Hiroshi Mimura

Operating Characteristics of Gear by Low Elastictity Polyacetal

The demands for noise reduction of plastic gears in OA machines, etc. were increased recently because of the strong demands of lowering noise level in environment. The usual way to reduce the noise of polyacetal gears, mainly used as plastic gears, was dependent on lubricant additives only. We newly developed low elastic polyacetal by modifying the molecule chain itself with adding lubricant. In this study, we investigated the characteristics of the gears using the new polyacetal above mentioned in operation on various conditions, and found out the profitability in characteristics of those gears by comparing with the gears made by Nylon 12 (PA 12), well known as a suitable plastic material for low noise gears.

Appraisal of Polybutylene Terephthalate Used as the Gear Material.

Polybutylene terephthalate (PBT) has slightly lower mechanical strength than polyacetal (POM), however, with its minimal water absorption and small friction coefficient this plastic is considered attractive for use. However, the fact that its elongation is large is a problem. Since a plastic gear having a large elongation is deformed by applying a large force to the gear and its tooth from is changed by it, abnormal wear may be generated in the gear. Thus, PBT has not been considered the best gear material. In this research, using gears manufactured with PBT, investigations on its fatigue strength as well as its sliding properties such as gear wear and change in tooth form were carried out using a running test. Furthermore, by comparing the results with those of POM gear, the use of PBT as a gear material was evaluated.

Noise Comparison of Polyacetal Gears Based on Differences of Gear Processing Methods

The mass production of plastic gears by injection molding has many advantages ; however, hobbing has been used in cases where injection molding could not be applied due to economical problems of mold shrinkage in the small-volume production of a wide variety of gears and technical problems in the processing of large-sized thick gears. It has been said that there are differences in the magnitude of noises between injection-molded plastic gears and hobbed plastic gears causing a difference in acoustic characteristics based on the difference of crystallinity of their gear surfaces. In this study, by using a polyacetal gear, and by measuring noises of both gears at each point of time when the properties of their gear surfaces were changed by the wear during a long period of operation, the difference of noises based on the difference of plastic gear processing methods was verified quantitatively based on experimental results.

Water Lubrication characteristics of Polyacetal Gears Filled with Carbon Fiber.

For a polyacetal with high strength and rigidity, we should select a composite polyacetal filled with, for example, carbon fiber. However, this may not be true in the case when this kind of composite polyacetal is used in water. That is, it is considered that the sliding characteristics might decrease because water enters into the bonding space of carbon fibers and polyacetal and of carbon fibers with each other. In this study, operation tests of a polyacetal gear filled with carbon fiber in water and with water drops were examined and the wear of the gear and the change of the tooth profile were determined. By comparing these results with those of an unfilled polyacetal gear, we investigated the usefulness of polyacetal gears filled with carbon fiber in water or steamy environments.