Many domestic universities and scientific research institutions are making efforts to research, so the research and development of ceramic bearings is imperative.

ABSTRACT：The present situation and future prospect of study on advanced ceramic bearing at domestic and abroad has been summarized. Preparing and machining technology for ceramic balls and ferrules has been expatiated. The direction to be r e searched on ceramic bearing is also presented in the paper.

KEY WORDS: ceramic bearing, rolling bearing, ceramic material, machining center

1. Necessity and urgency of developing new ceramic bearings

With the development of mechanical industry towards high precision, high efficiency and high automation, the working speed of various machines has been continuously improved. For example, the spindle speed of CNC machining center has increased from 5000 rpm in the 1980s to 25000 ~ 30000 RPM now. The DN value of high speed (the product of bearing diameter and speed) has reached 3 million, and the test value has reached 3.5-4 million. Machinery working in some high-tech fields and in some special environments, such as aerospace industry, nuclear energy industry, chemical industry, petroleum industry and food industry, need to work in special environments such as high temperature, high speed, corrosion resistance, vacuum, non-magnetic, oil-free lubrication and light weight. The current steel bearings cannot meet these requirements. The main problems of high-speed bearing are the decline of bearing life, the increase of temperature rise, the decline of stiffness and accuracy caused by high-speed, and the rolling ball will cause a great centrifugal force. The pressure between the ball and the outer race raceway caused by this centrifugal force even exceeds the effect of external load, so as to reduce the service life of the bearing. At the same time, if the speed is too high, a large gyroscopic torque will act on the ball, causing the ball to slide with the ferrule, increasing the friction torque, increasing the heat and generating additional pressure on the cage, resulting in large temperature rise and damage to the cage. Therefore, the needs of high-speed machinery, high-temperature corrosion resistance and other special working environments cannot be met only by improving the structure and lubrication conditions of the current steel bearings. We must make a fundamental breakthrough and innovation in ceramic materials and research and develop new materials of bearings in order to meet the requirements of the development of high-tech and industrial production.

As a high-tech material, modern ceramic engineering is also developing rapidly. It has a series of advantages such as wear resistance, high temperature resistance, corrosion resistance, low non-magnetic density (only about 40% of the bearing steel), small thermal expansion coefficient (25% of the bearing steel) and large elastic coefficient (1.5 times of the bearing steel). Ceramic bearings made of ceramic materials are very suitable for working in special environments such as high speed, high temperature and corrosion resistance. Because of its low density, using ceramic ball as rolling element can greatly reduce the centrifugal force and gyro torque at high speed, so as to greatly reduce the pressure and friction torque on the outer ring of the bearing and improve the service life of the bearing. The experimental research shows that the service life of hybrid ceramic bearing can be increased by 3 ~ 6 times and the temperature rise can be reduced by 35% ~ 60% compared with the steel bearing with the same specification and accuracy grade. There are two kinds of ceramic bearings: hybrid ceramic bearing and all ceramic bearing. The former is made of ceramic materials as the ball or column of rolling element, while the inner and outer rings are still made of bearing steel; Or both the rolling element and the inner ring are made of ceramic materials, while the outer ring is still made of bearing steel; The all ceramic bearing is made of ceramic with rolling elements and inner and outer rings. At present, the high-speed ceramic bearings developed and applied internationally are mainly hybrid, that is, the ball or column is made of ceramic materials, and the inner and outer rings are still made of bearing steel. Because the ceramic material is brittle and easy to be damaged, and the reliability of making inner and outer rings is poor, the all ceramic bearing has not yet reached the practical stage.

2. Overview of ceramic bearing research at home and abroad

Internationally, since the United States successfully developed ceramic bearings in 1972, Japan and Germany have caught up and developed rapidly. Ceramic bearings have made remarkable achievements from scratch, from experimental research to industrial production, from high confidentiality to public sales. This is a high-tech product of the intersection of mechanical manufacturing technology and material science. At the 16th Japan International Machine Tool Expo in Tokyo in 1992, ceramic rolling bearing series physical products were publicly displayed and accepted orders from users. Some machine tool manufacturers also exhibited machining centers and ultra-precision machine tools with ceramic bearings for high-speed cutting. The application of ceramic bearings in machine tools is developing fastest in Japan. In recent years, mc56-5xa machining center (15000 RPM) produced by Muye company, mv-65a / 40 machining center (25000 RPM) produced by Mori Seiji company, uhs10 machining center (100000 RPM) produced by Niigata iron company, HPM ultra precision lathe (150! 150mm) produced by kyocers company in Japan and other spindle components have adopted hybrid ceramic rolling bearings. Fag and SKF produced a series of spindle units 1 with ceramic bearings. The service temperature of ceramic rolling bearings successfully developed in the United States has reached 800 ∀ and the DN value of mixed ceramic bearings in Japan has reached 2.5 million and that of all ceramic bearings has reached 2.12 million. At present, in addition to the application of ceramic bearings in the main bearings of precision machine tools, ceramic bearings are also used in oxygen-fired turbine engines, vacuum pumps and other machinery in the world. However, mixed ceramic bearings (Si3N4 ceramics for balls or columns) are mainly used. All ceramic bearings are still in the experimental stage and have not been applied in practice.

