Abstract: From the five aspects of bearing structure design, cage, material, lubrication and sealing, and standard, this paper summarizes the technical status and development of key bearings for rail transit vehicles such as traction motor bearings, axle box bearings, gear box bearings, etc. Reference from domestic counterparts.
During the “Twelfth Five-Year Plan” period, the total mileage of China’s national railways will increase from the current 91,000 km to about 120,000 km. Among them, the construction of high-speed rail and urban rail transit has shown an unprecedented development situation. A new round of rail transit construction has been gradually launched nationwide, and China has become the world’s largest urban rail transit market.
Bearings are the key components for vehicle operation safety, and their performance and reliability need to meet higher requirements. At present, my country’s rail transit vehicle bearings are in a period of technological upgrading. There is still a certain gap between the technical level of China’s bearings and the international advanced level. For example, bearings for high-speed trains and high-speed trains all rely on foreign imports. Therefore, it is imperative to learn advanced bearing technology and accelerate the localization of bearings for rail transit vehicles.
Here, from the five aspects of bearing structure design, cage, material, lubrication and sealing, and standard, the technical development of traction motor bearings, axle box bearings, gear box bearings and other key bearings of rail transit vehicles is introduced for reference. .
1 Structural design of bearings
The technical development of bearings for rail transit vehicles should conform to the development direction of safe vehicle operation, high speed, extended maintenance period, and improved operating comfort.
- 1 Traction motor bearing
- 1. 1 Technical features
Traction motor is one of the key equipments to realize the high speed of rail transit vehicles. With the development of power electronic equipment and control technology, traction motors are gradually converted from DC motors to asynchronous motors, and direct-drive traction motor systems are also being developed to meet the needs of small, lightweight, energy-saving, maintenance-saving, and long-life motors. Requirements. The speed of the traction motor is generally 2 000 to 4 000
r/min , the speed of the new high-speed railway traction motor can reach 6 000 r/min, and the high-speed traction motor above 7 000 r/min has been developed. The traction motor bearing mainly bears radial load. Due to the influence of the vibration of the bogie during operation, the bearing also needs to bear a certain amount of load.
Dynamic load, realize frequent operation and stop cycle operation. Compared to axlebox bearings and gearbox bearings, traction motor bearings have a much longer calculated life. The general configuration of bearings is as follows: cylindrical roller bearings are used on the gear side (transmission side), deep groove ball bearings are used on the counter gear side (commutator side), and grease lubrication is mostly used for lubrication [2-4]. In order to realize the high-speed rotation of the bearing and effectively restrain the temperature rise of various parts of the traction motor, the heat treatment process of the bearing, the insulation performance, the heat resistance and durability of the grease, and the guiding method of the cage have been improved [2-3].
- 1. 2 Insulated bearings
In traction motors, the galvanic corrosion caused by the current passing through the bearing is extremely harmful and may cause premature aging of the grease in the bearing and even damage to the bearing. In order to prevent the phenomenon of electric corrosion, considering the price and reliability, insulated bearings are adopted as the solution. There are generally three types of insulated bearings used in traction motors: hybrid ceramic bearings, ceramic sprayed bearings, and resin-coated bearings .
(1) Hybrid ceramic insulated bearings, which use ceramic materials to make rolling elements or rings, are technically difficult due to the preparation and processing of ceramics, but have excellent insulation and high-speed performance. Ceramic rolling elements or ferrules can completely cut off the current path, which can better prevent electrical corrosion damage. In addition, the contact between the ceramic and the steel does not “micro-weld” in poor lubrication conditions, and the bearing will not seize even in the state of complete lubrication failure. However, considering that the brittle failure of ceramics can cause serious consequences, hybrid ceramic insulated bearings are rarely used in high-speed trains. At present, the domestic hybrid ceramic insulating bearing is still in the research and trial production stage [2-5].
