General Design Principles for DuPont Engineering Polymers - Module

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frequently assembled and disassembled this solution is quite convenient. For example, it is used successfully for assembling a thermostat body onto a radiator valve (see Figure 10.14). Here a metal ring is used to insure retention. The toothed pulley in Figure 10.15 is not subjected to significant axial load. A snap-fit provided with slots is, therefore, quite adequate. It allows a deeper groove and, therefore, a higher thrust bearing shoulder, which is advantageous since it is subject to wear. Pressure operated pneumatic and hydraulic diaphragm valves or similar pressure vessels sometimes require higher retaining forces for snap-fits. This can be achieved by means of a positive locking undercut as shown in Figure 10.16. A certain number of segments (usually 6 or 8) are provided with a 90° undercut, ejection of which is made possible through corresponding slots. In the portions between the segments there are no undercuts. This design provides very strong snap-fit assemblies, the only limitation being elongation and force required during assembly. It is also conceivable to preheat the outer part to facilitate the assembly operation. Figure 10.13 Slotted snap-fit Figure 10.14 Snap-fit thermostat body 74 Figure 10.15 Tooth belt pulley Figure 10.16 Tooth belt pulley
Cantilever Lug Snap-Fits The second category into which snap-fits can be classified is based on cantilevered lugs, the retaining force of which is essentially a function of bending stiffness. They are actually special spring applications which are subjected to high bending stress during assembly. Under working conditions, the lugs are either completely unloaded for moving parts or partially loaded in order to achieve a tight assembly. The typical characteristic of these lugs is an undercut of 90° which is always molded by means of side cores or corresponding slots in the parts. The split worm gear of Figure 10.18 shows an example in which two identical parts (molded in the same cavity) are snapped together with cantilevered lugs that also lock the part together for increased stiffness. In addition, the two halves are positioned by two studs fitting into mating holes. The same principle is especially suitable for noncircular housings and vessels of all kinds. For example, there is the micro-switch housing in Figure 10.19 where an undercut in the rectangular housing may not be functionally appropriate. A similar principle is applied to the ball bearing snapfit in Figure 10.17. The center core is divided into six segments. On each side three undercuts are molded and easily ejected, providing strong shoulders for the heavy thrust load. Cantilevered lugs should be designed in a way so as not to exceed allowable stresses during assembly operation. This requirement is often neglected when parts in Delrin ® acetal resin are snapped into sheet metal. Too short a bending length may cause breakage (see Figure 10.20). This has been avoided in the switch in Figure 10.21, where the flexible lugs are considerably longer and stresses lower. Cantilevered snap-fit lugs should be dimensioned to develop constant stress distribution over their length. This can be achieved by providing a slightly tapered section or by adding a rib (see Figure 10.22). Special care must be taken to avoid sharp corners and other possible stress concentrations. To check the stress levels in a cantilevered lug, use the beam equations: Stress S = FLc l Deflection y = FL3 3El The allowable amount of strain depends on material and on the fact if the parts must be frequently assembled and disassembled. Table 10.06 shows suggested values for allowable strains. 75 Table 10.06 Suggested Allowable Strains (%) for Lug Type Snap-fits Allowable strain Used once Used Material (new material) frequently Delrin ® 100 8 2–4 Delrin ® 500 6 2–3 Zytel ® 101, dry 4 2 Zytel ® 101, 50% RH 6 3 Zytel ® GR, dry 0.8–1.2 0.5–0.7 Zytel ® GR, 50% RH 1.5–2.0 1.0 Rynite ® PET GR 1 0.5 Crastin ® PBT GR 1.2 0.6 Hytrel ® 20 10 Figure 10.17 Snap-fit ball bearing Figure 10.18 Snap-fit worm gear
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dDuPont Engineering Polymers Genera
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8—Gears . . . . . . . . . . . . .
Page 5 and 6:
1—General Introduction This secti
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Prototyping the Design In order to
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2—Injection Molding The Process a
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As a general rule, use the minimum
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Figure 3.11 Cored holes Figure 3.12
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Figure 3.19 Mold-ejection of rounde
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• Zytel ® nylon resin—Parts of
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4—Structural Design Short Term Lo
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Form of section Area A d d R R 1 1
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Table 4.03. Formulas for Stresses a
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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 and 52: 8—Gears Introduction Delrin ® ac
Page 53 and 54: 9—Gear Design Allowable Tooth Ben
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: Undercut Snap-Fits In order to obta
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
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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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