Time : November 14, 2023
With the development of the mechanical industry towards high-precision, high-efficiency, and highly automated directions, the working speed of various machines is constantly increasing. Currently, the spindle speed of CNC machine tools has increased from 3,000 to 5,000 r Π Min developed to 15,000 to 70,000 r Π Min. For example, the spindle speed of Boston Digtal’s vertical machining center in the United States is 60,000 r Π Min, a high-speed CNC boring and milling machine from Mikron, Switzerland, with a spindle speed of 42,000 r Π Min. Machinery that operates in certain high-tech fields and special environments, such as aerospace, nuclear energy, chemistry, petroleum, and other industries, needs to work in special environments such as high temperature, high speed, corrosion resistance, vacuum, non-magnetic, oil-free lubrication, and light weight. Current steel bearings cannot meet these requirements. High speed rotating bearings generate a significant centrifugal force during rolling, causing the pressure between the ball and the outer ring raceway to exceed the external load, thereby shortening the service life of the bearing. At the same time, under the action of a large centrifugal force, the contact angle between the ball and the outer ring raceway decreases, or even drops to zero; The contact angle between the ball and the inner raceway increases. The former reduces the axial stiffness of the bearing, while the latter reduces the radial stiffness. In addition, excessive rotational speed will exert a significant gyroscopic torque on the ball, causing sliding between the ball and the ring, increasing frictional torque, causing heating and cage damage. Therefore, for high-speed machinery, relying solely on improving the structure and lubrication conditions of steel bearings is far from meeting the requirements. Further research and development of new bearings are needed to adapt to the high-speed transmission requirements of high-tech and industrial production development.
However, in high-speed and high-power spindle structures, bearings that can be used include magnetic levitation bearings, liquid dynamic and static pressure bearings, or ceramic ball bearings. Magnetic levitation bearing is a new type of intelligent bearing that utilizes electromagnetic force to suspend the spindle without mechanical contact or lubrication. It is an ideal support component for the spindle of ultra high speed machine tools, but due to the complex electrical control of magnetic levitation bearings and the high manufacturing cost of the entire bearing, it has not been promoted and applied until now, and is only used in high speed situations. Liquid dynamic and static bearing refers to a bearing structure that can produce a mixed support effect of dynamic and static pressure. This type of bearing must be specially designed and manufactured according to a specific machine tool, with low standardization and limited use.
Silicon nitride (Si3N4) is a newly synthesized inorganic material that differs greatly from traditional ceramics. It has a series of advantages such as wear resistance, high temperature resistance, corrosion resistance, non magnetism, low density (only about 40% of steel), low thermal expansion coefficient (25% of bearing steel), and high elasticity coefficient (15 times of bearing steel). The bearing industry mainly utilizes some of the excellent physical, chemical, and mechanical properties of this ceramic to produce high-speed precision bearings or rolling bearings suitable for working in special environments. Due to its lower density than bearing steel, ceramic balls are used as rolling elements. When rotating at high speeds, the centrifugal force generated by the rolling element is small, and the gyroscopic torque can be reduced, greatly reducing the pressure and friction torque on the outer ring of the bearing. Therefore, it can reduce heat generation and improve service life. Due to its high hardness, good rigidity, and small deformation of the bearing, ceramic material (Si3N4) is used as the rolling element. Compared with steel bearings of the same specification and accuracy level, the mixed ceramic ball bearing can increase its speed by 25% to 35%, increase its lifespan by 3-6 times, and reduce its heating rate by 35% to 60%. Ceramic ball bearings are better performing and more economical bearings in ultra high speed spindles, and have been increasingly widely used.
Ceramic bearings are divided into full ceramic bearings and hybrid ceramic bearings. All ceramic bearings refer to bearings made entirely of ceramic materials. Hybrid ceramic bearings refer to bearings where one part is made of ceramic material and the rest is made of bearing steel. Hybrid ceramic bearings can be further divided into three types: ① bearings with rolling elements made of ceramic material and the rest made of bearing steel; ② The rolling element and outer ring are made of ceramic material while the rest are made of bearing steel; ③ The rolling element and inner ring are made of ceramic material while the rest are made of bearing steel. The commonly referred to hybrid ceramic bearings or hybrid bearings refer to bearings with ceramic rolling elements and bearing steel rings. At present, the high-speed ceramic bearings developed and applied internationally are mainly hybrid types. Due to the brittleness of ceramic materials, easy damage, and poor reliability in making inner and outer rings, all ceramic bearings have not yet reached the practical stage.
