Rolling bearing

Abstract

To provide a rolling bearing excellent in indentation removing ability, long-term durability, and manufacturing cost even in use under the conditions that foreign matters are mixed therein.SOLUTION: A rolling bearing 1 includes an inner ring 2 and an outer ring 3, and a plurality of rolling elements 4 laid between the inner and outer rings. It rotatably supports a shaft inserted through the inner ring 2 and receives a radial load from the shaft or a shaft box. At least one of the rolling elements 4 is a rolling element with hard film 41 on the surface of which a hard film 6 is formed, and the rest is a standard rolling element 42 on the surface of which the hard film is not formed. The number of the rolling elements with hard film 41 is smaller than 20% of the total number of the rolling elements 4. When there are two or more rolling elements with hard film 41, the rolling elements with hard film 41 are arranged neighboring each other in the peripheral direction.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a rolling bearing having a hard film formed on the surface of at least one rolling element.

Background Art

[0002] The life of a rolling bearing is affected by the use environment, use load, lubrication conditions, bearing material, etc. For example, in a rolling bearing used under conditions where foreign matter is likely to enter the bearing, the influence of the foreign matter on the life of the rolling bearing is great.

[0003] In large vehicles for construction and mining, a speed reducer is combined, and steel-based foreign matter is likely to be generated due to the pitching of the gears attached to the speed reducer. In addition, dust and the like associated with the use environment such as in mines become foreign matter from the outside for the bearing. When a helicopter or an electric vertical takeoff and landing aircraft that is expected to be put into practical use in the future takes off and lands in a desert area or a coastal area, the possibility of foreign matter entering the bearing is also considered. If the operation continues with foreign matter mixed in the bearing, indentations may occur on the rolling surfaces of the inner and outer rings that the rolling elements of the bearing contact. And when swelling occurs due to the indentation, peeling occurs starting from there, which may lead to a shortening of the life of the bearing.

[0004] Here, in Patent Document 1, an indentation individual response life for an indentation is estimated based on the swelling height of all the indentations formed on the rolling surface, and the life of the rolling bearing is estimated by multiplying each estimated indentation individual response life by a predetermined correction coefficient. In Patent Document 1, it is suggested that when the swelling height of the indentation is 1.0 μm or less, indentation-originated peeling does not occur, and thus the indentation does not affect the life of the bearing when the swelling height of the indentation is 1.0 μm or less.

[0005] Furthermore, Patent Document 2 describes a technique for removing indentation protrusions by forming a hard film on each component of a rolling bearing. Specifically, it describes a test in which a diamond-like carbon (DLC) film is formed on one component and this component is slid against a mating material on which indentations have been formed, and it is shown that the DLC film can remove the indentation protrusions. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 5615726 [Patent Document 2] Japanese Patent Publication No. 2020-046068 [Overview of the project] [Problems that the invention aims to solve]

[0007] Incidentally, according to the above-mentioned Patent Document 2, the higher the hardness of the hard film, the better the ability to remove indentations. However, on the other hand, the hard film may become more prone to peeling, and the rolling surface other than the raised indentation may be more susceptible to wear on the mating member. In particular, when used under conditions where foreign matter is present, the roughening of the hard film surface may increase its aggression towards the mating material, potentially reducing the long-term durability of the bearing and shortening its lifespan. On the other hand, applying a hard film to all members may increase manufacturing costs.

[0008] This invention was made to address these problems and aims to provide a rolling bearing that offers superior indentation removal capabilities, long-term durability, and low manufacturing costs, even when used under conditions where foreign matter is present. [Means for solving the problem]

[0009] The rolling bearing of the present invention comprises an inner ring and an outer ring, and a plurality of rolling elements interposed between the inner and outer rings, and rotatably supports a shaft inserted through the inner ring and receives a radial load from the shaft or shaft housing, wherein at least one of the rolling elements is a hard-coated rolling element with a hard film deposited on its surface, the remainder are standard rolling elements without a hard film deposited on their surface, the number of hard-coated rolling elements is less than 20% of the total number of rolling elements, and if there are two or more hard-coated rolling elements, these hard-coated rolling elements are arranged adjacent to each other in the circumferential direction.

