General Design Principles for DuPont Engineering Polymers - Module

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Problem #2 An existing spring of Delrin ® acetal resin has the following dimensions: L = 15.2 mm (0.6 in) h = 2.54 mm (0.1 in) b = 10.2 mm (0.4 in) Determine load (W) to deflect the spring 1.14 mm (0.045 in) (y) Solution L/h = 15.2/2.54 = 6 y/L = 1.14/15.2 = 0.075 Find chart spring rate Wc/y from Figure 6.02 at intersect of L/h and y/L lines. So Wc/y = 80 N/mm (450 lb/in) Then Wc = 80 y = 80 × 1.14 = 91.2 N (20.25 lb) Since the chart is based on a spring width of 25.4 mm (1.00 in), the actual load for a spring 10.2 mm (0.40 in) wide would equal: W = (10.2) 91.2 = 36.6 N (8.1 lb) (25.4) 38 Figure 6.02 Spring rate chart for cantilever springs of Delrin ® acetal resin at 23°C (73°F), intermittent loading b = 1 in
7—Bearings Bearings and bushings of Zytel ® nylon resin and Delrin ® acetal resin are used in numerous commercial applications. Zytel ® is uniquely suited to abrasive atmospheres such as in cement plants and in factories where dust is a constant problem. Bearings of Zytel ® have been used successfully under diverse environmental conditions including the presence of various oils, greases, chemicals, reagents and proprietary compositions, many of which are harmful to other types of plastic materials. Bearings of Delrin ® acetal resin offer the unique characteristic of no “Slip-stick,” or the static friction coefficient being equal or lower than dynamic. Typical applications are hemispherical bearings for auto ball-joints; food mixer housing-bearing combinations; wear surfaces on integral gear-spring-cam units for calculators; clock bushings and many others. The extensive use of Delrin ® acetal resin as a bearing material has developed because of its combination of low coefficient of friction with self-lubricating properties, good mechanical properties and dimensional stability in many chemical media. The performance of bearings depends on a number of factors. Shaft Hardness and Finish If a metal shaft runs in a bearing of Delrin ® acetal resin or Zytel ® nylon resin, the most important parameter is the surface hardness of the shaft. For unlubricated bearings of Delrin ® acetal resin or Zytel ® nylon resin on metal, the metal should be as hard and smooth as consistent with bearing-life requirements and bearing cost. Common centerless ground shafts of steel are acceptable, but increased hardness and finish Figure 7.01 Wear of Delrin ® 500 against various materials* Wear Delrin ® Mild Steel Delrin ® 1 2 3 Hard Steel Time of Test 39 will improve bearing life. At one set of test conditions* a steel shaft of R60 hardness and 4 μin (AA) finish should extend bearing life twofold relative to a shaft of R22, 16 μin (AA). The actual wear performance will change with the speed and load and the type of mating surface material. Soft steel or stainless steel, as well as all nonferrous metals, do not run well with plastic bearings, even those with a so-called “self-lubricating” filler. It is only a question of load, speed and time until wear increases rapidly, leading to premature failure. Shaft hardness becomes more important with higher PV values and/or the expected service life. A highly polished surface does not improve wear behavior if the hardness is insufficient. There are nevertheless a great number of bearing applications which perform satisfactorily against soft metals running at low speed and load, such as bearings for clocks and counter mechanisms. Delrin ® acetal resin generally performs better than other plastics against soft metals. If a bearing fails, it is, however, most important to carefully check the hardness of the metal shaft surface since it may account partially for unsatisfactory performance. Bearing Surface The influence of mating surface on wear rate between Delrin ® acetal resin and various materials is shown in Figure 7.01. A dramatic reduction in wear can be seen as material hardness increases between curves 1, 2 and 3. The most dramatic differences can be seen in curve 4 where Delrin ® acetal resin is matched with Zytel ® 101 nylon. *Thrust washer test non-lubricated; P, 2 MPa (300 psi) V, 50 mm/s (10 fpm), AISI 1080 carbon steel. Delrin ® Tungsten Carbide 5 Delrin ® Hard Steel with slots Delrin ® —Zytel ® 101 4
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: Cantilever Springs Tests were condu
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 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
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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
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B) Commercial linear and angular we
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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
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The polishing operation is performe
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General Design Principles for DuPont Engineering Polymers - Module

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