Engineering plastics are widely utilized in parts of mechanisms—such as gears, bearings, cams, rollers for sliding doors, and other types of rollers—of various equipment such as audio visual systems, office automation equipment, consumer electronics, automobile parts, precision machinery components, toys, and stationery. Most engineering plastics are made from crystalline resins, which exhibit superior resistance to wear and fatigue. Polyoxymethylene (POM) accounts for the majority of these plastics made from crystalline resins.¹⁾ In comparison to other engineering plastics in the same price range, such as polyamide (PA) and polybutylene terephthalate (PBT), POM offers both higher strength and greater rigidity in the 60 to 80℃ (from 140°F to 176°F) temperature range, which is typical for specifications of automobile parts and office automation equipment. This allows it to be used unfilled. Being an unfilled material enables the manufacture of components that exhibit lower wear and higher precision, making it arguably the best choice of material for use in parts of mechanisms.

Although Polyplastics is a manufacturer of materials, since our founding our offerings to the market have included proposals on design technology, in addition to materials. As a result, our DURACON^® POM maintains a greater than 60% share of the Japanese market for POM resins.

Supporting design technology for plastic gears—which is the theme of this paper—is one such offering. There is a tendency to believe that design technology falls outside the scope of work of a materials manufacturer. However, as an expert in engineering plastics, Polyplastics has contributed to improvements in the quality of products across a wide variety of fields, through our design technology solutions.

For example, during the time at the end of the 20th century, when development of video tape recorders (VTRs) was tremendously popular in Japan, all manufacturers of home electronics were grappling with the issue of how to reduce noise from plastic gears. In order to achieve these noise reductions, manufacturers need to properly understand the particular ways in which plastics behave differently from metals. We take pride in the fact that our design technology support made a major contribution to plastic gear noise reductions achieved during this period. We have now developed these technologies even further, taking them beyond the scope of home electronics products such as VTRs and applying them to every conceivable field.

Here we will introduce some of our plastic gear noise reduction technologies.

While it cannot reveal all of the causes of gear noise, it is important to perform frequency analysis to obtain the minimum level of information needed to investigate the causes behind occurring noises. Figure 1 shows the frequency spectrum of noise generated when two POM gears (standard unfilled POM copolymer) mesh during operation. In this experiment, the gear noise is comprised of the three types of sound shown in the figure. B and C are the natural frequencies of the equipment and the plastic gears, and these are generally low levels of noise in most cases. The important one is A, which is typically known as gear mesh

There are three typical causes of mesh frequency in plastic gears.

        (1) Repeated changes in the number of pairs of teeth meshing with each other

        (2) Reversal of friction vector for pitch points

        (3) Pitch compatibility with the mating gear

(1) is a general idea that also applies to metal gears, but (2) and (3) are causes that are particular to plastics. Below are brief descriptions of these causes and the ways to counteract them.

There is a parameter for gears called contact ratio, and for a standard spur gear this ratio is above one but less than two. Figure 2 shows that as the gears turn, they alternate between meshing with one pair of teeth and two pairs of teeth. Since there is a difference in the extent to which the teeth deform depending on whether torque is absorbed by one or two pairs of meshing teeth, the deformation fluctuates cyclically as each tooth meshes, as shown in Figure 3. Gear mesh frequency occurs as a consequence. This can be counteracted by using materials with higher stiffness. However, in the case of plastic, this almost always has the reverse effect. The reason is that plastic gears are normally filled with glass fibers, and resulting factors caused by shrinkage anisotropy, such as reduced precision and increased surface roughness, lead to an increase in noise. Conversely, there is also the approach of using extremely soft resins to promote deformation so that there are always two pairs of teeth meshing, but this approach poses limitations in terms of strength. Therefore, typical designs adopt a contact ratio of at least 2, alternating between two and three pairs of meshing teeth. Helical gears and high-tooth gears are more specific examples of such methods, and we encourage you to refer to technical literature on gears to learn more about these.

The torque of gears is transferred in a vertical direction relative to the surface of the teeth. This direction of transmission of the force is typically called the normal line, and the ideal scenario is for the direction of transmission of the force associated with the rotation of the gears to remain constant. However, in reality, the sliding friction of the teeth causes the direction of transmission of the force to change constantly. The teeth contact each other with curved surfaces, and though at a glance it appears they are rolling, the teeth are actually "approaching" each other diametrically as they start to mesh, after which they finish meshing by "receding" diametrically. This brings about sliding. The moment that this "approaching" changes to "receding" is when the direction of sliding friction reverses. The relation to the normal line is as shown in figure 4; the direction of synthetic driving force of the normal line and friction vector instantaneously changes each time one tooth meshes with another. In these cases, using materials with a low coefficient of friction is an effective measure against noise, as reducing frictional force allows changes in synthetic driving force along the normal line to be minimized. DURACON M90-44 is the most widely-used grade in the manufacture of plastic gears in Japan. However, DURACON NW-02-which was developed for use in gears-offers exceptional gear noise improvements (figure 5). It also has a track record of approximately 20 years of use in a wide variety of applications, particularly in devices where noise is a major consideration, such as copiers, laser printers, and inkjet printers. DURACON LW-02 is a material that achieves further improvements in this area, offering noise reductions of over 10 dB compared to DURACON M90-44, as shown in figure 5.

Ideally, it would be possible to accurately calculate the level of shrinkage that injection-molded gears experience during the molding process. However, there are virtually no gears for which this is perfectly estimated. Unlike metal gears, which exhibit less shrinkage, there are many cases with plastic gears where the normal pitch-a factor that is critical to gear meshing-is not as it was designed. As figure 6 shows, the normal pitch of the driving gear and the driven gear must be equal for them to mesh correctly. Normal pitch is expressed by the following formula.

Normal pitch = mΠcosα (Formula 1)

m: module   Π: ratio of a circle's circumference to its diameter   α: pressure angle

Noise reduction for plastic gears requires methods that take into account the particular ways in which plastics behave. As a leading manufacturer of POM, Polyplastics not only offers materials solutions, but also design solutions for noise reduction. We also actively offer comprehensive solutions that include gearbox materials and design. We are continuously developing new technologies that we are confident will contribute to improving the quality of our entire range of products.

5th September 2017