General Design Principles for DuPont Engineering Polymers - Module

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• It must also be understood that any large opening in the web, especially if located close to the teeth, will show up on a gear measuring device and may cause noise or accelerated wear on fast running gears (see Figure 9.13). • Figures 9.14 and 9.15 demonstrate how design and gating sometimes can determine whether a gear fails or performs satisfactorily. Both are almost identical windshield wiper gears molded onto knurled shafts. The gear on Figure 9.14 is center gated and does not create a problem. • The gear shown in Figure 9.15 is filled through 3 pinpoint gates in the web. In addition, the three holes provided for attaching a metal disc are placed close to the hub. As a result, the thicker hub section is poorly filled and the three weld lines create weak spots, unable to withstand the stresses created by the metal insert and the sharp edges of the knurled surface. Figure 9.13 Holes and ribs in molded gears Figure 9.14 Center gated gear Gate High Shrinkage Low Shrinkage 56 Figure 9.15 Web gated gear 3 Gates Accuracy and Tolerance Limits As discussed previously, plastic gears, because of their resilience, can operate with broader tolerances than metal gears. This statement should not be taken too generally. Inaccurate tooth profiles, out-ofroundness and poor tooth surfaces on plastic gears may well account for noise, excessive wear and premature failure. On the other hand, it is useless to prescribe tolerances which are not really necessary or impossible to achieve on a high production basis. The main problem in producing accurate gears in plastic is, of course, mold shrinkage. The cavity must be cut to allow not only for diametral shrinkage but also the effect of shrinkage on tooth profile on precision gears, this fact must be taken into account, requiring a skillful and experienced tool maker. With the cavity made correctly to compensate for shrinkage, molding conditions must be controlled to maintain accuracy. The total deviation form the theoretical tooth profile can be measured by special equipment such as used in the watch industry. An exaggerated tooth profile is shown in Figure 9.16. It includes the measurement of surface marks from the cavity as well as irregularities caused by poor molding conditions. Figure 9.16 Measurement of tooth profile error Weld lines
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
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dDuPont Engineering Polymers Genera
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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 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: The following additional examples i
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
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For this reason, if the strength of
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Figure 11.64A shows a variation whi
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3. Vibrations, generated either by
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If one part is only vibrated at twi
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Another joint design, with flash tr
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Figure 11.82. A linear welded motor
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B) Commercial linear and angular we
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Hot Plate Welding of Zytel ® One o
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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
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The polishing operation is performe
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