Material friction and wear, structural design, surface treatment and lubrication application of rolling bearing

Abstract: The overview of tribology is described briefly. The applications of tribology in rolling bearings are discussed from friction and wear of material, structural design, surface treatment and lubrication of rolling bearings. The difference is analyzed between bearing enterprises in China and foreign well － known bearing enterprises about tribology and application of the bearings, and the appropriate suggestions are put forward．

Key words: rolling bearing; tribology; application

As an emerging discipline, tribology includes the science of friction, wear and lubrication. It is a science and technology that mainly studies the theory and practice of friction, wear and lubrication of the surface of the friction pair moving relatively in the process of friction and wear (including lubricating materials and methods, design and calculation, friction mechanism and fault diagnosis, monitoring and prediction, as well as the state of friction materials and surfaces). Tribology involves mathematics (Analytical Mathematics and Computational Mathematics), mechanics, physics, surface engineering, materials science, chemistry, fluid dynamics, thermodynamics and other disciplines. Friction and wear are extremely complex behaviors that occur on the contact interface. They are affected by many factors, such as working parameters, environmental conditions, friction pair materials, lubrication technology and so on. There are complex coupling and interaction between various factors. The friction and wear process is irreversible and dissipative. Therefore, tribology has the characteristics of complexity, systematicness and subject comprehensiveness.

Friction phenomenon was discovered very early, but due to various reasons, the research progress of tribology was very slow. Until the 15th century, Da Vinci introduced the research on sliding bearing wear and came to the conclusion that the wear increased with the increase of load; In 1785, based on previous studies, Coulomb explained dry friction with the concept of mechanical meshing, and finally put forward the friction theory; In the 19th century, there was no outstanding progress in the study of friction and wear; In the 20th century, hardy W B and Tomlinson g a successively put forward the "adhesion theory" of friction, which believed that the molecular adhesion acting on the contact surface produced friction. Literature [6] cited the concept of adhesion in the study of dry friction with materials, and formally put forward the adhesion theory in 1950. At the same time, fttchsel m proposed that dry friction is a deformation process of contact surface and subsequent stripping process after metal deformation. Taber believes that the role of shear and ploughing in friction behavior is the molecular mechanical duality of friction. After World War II, with the development of industry, science and technology, the working conditions of the machine developed towards high-speed and heavy load. Special requirements were put forward for the reliability and service life of machine parts, the efficiency and energy consumption of the machine, and the work in specific environment (such as high temperature, corrosive medium, etc.), which increased people's interest in friction, wear and lubrication. Reference [2] first proposed the metal transfer between contact surfaces in the process of friction, and studied the relationship between transfer and wear. Literature [7] studies the dynamic characteristics of oxidative wear and establishes a balance between the wear process and migration (being worn off): during friction, the soft metal transfers to the hard metal, and the transferred metal is oxidized rapidly until the oxide film limits the further transfer of the metal, and then the oxide film is worn off, which is a circular process. In the study of object lubrication, scholars first observed the phenomenon of fluid dynamic pressure. In 1886, Reynolds established Reynolds equation to characterize the mechanical properties of fluid lubrication film on the basis of summarizing the above phenomena, which laid the theoretical foundation of fluid lubrication. In the 1950s, computers were widely used in science, and the theory of hydrodynamic lubrication of contact elastic fluid developed rapidly. In recent years, solid lubrication has made great progress. On the research of wear, after scholars put forward the adhesion theory in the 1950s, various surface analysis instruments have been applied one after another, and the wear research has developed rapidly. In the process of comprehensively studying the relationship between friction, lubrication and wear, tribology has gradually formed a discipline. At present, the research and development of tribology has changed from static characteristic research to dynamic process research, from qualitative relationship to quantitative relationship, from macro phenomenon analysis to micro mechanism research, from single discipline decentralized research to multi-disciplinary comprehensive research on many influencing factors of tribology system. With the rapid development of industry, tribology is more and more widely used in industry. The research of Tribology in China rose in 1956. After more than 60 years of development, it has become more and more influential in the world. In September 2017, Beijing successfully held the sixth world tribology conference.

At present, tribological research can be divided into seven directions: material friction and wear, lubrication theory and application, biological and bionic tribology, micro nano tribology, surface engineering tribology, industrial tribology and tribological design. As an important basic part, rolling bearing is closely related to the above tribological plates, such as bearing design (tribological design), friction and wear of bearing materials, bearing lubrication, surface treatment of bearing ring (surface engineering tribology), and bearing application (Industrial Tribology). It can be seen that the development of rolling bearing is inseparable from tribology.

