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Suitable for use in vacuum conditions - ball bearings for X-ray tubes

by:JNSN     2022-03-12
The ball bearing specially designed by NSK for rotating anode of X-ray tube is used in harsh working conditions of high vacuum, high temperature and high speed. The structure of the X-ray tube is shown in Figure 5. When the internal pressure is less than or equal to 0.13mPa {10-6 Torr}, thermionic electrons flow from the cathode to the anode, and X-rays are generated on the anode. The rotor is one of the components of the motor, which can be driven by an external electromagnet, and the rotational speed is mostly 3000~10000min-1. The rotation mode of the anode is divided into two types: inner ring rotation and outer ring rotation (see Figure 6). Generally speaking, the rigidity of the inner ring rotation is high, and the bearing temperature is low, but its structure is more complicated. Due to the heating of the anode, the bearing temperature on one side of the anode can reach up to 400~500C, and the bearing on the other side also reaches 200~300C. Therefore, the bearing type must use high-speed tool steel with excellent heat resistance. Since X-ray tubes are mostly used in medicine, they must be rotated statically. However, since it is difficult to improve the rigidity in structure, and the bearing clearance changes greatly due to severe temperature changes, it is not conducive to vibration reduction. Therefore, the design of the bearing and its periphery must be very careful. The inner diameter of the bearing is mostly 6-10 mm, and its structure is shown in Figure 7. (a) With stamping cage (b) The outer ring groove is cylindrical surface (c) In order to alleviate the axial dislocation of the inner and outer rings caused by thermal expansion, the outer ring unilateral groove is designed as a cylindrical surface. In addition, (b) and (c) usually use full complement ball bearings. Under high vacuum conditions where conventional lubricating oil or grease cannot be used, solid lubricant coated ball bearings are used. See Table 15 for their nominal code and main dimensions. This type of bearing is divided into two types: with cage type and full complement steel ball type. For special purposes, bearings with stop rib or dust cover are also used. For bearings with cages, selecting cages of appropriate shape and material can ensure stable rotation with low torque at low speeds. However, at high speed, the sliding friction between the cage and the balls increases, so a full complement steel ball bearing is suitable. Compared with bearings with cages, full-complement bearings have a slightly larger rotational torque due to the contact of steel balls with each other, but have less wear and torque changes. Therefore, full complement steel ball type ball bearings can be used from low speed to high speed. The solid lubricants used are soft metals such as silver (Ag) and lead (Pb), and sheet-like structures such as molybdenum disulfide (MoS2). The friction and wear conditions of the bearings lubricated with this kind of solid lubricant film in the speed range of 100 to 9000 min-1 are shown in Table 16 and Figures 8 to 10. It can be seen from Table 16 that silver is suitable for low wear conditions, and lead or molybdenum disulfide is suitable for conditions requiring low torque. The biggest difficulty of ball bearings for X-ray tubes is the lubrication method. Due to vacuum and high temperature, the following solid lubrication methods are mostly used. (1) Laminate solid lubricants such as molybdenum disulfide on the surface of the cage pockets. (2) Soft metal (silver or lead) film is plated on the surface of the steel ball and the inner and outer ring channels. Method (2) is mostly used for full complement steel ball bearings, and the coating methods include electroplating and ion plating. The durability test results of soft metal-coated ball bearings under vacuum conditions are as follows. Figure 11 is a comparison of the endurance time of a ball bearing with an inner diameter of 8mm and an outer diameter of 22mm under the conditions of 0.13mPa {10-6Torr}, speed of 9000min-1, and axial load of 20N{2kgf} at room temperature. Figure 12 shows the variation of torque with time in the test. When ceramic balls are used and there is no lubrication, the channel wear is serious, and after the channel is coated with a lubricating oil film, the torque change is small and very stable. Figure 13 shows that when the bearing seat temperature rises to 300C, the ball bearing with inner diameter of 9.5 mm and outer diameter of 22 mm is subjected to an axial load of 5N{0.5kgf} or 20N{2kgf} and a speed of 9000min-1. Example of a spin test.
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