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The helix angle is the angle between the axis (bore) of a helical gear and an (imaginary) line tangent to the tooth.
The helix angle will be between 0º and 90º. (See Figure 3.3)

The shaft angle of a pair of crossed helical gears is the angle that lies between the ends of the shafts that rotate in opposite directions. (See Figure 3.3A)

Note: There are two different angles between intersecting shafts (one being 180º minus the other). However, only the angle that meets the above definition is designated as the shaft angle.
Note that in the two diagrams to the right that although the shaft axes lie in the same direction, the shaft angles are not the same because the shaft rotations are different. (See Figure 3.3A, 3.3B)

IMPORTANT: Either the correct shaft angle – or one of the angles between the shafts and the direction of rotation of each shaft – must be provided before helical gears can be designed to fulfill specific application requirements

The pitch diameter of a helical pinion (which, you will remember from our introduction to Gearology, is the smaller of two mating gears) and mating gear for a given ratio and center distance may be determined using the following formulas:
Pinion pitch diameter (d) = 2C ÷ ratio + 1
Gear pitch diameter (D) = d x ratio

Note: These formulas are not applicable to crossed axis helical gears with unequal helix angles.