(2) Ceramic spraying insulated bearings, using plasma spraying method, use alumina-based ceramic powder to spray and insulate the outer diameter surface of the outer ring. In order to prevent high-frequency current from flowing and causing electrical corrosion, the thickness of the sprayed film should be greater than 0. 3 mm [3,5]. The insulating bearings produced by NTN Company use multi-layer coatings of ceramic and metal on the outer surface, and the thickness of each layer is 0.0. About 2 mm; depending on the application, only one layer of ceramic coating can be used . At present, this type of ceramic insulating bearing has been successfully used in Japan’s Shinkansen 300 series, 500 series, 700 series EMUs and my country’s “Harmony” EMUs and high-speed trains .
(3) For resin-coated insulated bearings, the outer surface of the outer ring is sprayed with insulating materials such as PPS resin by injection molding. PPS resin insulated bearings have excellent electrical insulation performance, low price and wide application. Although PPS insulated bearings can prevent electrical corrosion, due to the low thermal conductivity of PPS resin, compared with non-insulated bearings, high temperatures are easily formed inside the bearing . In order to maintain the excellent performance of PPS insulated bearings and improve thermal conductivity in the application, non-conductive high thermal conductivity fillers are added to the original glass fiber reinforced PPS resin to improve the thermal conductivity of the resin [3,5].
- 2 axlebox bearings
The axle box bearing needs to bear the body weight and load of the entire vehicle, and also needs to bear the forces in all directions generated by the swing of the vehicle during operation. In addition to bearing static and dynamic radial loads, it also bears non-constant axial loads. With the high speed of the vehicle, in order to ensure the running stability of the vehicle, the structural parts of the axle are required to be as lightweight and miniaturized as possible, that is, the compact design of the axle box bearing is required. Moreover, the lightening of vehicles is closely related to energy saving, which can effectively reduce the impact of vehicles on the rails [3, 6].
In recent years, axlebox bearings for high-speed rail vehicles at home and abroad have developed from oil bath lubricated cylindrical roller bearings with ribs to grease lubricated maintenance-free sealed double row tapered roller bearings, which can bear both radial and axial loads , and meet the requirements of compact structure and light weight. The use of grease-lubricated seals can simplify maintenance and improve operational stability by reducing the axial clearance of the bearing . SKF has developed the Compact Tapered Roller Bearing Unit (CTBU) by applying FEM finite element analysis to the design of axlebox bearings. Plastic cages and low friction contact seals are used in the bearing design to shorten the axle, reduce axle bending caused by the bearing, and reduce unsprung mass. Sensors for monitoring speed and bearing status are also installed in the CTBU assembly . Since the compact design can prolong the maintenance period, it can further improve the bearing performance and safety. It has the characteristics of low operating temperature, long grease life, and optimal sealing performance. It has been widely used and is also the development direction of axle box bearings  .
- 3 Gearbox bearings
The torque of the traction motor of the electric vehicle is first transmitted to the pinion shaft on the gear device through the coupling, and then transmitted by the pinion to the large gear press-fitted on the axle, and finally converted into the driving force or braking force of the wheel/rail system . Bearings for gear devices mostly use a pair of single-row tapered roller bearings , and use lubricating oil together with gears. The function of the gearbox bearing is to act as a support. While bearing the running vibration, it can smoothly transmit the rotational force to the axle. The vibration load and rotational speed need to be considered in the design. The axial clearance of tapered roller bearings needs to be precisely adjusted during assembly. In addition to improving the pinion structure to control the axial clearance, it is also possible to use bearings that do not require adjustment of the axial clearance during assembly, such as a combination of three-point or four-point contact ball bearings and cylindrical roller bearings. Technological developments in recent years include: improving the structure to control bearing clearance; reducing the surface roughness of the contact surface; improving the lubricating structure to prevent inner ring burns; Stabilization heat treatment .
- 1 Cage structure
Traction motor bearings use cages guided by rolling elements. 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 grease base oil to the outer ring raceway surface . The CTBU, which represents the development direction of axlebox bearings, adopts a plastic cage and has a unique inspection window on the window hole of the cage to facilitate the inspection of the inner ring raceway and large rib. The pinion shaft in the gearbox is used as the input shaft, which is subjected to vibration from the coupling under high-speed rotation, which is easy to cause the bearing cage to suffer from wear and even fatigue fracture. Therefore, a reinforced cage with a nitrocarburized surface layer is used. frame [3,7].
