General Design Principles for DuPont Engineering Polymers - Module

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solution out of many other possible designs. It is essential that the springs do not remain accidentally in the loaded position. In the example shown, this is achieved by providing three pivoting springs. • For specific requirements, it is also possible to combine gears with sliding couplings. The example shown in Figure 8.06 is a gear in Delrin ® acetal resin snapped onto a shaft in Zytel ® 101, in which Figure 8.01 Ratchet and gear combined Figure 8.04 Backlash-free gear Figure 8.02 Impact resistant web Figure 8.05 Torque limiting gear Figure 8.03 Backlash-free gear Figure 8.06 Gear with sliding coupling 48 the split hub acts as a coupling for a small dial setting device. If, as in this case, the required torque is very low, stress relaxation in the springs will not endanger perfect functioning for a sufficient length of service life. If, on the contrary, a constant torque must be transmitted for a long period of time, an additional metal spring around the hub would be required to keep the force constant.
9—Gear Design Allowable Tooth Bending Stress The key step in gear design is the determination of allowable tooth bending stress. Prototyping of gears is expensive and time consuming, so an error in the initial choice of tooth bending stress can be costly. For any given material, the allowable stress is dependent on a number of factors, including: • Total lifetime cycle • Intermittent or continuous duty • Environment—temperature, humidity, solvents, chemicals, etc. • Change in diameter and center to center distance with temperature and humidity • Pitch line velocity • Diametral pitch (size of teeth) and tooth form • Accuracy of tooth form, helix angle, pitch diameter, etc. • Mating gear material including surface finish and hardness • Type of lubrication Selection of the proper stress level can best be made based on experience with successful gear applications of a similar nature. Figure 9.01 plots a number of Figure 9.01 Speed versus stress: typical gear applications placed on curve Peripheral speed, m/s 4.5 4.0 – 3.5 – 3.0 – 2.5 – 2.0 – 1.5 – 1.0 – 0.5 – 250 I 0 0 2 500 I 750 I 1,000 I Hand drill press 300W 1,250 I Tooth stress, psi 1,500 I Carpet cleaning device (bevel gear drive) Meat grinder (1st reduction) Hand drill press 130W Tooth stress, N/mm2 4 6 8 10 12 14 16 18 20 22 49 successful gear applications of Delrin ® acetal resin and Zytel ® nylon resin in terms of peripheral speed and tooth bending stress. Note that all of these applications are in room temperature, indoor environments. For similar applications operating at temperatures higher than 40°C (104°F), the allowable stress should be multiplied by the ratio of the yield strength at the higher temperature to the yield strength at 40°C (104°F). Since fatigue endurance is reduced somewhat as temperature increases, this effect must also be considered. Where very high temperatures are encountered, thermal aging may become a factor. All of this data is in the product modules. Where suitable experience is not available, the allowable tooth stress must be based on careful consideration of all the factors previously outlined, and on available test data on the gear material of choice. A number of years ago, DuPont commissioned a series of extensive gear tests on gears of Delrin ® acetal resin and Zytel ® nylon resin. This data can be combined with environmental operating conditions to arrive at an initial tooth bending stress. Whether similar experience exists or not, it is essential that a prototype mold be built and the design carefully tested in the actual or simulated end-use conditions. 1,750 I 2,000 I 2,250 I Meat grinder (2nd and 3rd reduction) 2,500 I 2,750 I Planetary gear drive (washing machines and general industrial purposes) Planetary gear drive for washing machines Planetary gear drive 3,000 I – 14 – 12 – 10 – 8 – 6 – 4 – 2 Peripheral speed, ft/sec
Page 1 and 2: dDuPont Engineering Polymers Genera
Page 3 and 4: 8—Gears . . . . . . . . . . . . .
Page 5 and 6: 1—General Introduction This secti
Page 7 and 8: Prototyping the Design In order to
Page 9 and 10: 2—Injection Molding The Process a
Page 11 and 12: As a general rule, use the minimum
Page 13 and 14: Figure 3.11 Cored holes Figure 3.12
Page 15 and 16: Figure 3.19 Mold-ejection of rounde
Page 17 and 18: • Zytel ® nylon resin—Parts of
Page 19 and 20: 4—Structural Design Short Term Lo
Page 21 and 22: Form of section Area A d d R R 1 1
Page 23 and 24: Table 4.03. Formulas for Stresses a
Page 25 and 26: Form of bar; manner of loading and
Page 27 and 28: Figure 4.01 Creep Stress (S), MPa (
Page 29 and 30: Figure 4.05 Creep in flexure of Zyt
Page 31 and 32: Example 1 It there are no restricti
Page 33 and 34: Determine the moment of inertia and
Page 35 and 36: Figure 4.11 Stress curves The compu
Page 37 and 38: Rim Design Rim design requirements
Page 39 and 40: Figure 5.08 Chair Seat Zytel ® 71
Page 41 and 42: Cantilever Springs Tests were condu
Page 43 and 44: 7—Bearings Bearings and bushings
Page 45 and 46: debris is moved around continuously
Page 47 and 48: Table 7.01 Coefficient of Friction*
Page 49 and 50: Figure 7.15: Connecting rod spheric
Page 51: 8—Gears Introduction Delrin ® ac
Page 55 and 56: Delrin ® 500. See the section on D
Page 57 and 58: Figure 9.06 Sizes of gear teeth of
Page 59 and 60: The following additional examples i
Page 61 and 62: In practice, the most commonly used
Page 63 and 64: Driving Gear Meshing Gear Table 9.0
Page 65 and 66: A full-throated worm gear is shown
Page 67 and 68: Worm Material Gear Material Possibl
Page 69 and 70: 10—Assembly Techniques Introducti
Page 71 and 72: For these materials, the finer thre
Page 73 and 74: Table 10.01 Self-Threading Screw Pe
Page 75 and 76: Figures 10.07 and 10.08 show calcul
Page 77 and 78: Undercut Snap-Fits In order to obta
Page 79 and 80: Cantilever Lug Snap-Fits The second
Page 81 and 82: 11—Assembly Techniques, Category
Page 83 and 84: Figure 11.03 Drill spindle position
Page 85 and 86: Inertia Welding By far the simplest
Page 87 and 88: Machines for Inertia Welding The pr
Page 89 and 90: This is not always easy, because ap
Page 91 and 92: The holder a, will have equal and o
Page 93 and 94: Calculations for Inertia Welding To
Page 95 and 96: If, for example, the angles of the
Page 97 and 98: Welding Double Joints The simultane
Page 99 and 100: Figure 11.37 Commercial bench-type
Page 101 and 102: Heat is generated throughout the pa
Page 103 and 104:
Figure 11.45 Tapered or stepped hor
Page 105 and 106:
Weld strength is therefore determin
Page 107 and 108:
) General Part Design The influence
Page 109 and 110:
eticule in the eye piece is suitabl
Page 111 and 112:
For this reason, if the strength of
Page 113 and 114:
Figure 11.64A shows a variation whi
Page 115 and 116:
3. Vibrations, generated either by
Page 117 and 118:
If one part is only vibrated at twi
Page 119 and 120:
Another joint design, with flash tr
Page 121 and 122:
Figure 11.82. A linear welded motor
Page 123 and 124:
B) Commercial linear and angular we
Page 125 and 126:
Hot Plate Welding of Zytel ® One o
Page 127 and 128:
Figure 11.94 Applications of riveti
Page 129 and 130:
Tolerances may be difficult to hold
Page 131 and 132:
are used in either hand- or power-o
Page 133 and 134:
ather than scrape, these drills wil
Page 135 and 136:
The polishing operation is performe
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