Abstract: As a special type of sliding bearing, joint bearings are divided into lubrication type and self-lubricating type. The author of this article analyzes and discusses the development history and performance testing of self-lubricating joint bearings both domestically and internationally.
Keywords: self-lubricating joint bearings; Development history; Self lubricating materials; performance test
Joint bearings, also known as spherical sliding bearings, are a typical radial joint bearing composed of an outer ring with a spherical surface and an inner ring with an outer spherical surface. They have the characteristics of simple structure, small volume, and large bearing capacity. Self lubricating joint bearings are joint bearings with self-lubricating materials, which have excellent characteristics such as low friction coefficient, long service life, impact resistance, corrosion resistance, and no need to add lubricants during operation. They are widely used in engineering machinery, heavy-duty vehicles, water conservancy facilities, aerospace, military machinery and other fields. Therefore, in some aviation industry developed countries such as the United States, France, Germany, and the United Kingdom, the research on self-lubricating joint bearings started very early. After years of systematic research and testing, a variety of series products that can adapt to various working conditions have been formed. The research on self-lubricating joint bearings in China started late and the basic research is weak. Although some domestic products can replace foreign products in some fields, there is still a significant gap in product quality, performance, and product serialization compared to similar foreign products, especially in some cutting-edge fields such as aerospace where self-lubricating joint bearing technology is close to blank, This has become an important bottleneck affecting China's breakthroughs in these fields.
1. The development history of self-lubricating joint bearings
Self lubricating joint bearings have a history of nearly a hundred years of application in foreign countries. China only began research in this area in the 1970s, and there is now a significant gap compared to advanced industrial countries. The performance of self-lubricating joint bearings depends on the self-lubricating material, dual surface treatment, and manufacturing process of the bearings. Foreign countries are far ahead in related areas and their technology is very mature.
1.1 Development of self-lubricating materials
Self lubricating materials play an important role in the technology of self-lubricating joint bearings, and their performance directly affects the working performance and service life of self-lubricating joint bearings.
Early self-lubricating joint bearings prepared lubricating films on the sliding surface to achieve friction reduction effects. For example, ELGES and NTN company Z have conducted research in this area. They coated PTFE coatings on the inner surface of the outer ring or the outer surface of the inner ring to prepare joint bearings with low friction coefficient and high wear resistance.
SKF has developed a self-lubricating joint bearing with solid lubricating material embedded on the inner surface of the outer ring. This type of joint bearing has a low friction coefficient, and it is still used in some fields.
White in the United States in 1955 Charles S et al. invented a fabric type self-lubricating material, which is a composite woven material composed of fibers with low friction coefficient and other fibers. The woven material is bonded together with suitable materials. Due to the requirement that the matrix material has the ability to resist deformation at higher working temperatures, thermosetting resin is chosen. Due to the poor adhesion of low friction fiber materials, weaving is adopted to place most of the anti friction fibers on the working surface of the pad, while the fibers with good adhesion are on the bonding surface. This lubricating material has good anti friction and load-bearing capacity at the same time. In addition, this woven lubricating material has strong designability, and suitable weaving methods can be adopted according to specific working conditions, selecting appropriate weaving materials and matrix materials. This design concept is still one of the basic design concepts for self-lubricating joint bearings and self-lubricating pads Z.
1956 White Charles S weaves PTFE fibers and adhesive fibers into bearing lubrication materials, and fills them with adhesive resin sources to form fabric type composite materials.
In 1958, Richard et al. also proposed a woven lubricating material. Woven self-lubricating pads are currently the main self-lubricating material for self-lubricating bearings, especially in cutting-edge fields such as aerospace. After years of research in developed industrial countries in the West, self-lubricating pad technology has matured and improved, forming a series of products that can adapt to various working conditions. SKF Company has introduced a new type of self-lubricating pad for its newly launched TX self-lubricating joint bearings, The performance of self-lubricating pads has been improved by improving the fabric structure and matrix materials. This self-lubricating pad has greatly improved its load-bearing, anti-wear, friction reduction, and moisture resistance performance, as well as the working life of the pad. This has greatly improved the performance of TX self-lubricating joint bearings compared to similar products and other company products in the past. RBC Company has newly launched a variety of high-performance self-lubricating pads, which are suitable for different working conditions such as high-speed light load, low-speed heavy load, high temperature, low friction, etc. At the same time, they are also suitable for forming friction surfaces with steel, aluminum, titanium, etc. Overall, the self-lubricating pads of woven structures in foreign countries have been serialized and are suitable for applications under different working conditions (low speed, heavy load, high speed, high temperature, etc.).
Due to the complex manufacturing process and high cost of self-lubricating pads for fabric structures, research on non fabric self-lubricating materials, especially injection molded self-lubricating materials, has been promoted. In 1978, McClos key Albert R invented a non woven self-lubricating liner made of PTFE fiber and phenolic resin. 2005 Ampuero Auza Jamime A et al. invented a non woven self-lubricating liner composed of fluoropolymer fibers and other wettable organic structural fibers. The manufacturing process is simple, the price is cheap, and compared with traditional woven structural liners, it can adapt to higher speeds and have a longer working life.
At present, the commonly used self-lubricating pads in mass production of joint bearings include not only fabric type self-lubricating pads and injection molded polymer anti friction and wear materials, but also sintered porous materials (such as sintered bronze, storing lubricating oil or grease in the holes), copper mesh plastic composite materials, etc. These two types of self-lubricating pads are mainly used in civil mechanical equipment with low load and low transmission requirements.