After 1990, China carried out basic research on ceramic bearings in some universities, such as Guangdong Institute of technology, Northeast University, Shenyang Institute of construction and engineering, Northwest University of technology and Tianjin University, and carried out research on precision machining of ceramic balls. Harbin University of technology carried out research and development and performance test of hybrid ceramic bearings (made of Si3N4 balls and supplied by Shandong Zibo Ceramic Research Institute), Guangdong Institute of technology and Luoyang Bearing Research Institute cooperate in the research and development of ceramic bearings. In short, the research of ceramic bearings started late in China. At present, the mixed ceramic bearings are still in the experimental research stage, and no products have been successfully developed. In addition to doing some basic research, all ceramic bearings have not carried out any experimental work.

From the situation of the mixed ceramic bearing that has been applied internationally, the effect is very significant. The most prominent is that it solves the problem of developing new machine tools that can process at high speed and ultra-high precision, and the service life of the bearing is 3 ~ 6 times longer than that of the steel bearing. At present, the price ratio of hybrid ceramic bearing to steel bearing is reduced from 10:1 to 2.5:1.

3. Preparation and processing technology of ceramic balls

3.1 Preparation technology of ceramic balls

At present, the domestic ceramic balls mainly include Si3N4 ceramic balls, ZrO2 ceramic balls and Al2O3 ceramic balls. The units that can prepare ceramic ball blanks are mainly concentrated in scientific research institutions, such as Shandong industrial ceramics research and Design Institute and Luoyang Bearing Research Institute. At present, they are still in the stage of small batch production in the laboratory and have not yet formed industrialized mass production. In the process of preparing ceramic ball blank, in order to improve the shape accuracy, dimensional accuracy and high density of ceramic ball blank, the method of hot isostatic pressing is generally adopted, that is, the blank is initially densified by transferring pressure with high-pressure gas to form ceramic ball blank, and then sintered at high temperature in high-pressure nitrogen, argon or hydrogen; Or cold isostatic pressing machine is used to carry out cold isostatic pressing on ceramic powder first, and then pressureless sintering.

3.2 Processing technology of ceramic balls

At present, the demand for ceramic balls is very small, so batch processing has not been formed. The units conducting ceramic ball research have the ability to process ceramic balls in small batches, and there are few units that can actually process ceramic balls in batch. For example, Shandong Wulian steel ball factory has jointly built a ceramic ball finishing production workshop with Taiwan Province, which has the ability to supply high-precision (G5, G10, G16, G20, etc.) ceramic balls in batch, However, due to the small domestic demand for ceramic balls, the production workshop is basically in the state of shutdown. Because its minimum processing capacity is 30000 ceramic balls at one time, and the enterprise is unwilling to stock, it cannot meet the requirements of small purchase volume of most users.

The processing method and equipment of general ceramic ball are similar to that of steel ball blank, and the difference lies in the difference between abrasive tools and abrasives. Because the shape and size errors of ceramic ball blank are large and there are many surface defects, the processing of ceramic ball generally goes through two stages: grinding and grinding. The grinding is completed on the ball grinding machine, and SiC grinding wheel or diamond grinding wheel can be used. The precision of Si3N4 ceramic ball after grinding is: the diameter variation and spherical deviation are 70-80 m, and the surface roughness is ra2-3um. In addition, the ceramic ball can be ground directly on the ordinary ball grinding machine, but the grinding abrasive is different from the steel ball. The grinding is divided into rough grinding, semi fine grinding, fine grinding and super fine grinding. The abrasive used in rough grinding and fine grinding, its particle size and the surface roughness that can be achieved are listed in Table 1.

The liquid guide can generally be mixed with petroleum, grease, engine oil, wax, latex water and alcohol in a certain proportion, with a certain viscosity, and then mixed with abrasive to form grinding liquid. After grinding, the accuracy of Si3N4 ceramic ball is: the diameter variation and spherical deviation are 0.04-0.08um, and the surface roughness is Ra0 004-0.008um, which has reached and exceeded the quality standard of grade 3 ball.

