4.0 INVOLUTOMETRY

Basic calculations for gear systems are included in this section for ready reference in design. More  advanced calculations are available in the listed references.

4.1.1 GEAR NOMENCLATURE*

ACTIVE PROFILE is that part of the gear tooth profile which actually comes in contact with the profile of its mating tooth along the line of action.

ADDENDUM (a) is the height by which a tooth projects beyond the pitch circle or pitch line; also, the radial distance between the pitch circle and the addendum circle (Figure 1.1); addendum can be defined as either nominal or operating.

AXIAL PITCH (pa) is the circular pitch in the axial plane and in the pitch surface between corresponding sides of adjacent teeth, in helical gears and worms. The term axial pitch is preferred to the term linear pitch. (Figure 1.7) 

AXIAL PLANE of a pair of gears is the plane that contains the two axes. In a single gear, an axial plane may be any plane containing the axis and a given point.

BASE DIAMETER (D_b = gear, and d_b = pinion) is the diameter of the base cylinder from which involute tooth surfaces, either straight or helical, are derived. (Figure 1.1); base radius (R_b = gear, r_b = pinion) is one half of the base diameter.

BASE PITCH (p_b) in an involute gear is the pitch on the base circle or along the line-of-action. Correspcndng sides of involute gear teeth are parallel curves, and the base pitch is the constant and  fundamental distance between them along a common normal in a plane of rotation. (Figure 1.8)

BASIC RACK is a rack that is adopted as the basis for a system of interchangeable gears.

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
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*Portions of this section are repented with permission from the Barber-Colman Co., Rockford, Ml.

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