Gear Terminology

American Gear Manufacturers Association
American National Standards Institute
Addendum (ADD)
the radial or perpendicular distance between the pitch circle and the top of the teeth
Accuracy of the alignment of shafts on which gears are mounted is a critical factor in their life and performance. Shafts should be set parallel on centers and co-plainer in spur and helical gear sets, and perpendicular in most bevel and worm sets. Misalignment of shafts is one of the most common causes of premature gear failure or other performance issues such as noise.
Axial Pitch (AP)
Axial pitch is linear pitch in an axial plane and in a pitch surface. In helical gears and worms, axial pitch has the same value at all diameters. In gearing of other types, axial pitch may be confined to the pitch surface and may be a circular measurement. The term axial pitch is preferred to the term linear pitch. The axial pitch of a helical worm and the circular pitch of its worm gear are the same.
is the amount by which the width of a tooth space exceeds the thickness of the engaging tooth on the pitch circles.
Backlash Variation
is the difference between the maximum and minimum backlash occurring in a whole revolution of the larger of a pair of mating gears.
Base Circle
The base circle is the circle from which involute tooth profiles are derived.
Base Diameter (BD)
Base diameter is the diameter of the base circle of an involute gear.
Base Helix Angle (BHA)
Base helix angle is the helix angle on the base cylinder of involute helical teeth or threads.
Base Pitch (BP), normal
Normal base pitch in an involute helical gear is the base pitch in the normal plane. It is the normal distance between parallel helical involute surfaces on the plane of action in the normal plane, or is the length of arc on the normal base helix. It is a constant distance in any helical involute gear.
Bevel Gears
are gears of conical form designed to operate on intersecting axes. These gears allow transmission of power "around corners" so to speak as their shafts intersect. Most commonly the shafts will intersect at a 90° angle. Straight bevel gears (as opposed to spiral bevel gears) have teeth that are straight, radially from the gears center point.
the diameter of the hole in the center of a gear, bushing, bearing, etc.
Brinell Hardness Number (BHN)
a measure of the hardness of a material such as steel, measured on the Brinell scale
Bull Gear
When considering a set of two gears one will usually be smaller than the other. The smaller gear is referred to as the pinion. The larger gear is sometimes called the “Bull” gear.
a heat treating process, which allows the surface of a low carbon steel gear to be hardened to high hardness but to only a shallow depth. This process is usually applied "all over" or to the entire surface of the part. This type of treatment makes post heat-treat machining processes difficult. If desired, specific areas of parts can be masked prior to carburizing to prevent the introduction of carbon into the steel's surface and thereby preventing hardening of the masked areas when quenched.
Center Distance (CD)
is the shortest distance between non-intersecting axes of mating gears.
Chordal Tooth Thickness
is the thickness of the tooth on the pitch circle measured as a chord.
Circular Pitch (CP)
is the distance along the pitch line between corresponding profiles of adjacent teeth.
Circular Thickness
Circular thickness is the length of arc between the two sides of a gear tooth, on the specified datum circle.
is the amount by which the dedendum in a given gear exceeds the addendum of its mating gear. In other words, it is the distance between the tip of one tooth and the bottom or root of its mate.
When two diameters share a common center they are said to be concentric. Deviations from theoretical or perfect concentricity are called eccentricity.
Crowned Teeth
Crowned teeth have surfaces modified in the lengthwise direction to produce localized contact or to prevent contact at their ends. Crowning can be applied to all types of teeth.
D & T
drill and tap
Dedendum (DED)
is the radial or perpendicular distance between the pitch circle and the bottom.
Diametral Pitch (DP)
is the ratio of the number of teeth to the number of inches in the pitch diameter. There is a fixed relation between diametral pitch (DP) and circular pitch (CP), namely CP = pi/DP.
Distance Through Bore
describes the overall width of a gear measured axially at the center hole.
Double Helical Gears
Double helical gears have teeth of both right hand and left hand on each gear. The teeth are separated by a gap between the helices. Where there is no gap, they are known as herringbone.
