Abstract
Focusing on the low friction and high-speed precision of bearings, the factors and key points affecting low friction and high-speed precision were analyzed. Taking new energy drive motors as an example, the requirements for low friction and high speed were listed.
Introduction
Bearings are essential basic components in modern industry, known as the "heart" of mechanical equipment. Their load-bearing capacity directly affects the technical level of equipment and is an important standard for measuring a country's scientific and industrial strength. But with the development and evolution of downstream industries, traditional rolling bearings are becoming increasingly "overwhelmed". The trend towards large-scale and high-speed equipment has posed new challenges to bearing technology due to more stringent operating conditions and high reliability requirements.
After more than 40 years of development, China's bearing industry has made significant progress in both bearing products and manufacturing levels, and has established a considerable scale and development foundation. With the advancement and development of technology, higher and higher requirements have been put forward for the performance and demands of bearings. This article mainly discusses the development trend of bearing technology from two aspects: low friction and high-speed precision.
Low friction
Friction technology is the core technology of bearings, and the significant technical feature of the new generation of bearings is anti friction design. Compared to sliding bearings, rolling bearings themselves belong to "anti friction bearings". The challenge of the new generation of anti friction design is to significantly reduce the already extremely low friction torque, with a maximum reduction of up to 80%.
In the application of new energy vehicles, low friction bearings are required to reduce energy loss; In aerospace application requirements, bearings are required to have low-temperature friction resistance and other properties.
The friction of a bearing is not constant, it depends on a certain friction phenomenon that occurs in the lubricating film between the rolling elements, raceway, and cage. The technical indicators that affect low friction of bearings mainly include aspect ratio, bearing clearance, bearing wall thickness, and special designs suitable for extreme (extreme) working conditions.
1) Width to diameter ratio: For radial bearings, the width to diameter ratio (B/d) is usually between 0.35 and 1.5, while for thrust bearings, the size to diameter ratio (D2/D1) is usually taken as ≤ 2. Choosing a larger value will increase the bearing capacity, but at the same time, it will increase the deformation of the shaft and the sensitivity of the two bearing holes to different axial degrees.
2) Bearing clearance: Bearing clearance has a significant impact on the working performance of bearings. Excessive clearance may lead to increased wear and reduced operational accuracy, while insufficient clearance may cause bearing overheating and excessive temperature rise, requiring sufficient clearance for fitting. The diameter clearance of general non lubricated bearings is preferably not less than 0.075mm.
3) The influence of bearing wall thickness on operational performance becomes increasingly evident with the increase of bearing volume. Under the condition of ensuring strength permission, the wall thickness should be minimized as much as possible to reduce the impact on operational performance.
4) Extreme (extreme) condition design: In extreme conditions such as low-temperature vacuum environments, specially designed bearings are required to adapt to extremely low temperatures and high-pressure environments. For example, the application of solid lubricated rolling bearings in the aerospace field has achieved friction testing and high-resolution measurement of friction factors (coefficients) under various extreme working conditions such as extremely low temperature/wide temperature range, high vacuum, etc. through high-power combined cooling and temperature control of the inner and outer rings, and radial and axial combined electromagnetic loading technology.
High speed precision
The high-speed and low-speed of bearings are not measured by the rotational speed of the bearing itself, but by its linear velocity. A bearing with a general linear speed of 60 meters per second can be considered a high-speed bearing. High speed bearings typically have a smoother surface and a very small distance between the inner and outer rings, thus requiring higher precision for the bearings. The main factors affecting the high-speed performance of bearings include bearing type, size, load, lubrication method, material and type of cage, bearing load, preload force, driving method, size of bearing clearance, and bearing combination.
1) Bearing type and size: Different types of bearings have different structures and materials. The larger the size, the greater the moment of inertia, which will affect the speed of the bearing.
2) Load: The greater the load borne by the bearing, the greater the frictional resistance, which affects the speed.
3) Lubrication method: Appropriate lubrication method can reduce friction and wear, and increase the speed of bearings.
4) The material and type of the cage can also affect the rotational speed of the bearing.
5) Bearing load: The ultimate speed of a rolling bearing is the highest speed achieved under certain load and lubrication conditions, which is closely related to the bearing type, size, direction, lubrication method, clearance, cage structure, cooling conditions, etc.
6) Pre tightening force: Excessive pre tightening force can increase the contact stress between the rolling elements and the raceway surface, increase heat generation, and further increase the internal pre tightening force of the bearing, making the bearing more prone to wear and reducing the maximum speed.
7) Drive mode: Different spindle drive modes can affect the maximum speed of the bearing. For example, when driving an electric spindle, if outer cylinder cooling is used simultaneously, it may cause an increase in the temperature difference between the inner and outer rings of the bearing, increase the internal load, and reduce the maximum speed.
8) The size of bearing clearance: The size of clearance has a significant impact on the operational performance and lifespan of bearings, and appropriate clearance can improve the bearing's load-bearing capacity and maximum speed.
9) Bearing combination: By supplying a large amount of oil, the heat generated by the bearings can be quickly removed, which can increase the maximum speed.
Conclusion
Taking the application of electric drive systems in new energy vehicles as an example, bearings need to meet the characteristics of long life, creep resistance, high speed, low friction, and low vibration (noise). The starting torque requirement is below 0.002 N.m, and the speed DmN value requirement is above 1.8 million mm · r/min. In terms of low friction bearings, SKF, NSK, JTEKT, BLH bearings, etc. have a leading market share in the global market. In terms of high-speed precision small and medium-sized bearings, SKF, NTN, BHL bearings, etc. have a leading market share in the global market.
