The following terms are used when describing the dimensions of a gear tooth:
Addendum: the distance from the top of a tooth to the pitch circle. (See Figure 2.2C)
Dedendum: the distance from the pitch circle to the root circle. It equals the addendum + the working clearance. (See Figure 2.2C)
Whole Depth: the distance from the top to the bottom of the gear tooth.
Working Depth: the total depth of a tooth space. It is equal to the addendum + the dedendum (or the working depth +
the variance).
Working Clearance: the distance from the working depth to the root circle. (See Figure 2.2C)
As noted above, spur gears are measured according to their diametral pitch – the number of teeth per inch of pitch diameter.
Example: A gear with a 1" pitch diameter and 12 teeth is a 12-pitch gear. (See Figure 2.2D)
Example: A gear with a 1" pitch diameter and 20 teeth is a 20-pitch gear. (See Figure 2.2E)
Example: A gear with a 1-1/2" pitch diameter and 72 teeth is a 48-pitch gear (72 ÷ 1.5). (See Figure 2.2F)
