High-speed spindle bearing technology for machine tools

Table 1 Comparison of characteristics of rolling bearings for main shafts Tapered roller bearings Single row cylindrical roller bearings Double row cylindrical roller bearings Angular contact ball bearings Mounting in pairs or groups Angular contact ball bearings Load carrying capacity Radial â–¡ â–¡ â–³ â–¡ â–¡ Axial â–¡ â† â˜“ â˜“ â†â†â† â–³â†’ Joint â—‹ â˜“ â˜“ â–¡ â–¡ High-speed performance â–¡ â–¡ â–³ â—‹ â–¡ High-accuracy â–¡ â–¡ â–¡ â—‹ â—‹ Low noise and low torque â–¡ â–³ â–³ â—‹ â–¡ Rigidity â—‹ â–¡ â—‹ â–³ â–¡ å¤‡æ³¨ Remark 2 sets of opposed use N(NN) Type and NU (NNU) type contact angles are C (15Â°C), AC (25Â°C), 60Â°C, 2 sets or more combined use â—‹ â–¡ â–³ â–½ â˜“ â† â†” â†” â†” ä¸å¯ ä¸å¯ ä¸å¯ ä¸å¯ Unidirectional bidirectional Table 2 Ceramics Performance comparison of material (Si3N4) and bearing steel (Gcr15) Project GCr15 Si3N4 Specific heat resistance (Â°C) 180 800 4.44 Density (g/cm3) 7.8 3.2 0.41 Linear expansion coefficient (1/Â°C) 12.5Ã—10-6 3.2Ã—10 -6 0.26 Elastic Modulus (MPa) 20.8Ã—104 31.4Ã—104 1.52 Vickers Hardness (HV) 700 to 800 1300 to 1800 Poisson's Ratio 0.3 0.26 0.87 Thermal Conductivity [W/(mÂ·K)] 4.9 to 50.2 29.3 - Good corrosion resistance - Strong magnetic non-magnetic - Conductive conductive insulation - 1 Development of high-speed spindle bearings To adapt to machine tools High speed shaft required, generally more rigid and excellent in high speed performance of the angular contact ball bearings, cylindrical roller bearings secondly (see Table 1). At the same time, gradually develop a suitable lubrication system. From the expression dmN value of the high-speed performance of the main shaft bearing (dm is the diameter of the bearing pitch circle mm, and N is the rotation speed r/min), the dmN value under the grease lubrication condition is 50Ã—104 or less. After the development of oil and gas lubrication, the dmN value has reached more than 100Ã—104. Since then, angular contact ball bearings with rolling elements made of ceramics have been developed in bearings, achieving a dmN value of 200Ã—104. After the development of jet lubrication in the 1990s, the dmN value could reach 300Ã—104. 2 High-speed spindle bearing technology High-speed angular contact ball bearings As can be seen from Table 1, tapered roller bearings, single-row, double-row cylindrical roller bearings are inferior to angular contact ball bearings in terms of high-speed performance. The angular contact bearing is a bearing having a contact angle. The larger the contact angle is, the better the axial rigidity is. However, since the gyro slip and the spin slip between the ball and the raceway are also large, the heat generation also increases. In order to improve the speed performance, the method is to reduce the size (or mass) of the ball, change the curvature coefficient of the channel to reduce the centrifugal force of the ball, reduce the internal load generated during high-speed operation, and increase the number of balls to increase the rigidity. Ceramic Ball Angular Contact Ball Bearings In order to reduce the mass of the ball and increase the speed, a hybrid ceramic ball bearing with silicon nitride (Si3N4) ceramics for the rolling system was introduced. The performance is shown in Table 2. As a high-speed spindle bearing material, ceramic (Si3N4) has the following advantages: Light weight. Because the density is lower than that of the bearing steel, the centrifugal force generated by the rolling elements at the time of high-speed rotation is small, and the rotating torque can be reduced, so that the temperature rise can be reduced and the life can be improved. The good thermal conductivity makes it difficult for the rolling elements of the ceramic material to stick to the metal at high speeds. In the case of good lubrication conditions resistant to burns. Thermal expansion is small. When the rolling element is in contact with the inner ring, it is less likely to cause an increase in the pre-tightening force, resulting in reduced play and burning. Due to the high hardness and rigidity, the deformation of the bearing is small, and the rigidity of the spindle is also improved. Therefore, in summary, using ceramic material (Si3N4) as the rolling element, the speed can be increased by about 25% to 35% compared with the rolling element of the bearing steel, and the life can be increased by about 3 times. When a new type of hybrid angular contact ball bearing rotates at a high speed, the inner ring will expand due to centrifugal force, and the contact stress with the rolling elements will increase, which will increase the internal preload, reduce the clearance, and increase the heat generation. In response to this problem, a new hybrid ceramic ball bearing with a stainless steel inner ring has recently been developed. Since the linear expansion coefficient of stainless steel is 20% smaller than that of bearing steel, it can further control the increase in preload due to the expansion of the inner ring when the bearing rotates at a high speed. Under sufficient lubrication conditions, the dmN value can be increased by 1.2 times under fixed preload. Hybrid angular contact ball bearings with inner rings of ceramics have recently been introduced. In the case of positioning preloading, hybrid angular contact ball bearings of ceramic material are also used for the inner ring. Because the inner ring also uses ceramic materials, the bearing inner diameter or the raceway centrifugal expansion is small, the increase of pre-tightening is also small, and the rigidity is good, the contact area of â€‹â€‹the ball and the raceway is small, so the heat and expansion are also smaller and can be smaller than only the ball. Ceramic bearings reach higher speeds. However, it is precisely because of the small centrifugal and expansion during high-speed rotation, if the mating stress between the metal spindle and it is too large, it may be damaged or even broken. 3 High Speed â€‹â€‹Spindle Bearing Lubrication The trend of high-speed development of spindle bearings places higher demands on lubrication. Traditionally, the dmN value is less than 50Ã—104 for grease lubrication. It is widely used due to its simple and economical use. It requires no special maintenance and does not require follow-up replenishment. Most of them are lubricated for life. With the increase of the rotational speed, when the dmN value reaches 100Ã—104 or more, lubrication is performed using oil and gas and oil mist. Compared with the grease lubrication, the temperature rise is small and can be rotated at a higher speed, thus becoming the main lubrication method. However, although the dmN value of the jet lubrication can reach 250Ã—104, a large amount of lubricating oil is required, the power loss is large because of the stirring resistance, and the complicated auxiliary equipment is required, and the cost is high, so the lubrication method is generally not used. From the machine tools exhibited at the 18th Tokyo International Machine Tool Expo (1996) and the 19th Osaka International Machine Tool Expo (1998), CNC machine tools and machining centers using ceramic rolling elements, oil and gas lubrication, and high-power electric spindles for oil mist lubrication have taken up. About 85% of the entire exhibition machine tools. 4 Conclusion For high-efficiency machining of machine tools, increasing the speed of spindle bearings is the key to achieving high-speed, practical spindles. With the development of high speed, the rolling elements are made of ceramic materials, the dmN value is about 200Ã—104, and the numerically-controlled machining centers with the dmN value of about 100Ã—104 under the condition of grease lubrication are increasing. It has become a development trend, using oil and gas and oil. The mist lubrication method will also become the development direction of high-speed spindle bearing lubrication in the future.