This month we are posting a list of gear terms that I hope will help.

Active Profile  -  is the part of the gear tooth profile which actually comes in contact with the profile of its Mating tooth along the line of action.

Addendum  -  is the radial or perpendicular distance between the pitch circle and the top of the teeth.

Arc of Action  -  is the arc of the pitch circle through which a tooth travels from the time it first makes contact with a mating tooth until contact with the mating tooth ceases.

Arc of Approach  -  is the arc of the pitch circle through which a tooth travels from the time it first makes contact with a mating tooth until it is in contact at the pitch point.

Arc of Recession  -  is the arc of the pitch circle through which a tooth travels from the time it is in contact with a mating tooth at the pitch point until contact ceases.

Axial Pitch of Worm  -  is the distance parallel to the axis of the worm between corresponding sides of adjacent teeth. The term axial pitch is preferred to the term linear pitch.

Axial Plane  -  is the plane that contains two axes. In a single gear, an axial plane may be any plane (of a pair of gears) containing the axis and a point of its diameter.

Axial Pressure Angle of Worm  -  is the angle in a axial plane between the side of the thread and a line perpendicular to the axis.

Axial Thickness Of Worms  -  is the distance parallel to the axis between two pitch line elements of the same tooth.

Backlash  -  is the amount by which the width of a tooth space exceeds the thickness of the engaging tooth on the pitch circles.

Base Diameter  -  is the diameter of the circle from which the involute is generated.

Base Helix Angle  -  is the helix angle on the base cylinder.

Base Pitch  -  is the circular pitch taken in the circumference of the base circle and is also the (normal to involute) distance along the line of action between two successive and corresponding involute profiles.

Bore Diameter  -  is the diameter of the hole in the gear.

Bottom Land  -  is the surface of the gear between the flanks of adjacent teeth.

Center Distance  -  is the shortest distance between non-intersecting axes of mating gears.

Chordal Addendum (for tooth calipers)  -  is the radial distance from the circular thickness chord to the top of the tooth.

Chordal Addendum (normal)  -  is the perpendicular distance from the normal thickness chord to the top of the tooth.

Chordal Thickness  -  is the length of the chord sustended by the circular thickness arc.

Circular Thickness  -  is the thickness of the tooth on the pitch circle.

Clearance  -  is the amount by which the dedendum in a given gear exceeds the addendum of its mating gear.

Contact Ratio  -  is the ratio of the arc of action to the circular pitch.

Contact Ratio Face  -  is the ratio of the face advance to the circular pitch.

Contact Ratio Total  -  is the ratio of the sum of the arc of action and the face advance to the circular pitch.

Dedendum  -  is the radial or perpendicular distance between the pitch circle and the bottom of the tooth space.

Diametral Pitch  -  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 = n/DP.

Edge Round  -  is the radius on the circumferential edge of the gear tooth-to break sharp corner.

Effective Face Width  -  is the width of face which comes actually into contact with a mating gear.

Equivalent Diameter  -  is the diameter of curvature of the pitch surface in the plane normal to the pitch line element.

Face Advance  -  is the distance on the pitch circle that a gear tooth travels from the time pitch point contact is made at one end of the tooth until pitch point contact is made at the other end.

Face Width  -  is the width of the pitch surface. For herringbone gears, this includes the width of the surfaces containing both of the helices and the groove width.

Gear Ratio  -  is the ratio of the numbers of teeth in mating members.

Groove Depth  -  is the depth of the clearance groove between helices of herringbone gears.

Groove Width  -  is the width of the clearance groove between helices of herringbone gears.

Helix Angle  -  is the angle between a tangent to the helix and an element of the cylinder.

Hub Diameter  -  is the diameter of the central part of the gear body surrounding the bore and extending beyond the web, spokes, or body.

Interference  -  is the contact between mating teeth at some point other than along the line of action.

Internal Diameter  -  is the diameter of that circle which contains the tope of the teeth on a internal gear.

Involute Teeth  -  (of gears, helical gears, and worms) are those in which the active portion of the profile in the transverse plane is the involute of a circle.

Lead  -  is the axial advance of a helix for one complete turn, as is the threads of cylindrical worms and teeth of helical gears.

Lead Angle  -  is the angle between a tangent to the pitch helix and a plane of rotation.

Length of Action  -  is the distance on a involute line of action through which the point of contact moves during the action of the tooth profiles.

Line of Action  -  is that portion of the common tangent to the base circles along which contact between the mating involute teeth occurs.

