Time : September 28, 2023
The bearings discussed in this article refer to rolling bearings, which are important mechanical basic components with a wide range of applications. The service conditions, operating accuracy, and service life of various machinery are often connected to bearings, and sometimes even depend on the parameters of the bearings. Therefore, mechanical innovation often involves bearing innovation, constantly launching high-end products with superior performance compared to widely used traditional products.
The improvement and expansion of bearing performance cannot be separated from three factors: design, technology, and materials. Especially in material technology, the promotion of bearing innovation is particularly evident. In the past 20 to 30 years, the application of high-tech ceramics has led to the development of high-end bearings with new performance. Some of these bearings have gradually entered the mainstream product line and are increasingly showing their irreplaceable potential in civil and national defense industries.
The high-tech ceramics used in the development of ceramic bearings, also known as structural ceramics, fine ceramics, or advanced ceramics, mainly include silicon nitride, silicon carbide, zirconia, and alumina. Among them, silicon nitride is commonly used, which has many properties suitable for bearing manufacturing: silicon nitride has a high hardness; Has good corrosion resistance; The small density greatly reduces the centrifugal force of the bearing ball during operation, and the plastic deformation is very small. In addition, the linear expansion coefficient of silicon nitride is lower than that of bearing steel, and the temperature rise during bearing operation will not cause jamming. At the same time, silicon nitride is non-conductive and non magnetic, and operates under conditions such as strong magnetic fields, demonstrating superiority. The inferior performance of ceramics compared to steel is mainly due to its lower bending strength. However, through strict ceramic manufacturing processes, the strength of silicon nitride can currently reach 800-1000MPa, which is already suitable for applications in bearings.
The development of ceramic bearings as products began in 1984 with the collaboration of Toshiba Materials and Guangyang Seiko in Japan; In 1992, silicon nitride materials suitable for acid and alkali solutions were developed; In 1993, corrosion-resistant bearings were manufactured; In 1995, the excellent performance and high reliability of ceramic bearings were recognized, and NASA first used them on space shuttle turbopumps; In the same year, ceramic bearings were also used in automotive turbochargers; In 1999, ceramic bearings were successfully applied in the production of hard drives; After the 21st century, the application of ceramic bearings has further expanded, and many world-renowned enterprises have introduced their newly developed ceramic bearing products.
The world-renowned SKF company has launched the EXPLORER bearing series, which can work even under poor lubrication conditions, and reduces vibration, friction, and heat generation, maintaining operating accuracy and dimensional stability, thereby completely improving traditional maintenance operations and simplifying pump structural design.
This type of bearing is suitable for pumps, compressors, fans, wind turbines, gearboxes, and turbines. Its lifespan is three times longer than that of similar steel bearings from any other manufacturer.
The pump industry market in the UK has an annual sales revenue of approximately 980 million pounds, of which 80% are centrifugal pumps. The average service life of each bearing set is only one year, and bearing failure accounts for 60-70% of pump failure rates. The X-Life Ultra series hybrid bearings developed by Barden, a subsidiary of FAG in the UK, increase the bearing life and save on pump service costs. For example, the fuel pump bearings of the main engine of NASA’s space shuttle were originally replaced after each shuttle flight, costing 100000 pounds. After using Barden’s hybrid bearings, the shuttle was still intact six times, saving at least 80% of the cost. FAG/Barden’s X-Life Ultra series can also be used as machine tool spindle bearings, with a lifespan 10 times longer than the same steel bearings.
There are also various reports on the application of ceramic bearings in different industries, including pharmaceutical machinery, food machinery, textile machinery, high-speed vacuum pumps, sports equipment such as ice skates and bicycles. The famous engine companies Pratt and Whitney claim to have installed hybrid ceramic bearings on the engines of the new generation of US Air Force fighter jets.
