Touchdown bearing

Abstract

To improve bearing life in touchdown bearings used under lubrication with solid lubricants only. [Solution] A touchdown bearing used in a magnetic bearing device that provides non-contact support for a rotating member, which rotatably supports the rotating member when the power supply to the magnetic bearing device is cut off, comprises an inner ring, an outer ring, and a plurality of rolling elements interposed between the inner ring and the outer ring, wherein at least the raceway surfaces of the inner ring and the outer ring are super-finished surfaces, and at least the raceway surfaces have a solid lubricant coating, the solid lubricant coating is a molybdenum disulfide plating coating, and at least the raceway surface of the inner ring or the outer ring has a molybdenum coating as an underlayer for the solid lubricant coating, and the thickness of the solid lubricant coating is 0.5 μm to 6 μm.

Description

[Technical Field]

[0001] The present invention mainly relates to a touchdown bearing used in an apparatus provided with a magnetic bearing. [Background Art]

[0002] A magnetically levitated turbomolecular pump used in semiconductor manufacturing apparatuses, liquid crystal panel manufacturing apparatuses and the like is provided with a so-called touchdown bearing along with a magnetic bearing. A magnetic bearing is a bearing that supports the rotating shaft of a turbomolecular pump in a non-contact manner by magnetic force, and can be operated oil-free. A touchdown bearing is a rolling bearing that contacts and supports the rotating shaft when the magnetic bearing stops due to a power outage, failure or the like during operation of the turbomolecular pump. When a touchdown occurs, the rotating ring of the touchdown bearing contacts the rotating shaft of the turbomolecular pump and rapidly rotates at a high speed, so lubrication is extremely important for the touchdown bearing.

[0003] Since a turbomolecular pump is operated in a vacuum environment, it is necessary to avoid using liquid lubricants such as lubricating oil and grease for the touchdown bearing in order to avoid contamination caused by outgassing and dust generation. For this reason, solid lubricants such as molybdenum disulfide (MoS2) are used. For example, Patent Document 1 proposes that in a touchdown bearing, a hardened layer is formed by causing spherical or substantially spherical metal particles to collide with the inner diameter surface and end face of an inner ring, and a lubricating coating formed of a solid lubricant is formed on the surface of the hardened layer. Further, Patent Document 2 proposes a surface treatment method for forming a solid lubricant coating with high adhesion strength on the surface of a sliding portion of a sliding member. [Prior Art Documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Laid-Open No. 2009-024846 [Patent Document 2] Japanese Patent Laid-Open No. 2024-093574 [Summary of the Invention] [Problems that the invention aims to solve]

[0005] Conventionally, as a surface modification treatment to improve the durability of bearing surfaces and the adhesion of coatings to bearing surfaces, blast processing including shot peening is often used before coating formation, as described in Patent Documents 1 and 2. When such physical pretreatment is performed, minute irregularities are formed on the bearing surface, which is beneficial because the liquid lubricant can be retained in these irregularities under lubrication with a liquid lubricant. However, in bearings used in a vacuum environment, such as touchdown bearings, liquid lubricants are not used, so it is conceivable that the irregularities on the bearing surface will not exert the same effect as in a lubrication environment with a liquid lubricant.

[0006] To confirm the durability of the surface coating with and without pre-treatment blasting, the inventors conducted a friction and wear test using the ball-on-disk method in accordance with JIS R 1613:2010 (rotation speed: 190 rpm, weight: 2 GPa, no lubrication). As disc test pieces, ground stainless steel plates (surface roughness Ra: 0.5 μm) were used, and two samples were prepared: one with a solid lubricating coating applied after blasting (surface roughness Ra: 0.5 μm after coating formation), and another with a solid lubricating coating applied without blasting (surface roughness Ra: 0.5 μm after coating formation). Compared to the sample with blasting, the sample without blasting had a 1.4 times longer operating time until metal-to-metal contact occurred, i.e., until the friction coefficient reached 1.1 or higher, indicating that the solid lubricating coating had higher durability. Therefore, the inventors considered that, in the case of bearings using only solid lubricants, surface irregularities in the bearing only increase frictional resistance and can lead to a decrease in bearing life, and thus it is desirable to apply solid lubricants without blasting.

