Abstract: From the five aspects inculding structural design，cage，material, lubrication and seal, and standard, the technical status and development of key bearings for rail transit vehicles are summarized，such as traction motor bearings, axle bearings and gearbox bearings, which provide reference for domestic industry.
Key words: high-speed rail transit; rolling bearing; structure; material; cage; lubrication; seal; standard
During the 12th Five Year Plan period, the total mileage of China's national railway operation will increase from the current 91000 km to around 120000 km. Among them, the construction of high-speed rail and urban rail transit has shown an unprecedented development situation, and a new round of rail transit construction has gradually begun nationwide. China has become the world's largest urban rail transit market.
As a key component for vehicle operation safety, bearings need to meet higher performance and reliability requirements. At present, bearings for rail transit vehicles in China are in a period of technological upgrading and upgrading, and there is still a certain gap between the technical level of bearings in China and the international advanced level. For example, bearings for high-speed trains and high-speed trains still rely entirely on foreign imports. Therefore, it is imperative to learn advanced bearing technology and accelerate the localization of bearings for rail transit vehicles.
Here, the technical development of key bearings for rail transit vehicles such as traction motor bearings, axle box bearings, and gearbox bearings is introduced from five aspects: structural design of bearings, cage, materials, lubrication and sealing, and standards, for reference.
1. Structural design of bearings
The technological development of bearings for rail transit vehicles should comply with the development direction of safe vehicle operation, high-speed, extended maintenance period, and improved operational comfort.
1.1 Traction motor bearings
1.1.1 Technical Features
Traction motor is one of the key equipment for achieving high-speed rail transit vehicles. With the development of power electronic equipment and control technology, traction motors are gradually transitioning from DC motors to asynchronous motors, and direct drive traction motor systems are also being developed to meet the requirements of small, lightweight, energy-saving, maintenance saving, and long service life of motors. The speed of traction motors is generally between 2000 and 4000 r/min, and the speed of new high-speed railway traction motors can reach 6000 r/min. High speed traction motors above 7000 r/min are already being developed.
The traction motor bearings mainly bear radial loads. Due to the influence of bogie vibration during operation, the bearings also need to bear a certain dynamic load to achieve frequent operation and stop cyclic operations. Compared with axle box bearings and gearbox bearings, the calculated life of traction motor bearings is much longer. The general configuration of bearings is: cylindrical roller bearings are used on the gear side (transmission side), and deep groove ball bearings are used on the reverse gear side (commutator side). The lubrication method is mostly grease lubrication [2-4]. In order to achieve high-speed rotation of bearings and effectively suppress the temperature rise of various components of traction motors, improvements have been made to the heat treatment process, insulation performance, heat resistance and durability of lubricating grease, as well as the guiding method of the cage.
1.1.2 Insulated bearings
In traction motors, the electrical corrosion generated by the current passing through the bearings is extremely harmful, which may cause premature aging of the lubricating grease inside the bearings and even cause bearing damage. To prevent electrical corrosion, considering price and reliability, insulated bearings are adopted as the solution. There are generally three types of insulation bearings used for traction motors: hybrid ceramic bearings, ceramic spray bearings, and resin coated bearings.
(1) Hybrid ceramic insulated bearings use ceramic materials to make rolling elements or rings. Due to the high technical difficulty involved in the preparation and processing of ceramics, they have excellent insulation and high-speed performance. Ceramic rolling elements or ferrules can completely cut off the current path and effectively prevent electrical corrosion damage. In addition, the contact between ceramics and steel will not experience "micro welding" under poor lubrication conditions, and even in the state of complete lubrication failure, the bearing will not get stuck. However, considering the serious consequences of brittle failure of ceramics, hybrid ceramic insulated bearings are less commonly used in high-speed trains. At present, the hybrid ceramic insulated bearings in China are still in the research and trial production stage.
