A helical gear has two major circles:
1) the outside circle
2) the pitch circle.
The outside circle is the distance around the outer edge of the gear’s teeth. (1 and 2) The diameter of the outside circle is called the outside diameter. (See Figure 3.1)
The pitch circle is the imaginary circle found at the point where the teeth of two gears mesh (come in contact, See 2 and 4).The diameter of the pitch circle is called the pitch diameter. (See Figure 3.1A)
BASIC PHYSICAL DIMENSIONS
Data regarding the basic dimensions of gears (as shown below), whether you are looking for information on plain style/no hub gears (See Figure 3.2A) or plain style/with hub gears. (See Figure 3.2B)
CENTER DISTANCE
As you will remember from Chapter 2, the center distance of two mating gears (helical gears and spur gears alike) is the distance between the centers of the gears, or half the sum of the two pitch diameters.
Example: If the center distance is designated as C, and the two pitch diameters are designated as D and d, then: C = D+d ÷ 2. Therefore, if you have two mating helical gears, one (D) with a 4” pitch diameter and one (d) with a 2” pitch diameter, then the center distance (C) will be 3” (4 + 2 ÷ 2 = 3).
