In recent years, China's high-speed train industry has developed rapidly. As a key core component, the process of localizing axle box bearings is limited by insufficient fatigue performance. At present, high-speed train bearings rely entirely on imports, and fatigue of domestically manufactured bearing rings has become an urgent problem that must be addressed. The fatigue performance of axle box bearing rings is mainly determined by the metallurgical quality of the bearing steel and the heat treatment process. G20CrNi2MoA steel is a high-quality carburized bearing steel for manufacturing the inner and outer rings of this type of bearing. Although domestic smelting levels have gradually caught up with international levels, there is still a significant gap in carburizing heat treatment technology, and the hardness distribution of the carburized layer is the key factor determining the rolling contact fatigue life of carburized steel bearings. To address this dilemma, the outer ring of the domestic G20CrNi2MoA bearing is the core research object of this study, aiming to explore the influence of the hardness distribution of the carburized layer on the bearing's rolling contact fatigue performance and to optimize the process.

A systematic research system of experimental verification, simulation, and intelligent optimization was constructed. First, the key factors affecting performance were determined through a rolling contact fatigue test, after which the formation mechanism of the carburized layer was analyzed via simulated heat treatment. Subsequently, a model combining a genetic algorithm (GA) and a backpropagation (BP) neural network was introduced to optimize the parameters.

Using this system, innovative results were achieved. The results confirm that a carburized layer with a moderate depth is key to extending the fatigue life of the outer ring, and the model provides a more accurate solution for the hardness distribution curve of the optimal carburized layer (surface hardness: 693 HV; depth of carburized layer: 1.71 mm). Based on this reverse optimization, a complete carburized heat treatment scheme is obtained. The key parameters include a long infiltration time of 16.5 h, a diffusion temperature of 930 ℃, a diffusion time of 6.54 h, a carbon diffusion potential of 1.05%, and an isothermal time of 3.6 h. Targeted tests verify that the rolling contact fatigue life of the outer ring of the bearing treated using this process is approximately 4.7% higher than that of the existing domestic bearing outer ring.

First, there is a significant nonlinear correlation between the rolling contact fatigue performance of the domestic G20CrNi2MoA bearing outer ring and the hardness distribution of the carburized layer, the depth of the carburized layer, and other parameters. Accurately regulating these parameters is key to improving performance. Second, the model combining a GA and a BP neural network provides an efficient and accurate technical approach for optimizing the carburizing heat treatment of bearings, thereby greatly reducing the cost and cycle time of traditional trial-and-error methods. Third, the carburizing heat treatment process proposed in this study effectively improves the fatigue performance of domestic bearing rings. It provides important support, with both theoretical value and practical guiding significance, for addressing the technical gap in the heat treatment of high-speed bearings in China, breaking the monopoly of foreign technology, and promoting the localization of high-speed train bearings. It also lays a foundation for subsequent research and the development of higher-speed grade bearings.

The internal flow channel design and oil immersion depth of gearboxes play a crucial role in determining the lubrication effectiveness of gears and the temperature rise within the gearbox. These effects intensify as train speeds increase. This study focuses on a specific high-speed rail aluminum alloy gearbox, using Simcenter STAR-CCM+ (hereinafter referred to as Star CCM+, a multi-physics simulation software) simulation software to develop a thermal-fluid-solid coupling simulation and analysis model. By integrating the simulation results with bench test data, this study aims to investigate the internal flow field and temperature field of the gearbox. The effects of various factors, including rotational speed, oil immersion depth, and steering direction, on the flow and temperature distributions within the gearbox are examined, providing insights into optimal operating conditions and potential design improvements.

To analyze the performance of the gearbox, parameterized simulation analyses were performed considering different rotational speeds, oil immersion depths, and steering directions. The distribution of the internal flow and temperature fields under these varying conditions was studied. The analysis also focused on the mass flow rate and temperature field of each flow channel. This comprehensive approach allowed for a detailed evaluation of the lubrication performance of the gearbox. The Star CCM+ simulation model was calibrated using experimental data from a 1∶1 test bench, where temperature measurements were taken at various points within the gearbox. These measurements were compared with the simulation results to ensure the accuracy and reliability of the simulation model. The study also incorporated detailed thermal conditions, including gear frictional power losses, bearing power losses, and forced convection heat transfer, to represent the true working conditions of the gearbox under different operational scenarios.