[0010] The diameter D1 of the above-mentioned hard-coated rolling element is characterized by being expressed by the following formula (1). (D2-A)-(B+C)≦D1≦D2-A (1) D1: Diameter of the hard-coated rolling element mentioned above. D2: Diameter of the standard rolling element mentioned above A: Amount of movement of the axis's center position toward the maximum load position. B: Allowable level of indentation elevation C: Difference in diameter between the above standard rolling elements

[0011] The rolling bearing is characterized in that the ratio of the radial load applied to the rolling bearing to the basic static radial load rating is 10% or more and less than 35%.

[0012] The above-mentioned hard film is characterized by being a DLC film.

[0013] The DLC film described above is characterized by having an indentation hardness of 20 GPa or higher, as measured by the ISO 14577 method.

[0014] The rolling element described above is characterized by being a tapered roller, a cylindrical roller, a self-aligning roller, or a ball.

[0015] The rolling bearing described above is characterized by being mounted on an electric vertical takeoff and landing aircraft that has multiple drive units having rotors and motors for rotating the rotors, and which flies by the rotation of the rotors, and is a bearing that supports the rotating shaft in the drive unit. [Effects of the Invention]

[0016] The rolling bearing of the present invention rotatably supports a shaft inserted into an inner ring and is a bearing that receives a radial load from the shaft or a shaft housing. Among the rolling elements, at least one or more are rolling elements with a hard film formed on the surface, and the rest are standard rolling elements without a hard film formed on the surface. The number of rolling elements with a hard film is less than 20% of the total number of rolling elements. When there are two or more rolling elements with a hard film, these rolling elements with a hard film are arranged adjacent to each other in the circumferential direction. Therefore, the hard film-bearing rolling element removes the swelling of the indentation, and it is possible to prevent the short life due to the peeling of the swelling part of the indentation of the mating material. In addition, damage to the rolling surface other than the swelling of the indentation of the mating material can be suppressed, and peeling of the hard film can be suppressed. Furthermore, compared with the case where a hard film is formed on all the rolling elements, cost reduction can be achieved. Thereby, the rolling bearing of the present invention is excellent in the ability to remove indentations, long-term durability, and manufacturing cost even when used under conditions where foreign matter is mixed in.

[0017] Further, since the diameter D1 of the rolling element with a hard film is represented by the above formula (1), while ensuring the ability to remove the swelling of the indentation due to the hard film, peeling of the hard film can be further suppressed.

[0018] Since the hard film is a DLC film, it is excellent in low friction, wear resistance, chemical stability, surface smoothness, etc., and is excellent in the ability to remove indentations.

[0019] Since the indentation hardness measured by the ISO14577 method of the DLC film is 20 GPa or more, it is excellent in hardness and further excellent in the ability to remove indentations.

[0020] Since the rolling element is, for example, a tapered roller, it can be suitably used for bearings in applications such as large vehicles and electric vertical takeoff and landing aircraft that are used in environments where foreign matter is likely to be mixed in and are subject to heavy loads and impact loads.

[0021] The rolling bearing is equipped with a plurality of drive units each having a rotating blade and a motor for rotating the rotating blade, and is mounted on an electric vertical takeoff and landing aircraft that flies by the rotation of the rotating blade. Since it is a bearing that supports the rotating shaft in the drive unit, even when used under conditions where foreign matter is particularly likely to混入, such as in desert areas or coastal areas, it has excellent indentation removal ability and long-term durability, enabling the electric vertical takeoff and landing aircraft to take off and land in more free places.