(2) Ceramic spray insulated bearings, using plasma spraying method, use ceramic powder mainly composed of aluminum oxide to spray and insulate the outer diameter surface of the outer ring. In order to prevent high-frequency current from flowing through and causing electrical corrosion, the thickness of the sprayed film should be greater than 0.3 mm. The insulated bearings produced by NTN company use multiple layers of ceramic and metal coatings on the outer surface, with each layer thickness around 0.2 mm; Depending on the usage situation, only one layer of ceramic coating can be used. At present, this type of ceramic insulated bearing has been successfully applied to the 300 series, 500 series, and 700 series high-speed trains of Shinkansen in Japan, as well as the "Harmony" high-speed trains and other high-speed trains in China.
(3) Resin coated insulated bearings are sprayed with insulation materials such as PPS resin on the outer surface of the outer ring through injection molding. PPS resin insulated bearings have excellent electrical insulation performance, low price, and are widely used. Although PPS insulated bearings can prevent electrical corrosion, due to the low thermal conductivity of PPS resin, compared to non insulated bearings, high temperatures are easily formed inside the bearings. In order to maintain the excellent performance of PPS insulated bearings and improve thermal conductivity in applications, non-conductive high thermal conductivity fillers are added to the original glass fiber reinforced PPS resin to improve the thermal conductivity of the resin.
1.2 Axle box bearings
The axle box bearing needs to bear the weight and load of the entire vehicle body, as well as the forces generated by the vehicle's swing in various directions during operation. In addition to bearing static and dynamic radial loads, it also bears non constant axial loads. With the high-speed development of vehicles, in order to ensure the stability of vehicle operation, it is required that the structural components of the axle should be as lightweight and miniaturized as possible, that is, the axle box bearings need to achieve compact design. Moreover, the lightness of vehicles is closely related to energy conservation, which can effectively reduce the impact of vehicles on the railway track.
In recent years, the axle box bearings of high-speed rail vehicles both domestically and internationally have evolved from oil bath lubricated cylindrical roller bearings with retaining edges to grease lubricated maintenance free sealed double row tapered roller bearings, which can withstand both radial and axial loads and meet the requirements of compact and lightweight structures. The use of grease lubricated sealing can simplify maintenance and improve operational stability by reducing the axial clearance of the bearings.
SKF Company applied the FEM finite element analysis method to the design of axle box bearings and developed a compact tapered roller bearing component (CTBU). Plastic cages and low friction contact seals are used in the bearing design to shorten the axle, reduce axle bending caused by the bearings, and reduce unsprung mass. The CTBU component is also equipped with sensors to monitor speed and bearing status. Due to its compact design, which can extend maintenance cycles and further improve bearing performance and safety, it has the characteristics of low operating temperature, long service life of lubricating grease, and optimal sealing performance. It has been widely used and is also the development direction of axle box bearings.
1.3 Gearbox bearings
The torque of the electric vehicle traction motor is first transmitted to the small gear shaft on the gear device through the coupling, and then transmitted to the large gear pressed onto the axle by the small gear. Finally, it is converted into the driving force or braking force of the wheel/rail system. The bearings used in gear devices mostly use a combination of a pair of single row tapered roller bearings, which use lubricating oil together with the gears.
The function of the gearbox bearing is to serve as a support, while bearing operating vibration, to smoothly transmit rotational force to the axle. When designing, vibration load and speed need to be considered. The axial clearance of tapered roller bearings needs to be accurately adjusted during the assembly process. In addition to improving the pinion structure to control the axial clearance, bearings that do not need to adjust the axial clearance during assembly can also be used, such as the combination of three-point or four point contact ball bearings and cylindrical roller bearings. In recent years, technological developments have included: improving the structure to control bearing clearance; Reduce the surface roughness of the contact surface; Improve lubrication structure to prevent inner ring burns; To prevent the creep of the inner ring caused by the stirring and heating of the lubricating oil, the inner ring is subjected to size stabilization heat treatment.
