Pitch.—This term is the most difficult to understand. When two gears of equal size mesh together, the pitch line, or the pitch circle, as it is[p. 123] also called, is exactly midway between the centers of the two wheels.
Now the number of teeth in a gear is calculated on the pitch line, and this is called:
Diametral Pitch.—To illustrate: If a gear has 40 teeth, and the pitch diameter of the wheel is 4 inches, there are 10 teeth to each inch of the pitch diameter, and the gear is then 10 diametral pitch.
Circular Pitch.—Now the term "circular pitch" grows out of the necessity of getting the measurement of the distance from the center of one tooth[p. 124] to the center of the next, and it is measured along the pitch line.
Supposing you wanted to know the number of teeth in a gear where the pitch diameter and the diametral pitch are given. You would proceed as follows: Let the diameter of the pitch circle be 10 inches, and the diameter of the diametral pitch be 4 inches. Multiplying these together the product is 40, thus giving the number of teeth.
