The "evolution" of China's ultra-precision gear technology

Since the Sino-US trade friction, the chip "stuck" has become a topic discussed by the country, society, and academia, and the precision problem in the manufacturing field is an important part of the "stuck neck".

The breakthrough of China's ultra-precision gear level 1 precision is a typical representative of solving this precision problem, and it is also a typical case of my country's independent mastery of key technologies. Western developed countries have imposed a technological blockade on China. In order to independently master core technologies, Wang Liding, an academician of the Chinese Academy of Sciences, has spent more than 40 years and led his team to successfully develop ultra-precision standard gears with level 1 precision in the international standard for gears since 1999.

This article sorts out the "evolution" process of ultra-precision gears from level 4 to level 2, and level 2 to level 1, enriches the historical data of the development of China's ultra-precision gear technology, and shows the innovative spirit of the older generation of scientists who are brave enough to climb to the top and dare to be the first.

Gears are widely used in the transmission of mechanical equipment.

Among the gear accuracy levels, 6~8 are medium accuracy levels, which can be applied to industrial equipment such as machine tools and automobiles; 3~5 are high accuracy levels, which are mainly used in ultra-precision machine tools, instruments, ships, radars, and aerospace engines, etc., which have high-speed and high-smooth transmission requirements; 1~2 accuracy levels are ultra-precision levels, which are mainly used as national or international gear measuring instrument calibration and accuracy transfer entity benchmarks.

In the 1960s, in order to quickly establish the foundation of socialist industrialization and meet the needs of large-scale national defense engineering construction, gears were gradually mass-produced, and the requirements for gear accuracy became higher and higher.

In order to complete the manufacturing tasks of high-precision defense equipment and respond to the challenges of ultra-precision gear technology blockade, Wang Liding, a Chinese precision machinery and micro-nano machinery expert and academician of the Chinese Academy of Sciences, led the team for more than 40 years to gradually improve the gear accuracy from 7 levels to 1 level (the stage of gear accuracy from 4 precision level to 2 ultra-precision level, and from 2 to 1 ultra-precision level is the most critical period from the formation to the perfection of ultra-precision gear technology), enabling China to catch up from the backward position of ultra-precision gear technology to the world leader.

Ultra-precision gear technology

Evolution from level 4 to level 2

Background

In 1960, a department of the Ministry of National Defense required the Changchun Institute of Optics and Precision Instruments (now the Changchun Institute of Optics, Precision Mechanics and Physics of the Chinese Academy of Sciences, hereinafter referred to as Changguang Institute) to develop a photoelectric theodolite to track our and enemy aircraft and missiles.

This large-scale instrument and equipment contains a large number of gear transmissions, requiring the gear accuracy to reach level 6 or above, but at that time, the gear accuracy processed domestically was mostly level 7. Wang Liding, a research intern at Changguang Institute, participated in this project. He used the Y7131 conical grinding wheel gear grinder to grind and optimize the gears, successfully completing the national defense task, and then developed a level 4 standard gear on the Y7431 gear grinder.

Current gear standards correspond to precision grades

ISO1328-1:1995, ISO1328-2:1997 are international standards; DIN3962-1~3:1978 are German national standards; JISB1702-1:1998, JISB1702-2:1998 are Japanese national standards; ANSI/AGMA2015-1A01, ANSI/AGMA2015-2 A06 are American national standards; GB/T10095.1-2008, GB/T10095.2-2008 are Chinese national standards

In 1965, the country wanted to develop a new type of precision measurement radar, of which the shaft angle encoding data transmission gearbox used in the radar azimuth angle measurement system was developed by Changguang Institute.

Changguang Institute established a special research and development team to be responsible for the design, process and measurement of the gearbox. The research team had to independently complete the development of the gearbox. Wang Liding was a research intern of the group.

Problems and Solutions

The entire process of ultra-precision gear processing can be summarized as installing the original gear blank on the gear grinding machine, selecting and adjusting the machine parameters according to the parameters of the gear to be ground, combining the gear grinding process with manual technical operations, and finally measuring with a measuring instrument, the accuracy must reach the international gear standard level 3 or above.

Gear grinding machine is the core element of ultra-precision gear technology. The research group had two gear grinding machines at the time - a worm wheel gear grinding machine imported from the UK and a domestic machine Y7431. Since the latter can process gears with larger diameters, Wang Liding chose this machine for modification and refinement.

