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ADDENDUM (a) is the height by which a tooth projects beyond the pitch circle or pitch line.
BASE DIAMETER (Db) is the diameter of the base cylinder from which the involute portion of a tooth profile is generated.
BACKLASH (B) is the amount by which the width of a tooth space exceeds the thickness of the engaging tooth on the pitch circles. As actually indicated by measuring devices, backlash may be determined variously in the transverse, normal, or axial-planes, and either in the direction of the pitch circles or on the line of action. Such measurements should be corrected to corresponding values on transverse pitch circles for general comparisons.
BORE LENGTH is the total length through a gear, sprocket, or coupling bore.
CIRCULAR PITCH (p) is the distance along the pitch circle or pitch line between corresponding profiles of adjacent teeth.
CIRCULAR THICKNESS (t) is the length of arc between the two sides of a gear tooth on the pitch circle, unless other-wise specified.
CLEARANCE-OPERATING (c) is the amount by which the dedendum in a given gear exceeds the addendum of its mating gear.
CONTACT RATIO (mc) in general, the number of angular pitches through which a tooth surface rotates from the beginning to the end of contact.
DEDENDUM (b) is the depth of a tooth space below the pitch line. It is normally greater than the addendum of the mating gear to provide clearance.
DIAMETRAL PITCH (P) is the ratio of the number of teeth to the pitch diameter.
FACE WIDTH (F) is the length of the teeth in an axial plane.
FILLET RADIUS (rf) is the radius of the fillet curve at the base of the gear tooth.
FULL DEPTH TEETH are those in which the working depth equals 2.000 divided by the normal diametral pitch.
GEAR is a machine part with gear teeth. When two gears run together, the one with the larger number of teeth is called the gear.
HUB DIAMETER is outside diameter of a gear, sprocket or coupling hub.
HUB PROJECTION is the distance the hub extends beyond the gear face.
INVOLUTE TEETH of spur 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.
LONG- AND SHORT-ADDENDUM TEETH are those of engaging gears (on a standard designed center distance) one of which has a long addendum and the other has a short addendum.
KEYWAY is the machined groove running the length of the bore. A similar groove is machined in the shaft and a key fits into this opening.
NORMAL DIAMETRAL PITCH (Pn) is the value of the diametral pitch as calculated in the normal plane of a helical gear or worm.

To be continued….