The advantages of ceramic rolling bearings, the categories of ceramic bearings, hybrid ceramic bearings and full ceramic bearings.

Abstract: using ceramic materials to make bearings is a revolution of traditional bearings. This paper introduces the advantages of ceramic rolling bearings, the categories of ceramic bearings, hybrid ceramic bearings and full ceramic bearings in detail, and points out the development trend of ceramic bearings.

Key words: ceramic materials; Bearing manufacturing; ceramic bearing trend

1. Introduction

Rolling bearing is an important component in modern machinery. It is mainly used to support the shaft and parts on the shaft, which has a direct impact on the accuracy, efficiency and service life of the whole machine (Fig. 1). Compared with sliding bearing, rolling bearing has the advantages of low friction coefficient, low power consumption and high efficiency. According to survey and statistics, about 1 / 3 of the world's energy consumption is in different forms of friction, of which bearings account for about 1 / 10. The machine tool is developing towards high precision, long life and high-speed automation, and the requirements for the basic parts of the mechanical industry are becoming more and more stringent. The production practice has proved that the traditional bearing can not meet the requirements of the development of modern science and technology and industrial production by improving the structure or lubrication conditions of the bearing alone.

The world's first set of ceramic bearings was successfully developed by NASA in 1972. After more than 30 years of research and development, the products have made remarkable achievements from extreme confidentiality to public sales. Foreign countries have successfully developed ceramic rolling bearings with solid lubricant under high temperature conditions, as well as rolling bearings or full ceramic rolling bearings composed of special steel and ceramics lubricated by liquid or grease.

Fig. 1 Bearings

2. Physical Properties of Silicon Nitride Ceramics

In recent years, with the rapid development of science and technology, the service environment and conditions of bearings are becoming more and more diversified, and the requirements for the structure, material and performance of bearings are becoming higher and higher. Some high-tech fields and machinery working in some special environments, such as aerospace, nuclear energy, metallurgy, chemical industry, petroleum and other industries, need to work at high temperature, high speed, high precision, vacuum, non-magnetic, oil-free lubrication, strong acid Work in special environment such as strong alkali. These new requirements can not be met only by improving the structure or lubrication conditions of traditional metal bearings. We must develop new materials and make a fundamental breakthrough and innovation.

Using ceramic materials to make bearings is a revolution of traditional bearings. Because ceramic materials have excellent properties, can withstand the harsh working environment that metal materials and polymer materials are not competent for, and have all the important characteristics required by bearing materials, the application of ceramic materials in bearing manufacturing has become a hot spot in the development and application of high and new technologies in the world and a symbol of the technological revolution of mechanical materials.

The material used to manufacture ceramic rolling bearings mainly adopts Si₃N₄ ceramics. The density of Si₃N₄ is 40% of that of bearing steel, and its hardness is 2 ~ 3 times that of bearing steel. The high hardness improves its ability to resist wear, adhesion and erosion damage. The thermal expansion coefficient of Si₃N₄ is about 1 / 4 of that of bearing steel. Low thermal expansion coefficient can reduce the deformation of bearing under high temperature working conditions. The high temperature property of Si₃N₄ ceramics makes it more suitable for high-temperature industries and mines. Under the temperature that can make the bearing steel lose its original hardness and strength, the hardness and strength of Si₃N₄ ceramics will not be reduced, and the high-temperature strength is good. In addition, Si₃N₄ ceramics have good dimensional stability at extremely high temperature, while bearing steel can ensure its dimensional stability at high temperature only after special heat treatment. Si₃N₄ ceramics have strong corrosion resistance and are suitable for applications in water, acid and alkali media. Therefore, it has a wider application range than bearings composed of plastic, glass or stainless steel rolling elements. Therefore, it can be seen from the above performance analysis that Si₃N₄ ceramic has excellent comprehensive performance and is an ideal material for making bearings.

3. Advantages of Silicon Nitride Ceramic Bearing

Silicon nitride ceramic bearings (Figure 2) are mainly used in four aspects: high-speed bearings, high-temperature bearings, vacuum bearings and corrosion bearings. Silicon nitride ceramic bearings have the following advantages.