Brief Description of the Drawings

[0022] [Figure 1] It is a cross-sectional view showing a first embodiment of the rolling bearing of the present invention. [Figure 2] It is a plan view of the tapered roller seen from the axial direction. [Figure 3] It is a schematic diagram of the radial cross-section of the rolling bearing of the first embodiment. [Figure 4] It is a schematic diagram showing a mode of measuring the roller diameter of the tapered roller. [Figure 5] It is a cross-sectional view showing a second embodiment of the rolling bearing of the present invention. [Figure 6] It is a perspective view of an electric vertical takeoff and landing aircraft on which the rolling bearing of the present invention is mounted. [Figure 7] It is a partial cross-sectional view of the motor in the drive unit of the electric vertical takeoff and landing aircraft. [Figure 8] It is a rolling element load distribution diagram when the diameters of all rolling elements are the same. <0000i12>It is a rolling element load distribution diagram when there is one rolling element with a hard film. [Figure 10] It is a rolling element load distribution diagram when there is one rolling element with a hard film. [Figure 11] It is a rolling element load distribution diagram when two types of rolling elements are arranged alternately. [Figure 12] It is a rolling element load distribution diagram when three rolling elements with hard films are arranged continuously. [Figure 13] It is a rolling element load distribution diagram when five rolling elements with hard films are arranged continuously. [Figure 14] It is a rolling element load distribution diagram when seven rolling elements with hard films are arranged continuously. [Figure 15] This figure shows an example of measuring the height of an indentation. [Modes for carrying out the invention]

[0023] Hard films such as DLC films have residual stress within the film, and generally, the more hard the film hardens, the lower its peel resistance tends to be. Therefore, when applying hard films to products, it is important to achieve both the ability to remove indentation protrusions caused by increased indentation hardness of the hard film and the peel resistance of the hard film itself, as well as the suppression of wear on the rolling surface other than indentation protrusions on the mating material. In response to this, the inventors have found that in a rolling bearing comprising hard-film-coated rolling elements with a hard film deposited on their surface and standard rolling elements without a hard film deposited on their surface, the long-term durability of the rolling bearing can be improved by arranging the hard-film-coated rolling elements in a predetermined manner, thereby enabling them to exhibit their indentation removal ability even when used under conditions where foreign matter is present. Furthermore, the inventors have found that the peel resistance of the hard film can be further improved by keeping the diameter of each rolling element within a predetermined range.

[0024] (First Embodiment) As a first embodiment of the rolling bearing of the present invention, a tapered roller bearing will be described with reference to Figures 1 to 3. Figure 1 is an axial cross-sectional view of a tapered roller bearing. As shown in Figure 1, the tapered roller bearing 1 comprises an inner ring 2 having a tapered racing surface 2a on its outer circumference, an outer ring 3 having a tapered racing surface 3a on its inner circumference, a plurality of rolling elements (tapered rollers) 4 that roll between the racing surface 2a of the inner ring 2 and the racing surface 3a of the outer ring 3, and a cage 5 that holds the rolling elements 4 in pockets so that they can roll freely at regular intervals in the circumferential direction. Each racing surface of the inner and outer rings is tapered, with the diameter constituting the racing surface increasing and decreasing along the axial direction. The angle of the taper is not particularly limited, but the contact angle 2α of the outer ring is usually about 15° to 90° with respect to the axial direction.

[0025] In the tapered roller bearing 1, at least one of the multiple rolling elements 4 is a hard-coated rolling element 41, which has a hard film 6 deposited on its surface, and the remaining ones are standard rolling elements 42 that do not have a hard film deposited on their surface. The hard-coated rolling elements 41 have a hard film deposited on their rolling surface 4c, the small end face 4a which is the axial end face, and the large end face 4b, so that the hard film is deposited on the entire surface of the tapered roller. From the viewpoint of removing indentations, it is sufficient for the hard film to be formed on at least the rolling surface (outer surface of the roller) of the surface of the tapered roller.

