Polyplastics Co., Ltd., a leading global supplier of engineering thermoplastics, offers a range of design tips and techniques to help manufacturers reduce noise in helical gears made of polyoxymethylene (POM). Polyplastics is a leading global manufacturer of POM, under the DURACON (R) tradename, for diverse markets, including gears.
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Among the design findings, it is generally believed that the tilt of the gear axis is an important consideration and its elimination will have a positive impact on noise issues. Polyplastics tried to verify that assumption, and actually focused on the skewed shaft angle (offset error). The company learned that noise does not necessarily worsen when the axis tilts.
Instead, Polyplastics found that noise issues arise when changing the center distance, depending on whether there is a skewed shaft angle. Polyplastics examined the impact of the center distance for helical gears with a skewed shaft angle of 0 deg. Noise levels are lowest in the area where an increase in the center distance is between 0 and 0.6 mm, and then flattens out. On the other hand, noise increases beyond 0.6 mm because the transverse contact ratio is less than 1.0.
Polyplastics notes that helical gears have a phenomenon called an overlap ratio. The total contact ratio is the sum of a transverse contact ratio and an overlap ratio, and theoretically the total contact ratio must be 1.0 or higher for rotation at constant speed on the driven side. A transverse contact ratio less than 1 means that the length of the path of contact is less than one pitch. In other words, there is a space in which teeth do not touch within the space of one pitch. Findings show that helical gears will have a total contact ratio of 1 or higher and will keep operating at constant speed without noise issues.
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