2. Holder
2.1 Cage structure
The traction motor bearing adopts a cage guided by a rolling element. Compared with the original outer ring guide cage, it can reduce the bearing temperature rise caused by high-speed rotation, reduce wear, and improve the inflow of lubricating grease based oil to the outer ring raceway surface. CTBU, which represents the development direction of axle box bearings, adopts plastic cages and leaves unique inspection window holes on the window holes of the cages to facilitate inspection of the inner race raceway and large retaining edges.
The small gear shaft in the gearbox serves as the input shaft and is subjected to vibration from the coupling during high-speed rotation, which can easily cause wear and even fatigue fracture of the bearing cage. Therefore, a reinforced cage with a soft nitrided surface layer is used.
2.2 Plastic Cage
Plastic cages are made by injection molding, mostly filled with glass fibers in polyamide resin to improve mechanical strength, dimensional stability, and impact resistance. The addition of glass fibers in foreign raw materials during the polymer synthesis process has extremely high uniformity. However, domestic materials are modified by adding glass fiber, which results in poor uniformity and to some extent affects the performance and stability of the cage. The quality of plastic cages is about 1/6 that of steel cages. They have excellent mechanical properties, good stability, good self-lubricating effect, low friction coefficient, high impact fatigue resistance, and improved bearing performance under lean oil lubrication and high-speed operation conditions. Even if the bearing is in a damaged state, it will not cause rotation obstacles and can operate continuously.
In the oil free test conducted by foreign companies, even if the plastic cage is in a melted or fractured state, there is no phenomenon of "holding the shaft", which greatly delays the occurrence of faults, prevents or eliminates the occurrence of hot cutting shafts, avoids malignant accidents, and improves the safety of vehicle operation. SKF Company has been using plastic cages in tapered roller bearing components since 1990 and has gained a lot of experience and data in practical applications; The plastic cage developed by NTN Company using dynamic analysis of cage motion and advanced finite element analysis design methods has a lifespan of more than three times that of ordinary materials; The cylindrical roller bearing developed by FAG company uses polyamide material as its retainer, which closely matches the sealing of the layered annular structure, and uses special lithium soap lubricating grease with extreme pressure additives, resulting in a service life of 3000 000 km.
Chinese railways have adopted plastic cages as standard components. Although plastic cages have better performance than metal cages, there are limitations to the usage conditions of plastic cages based on their material characteristics, and full attention should be paid when using them.
3. Bearing material
3.1 Steel for railway bearings
Bearing steel is one of the key factors determining the lifespan and reliability of bearings. The bearing conditions in key parts of rail transit vehicles are harsh and variable. In addition to long contact fatigue life and high wear resistance, the performance of bearing steel also requires strong impact toughness and dimensional stability. The primary way to improve the service life of bearing steel is to improve its purity, as even a small amount of oxides, sulfides, or nitride inclusions in the steel can greatly reduce its service life. Therefore, advanced bearing steel often uses electroslag remelting method. In addition, Japan conducts the smelting of high-speed rail bearing steel based on the smelting process of bearing materials for aviation engines. Its ultra-high cleanliness SAE4320 steel (equivalent to China's G20CrNi2MoA steel) adopts a vacuum refining production method (electric furnace → outside furnace refining → RH vacuum degassing → continuous casting), which has been used on bearings of the 700 series high-speed trains on the 285 km/h Shinkansen line [9]. The research on heat treatment, processing technology, and surface modification of bearing metal materials in China is still in the exploratory and experimental stages.
In the process of developing steel for high-speed rail bearings, considerable efforts have been made to improve the composition and processing technology of bearing steel. At present, research has been conducted on using vacuum refining carburized bearing steel instead of electroslag remelting bearing steel and improving the purity of bearing steel materials.
The research direction for steel used in rail transit bearings mainly focuses on the following aspects:
(1) Research on heat treatment technology. In order to ensure the contact fatigue life of bainite treated materials, high fracture toughness, and high impact toughness, it is necessary to select and conduct experimental research on heat treatment technology;
(2) The selection of steel grades and development direction are to use GCr18Mo for bainite isothermal heat treatment, and G20Cr2Ni4A steel can also be considered;
(3) Research on smelting process: The oxygen content in steel is a key factor affecting the service life of bearings, requiring uniform oxide distribution, fine carbide particles, and micro dense microstructure. It is necessary to study a smelting process that can obtain low oxygen content while ensuring other quality.
