【Technology】Railway Vehicle Bearing Lubrication Technology

Bearings for railway vehicle axles, traction motors, and gear systems are currently being demanded to achieve higher speeds, lighter weight, and more compact dimensions to enhance performance. Simultaneously, there is a pressing need for extended disassembly inspection intervals and longer service life in bearing maintenance.

The following describes new trends in railway vehicle bearings and their lubrication technology against this backdrop.

1. Required Functions of Axle Bearings

In addition to bearing static and dynamic radial loads resulting from vehicle and load mass, axle bearings also must withstand non-constant axial thrust. As a component of the running gear, bearing failures can have a significant impact on vehicle operation. Consequently, meticulous consideration has long been given to the selection, design, and manufacture of bearing structures. Axle bearings have evolved to meet the needs of various vehicle generations, with recent demands for higher speeds, lighter weight, and maintenance-free axle bearings.

Regarding Shinkansen speed increases, JR Central's 300 Series Shinkansen EMUs began operating at a maximum speed of 270 km/h in 1992, and JR West's 500 Series Shinkansen EMUs began operating at 300 km/h in 1997. Even on existing lines, JR East's 651 Series Limited Express EMUs first began operating at 130 km/h in 1989. Since then, JR companies have competed to increase train speeds on existing lines. Axle bearings have undergone structural developments during these speed increases.

The second key requirement is lightweighting, especially with higher-speed trains. This lightweighting of the vehicle itself is crucial for energy conservation and minimizing impact on the track. Furthermore, reducing unsprung mass and intersprung mass (the mass between the axle spring and air spring) is believed to improve stability during high-speed operation, thus further necessitating lightweighting of axle bearings.

Recently, railway companies have been pushing for maintenance-free operation, especially in light of their management philosophy. According to a Ministry of Transport order, Japanese railway vehicles are inspected or repaired based on designated mileage or operating hours. Recently, extending vehicle maintenance intervals has become a trend. For example, Shinkansen vehicles currently undergo comprehensive inspections after less than 900,000 km or three years of operation; the goal is to extend this to 1.2 million km by the early 21st century. Currently, axle bearings are inspected during bogie inspections, but in the future, maintenance-free operation will be required after 1.2 million km.

2. Axle Bearing Life and Maintenance

Axle bearings are designed to last until delamination of the roller and raceway surfaces occurs. Regarding maintenance, axle bearings are disassembled and inspected during bogie inspections (less than one year of operation or 450,000 km) and comprehensive inspections (less than three years of operation or 900,000 km) for Shinkansen EMUs, and during key inspections (less than four years of operation or 600,000 km) and comprehensive inspections (less than eight years of operation) for existing EMUs. Furthermore, axle bearings are replaced on a planned basis after reaching a certain mileage (3 million km) or number of years (6-7 years) on Shinkansen trains. To date, bearing replacement has not been required due to delamination, but rather due to electrolytic corrosion. While bearings may be replaced during inspections if dents or scratches are detected, planned replacements account for the majority of these cases.

The lifespan of the grease and seals in grease-sealed tapered roller bearings is a key concern. Currently, Shinkansen vehicles undergo axle bearing inspections during every bogie inspection. However, on existing lines, sealed axle bearings are not removed for inspection until the wheel diameter reaches its limit (approximately 1 million km of average mileage) after a comprehensive inspection. Therefore, ensuring that the grease and seals achieve this minimum lifespan is crucial.

3. Traction Motor Bearings

The torque of an electric vehicle's traction motor is transmitted via a coupling to a pinion in the gear mechanism, which in turn transmits it to a large gear press-fitted onto the axle, ultimately generating the driving and braking forces between the wheel and rail. The bearings used in the rotating mechanism of the traction motor and gear mechanism are crucial. The coupling method between the traction motor and gear mechanism varies. A typical example of a parallel universal joint drive system shows the bearing locations. Electric vehicles operating on narrow-gauge lines use flexible plate couplings due to limited bogie space. Shinkansen electric vehicles operating on wider-gauge lines, however, use gear couplings. Recently, due to the compactness of traction motors, even existing trolley buses have adopted CFRP plate couplings that can produce the necessary flexible displacement within a short distance.