[0026] In this invention, the number of hard-coated rolling elements 41 is at least 1 and less than 20% of the total number of rolling elements 4 (number of hard-coated rolling elements 41 + number of standard rolling elements 42). For example, if the total number of rolling elements is 32, the number of hard-coated rolling elements will be less than 7. The above ratio of the number of hard-coated rolling elements 41 may be less than 15% or less than 10%. By reducing this ratio, the load on the hard-coated rolling elements can be further reduced, and the number of hard-coated rolling elements, which have high manufacturing costs, can be reduced, resulting in lower manufacturing costs.

[0027] When there is one hard-coated rolling element 41, standard rolling elements 42 are arranged on both sides of it in the circumferential direction. When there are two or more hard-coated rolling elements 41, they are arranged adjacent to each other in the circumferential direction. In other words, in this case, the hard-coated rolling elements 41 are arranged continuously in the circumferential direction. By arranging them in this way, the load on the hard-coated rolling elements 41 can be reduced compared to, for example, when hard-coated rolling elements 41 and standard rolling elements 42 are arranged alternately in the circumferential direction, and consequently the peel resistance of the hard coating can be improved.

[0028] Next, the relationship formula for the diameter D1 of the hard-coated rolling element in the rolling bearing of the first embodiment will be explained using Figures 2 and 3. Figure 2 is a plan view of the hard-coated rolling element 41 and the standard rolling element 42, viewed axially from the small end face 4a side. The diameter D1 of the hard-coated rolling element is preferably smaller than the diameter D2 of the standard rolling element 42, and it is more preferable that the diameter D1 of the hard-coated rolling element is expressed by the following formula (1). (D2-A)-(B+C)≦D1≦D2-A (1) D1: Diameter of the hard-coated rolling element D2: Standard rolling element diameter A: Amount of movement of the axis's center position toward the maximum load position. B: Allowable level of indentation elevation C: Diameter difference between standard rolling elements

[0029] Here, in the case where the rolling element is a tapered roller, as in the first embodiment, "diameter of the rolling element" refers to the "nominal diameter of the tapered roller." The nominal diameter of the tapered roller is defined based on JIS B1506 Annex 1 Table 2, and is the diameter of the virtual circle Ca where the rolling surface 4c, which is the outer diameter surface of the tapered roller, and the large end surface 4b intersect. In Figure 2, the diameter D1 of the hard-coated rolling element 41 is smaller than the diameter D2 of the standard rolling element 42.

[0030] The terms A and B in equation (1) above will be explained using Figure 3. Figure 3 is a schematic diagram of the radial cross-section of a rolling bearing. Note that the cage is omitted in Figure 3. The rolling bearing 1 rotatably supports a shaft 7 inserted through the inner ring 2 and receives a radial load from the shaft 7. In Figure 3, it is assumed that the rolling bearing 1 is subjected to a maximum radial load Fr in the direction of the arrow. In Figure 3, the position of the hard-coated rolling element 41 corresponds to the position where the maximum load is received (maximum load position).

[0031] As shown in Figure 3, when a radial load Fr is applied to the rolling bearing 1 from the shaft 7, the center position of the shaft moves from the reference point O to O'. This amount of movement corresponds to A in equation (1) above. The reference point O represents the center position of the shaft before the radial load is applied (unloaded). If the standard rolling element 42 is located at the maximum load position, then when the center position of the shaft 7 moves from the reference point O to O', the inner ring 2, the standard rolling element 42, and the outer ring 3 undergo elastic deformation by that amount. In other words, A can be said to be the amount of elastic displacement at the maximum load position, and this is equal to the sum of the deformation amounts of the inner ring 2, the standard rolling element 42, and the outer ring 3.

[0032] Here, as shown in equation (1) above, the diameter D1 of the hard-coated rolling element 41 is equal to or less than the value obtained by subtracting the amount of movement A of the axis center position toward the maximum load position from the diameter D2 of the standard rolling element 42. By defining the diameter D1 in this way, the load on the rolling element can be suppressed even when the hard-coated rolling element 41 is located at the maximum load position.

[0033] Furthermore, when foreign matter enters the bearing, an indentation M is formed on the racing surface 2a of the inner ring 2, as shown in Figure 3, which is higher than the racing surface. The height of this indentation M that does not affect the bearing's lifespan is defined as the allowable indentation height B. The allowable height B is set to, for example, 0.001 mm.