Since NASA successfully developed the first set of ceramic bearings in 1972, various industrial powers around the world have been competing to develop and develop a new generation of higher performance ceramic bearings. The development process has roughly gone through three stages. The first stage refers to the 1960s, mainly exploring which ceramics are suitable as bearing materials and preliminarily exploring the broad prospects of using silicon nitride as bearing materials. The second stage was from the 1970s to the mid-1980s, mainly exploring the impact of replacing steel rolling elements with hot-pressed silicon nitride ceramic rolling elements through extensive experiments on bearing performance and predicting the service life of hybrid ceramic bearings. The third stage refers to the 1980s, which mainly focuses on studying the performance of all ceramic bearings based on experiments, further improving the research on the performance of hybrid bearings, and making them preliminarily applied in practical industrialization. At the same time, a large amount of energy is also invested in the research of the design theory of ceramic bearings. From the results, the most prominent effect is to explore how to improve the service life and limit speed of bearings, providing basic components for the development of high-speed and high-precision machine tools. The high-speed performance of bearings can be easily measured by their speed factor Dm·N or D·N values (Dm represents the diameter of the bearing pitch circle, mm; N represents the rotational speed of the bearing, r/min. At present, the international Dm·N value is 2.5 × 106 mm·r/min. The ceramic bearing of min has entered the industrial production stage, and the Dm·N value of the bearing in the laboratory has reached 4 × 106 mm·r/min. Correspondingly, the dynamic design and analysis of high-speed bearings have reached a new level, and structural parameters of high-speed bearings with low heat generation and low friction have been determined.
The research on ceramic bearings started relatively late in China, and the research on ceramic bearings began in the late 1980s. Scientific and technological personnel conducted a large number of experiments on the processing and performance of ceramic bearings, and achieved gratifying results. During the 7th Five Year Plan period in China, research work on silicon nitride bearings began, mainly focusing on the manufacturing of silicon nitride ball billets, ceramic ball processing methods, and ceramic bearing tests. The test results showed that the rigidity of ceramic ball bearings is better than that of steel ball bearings, but their vibration is large. This is mainly due to some problems in the manufacturing and processing of ceramic balls. Ceramic ball bearings can reach extremely high Dm · N values, and ceramic bearings can withstand high temperatures Under insufficient lubrication and heat dissipation, it cannot work normally, but ceramic bearings are difficult to process and expensive. During the Ninth Five Year Plan period, research and development were conducted on the industrialization of ceramic bearings. However, the Dm·N value of ceramic bearings only increased to 2.0 × 106 mm·r/min (military bearings) has not yet reached the level of practical application abroad. In terms of analysis and research, in the late 1980s, the “General Bearing CAD Research” project conducted pseudo dynamic analysis of high-speed bearings, but there is still a certain gap from practicality.
At present, high-speed CNC machine tools mostly use hybrid ceramic ball bearings. Although their structure is simple, the movements and loads of their internal components are relatively complex, so there are still many factors to consider. With the increase of machine tool speed, further research on the high-speed performance of ceramic ball bearings is needed, in order to provide a reasonable basis for optimizing the design of various parameters of ceramic bearings.