- 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 glass fibers in foreign raw materials are added during the polymer synthesis process, and have extremely high uniformity. The domestic material adds glass fiber in the modification process, which has poor uniformity, which affects the performance and stability of the cage to a certain extent. The quality of the plastic cage is about 1/6 of that of the steel cage. It has excellent mechanical properties, good stability, good self-lubricating effect, low friction factor, and high impact fatigue resistance, which improves the bearing’s performance in lean oil lubrication and high-speed operation. performance under conditions . Even if the bearing is damaged, it will not cause a rotation obstacle and can run continuously . In the oil-free test conducted by foreign companies, the plastic cage did not “hold the shaft” even if it was in a melted or broken state, which greatly delayed the occurrence of failures, prevented or eliminated the occurrence of eager shafts, avoided vicious accidents, and improved the safety of vehicle operation. SKF company has used plastic cages in tapered roller bearing components since 1990, and has obtained a lot of experience and data in practical applications ; NTN company adopts dynamic analysis of cage motion and advanced finite element analysis and design methods The developed plastic cage has a lifespan of more than 3 times that of ordinary materials ; the cylindrical roller bearing developed by FAG is made of polyamide material for its cage, which is closely matched with the seal of the layered annular structure, and uses added The special lithium soap grease with extreme pressure additives enables its service life to reach 3 000 000 km . China Railway has adopted plastic cages as standard components . Although the performance of the plastic cage is better than that of the metal cage, according to its material characteristics, the use conditions of the plastic cage are limited, and full attention should be paid when using it.
3 Bearing material
- 1 Steel for railway bearings
Bearing steel is one of the key factors that determine the life and reliability of bearings. The working conditions of bearings in key parts of rail transit vehicles are harsh and changeable. For the performance of bearing steel, in addition to long contact fatigue life and high wear resistance, it is also required. It has strong impact toughness and dimensional stability . The primary way to improve the service life of bearing steel is to improve the purity of the steel, because even a small amount of oxides, sulfides or nitrides in the steel will greatly reduce its service life. Therefore, electroslag remelting is often used for advanced bearing steels. Law. In addition, Japan smelts high-speed rail bearing steel according to the smelting process of bearing materials for aero-engines, and its ultra-high cleanliness SAE4320 steel (equivalent to China’s G20CrNi2MoA steel) is produced by air refining (electric furnace → out-of-furnace refining → RH vacuum dehydration The method of gas → continuous casting) has been used on the bearings of the 285km/h Shinkansen 700 series high-speed train . In my country, the research on heat treatment, processing technology and surface modification of bearing metal materials is still in the stage of exploration and testing . In the process of developing high-speed rail bearing steel, considerable efforts have been made to improve the composition of bearing steel and its processing technology . At present, the vacuum refining carburized bearing steel has been studied to replace the electroslag remelting bearing steel and the process method to improve the purity of the bearing steel material . The research direction of bearing steel for rail transit mainly focuses on the following aspects:
(1) Research on heat treatment process, in order to ensure the contact fatigue life after bainite treatment, to ensure high fracture toughness and high impact toughness, it is necessary to select and test the heat treatment process [9-10];
(2) The selection of steel grades, the development direction is to use GCr18Mo for bainite isothermal heat treatment, and G20Cr2Ni4A steel can also be considered [9-10];
(3) Research on the smelting process, the oxygen content in the steel is a key factor affecting the service life of the bearing. It requires uniform oxide distribution, fine carbide particles, and micro-dense microstructure. It must be studied to obtain low oxygen content and ensure that the Other quality smelting process [9-10].