When two diameters share a common center they are said to be concentric. Deviations from theoretical or perfect concentricity are called eccentricity.
External Gear
An external gear is one with the teeth formed on the outer surface of a cylinder or cone.
Face Width (FW)
Face width is the length of teeth in an axial plane. For double helical, it does not include the gap.
Fillet Curve (root fillet)
The fillet curve (root fillet) is the concave portion of the tooth profile where it joins the bottom of the tooth space.
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.
machine elements that transmit motion by means of successively engaging teeth
Gear (wheel)
a gear (wheel) is a machine part with gear teeth. Of two gears that run together, the one with the larger number of teeth is called the gear.
Gear Ratio
is the ratio of the larger to smaller number of teeth in mating gears.
Helical Gears
Helical gear sets run on non-intersecting, parallel shafts. Instead of being parallel to the shaft axis (as with spur gears), the teeth of a helical gear spiral around the gears shaft axis. Placing the gear teeth on this kind of angle increases the amount of tooth surface in contact during operation. This allows for transmission of a higher load with a given size of gear set relative to spur gears. Use of the helix angle also creates end-thrust forces, which attempt to push the gears away from each other axially. These forces must be allowed for in any gear set design.
Helix Angle (HA)
is the angle between a tangent to the helix and an element of the cylinder. Unless otherwise specified, the pitch helix is referred to.
Herringbone Gears
Herringbone gear sets run on non-intersecting, parallel shafts. The tooth of a true herringbone gear is one continuous set of two opposing helices. The two helix angles come together in the center of the gear face to form a `V'. Gains in allowable load through increased tooth contact relative to spur gear sets of equal size are made. This is similar to helical gears, but with herringbone gears the end-thrust forces cancel themselves out. Cutting gears of this type is a difficult process which is made somewhat easier by machining a groove in the face at the point of the apex of the `V', creating a break in the middle of the herringbone gear tooth. This type of gear is technically a “Double- Helical” gear but is sometimes referred to as a herringbone if the center groove is narrow.
a machining process for making gears teeth and splines on a hobbing machine, which is a special type of milling machine. The teeth or splines are progressively cut into the workpiece by a series of cuts made by a cutting tool called a hob.
a shoulder or flange protruding from the side of a gear. Hubs provide width to a part, which is used, for mounting the part on a shaft.
a component in a mechanical power transmission system, which simply turns on its shaft without performing any ratio reduction or power transmission, functions.
Induction Hardening
Induction hardening is a form of heat treatment in which a metal part is heated by induction heating and then quenched. The quenched metal undergoes a transformation, increasing the hardness and brittleness of the part. Induction hardening is used to selectively harden areas of a part or assembly without affecting the properties of the part as a whole. Induction hardening allows for hardening teeth only, leaving other areas of a part available for further machining.
Inside Cylinder
The inside cylinder is the surface that coincides with the tops of the teeth of an internal cylindrical gear.
Inside Diameter (ID)
Inside diameter is the diameter of the addendum circle of an internal gear.
Internal Gear
An internal gear is one with the teeth formed on the inner surface of a cylinder or cone. For bevel gears, an internal gear is one with the pitch angle exceeding 90º. An internal gear can be meshed only with an external pinion.
Involute Profile
with an involute gear, the profiles of the teeth are involutes of a circle. The involute of a circle is the spiraling curve traced by the end of an imaginary taut string unwinding itself from that stationary circle called the base circle. The involute profile can be generated using a hobbing machine with a rack form.
Involute Roll Angle
Involute roll angle is the angle whose arc on the base circle of radius unity equals the tangent of the pressure angle at a selected point on the involute.
Keyless Shaft/Hub Connectors
devices designed to attach power transmission components such as gears to shafts using expansion and compression forces between the bore and shaft.
Keyway (KW)
a slot cut into both the bore of a gear and the shaft onto which the gear will be mounted. A square, steel `key' (straight or tapered) is inserted between the shaft and the gear, allowing power to be transmitted between the two components.
