LEAD (L) is the axial advance of a helix for one complete turn, as in the threads of cylindrical worms and teeth of helical gears. (Figure 1.11)
LENGTH-OF-ACTION (ZA) is the distance on an involute line of action through which the point of contact moves during the action of the tooth profiles. (Figure 1.8)
LEWIS FORM FACTOR (Y, diametral pitch; yc, circular pitch). Factor in determination of beam strength of gears.
LINE-OF-ACTION is the path of contact in involute gears. It is the straight line passing through the pitch point and tangent to the base circles. (Figure 1.12)
LONG- AND SHORT-ADDENDUM TEETH are those in which the addenda of two engaging gears are unequal.
MEASUREMENT OVER PINS (M). Distance over two pins placed in diametrically opposed tooth spaces (even number of teeth) or nearest to it (odd number of teeth).
NORMAL CIRCULAR PITCH, Pcn, is the circular pitch in the normal plane, and also the length of the arc along the normal helix between helical teeth or threads. (Figure 1.7)
NORMAL CIRCULAR THICKNESS (tn) is the circular thickness in the normal plane. In helical gears. it is an arc of the normal helix, measured at the pitch radius.
NORMAL DIAMETRAL PITCH (Pdn) is the diametral pitch as calculated in the normal plane.
NORMAL PLANE is the plane normal to the tooth. For a helical gear this plane is inclined by the helix angle, y, to the plane of rotation.
OUTSIDE DIAMETER (Do gear, and do = pinion) is the diameter of the addendum (outside) circle (Figure 1.1); the outside radius (Ro gear, ro pinion) is one half the outside diameter.
PITCH CIRCLE is the curve of intersection of a pitch surface of revolution and a plane of rotation. According to theory, it is the imaginary circle that rolls without slip with a pitch circle of a mating gear. (Figure 1.1)
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 (D = gear, d = pinion) 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. (Figure 1.1) The pitch radius (R = gear, r pinion) is one half
the pitch diameter (Figure 11).
PITCH POINT is the point of tangency of two pitch circles (or of a pitch circle and pitch line) and is on the line-of-centers. Also, for involute gears, it is at the intersection of the line-of-action and a straight line connecting the two gear centers. The pitch point of a tooth profile is at its intersection with the pitch circle. (Figure 1.1)
PLANE OF ROTATION is any plane perpendicular to a gear axis.