- 2 Application of ceramic materials
Higher requirements, such as high temperature, high speed, high precision, etc., can no longer meet the requirements only by improving the traditional metal bearing structure or improving the lubrication conditions, and it is the general trend to develop new materials. Studies have found that some ceramic materials have particularly good properties and can be used in harsh working environments where metal materials and polymer materials are difficult to handle. While possessing important properties necessary for bearing materials, the mass of ceramic materials is only 40% of that of steel, which contributes to the reduction of weight and speed of bearings. It can still ensure high strength and high hardness under high temperature conditions, so that the ceramic bearing has good wear resistance. The application of ceramic materials to bearing manufacturing has become a hot spot in the world’s high-tech R&D and application .
4 Lubrication and sealing
- 1 Lubrication
Compared with lubricating oil, grease has the advantages of simple handling, convenient use, and no oil storage device. Therefore, the bearings of high-speed trains are mainly lubricated by grease , but when the bearings are lubricated together with other parts such as gears, such as bearings and gears in the driving device of railway vehicles, oil lubrication is selected. It is very important to choose the right grease. The correct determination of bearing lubrication status has guiding significance for the formulation of train maintenance cycle. The decomposition detection cycle is determined by the bearing life, and the lubricant has the lowest life among the bearing parts. Extending the life of the lubricant can prolong the maintenance interval and reduce the cost. Therefore, the evaluation of the bearing grease status is an important basis for formulating the vehicle maintenance cycle.
- 1. 1 The development of grease
Grease is composed of a liquid base oil component and a thickener that can maintain the base oil component. There are also a small amount of additives in different greases. According to different thickeners, grease can be divided into sodium soap, calcium soap, lithium soap or barium soap, etc. In the past, sodium soap was mostly used. In recent years, lithium soap grease has gradually become the mainstream. DC motor bearings are generally lubricated with lithium-based grease. With the use of AC motors, in order to further improve the heat resistance and durability of bearings, complex lithium soap-based grease has been developed . According to Hitachi’s research, the lubrication quality of axle box bearings is determined by three elements: axle box structure, bearings and grease. While improving the structure of bearings and axle boxes, a new type of synthetic oil lubricating grease was developed with a mixture of alkyl phenyl ether and polyol ester as base oil, lithium complex as thickener and adding amine series and sulfur series additives. The bearing life of grease has reached three times that of the previous Shinkansen electric vehicle bearings, and has achieved 1 200 000 km non-disintegration operation . NTN has also developed lubricants with high performance additives
Lipid UPG2, the temperature rise and life indicators improved significantly . Synthetic oil grease has become the main research and development direction of high-speed railway locomotive bearing grease . my country’s rail transit vehicle bearing grease has been studied by several generations of scientific researchers, and has experienced calcium-sodium-based, lithium-based, lithium-calcium-based, extreme pressure type
The development process of lithium-calcium-based lubricants has basically reached the advanced level of similar foreign products. At present, the type IV lubricating grease for rolling bearings of railway vehicles draws on the technical characteristics of foreign lubricating greases, and adopts the highly refined paraffin base oil thickened with high-grade fatty acid lithium calcium soap. It has the characteristics of good mechanical stability, colloidal stability, extreme pressure and wear resistance, oxidation resistance, rust resistance, water resistance and long life, which can meet the requirements of railway freight cars for speed increase, heavy load, and extended maintenance period  .
- 1. 2 Bearing housing grease chamber structure
The bearing housing is the part that installs the bearing and fills the grease. The structure of the grease chamber has an important influence on the lubricating performance and life of the bearing. A simple annular grease chamber is widely used in China, and there is a lack of research and experimental analysis on the lubrication system. The successful foreign bearing housing grease chamber structure is designed with an auxiliary oil chamber in addition to the annular grease main oil chamber. The main oil chamber prevents the grease from aging caused by stirring and improves the inflow of the grease base oil in the bearing, the auxiliary oil chamber is used to increase the sealing amount of the grease, and the bypass hole and the open air hole use the negative pressure in the machine Prevent dust and rain from entering. try
Experiments show that increasing the depth of the main oil cavity of the grease chamber and increasing the contact area between the main oil cavity and the auxiliary oil cavity can help to improve the service life of the bearing grease [4, 12-14].
