FIGURE 33.1 Terminology of gear teeth. A, addendum circle; B, pitch circle; C9 clearance circle;
D, dedendum circle; E, bottom land; F, top land; G, flank; H, face; a = addendum distance;
b = dedendum distance; c = clearance distance;/? = circular pitch; t = tooth thickness; u = undercut
distance.
of meshing gears. Line E is the pressure line, and the angle § is the pressure angle.
The resultant force vector between a pair of operating gears acts along this line.
The pressure line is tangent to both base circles C at points E The operating diameters of the pitch circles depend on the center distance used in mounting the gears, but the base circle diameters are constant and depend only on how the tooth forms were generated, because they form the base or the starting point of the involute profile.
TABLE 33.1 Basic Formulas for Spur Gears
Quantity desired Formula Equation number
Diametral pitch Pd = N/d (33.1)
Module m m=d/N (33.2)
Circular pitch p p =∏d/N=∏m (33.3)
Pitch diameter, d or D d d=N/ Pd=mN (33.4)
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http://www.youerw.com/FIGURE 33.2 Layout drawing of a pair of spur gears in mesh. The pinion is the driver and
rotates clockwise about the axis at O. A, addendum circles; B, pitch circles; C, base circles; D,
dedendum circles; E, pressure line; F, tangent points; P, pitch point; a, initial point of contact; b,
final point of contact.
Line aPb is the line of action. Point a is the initial point of contact. This point is located at the intersection of the addendum circle of the gear with the pressure line.
Should point a occur on the other side of point F on the pinion base circle, the pinion
flank would be undercut during generation of the profile.
Point b of Fig. 33.2 is the final point of contact. This point is located at the intersection of the addendum circle of the pinion with the pressure line. For no undercutting of the gear teeth, point b must be located between the pitch point P and原文请+QQ3249'114优.文^论,文'网
point F on the base circle of the gear.
Line aP represents the approach phase of tooth contact; line Pb is the recess
phase. Tooth contact is a sliding contact throughout the line of action except for an
instant at P when contact is pure rolling. The nature of the sliding is quite different
during the approach action and the recess action; and bevel-gear teeth, for example,
are generated to obtain more recess action, thus reducing wear.
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