NTN has developed wheel hub bearings for more than 40 years, and the market share of automotive wheel hub bearings is also relatively large. We have introduced the development history of the company's wheel hub bearings before. Among them, the third-generation low-friction wheel hub bearing III is a new product developed and applied in recent years. In order to meet the requirements of low friction torque, through the improvement of lubrication and sealing technology, the torque of low friction wheel hub bearing III is reduced by 62% compared with conventional products. The following is a detailed introduction to the optimization of lubrication and sealing of low-friction wheel hub bearing III. 1. Introduction of low-torque wheel hub bearing 1.1 Structure of wheel hub bearing and factors affecting torque The structure example of the third-generation wheel hub bearing is shown in Figure 1. Figure 1 Structure example of the third-generation hub bearing The torque of the hub bearing consists of the rolling resistance of the bearing and the sliding resistance of the seal (outer seal and inner seal), each accounting for about 50%. Various factors affecting the torque of the wheel hub bearing are shown in Figure 2. Various anti-friction technologies include: optimizing the bearing structure parameters, improving the internal grease of the bearing to reduce rolling resistance; improving the sealing rubber material, optimizing the sealing structure and sealing interference to reduce sliding resistance. Fig. 2 Influencing factors of bearing torque 1.2 The evolution of low-friction wheel hub bearings is to reduce torque, and NTN wheel hub bearings have evolved into low-friction wheel hub bearings I, II and III. An overview of the evolution is shown in Table 1, and the torque reduction ratio relative to conventional products is shown in Figure 3. Table 1 Evolution of low-friction wheel hub bearingFig. 3 Evolution of torque reduction Compared with conventional products, the torque of the newly developed low-friction wheel hub bearing III is reduced by 62%. Torque reduction is achieved by applying previously developed technical parameters and newly developed grease (improved internal grease of low friction hub bearing II). 2. Development of lubricating grease 2.1 Functional requirements and technical parameters of new grease In addition to low friction, the internal grease of the hub bearing is also required to have the following functions: 1) Under the load, temperature and speed conditions of the hub bearing, by providing Sufficient oil film and prevent seizing, so as to maintain long service life; 2) Excellent anti-fretting wear performance at low temperature; 3) Good anti-rust and anti-leakage. Based on the above requirements, the technical parameters of the new grease are determined according to the ideas in Table 2. Table 2 Development ideas of new greases 2.1.1 Reduce the rolling contact point between the friction ball and the raceway while maintaining the bearing service life and also include the factor of sliding. Therefore, greases with low kinematic viscosity and low drag coefficient base oils are used to reduce friction caused by sliding. As the kinematic viscosity of the grease base oil decreases, the risk of insufficient oil film increases, a trade-off that can lead to channel damage. This is especially important under heavy load conditions as bearing temperatures increase and grease base oil viscosity decreases further. A base oil with a small drag coefficient is used to control the local temperature rise to maintain sufficient oil film thickness and high viscosity index. Improves evaporation and oxidation resistance to ensure long-term lubricity in heavy-duty areas for equal or better service life than conventional products. 2.1.2 Anti-fretting wear performance at low temperature When a newly manufactured vehicle is transported for long distances on a railway freight car, there will be tiny vibrations between the raceway and the ball of the hub bearing. As indicated by the arrows in Figure 4, the vibration squeezes out the grease between the channel and the ball, creating metal-to-metal contact and causing fretting wear. Fretting wear is more pronounced at low temperatures where grease tends to harden and reduce fluidity. Driving the vehicle in this condition may produce abnormal sounds. Figure 4 Mechanism of fretting wear To ensure the fluidity of the grease at low temperatures, low pour point base oils were used, and thickeners and additives were optimized to improve the anti-fretting wear performance at low temperatures. 2.2 Evaluation test A torque test was performed on the hub bearing with fresh grease filled and the seal with the rubber lip removed. The results are shown in Figure 5. Compared with the conventional product (the original 3rd generation hub bearing), the torque reduction ratio reached 56%, which verified the effect of reducing rolling resistance by improving the internal grease of the bearing. Figure 5 Torque test (room temperature) Reliability evaluation tests were also performed to verify the desired function, see Table 3. The bearings filled with the new grease met all the development objectives of the pilot project and performed all the functions required by the grease inside the bearing. Table 3 Reliability evaluation test of grease inside bearing Note: 1. Turning load: 0.6 G; 2. Speed ​​condition: vehicle speed 200 km/h; 3. Fretting wear test under -20 ℃ environment. 3. The development of the seal The structure comparison of the external seal and the internal seal adopted by the conventional seal and low-friction seal of the third-generation wheel hub bearing is shown in Figure 6. Fig. 6 The construction of conventional seals and low friction seals generally uses 3 contact lips to ensure the necessary protection against the penetration of muddy water. Whereas low friction seals use fewer contact lips to further reduce friction. The reduction of mud water resistance due to the reduction of the sealing lip is avoided by introducing a labyrinth structure and carefully designing the lip shape. Reduced friction is not only due to the structure,Also due to the rubber material with a low coefficient of friction, the optimized design of the lip contact surface and the lip grease. These products have a 70% reduction in sealing torque compared to conventional products, as shown in Figure 7. Figure 7 Seal torque test (room temperature) 4. Low friction hub bearing III combines the newly developed bearing grease with low friction seal, and the torque ratio of this bearing is shown in Figure 8.