Although the production of silicon nitride ceramic balls has laid the foundation for the development of ceramic bearings, the progress in the production of ceramic bearings in China is slow. Not only is the application range narrow and the quantity is small, but there is also a significant gap between producing high-performance high-end bearings. The reason is that the main driving force for this work is the material development unit rather than the bearing manufacturer. On the other hand, the development of ceramic bearings is a systematic technical issue. It is not necessary to replace steel balls with ceramic balls to become ceramic bearings. To truly manufacture high-quality ceramic bearings suitable for corresponding working conditions, it involves bearing structure design, selection of inner and outer ring materials, lubrication technology, and manufacturing technology of ceramic balls. Manufacturers may not necessarily pay sufficient attention to these issues:
2.1 Bearing structure design: Currently, ceramic bearings sold in the market are made by replacing steel balls with ceramic balls of the same size. When used in small bearings with light loads, the problem cannot be seen. However, when under heavy loads, the difference in material properties between ceramics and steel inevitably causes changes in the contact stress on the bearing surface under load, which will directly affect the contact fatigue life of the bearing and may even lead to early failure. Therefore, the design of the bearing structure must make appropriate adjustments to this issue in order to fully obtain the gain in bearing performance brought about by the application of ceramic balls.
2.2 Selection of inner and outer ring materials: This issue involves two aspects. Firstly, the selected material should be suitable for the working conditions of the bearing, such as temperature, corrosion, etc; The second is to match the steel surface of the inner and outer rings with the ceramic ball for better tribological performance.
Under normal temperature and non corrosive conditions, the widely used material Z is the standard high carbon chromium bearing steel GCr15. After years of effort, the GCr15 produced in China can meet the requirements of bearing manufacturing regardless of its purity and uniformity. However, experimental data shows that GCr15 is not a material that matches the Z-best tribological properties of silicon nitride ceramics, and nitrided steel may be better. In recent years, Germany has introduced high-speed bearings made of nitrided steel, claiming that only the combination of nitrided steel and ceramic balls can achieve this effect.
Under high temperature conditions, the high-temperature bearing steel used in China is Cr4Mo4V, which is equivalent to M50 in the United States. For corrosion-resistant bearings, China mostly uses stainless bearing steel 9Cr18, or 9Cr18Mo, which is equivalent to 440C in the United States. This is a high hardness martensitic stainless steel, but also due to its high carbon and chromium content, it is easy to cause early fatigue peeling and difficult to obtain silent performance, and further solutions are needed.
Considering that austenitic stainless steel has better corrosion resistance than martensitic stainless steel, and also has non magnetic properties, it is also useful to use austenitic stainless steel for carburization treatment to manufacture bearings. This type of bearing has better corrosion resistance than 440C in aqueous environments and acid and other media.
2.3 Lubrication technology: The lubrication technology of ceramic bearings is quite complex, and different lubrication technologies are required according to different service conditions. Lubricants include oil, grease, solids, gases, and the transmitted liquid medium. If lubrication problems cannot be solved, ceramic bearings cannot be successfully applied.
When the bearing is immersed in the transmission liquid medium for operation, it can only rely on medium lubrication. Due to the corrosion resistance of ceramics, ceramic bearings are capable of working in the transmission medium, which is an advantage. However, in some cases, the lubrication performance of the medium is quite poor, such as liquid natural gas, whose viscosity is only 0.2-0.3Cst, which is only one percent of the viscosity of No. 30 engine oil (30Cst). It is quite difficult to establish a lubricating film on the friction interface with such low viscosity. When there is no lubricating film, such as using steel bearings, the friction surface will have serious adhesion and bite. If using ceramic bearings, the friction coefficient between ceramics and steel during dry friction also reaches around 0.8, which is quite high, and the bearings cannot work due to this. At this point, the design of the retainer becomes important. If the material of the retainer can be continuously transferred to the friction surface of the bearing during operation, forming a solid lubrication transfer film, the problem may be solved.
For commonly used lubricants such as oil and grease, their lubrication effect on metal materials such as steel is achieved by forming a surface adsorption film and producing a chemical reaction film. For ceramics, ion bonded oxide ceramics (such as zirconia and alumina) can generate strong adsorption films, similar to iron oxide in steel materials. Polar molecular chains in lubricating oil are vertically arranged on the surface of the ceramic, providing effective lubrication. However, covalent bonded ceramics, such as silicon nitride and silicon carbide, exhibit poor lubrication performance as the polar molecular chains in lubricating oil are arranged parallel to the ceramic surface. As for chemical reaction films, regardless of ionic or covalent bond ceramics, they do not react with extreme pressure additives such as sulfur, phosphorus, and chlorine, and cannot form frictional chemical reaction films. This means that commonly used extreme pressure additives cannot improve the lubrication performance of ceramics. Therefore, we look forward to developing efficient lubricants that can affect the surface of ceramics.
