Machinery's Handbook, 31st Edition
Gears and Gearing 2205 Axial thickness is the distance parallel to the axis between two pitch line elements of the same tooth. Backlash is the shortest distance between the non-driving surfaces of adjacent teeth when the working flanks are in contact. Base circle is the circle from which the involute tooth curve is generated or developed. Base helix angle is the angle at the base cylinder of an involute gear that the tooth makes with the gear axis. Base pitch is the circular pitch taken on the circumference of the base circles, or the dis tance along the line of action between two successive and corresponding involute tooth profiles. The normal base pitch is the base pitch in the normal plane and the axial base pitch is the base pitch in the axial plane. Base tooth thickness is the distance on the base circle in the plane of rotation between invo lutes of the same pitch. Bottom land is the surface of the gear between the flanks of adjacent teeth. Center distance is the shortest distance between the non-intersecting axes of mating gears, or between the parallel axes of spur gears and parallel helical gears, or the crossed axes of crossed helical gears or worm gears. Central plane is the plane perpendicular to the gear axis in a worm gear, which contains the common perpendicular of the gear and the worm axes. In the usual arrangement with the axes at right angles, it contains the worm axis. Chordal addendum is the radial distance from the circular thickness chord to the top of the tooth, or the height from the top of the tooth to the chord subtending the circular thickness arc. Chordal thickness is the length of the chord subtended by the circular thickness arc. The dimension obtained when a gear tooth caliper is used to measure the tooth thickness at the pitch circle. Circular pitch is the distance on the circumference of the pitch circle, in the plane of rota tion, between corresponding points of adjacent teeth. The length of the arc of the pitch circle between the centers or other corresponding points of adjacent teeth. Circular thickness is the thickness of the tooth on the pitch circle in the plane of rotation, or the length of arc between the two sides of a gear tooth measured on the pitch circle. Clearance is the radial distance between the top of a tooth and the bottom of a mating tooth space, or the amount by which the dedendum in a given gear exceeds the addendum of its mating gear. Contact diameter is the smallest diameter on a gear tooth with which the mating gear makes contact. Contact ratio is the ratio of the arc of action in the plane of rotation to the circular pitch, and is sometimes thought of as the average number of teeth in contact. This ratio is obtained most directly as the ratio of the length of action to the base pitch. Contact ratio - face is the ratio of the face advance to the circular pitch in helical gears. Contact ratio - total is the ratio of the sum of the arc of action and the face advance to the circular pitch. Contact stress is the maximum compressive stress within the contact area between mating gear tooth profiles. Also called the Hertz stress. Cycloid is the curve formed by the path of a point on a circle as it rolls along a straight line. When such a circle rolls along the outside of another circle the curve is called an epicy cloid , and when it rolls along the inside of another circle it is called a hypocycloid . These curves are used in defining the former American Standard composite Tooth Form. 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 di- ameter in the plane of rotation, or the number of gear teeth to each inch of pitch diameter. Normal diametral pitch is the diametral pitch as calculated in the normal plane, or the diametral pitch divided by the cosine of the helix angle.
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