces at a point or line of contact where material is removed near the gear tooth tip.
• Tip Relief is an alteration to the tooth profile where a small amount of material is removed near the tooth tip.
• Tooth Thickness: The width of a tooth measured along the pitch circle.
• Tooth Space (Space Width) is the space between teeth measured along the pitch circle.
• Total Face Width is the actual width dimension of a gear blank.
• Transverse Plane is a plane perpendicular to the axial plane and to the pitch plane.
• Trochoid is the curve formed by a point on the extension of a circle as it rolls along a curve or line or the curve formed by a point on a perpendicular to a straight line as it rolls along the convex side of a base curve.
• True Involute Form Diameter is the smallest diameter on a tooth where the involute exists, typically where the involute tooth profile is tangent to the fillet curve; often referred to as the TIP diameter.
• Undercut is a condition in generated gear teeth in which the fillet curve lies inside a line tangent to the working profile at its lowest point.
• Whole Depth is the total depth of a tooth space, equal to the addendum plus the dedendum, also equaling the working depth plus clearance.
• Working Depth is the engagement depth of two gears, equivalent to the sum of their addendums.