In 1963, Wang Liding first replaced the sliding friction processing spindle of Y7431 with a dense bead rolling shaft system with high precision and small friction torque, which laid an important technical foundation for the subsequent development of coding gears in precision radars and was also a major innovative measure for machine tool refinement.

After the first National Mechanical Transmission Annual Conference, Wang Liding followed his colleagues to Shanghai Machine Tool Factory to investigate the indexing technology in battery equipment. He found that the magnetic disk rotary axis system in the magnetic recording machine they developed was composed of multiple high-precision steel balls, which directly guaranteed the accuracy of the magnetic recording disk.

This technology inspired Wang Liding to solve the problem of machine tool spindle structure design. He added 400 steel balls to the machine tool spindle, which greatly improved the spindle stiffness, ensured that the bearing was not easy to bend, and also promoted the error averaging effect. The spindle accuracy was reduced from the original 2 μm error to 0.5 μm, which is higher than the domestic production level.

Gear grinding machine tools can be divided into three technical modules according to their functionality: grinding wheel system, development system and indexing system.

Wang Liding believes that the indexing accuracy of machine tools is the key to improving gear accuracy, and the indexing disk is the core component of the indexing system, and its accuracy determines the indexing accuracy of the gear.

Structure of large flat wheel gear grinding machine

Wang Liding installed the high-precision polyhedron in the position of the gear of the gear grinding machine, and the indexing plate was installed behind the indexing plate. The precision of the polyhedron was transferred to the indexing plate by the precise shaft system, thereby increasing the indexing plate precision to 10 to 20 arc seconds, but the precision still needs to be further improved.

Wang Liding drew a mathematical model diagram of the indexing plate error, which is shown as a sine curve. He assumed that this sine curve was the sine error caused by the installation eccentricity, and drew an inverse sine curve in the opposite direction, thereby offsetting the actual sine to reduce the error of the ordinate. Adjusting the indexing plate according to the inverse and sine curves greatly improved the precision of the indexing plate. This is the "sine subtraction method" created by Wang Liding.

Afterwards, Wang Liding used a small file to grind the teeth of the indexing plate in turn. After two weeks of grinding day and night, the indexing plate precision reached the processing requirements.

However, ultra-precision gear processing needs to be carried out in a constant temperature, dust-proof and shock-proof experimental environment. The domestic ultra-precision gear laboratory equipment is simple and cannot meet the requirements of ultra-precision gear processing.

Wang Liding asked his assistant to do semi-finishing work during the day, and he did super-finishing in a relatively quiet environment at night. He would open the door of the laboratory at about 5 a.m. every day to allow the gears to cool down quickly, so that the laboratory staff could measure after work during the day.

A year later, Wang Liding and others completed the development task of the coding gear as scheduled. After that, Wang Liding participated in the development of 5 batches of radar coding gears, which were successively loaded on China's most precise radars, and no problems have occurred so far.

However, Wang Liding was not satisfied with the existing scientific research achievements. In order to change the passive situation in the application field of ultra-precision gears in China, he resolutely decided to continue to study and overcome the technical difficulties of ultra-precision gears until he developed a level 1 ultra-precision gear, so that China's ultra-precision gear technology would not only keep up with the pace of developed countries, but also be ahead of the international level.

In order to make the machine tool indexing system fully meet the requirements of developing level 2 precision gears, Wang Liding completely modified the indexing mechanism, added the end tooth automatic indexing mechanism, and equipped the end tooth automatic indexing mechanism on machine tools, especially precision machine tools, for gear grinding. This is a domestic first.

In 1977, Wang Liding modified the end gear indexing mechanism to the Y7413 machine tool. The modified machine tool can stably grind the "small modulus standard gear" with grade 2 accuracy in the West German gear standard DIN3962-1977.

The "small modulus standard gear" with grade 3 and grade 2 accuracy developed by him won the Major Scientific and Technological Achievement Award of the Chinese Academy of Sciences and the National Science Conference Award in 1978.

In addition, Wang Liding developed the "end gear automatic indexing mechanism" from 1976 to 1978 to replace the indexing plate on the traditional gear grinder.

He also conducted research on various modulus gears, among which the "medium modulus benchmark standard gear" won the first prize of the Chinese Academy of Sciences for Scientific and Technological Progress, and the China Institute of Metrology used it as a national gear accuracy physical benchmark.