3.2 Application of Ceramic Materials
Higher requirements, such as high temperature, high speed, high precision, etc., cannot be achieved solely by improving traditional metal bearing structures or improving lubrication conditions. Developing new materials has become an inevitable trend. Research has found that certain ceramic materials have particularly excellent performance and can be used in harsh working environments that are difficult for metal and polymer materials to handle.
While possessing the essential characteristics of bearing materials, the quality of ceramic materials is only 40% of that of steel, which helps to achieve lightweight and high-speed bearings. Under high temperature conditions, it can still ensure high strength and hardness, making ceramic bearings have good wear resistance. The application of ceramic materials in bearing manufacturing has become a hot topic in the world's high-tech research and application.
4. Lubrication and sealing
4.1 Lubrication
Compared with lubricating oil, lubricating grease has the advantages of simple processing, convenient use, and no need for oil storage devices. Therefore, high-speed train bearings are mainly lubricated with grease, but when bearings are lubricated together with other parts such as gears, such as bearings and gears in railway vehicle driving devices, oil lubrication is chosen. Choosing the right grease is very important. The correct judgment of bearing lubrication status has guiding significance for formulating train maintenance cycles. The decomposition detection cycle is determined by the service life of the bearing, and the lubricant has the lowest service life in the bearing parts. Extending the service life of the lubricant can extend the maintenance interval and reduce costs. Therefore, the evaluation of the status of bearing grease is an important basis for formulating vehicle maintenance cycles.
4.1.1 Development of Lubricating Greases
Lubricating grease is composed of a liquid base oil component and a thickener that can maintain the base oil component. There are also small amounts of additives present in different greases. According to the different thickening agents, fats can be divided into sodium soap, calcium soap, lithium soap, or barium soap based lubricants. In the past, sodium soap based lubricants were mostly used, but in recent years, lithium soap based lubricants have gradually become the mainstream. DC motor bearings are generally lubricated with lithium based grease. With the use of AC motors, a composite lithium soap based grease has been developed to further improve the heat resistance and durability of bearings.
Hitachi research suggests that the lubrication quality of axle box bearings is determined by three factors: axle box structure, bearings, and lubricating grease. While improving the structure of bearings and axle boxes, a new type of synthetic oil lubricating grease was developed using a mixture of alkyl phenyl ether and polyol esters as the base oil, composite lithium as the thickener, and amine and sulfur additives added. The bearing life of using this lubricating grease reached three times that of previous Shinkansen electric vehicle bearings, achieving 1200 000 km of non disassembly operation. NTN company has also developed lubricating grease UPG2 containing high-performance additives, which significantly improves temperature rise and lifespan indicators. Synthetic oil lubricating grease has become the main research and development direction of high-speed railway locomotive bearing lubricating grease.
After several generations of research by researchers, the bearing grease for rail transit vehicles in China has gone through the development process of calcium sodium based, lithium based, lithium calcium based, and extreme pressure lithium calcium based lubricants, basically reaching the advanced level of similar products abroad. At present, the Type IV lubricating grease for rolling bearings of railway vehicles draws on the technical characteristics of foreign lubricating greases and uses advanced fatty acid lithium calcium soap to thicken and refine paraffin based base oil. At the same time, additives such as antioxidants, rust inhibitors, extreme pressure and wear inhibitors are added, which have good mechanical stability, colloidal stability, extreme pressure and wear resistance, oxidation resistance, rust prevention, water resistance, and long service life, and can meet the requirements of railway freight car speed increase, heavy load Requirements such as extending maintenance period.