The simulation results showed that the lubrication and temperature control effects of the gearbox were most effective when the internal oil immersion depth was between 1.75 and 2.00 times the tooth height. It was found that insufficient lubrication occurred on the upper and right sides of the gearbox, highlighting areas that require design improvements. Additionally, the research revealed that increasing the oil immersion depth improves the flow and distribution of lubrication oil within the gearbox. However, a deeper oil immersion beyond the optimal range increases churning losses and heat generation. By adjusting the flow channel configuration and improving the number and distribution of the internal flow paths, the optimized design reduced the temperatures in critical areas, including the bearing and meshing zones, by approximately 5℃. This improvement was achieved by increasing the oil flow to the gears and bearings while enhancing the cooling effect on the gearbox walls.

This study demonstrates that a proper oil immersion depth is critical for maintaining effective lubrication and temperature control in high-speed rail aluminum alloy gearboxes. The results highlight that there is an optimal oil immersion depth range (1.75-2.00 times the tooth height) that ensures sufficient lubrication and effective cooling. Furthermore, the study reveals that there are areas within the gearbox, particularly on the upper and right sides, where lubrication is insufficient, suggesting that the design of the flow channels in these regions can be improved. The proposed modifications, such as the addition of more flow channels and optimizing their distribution, provide a substantial enhancement in the lubrication and cooling efficiency of the gearbox. These modifications result in a notable temperature reduction of approximately 5℃ in key areas, thereby demonstrating the effectiveness of the flow channel optimization strategy. This research provides insights into future gearbox design, particularly in optimizing lubrication systems and minimizing temperature rise to ensure the reliable operation of the system at high speeds.

Currently, iron or steel brake disks generally be selected to research braking noise. This research focuses mainly on the influence of the structural characteristics of the brake pads and brake disks on the braking noise, and few studies addresse how braking conditions or braking modes affect the noise characteristics. To clarify the causes of the abnormal noise in the braking process of different friction pairs, through a noise test of the multiscale bench braking test and the simulation method of complex eigenvalue analysis, this paper studys the braking noise characteristics and the generating mechanism of different friction pairs under different braking conditions and braking modes.

According to the scaled test scheme, a scaled test of three types of friction pairs is performed on the MM-1000/IV scaled test machine, and the braking noise characteristics of the friction pairs are compared and analyzed. For the selected friction pair of the SiCp/A356 composite brake disk and synthetic pad, a more realistic 1∶1 bench brake test is performed. On the basis of HyperMesh and Abaqus software, a finite element simulation model of the wheel-mounted disk foundation braking device with a wheel, disks, pads, pad holders, brake levers, etc. is constructed.

Under the same working conditions, the sound pressure levels (SPL) of the two SiCp/A356 composite friction pairs were lower than those of FE-928W friction pair. The maximum value of maximum sound pressure level (Max SPL) was 96 dB for the FE-928W friction pair and 87 dB for the BJTU-928W friction pair. The braking noise type of the FE-928W friction pair was mainly a multifrequency squeal, and the braking squealing phenomenon occured in most working conditions. The noise types of the BJTU-928W friction pair and the KNORR-928W friction pair were mainly wide area noise and single frequency squeal. The friction surfaces of different friction pairs had different degrees of surface scratching, material transfer, and peeling, which led to differences in the braking noise. The main frequency of the squealing noise of the friction pair between the SiCp/A356 composite brake disk and the synthetic brake pad was concentrated near 2.20 kHz, and the Max SPL could reach 110 dB. Brake pads were the main source of braking noise. The greater the energy inputed, the more times of high-temperature continuous braking, the greater the squealing sound pressure level, and the more severe the squeal. In contrast, when braking at a lower energy input level, the disk state would be restored, and quiet braking could be achieved during subsequent braking.

Through the experiments and simulations, the simulation method of complex eigenvalue analysis can better predict the tendency of braking noise, and the braking noise characteristics and causes of different friction pairs under different braking conditions and braking modes are clarified. This research provides a scientific basis for noise reduction strategies such as friction matching, structural design optimization, and braking mode improvement of the friction pair between the SiCp/A356 composite brake disk and the synthetic brake pad.