People first analyzed the performance of bearings using the principles of statics. Later, A. B. Jones proposed a pseudo statics analysis method based on this. Later, T. A. Harris developed a pseudo statics analysis method that fully considered the role of elastohydrodynamic lubrication and systematized it. The quasi-static analysis model is a nonlinear algebraic equation system that considers the effects of external forces, external moments, centrifugal forces, and rotational moments, lists the force balance equation and moment balance equation for each bearing component, and solves them using the Newton-Raphson method for analysis and calculation. However, the quasi-static analysis model has the following shortcomings: ① When calculating the angular velocity of rolling bodies, certain motion assumptions that constrain the direction of the angular velocity vector of rolling bodies need to be used. For ceramic ball bearings, the possible spin speed of the ball complicates the motion, resulting in the assumption of “ring control” of spin on the outer or inner ring of the ball. At high speeds, the centrifugal force of the ball causes a sharp increase in contact load on the outer ring, which in turn leads to outer ring control Do not consider the acceleration of bearing parts and assume that the speed of bearing parts is constant; ③ Failure to consider the motion analysis of the cage; ④ For the instability problem of rolling elements, the sliding and asymmetry of rolling elements are ignored; ⑤ Not considering the temporal variation of various physical quantities. Despite the aforementioned limitations of the quasi-static model, it is very effective in calculating the true load distribution, fatigue life, and bearing stiffness of bearings, which is of great help for the design of high-speed ceramic ball bearings. In addition, when conducting detailed dynamic simulations, the solution of the static equilibrium nonlinear algebraic equation system provides satisfactory initial conditions for determining the differential equation of motion of bearing parts, which is also the foundation of dynamic analysis.
The dynamic factors inside ceramic ball bearings that operate at high speeds have a significant impact on their performance. C. T. Walters first proposed a dynamic analysis model, P K. Gupta further developed kinetic analysis methods. In the dynamic analysis model, the differential equation of motion for each bearing component replaces the equilibrium equation in the quasi-static model. Therefore, dynamic models can be used to simulate the performance of bearings in real-time and can handle most problems related to equilibrium in quasi-static models. For example, there is no need to assume motion constraints such as loop control; The forces and moments generated by the interaction of various parts of the bearing determine the acceleration of each part; Any lubricant performance can be considered in the model. At the same time, real-time simulation of all external effects that vary over time and instability related to rolling elements and cages is also considered. The differential equations of motion for each component of the bearing are given in a fixed coordinate system. The model takes into account the sliding between the ball and the ring groove, as well as the integration of the rolling element and groove on each collision of the cage, as well as the drag and sliding forces of the rolling element at each contact point of the inner and outer ring groove, as well as the solution of the Hertz contact stress on each ball over time. Although Gupta’s method requires a large amount of computation, its model can accurately analyze the dynamic performance of rolling elements and cages. In this way, a computer program is developed to analyze the performance of bearings and output curves such as centrifugal force, gyroscopic torque, rolling ratio, friction torque, total heat output, service life, oil film thickness, stress, stiffness, and deformation. Of course, the accuracy and effectiveness of the model ultimately depend on experimental verification. On the electric spindle of a high-speed grinding machine, bearing heat generation and vibration tests were conducted on high-speed angular contact ceramic ball bearings under different rotational speeds, different loads, and different guide clearances and pocket clearances of the cage. Oil cut-off tests and limit speed tests were also conducted. Finally, the test data was processed and analyzed. A large number of test results were compared with calculation results, and actual bearing usage experience was used for analysis Judge and propose reasonable structural parameters for high-speed ceramic bearings.
In summary, for dynamic analysis of high-speed ceramic ball bearings, this method can be used for real-time simulation of bearing performance and can handle most related problems in static models. The quasi-static analysis method is not suitable, but it can serve as the basis for dynamic analysis. Although dynamics is feasible, the computational complexity of dynamic model analysis is too large, and a large number of experiments are required. Not only is the development cost high, but the development cycle is long. With the rapid development of computer simulation technology, using computer simulation methods for dynamic performance analysis instead of countless repeated experiments can greatly reduce development costs and increase product reliability. These require further research by bearing workers.
More about XZBRG Ceramic Balls:
Xinzhou Bearing provide a wide range of ceramic ball options across a variety of sizes. Material options include Silicon nitride (Si3N4), Zirconia (ZRO2), Alumina (Al2O3) and Silicon carbide (SIC) from 0.4mm to just over 115mm in diameter with the most common sizes in between. Precision grades provided are 5, 10, 16, 25 and 100.
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