[0007] Therefore, the objective of the present invention is to improve the bearing life of a touchdown bearing used under lubrication by solid lubricant alone, in order to solve the above problems. [Means for solving the problem]

[0008] Furthermore, the touchdown bearing according to the present invention is A touchdown bearing used in a magnetic bearing device equipped with a magnetic bearing that non-contactually supports a rotating member, which rotatably supports the rotating member when the power supply to the magnetic bearing device is stopped, It comprises an inner ring, an outer ring, and a plurality of rolling elements interposed between the inner ring and the outer ring, At least the raceway surfaces of the inner ring and outer ring are super-finished surfaces, At least the raceway surface has a solid lubricant coating, The solid lubricant coating is a molybdenum disulfide plating coating. At least the raceway surface of the inner or outer ring has a molybdenum coating as an underlayer for the solid lubricant coating. The thickness of the solid lubricant coating is 0.5 μm to 6 μm.

[0009] Here, superfinishing is a process in the bearing manufacturing process in which a special device is used to precisely polish an intermediate product after grinding. This process involves applying a superfinishing abrasive to the surface of a rotating intermediate product while oscillating it. By applying superfinishing, in conjunction with the formation of a solid lubricating film described later, the coefficient of friction on the bearing surface is reduced and wear resistance is improved, thus reducing damage sustained during touchdown. Here, the surface roughness Ra of the superfinished surface is approximately 0.01 to 0.15 μm. More preferably, the surface roughness Ra of the superfinished surface may be 0.03 μm or less. Surface roughness can be measured, for example, using a rotary talisurf.

[0010] With this configuration, in the touchdown bearing, at least the raceway surface is a super-finished surface, and a solid lubricating film is provided on it. This reduces the frictional resistance of the bearing surface during touchdown, thereby improving the bearing life.

[0011] Furthermore, the solid lubricant coating is a molybdenum disulfide plating. In touchdown bearings used under lubrication solely by solid lubricants, impact loads and high-speed rotation are expected during operation. By adopting a molybdenum disulfide plating as the lubricant coating for touchdown bearings, excellent lubrication under high loads can be achieved while reducing the cost of coating formation.

[0012] Furthermore, the presence of a molybdenum coating as an underlayer between the bearing surface and the molybdenum disulfide coating as a lubricating film can improve the adhesion of the lubricating film to the bearing surface.

[0013] Furthermore, because the thickness of the solid lubricant coating is in the range of 0.5 μm to 6 μm, the coating has a good lifespan, and even when the coating peels off, the gap in the bearing does not increase, making it possible to suppress bearing vibration. [Brief explanation of the drawing]

[0014] [Figure 1] This is a schematic partial longitudinal cross-sectional view showing an exemplary turbomolecular pump equipped with a touchdown bearing according to the present invention. [Figure 2] This is a schematic longitudinal cross-sectional view showing a touchdown bearing according to one embodiment of the present invention. [Figure 3] Figure 2 shows an example of the area where a solid lubricating coating is applied to a touchdown bearing. [Figure 3A] Figure 3 shows a magnified view of the underlayer and solid lubricating film on the surface of the touchdown bearing. [Figure 4] This is a schematic longitudinal cross-sectional view showing a face-to-face combination touchdown bearing according to one embodiment of the present invention. [Figure 5] Figure 4 is a schematic longitudinal cross-sectional view showing an example of a touchdown bearing installed in a turbomolecular pump, illustrating the state where the rotation axis of the turbomolecular pump is not in touchdown position. [Figure 6] Figure 5 shows the turbomolecular pump in a state where its rotating shaft has touched down. [Figure 7]Fig. 4 is a diagram showing an example of a range where a solid lubricant coating is provided in the touchdown bearing. MODE FOR CARRYING OUT THE INVENTION

[0015] Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In the following description, the axial direction of the central axis of a touchdown bearing (hereinafter also simply referred to as "bearing") is referred to as "axial direction", the radial direction of the bearing is referred to as "radial direction", and the circumferential direction around the central axis of the bearing is referred to as "circumferential direction". Further, the vertical direction and the horizontal direction respectively match the description directions in the drawings.

