General Design Principles for DuPont Engineering Polymers - Module

In practice, the most commonly used method for
checking gear accuracy is a center distance measuring
device as shown in Figure 9.17.
The plastic gear meshes with a high precision metal
master gear, producing a diagram of the center distance
variations as shown in Figure 9.18.
This diagram enables the designer to evaluate the
accuracy of the gear and to classify it according to
AGMA or DIN specifications.
AGMA specification No. 390.03 classifies gears into
16 categories, of which class 16 has the highest
precision and class 1 the lowest. Molded gears usually
lie between classes 6 and 10 where class 10 requires
superior tool making and processing.
Similarly, DIN specification No. 3967 classifies gears
into 12 categories, of which class 1 is the most precise
and class 12 the least. Molded gears range between
classes 8 and 11 in the DIN categories.
The total error as indicated in Figure 9.18 may be due
in part to an inaccurate cavity, inadequate part gating
or poor processing.
If several curves of a production run are superimposed,
as in Figure 9.19, the distance “T” between
the highest and the lowest indicates the molding
tolerances.
Figure 9.17 Center distance measuring instrument
Figure 9.18 Center distance variation diagram
57
Figure 9.19 Molding tolerances from center distance
diagram
Backlash and Center Distances
As shown in Figure 9.20, backlash is the tangential
clearance between two meshing teeth. Figure 9.21
provides a suggested range of backlash for a first
approach.
Figure 9.20 Measurement of backlash
Backlash
Figure 9.21 Suggested backlash for gears of Delrin ®
and Zytel ®
Module, mm
0.002
3
2.0
1.5
1.25
1
0.85
0.05
Backlash, in
0.004 0.006
0.10 0.15
Backlash, mm
10
20
30
Diametral pitch, in