(1) High speed. The weight of ceramic material is only 40% of the weight of the same steel, and the low density can realize the lightweight and high-speed of the bearing, so that when the ceramic bearing rotates at high speed, it can inhibit the increase and slip of rolling body load caused by centrifugal force. The rotating speed of ceramic bearing is 1.3 ~ 1.5 times that of steel bearing, and its DN value can reach 3 million. For example, when the contact angle between the sliding bearing and the rolling bearing is small, it is not only beneficial to the aircraft, but also has a certain damage to the rolling surface of the rolling bearing.

(2) High rigidity. The elastic modulus of silicon nitride ceramics is much higher than that of metal, which is 1.5 times that of metal. Therefore, the elastic deformation after stress is small and the rigidity of relative load is high, which can increase the stiffness by about 1.5% ~ 20%, so as to reduce the vibration of machine tool. It has been well applied in high-precision system, such as spindle of ultra precision machine tool, high-precision aerospace bearing and so on.

(3) Wear resistance. Because ceramic materials have few dislocations, low mobility and high hardness, which is generally more than twice as hard as metal, they can reduce wear and make ceramic bearings have good wear resistance.

(4) High mechanical strength. The tensile strength and bending strength of silicon nitride ceramics are equivalent to that of metals; The compressive strength is very high, about 5 ~ 7 times that of metal materials. Especially under high temperature conditions, it can still maintain high strength and hardness. Even at 1,200 ℃, the strength remains basically unchanged. When there are foreign matters hybrid in, ceramic balls rarely peel off and fail.

(5) Low fever. Because the friction coefficient of silicon nitride ceramic material is small, which is about 30% of that of standard bearing steel, compared with metal materials, silicon nitride ceramic has poor thermal conductivity. Therefore, ceramic bearings produce less heat when working, which can prolong the service life of grease.

(6) Low thermal expansion. The thermal expansion coefficient of silicon nitride ceramic is about 20% of that of bearing steel, so the size change of ceramic bearing with temperature is small, and the thermal preload is low, so as to avoid fatigue spalling failure caused by excessive heat accumulation, which is beneficial to use in the environment with large temperature change.

Figure 2 silicon nitride ceramic bearing

(7) Corrosion resistance. The inactive chemical properties of ceramic materials make ceramic bearings have certain corrosion resistance. Therefore, ceramic bearings can be used in all applications where steel bearings fail in advance due to lack of chemical resistance, such as machinery used in chemical machinery and equipment, food, marine and other departments and atomic energy equipment.

(8) Non magnetic. In the strong magnetic environment, when using steel bearings, the micro powder worn down from the bearing itself is adsorbed between the rolling body and the raceway surface, which becomes the main reason for the early peeling damage of the bearing and the increase of noise. Because the ceramic bearings are completely non-magnetic and have normal bearing capacity, they can be used in the occasions where completely non-magnetic bearings are needed.

(9) Insulation. The ceramic material has high resistivity and can be used as a better insulating material to prevent the bearing from arc damage.

4. Categories of Ceramic Bearings

According to the application of ceramic materials in bearing parts, ceramic rolling bearings can be divided into three categories: the first category is that the rolling elements are made of ceramic materials, while the inner and outer rings are still made of bearing steel. The second type is ceramic material for rolling element and inner ring, and bearing steel for outer ring. The third type is that the rolling elements and inner and outer rings are made of ceramic materials. The first and second types are called hybrid ceramic bearings, and the third type is called full ceramic bearings.

4.1 Hybrid Ceramic Bearings

The most common form of hybrid ceramic bearing is the angular contact ball bearing equipped with silicon nitride ball, which can operate effectively at high speed under both radial and axial loads. However, the axial load can only be applied from one direction, so these bearings are usually installed in pairs and preloaded to ensure the correct contact angle. Compared with deep groove ball bearings, angular contact ball bearings have larger openings at one end, so reinforced phenolic resin cages are usually used. Some hybrid bearing products only change the steel ball into silicon nitride ball in terms of material, but on the other hand, the geometric dimension of the channel has also been improved to optimize the bearing performance.