[0016] As an example of a magnetic bearing device provided with the touchdown bearing according to the present invention, a turbomolecular pump 1 is shown in Fig. 1. A turbomolecular pump is a vacuum pump that discharges gas molecules when a rotor provided with moving blades rotates at high speed. The turbomolecular pump 1 includes a housing 3, a rotor 5, a motor 7, magnetic bearings 9a to 9d, and touchdown bearings 11 and 13 which are rolling bearings. The rotor 5 includes a rotating shaft 5a that rotates around a rotation axis C, and a flange portion 5b that protrudes in the radial direction at one end of the rotating shaft 5a. In the example of Fig. 1, the rotation axis C of the rotating shaft 5a coincides with the vertical direction. Among the bearings 11 and 13 mounted on the housing 3, the bearing 11 provided on the upper side of the rotating shaft 5a is configured as a deep groove ball bearing as shown in Fig. 2. The bearing 13 provided on the lower side of the rotating shaft 5a is configured as double-row angular contact ball bearings 13A and 13B as shown in Fig. 4.

[0017] During normal operation of the turbomolecular pump 1, the magnetic bearings 9a to 9d shown in Fig. 1 are energized to generate magnetic force, whereby the rotating shaft 5a, which is a rotating member, is supported in a non-contact state, and the rotor 5 is driven by the motor 7 to rotate at high speed. The magnetic bearings 9a and 9b are radial magnetic bearings that apply magnetic force to the rotating shaft 5a in the radial direction. The magnetic bearings 9c and 9d are axial magnetic bearings that are disposed to sandwich the flange portion 5b and apply magnetic force in the axial direction. As the rotor 5 rotates, gas molecules taken in from an intake port (not shown) are struck away by the moving blades of the rotor 5 and discharged from the exhaust port 15, thereby performing vacuum evacuation.

[0018] During operation of the turbomolecular pump 1, if energization to the magnetic bearings 9a to 9d is stopped due to a power outage, abnormality of the control system, or the like, the magnetic force of the magnetic bearings 9a to 9d is lost, and the rotating shaft 5a can no longer be supported in a non-contact state. In this case, the rotating shaft 5a transitions from the floating state shown in Fig. 5 to a state of contact (touchdown) with the bearing 13 shown in Fig. 6, and rapidly accelerates the inner ring of the bearing 13. The bearing 11 in Fig. 1 receives the radial load of the inclined rotating shaft 5a, and the bearing 13 receives the axial load and the radial load of the rotating shaft 5a. Since the bearings 11 and 13 rotatably support the rotating shaft 5a, contact between the rotating shaft 5a and the magnetic bearings 9a to 9d as well as the housing 3 is prevented, and damage to the turbomolecular pump 1 is prevented.

[0019] As described above, the bearings 11 and 13 are configured such that the rotating ring (inner ring or outer ring) does not come into contact with the rotating member (shaft or housing) during normal operation of the turbomolecular pump 1, and when an abnormality such as a power outage or failure occurs and the rotating shaft 5a is no longer supported by the magnetic bearings 9a to 9d, the bearings operate in contact with the rotating shaft 5a. Hereinafter, the upper bearing 11 will first be described in detail.

[0020] The bearing 11 shown in Figure 2 comprises an inner ring 17 having a raceway surface 17aa on its outer circumferential surface 17a, an outer ring 19 having a raceway surface 19aa on its inner circumferential surface 19aa, and a plurality of balls acting as rolling elements 21 interposed between the raceway surfaces 17aa of the inner ring 17 and 19aa of the outer ring 19. In this example, the inner ring 17 is a rotating ring, and the outer ring 19 is a stationary ring. The inner ring 17 is installed so as to have an axial gap with respect to the rotating shaft 5a during normal operation, and when a touchdown occurs, the protruding end face 5c of the rotating shaft 5a comes into contact with the end face 17c of the inner ring 17. The outer ring 19 is fixed to the housing 3.

[0021] The inner ring 17 and outer ring 19 of the bearing 11 are preferably made of stainless steel. When the bearing 11 is used as a touchdown bearing in a vacuum environment, rust-preventive oils commonly used in bearings cannot be used because they can cause contamination in the vacuum environment. For this reason, it is necessary to select a material with excellent corrosion resistance, and stainless steel is preferable. Examples of stainless steel include martensitic stainless steel (such as SUS440C) and austenitic stainless steel. From the viewpoint of corrosion resistance, martensitic stainless steel is particularly preferred. However, if surface treatment is performed to suppress rusting, bearing steel (such as SUJ2) can also be used.