4. Traction Motor Bearing Function

Traction motor bearings in electric vehicles withstand radial loads generated by the mass of the rotor shaft and coupling, rotate at high dmN values (the product of the pitch diameter of the bearing rolling elements and the rotational speed), and frequently and alternately start and stop. Due to the influence of bogie vibration during operation, the bearings are subject to dynamic loads. Due to their relatively low static mass, the calculated life of traction motor bearings is significantly longer than that of axle bearings and gear bearings. Typically, these bearings use a cylindrical rolling bearing on the gear side and a deep-groove ball bearing on the opposite side, which also supports the axial movement of the rotor shaft and is grease lubricated. Recent electric vehicles are equipped with VVVF-controlled AC motors, achieving lightweight and compact motors while maintaining high speeds. The relationship between the maximum operating speed of Shinkansen and existing electric vehicles and the dmN value of the traction motor bearings (gear side) is discussed. The dmN values of the bearings used in traction motors for Shinkansen and existing railway vehicles are essentially the same. The corresponding gear ratios between the large and small gears are approximately 2-3 for Shinkansen electric vehicles and 4-7 for existing railway vehicles, twice as much. Therefore, the cumulative number of bearing rotations per vehicle traveled for a given distance is approximately twice that of the Shinkansen electric vehicles.

When using AC motors, efforts are made to improve the heat resistance and durability of the bearings and grease to address the high-speed rotation of the bearings and the temperature rise of the traction motor components. DC motors are typically lubricated with lithium-based grease, a common grease used for bearings in rotating machinery. With the adoption of AC motors in traction motors, lithium-complex soap greases have been developed to improve heat resistance and durability. These greases are widely used in vehicles using AC motors, including the 300 series Shinkansen electric vehicles. They are also widely used in traction motors for newer existing railway electric vehicles using AC motors. Regarding bearings, changes in the cage guidance method have reduced grease degradation; bearing insulation has prevented electrolytic corrosion; and the application of dimensional stabilization heat treatment has suppressed bearing dimensional fluctuations. The cage is a bearing component that precisely maintains the rotational motion of the rolling elements. Its rotational guidance can be achieved by either guiding along the outer ring inner diameter surface (outer ring guidance) or by using the rolling element (roller) raceway (roller-guided guidance). With an outer ring guidance, the cage forms sliding contact with the outer ring inner diameter surface. If lubrication performance decreases due to grease aging and electrolytic corrosion, the cage guide surface and the outer ring inner diameter surface are susceptible to wear. With a rolling element guidance method, sliding contact with the raceway surface ensures greater lubrication, preventing grease degradation even under reduced lubrication conditions. Coupled with advances in cage manufacturing technology and cost reductions, rolling element-guided cages are now being used in traction motor bearings. Various countermeasures have been implemented to improve the effectiveness of electrolytic corrosion prevention, and bearing insulation has been identified as the most effective method. Bearing insulation treatment involves coating the outer diameter and end faces of the outer ring with insulating materials such as ceramics primarily composed of alumina and PPS resin (polyphenylene sulfide). The ceramic is applied by spraying, while the PPS is applied by injection molding, ensuring strong adhesion.

Ceramic-coated insulated bearings were first used in the main motor bearings of the 300 series Shinkansen, and their application has since expanded to other Shinkansen electric vehicles. While PPS has a drawback of somewhat limited toughness, it offers heat resistance, dimensional stability, and good moldability. Glass fiber is added during molding to improve toughness. Alumina ceramics are superior to PPS resin in terms of physical properties such as thermal, mechanical, and dielectric breakdown characteristics. However, while PPS is slightly inferior, this is not a problem in practical application. Furthermore, due to its cost advantages, PPS resin-coated insulated bearings were first used in JR East's E209 series electric vehicles and are now widely used in new existing line electric vehicles equipped with AC motors.

During the development of insulated bearings, not only were the characteristics necessary for bearing rotation tested, but disassembly and assembly tests with the axlebox were also conducted. Furthermore, a series of evaluation tests were conducted from a maintenance perspective, including chemical cleaning of the bearings and liquid immersion tests.

