9.1 DEFINITIONS

Spur gears are used to transmit rotary motion between parallel shafts. They are cylindrical, and the teeth are straight and parallel to the axis of rotation.

The pinion is the smaller of two mating gears; the larger is called the gear or the wheel.

The pitch circle, B in Fig. 9.1, is a theoretical circle upon which all calculations are based. The operating pitch circles of a pair of gears in mesh are tangent to each other.

Figure 9.1: Terminology of gear teeth. A, addendum circle; B, pitch circle; C, clearance circle; D, dedendum circle; E, bottom land; F, top land; G, flank; H, face; a = addendum distance; b = dedendum distance; c = clearance distance; p = circular pitch; t = tooth thickness; u = undercut distance.

The circular pitch, p in Fig. 9.1, is the distance, measured on the theoretical pitch circle, from a point on one tooth to a corresponding point on an adjacent tooth. The circular pitch is measured in inches or in millimeters. Note, in Fig. 9.1, that the circular pitch is the sum of the tooth thickness t and the width of space.

The pitch...