- 1. 3 Quantitative management of bearing grease
Using appropriate grease injection process and grease injection quantity can effectively prolong the service life of bearings. The amount of grease filled is related to the bearing temperature rise and service life. In order to ensure the performance and characteristics of the traction motor insulated bearing, the grease should be filled quantitatively [4, 15].
- 1. 4 Non-disassembly maintenance
Non-disassembly maintenance means that the inspection, cleaning, and filling of new grease for bearing failures are all carried out in a non-disassembled state. In response to the requirements of non-disassembly maintenance, in addition to developing long-life greases that extend the disassembly cycle, it is also necessary to study a non-disassembled bearing structure that can be cleaned without disassembly. For the vehicle traction motor running to the inspection distance, the bearing is sub-inspected by vibration analysis and fluorescent X-ray in a completely non-disassembled state; the bearing that continues to be used is cleaned with high temperature, high pressure water and vacuum composite device and filled with new grease [3-4, 16].
- 2 seal
There is a sealing device on the bearing, which is used to prevent the lubricant of the bearing from leaking to the outside, and to prevent the loss of lubrication due to the intrusion 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 ring, spring, etc. The wedge-shaped lip of the oil seal is in contact with the oil retaining ring to ensure the sealing performance. Through experimental research, it has been confirmed that the durability and mechanical strength of oil seals made of fluorinated rubber are better than those made of acrylate rubber. In addition, in order to meet the needs of high speed, the sealing lip and the shaft are restrained from peeling off, and the filler is improved; in order to prevent the change of the lip posture caused by the change of air pressure when the train enters and exits the tunnel, the size and shape of the oil seal lip are improved. Improved lip stiffness [17-18]. The compact tapered roller bearing developed by SKF has a built-in low-contact seal, which is based on the mechanism of low-friction rubber seals, and uses a combination of labyrinth seals, lip seals, and slinger parts to reliably isolate pollution. , to achieve long life and energy saving of grease. The seal seat and gasket are eliminated in the bearing design, and the seal is built in the middle of the inner and outer rings of the bearing, which saves space and restrains the bending of the axle. A polymer liner is designed on the mating surface of the inner ring and the back cover to avoid the increase of the axial gap caused by the frictional corrosion between the steel and the steel, thereby preventing foreign matter from entering the bearing .
5 Relevant standards
The United States generally uses the Association of American Railroads (AAR) standards, which take North American trucks as the application object . EU member states have begun to use general railway bearing standards, mainly including: rolling bearing standard EN12080 for axle bearings, grease standard EN12081 and performance test standard EN12082, the basic content is shown in Table 1. In addition, EU member states have established standards for reliability, availability, maintainability and safety (RAMS) throughout the entire product life cycle (from feasibility studies to product end-of-life).
Japan has developed a state-owned railway standard (JRS), including the standard content of railway bearings and lubricants, but the standard was abolished in 1982. At present, the Japan Railway Rolling Stock Industry Association is promoting the formulation of a public body standard (JRIS), which includes the life calculation method and performance test method for axle bearings . The above-mentioned RAMS standardization process has also been carried out in Japan .
At present, my country’s high-speed rail transit is developing by leaps and bounds. The continuous introduction of related new products and technologies has accumulated a lot of experience in the technical application of rail transit vehicle bearings, which will greatly promote the development of domestic rail transit vehicle bearing independent innovation technologies. Promote the progress of key technologies, common technologies and supporting technologies for rail transit vehicle bearings. Domestic professional railway bearing manufacturers, scientific research institutions, universities, etc. have invested in and declared relevant research projects on high-speed railway bearings to the state, and obtained corresponding support from the state. Through the technical research and collection of technical papers about rail transit vehicle bearings at home and abroad, the development status and key technologies of rail transit vehicle bearings at home and abroad are summarized, and it is hoped that it can serve as a reference for the counterparts in the be