| 18.6.2 Environmental and
Tolerances In any discussion of tolerances for plastic gears, it is necessary to distinguish between manufacturing conditions tolerances and dimensional changes due to environmental conditions. As far as manufacturing is concerned, plastic gears can be made to high accuracy, if desired . For injection molded gears, Total Composite Error can readily be held within a range of roughly 0.075-0.125 mm, with a corresponding Tooth-to-Tooth Composite Error of about 0.025-0.050 mm. Higher accuracies can be obtained if the more expensive filled materials, mold design, tooling and quality control are used. In addition to thermal expansion changes, there are permanent dimensional changes as the result of moisture absorption. Also, there are dimensional changes due to compliance under load. The coefficient of thermal expansion of plastics is on the order of four to ten times those of metals (see Tables 18-3 and 18-10). In addition, most plastics are hygroscopic (i.e., absorb moisture) and dimensional changes on the order of 0.1% or more can develop in the the course of time, if the humidity is sufficient. As a result, one should attempt to make sure that a tolerance which is specified is not smaller than the inevitable dimensional changes which arise as a result of environmental conditions. At the same time, the greater compliance of plastic gears, as compared to metal gears, suggests that the necessity for close tolerances need not always be as high as those required for metal gears. 18.6.3 Avoiding Stress Concentration In order to minimize stress concentration and maximize the life of a plastic gear, the root fillet radius should be as large as possible, consistent with conjugate gear action. Sudden changes in cross section and sharp corners should be avoided, especially in view of the possibility of additional residual stresses which may have occurred in the course of the molding operation. 18.6.4 Metal Inserts Injection molded metal inserts are used in plastic gears for a variety of reasons: 1. To avoid an extra finishing operation. 2. To achieve greater dimensional stability, because the metal will shrink less and is not sensitive to moisture; it is, also, a better heat sink. 3. To provide greater load-carrying capacity. 4. To provide increased rigidity. 5. To permit repeated assembly and disassembly. 6. To provide a more precise bore to shaft fit. Inserts can be molded into the part or subsequently assembled. In the case of subsequent insertion of inserts, stress concentrations |
may be present
which may lead to cracking of the parts. The interference limits for press
fits must be obeyed depending on the material used; also, proper minimum
wall thicknesses around the inserts must be left. The insertion of inserts
may be accomplished by ultrasonically driving in the insert. In this case,
the material actually melts into the knurling at the insert periphery. Inserts are usually produced by screw machines and made of aluminum or brass. It is advantageous to attempt to match the coefficient of thermal expansion of the plastic to the materials used for inserts. This will reduce the residual stresses in the plastic part of the gear during contraction while cooling after molding. When metal inserts are used, generous radii and fillets in the plastic gear are recommended to avoid stress concentration. It is also possible to use other types of metal inserts, such as self-threading, self-tapping screws, press fits and knurled inserts. One advantage of the first two of these is that they permit repeated assembly and disassembly without part failure or fatigue. 18.6.5 Attachment of Plastic Gears to Shafts Several methods of attaching gears to shafts are in common use. These include splines, keys, integral shafts, set screws, and plain and knurled press fits. Table 18-21 lists some of the basic characteristics of each of these fastening methods. 18.6.6 Lubrication Depending on the application, plastic gears can operate with continuous lubrication, initial lubrication, or no lubrication. According to L.D. Martin ("Injection Molded Plastic Gears", Plastic Design and Processing, 1968; Part 1, August, pp 38-45; Part 2, September, pp. 33-35): 1. All gears function more effectively with lubrication and will have a longer service life. 2. A light spindle oil (SAE 10) is generally recommended as are the usual lubricants; these include silicone and hydrocarbon oils, and in some cases cold water is acceptable as well. 3. Under certain conditions, dry lubricants such as molybdenum disulfide, can be used to reduce tooth friction. Ample experience and evidence exist substantiating that plastic gears can operate with a metal mate without the need of a lubricant, as long as the stress levels are not exceeded. It is also true that in the case of a moderate stress level, relative to the materials rating, plastic gears can be meshed together without a lubricant. However, as the stress level is increased, there is a tendency for a localized plastic-to-plastic welding to occur, which increases friction and wear. The level of this problem varies with the particular type of plastic. |
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Table 18-21 Characteristics of Various Shaft Attachment Methods |
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