Abstract: This article focuses on the characteristics and main failure modes of bearings used in driving motors for new energy vehicles, and proposes design solutions based on these analyses, in order to provide a reference for the design of motor bearings for new energy vehicles.
Keywords: new energy vehicles; motor bearings; characteristics; design
1、Characteristics of driving motors
(1) Large starting torque, strong speed regulation.
(2) High efficiency with a wide high-efficiency range.
(3) Large instantaneous power, strong overload capacity.
(4) High power density, small size, and light weight.
(5) High modularity and integrated development direction, with the motor, controller, and reducer integrated into one.
(6) Good environmental performance.
2、 Research direction and main design concepts of driving motor bearings
2.1 Research direction of driving motor bearings
(1) High speed, with the maximum speed reaching 15,000-20,000 r/min, and the Dn value generally above 700,000, and even up to 1 million.
(2) Abrupt changes in speed.
(3) Low energy consumption, low starting torque, and low-friction bearing design.
(4) Low noise and low vibration.
(5) Good resistance to high and low temperatures.
(6) High reliability and fatigue life.
2.2 Brief introduction of the main design concepts of bearings
In traditional designs, a mathematical model is established, the objective function and constraints of the bearing design are determined, and the bearing internal design details are obtained using traditional coefficient values and traditional component structures. However, for driving motor bearings, their special characteristics require a breakthrough in traditional design concepts, and the bearing structure design, coefficient values, material selection, heat treatment requirements, and component structural forms are all different from those of ordinary bearings.
In this paper, the main failure modes, causes, and designs of front and rear bearings for driving motors are briefly explained. Due to the technical confidentiality of specific design details and the vast differences in specific operating conditions, this paper only describes research directions.
3 Main Failure Forms and Causes of Bearings
(1) Bearing temperature rise, heat generation or even seizing. This can be caused by poor selection of bearing clearance, lubricating grease, lubrication status, excessive speed, excessive load, etc.
(2) Bearing groove pressure marks or even biased wear caused by poor installation. When the installation is not good, the groove will be stressed, the installation is not in place, or the coaxial degree of the front and rear bearing seat holes is not good, pressure marks and biased wear may occur, and the vibration and noise of the bearing during operation may increase.
(3) Bearing vibration and abnormal noise. This can be caused by improper selection of clearance value, deformation of mating parts due to poor processing accuracy, poor coaxiality of front and rear seat holes, poor material selection of the cage, injury to the bearing’s motion surface, and so on.
(4) Peeling of the bearing’s motion surface. This can be caused by improper selection of bushing material, improper treatment of the bushing’s motion surface, excessive load, improper clearance value setting, poor lubrication, failure of sealing, and so on.
(5) Bearing failure caused by poor sealing, resulting in leaking grease. The sealing structure is a key design factor for deep groove ball bearings used in the drive motor. The high-speed operation and temperature rise of the bearing can cause the lubricating grease to become thinner and be thrown out more easily due to centrifugal force. Therefore, how to design a reasonable sealing structure to address the current bearing leakage problem in the market is a key consideration.
(6) Cage fracture. This can be caused by improper selection of cage material, improper design of structure, or bearing failure due to poor selection of lubrication and clearance.
(7) Failure caused by poor lubrication.
(8) Failure caused by poor lubricating grease selection and poor sealing structure, resulting in grease leakage.
4、Bearing Design Considerations
- 1 Low Friction Design The primary concern in designing high speed bearings is low friction design, given the high speed requirements of the driving motor. Adjusting the values of the friction torque and the center diameter Dpw of the deep groove ball bearing can reduce friction torque, lower friction, and increase speed by adjusting the values of these two parameters, which are directly proportional to the center radius Dpw and inversely proportional to the ball radius Dw.
4.2 Optimizing the Interior Surface Structure of Bearings The high-speed nature of bearings requires the optimization of motion surface structure, with various coefficient values not selected according to traditional forms or data to reduce friction between the motion surface and the steel ball, increase speed, and reduce power consumption. Additionally, innovative structural designs provide an easier way to improve oil passage flow between the steel ball and the moving surface.
4.3 Surface Treatment of Materials Due to the development of new material science technologies, coating, ion implantation or special heat treatment of motion surfaces can increase surface hardness and wear resistance and reduce friction. Special heat treatment of the raceway surface can greatly increase the bearing’s fatigue life by altering the residual stress state of the surface.
4.4 Special Cage Construction This mainly refers to polyamide cages, and how to overcome the deformation caused by greater centrifugal force at high speeds is currently a problem that needs to be solved. The structure of the cage should be designed to greatly reduce stirring resistance to the lubricating grease and have sufficient strength to achieve low friction and high speed requirements. There are currently several special structures that have been experimentally validated and meet high-speed and low friction requirements. However, further exploration and research may result in newer and more suitable structures being developed.