Lead (L)
is the axial advance of a helix for one complete turn, as in the threads of cylinder worms and teeth of helical gears.
Left or Right Flank
It is convenient to choose one face of the gear as the reference face and to mark it with the letter “I”. The other non--reference face might be termed face “II”. For an observer looking at the reference face, so that the tooth is seen with its tip uppermost, the right flank is on the right and the left flank is on the left. Right and left flanks are denoted by the letters “R” and “L”, respectively.
Length Through Bore (LTB)
measurement from one edge of the bore to the other.
Lightener Holes
are holes cut into the web of larger gears to reduce their weight, without reducing load carrying capacity. Lightener holes reduce shaft loads and shipping costs.
Line of Centers
The line of centers connects the centers of the pitch circles of two engaging gears; it is also the common perpendicular of the axes in crossed helical gears and worm gears. When one of the gears is a rack, the line of centers is perpendicular to its pitch line.
Measurement Over Pins
Measurement over pins is the measurement of the distance taken over a pin positioned in a tooth space and a reference surface. The reference surface may be the reference axis of the gear, a datum surface or either one or two pins positioned in the tooth space or spaces opposite the first. This measurement is used to determine tooth thickness.
Module (MOD)
the ratio of the pitch diameter in millimeters to the number of teeth; a measurement of gear pitch expressed in metric
Mounting Distance (MD)
is the distance, for assembling bevel gears or hypoid gears, from the crossing point of the axes to a location surface of a gear, which may be at either back or front.
Operating Pitch Diameters
The pitch diameters determined from the numbers of teeth and the center distance at which gears operate.
Outside (Tip or Addendum) Cylinder (or Circle)
The outside (tip or addendum) cylinder is the surface that coincides with the tops of the teeth of an external cylindrical gear.
Outside Diameter (OD)
The diameter of the circle which contains the tops of the teeth of an external gear.
Parallel Axis Gears
Gears which operate on parallel axes. External helical gears on parallel axes have helices of opposite hands. If one of the members is an internal gear, the helices are of the same hand.
When considering two gears that run together, the one with the fewer teeth is called the pinion.
Pitch is the distance between a point on one tooth and the corresponding point on an adjacent tooth. It is a dimension measured along a line or curve in the transverse, normal, or axial directions. In general terms, pitch is a measure of the size of the teeth of a gear; the larger the pitch, the larger the teeth. Several systems are used to designate pitch in gears, such as Diametral Pitch, Circular Pitch and Module.
Pitch Angle
The angle between an element of a pitch cone and its axis.
Pitch Circle (operating)
A pitch circle (operating) is the curve of intersection of a pitch surface of revolution and a plane of rotation. It is the imaginary circle that rolls without slipping with a pitch circle of a mating gear.
Pitch Cylinder
A pitch cylinder is the imaginary cylinder in a spur or helical gear that rolls without slipping on a pitch plane or pitch cylinder of another gear.
Pitch Diameter (PD)
is the diameter of the pitch circle. In parallel shaft gears, the pitch diameters can be determined directly from the center distance and the number of teeth by proportionality. Operating pitch diameter is the pitch diameter at which the gears operate.
Pitch Line
The pitch line corresponds, in the cross section of a rack, to the pitch circle (operating) in the cross section of a gear.
Pitch Plane
The pitch plane of a pair of gears is the plane perpendicular to the axial plane and tangent to the pitch surfaces. A pitch plane in an individual gear may be any plane tangent to its pitch surface. The pitch plane of a rack or in a crown gear is the imaginary planar surface that rolls without slipping with a pitch cylinder or pitch cone of another gear. The pitch plane of a rack or crown gear is also the pitch surface.
Pitch Point
The pitch point is the point of tangency of two pitch circles (or of a pitch circle and pitch line) and is on the line of centers.
