China issues its first plastic gear industry standard

China's national standardization administration has published the national standard for plastic gear accuracy, GB/T 38192-2019, defining and specifying the allowable values of tooth-flank deviations and radial composite deviations for injection-molded plastic cylindrical gears.

Dr. Shi Zhaoyao, head of the drafting group, said the standard fills a gap, has been reviewed by the national gear standardization technical committee, reaches advanced international level and answers a decades-old wish of the plastic gear industry.

Standards are what a mature market needs. Gears are among the most widely used mechanical transmissions, with a long history. Since nylon was developed in 1935, polymer synthetics have advanced rapidly and a new material entered gear making: plastic gears.

Compared with metal gears, plastic gears are light, have low inertia, run quietly and are self-lubricating. They are molded in tools, which is efficient and inexpensive, and after decades of development are used in automotive electrical systems, instruments, service robots, home appliances, office automation and smart home devices.

Especially in the last twenty years, as high-strength, wear-resistant engineering plastics were developed, plastic gears have moved toward higher loads and wider application, replacing steel in many fields. Their rapid growth is a worldwide trend.

Yet China long lacked a plastic gear standard, above all the basic accuracy standard, which held the industry back and left China importing large quantities of high-end plastic gears.

Breaking through requires work on gear design, plastic materials, injection mold design and making, new machines and processes, and gear testing — all of which depend on a standardization system. Because plastic gears differ greatly from metal gears in design, manufacturing and application, drafting their own accuracy standard is far from simple.

The standard gives a basis for measurement and grade classification, helping to ensure quality and reliability, guide orderly competition, support innovation and raise the technical level of the industry. It also helps companies widen purchasing channels, reduce dependence on imports, save time and cut purchasing and maintenance costs.

In drafting, the team adopted the latest ISO accuracy system — terminology, grade concepts and tolerance formulas — and combined it with China's mature technology and experience to create a Chinese system for calculating, measuring and evaluating injection-molded gear accuracy.

Injection-molded gears are special: small module, teeth formed in one shot by the mold, ribbed structures. Their accuracy and measuring methods therefore cannot simply follow ISO 1328 and ISO/TR 10064. The Chinese standard is not equivalent to ISO but stays consistent with it at the macro level, as close to international practice as possible, and was drafted after studying ISO 1328 and Japanese, American and German standards together with domestic industry conditions.

Wang Zhigang, secretary general of the national gear standardization technical committee, said the work was far from easy, from project approval in 2015 to completion, and is a landmark summary of China's plastic gear industry. He hopes to see more innovation in small-module plastic gears and, next, progress in standardizing strength and life testing.