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Angular contact ball bearings for CNC machine tools
During the design and manufacturing process of machine tools, choosing the right spindle bearing is crucial to ensure machining accuracy and improve equipment performance. Angular contact bearings are often used in the spindle system of machine tools due to their unique design and performance characteristics. Precision angular contact ball bearings are widely used, and their rolling elements are balls. Because they are point contact (different from the line contact of roller bearings), they can provide higher speeds, less heat generation and higher rotation accuracy.
In some ultra-high-speed spindle applications, hybrid bearings with ceramic balls (generally Si3N4 or Al2O3) are also used. Compared with traditional hardened steel balls, the ceramic ball material itself gives ceramic ball bearings the advantages of high rigidity, high speed, high temperature resistance and long life, thus meeting the high customer demand for machine tool bearing products. Angular contact ball bearings have a high limit speed, can bear radial loads and axial loads at the same time, and can also bear pure axial loads, with high rigidity and load-bearing capacity. Its axial load capacity is determined by the contact angle and increases with the increase of the contact angle.
Standard contact angles are 15° (indicated by suffix C), 25° (indicated by suffix AC), 40° (indicated by suffix B) and 30° (indicated by suffix A). A single single-row angular contact ball bearing can only bear unidirectional axial loads. When bearing radial loads, it will cause additional axial forces, so the bearings are generally used in pairs. DB (back-to-back) combination and DF (face-to-face) combination can bear radial loads and bidirectional axial loads. The bearing has a large load capacity, strong rigidity, stable rotation, high limiting speed, and adjustable clearance pre-interference, so it is particularly suitable for high-speed precision machine tools.
Double-row angular contact ball bearings
Double-row angular contact ball bearings are basically single-row bearings that are back-to-back or face-to-face, with the outer ring or inner ring integrated. Compared with single-row angular contact ball bearings, they have better rigidity and can withstand bidirectional axial loads. At the same time, they can also withstand a certain tipping moment. Their use effect is similar to that of two single-row angular contact ball bearings configured back-to-back.
Four-point contact ball bearings
Four-point contact ball bearings are a type of separable bearing, or an angular contact ball bearing that can withstand bidirectional axial loads. When there is no load or a pure radial load, the steel ball and the ferrule are in four-point contact, which is the origin of the name. When there is only a pure axial load, the steel ball and the ferrule become two-point contact and can withstand bidirectional axial loads. Four-point contact ball bearings can also withstand moment loads, and have the functions of single-row angular contact ball bearings and double-row angular contact ball bearings. Four-point contact ball bearings are generally suitable for those occasions where there is two-point contact under pure axial loads or combined loads with large axial loads. Four-point contact ball bearings have a high limit speed and are also suitable for those occasions where high-speed operation is required. The contact angle of four-point contact ball bearings is 35° or 45°.
Angular contact bearing grades
Bearings are the core components of machine tool spindles and rotary tables, and play a vital role in the performance of machine tools. In order to maintain the processing capacity of machine tools, high-performance bearings must be used. The precision of the bearings used on the spindle of machine tools should be ISOP5 or above (P5 or P4 is the ISO precision, usually from low to high (P0, P6, P5, P4, P2), and for the spindle support of high-speed and high-precision machine tools such as CNC machine tools and machining centers, ISOP4 or above precision should be selected. Spindle bearings include angular contact ball bearings, tapered roller bearings, and cylindrical roller bearings.
Advantages of angular contact bearings
Their performance directly affects the processing accuracy and equipment stability. Angular contact bearings have the following characteristics:
High rigidity: The contact angle design of angular contact bearings makes them have high rigidity, which can effectively reduce the vibration and deformation of the spindle.
High precision: The manufacturing precision of angular contact bearings is high, which can ensure the stability and precision of the spindle during operation.
High load capacity: Angular contact bearings can withstand large radial and axial loads and are suitable for high-load processing occasions.
Long life: Angular contact bearings have good lubrication and sealing performance, which can reduce wear and extend service life.
Selection of angular contact bearings
The machine tool spindle is the heart of the entire machine tool. It is the key part to achieve workpiece processing accuracy and directly affect the performance of the machine tool. Today’s high-speed and efficient production methods have put forward higher requirements on the speed, accuracy, load capacity and running noise of the spindle. Therefore, the spindle designers and manufacturers face huge challenges, and the correct bearing selection analysis can help spindle manufacturers cope with the above challenges. Different types of bearings have their own applicable application environments, so it is difficult to formulate a universal selection rule. For the selection of precision bearings, it is necessary to comprehensively consider the following factors: accuracy, rigidity, speed and load, etc. According to the specific application requirements, determine which factor is important, and then make targeted selections. When selecting angular contact bearings, the following factors need to be considered:
Load type: Select the appropriate angular contact bearing type according to the load type (radial load, axial load or combined load) borne by the spindle.
Contact angle: The size of the contact angle affects the bearing’s load capacity and rigidity. It is necessary to select the appropriate contact angle according to the specific application requirements.
Size and accuracy: The size and accuracy grade of the bearing should match the design requirements of the machine tool spindle.
Lubrication and sealing: A good lubrication and sealing system can improve the service life and reliability of the bearing.
Installation and maintenance of angular contact bearings
The selection of angular contact bearings also depends on the specific application requirements and machine tool design. Through reasonable selection and maintenance of angular contact bearings, the processing accuracy and stability of machine tools can be significantly improved and the service life of the machinery can be extended.