[0022] The rolling elements 21 of the bearing 11 are preferably made of ceramics. When the bearing 11 is used as a touchdown bearing, when touchdown occurs, the rotating shaft 5a comes into contact with the rotating ring (inner ring 17 in this example), causing a rapid acceleration of the rotating ring (inner ring 17) of the bearing 11. At this time, friction occurs between the rolling elements 21 and the bearing raceways 17 and 19, or between the balls in a ball bearing, causing rapid heat generation, which often leads to the rolling elements 21 being the first to be damaged. For this reason, it is preferable to use ceramics as a material with excellent wear resistance. Examples of ceramics include silicon nitride, silicon carbide, aluminum oxide (alumina), zirconium oxide (zirconia), sialon, and glass. However, depending on the specifications of the device to which the bearing 11 is applied, other materials such as stainless steel or bearing steel may be used if there are no problems in use.

[0023] In the bearing 11, at least the raceway surfaces 17aa and 19aa of the inner ring 17 and outer ring 19 are super-finished surfaces.

[0024] In this embodiment, as shown by the dashed line in Figure 3, the parts of the inner ring 17 and outer ring 19 other than the raceway surfaces 17aa and 19aa are also super-finished, similar to the raceway surfaces 17aa and 19aa. For example, the entire outer circumferential surface 17a, inner circumferential surface 17b, end face 17c of the inner ring 17, and the inner circumferential surface 19a of the outer ring 19, which are parts of the bearing 11 where contact and sliding with the rotating shaft 5a may occur, are also super-finished. On top of that, as shown in Figure 3A, a solid lubricating film LC is provided via an underlayer BC.

[0025] The bearing 11 has a solid lubricating film LC on at least the raceway surfaces 17aa and 19aa via an underlayer BC. By having a solid lubricating film LC on the bearing raceway surfaces 17aa and 19aa, where friction with the rolling elements 21 occurs during bearing rotation, friction during touchdown, as described later, can be suppressed. The extent to which the solid lubricating film LC and underlayer BC are provided can be appropriately determined according to the specifications and requirements of the bearing.

[0026] The undercoat BC and solid lubricating film LC may be applied to the entire surface of the inner ring 17 and the outer ring 19. However, from a cost reduction perspective, the coating treatment may be omitted for parts that do not come into contact with the rotating shaft 5a. In this example, as shown in Figure 3, the undercoat BC and solid lubricating film LC are applied to the entire surface of the inner ring 17 and at least the inner circumferential surface 19a of the outer ring 19.

[0027] In this embodiment, the solid lubricating film LC and the underlayer BC are formed by a process in which granular material forming the film is sprayed with compressed gas, and the granular material is diffused and penetrated into the surface to be treated to form a plating film. The particle size of the sprayed granules may be, for example, #100 to #800, and the spraying pressure may be, for example, 0.2 MPa or higher. In this example, the underlayer BC is formed by spraying granular underlayer material onto the bearing surface which has been superfinished, and the solid lubricating film LC is formed by spraying granular solid lubricant onto the underlayer BC. Note that, in order to simplify the manufacturing process, it is also possible to omit the underlayer BC and form only the solid lubricating film LC.

[0028] In this example, molybdenum is used as the base material, and molybdenum disulfide is used as the solid lubricant. In this example, since a solid lubricant film LC is formed by the above surface treatment, the solid lubricant film LC is a molybdenum disulfide plating film. Molybdenum disulfide exhibits excellent lubricity under high loads and is also advantageous in terms of manufacturing cost because it does not require chemical synthesis. Furthermore, the presence of a molybdenum film as an underlayer BC between the bearing surface and the molybdenum disulfide film can improve adhesion to the bearing surface. However, the material used to form the film is not limited to this. For example, when using molybdenum disulfide as a solid lubricant, a molybdenum alloy can also be used as the base material. In addition, tungsten disulfide or soft metals such as gold, silver, and lead may be used as the solid lubricant. The solid lubricant may be composed of a single material, or it may be used in combination with other solid lubricants as long as it does not impair the effects of the present invention. The solid lubricating film LC can also be formed by well-known methods other than the above-mentioned process, such as sputtering, shot blasting, and dispersion coating.

[0029] Furthermore, the surface portion to be superfinished is not subjected to mechanical processing such as shot peening, which involves spraying granular abrasive material as a pretreatment before coating formation to roughen and harden the surface through plastic deformation. For this reason, the surface hardness of the superfinished surface of the bearing 11 may be, for example, 56 to 67 HRC on the Rockwell hardness scale. The surface hardness may also be a value measured by, for example, the Rockwell hardness test (JIS Z 2245).