[0034] In equation (1) above, C represents the difference in diameters between standard rolling elements. Specifically, it represents the difference between the diameter of the largest standard rolling element and the diameter of the smallest standard rolling element among several standard rolling elements. Furthermore, if the rolling elements are tapered rollers, the difference in diameters C between standard rolling elements represents the maximum difference in roller diameters between the tapered rollers that are standard rolling elements.

[0035] Here, the method for measuring the diameter of a tapered roller will be explained using Figure 4. The diameter of the rolling element 4 is the length perpendicular to the roller axis at a position a distance of any length in the direction of the roller axis from the large end face 4b of the rolling element 4.

[0036] As shown in Figure 4, the rolling element 4 to be measured is supported by a V-block 8 on its rolling surface 4c so that the roller axis is horizontal. The rolling element 4 is fixed in position in the roller axis direction by contact with a positioning member 9 having a projection on its large end surface 4b. The contact position of the positioning member 9 on the rolling element 4 is preferably approximately in the center of the radial range of the large end surface 4b where the large flange of the inner ring makes contact when the rolling element 4 rolls on the running surface. The roller diameter is obtained by measuring the rolling surface 4c at an arbitrary length L in the roller axis direction from the large end surface 4b using a dial gauge 10 from a direction perpendicular to the roller axis.

[0037] Here, as shown in equation (1) above, the "diameter D1 of the hard-coated rolling element 41" is equal to or greater than the value obtained by subtracting the "allowable indentation height B" and the "difference in diameter between standard rolling elements C" from "D2-A". By defining the diameter D1 in this way, it is possible to reduce the load on the hard-coated rolling element 41 at the maximum load position while making it easier for the hard film to come into contact with the indentation, thereby achieving excellent indentation removal performance.

[0038] As described above, by making the diameter D1 of the hard-coated rolling element 41 satisfy equation (1) above, the load on the hard-coated rolling element 41 can be reduced, and the delamination resistance of the hard film under foreign matter contamination and the wear of the rolling surface can be reduced while removing the raised indentations. Furthermore, by making the diameter D1 of the hard-coated rolling element 41 smaller than the diameter D2 of the standard rolling element 42, the load on the hard-coated rolling element 41 is reduced, and the opportunities for the hard film to contact the rolling surface are reduced, making wear of the rolling surfaces of the inner and outer rings and delamination of the hard film less likely to occur. In this way, by ensuring the frequency of contact between the hard film and the indentations formed on the rolling surfaces of the inner and outer rings, while relatively reducing the frequency of contact between the hard film and the rolling surface other than the raised indentations, only the raised indentations can be effectively worn away.

[0039] The hard film deposited on the hard-filmed rolling element can be freely selected as long as it has a higher indentation hardness than the surface hardness of the rolling element before deposition. Examples of hard films that can be selected include titanium-based hard films, chromium-based hard films, carbon-based hard films, alumina-based hard films, and zirconia-based hard films. For example, from the viewpoint of low friction, wear resistance, chemical stability, and surface smoothness, a DLC film, which is a carbon-based hard film, is preferred. In addition, a chromium nitride film can be used from the viewpoint of corrosion resistance and low aggressiveness against other materials.

[0040] The hardness of the hard film is not particularly limited, but from the viewpoint of indentation removal, it is preferable that the indentation hardness measured by the ISO 14577 method be 10 GPa or higher, more preferably 15 GPa or higher, and even more preferably 20 GPa or higher. The upper limit of the indentation hardness is not particularly limited, but for example it may be 50 GPa or less, or 40 GPa or less.