5. New Maintenance Method for Traction Motor Bearings

Initiated by the introduction of AC motors in the 300 Series Shinkansen electric vehicles, a non-disassembly inspection method, primarily focusing on the bearings, was developed to replace the conventional disassembly inspection method. For traction motors used in vehicles that have reached the inspection interval, bearings are inspected for abnormalities without disassembly, and grease is cleaned and refilled in re-used bearings. Bearing abnormalities are detected through vibration analysis and on-site grease analysis using fluorescent X-rays. Grease cleaning is performed using a combination of high-temperature, high-pressure water, and vacuum suction.

6. Function of Gear Bearings

Gear bearings utilize a pair of single-row tapered roller bearings for both the gear and pinion. The pinion bearing consists of a frontal assembly, one on the traction motor side, and one on the wheel side. Due to different couplings, the gear bearing may have both a back-to-back and a frontal assembly. Gear bearings function as a support mechanism, both absorbing vibrations during operation and smoothly transmitting rotational force to the axle. These bearings operate under stringent conditions, with load and speed, including vibration conditions, as the primary design considerations. They are lubricated with a bath lubrication system using No. 80 or No. 90 gear oil. Bearing loads are calculated based on the gear unit's rated torque, taking into account the gear meshing reaction force, including the dynamic load factor, and the vibration acceleration during operation, with the inertia of the pinion shaft and gear (transmission) case as the primary load factors. The axial and radial bearing loads are calculated based on the gear transmission's dimensional factors. In recent years, the gearboxes of Shinkansen electric vehicles from the 300 series onwards have been converted from cast steel to aluminum alloy (JISAC4C). This has resulted in miniaturization, with a mass of approximately 1/2 of the previous model. This reduction in unsprung mass has also contributed to lower bearing loads. Recently, the size of Shinkansen electric vehicle bearings has been miniaturized alongside the lightweighting of bogie components.

Tapered roller bearings form rolling and sliding contact between the large inner ring ribs and the large end faces of the tapered rollers. The PV value, the product of the surface pressure at the contact point and the sliding velocity V, serves as a guide for determining the risk of thermal seizure and galling. While there appear to be some differences in surface pressure and sliding velocity at the contact point between Shinkansen electric vehicles and existing rail vehicles, the PV value shows no significant difference.

7. Improving the Performance of Gear Bearings

Gear bearings, particularly pinion bearings, are subject to vibration during vehicle operation, generating various high-frequency alternating stresses in various parts of the cage. Ensuring the reliability of cage fatigue strength is a particularly challenging issue as Shinkansen electric vehicles increase in speed. The fatigue strength of the cage has been significantly improved by increasing the thickness of the retainer plate to enhance rigidity and reduce stress, and by applying a soft nitriding treatment to the retainer surface to improve wear resistance and fatigue strength.

Shinkansen electric train gear systems, characterized by high-speed, continuous operation, experience high temperature rises due to agitation of the lubricating oil. To prevent creep of the bearing inner ring caused by the increased inner diameter over extended use, the bearing inner ring is typically subjected to dimensional stabilization heat treatment. Inner ring rib burns have been reported in existing electric train pinion bearings. During startup of high-ratio electric trains, the gears accelerate rapidly, and the rapid temperature rise can easily reduce the bearing axial clearance, leading to oil film rupture between the friction surfaces of the large inner ring rib and the roller end faces. While this does not normally present a problem during operation, during winter startups, due to the low outdoor temperature and high lubricating oil viscosity, lubricant circulation can be difficult. To prevent inner ring rib burn, efforts should be focused on increasing the lower limit of the bearing's axial clearance management value, improving the lubrication structure within the gearbox, and, in cold regions, appropriately reducing the lubricant viscosity. Furthermore, improvements in bearing design can be made, such as moving the contact point between the inner ring's large rib and the roller's large end face closer to the center of rotation, reducing the sliding velocity at the contact point, and improving the surface roughness of the contact surface.

8. Conclusion

Railway bearings have evolved to meet the needs of each era. Future challenges lie in achieving longer maintenance-free operation and more accurate understanding of operating conditions, primarily based on actual loads. Furthermore, with significant advances in bearing materials, studying the tribology of sliding contact areas, such as rollers and ribs or cages, is crucial for improving rolling fatigue strength.