This page lists the standard US nomenclature used in the description of mechanical gear construction and function, together with definitions of the terms. The terminology was established by the American Gear Manufacturers Association (AGMA), under accreditation from the American National Standards Institute (ANSI).
Addendum
The addendum is the height by which a tooth of a gear projects beyond (outside for external, or inside for internal) the standard pitch circle or pitch line; also, the radial distance between the pitch diameter and the outside diameter.
Addendum angle Addendum angle in a bevel gear, is the angle between face cone and pitch cone.
Addendum circle
The addendum circle coincides with the tops of the teeth of a gear and is concentric with the standard (reference) pitch circle and radially distant from it by the amount of the addendum. For external gears, the addendum circle lies on the outside cylinder while on internal gears the addendum circle lies on the internal cylinder.
Pressure angle
Apex to back
Apex to back, in a bevel gear or hypoid gear, is the distance in the direction of the axis from the apex of the pitch cone to a locating surface at the back of the blank.
Back angle The back angle of a bevel gear is the angle between an element of the back cone and a plane of rotation, and usually is equal to the pitch angle.
Back cone
The back cone of a bevel or hypoid gear is an imaginary cone tangent to the outer ends of the teeth, with its elements perpendicular to those of the pitch cone. The surface of the gear blank at the outer ends of the teeth is customarily formed to such a back cone.
Back cone distance Back cone distance in a bevel gear is the distance along an element of the back cone from its apex to the pitch cone.
Backlash
In mechanical engineering, backlash is the striking back of connected wheels in a piece of mechanism when pressure is applied. Another source defines it as the maximum distance through which one part of something can be moved without moving a connected part. It is also called lash or play. In the context of gears, backlash is clearance between mating components, or the amount of lost motion due to clearance or slackness when movement is reversed and contact is re-established. In a pair of gears, backlash is the amount of clearance between mated gear teeth.
Backlash is unavoidable for nearly all reversing mechanical couplings, although its effects can be negated. Depending on the application it may or may not be desirable. Reasons for requiring backlash include allowing for lubrication and thermal expansion, and to prevent jamming. Backlash may also result from manufacturing errors and deflection under load.
Base circle
The base circle of an involute gear is the circle from which involute tooth profiles are derived.
Base cylinder
The base cylinder corresponds to the base circle, and is the cylinder from which involute tooth surfaces are developed.
Base diameter
The base diameter of an involute gear is the diameter of the base circle.
Bevel gear
Bull gear The term bull gear is used to refer to the larger of two spur gears that are in engagement in any machine. The smaller gear is usually referred to as a pinion.
Center distance
Center distance (operating) is the shortest distance between non-intersecting axes. It is measured along the mutual perpendicular to the axes, called the line of centers. It applies to spur gears, parallel axis or crossed axis helical gears, and worm gearing.
Central plane
The central plane of a worm gear is perpendicular to the gear axis and contains the common perpendicular of the gear and worm axes. In the usual case with axes at right angles, it contains the worm axis.
Circular Pitch The Circular Pitch defines the width of one tooth and one gap measured on an arc on the pitch circle; in other words, this is the distance on the pitch circle from a point on one tooth to the corresponding point on the adjacent tooth. This is equal to π divided by the Diametral Pitch.
CP = Circular Pitch in inches
DP = Diametral Pitch
CP = π / DP Machinery's Handbook Twenty-Fifth Edition, by Erik Oberg, Franklin D. Jones, Holbrook L. Horton, and Henry H Ryffle, 1996, Industrial Press Inc.
Composite action test
The composite action test (double flank) is a method of inspection in which the work gear is rolled in tight double flank contact with a master gear or a specified gear, in order to determine (radial) composite variations (deviations). The composite action test must be made on a variable center distance composite action test device. and this is composite action test for double flank
Cone distance
Cone distance in a bevel gear is the general term for the distance along an element of the pitch cone from the apex to any given position in the teeth.
Outer cone distance in bevel gears is the distance from the apex of the pitch cone to the outer ends of the teeth. When not otherwise specified, the short term cone distance is understood to be outer cone distance.
Mean cone distance in bevel gears is the distance from the apex of the pitch cone to the middle of the face width.
Inner cone distance in bevel gears is the distance from the apex of the pitch cone to the inner ends of the teeth.
Conjugate gears Conjugate gears transmit uniform rotary motion from one shaft to another by means of gear teeth. The normals to the profiles of these teeth, at all points of contact, must pass through a fixed point in the common centerline of the two shafts. Usually conjugate gear tooth is made to suit the profile of other gear which is not made based on standard practice.
Crossed helical gear A crossed helical gear is a gear that operate on non-intersecting, non-parallel axes.
The term crossed helical gears has superseded the term spiral gears. There is theoretically point contact between the teeth at any instant. They have teeth of the same or different helix angles, of the same or opposite hand. A combination of spur and helical or other types can operate on crossed axes.