Figure 3 hybrid ceramic bearing

Most of the hybrid ceramic bearings used in modern industry are rolling elements, that is, the inner and outer rings of the ceramic ball are the structure of bearing steel (Figure 3). The failure of hybrid ceramic bearing is similar to that of steel bearing, which is manifested in the failure of steel ferrule and fatigue spalling of ceramic rolling element. The failure of ceramic rolling element is due to the defects of the material itself, such as uneven density, pores, impurities and microcracks formed on the machined surface. When the contact stress between the rolling element and the ring groove is the same, the service life of the hybrid ceramic bearing can reach or even exceed that of the steel bearing with the same specification. At high speed, the service life of ceramic ball bearing is 3 ~ 6 times longer than that of steel bearing. The rotating speed of hybrid ceramic bearing is 60% higher than that of high-speed steel bearing, the temperature rise of bearing is reduced by 35% ~ 60%, and the stiffness is increased by 11%. In addition, due to the small affinity between ceramics and steel molecules, small friction coefficient, certain self-lubricating performance and good operation performance, the hybrid ceramic bearing can effectively prevent burning and sticking caused by oil film damage.

Because the ball will produce great centrifugal force when rotating at high speed, the centrifugal force will cause pressure between the ball and the outer race raceway. Sometimes this pressure even exceeds the effect of external load and aggravates the load of the bearing. This centrifugal force is directly proportional to the density of the rolling element material, while the density of Si₃N₄ ceramics used to manufacture bearing rolling elements is only 40% of that of steel, which is conducive to the realization of high speed.

4.2 Full ceramic Bearing

The so-called full ceramic bearing means that the rolling element and inner and outer rings are made of ceramic materials (Figure 4). Compared with steel bearings, full ceramic bearings are more resistant to corrosion, high temperature, wear and high stiffness. In the aerospace industry, ceramic rolling bearings have excellent high-speed performance. Under high temperature environment, full ceramic bearings can work reliably at 800 ~ 1,000 ℃. In corrosive media, full ceramic bearings can show their unique advantages. All kinds of acid resistant pumps, vacuum pumps, centrifugal pumps and turbomolecular pumps used in the chemical industry have applied full ceramic bearings. In addition, the motor industry and electric locomotives use full ceramic bearings as insulated bearings, and aerospace vehicles use full ceramic bearings to reduce weight and improve flight speed.

Silicon nitride full ceramic bearing rings and rolling elements are made of silicon nitride ceramic materials, and the retainer uses polytetrafluoroethylene as the standard configuration. Generally, RPA66-25, PEEK, PI and phenolic sandwich bakelite pipes can also be used. Ceramic quality assurance holder has the advantages of wear resistance, high strength, corrosion resistance and self lubrication. The full ceramic bearing with ceramic holder can be used in harsh environments such as high corrosion, ultra-high, low temperature and high vacuum. Its common ceramic materials are ZrO₂ and Si₃N₄. Compared with ZrO₂ material, Si₃N₄ full ceramic bearing can be suitable for higher speed, load capacity and higher ambient temperature, At the same time, it can provide precision ceramic bearings for high-speed, high-precision and high-rigidity spindles.

Figure 4 full ceramic bearing

One side of the fully loaded ball type full ceramic bearing is provided with a ball notch. Due to the design of no cage structure, more ceramic balls can be loaded than the bearing with standard structure, so as to improve its load capacity. In addition, the limitation of cage material can be avoided, and the corrosion and temperature resistance of ceramic cage type full ceramic bearing can be achieved. This series of bearings is not suitable for high speed. During installation, pay attention to install the notch surface at the end that does not bear axial load.