2.4 Manufacturing technology of ceramic balls: Currently, the precision of silicon nitride ceramic balls supplied in the domestic market can reach G5 level, and some specifications can even reach G3 level. The diameter difference of the balls has been controlled within 80nm (nanometers), so the ultra precision machining level of domestic ceramic balls is quite good, which can meet the needs of bearing manufacturing. However, it is still important to note that the machining accuracy of bearing balls continues to improve. Currently, ceramic balls are processed using a ball mill that processes steel balls. This single disc rotating ball mill with V-shaped grooves has a grinding trajectory that is a circular ring, rather than unfolding into a spherical surface, according to recent analysis by scholars. This will be a difficult problem for high-precision spherical machining. In addition, in terms of sintering technology, hot isostatic pressing (HIP) is commonly used abroad, while gas pressure sintering (GPS) technology is mostly used domestically, and HIP sintering is not commonly used. The composition control and manufacturing process of green balls have a crucial impact on the quality of finished balls. There is still room for exploration on how to manufacture ceramic balls with better performance at lower costs. The commercial production of ceramic cylindrical rollers has not yet been completed and needs to be filled.
Ceramic bearings, due to their outstanding performance, can become high-end bearing products with high added value. At present, the production volume of bearings in China is large, with an annual output of over 3 billion sets and a large number of exports to overseas markets. However, the exports are mainly low-end bearings, and a considerable portion of the high-end bearings needed in China also need to be imported. Therefore, strong bearing manufacturers should invest in developing the high-end product market, and ceramic bearings can also be an option. Based on the current technical conditions in China, there are approximately the following types of ceramic bearings with promising application prospects that can be developed: bearings for sports goods: roller skate bearings, high-end bicycle bearings; Corrosion resistant bearings for chemical pumps, vacuum pump bearings; High speed bearings for textile machines; Micro and small bearings for household appliances such as air conditioners and vacuum cleaners; Low noise bearings for motors; Bearings for hard drives; High speed and high-precision bearings for machine tool spindles, high-speed bearings for electric spindles, and bearings for ball screws; High temperature and high-speed bearings for engine cylinder pressure devices; High speed and heavy-duty bearings for automotive transmission and railway locomotives.
From the progress made in the development and application of ceramic bearings, it is evident that the emergence of ceramics as a new material has promoted the innovation of bearings, which are important mechanical basic components. In the future, advanced material technology still needs to be applied in different aspects to promote the improvement of ceramic bearing products. For example, using failure analysis technology to study the causes of bearing damage to improve its fatigue life, and using non-destructive testing technology to inspect the micro defects of ceramic balls to improve their reliability in use. Only in this way can we manufacture products with higher technological content, and safely use ceramic bearings under more severe conditions, such as as for use as aviation engine spindle bearings. These crucial and quite difficult technical problems cannot be easily learned from materials, and perhaps the pioneers have not yet solved them. They still rely on their own continuous exploration and innovation.
More about XZBRG Vollkeramik-Schrägkugellager:
Vollkeramik-Kugellager, die vollständig aus Keramikmaterial bestehen. Innere/äußere Laufringe und Kugeln bestehen entweder aus Siliziumnitrid (Si3N4), Zirkoniumoxid (ZrO2) oder Siliziumkarbid (SiC). Sie sind vollrollig (ohne Käfig) oder mit einem Käfig aus PEEK oder PTFE erhältlich. Vollkeramiklager sind für Anwendungen mit mittlerer Belastung und mittlerer Geschwindigkeit. Es ist nicht möglich, die Rundheit des Innen- und Außenrings zu erreichen, die bei Präzisionsstahllagern zu finden ist, daher haben Vollkeramiklager niedrigere Drehzahlwerte.
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