General Design Principles for DuPont Engineering Polymers - Module

Figure 9.05 Gear nomograph (British units)
From a strictly functional and technical point of view,
there is no reason to choose a larger tooth size than
required. In the case of plastic gear design, the tooth
size is often chosen smaller than necessary for the
following reasons:
• Smaller teeth, for a given diameter, tend to spread
the load over a larger number of teeth
• Less critical molding tolerances
• Less sensitivity to thermal variations, post molding
shrinkage, and dimensional stability
• Coarse module teeth are limited by higher sliding
velocities and contact pressures
Actual sizes of gear teeth of different diametral
pitches are shown in Figure 9.06. This can be used to
quickly scale an existing spur gear to determine the
diametral pitch. Table 9.03 may also be helpful in
understanding the various gear relationships.
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Having selected the diametral pitch, the face width is
determined by extending a line from the pitch through
the point on the Reference line previously determined
from the allowable stress and tangential force. Obviously,
the nomographs can be used to determine any
one of the four variables (S, F, f, or MP d) if any three
are known.
Item Equations
Pitch Diameter (Dp) Diametral Pitch (Pd) Circular Pitch (Pc) Center Distance (C)
Table 9.03
Gear Relationships
are related by D p × P d = N
are related by P c × P d = π
are related by C = Np + Ng Pc 2π
where: N or N g = number of teeth on the gear
N p = number of teeth on the pinion