When a plastic gear is used, there may be a problem as to which is better : intermeshing with plastic gears, or intermeshing with a plastic gear and a steel gear. In the former case, the wear is small since the coefficient of friction of the former is generally smaller than that of the latter. On the other hand, in the latter case, the temperature at the plastic gear becomes lower than in the former case, because the generated heat in the plastic gear conducts to the steel gear with good thermal conductivity, and it dissipates from the steel gear. This is advantageous for the plastic gear since it has a great defect in resistance to heat. In these ways, each intermeshing has its respective merits. In order to further study it in detail, we operated the plastic gear in both intermeshings and investigated the wear of the tooth, the change in tooth form, the temperature at the tooth, and the like.
The design methodology in terms of plastic gears is essentially based on those developed for steel gears, in spite of the fact that various kinds of design methodology have been proposed for plastic gears in recent years. Recently, several problems have been pointed out in relation to this design methodology since there are several experimental evidence, none of which can be clearly explained by the existing methodology. Therefore Tsukamoto, one of our co-workers, has performed many experiments and has proposed an entirely new strength formula, adopting the quantity referred to as the product of the contact period of teeth and loading torque, based on experimental results. The present study is aimed at establishing a practical gear strength design formula by developing Tsukamoto's thory, which is not sufficiently clear in its physical definitions. In this report, a calculation method of the fatigue strength of a gear due to strain energy is proposed.
Recently, plastics, filled with various fibers to make fiber-reinforced composites, have been investigated, and their basic properties have been fairly extensively examined. However, the various properties of these plastics when they are applied to gears have scarcely been revealed. In this study, at first, we prepared three kinds of plastic gears with different fiber-filling rations and made running tests to examine abrasion of the teeth, change of the tooth profiles, temperature of the teeth and noise of the gear. Then we, compared these results with the running-test results of the MC nylon gear which had already been reported, to clarify as much as possible the operating characteristics of plastic gears filled with glass fibers.
In the first report, the .method of calculating the fatigue strength of plastic gears using strain energy was proposed; however, the conditions only extended to gear life in the normal temperature environment. Recently, the use of plastic gears has expanded, so that they are used frequently in elevated temperature environments. For example, the plastic gears used to drive the drying drum of a paper-making machine perform at the ambient temperature of about 90°C. Owing to these facts, the method of estimating the operation life of plastic gears is proposed in this report. Here, the change of the properties of teeth due to temperature difference was taken into account using the calculation method from the previous report in order to determine the type of teeth which can cope with the plastic gears in the environment of both elevated temperature and normal temperature.
Polyacetal homopolymer is excellent in its wear resistance, mechanical strength, and so on, as is polyacetal copolymer, and it is a plastic which works well in applications to mechanical parts. Also in this plastic, the material in which friction was decreased has been developed, although insufficient evaluation has been made with regard to applications to gears. Because of this, in this study, as for the polyacetal copolymer reported already, gears were made of the polyacetal homopolymer in which friction was decreased by filling them with fluorine-containing resin, and by carrying out the operation test under various conditions. The wear and temperature of the teeth, the change in the tooth profile, the surface roughness of the teeth and so on were examined. The results were compared with the experimental results of the dears made of the polyacetal homopolymer without filling, and its usefulness was examined.