[0041] The above numerical range for indentation hardness is based on the findings described in Patent Document 2 mentioned above. Patent Document 2 describes the results of an indentation removal test performed using a two-cylinder testing machine with one test piece having a hard film formed on it and the other mating material having an indentation formed on it. This two-cylinder testing machine is equipped with a drive-side test piece and a driven-side test piece that rolls and slides in contact with it. Each test piece (ring) is supported by a support bearing and a load is applied by a load spring. A hard film is formed only on the driven-side test piece, and foreign matter is mixed between the drive-side and driven-side test pieces to promote the peeling of the hard film, and the peel resistance of the hard film after operation is evaluated.

[0042] Table 1 below shows the relationship between the indentation hardness of the hard film and its ability to remove indentation protrusions. Figure 15 shows an example of measuring the height of the indentation protrusion before and after the indentation removal test. In Figure 15, height A represents the height of the indentation protrusion before rotating the bearing a predetermined number of times, and height B represents the height of the indentation protrusion after rotating the bearing a predetermined number of times.

[0043] [Table 1]

[0044] The results in Table 1 show that the removal performance of indentation bumps is improved by forming a hard film with a predetermined indentation hardness. While the peel resistance tends to decrease as the indentation hardness of the hard film increases, the peeling of the hard film can be further suppressed, for example, by defining the diameter D1 of the hard-film-coated rolling element as shown in equation (1) above.

[0045] The indentation hardness of the hard film can be measured using, for example, Agilent Technologies' Nanoindenter (G200). The measured value is obtained by taking the average value at a depth unaffected by surface roughness (a point where the hardness is stable), and then averaging the values ​​obtained from multiple points on the surface of the hard film.

[0046] The thickness of the hard film is not particularly limited, but it is preferably between 0.5 μm and 3.0 μm. By keeping the film thickness within this range, it is possible to achieve a balance between abrasion resistance, mechanical strength, and peel resistance.

[0047] When the hard film is a DLC film, it is preferable that the DLC film has a three-layer structure consisting of a base layer directly deposited on the surface of the rolling element that serves as the substrate, a mixed layer mainly composed of WC (tungsten carbide) and DLC deposited on the base layer, and a surface layer mainly composed of DLC deposited on the mixed layer. By making the film structure of the hard film a three-layer structure as described above, it is possible to avoid abrupt changes in physical properties (hardness, elastic modulus, etc.). The DLC film with this three-layer structure will be described below.

[0048] The above-mentioned underlayer is a layer that is directly formed on the surface of the rolling element that serves as the base material. The material and structure are not particularly limited as long as they can ensure adhesion to the base material, and for example, Cr (chromium), W (tungsten), Ti (titanium), and Si (silicon) can be used as materials. Among these, it is preferable that the underlayer contains Cr because it has excellent adhesion to the bearing member that serves as the base material (for example, high-carbon chromium bearing steel). Furthermore, the base layer is preferably composed mainly of Cr and WC, taking into consideration adhesion with the mixed layer. WC has intermediate hardness and elastic modulus between Cr and DLC, and is less prone to residual stress concentration after film formation. In particular, it is preferable to have a gradient composition in which the Cr content decreases and the WC content increases from the substrate side to the mixed layer side. This results in excellent adhesion between the substrate and the mixed layer on both sides.

[0049] The above-mentioned mixed layer serves as an intermediate layer interposed between the substrate layer and the surface layer. As mentioned above, the WC used in the mixed layer has intermediate hardness and elastic modulus between Cr and DLC, and is less prone to residual stress concentration after film formation. The mixed layer has a gradient composition in which the WC content decreases and the DLC content increases continuously or stepwise from the substrate layer side to the surface layer side, resulting in excellent adhesion to both the substrate layer and the surface layer. Furthermore, the WC and DLC are physically bonded within the mixed layer, preventing damage within the mixed layer. In addition, the DLC content is higher on the surface layer side, resulting in excellent adhesion between the surface layer and the mixed layer. Thus, the above-mentioned mixed layer is a layer that uses WC to anchor and bond highly non-adhesive DLC to the substrate layer side.