Line of Centers  -  is the straight line through the center of tangent pitch circles.

Module  -  is the ratio of the pitch diameter in millimeters to the number of teeth.

Normal Chordal Thickness  -  is the length of the normal thickness chord between pitch line elements of a tooth.

Normal Circular Thickness  -  is the circular thickness in the normal plane. In helical gears it is an arc of the normal helix.

Normal Circular Pitch  -  is the shortest distance on the pitch surface between corresponding pitch line elements of adjacent teeth.

Normal Diametral Pitch  -  is the Diametral pitch corresponding to the normal circular pitch and calculated in the normal plane.

Normal Pressure Angle  -  is the pressure angle in a plane normal to the pitch line element.

Normal Tooth Profile  -  is the outline formed by the intersection of a tooth surface and a plane perpendicular to the pitch line element.

Normal Tooth Profile  -  is the number of profile contacts in a pair of mating gears at any given instant.

Number of Threads in Worm  -  in the number of starts or separate threads on the worm.

Outside Diameter  -  is the diameter of the circle which contains the tops of the teeth of external gears.

Outside Radius  -  is the radius of the circle which contains the tops of the teeth of external gears.

Pitch, Circular  -  is the distance on the circumference of the pitch circle between corresponding points of adjacent teeth.

Pitch Circle  -  is the circle through which the pitch point having its center of the axis of the gear.

Pitch Cylinder  -  is the imaginary cylinder in a gear that rolls without slipping on a pitch cylinder or pitch plane of another gear.

Pitch Diameter  -  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 Helix  -  is the helix formed by the intersection of the surface of a helical tooth or thread with the pitch cylinder.

Pitch Line Element  -  is a line curved or straight, formed by the intersection of the pitch surface and the tooth surface.

Pitch Plane  -  is the plane which is tangent to both the pitch cylinder of the worm and the pitch cylinder of the gear.

Pitch Radius of Gear or Pinion  -  is the radius of the pitch circle.

Pitch Point  -  is the intersection between the axes of the line of centers and the line of action.

Pitch Surface  -  is the surface of the pitch cylinder, which rolls with the surface of the mating member.

Plane of Rotation  -  is any plane perpendicular to a gear axis.

Pressure Angle  -  is the angle between a tooth profile and the radial line at its pitch point. In involute teeth, pressure angle is often described also as the angle between the line of action and the line tangent to the pitch circle. Standard pressure angles are established in connection with standard gear-tooth proportions.

Root Circle  -  is the circle containing the bottom of the tooth spaces.

Root Diameter  -  is the diameter of the circle which contains the roots of the teeth.

Space Bottom  -  is a line joining two fillets of adjacent tooth profiles in the same plane.

Throat Diameter (of worm gear)  -  is the outside diameter of worm gear in central plane.

Teeth, left hand  -  are the teeth which twist to the left or in a counterclockwise direction as they recede from the observer.

Teeth, right hand  -  are the teeth which twist to the right or in a clockwise direction as they recede from the observer.

Tooth Bearing  -  is that portion of the tooth surface which actually comes in contact.

Tooth Chamfer  -  is the bevel between the end of a tooth and the tooth surface, to break the sharp edge.

Tooth Face  -  is the surface between the pitch line element and the top of the tooth.

Tooth Fillet  -  is the curved surface of the tooth flank joining it to the bottom land.

Tooth Flank  -  is the surface between the pitch line element and the bottom land.

Tooth Surface  -  is the total area including the tooth face and the tooth flank.

Tooth Thickness on Base Circle  -  is the distance on the base circle between involutes of the same tooth.

Tooth Top  -  is a line joining the outer ends of two adjacent tooth profiles in the same place. In internal gearing it is the inner ends of the teeth.

Top Land  -  is the surface of the top of the tooth.

Transverse Circular Thickness  -  is the circular thickness in the transverse plane.

Transverse Pitch  -  is the distance between corresponding pitch line elements of adjacent teeth in the plane of rotation.

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.

Transverse Pressure Angle  -  is the pressure angle in the transverse plane.

Undercut  -  is a condition in generated gear teeth when any part of the fillet curve lies inside of a line drawn tangent to the true involute form at its lowest point. Undercut may be deliberately introduced to facilitate finishing operations.

Whole Depth  -  is the radial distance between the outside circle and the root circle.

Working Depth  -  is the greatest depth to which a tooth of one gear extends into the tooth space of the mating gear.