In China, relevant universities, research institutes, and enterprises have also conducted research on woven structure self-lubricating pads. Fujian Longxi Bearing (Group) Co., Ltd. has been developing PTFE fabric type self-lubricating materials since the late 1980s, with a static load capacity of over 450MPa and a transport capacity of over 200MPa. The friction coefficient Z is as low as 0.03 under heavy loads and low speeds, and the performance indicators are at the leading level in China.
In recent years, some domestic scholars have achieved certain results in the research of non woven self-lubricating materials and their applications in bearings, which have improved the bearing capacity, reduced friction coefficient, PV value limit, service life and other performance to varying degrees. However, research on self-lubricating pads applied in cutting-edge fields is still very weak.
1.2 Development of Surface Treatment Technology
Self lubricating pads are generally pasted on the inner spherical surface of the outer ring of a joint bearing, and the outer spherical surface of the inner ring is generally treated with special processes such as chrome plating, ion implantation, ceramics, etc., so that when the self lubricating pad forms a friction pair with the outer spherical surface of the inner ring, it can achieve better working performance. The material of the inner ring of self-lubricating joint bearings can be stainless steel, aluminum alloy, and titanium alloy. The use of titanium alloy in the inner ring of self-lubricating joint bearings can reduce weight. Due to the lower hardness of titanium alloy compared to stainless steel and copper alloy, the outer surface of the inner ring is more prone to scratches during operation, and the damaged outer surface also accelerates the damage and wear of the self-lubricating gasket. In 2005, Smith et al. proposed surface treatment of the outer spherical surface of the titanium alloy inner ring to form a nitride infiltration layer, which was processed to reduce the surface roughness to less than 18nm; In 2006, Lopes et al. formed a TiN treated layer on the outer spherical surface of the titanium alloy inner ring using PVD method. In order to improve the performance of self-lubricating joint bearings, some scholars have proposed using ceramic materials for the inner ring, so that the working pair formed with self-lubricating pads has better performance.
The material of the inner and outer rings of SKF's aviation self-lubricating joint bearings is stainless steel, and the outer ring is cold extruded. The inner spherical surface of the outer ring is glued with PTFE fabric lining, and the outer spherical surface of the inner ring is treated with ceramic coating. This self-lubricating joint bearing has a larger bearing capacity and longer service life.
2. Self lubricating joint bearing test
The main failure mode of self-lubricating joint bearings is wear. Therefore, conducting performance tests on joint bearings and studying the friction and wear performance of joint bearings is a fundamental work, which can improve the understanding of self-lubricating joint bearings, accumulate a foundation for improvement and enhancement, and also provide guidance for use.
A lot of work has been done in the research of experimental methods, equipment, and experiments in China, but there is still a certain gap compared to foreign countries. There is still a lack of testing machines for combined loading and combined motion, and the experimental work is not sufficient. The theoretical research is not deep enough.
2.1 Test methods
The tests of joint bearings mainly include static load test, dynamic load test, high temperature test, low temperature test, medium resistance test, etc. In China, the national industry standards, joint bearing static load test regulations, and joint bearing dynamic load and life test regulations mainly drafted by Fujian Longxi Bearing (Group) Co., Ltd; The technical specifications of foreign US and EU navigation marks both specify the testing of static and dynamic loads on joint bearings.
2.2 Test equipment
The testing equipment for joint bearings mainly includes static load and dynamic load testing equipment. Static load testing equipment is generally modified from universal material testing machines, while dynamic load testing equipment needs to be specially developed. Currently, there are constant load testing machines, alternating load testing machines, combined motion and loading testing machines, and testing machines that can simulate high and low temperatures, dust, water vapor, and other environments. Fujian Longxi Bearing (Group) Co., Ltd. has invested heavily in testing equipment and has established CNAS accredited laboratories. It has various joint bearing life performance testing equipment for constant load, alternating load, high and low temperature, dust, and water vapor simulation environments. It can measure performance indicators such as friction coefficient, wear amount, and temperature online, and automatically draw the curve between friction and wear performance indicators and motion time. It is a complete domestic Z A large-scale joint bearing performance testing laboratory has conducted a large number of joint bearing life performance tests.
2.3 Analysis technology for friction and wear surface of self-lubricating joint bearings
The analysis techniques for friction and wear of self-lubricating joint bearings mainly include surface and structure analysis and surface and atomic state analysis. Surface and structural analysis mainly uses transmission electron microscopy and scanning electron microscopy to observe the morphology and failure characteristics of the friction surface; Using electron diffraction and X-ray diffraction techniques to determine surface structure and composition. Surface and atomic state analysis mainly involves Auger electron spectroscopy, photoelectron spectroscopy, scanning probe microscopy, scanning tunneling microscopy, atomic force microscopy, etc. to analyze the composition and distribution of the friction surface.
3. Conclusion
3.1 Self lubricating joint bearings have been applied in many fields due to their large load-bearing capacity, light weight, no need for manual lubrication, and excellent wear performance and friction torque. Due to the low level of research on it in China, it is believed that with the gradual maturity of this product, research work will also become more in-depth, and self-lubricating joint bearings will be widely used.
3.2 Experimental research on joint bearings is very important, as it can reveal the friction and wear mechanism of joint bearings, analyze the forms of friction and wear, accumulate a foundation for improving the performance of joint bearings, and provide guidance for the application of joint bearings. It is necessary to increase the development and investment of testing equipment and conduct in-depth experiments.
2023 December 1st Week Marginal Product Recommendation:
Margianl LCB Plastic Clip Linear Bushings：
Made of high anti-wear material EPB13;
2.Maintenance-free, self lubricating;
3. Chemical resistant;
4. Dust resistant and quit running;
5. Installation by force fitting;
6. The E10 inner tolerance ensues only after the press fit.
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