General Design Principles for DuPont Engineering Polymers - Module

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Protection against Dirt Penetration Bearings in Delrin ® acetal resins, Zytel ® nylon resins and Rynite ® PET thermoplastic polyester resins, although more tolerant than metals, work more satisfactorily when they are protected against penetration of dust, dirt and water. The benefit of initial lubrication may be lost completely by the action of dirt particles penetrating between the rubbing surfaces, thus forming an abrasive paste with the lubricant. Bearings running at a high PV value should therefore be protected by means of felt rings or rubber seals which in addition to keeping out dirt, retain the lubricant. To say that plastic bearings are not affected by dirt is absolutely wrong. Wear debris from the metal shaft as well as dirt particles can be embedded into the plastic bearings and substantially reduce service life. It is not always practical to protect a bearing efficiently in the described way. This does not necessarily mean that plastic bearings will not function in adverse environments. The designer must be aware of these problems and make allowances with regard to bearing pressure, velocity and service life. Successful applications of this nature can be found in conveyors, chains and textile machine bearings, for example. In environments where dust and dirt must be tolerated, Zytel ® nylon resin is the preferred material. In such applications, it may prove beneficial to eliminate initial lubrication. To summarize, important design considerations affecting the performance and life of bearings in Delrin ® acetal resin, Zytel ® nylon resin and Rynite ® PET thermoplastic polyester are: • Surface hardness and finish of the metal shaft • Geometric accuracy of bearing bore • Correct clearance • Grooves or holes in the sliding surface • Initial lubrication • Means to protect the bearing against dirt penetration and to retain the lubricant. Calculation of Bearings A plastic bearing subjected to a continuously but slowly increasing load and/or speed reaches a point where it fails due to excessive temperature rise. This limit is defined as the maximum PV value, and it is often used to compare various plastic materials as far as wear behavior is concerned. The surface temperature in a bearing is not only a function of load, speed and coefficient of friction, but also of heat dissipation. The latter depends very much on the overall concept 42 of the testing device, for which no international standards exists. The comparison of such values is therefore pointless unless they are determined under exactly the same conditions. Even if this is the case, they are of no great help to designers for the following reasons: • A bearing running close to the PV limit thus determined usually has so much wear that the value can only be applied in very special cases. • PV limits of plastic materials measured under specific lab conditions may, for many reasons, prove to be completely different or even reversed in practical applications. • Many so-called “special bearing compositions” with fillers show higher PV limits than the base resin. This result is merely due to the fact that blended fillers reduce the coefficient of friction, thus generating less heat and allowing the bearing to run at a higher PV value. This fact does not guarantee less wear. Tests have shown that most unfilled resins have better wear behavior than the same resins with fillers. This is significant because it is a widespread but erroneous belief that a low coefficient of friction also means better wear resistance. Delrin ® AF acetal resin is an exception, as both lower friction and wear result from the addition of Teflon ® fibers. Filled resins may prove to be preferable for very specific applications where a low coefficient of friction is essential. For instance, highly loaded bearings running for very short periods at a time and for which a limited service life is expected. The coefficient of friction is not a material constant but rather a response of the bearing surface to a dynamic event. The coefficient of friction data in Table 7.01 shows a comparison of Delrin ® acetal resin and Zytel ® nylon resin against steel, etc., at a specific set of conditions. A low non-lubricated coefficient of friction can mean lower horsepower and smoother performance to the designer. Table 7.01 also illustrates the low stick-slip design possibilities with Delrin ® acetal resin with the static coefficient of friction lower than the dynamic, and particularly when running against Zytel ® nylon resin. The diagram in Figure 7.07 suggests reasonable PV values which take into account the mating material and the degree of post machining. It does not include, however, severe running conditions at higher temperatures or the presence of dirt, dust, textile particles or other abrasive materials. Table 7.02 shows how to choose the proper curve in Figure 7.07. This guide is based on the assumption that the design, bearing clearance, bore accuracy, processing assembly and load distribution are correct.
Table 7.01 Coefficient of Friction* Static Dynamic Delrin ® acetal resin on steel Delrin ® 100, 500, 900 0.20 0.35 Delrin ® 500F, 900F — 0.20 Delrin ® 500 CL 0.10 0.20 Delrin ® AF Delrin 0.08 0.14 ® on Delrin ® Delrin ® 500/Delrin ® 500 Delrin 0.30 0.40 ® on Zytel ® Delrin ® 500/Zytel ® 101 L Zytel 0.10 0.20 ® on Zytel ® Max. 0.46 0.19 Min. Zytel 0.36 0.11 ® on Steel Max. 0.74 0.43 Min. Rynite 0.31 0.17 ® PET on Rynite ® PET Max. 0.27 Min. Rynite 0.17 ® PET on Steel Max. 0.20 Min. Hytrel 0.17 ® on Steel Hytrel ® 4056 on Steel 0.32 0.29 Hytrel ® 5556 on Steel 0.22 0.18 Hytrel ® 6346 on Steel 0.30 0.21 Hytrel ® 7246 on Steel 0.23 0.16 PC on Steel 0.50 – ABS on Steel 0.50 – PBT on Steel 0.40 – PBT on PBT 0.40 * Data on Zytel ® nylon resin and Delrin ® acetal resin determined via the Thrust Washer test, non-lubricated, 23°C (73°F); pressure 2.1 MPa (300 psi); velocity 3 m/min (10 fpm). Data on Rynite ® PET thermoplastic polyester determined in accordance with ASTM D1894. Figure 7.07 Bearing pressure versus velocity** Velocity, m/min 200 120 100 80 60 40 20 10 8 6 **See Table 7.02 for guide to the proper curve to use. 4 *1 *2 *3 *4 *5 43 Table 7.02 Guide to Curves in Figure 7.07 to Determine Max. Allowable PV Values Bore Shaft Molded Machined Hardened and ground steel —Chrome plated Rc >50 Stainless steel—Anodized 3 4 aluminum Rc 30–35 2–3 3 Soft steel—Ground stainless steel 2 2–3 Cold drawn steel unmachined Non ferrous metals—Diecast 1–2 2 alloys 1 or less — Delrin ® on steel Rc >70, dry 5 Delrin ® on steel, Rc
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: debris is moved around continuously
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
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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