5. Development Trend of Ceramic Bearings

The development of ceramic bearings is in the ascendant, and the practicability of nano materials and composites has brought greater vitality to the development of ceramic bearings. According to statistics, according to the calculation that special bearings account for 10% of the total bearing market demand, the total annual demand of Chinese bearings is about 2.1 billion sets. If ceramic bearings can account for 10% of the market share of special bearings, 21 million sets need to be produced every year. If the investment is 150-200 million yuan to build a ceramic bearing factory with an annual output of 400,000 sets of production capacity, 52 ceramic bearing factories of the same scale need to be built to meet the demand of the Chinese market. It can be seen that the product market gap is very large.

In practical application, the service life of ceramic bearing is 5 ~ 10 times that of heat-resistant steel bearing. Moreover, the price of products with the same specifications is significantly lower than that in foreign countries. It is reported that taking the outer diameter of 32mm as an example, the price of ceramic ball bearings in the United States is US $80, while the price of Chinese ceramic ball bearings is only 110 yuan (about US $12), which has strong international market competitiveness among products of the same quality.

Using ceramic materials to make bearings can greatly expand the application range of rolling bearings in various fields. At present, there are many companies that research, produce and sell ceramic bearings all over the world, but most of them are hybrid bearings, that is, the bearing rolling elements are made of ceramic materials and the inner and outer rings are made of steel materials. The continuous influx of large foreign bearing enterprises will have a greater impact on China's bearing market. Therefore, it has become an important topic to transform the traditional bearing industry with new materials and technologies, improve the technical content and added value of Chinese bearing products, especially stimulate the market competition and viability of ceramic bearings.

China started late in the research of ceramic bearings. In 1985, the state began to include the research and development of ceramic ball bearings in the key scientific and technological projects, and invested a lot of funds. Some scientific research institutes and enterprises have also made beneficial exploration and achieved gratifying results, but they are still in the experimental research stage. The main reasons affecting the wide application of ceramic materials in ball bearings are their difficult machinability and high manufacturing cost. The research of ceramic bearings needs to be further explored in the following aspects: first, the rolling contact performance of ceramic bearings under wider application range and worse lubrication conditions. The second is to study the structural matching design of ceramic bearing related parts, as well as the reliability and economy of processing. The third is the nondestructive testing method and damage prediction technology of ceramic bearing related parts; Fourth, formulate the inspection standards of ceramic bearings. It is believed that it is not far to enter the practical stage of ceramic bearing, and its application prospect is very broad.

More about FRC Ceramic Bearings:

As an important mechanical basic part, ceramic bearings are leading in the world of new materials due to their excellent performance that metal bearings cannot match, such as high temperature resistance and super strength.

Advantages of Ceramic Bearings:

First, because ceramics are almost not afraid of corrosion, ceramic rolling bearings are suitable for operation in harsh conditions full of corrosive media.

Second, because the density of the ceramic rolling ball is lower than that of steel and the weight is much lighter, the centrifugal effect of the outer ring during rotation can be reduced by 40%, thereby greatly extending the service life.

Third, ceramics are less affected by thermal expansion and contraction than steel, so when the bearing clearance is constant, the bearing can be allowed to work in an environment where the temperature difference changes more drastically.

Fourth, because the elastic modulus of ceramics is higher than that of steel, it is not easy to deform when subjected to force, so it is beneficial to improve the working speed and achieve higher precision.

Ceramic bearings have the characteristics of high temperature resistance, cold resistance, wear resistance, corrosion resistance, anti-magnetic and electrical insulation, oil-free self-lubrication, and high speed. It can be used in extremely harsh environments and special working conditions, and can be widely used in aviation, aerospace, navigation, petroleum, chemical, automotive, electronic equipment, metallurgy, electric power, textiles, pumps, medical equipment, scientific research and defense and military fields. High-tech products for new material applications.

The rings and rolling elements of ceramic bearings are made of all-ceramic materials, including zirconium oxide (ZrO2), silicon nitride (Si3N4), and silicon carbide (Sic). The retainer is made of polytetrafluoroethylene, nylon 66, polyetherimide, zirconia, silicon nitride, stainless steel or special aviation aluminum, thereby expanding the application surface of ceramic bearings.

Welcome new and old customers to contact us and know more about our high precision miniature bearings!