4.5 Sealing Structure Seal friction is also one of the factors affecting bearing friction. The design of the sealing structure plays a critical role in guaranteeing sealing performance, reducing friction and increasing speed. The high-speed nature of driving motor bearings requires the sealing structure to be treated as one of the key points in the design. Currently, many driving motor bearings place more emphasis on high speed and consideration of high-temperature pressure differential and adopt non-contact seals, which have an absolute advantage in low friction and high speed. However, at high speeds, the lubricating grease can be thrown out by the centrifugal force, resulting in grease leakage. When the bearing lubrication is poor, various failures also occur. When using a seal gasket, the sealing effect is better than that of a dust cover, but if the sealing structure is not well designed, it can increase friction, resulting in higher energy consumption and not being conducive to high-speed operation. To design a structure that meets high speed and sealing requirements, exploration should be carried out in the form of sealing grooves and sealing lip, irrespective of the structure, which should be a light contact method.
4.6 Bearing Clearance Selection Many forms of failure mention poor clearance settings, making the optimal selection of bearing clearance critical. The combination of driving motor bearings is generally overfit with the shaft on the inner ring and the outer ring with an interference fit with the seat hole. In theoretical calculations, the clearance loss due to assembly, the clearance loss caused by temperature rise, and the load slightly compensate for the clearance value. The high-speed, low friction bearing design generally requires using a large clearance, but too large a clearance can cause vibration noise. Most bearings use a C3 group clearance, but the design should also be based on specific operating conditions.
4.7 Material Selection Due to the high performance and high reliability requirements of driving motor bearings, material selection differs from that of traditional bearings.
4.7.1 Selection of High-Quality Bearing Steel In recent years, due to the improvement of steel smelting technology, the oxygen content has greatly decreased, and the bearing life has doubled. According to traditional bearing life calculations, for 6004 bearings: Cr = 9400N, C0r = 5000N, Pr is set at a maximum of 2150N, and the speed is set at a maximum of 7000 rpm, L10=(9400/2150)3 = 83.57 (million revolutions), and Lh=199h according to the bearing life requirements of 160,000 km is calculated at a speed of 100 km/h, requiring 1600 hours. In general, the experimental life is 3-8 times the theoretical value, while actual experimental results have already exceeded ten times. For driving motor bearings, high-quality bearing steel should be selected for the raw material in the collar because the oxygen content and non-metallic inclusions control more strictly and play an important role in guaranteeing bearing life.
4.7.2 Lubricating Grease Selection
Lubricating grease is one of the key factors affecting bearing life. Lubricating grease is composed of base oil, thickening agent, and additives. The selection of lubricating grease should be based on the operating conditions of the bearings.
(1) Base oil. Bearings require lubricating grease with high Dn value for high-speed operation. The choice of Dn value should generally be above 700,000. Lower base oil viscosity can reduce frictional heat and bearing temperature rise, and can reduce the starting torque of bearings at low temperatures, thereby reducing energy consumption. However, too low base oil viscosity will reduce the high-temperature resistance, load-carrying capacity, and anti-peel strength of lubricating grease. Therefore, finding a suitable base oil viscosity value through repeated tests is one aspect of selecting suitable lubricating grease to withstand the high and low-temperature resistance of the drive motor. The low-temperature limit of the drive motor bearings is generally -40°C to -50°C, but the high-temperature environment temperature is not too high, generally around 90°C. Therefore, lubricating grease with good viscosity-temperature performance and low-temperature performance should be selected.
(2) Thickening agent. Good shear resistance and oil separation capability of lubricating grease are beneficial for high-speed lubricating grease to perform well, which depends on the performance and structure of the thickening agent. When the bearing runs at high speed, the fibrous structure of the thickening agent is severely sheared and destroyed, causing the lubricating grease to become thin due to shear, so a lubricating grease with a harder and higher oil separation rate should be selected.
(3) Additives. The sudden acceleration and deceleration characteristics of the bearings cause the lubricating grease to suffer more impact and agitation, which has a negative impact on the life of the lubricating grease. Therefore, lubricating grease should have high-activity additives, and extreme pressure additives can also be selected. Although the radial load on the bearing is not large, it can optimize the carrying capacity of the moving surface to a certain extent and provide longer service life for the lubricating grease.
Some people have proposed that electric corrosion is also a form of bearing failure, but currently, this kind of failure in drive motor bearings is not common. The response to this failure is to select lubricating grease with conductive material as an additive, but there is a risk of increasing noise after actual addition. The selection of lubricating grease is ultimately verified through experiments.
4.7.3 Sealing Materials
Currently, both sealing gaskets and dust covers are used, but dust covers are more common. Non-contact sealing does not require high grease leakage, and it is suitable for clean environments. On the other hand, the selection of gasket materials is also crucial. Taking into account the wear resistance and compatibility of gasket materials with lubricating grease, cost should also be considered under the premise of satisfying performance.
4.7.4 Cage Materials
Currently, the speed of passenger car drive motors in China is generally between 11,000 and 14,000 rpm, while overseas has reached 16,000 to 20,000 rpm. Currently, steel and nylon cages are both available on the market, with steel wavy cages having high strength and better high and low-temperature adaptability. However, it cannot be denied that it has a higher coefficient of friction with steel balls, which is not conducive to bearings running at higher speeds. Nylon cages are more suitable for high-speed operation due to their superior self-lubricating performance and very low friction coefficient with steel balls. Ordinary PA66 or PA46 have corresponding temperature limits, but currently, the ambient temperature of the drive motor bearings is still within their application range. Otherwise, other higher-temperature-resistant materials should be considered. Therefore, the selection of cage materials should be based on specific operating conditions.