1.Application of Tribology in rolling bearing

As an important basic component, rolling bearing is widely used in various industrial fields. There are many friction pairs in rolling bearings, such as the friction between rolling element and inner and outer ring raceway surface, the friction between rolling element and cage pocket hole, the friction between lubricant and rolling element, the friction between lubricant and inner and outer ring raceway surface, and even the friction of sealing ring. According to the generation mechanism of massage friction, the friction of rolling bearing can be divided into pure rolling friction and sliding friction (differential, spin and other sliding friction on the rolling contact surface; the sliding friction at the sliding contact part includes the sliding between the rolling body and the pocket hole of the cage; the sliding between the cage and the guide surface of the ferrule; the sliding between the end face of the roller and the retaining edge of the ferrule; the sliding between the seal and the ferrule in the sealed bearing, etc.) The viscosity of the lubricant blocks friction. Compared with the current research direction of tribology, this paper discusses the application of Tribology in rolling bearing from four aspects: the friction and wear of rolling bearing materials (corresponding to the friction and wear direction of tribological materials), the design of rolling bearing (corresponding to the tribology direction of tribology design), the surface treatment of rolling bearing (corresponding to the tribology direction of Surface Engineering) and the lubrication of rolling bearing (corresponding to the lubrication theory and application direction of Tribology).

1.1 friction and wear of materials

The service life of rolling bearing is greatly affected by the performance of bearing materials, especially in harsh environment. However, due to the large number of friction pairs of rolling bearing, friction will produce wear. If the wear reaches a certain degree, the bearing will fail. Wear failure is one of the main forms of rolling bearing failure. The wear properties of Cr4Mo4V were studied in reference [10]. It is considered that the pits of adhesion wear and fatigue wear on the material surface will cause surface cracks; According to the tribological analysis of WEC failure of rolling bearing in document [11], it is considered that WEC failure is the product of friction material, friction machinery and tribochemical micro balance. Therefore, studying the friction and wear of bearing materials, exploring the wear mechanism of bearing materials and seeking to reduce the wear of rolling bearing from the perspective of materials is one of the methods to effectively improve the service life of rolling bearing.

1.2 structural design (Tribology)

With the wide application of rolling bearing, especially under harsh conditions, higher requirements are put forward for the design of rolling bearing. The purpose of rolling bearing design should be to meet the requirements of application conditions. For example, the high-speed angular contact ball bearing developed by SKF company is to meet the requirements of high speed (dm · n = 1.05 ×  106 mm · r / min), the bearing design has made the following changes: improve the contour morphology of the channel surface, change the contact angle, improve the pocket morphology of the cage, finally reduce the sliding between the ball and the channel surface, reduce the pocket force of the cage, and then reduce the friction between the cage and the ball, and finally develop a high-performance bearing that meets the requirements. The high performance low friction seal developed by NSK through bearing seal design is shown in Figure 1. The design of rolling bearing not only meets the requirements of customers, but also realizes the effect of minimizing the friction of each friction pair. It can be seen that the application of tribology is helpful to improve the design of rolling bearing.

Fig． 1 Sealing structure of new bearing developed by NSK

1.3 surface treatment and Tribology

With the in-depth statistical analysis of the failure of rolling bearing, the failure often occurs in the surface and sub surface of the ring or rolling element; In addition, under the background of saving energy and reducing pollution all over the world, the surface treatment technology of rolling bearing has developed rapidly. The development trend of rolling bearing surface treatment technology is considered in the literature. Whether traditional surface treatment technologies, such as carburizing, nitriding, PVD and CVD, or emerging surface treatment technologies, such as ion implantation, laser surface modification and ultrasonic surface modification, all have performance evaluation, namely wear resistance, which must pass the friction test. The effect of N ion implantation on the surface wear properties of 9Cr18, the effect of metal ion implantation on the wear properties of Cr4Mo4V and the surface wear properties of M50 treated with ZnO composite layer were studied in reference [15-17]. The comparison results of surface wear of 9Cr18 with different N ion treatment processes are shown in Figure 2. Literature [18] uses natural antioxidants for surface treatment, which significantly reduces the friction coefficient (Fig. 3). By studying the tribological characteristics of the surface treated layer, we can master the performance and damage mechanism of the modified layer, and finally improve the surface treatment process and improve the performance and service life of the bearing. Therefore, tribology is widely used in the surface treatment technology of rolling bearing.

 Fig． 2 Comparison of surface wear amount of 9Cr18 by different N ion treatment processes

Fig． 3 Comparison of friction coefficients on bearing surface treated with natural antioxidants

1.4 lubrication (tribological lubrication theory)

The excellent performance of rolling bearings is inseparable from the role of lubrication. The failure of rolling bearing is largely due to lubrication failure, that is, the change of lubrication state (lubricant pollution and the change of lubricating oil film thickness). Generally speaking, the rolling bearing is well lubricated, and the lubricating oil film can completely isolate the rolling body and ferrule, making it in the state of elastohydrodynamic lubrication; However, during the actual operation of the bearing, the rolling bearing is often in the state of mixed lubrication, and even in the state of dry friction when the lubrication fails, which is related to the change of oil film thickness in the lubrication theory of the rolling bearing. The lubrication theory is constantly developing and improving and applied to rolling bearings. The lubrication theory of rolling bearings is discussed in detail in document [19]. According to the research of literature [20], the oil film thickness h_T = h_R (thickener material) + h_RHL (hydroelastic dynamic lubrication film); By defining the life of lubricating grease and according to the life test results, the empirical formula of oil change time is

L10 = 2.7L₀₁.