                     Table 1 Selection of abrasives for grinding Si3N4 ceramic balls

The V-groove grinding method has been experimentally studied by units such as northeast university and Northwest University of technology in China. The disadvantages of this method are low grinding efficiency and slow spherical error correction. Based on the traditional V-groove grinding method, Kanazawa University of Japan has designed a spin return angle control grinding method, which is characterized by controlling the spin angle of the ball, enhancing the spin motion of the ball, and improving the sphericity and submergence efficiency. Subsequently, Japan developed a new method for grinding ceramic balls - magnetic fluid grinding method, which has high grinding efficiency, about 50 times that of ordinary V-groove grinding method. In 1997, based on the research at home and abroad, Northeast University tested a new grinding method of ceramic ball, namely conical grinding method, and systematically studied the grinding mechanism, grinding technology and accuracy of ceramic ball.

4. Preparation and processing technology of ceramic ferrule

4.1 Preparation technology of ceramic ferrule

At present, the ceramic rolling bearing ring is mainly made of Si3N4 ceramic, and its preparation technology is much more complex than that of ceramic ball. The better preparation method is to press the ceramic powder into the green bearing ring with certain strength and hardness by using the cold isostatic press and mold of certain shape, then use the lathe to process the raceway and chamfer on the inner and outer bearing rings, and finally carry out pressureless sintering to obtain the required ceramic bearing ring blank, For further finishing. Domestic research in this field started late, and there is no ceramic bearing ring that can be prepared and actually used by this production process. At present, Shandong University of technology is developing a new type of ceramic material for bearing by taking advantage of the technical advantages of developing ceramic cutting tools for many years and advanced equipment foundation (such as cold static press and hot pressing sintering furnace), so as to overcome the shortcomings of low strength and poor fatigue resistance of ceramic bearing ring. At present, most ceramic bearing rings are prepared by hot pressing sintering process. The disadvantage of this method is that the raceway and chamfering cannot be machined in advance and are processed by grinding. In this way, the production cost is high and the processing is difficult.

4.2 Processing technology of ceramic ferrule

The processing technology of ceramic bearing ring is very different from that of steel bearing ring. Since the second issue of China Ceramic Industry in 1999, ceramic bearings are generally only used in high-speed precision parts and mechanical equipment working in special environment (such as high temperature, corrosion resistance, light weight, non-magnetic, etc.), so their production batch cannot be large. Therefore, the grinding of ceramic rings is generally carried out on high-stiffness cylindrical or cylindrical grinding machines. The processes are divided into rough grinding, fine grinding, grinding and polishing. The coarse grinding particle size is 120 / 140, and the fine grinding is 270 / 400, the grinding is carried out on the grinder with fine-grained fine grinding pieces. The polishing operation generally adopts soft polisher and diamond powder with fine grinding particles (4um). After polishing, the surface roughness of ceramic ferrule can reach Ra0 04-0.1um. Moreover, the ceramic ferrule is generally non-conductive and cannot be machined electrically; No magnetic conduction, magnetic fixture cannot be used. Therefore, generally, the pressure disc is used for mechanical clamping during rough grinding, and the expanded tire mandrel is used for hydraulic clamping during fine grinding. Generally, diamond grinding wheel should be used, and the diamond grinding wheel with ceramic binder has the best grinding effect. In order to achieve low surface roughness (ra0.1um), diamond honing is sometimes used.

5. Outlook

Although China's research on ceramic bearings started late, and the current research is still in the experimental stage, especially the research on all ceramic bearings has just begun, it has attracted the great attention of our government. Many domestic universities and scientific research institutions are making efforts to research, so the research and development of ceramic bearings is imperative. In order to make this high-tech product practical in China as soon as possible, we must focus on the following aspects:

(1) Based on reliability design, ceramic materials for inner and outer rings of new ceramic bearings are researched and developed, which are characterized by high strength (≥2000mPa), high hardness, high fracture toughness (≥12MPa. M1 / 2) and good fatigue resistance.

(2) Study the new forming process of ceramic ferrule to further reduce the amount of machining and reduce the production cost.

(3) The structural parameters of the new ceramic bearing are designed and studied to solve the matching problem between the ceramic bearing and the steel shaft and the cast iron box.

(4) Colleges and universities and scientific research institutions should cooperate with existing large bearing manufacturing enterprises, make full use of the existing production equipment, testing instruments, technical force and marketing advantages of the factory, and speed up the industrialization of ceramic bearings. For example, Shandong University of technology is cooperating with Jinan Bearing Factory to develop and produce ceramic bearings.

More about FRC Ceramic Bearings:

Ceramic bearings are comprised of ceramic rolling elements (balls) enclosed in a ferrous (typically steel) inner and outer race. Since these bearings are constructed from many materials, they often operate in hybrid capacities. We also build ceramic bearings , but they are not as common because they are less durable. These bearings offer several benefits over conventional steel bearings, including enhanced acceleration capability, increased stiffness, and reduced friction. Hybrid ceramic bearings also offer the benefit of not allowing electrical currents to run through the bearing, which allows for prolonged use.

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