General Design Principles for DuPont Engineering Polymers - Module

Table 9.04
Tooth Form, Factor Load Near the Pitch Point
Number Teeth 14 1 ⁄2° 20° Full Depth 20° Stub
14 — — 0.540
15 — — 0.566
16 — — 0.578
17 — 0.512 0.587
18 — 0.521 0.603
19 — 0.534 0.616
20 — 0.544 0.628
22 — 0.559 0.648
24 0.509 0.572 0.664
26 0.522 0.588 0.678
28 0.535 0.597 0.688
30 0.540 0.606 0.698
34 0.553 0.628 0.714
38 0.566 0.651 0.729
43 0.575 0.672 0.739
50 0.588 0.694 0.758
60 0.604 0.713 0.774
75 0.613 0.735 0.792
100 0.622 0.757 0.808
150 0.635 0.779 0830
300 0.650 0.801 0.855
Rack 0.660 0.823 0.881
Figure 9.07 Comparison of tooth profiles
Designing for Stall Torque
There are many applications where the gear must be
designed to withstand stall torque loading significantly
higher than the normal running torque, and in some
cases this stall torque may govern the gear design. To
determine the stall torque a given gear design is
capable of handling, use the yield strength of the
material at expected operating temperature under stall
conditions. A safety factor does not normally need be
applied if the material to be used is either Zytel ® nylon
54
resin or Delrin ® 100, as the resiliency of these materials
allows the stall load to be distributed over several
teeth. Again, adequate testing of molded prototypes is
necessary.
Gear Proportions
Once the basic gear design parameters have been
established, the gear design can be completed. It is
very important at this stage to select gear proportions
which will facilitate accurate moldings with minimum
tendency for post molding warpage or stress relief.
An ideal design as far as molding is concerned is
shown in Figure 9.08.
For reasons of mechanical strength, it is suggested that
the rim section be made 2 times tooth thickness “t.”
The other sections depend both on functional requirements
and gate location. If, for some reason, it is
desirable to have a hub section “h” heavier than the
web, then the part must be center gated in order to fill
all sections properly, and the web “w” would be 1.5t.
If the gate must be located in the rim or the web, then
web thickness should equal hub thickness, as no
section of a given thickness can be filled properly
through a thinner one. The maximum wall thickness
of the hub should usually not exceed 6.4 mm ( 1 ⁄ 4 in).
For minimum out-of-roundness, use center gating.
On gears which are an integral part of a multifunctional
component or which have to fulfill special
requirements as shown in Figures 8.01–8.06, it could
be impossible to approach the ideal symmetrical shape
as shown in Figure 9.08, in which case the assembly
must be designed to accept somewhat less accuracy in
the gear dimensions.
Figure 9.08 Suggested gear proportions
h
t
Web
Hub
D
1.5 D
2t
L
Note: L = D
1.5t