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Suitable for use under vacuum conditions-X-ray tube special ball bearings

by:JNSN     2021-12-25
The designed X-ray tube rotating anode special ball bearing is used in the severe 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 ≦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 electromagnetic, and the speed is mostly 3000~10000min-1. There are two types of anode rotation: inner ring rotation and outer ring rotation (see Figure 6). Generally speaking, the rotation stiffness of the inner ring is higher and the bearing temperature is also lower, but its structure is more complicated. Due to the anode heating, the bearing temperature on one side of the anode can reach up to 400~50C, and the bearing on the other side can reach 200~30C. Therefore, the bearing type must use high-speed tool steel with excellent heat resistance. Since X-ray tubes are mostly used in medical treatment, they must be rotated statically. However, due to the structure, the rigidity is difficult to improve, and the temperature change causes a large change in the bearing clearance, which is not conducive to damping. Therefore, the design of the bearing and its periphery must be extremely cautious. 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 channel is cylindrical (c) In order to alleviate the axial misalignment of the inner and outer rings caused by thermal expansion, the outer ring unilateral channel 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 codes and main dimensions. This type of bearing is divided into two types: cage type and full steel ball type. For special purposes, bearings with stop ribs or with dust cover are also used. For bearings with cages, selecting a cage of appropriate shape and material can ensure stable rotation with low torque at low speeds. However, at high speeds, the sliding friction between the cage and the ball becomes larger, and a full complement steel ball bearing is appropriate. Compared with bearings with cages, full complement bearings have a slightly larger rotational torque due to the mutual contact of steel balls, but have less wear and less torque change. Therefore, the full complement steel ball type ball bearing can be used from low speed to high speed. The solid lubricants used are soft metals such as silver (Ag) and lead (Pb) and flake structures such as molybdenum disulfide (MoS2). The friction and wear conditions of bearings lubricated with this kind of solid lubricant film in the speed range of 100~9000min-1 are shown in Table 16 and Figures 8~10. It can be seen from Table 16 that silver is suitable for low-wear conditions, and lead or molybdenum disulfide is suitable for low-torque conditions. The biggest difficulty of X-ray tube special ball bearings is the lubrication method. Due to vacuum and high temperature, the following solid lubrication methods are mostly used. (1) Stick a solid lubricant such as laminated molybdenum disulfide on the surface of the cage pocket. (2) Plating a soft metal (silver or lead) film 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, ion plating, etc. The endurance test results of soft metal coated ball bearings under vacuum conditions are as follows. Figure 11 is a comparison of the durability time of a ball bearing with an inner diameter of 8mm and an outer diameter of 22mm at a speed of 9000min-1 and an axial load of 20N{2kgf} under 0.13mPa{10-6Torr}, and an axial load of 20N{2kgf}. Figure 12 shows the torque change over time in the test. When ceramic balls are used without lubrication, the channel wear is severe, and after the channel is coated with a lubricant film, the torque change is small and very stable. Figure 13 shows that when the bearing seat temperature rises to 300 °C, the ball bearing with an inner diameter of 9.5 mm and an outer diameter of 22 mm bears an axial load of 5N{0.5kgf} or 20N{2kgf} and a speed of 9000min-1 An example of a rotation test.
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