Ultra-precision gear technology

"Evolution" from grade 2 to grade 1

Background

In 1985, China officially launched the reform of the science and technology system. In the first stage, it took the development of technology markets as a breakthrough, guided scientific and technological work to economic construction, and promoted the close integration of science and technology with the economy.

The National Natural Science Foundation was established in 1986 to promote the development of basic research in natural sciences.

Against this background, Wang Liding applied for the National Natural Science Foundation project with the topic of "Research on the Formation Law of Involute Error and the Optimal Forming Scheme" to develop a Class 1 precision standard gear. "In scientific research, we must compare with the world and contribute to China's scientific cause."

Innovation

Common gear tooth profiles include cycloid, arc, involute and other curves. Involute gears have the advantages of smooth transmission, low vibration, constant output speed and no fluctuation. They are the most widely used gears in modern mechanical devices.

Ultra-precision gears are designed with involute tooth profiles. The key to reducing gear tooth profile errors is to improve the accuracy of machine tool involute cams.

In 1993, Wang Liding not only developed a double-roller involute grinding device, but also further designed and manufactured an involute measuring device, providing calibration instruments for the China Institute of Metrology.

The high-precision involute template developed by Wang Liding can not only be used for machine tools to grind first-level precision gear shapes, but also serve as a benchmark for involute precision transmission of the China Institute of Metrology. This achievement won the third prize of the National Science and Technology Progress Award in 1999.

Through the application of innovative technologies such as dense bead rolling shaft system, "sine subtraction method", end tooth indexing mechanism and involute template grinding and measuring device, the accuracy of the gear grinding machine tool has met the requirements for grinding first-level precision ultra-precision gears, but the grinding of ultra-precision gears still needs innovation in processing technology.

Wang Liding imitated the error law of the indexing plate, rotated the gear 180° and then ground half of the gear, stopped grinding the remaining part of the gear, and increased the gear accuracy from the original 60″ to 45″. He named this gear grinding process "transposition method".

Wang Liding believed that the key to the invention of the "transposition method" was to pay special attention to experimental phenomena. He placed a desk next to the machine tool to facilitate analysis and recording while processing gears. He also listened to the sound of gear grinding to judge the degree of gear processing.

Only by paying special attention to various phenomena and breaking through problems one by one in work practice can we carry out technological innovation and make continuous progress.

In addition, in conjunction with the new process, Wang Liding led the team to successfully develop 5 types of standard 1-level precision standard gears formulated by the International Organization for Standardization (ISO standard), and conducted a scientific and technological achievement appraisal in 2016, with the conclusion that "the precision index has reached the international leading level."

Significance

China is the first country to master the core technology of 1-level precision benchmark standard gears, so its successful research and development has important scientific research value and application prospects.

In terms of scientific research value

The gear grinding machine tool Y7125 and its gear grinding process developed by Wang Liding fill the gap in the manufacturing process of grade 1 precision gears at home and abroad, establish China's leading position in the field of ultra-precision gears in the world, break through the manufacturing quality difficulties of ultra-precision gears, improve the technical level of China's gear manufacturing industry, and lay a solid foundation for China to become a gear manufacturing power;

Grade 1 ultra-precision gears are used as gear precision transmission benchmarks by the China Institute of Metrology and enterprises to identify ordinary precision gears or gear measuring instruments, improve gear detection efficiency, and are suitable for large-scale production;

Ultra-precision gear technology plays a key role in many important fields such as industrial production, aerospace and military equipment, and has an important strategic position.

In terms of application prospects

The high-precision involute template developed by Wang Liding in the 1990s is mainly used to refine involute cams to grind gears, while the benchmark-level gear involute template and gear helix template specifically refer to gear involute and helix templates with grade 1 and above precision, which are used as national-level gear involute and helix value transmission benchmarks.

In addition, as a representative of high-end intelligent manufacturing, industrial robots have a significant increase in production demand, but China's reducers started late and mainly rely on imports, which has greatly restricted the development of China's industrial robots.

Since reducers contain a large number of gear transmissions, precision reducers have extremely high requirements for gear accuracy. Therefore, accelerating the localization of precision reducers has become a major topic in China's future industrial robot development strategy and a major research direction in the field of ultra-precision gear technology.