General Design Principles for DuPont Engineering Polymers - Module

Test results on gears of Delrin ® acetal resin and Zytel ®
nylon resin are shown in Figures 9.02 and 9.03.
Figure 9.02 is based on injection molded, continuously
lubricated gears of Zytel ® 101 running against
each other. Figure 9.03 is based on injection molded,
continuously lubricated gears of Delrin ® 500 running
against each other. The lines have been reduced 25
percent below the lines which represent actual failure
of the gears on test. The tests were run at room
temperature conditions. Cycle life is simply the total
revolutions of the gear. Caution should be exercised in
using this data for gear teeth larger than 16 pitch or
smaller than 48 pitch.
Since this data was based on injection molded gears
running in a room temperature environment with
continuous lubrication, a design factor “K,” Tables
9.01 and 9.02 should be applied to compensate for cut
gears vs. molded gears, initial lubrication vs. continuous,
pitch line velocity and tooth size (diametral pitch)
Figure 9.02 Maximum bending stresses for gear teeth
of Zytel ® nylon
Bending Stress, × 1,000 psi
Cycle Life, Millions
Figure 9.03 Maximum bending stresses for gear teeth
of Delrin ®
50
Table 9.01
Values of Design Factor K, Zytel ®
P.L. Velocity,
Teeth Lubrication* fpm Pitch K Factor
Molded Yes below 4000 16–48 1.00
Molded Yes above 4000 16–48 0.85
Molded No below 1600 16–32 0.70
Molded No above 1600 16–32 0.50
Molded No below 1000 32–48 0.80
Cut Yes below 4000 16–48 0.85
Cut Yes above 4000 16–48 0.72
Cut No below 1600 16–32 0.60
Cut No above 1600 16–32 0.42
Cut No below 4000 32–48 0.70
* Yes—refers to continuous lubrication
No—refers to initial lubrication
Table 9.02
Values of Design Factor K, Delrin ® *
MTL Mating Gear
Teeth Material Material Lubrication** Pitch K Factor
Molded 500 500 Yes 20–32 1.00
Molded 500 500 No 20 0.45
Molded 500 500 No 32 0.80
Molded 100 100 Yes 20–32 1.40
Molded 100 100 No 20 0.50
Molded 100 Hob Cut Steel Yes 20 1.20
Molded 100 Hob Cut Steel No 20 0.80
* Values based on maximum pitch line velocity of 1600 fpm.
** Yes—refers to continuous lubrication
No—refers to initial lubrication
variations. Thus, multiply the allowable tooth bending
stress obtained from Figures 9.02 and 9.03 by “K.” In
addition, the allowable tooth bending stress should be
modified by the ratio of the yield strength at operating
temperature to the yield strength at room temperature.
Again, the effect of environment on fatigue and
thermal aging must also be considered.
The “K” factors in Table 9.01 are based on molded
gears of Zytel ® nylon resin running against each other
and cut gears of Zytel ® nylon resin running against cut
gears of Zytel ® nylon resin. Zytel ® nylon resin running
against steel should provide higher “K” factors
because steel is a good conductor of heat and the heat
buildup due to friction is less. There is no test data.
However, many years experience in a multitude of
steel to Zytel ® nylon resin applications would support
this contention. As can be seen from Table 9.02, a
higher “K” factor does exist when Delrin ® acetal resin
is run against steel rather than itself with initial
lubrication. It should be noted that Delrin ® acetal resin
does not run against itself as well as Zytel ® nylon
resin does against itself when there is little or no
lubricant present. Also note that Delrin ® 100 performs
about 40 percent better in gear applications than