[0050] The above surface layer is a film mainly composed of DLC. It is preferable that the surface layer has a relaxation layer adjacent to the mixed layer. This relaxation layer is obtained by continuously or stepwise changing at least one of the deposition condition parameters (amount of hydrocarbon gas introduced, vacuum level, bias voltage) to avoid abrupt changes in these parameters when the deposition condition parameters (amount of hydrocarbon gas introduced, vacuum level, bias voltage) differ between the mixed layer and the surface layer. More specifically, the deposition condition parameters at the time of outermost layer formation of the mixed layer are used as the starting point, and the final deposition condition parameters of the surface layer are used as the endpoint, with each parameter being continuously or stepwise changed within this range. This eliminates abrupt differences in physical properties (hardness, elastic modulus, etc.) between the mixed layer and the surface layer, further improving the adhesion between the mixed layer and the surface layer. Furthermore, by continuously or stepwise increasing the bias voltage, the graphite structure (sp) in the DLC structure can be improved. 2 ) and diamond structure (sp 3 The composition ratio shifts towards the latter, and the hardness slopes (increases).

[0051] The above-described three-layer DLC film is obtained by depositing a base layer, a mixed layer, and a surface layer on a rolling substrate in that order. These layers are preferably formed using an unbalanced magnetron sputtering (UBMS) apparatus with Ar gas as the sputtering gas. In this method, a dense film (layer) can be formed due to the ion-assisted effect, where Ar ions and electrons reach the substrate along magnetic field lines that extend close to the substrate, resulting in a greater number of ionized targets reaching the substrate compared to conventional sputtering. Suitable targets include Cr targets, WC targets, and graphite targets.

[0052] Furthermore, the structure of the hard film is not limited to a multilayer structure in which three types of layers are deposited as described above. A layer of one type of composition may be deposited one or more times under the same or different deposition conditions, or layers of multiple types of compositions may be deposited multiple times under the same or different deposition conditions.

[0053] In the tapered roller bearing 1 shown in Figure 1, the bearing components, namely the inner ring 2, outer ring 3, rolling element substrate to be coated, standard rolling elements 42, and cage 5, are made of ferrous material. Any steel commonly used for bearing components can be used as the ferrous material, such as high-carbon chromium bearing steel, carbon steel, tool steel, and martensitic stainless steel.

[0054] Furthermore, the surface roughness Ra of the surface on which the hard film is formed is preferably 0.05 μm or less. If the surface roughness Ra exceeds 0.05 μm, it becomes difficult to form the hard film at the tips of the roughness protrusions, resulting in locally reduced film thickness.

[0055] The uniformity of the hard film thickness (the difference between the maximum and minimum values ​​of multiple measured film thicknesses divided by the average value) is preferably between 0 and 0.05. To ensure good uniformity of film thickness, sputtering of the hard film can be performed, for example, with the rolling element supported at two points, or while the rolling element is rotating. Furthermore, polishing may be performed after the hard film is formed to improve the uniformity of the film thickness.

[0056] (Second Embodiment) A rolling bearing according to a second embodiment of the present invention will be described with reference to Figure 5. Detailed explanations of matters described using Figures 1 to 4 will be omitted. Figure 5 is an axial cross-sectional view of a deep groove ball bearing equipped with hard-coated rolling elements, in which a hard film is deposited on the surface of the rolling elements.

[0057] As shown in Figure 5, the deep groove ball bearing 11 comprises an inner ring 12 having a racing surface 12a on its outer circumference, an outer ring 13 having a racing surface 13a on its inner circumference, and a plurality of rolling elements 14 that roll between the racing surfaces 12a and 13a. The rolling elements 14 are held at regular intervals by a cage 15. The axial openings at both ends of the inner and outer rings are sealed by a sealing member 16, and grease 17 is sealed in the bearing space. As the grease 17, known grease for rolling bearings can be used.

