Source: Clifford E. Adams, "Plastic Gearing", Marcel Dekker, Inc. New York, 1986. Ref. 13

20.3 Pressure Angles

Pressure angles of 14½^o, 20^o and 25^o are used in plastic gears. The 20^o pressure angle is usually preferred due to its stronger tooth shape and reduced undercutting compared to the 14½^o pressure-angle system. The 25^o pressure angle has the highest load-carring ability, but is more sensitive to center-distance variation and hence runs less quietly. The choice is dependent on the application.

20.4 Diametral Pitch

The determination of the appropriate diametral pitch is a compromise between a number of different design requirements. A smaller pitch number is associated with larger and stronger teeth. For a given pitch diameter, however, this also means a smaller number of teeth with a correspondingly greater likelihood of undercut at very low tooth numbers. Larger teeth are generally associated with more sliding than smaller teeth.
On the other side of the coin, larger pitch numbers, which are associated with smaller teeth, tend to provide greater load sharing due to the compliance of plastic gears. However, a limiting condition would eventually be reached when mechanical interference occurs as a result of too much compliance. Smaller teeth are also more sensitive to tooth errors and may be more highly stressed.
A good procedure is probably to size the pinion first, since it is the more highly loaded member. It should be proportioned to support the required loads, but should not be overdesigned.

20.5 Design Equations For Plastic Spur, Bevel, Helical and Worm Gears

20.5.1 General Considerations—The load-carrying capacity of a plastic spur gear is reached under a limiting load, which is determined either by bending fatigue strength or wear(surface durability). The latter is measured by contact stress. The character of the limiting load depends on the presence or absence of lubrication and its nature, as shown in Table 1.48