Comparative analysis of the development of high-speed CNC machine tool spindle bearings in China and abroad
Due to the limitations of processing equipment, manufacturing technology and related technologies, the overall level of China's machine tool bearing industry is quite different from that of other advanced manufacturing countries. Although precision machine tool bearings have excellent performance and high reliability, they are expensive and have long delivery times, and suppliers of advanced machine tool equipment may take measures such as restricting supply, which seriously hinders the development of China's major equipment manufacturing industry, and affect the operation of China's economy. At the same time, affected by the overall development of China's domestic high-end CNC machine tool industry, the current high-end CNC machine tool spindle bearing market is mainly occupied by the world's Top bearing manufacturers. Only a few bearing companies in China have developed some products, but It is still difficult to apply and practice in core high-end CNC machine tools.
The lack of functional components of high-end machine tools has become the biggest obstacle to the development of high-end CNC machine tools in China. During the "Made in China 2025" period, one of the main goals of the machine tool industry is to promote the industrialization of high-end CNC machine tool functional components. Therefore, the development of important functional components of precision high-end CNC machine tools (high reliability, high speed, ultra-precision bearings) can effectively improve the autonomy rate of important functional components of China's high-end CNC machine tools and meet the needs of China's machinery, aerospace and other industrial development. It is helpful to realize the industrial goal of "Made in China 2025" and China's strategic goal of revitalizing the equipment manufacturing industry.
After years of development, China's machine tool bearing industry has formed a considerable scale, but there is still a certain gap between the life and precision of China's domestic high-end machine tool bearing products and the international advanced level. This is mainly reflected in three aspects.
The first aspect is the high speed
The angular contact ball bearings used in European and American machine tool spindles are mainly lubricated by oil and gas, the dmn value can reach up to 4X106 mm r min-1, the average dmn value of oil lubrication can reach 2.0X106~2.5X106 mm r min-1, and the grease lubricated dmn The value can reach up to 2.0X106 mm·r·min-1, and the cylindrical roller bearing adopts oil-air lubrication and the dmn value can reach 2.0X106~2.5X106 mm·r·min-1. Taking angular contact ball bearings as an example, the dmn value of oil-air lubricated bearings in my country is 1.2X106~1.8X106 mm r min-1, and the dmn value of grease lubrication is 1.0X106 mm r min-1. There is still a certain gap.
The second aspect is the level of precision
The tolerance level of machine tool spindle bearings is generally above P5, and the spindle bearings used for high-speed and high-precision machine tools such as CNC machine tools and machining centers need to use P4 and above precision. The "standard tolerance class" of SKF's angular contact ball bearings for machine tool spindles is P4A or P7, that is, the rotational accuracy is P2, and the dimensional accuracy is P4. The "standard tolerance class" of cylindrical roller bearings is SP, that is, the rotational accuracy is P4. Dimensional accuracy is P5. The "standard tolerance level" of FAG's angular contact ball bearings for machine tool spindles is P4S, and some indicators are higher than P4. NSK's spindle bearings for high-precision lathes have a "standard tolerance class" up to P47.
The third aspect is long precision life
The service life of machine tool spindle bearings is usually not evaluated by fatigue life, but by accuracy life or wear life. This is because the contact stress between the main shaft bearing rolling body and the raceway is generally lower than its fatigue limit (for example, it is generally specified as 1500 MPa, and the reference value of ball bearings is 2200 MPa), and the operating conditions such as lubrication and sealing are relatively good , that is, it is generally considered to be a bearing in an "ideal state", while the long-precision life of domestic bearings is low. In addition, in terms of product innovation, R&D and testing of high-reliability, high-speed and ultra-precision bearings for high-end CNC machine tools, there are also gaps in China's bearings, such as the lack of basic theoretical common technical research, the low level of enterprise product R&D and testing, and the construction of innovation systems. And operation, investment in research and development funds, talent development and other aspects are still at a low level. In terms of actual manufacturing technology, China's manufacturing technology has developed slowly, and many key technical problems have not been achieved. Substantial breakthroughs have been made.
Factors Affecting Accuracy of High Speed Angular Contact Ball Bearings
The accuracy of high-speed angular contact ball bearings is mainly affected by three factors
The first point is errors caused by the external geometry of the bearing components
During the manufacturing process of bearing components, geometric shape errors generally occur, such as roundness errors of inner and outer ring raceways, spherical errors of rolling elements, etc., which will affect the vibration, operational reliability and wear of bearings. Under the action of load, the rotation accuracy of angular contact ball bearings will fluctuate due to the roundness error of the ring raceway and the spherical error of the steel ball, which is a relatively common geometric shape error. In this case, the force distribution of the ring is uneven, and the wear of some parts will be accelerated, so the accuracy of the bearing will decrease.
The second point is the surface features
The surface characteristics of parts include many aspects, such as waviness, surface roughness, surface geometry, etc., which mainly depend on the process of equipment manufacturing. For angular contact ball bearings, the waviness and surface roughness of the ring raceway and the steel ball surface will affect the accuracy of the bearing. The waviness of the bearing surface is mainly due to the periodic undulations formed on the surface of the component by the partial vibration of the grinding wheel during the bearing manufacturing process. The wavelength of the waviness will be greater than the Hertz contact width, so when the bearing rotates at a high speed Affects the accuracy of bearing rotation. In addition, surface roughness is also an index for evaluating bearing quality. The actual wavelength of roughness usually needs to be less than the Hertz contact width. The specific roughness value is subject to the requirements. Too large or too small will affect the bearing accuracy.
The third point is the working condition
The external working conditions of the rolling bearing will affect the accuracy of the bearing, such as the mode of assembly, the type of load, lubrication, etc. In the actual application process, the working conditions should be adjusted reasonably according to the actual situation, so that the bearing can achieve sufficient precision.
To ensure the rigidity of the spindle of high-speed CNC machine tools, it is necessary to use larger-sized angular contact ball bearings. In this case, there will not be a large contact stress between the rolling elements and the raceways of the inner and outer rings. However, the failure of the bearing is often due to the serious wear between the bearing raceway, the rolling body and the cage, resulting in a reduction in the bearing accuracy, so its working performance does not meet the requirements. The first is the problem of the contact angle of angular contact ball bearings. Once the difference in contact angle between the inner and outer rings of the bearing becomes larger, the cage will wear more intensely until the bearing is damaged. Therefore, the difference in contact angle between the inner and outer rings of the bearing should be reduced. The second is the spin-to-roll ratio, which will cause sliding wear on the contact surface between the steel ball and the raceway. Only the smaller the roll-to-roll ratio is, the lower the sliding wear will be. The third is the bearing stiffness. It is necessary to select the bearing stiffness according to the requirements of different types of machine tools, and to reduce the vibration as the standard, so that the critical speed of the bearing is far away from the working speed of the spindle system. The last is the contact stress of the contact surface. The greater the contact stress on the rolling surface, the more severe the wear will occur, so it is necessary to minimize the contact stress on the contact surface.
High-speed CNC machine tool spindle bearings are usually equipped with engineering plastic cages, and the precision injection molding process, testing, and application technology research of plastic cages are very important. If any demand for high speed spindle bearings or Super precision Bearings, please contact with us directly, our experts team will supply final bearing solutions for your applications.