Installation: The installation of angular contact bearings should be carried out in strict accordance with technical specifications to ensure the accurate position of the bearing and avoid damage caused by improper installation.
Lubrication: Regularly check and maintain the lubrication system of the bearing to ensure that the bearing is working in a good lubrication condition.
Inspection: Regularly check the operating status of the bearings, and promptly discover and deal with possible problems, such as wear, looseness, etc.
Angular contact bearing manufacturers
AUBEARING’s precision angular contact ball bearings are universally matched and can be interchanged with bearings of the same type and size from other brands in groups. At the same time, AUBEARING controls the axial preload deviation of each row of bearings within a smaller range to achieve precise preload force and rigidity.
15° angular contact ball bearing can bear radial load and certain axial load. There are two types of structures: single row and double row. The single row has different sizes with nominal diameters from 40mm to 650mm; the double row has different sizes with nominal diameters from 100mm to 400mm. This type of bearing generally consists of a pair of bearings arranged back-to-back, face-to-face or in series. The axial clearance should be within 2 to 4 μm. This type of bearing can generally only bear axial force in one direction.
The 25° angular contact ball bearing has a contact angle greater than 15°, so it can withstand axial forces in both directions. The structure of this kind of bearing is also available in single row and double row, and its size range is the same as that of 15° angular contact ball bearings. The axial clearance of such bearings should be within 3 to 6 μm.
The 40° angular contact ball bearing has a larger contact angle and can withstand axial forces in three directions. The structure of this kind of bearing also has two types: single row and double row, and its size range is the same as the first two bearings. The axial clearance of such bearings should be within 5 to 10 μm.
Standard series of spindle bearings
This series is divided into P4 and P4S grades. The inner and outer rings and rolling elements are generally made of 52100 fully hardened steel. Under certain conditions, the speed of grease lubrication can reach 650,000 DN, and the speed of oil lubrication can reach 1,000,000 DN. Compared with the high-speed series, the rolling element diameter of the standard series is larger and the preload is higher. It is suitable for occasions with large cutting loads and high requirements for system dynamic rigidity.
In addition, grease is an important condition to be considered in the selection. Too much grease will seriously affect the heat dissipation of the bearing, and the operating temperature will rise sharply, which will cause the grease life to drop rapidly and shorten the bearing life; too little grease will not form a certain lubricating oil film to separate the shaft ring and the rolling element. The friction between them will cause a higher temperature rise, which will also affect the bearing life. For different speeds, the recommended amount of grease is also different. When the speed is less than 500,000 DN, the amount of grease added should be 25% – 40% of the free space inside the bearing; when the speed is higher than 500,000 DN, the amount of grease added should be 15% – 20% of the free space inside the bearing. It should be noted that there are two types of cages in the standard series: nylon cages guided by rolling elements and bakelite cages guided by outer rings. The internal free space of the two types of bearings is different, and they should be distinguished when calculating the amount of grease added.
Spindle bearings in the ceramic ball series
This series is P4S grade, the inner and outer rings are generally 52100 fully hardened steel, and the rolling elements are ceramic materials. Under certain conditions, the speed of grease lubrication can reach 750,000 DN, and oil lubrication can reach 1,500,000 DN. The dynamic radial and axial load capacity is equivalent to the standard series, but the rigidity is increased by 50%; the weight of ceramic is 60% lighter than that of steel, which reduces the centrifugal force of the rolling element, thereby reducing the slip and spin phenomenon, and the heat generated between the rolling element and the raceway is also reduced accordingly, extending the bearing life; at the same time, the thermal expansion coefficient of the ceramic ball is low, and when the temperature rises during high-speed operation, the preload force is smaller than that of the standard series steel ball, and the heat generation is smaller. After comparison, the speed of the ceramic ball series is about 1.5 times that of the standard series, and the theoretical operating life can be as high as 3-5 times; when high speed, high load capacity and high rigidity need to be met at the same time, the ceramic ball series should be considered, and the amount of grease should be controlled within 15% – 20% of the free space inside the bearing.
High-speed series spindle bearings
This series is P4S grade, the inner and outer rings are generally 52100 fully hardened steel, and the rolling elements can be selected from 52100 fully hardened steel or ceramic materials, and there are two types: open and double-sided non-contact sealed. Under certain conditions, the speed of grease lubrication can reach 850,000 DN-1,000,000 DN, and oil lubrication can reach 1,700,000 DN-2,000,000 DN.
The high-speed series is usually used in high-speed CNC machining centers and high-speed milling machines because it can meet the requirements of low heat generation, low operating temperature and high rigidity. The main advantages of this series are: First, the diameter of the rolling element is reduced and the number is increased. The precisely designed raceway shape reduces the contact area between the rolling element and the raceway, and reduces frictional heat generation;
Second, the centrifugal force on the small-sized rolling element is reduced during high-speed operation, the slip and spin phenomenon is weakened, and the heat generation in the contact area is reduced;
Third, both the inner and outer rings have high and low shoulders, which provide a open channel for the lubricant, which can promptly take away the heat generated during operation;
Fourth, the shear effect of the lubricant on the rolling element is reduced, and the heat generation is reduced. When the speed requirement is higher, you can also choose the high-speed series of ceramic balls with higher limit speed and dynamic rigidity to further reduce the heat generation during operation, thereby achieving a lower operating temperature.
Conclusion
Angular contact ball bearings are ideal for high-precision mechanical equipment such as CNC machine tools because of their ability to withstand both radial and axial loads, and their good performance at high speeds. Correct selection and installation are essential to ensure long-term stable operation of the equipment.