The main bearing for China's self-researched 3m diameter shield machine, successfully used in the Shenyang Metro project.

The shield machine, known as the "king of the world's construction machinery", is a powerful tool for tunnelling and is used in subways, high-speed railways, water conservancy and other infrastructure projects.

As a major infrastructure country, although China has achieved the localization of shield machines, but the core components of shield machines - the main bearings have long relied on imports. Excitingly, this status quo will be reversed.

Recently, the scientific research team of the Institute of Metals of the Chinese Academy of Sciences led the research and development of the main bearing for the 8m diameter super-large shield machine successfully. This is the first set of China's largest diameter, the largest single weight shield machine with the main bearing, through the full localization of the super-large shield machine and independent controlled manufacturing of the "last kilometre". It is understood that the 41-tonne main bearing will be installed in a 16-metre diameter super-large shield machine for tunneling.

So, what is the difficulty in designing and manufacturing the main bearing, which is described as the "heart" of the shield machine? How was the key core technology overcome? This reporter has recently interviewed the comrades in charge of the R&D team of the Institute of Metals, Chinese Academy of Sciences.

 

Researchers and processors exchange details of the reaming process for large sleeve forgings on site.

 

The scene of a large collar ring tie. (Photo by Institute of Metals, Chinese Academy of Sciences)

 

High-end bearing long-term dependence on imports is the pain point of China's bearing industry

Shield machine by the main bearing "stuck in the neck"

For many mechanical devices, bearings are a very critical presence. We are surrounded by common bicycles, cars, high-speed rail, wind turbines, aviation engines, and so on, where the need to rotate, are inseparable from this core component. Its main function is to support the mechanical rotating body, reduce the coefficient of friction in the process of movement, and to ensure the accuracy of rotation.

Take the shield machine as an example. The main bearing is the core key component of the shield's blade drive system and is known as the "heart" of the shield. During the tunneling process of the shield machine, the main bearing "holds" the rotating blade and provides rotational support for the blade. If the shield is compared to a shovel being dug, then the main bearing is equivalent to the hands that control it.

Li Dengzhong, a researcher at the Institute of Metals, Chinese Academy of Sciences, specialises in metal materials and is also the head of the strategic pioneering science and technology project "Autonomous Controlled Manufacturing of High-end Bearings". As a professional, he knows the importance of bearings to a country's industry, especially the shield machine main bearings, marine engineering bearings, high-speed railway bearings, wind turbine bearings and other high-end bearings. "It can be said that high-end bearings are related to national economic security and represent the manufacturing level of basic components of a country." Li Dengzhong told reporters.

However, high-end bearing long-term dependence on imports is the pain point of China's bearing industry, but also China's industry is "stuck" one of the blockage.

It is understood that the 15.03m super large diameter shield machine "Zhenxing" used for Nanjing and Yan Road river crossing, the 16.07m super large diameter shield machine "Canal" used for Beijing East Sixth Ring Road, the Chinese Xingye Express Line used for Zhuhai tunneling The localization rate of a large number of shield machines has reached 98%, such as the first oversized synchronous tunneling machine "Xingye" and the 16.09-meter diameter shield machine "Ju Li No.1" used for tunneling the Jingjiang River Crossing in Jiangyin, but the main bearings are still imported.

"By importing, we not only can't buy the best bearings, but are also constrained in terms of technical service, delivery cycle or price." Li Dianzhong said, this is also related to the field of scientific researcher's heart "pain".

Why China can not produce their own high-end bearings?

"The key lies in the manufacture of bearing materials and large roller processing accuracy is not pass, the whole process technology chain is not coherent." Li Dengzhong told reporters that this is precisely the shield machine with a large diameter main bearing independent controlled manufacturing for a long time but not into the main reason.

According to reports, large shield machine in the boring process, only forward, not backward, once the main bearing failure, will cause serious losses, light in the boring area to remove the ground structures, newly constructed receiving well "plan out" shield machine after replacing the main bearing; heavy lead to the construction of the tunnel is null and void, shield machine forever buried underground. As a result, the main bearings have to have a very high load carrying capacity and reliability. This places high demands on the bearing steel used in the manufacture of main bearings for shield machines, as well as on the design, machining accuracy and lubricating grease of the complete set of main bearings.