In addition, the application environment of rolling bearings is diverse, and no specific lubricant can meet the lubrication state of all rolling bearings. Therefore, the lubrication of rolling bearings also includes lubricant research and application. For example, NTN adds an inhibitor to the lubricant to form an oxide film, which inhibits h from entering the steel matrix and increases its service life by about 4 times. Literature [22] studied the effect of oil chemical composition on rolling contact fatigue wear. It is considered that increasing the soap content of thickener can increase the oil film thickness and reduce contact wear (Fig. 4).

It can be seen that the lubrication theory in tribology is closely related to the lubrication of rolling bearings, and tribology is helpful to select the appropriate lubricant for the actual working conditions.

2. Application prospect of Tribology in rolling bearing

2.1 gap with foreign well-known bearing enterprises

Fig． 4 Wear volume for different lubrication tests

World famous bearing enterprises pay more attention to the application of Tribology in rolling bearings and widely carry out relevant research. In 2012, SKF invested 180 million euros in research and development. Many important innovations are the results of tribology, such as SKF energy-efficient bearings, wear-free coatings, ceramic bearings, copper and polymer cages, ultra smooth bearings for special applications, surface reticulation, grease and lubricant specifications and low friction seals. Schaeffler company has always attached importance to tribology and has formulated the tribological standard of bearings. NSK takes tribology knowledge as one of the four core technologies (Tribology technology, analysis technology, material technology and Mechatronics Technology). NTN company takes tribology as the basis of all its technologies and extends them. Its technology tree is shown in Figure 5. Jtekt company innovates through material development and processing technology based on tribology. NMB, which produces the world's smallest ball bearing (with an outer diameter of only 1.5 mm), believes that friction (lubrication) technology is essential to reduce friction and wear. The lubrication technology accumulated in nearly half a century ensures that it carries out lubrication design according to product application. It can be seen that foreign famous bearing enterprises pay more attention to tribology and its application in rolling bearings.

Fig． 5 Technical tree of NTN

Domestic bearing enterprises pay less attention to the application of Tribology in rolling bearings, and most domestic tribology researchers are not engaged in bearing related work, while bearing practitioners have less systematic knowledge of tribology. This situation is completely opposite to the development of tribology abroad. Therefore, in terms of the application of Tribology in rolling bearing, there is a large gap between China's bearing enterprises and foreign bearing enterprises, which may be one of the reasons why the quality of China's high-end bearing products is lower than that of foreign bearing enterprises. Therefore, how to effectively apply tribology to rolling bearings, improve the friction loss of rolling bearings (low friction, low energy consumption), and improve the performance and service life of rolling bearings is one of the directions of bearing development in the future.

2.2 outlook

At present, the development of rolling bearing tends to be highly reliable and long-life on the one hand, and to save energy consumption and reduce environmental pollution on the other hand. The use of tribological knowledge can reduce the friction of rolling bearing friction pairs, improve rolling bearing materials, rolling bearing design, bearing surface treatment technology, improve the lubrication state of rolling bearings, improve bearing performance, and then prolong the service life of rolling bearings. Therefore, the following suggestions are put forward for the application of Tribology in rolling bearings:

1) Carry out antifriction and wear resistance design of rolling bearings. Using tribological knowledge to optimize bearing design and improve the friction state of friction pair in rolling bearing.

2) Carry out the research and development of lubricant and lubrication theory of rolling bearing. There are many types of rolling bearings and different application environments. Through the basic research of lubrication theory, explore the best lubrication mode of corresponding bearings and develop lubricants with excellent performance, so as to improve bearing performance and improve bearing life.

3) Research and development of self-lubricating bearing materials. The development of rolling bearings is inseparable from the development of bearing materials. However, in harsh environments (such as aerospace), traditional lubrication methods are no longer suitable, and novel lubrication methods (such as self lubrication) are more important. At present, the self lubrication of bearings is mainly realized by solid lubrication on the bearing surface or soaking the cage with oil. The development of bearing self-lubricating materials with superior performance is one of the effective ways.

4) Research on surface engineering technology of rolling bearing. At present, bearing surface engineering technology is one of the hotspots of bearing life extension. Carry out systematic research on rolling bearing surface engineering technology, such as ion implantation, surface modification (spraying, ultrasonic processing), so as to expand the application scope of rolling bearing in harsh environment and improve the service life of rolling bearing at the same time.

3. Conclusion

Tribology is closely related to bearings. The use of tribological knowledge is beneficial to improve the performance and service life of rolling bearings. Foreign well-known bearing enterprises pay more attention to the application of Tribology in rolling bearings, while domestic bearing enterprises do not know enough about tribology, there is less connection between tribology and rolling bearings, and there is a large gap with foreign bearing enterprises. At present, the domestic bearing output is large and the quality is not high. In order to realize the transformation of the bearing industry from a large output country to a strong quality country, we need to pay attention to the application of Tribology in rolling bearings. As long as efforts are made to improve, the application of Tribology in rolling bearings will be in the forefront of bearing enterprises in the world in the future.

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