4.1.2 Structure of bearing box grease chamber
The bearing box is a component used to install bearings and fill them with lubricating grease. The structure of the lubricating grease chamber has a significant impact on the lubrication performance and lifespan of bearings. Simple annular grease chambers are commonly used in China, but there is a lack of research and experimental analysis on lubrication systems. The successful bearing box grease chamber structure in foreign countries is designed with not only an annular grease main oil chamber, but also a secondary oil chamber. The main oil chamber prevents the aging of lubricating grease caused by stirring and improves the inflow of lubricating grease based oil into the bearing. The auxiliary oil chamber is used to increase the sealing amount of lubricating grease, and the bypass hole and open air hole use the negative pressure inside the machine to prevent dust and rainwater from entering. Experiments have shown that increasing the depth of the main oil chamber in the grease chamber and increasing the contact area between the main and auxiliary oil chambers can help improve the service life of bearing grease.
4.1.3 Quantitative management of bearing grease
Adopting appropriate grease injection process and amount can effectively extend the service life of bearings. The filling amount of lubricating grease is related to the temperature rise and lifespan of bearings. To ensure the performance and characteristics of traction motor insulation bearings, quantitative filling of lubricating grease should be carried out.
4.1.4 Non disassembly maintenance
Non disassembly maintenance refers to the inspection, cleaning, and filling of new lubricating grease for bearing faults all carried out in a non disassembly state. In response to the requirements of non disassembly maintenance, in addition to developing long-life lubricating greases that extend the disassembly cycle, it is also necessary to study non disassembly bearing structures that can be cleaned with lubricating grease without disassembly. For the traction motor of the vehicle running to the inspection distance, conduct component inspection of the bearing through vibration analysis and fluorescent X-ray in a completely non disassembled state; Clean and fill the bearings that will continue to be used with new lubricating grease using a high-temperature, high-pressure water, and vacuum composite device.
4.2 Sealing
There is a sealing device on the bearing, which is used to prevent the lubricant of the bearing from leaking outward and to prevent the loss of lubrication due to the invasion of foreign objects such as dust and water. A sealing device called oil seal is used on the axle box bearing, which is composed of rubber material, metal rings, springs, etc. The wedge-shaped lip of the oil seal is in contact with the oil baffle ring, ensuring sealing performance. Through experimental research, it has been confirmed that the durability and mechanical strength of fluorinated rubber oil seals are superior to those of acrylic rubber oil seals. In addition, in order to meet the demand for high-speed and suppress the peeling of the sealing lip and shaft, the filling agent has been improved; To prevent changes in lip posture caused by changes in air pressure when trains enter and exit the tunnel, the size and shape of the oil seal lip have been improved to increase the lip stiffness.
The compact tapered roller bearing developed by SKF company comes with a built-in low contact seal, which is based on the mechanism of low friction rubber sealing. It combines labyrinth seals, lip seals, and oil throwing plate components to reliably isolate pollution and achieve long service life and energy-saving of lubricating grease. The sealing seat and gasket have been removed from the bearing design, and the sealing element is embedded between the inner and outer rings of the bearing, saving space and suppressing axle bending. A polymer pad is designed on the mating surface between the inner ring and the rear cover to avoid the increase in axial clearance caused by friction and corrosion between steel and prevent foreign objects from entering the bearing.
5. Conclusion
At present, China's high-speed rail transit is achieving leapfrog development, and the continuous introduction of relevant new products and technologies has accumulated a lot of experience for the technical application of rail transit vehicle bearings. This will greatly promote the development of independent innovation technology for domestic rail transit vehicle bearings, and promote the progress of key technologies, common technologies, and supporting technologies for rail transit vehicle bearings.
Domestic professional railway bearing manufacturers, research institutions, universities, etc. have invested in and applied for relevant research projects on high-speed rail bearings to the country, and have received corresponding support from the country. Through technical research and collection of technical papers on bearings for rail transit vehicles both domestically and internationally, this paper summarizes the current development status and key technologies of bearings for rail transit vehicles both domestically and internationally. It is hoped that this can provide some reference and reference for peers in the bearing industry.
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2023-08-14