Plain Bore
This term is used to indicate that the center hole in a gear has no keyway, set screw(s), bushing or any other type of machined preparation for shaft mounting. Plain bores may be machined to a specified tolerance or a nominal tolerance. Plain bores are also referred to as mandrel bores or stock bores.
Pressure Angle (PA)
Pressure angle is in general the angle at a pitch point between the line of pressure which is normal to the tooth surface, and the plane tangent to the pitch surface. The pressure angle gives the direction of the normal to the tooth profile. The pressure angle is equal to the profile angle at the standard pitch circle and can be termed the “standard” pressure angle at that point.
Profile Angle
Profile angle is in general the angle at a specified pitch point between a line tangent to a tooth surface and the line normal to the pitch surface (which is a radial line of a pitch circle). This definition is applicable to every type of gear for which a pitch surface can be defined. The profile angle gives the direction of the tangent to a tooth profile. In spur gears and straight bevel gears, tooth profiles are considered only in a transverse plane, and the general terms profile angle and pressure angle are customarily used rather than transverse profile angle and transverse pressure angle. In helical teeth, the profiles may be considered in different planes, and in specifications it is essential to use terms that indicate the direction of the plane in which the profile angle or the pressure angle lies, such as transverse profile angle, normal pressure angle, axial profile angle.
Hardness of steel measured on the Rockwell ‘C’ scale.
A straight length of square or rectangular steel bar with teeth on one side, across which a pinion or gear is driven. Technically a gear rack is a gear with an infinite pitch diameter. Racks can have either spur or helical teeth.
Root Cylinder (or Circle)
The root cylinder is the imaginary surface that coincides with the bottoms of the tooth spaces in a cylindrical gear.
Root Diameter (RD)
The diameter of the bottoms of the tooth spaces of a gear.
Span Measurement
Span measurement is the measurement of the distance across several teeth in a normal plane. As long as the measuring device has parallel measuring surfaces that contact on an unmodified portion of the involute, the measurement will be along a line tangent to the base cylinder. It is used to determine tooth thickness.
Speed Increaser
A speed increaser is a gear or series of gears combined in such a manner to increase the speed of a mechanical system. The speed of a system increases in direct proportion to the reduction of the torque. If you increase the rotation without reducing the torque of the system, you increase the speed.
Speed Reducer
A speed reducer is a gear or series of gears combined in such a manner to increase the torque of a mechanical system. The torque of a system increases in direct proportion to the reduction of the rotations per minute. If you decrease the rotation without slowing down the system, you increase the force generated.
Spiral Bevel Gears
In similar fashion to helical gears, higher potential load transmission relative to straight bevel gears is achieved with spiral bevel gears by cutting the teeth in a curved shape.
Gears can be manufactured by several methods, which allow them to be split in half. Splitting allows for installation of the parts without disassembling the machinery they are going into.
Spur Gears - Gears that are cylindrical in form, with teeth that are straight and parallel to the axis.
Tip Relief
Tip relief is a modification of a tooth profile whereby a small amount of material is removed near the tip of the gear tooth.
Total Face Width
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.
A transverse plane is perpendicular to the axial plane and to the pitch plane. In gears with parallel axes, the transverse plane and plane of rotation coincide.
Undercut is a condition in generated gear teeth when any part of the fillet curve lies inside of a line drawn tangent to the working profile at its point of juncture with the fillet. Undercut may be deliberately introduced to facilitate finishing operations. With undercut the fillet curve intersects the working profile. Without undercut the fillet curve and the working profile have a common tangent.
Worm & Worm Gear
Worm gear sets run on non-intersecting, perpendicular shaft axes and provide high ratios of reduction. The worm is a threaded cylindrical shaft, which drives the worm gear (also referred to as the worm wheel). Worm gears look somewhat similar to helical gears except that they have a curved throat recessed in their face to allow the worm access to the flanks of the gear teeth. Worm gears are almost always made from bronze or cast iron with the worms made from steel. This combination of materials allows for the sliding action between the gears. Worms may have a single start (continuous thread) or multiple starts.






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