Crossing point The crossing point is the point of intersection of bevel gear axes; also the apparent point of intersection of the axes in hypoid gears, crossed helical gears, worm gears, and offset face gears, when projected to a plane parallel to both axes.
Crown circle The crown circle in a bevel or hypoid gear is the circle of intersection of the back cone and face cone.
Crowned teeth
Crowned teeth have surfaces modified in the lengthwise direction to produce localized contact or to prevent contact at their ends.
Diametral Pitch The Diametral Pitch (DP) is the number of teeth per inch of diameter of the pitch circle. The units of DP are inverse inches (1/in).
DP = Diametral Pitch
PD = Pitch Circle Diameter in inches
CP = Circular Pitch in inches
n = Number of Teeth
DP = n / PD
The Diametral Pitch (DP) is equal to π divided by the Circular Pitch (CP).
DP = 3.1416 / CP
Dedendum angle Dedendum angle in a bevel gear, is the angle between elements of the root cone and pitch cone.
Equivalent pitch radius
Equivalent pitch radius is the radius of the pitch circle in a cross section of gear teeth in any plane other than a plane of rotation. It is properly the radius of curvature of the pitch surface in the given cross section. Examples of such sections are the transverse section of bevel gear teeth and the normal section of helical teeth.
Face (tip) angle Face (tip) angle in a bevel or hypoid gear, is the angle between an element of the face cone and its axis.
Face cone The face cone, also known as the tip cone, is the imaginary surface that coincides with the tops of the teeth of a bevel or hypoid gear.
Face gear
A face gear set typically consists of a disk-shaped gear, grooved on at least one face, in combination with a spur, helical, or conical pinion. A face gear has a planar pitch surface and a planar root surface, both of which are perpendicular to the axis of rotation. It can also be referred to as a face wheel, crown gear, crown wheel, contrate gear or contrate wheel.
Face width
The face width of a gear is the length of teeth in an axial plane. For double helical, it does not include the gap.
Total face width is the actual dimension of a gear blank including the portion that exceeds the effective face width, or as in double helical gears where the total face width includes any distance or gap separating right hand and left hand helices.
For a cylindrical gear, effective face width is the portion that contacts the mating teeth. One member of a pair of gears may engage only a portion of its mate.
For a bevel gear, different definitions for effective face width are applicable.
Form diameter
Form diameter is the diameter of a circle at which the trochoid (fillet curve) produced by the tooling intersects, or joins, the involute or specified profile. Although these terms are not preferred, it is also known as the true involute form diameter (TIF), start of involute diameter (SOI), or when undercut exists, as the undercut diameter. This diameter cannot be less than the base circle diameter.
Front angle The front angle, in a bevel gear, denotes the angle between an element of the front cone and a plane of rotation, and usually equals the pitch angle.
Front cone The front cone of a hypoid or bevel gear is an imaginary cone tangent to the inner ends of the teeth, with its elements perpendicular to those of the pitch cone. The surface of the gear blank at the inner ends of the teeth is customarily formed to such a front cone, but sometimes may be a plane on a pinion or a cylinder in a nearly flat gear.
Gear center A gear center is the center of the pitch circle.
Gear range The gear range is difference between the highest and lowest gear ratios and may be expressed as a percentage (e.g., 500%) or as a ratio (e.g., 5:1).
Heel
The heel of a tooth on a bevel gear or pinion is the portion of the tooth surface near its outer end.
The toe of a tooth on a bevel gear or pinion is the portion of the tooth surface near its inner end.
Helical rack A helical rack has a planar pitch surface and teeth that are oblique to the direction of motion.
Helix angle Helix angle is the angle between the helical tooth face and an equivalent spur tooth face. For the same lead, the helix angle is greater for larger gear diameters. It is understood to be measured at the standard pitch diameter unless otherwise specified.
Herringbone gear
Hobbing Hobbing is a machining process for making gears, splines, and sprockets using a cylindrical tool with helical cutting teeth known as a hob.
Index deviation The displacement of any tooth flank from its theoretical position, relative to a datum tooth flank.
Distinction is made as to the direction and algebraic sign of this reading. A condition wherein the actual tooth flank position was nearer to the datum tooth flank, in the specified measuring path direction (clockwise or counterclockwise), than the theoretical position would be considered a minus (-) deviation. A condition wherein the actual tooth flank position was farther from the datum tooth flank, in the specified measuring path direction, than the theoretical position would be considered a plus (+) deviation.
The direction of tolerancing for index deviation along the arc of the tolerance diameter circle within the transverse plane.
Inside cylinder
The inside cylinder is the surface that coincides with the tops of the teeth of an internal cylindrical gear.
Inside diameter
Inside diameter is the diameter of the addendum circle of an internal gear, this is also known as minor diameter.
Involute gear
Involute polar angle
Expressed as θ, the involute polar angle is the angle between a radius vector to a point, P, on an involute curve and a radial line to the intersection, A, of the curve with the base circle.
Involute roll angle