In 2020, the Chinese Academy of Sciences (CAS) launched a strategic pioneering science and technology project on "Independent and Controlled Manufacturing of High-end Bearings", in which seven CAS-affiliated research units, including the Institute of Metals of CAS and Lanzhou Institute of Chemical Physics, formed an established team and joined hands with CCCC Tianhe of China Communications Construction Group Co. Ltd. of China Communications Construction Group Co., Ltd. and other 20 units to carry out collaborative research and solve 12 key core technology problems in the preparation of main bearing materials, precision processing and complete set design, and developed a total of 10 sets of main bearings for shield machines with diameters ranging from 3m to 8m, which can be used in shield machines of 6m to 16m, breaking the "last kilometer" of independent and controllable manufacturing of shield machines. kilometre".

"The reality tells us that we still have to make our own high-end bearings, and we have to get out of our own technical route." Li Dengzhong told reporters, "If you only rely on copying foreign technology route, not only easy to 'like the shape of God is not like', and always only follow others run."

High purity, high homogeneity, high toughness, high wear resistance

Low oxygen rare earth steel "point stone into gold"

High-end bearing "neck" problem, the root cause in the material, the difficulty is also in the material.

 

China's own research on the 8m diameter main bearing for super-large shield machines.

 

It is understood that the main bearing material of the shield machine mainly includes bearing steel, lubricating oil, cage and so on. Among them, the most central issue is the bearing steel material.

So, what is the standard of good steel? Hu Xiaoqiang, a researcher at the Institute of Metals of the Chinese Academy of Sciences and chief engineer of shield machine main bearing technology, gave the reporter a list of several key words: high purity, high homogeneity, high toughness, high wear resistance.

High purity refers to the high purity of steel, less impurities; high homogeneity refers to the composition of different parts of the parts, hardness, etc. relatively uniform; high toughness refers to good strength and toughness, high load-bearing capacity and stability; high wear resistance refers to the use of bearing loss is small, long service cycle.

"Like the diameter of 8 meters of the main bearing in the operation process to carry the maximum axial force of up to 105 kN, if an adult Asian elephant weighs 4 tons, this axial force is equivalent to the gravity of 2500 adult Asian elephants." Hu Xiaoqiang further explained.

It was clear that ordinary steel would not be able to meet this and a special steel that could meet the above criteria needed to be found. Li Dengzhong thought of rare earths.

"There is a lot of research showing that adding trace amounts of rare earths to steel can significantly improve the toughness and plasticity, wear resistance, heat resistance and corrosion resistance of steel." Li told reporters that in the industrial field, rare earths are known as "industrial vitamin", and rare earths happen to be China's advantageous resources.

 

Researchers discuss the microstructure of the material.

However, rare-earth steel in industrial production encountered two major problems: First, the process does not go smoothly, there is a serious blockage of the gate problem; Second, the rare earth added in the steel, steel performance fluctuates dramatically, there is a problem of poor stability. This is also the cause of China's rare earth steel research and application once from hot to cold reasons.

Li Dengzhong they certainly faced the same problem. "We tried commercial rare earths of various purity, but the rare earth steel produced in combination with steel was still unstable in performance." To find out why, Li recalls, he and his team members travelled almost all over the country's rare earth production areas and went to production sites to observe the rare earth smelting process.

Finally, on run after run, they discovered the key to the problem: it turned out that there was a conceptual deviation between the rare earths made by the processing plant and the rare earths he needed.

"The manufacturer, in order to make the rare earth more pure, separated out the iron and carbon elements, which are indispensable ingredients for steel making, and in turn ignored the oxygen elements, etc., which should be removed." Li Dianzhong told reporters that after a lot of research, they found that the root cause of rare earth steel performance fluctuations and gate clogging problems lies in the oxygen content, "not only the oxygen content in the steel affects the performance of rare earth steel, the oxygen in the rare earth and the inclusions produced by the oxygen in the rare earth also have a great impact on the performance of rare earth steel."

After extensive experiments, calculations and characterisation, they revealed the main mechanism of the role of rare earths in steel and prepared low-oxygen rare earth steels with superior performance and stability by controlling the oxygen content.

The secret of this key technology is to control both the oxygen content of the steel and the oxygen content of the added rare earths, hence the term "double low oxygen".

