General Design Principles for DuPont Engineering Polymers - Module

Figure 9.06 Sizes of gear teeth of different diametral pitches, inches
Lewis Equation
For the convenience of those who might prefer the
Lewis Equation to the nomographs, or as a check
against the nomographs, the equation is as follows:
Sb = FPd fY
where: Sb = bending stress, psi
F = tangential force (see p. 51)
Pd = diametral pitch
f = gear face width, in
Y = form factor, load near the pitch point
(see Table 9.04)
In order to determine the form factor, Y, from Table
9.04, it is necessary to select the tooth form.
53
Selection of Tooth Form
The most common tooth forms are shown in Figure
9.07. 20° pressure angle yields a stronger tooth than
14 1 ⁄ 2°, and the 20° stub is stronger than the 20° full
depth. Choice of the best tooth form can be influenced
by pitch line velocity. The greater the pitch line
velocity, the more accurate the teeth must be for quiet
operation. Because of the mold shrinkage characteristic
of thermoplastics, smaller teeth can be held to more
accurate tooth profile, generally speaking. Variation in
center to center distance might also be a factor. The
14 1 ⁄ 2° pressure angle gives a thinner tooth and thus
better accuracy, however, the 20° full depth system is
generally recommended for gears of Delrin ® acetal
resin and Zytel ® nylon resin. It produces a strong tooth
with good wear characteristics. The 20° stub tooth will
carry higher loads, but gear life will be shorter than
the 20° full depth. Other gear systems can be used but
no advantage has been found in their use.