GEAR NOMENCLATURE

ADDENDUM (a)  is the height by which a tooth projectsbeyond the pitch circle or pitch line.

BASE DIAMETER (Db) is the diameter of the base cylinderfrom which the involute portion of a tooth profile is generated.

BACKLASH (B) is the amount by which the width of a toothspace 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 circlesor on the line of action. Such measurements should becorrected to corresponding values on transverse pitch circlesor general comparisons.

BORE LENGTH  is the total length through a gear, sprocket, orcoupling bore.

CIRCULAR PITCH (p) is the distance along the pitch circle orpitch 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 otherwise specified.

CLEARANCE-OPERATING (c) is the amount by which thededendum in a given gear exceeds the addendum of its mating gear.

CONTACT RATIO (mc) in general, the number of angularpitches 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.

DIAMETRICAL 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 baseof the gear tooth.

FULL DEPTH TEETH are those in which the working depthequals 2.000 divided by the normal diametrical pitch.

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 wormsare 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 shortaddendum.

KEY WAY 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 DIAMETRICAL PITCH (Pn) is the value of the diametrical pitch as calculated in the normal plane of a helical

gear or worm.

NORMAL PLANE is the plane normal to the tooth surface at a pitch point and perpendicular to the pitch plane. For a helical gear this plane can be normal to one tooth at a point laying in the plane surface. At such point, the normal plane contains the line normal to the tooth surface and this is normal to the

pitch circle.

NORMAL PRESSURE ANGLE (øn) in a normal plane of helical

tooth.

OUTSIDE DIAMETER (Do) is the diameter of the addendum

(outside) circle.

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