"Among the 12 key core technologies, the successful development of low-oxygen rare-earth steel is the key to the key." Li told reporters, 1 tonne of steel only need to add about 100 grams of rare earths, the cost only increased by a dozen dollars, but fatigue life can be increased by an order of magnitude. Compared with the bearing steel without rare earth, the rare earth bearing steel tensile fatigue life increased by more than 40 times, rolling contact fatigue life increased by 40%, effectively solving the bottleneck problem in the industrial application of rare earth steel.

In this rare-earth steel technology battle, the researchers "point stone into gold". By the relevant departments of the project, the Institute of Metals of the Chinese Academy of Sciences led the development of a number of rare earth steel industry standards, and gradually promote the application. At present, the comprehensive mechanical properties of rare-earth bearing steel produced by the relevant cooperative enterprises are better than imported products.

Breakthrough in roller precision machining technology

The service life of the 8m diameter main bearing exceeds 10,000 hours

According to the introduction, the shield machine main bearing is usually composed of collars, cages, rollers and other components. "The composition is not complicated, but the process is very complex." Hu Xiaoqiang told reporters, to turn high-end materials into high-end bearings, need to go through forging, machining, heat treatment and other more than 100 processes, any one link is not done well, the end will lead to the service life of the bearing is not long, performance out of control, "to make a good bearing, every link can not fall off the chain."

Roller, is the shield machine main bearing operation to bear the load of the key components, but also is the weakest parts of the large roller bearings. High-end bearings require extremely high machining accuracy for rollers.

"Rollers are the largest number of components in a bearing, including three types of radial rollers, main push rollers and secondary push rollers." Hu Xiaoqiang said that there are more than 400 rollers in a 3m diameter main bearing for shield machines, and even more rollers in an 8m main bearing, with nearly 1,000. Its manufacturing quality has a great impact on the working performance of the bearing and is the main factor affecting its service life. "The diameter error, surface finish and other indicators of these hundreds of rollers have to be controlled within 1 micron, and exceeding this value will lead to deviations in the shield's operation."

Hu Xiaoqiang had worked with team members to do research and analysis on the quality and production of the rollers, and found that the accuracy of the rollers in the imported main bearings was very high, both in terms of roughness, hardness uniformity, and the working surface plain line were very good, but domestic due to equipment limitations, large roller processing accuracy can only reach two, can not achieve a level of precision processing.

Deep into the production line, and the backbone of the joint brainstorming ...... Hu Xiaoqiang together with team members successfully overcome the main bearing high-precision processing and accuracy retention problems, the development of more than 100 mm diameter of the first-class rollers, so that China's bearing industry breakthrough of a large roller precision machining technology.

This is just one of the examples. This is how, with the support of the pioneering special project, CAS Institute of Metals integrated 12 teams within the Institute for bearing steel, heat treatment and cage, and brought together the strength of seven CAS institutes to form a full chain team covering the whole life cycle of R&D, materials, manufacturing, evaluation and service, achieving innovation from 0 to 1 and from materials to components.

"At first, we had to run through almost half of China to make a good bearing." For example, forging was in Guangdong, turning in Shandong, heat treatment in Liaoning, grinding in Zhejiang, assembly in Heilongjiang and Zhejiang, and field testing of the bearing had to return to Guangdong, Li Dengzhong said. Now, the whole process of processing and manufacturing, assembly and commissioning, testing and evaluation is independent and controllable "one-stop" service.

The development of the bearings took three years. The research team used 1,467.4 tonnes of rare earth bearing steel to develop 41 large rings, 7,996 rollers and 492 sections of copper and steel composite cages, with 369,000 welded seams alone. Eventually, domestically produced shield machine main bearings covering diameters from 3m class to 8m class were born one by one.

At present, the 3m diameter main bearings for shield machines have been successfully applied in the Shenyang Metro project. The 8m diameter main bearings for super large shield machines have also been delivered to the shield machine manufacturer in question, and "will have a service life of more than 10,000 hours and a continuous digging length of more than 10km." Li told reporters that the next goal of the research team is to create main bearings for shield machines with a service life of more than 15,000 hours and a digging length of more than 15 kilometres, and to develop main bearings for shield machines of different diameters according to demand.

Through the technology chain, create innovation chain, docking industry chain ...... This is a vivid practice of China's scientific and technological self-reliance and support to ensure the development of the whole life cycle of the industry, but also for China's high-end basic components to provide a good model of research.

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