[0030] The thickness of the solid lubricating film LC may be 0.5 μm to 6 μm. By setting the thickness of the solid lubricating film LC within this range, the film can withstand wear for a longer period, thus extending the lifespan of the bearing. In addition, the gap in the bearing that occurs when the film peels off can be reduced, thereby suppressing bearing vibration. The thickness of the solid lubricating film LC may be a value measured, for example, using a calorometer.

[0031] Next, the double-row bearing 13 located below the rotating shaft 5a in Figure 1 will be described. As shown in Figure 4, the bearing 13 consists of double-row angular contact ball bearings 13A and 13B installed in a face-to-face combination. Each angular contact ball bearing 13A or 13B comprises an inner ring 23 having a raceway surface 23aa on its outer circumferential surface 23a, an outer ring 25 having a raceway surface 25aa on its inner circumferential surface 25a, and a plurality of balls acting as rolling elements 27 interposed between the raceway surface 23aa of the inner ring 23 and the raceway surface 25aa of the outer ring 25. In this example, the inner ring 23 is a rotating ring, and the outer ring 25 is a stationary ring. The inner ring 23 is installed so as to have a gap with respect to the rotating shaft 5a during normal operation. As shown in Figure 6, when a touchdown occurs, the protruding end face 5c of the rotating shaft 5a collides with the upper end face 23c of the inner ring 23, and depending on the inclination of the rotating shaft 5a, the inner circumferential surface 23b of the inner ring 23 comes into contact with the outer circumferential surface of the rotating shaft 5a. The outer ring 25 is fixed to the housing 3.

[0032] The material, surface treatment, and areas where the underlayer BC and solid lubricant film LC are formed on the bearing 13 are the same as those of the bearing 11 described above. In other words, in each bearing 13, at least the raceway surfaces 23aa and 25aa of the inner ring 23 and outer ring 25 are super-finished surfaces, and the solid lubricant film LC is formed on these raceway surfaces via the underlayer BC shown in Figure 3A. In this example, as shown by the dashed line in Figure 7, the underlayer BC and solid lubricant film LC are formed on the entire surface of the inner ring 23 and at least the inner circumferential surface 23b of the outer ring 25. Unlike the bearing 11 described above, the bearing 13 is subjected to axial and radial loads from the rotating shaft 5a, so it is preferable to apply the underlayer BC and solid lubricant film LC to the upper end face 23c and lower end face 23d of the inner ring 23.

[0033] In the above, ball bearings are used as bearings 11 and 13, but it is also possible to use bearings with cages (not shown). When cages are used, the frictional resistance when the rolling elements rotate is reduced, which can improve the bearing life.

[0034] As described above, by not performing blasting on bearings 11 and 13, the surface roughness of the sliding parts of the bearings is suppressed, which reduces frictional resistance during sliding and improves bearing life. Furthermore, even though the surface of the sliding parts of the bearings is not blast-treated, the solid lubricating film LC shown in Figure 3A can be adhered and retained well. This is presumed to be because the area where the solid lubricating film LC is applied is super-finished, which suppresses damage due to friction during contact and sliding in that area, thereby suppressing wear and detachment of the solid lubricating film LC. [Explanation of Symbols]

[0035] 1…Turbomolecular pump (magnetic bearing device) 9a~9d...Magnetic bearings 11, 13… Touchdown bearings 17, 23… Inside 17a, 23a... Inner ring outer circumference 17aa, 23aa... Inner ring track surface 19,25…outer ring 19a, 25a... Inner surface of the outer ring 19aa, 25aa... Outer ring raceway surface 21, 27… Rolling elements LC…Solid lubricant coating BC…base layer

Claims

[Claim 1] A touchdown bearing used in a magnetic bearing device equipped with a magnetic bearing that non-contactually supports a rotating member, which rotatably supports the rotating member when the power supply to the magnetic bearing device is stopped, It comprises an inner ring, an outer ring, and a plurality of rolling elements interposed between the inner ring and the outer ring, At least the raceway surfaces of the inner ring and outer ring are super-finished surfaces, At least the raceway surface has a solid lubricant coating, The solid lubricant coating is a molybdenum disulfide plating coating. At least the raceway surface of the inner or outer ring has a molybdenum coating as an underlayer for the solid lubricant coating. A touchdown bearing in which the thickness of the solid lubricant coating is 0.5 μm to 6 μm.

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