General Design Principles for DuPont Engineering Polymers - Module

More documents

Recommendations

Info

The wear performance of Delrin ® 500, Delrin ® 900 F and Delrin ® 500 CL are illustrated in Figure 7.02 against mild steel. Comparable data have also been obtained to show the suitability of Delrin ® acetal resin with aluminum and brass. When loads and operating speeds are low, as in clocks and hand-operated window crank drives, anodized aluminium and hard brass can be used as bearing surfaces with Delrin ® acetal resin. Figure 7.03 shows wear of Zytel ® 101 running against steel without lubricant at a PV of 3000. The actual wear performance of specific resins will vary depending upon load, speed, mating surface, lubrification and clearance. Wear data can be found in the product modules. A bearing surface should always be provided with interruptions to allow wear debris to be picked up and, as much as possible, to be removed from the rubbing Figure 7.02 Wear of Delrin ® 500 against mild steel* Wear Delrin ® 500 Delrin ® 900 F Time of Test Delrin ® 500 CL *Thrust washer test; non-lubricated; P, 0.4 MPa (5.7 psi); V, 0.95 m/s (190 fpm) Note: Delrin ® 900 F was previously coded Delrin ® 8010. Figure 7.03 Typical wear of Zytel ® 101 against ground carbon steel No lubricant, PV = 3,000 lb ft × in2 min 40 surface. This can be achieved by means of longitudinal slots or simply by radial holes depending on the design possibilities. Extensive tests have proven beyond any doubt that maintenance of a clean rubbing surface increases service life considerably (Figure 7.01, curve 5). Axial grooves shown in Figure 7.04 may constitute the single most useful design improvement to a plastic bushing or bearing. Since significant increases in service life will be dependent on the volume of abrasive particles removed from the wear interface, the following design guidelines should be helpful: 1) use of a minimum of three grooves; 2) make them as deep as technically feasible; 3) keep width at about 10 per cent of shaft diameter; and 4) use through-holes where wall is too thin for grooves. Figure 7.04 Typical slotted bearings Accuracy Another factor affecting bearing life is concentricity. A simple sleeve molded for the purpose of being press-fitted into a metal frame as shown on Figure 7.05 may be sufficiently accurate. Most plastic bearings are, however, part of a whole unit or combined with other components. Figure 7.06 shows three typical examples where it becomes difficult or even quite impossible to mold a perfect round and/or cylindrical bore due to part geometry affecting uniform mold shrinkage. Thus, the load is carried by only a portion of the surface, causing high local specific pressure and immediate initial wear. With high PV value bearings, this may prove to be disastrous because the wear
debris is moved around continuously, thus accelerating wear and shortening service life. On low PV value bearings or ones which are operated only occasionally, this may be of no importance. High performance bearings are often post-machined in order to obtain a perfect cylindrical and round bore, which improves performance significantly. Figure 7.05 Press-fitted sleeve bearing Figure 7.06 Typical designs for integral bearings 41 Bearing Clearance Plastic bearings generally require larger running clearances than metal bearings, due primarily to the much higher coefficient of linear thermal expansion of the plastic material. The designer must also take into consideration the fact that post-molding shrinkage can reduce the diameter of the bearing after it is in service, and particularly if the service temperature is elevated. Post-molding shrinkage can be minimized by proper molding conditions. The engineer should include in his specifications a limit on post-molding shrinkage. This can be checked on a Q/C basis by exposing the part for approximately one hour to a temperature approximately 28°C (50°F) above the maximum use temperature, or 17°C (30°F) below the melting point, whichever is lower. Bearing clearances, when not carefully checked and controlled, are the most frequent causes of bearing failures. Bearing diametral clearance should never be allowed to go below 0.3–0.5% of the shaft diameter. Where the application requires closer running or sliding clearance, split bearing inserts of Delrin ® acetal resin or Zytel ® nylon resin have been used extensively. Here, the dimensional effect of the environment on bearing clearance need be considered only on the wall thickness rather than on the diameter. Lubrication The main reason for using bearings of Delrin ® acetal resin and/or Zytel ® nylon resin is to achieve good wear performance under completely dry running conditions (for instance in the food industry), or with initial lubrication (speed reducer bearings of all kinds in closed housings, like appliances). Continuously lubricated bearings are rarely encountered in the products where plastics are used and are not discussed here. Initial lubrication should always be provided. It not only facilitates the run-in period but increases total service life. Unless special steps are taken to retain the lubricant, it will have a limited effectiveness in time, after which the bearing must be considered as dry running. Consequently, an initially lubricated bearing where the lubricant cannot be retained cannot sustain a higher load but it will last longer and show better wear behavior. Initial lubrication is particularly helpful in running against soft metals.
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: 7—Bearings Bearings and bushings
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
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
show all

General Design Principles for DuPont Engineering Polymers - Module

Create successful ePaper yourself

Delete template?

Save as template?