[0058] The rolling bearing in Figure 5 comprises rolling elements 141 with a hard film 18 deposited on their surface and standard rolling elements 142 without a hard film deposited on their surface. The hard film 18 is deposited on the entire surface of the balls of the rolling elements 141. In this configuration as well, the number of hard-filmed rolling elements 141 is less than 20% of the total number of rolling elements 14, and if there are two or more hard-filmed rolling elements 141, they are arranged adjacent to each other in the circumferential direction. Furthermore, in the deep groove ball bearing 11, it is preferable that the diameter D1 of the rolling elements 141 is smaller than the diameter D2 of the standard rolling elements 142, and it is especially preferable that the relationship in the following formula (1) is satisfied. (D2-A)-(B+C)≦D1≦D2-A (1) D1: Diameter of the hard-coated rolling element D2: Standard rolling element diameter A: Amount of movement of the axis's center position toward the maximum load position. B: Allowable level of indentation elevation C: Diameter difference between standard rolling elements

[0059] In formula (1) above, D1 and D2 represent the diameters of the balls of the hard-coated rolling element 141 and the standard rolling element 142, respectively. A and B in formula (1) above are those described in the first embodiment. In addition, C in formula (1) above represents the maximum difference in diameter between the balls of the standard rolling element 42. In the second embodiment as well, by satisfying formula (1) above, the bearing becomes even better in terms of indentation removal ability, long-term durability, and manufacturing cost, even when used under conditions where foreign matter is present.

[0060] Figures 1 to 5 show tapered roller bearings and deep groove ball bearings as rolling bearings, but this technology can also be applied to other types of bearings such as angular contact ball bearings, cylindrical roller bearings, self-aligning roller bearings, and needle roller bearings.

[0061] The applications of the rolling bearing of the present invention are not particularly limited, but it is especially suitable for applications where foreign matter is likely to enter, as it can prevent the bearing from becoming prematurely damaged. For example, it can be used in axle support devices (wheel support devices) of construction machinery. More specifically, it can be used in axle bearings for large dump trucks used in mining. In this truck, the rotation of the shaft is transmitted to the drive wheels via a planetary gear mechanism. A spindle forming a fixed axle is located on the outside of the shaft, and a tire wheel is located on the outside of the spindle via a rolling bearing. Since the rolling bearing is located near the planetary gear mechanism, steel-based foreign matter due to gear pitting is likely to enter the inside of the rolling bearing. However, because it has the hard-film-coated rolling elements described above, the hard film has excellent resistance to peeling.

[0062] Other potential applications include, for example, flying cars, which have recently attracted attention as an alternative mode of transportation to automobiles. Flying cars are expected to solve various social problems and are anticipated to be used in a wide range of situations, including local and inter-regional transportation, tourism and leisure, emergency medical care, and disaster relief.

[0063] Vertical take-off and landing (VTOL) aircraft are attracting attention as a type of flying car. Because VTOL aircraft can ascend and descend vertically between the air and the take-off / landing area, they do not require runways and are highly convenient. In particular, in recent years, due to social demands for CO2 reduction, electric VTOL aircraft (eVTOL), which fly using batteries and motors, have become the mainstream of development.

[0064] An electric vertical take-off and landing (VTOL) aircraft equipped with the rolling bearings of the present invention will be described with reference to Figure 6. The VTOL aircraft 21 shown in Figure 6 is a multirotor having a main body 22 located in the center of the aircraft and four drive units 23 arranged in the front, rear, left, and right. The drive units 23 are devices that generate lift and thrust for the VTOL aircraft 21, and the VTOL aircraft 21 flies by being driven by the drive units 23. In the VTOL aircraft 21, there may be multiple drive units 23, and it is not limited to four.

[0065] The main body 22 has a living space where a crew (for example, 1 to 2 people) can board. This living space is equipped with control systems for determining the direction of travel and altitude, as well as instruments that indicate altitude, speed, and flight position. Four arms 22a extend from the main body 22, and a drive unit 23 is provided at the tip of each arm 22a. In Figure 6, each arm 22a is integrally provided with an annular section that covers the rotational circumference of the rotor blade 24 in order to protect the rotor blade 24. In addition, skids 22b that support the aircraft during landing are provided at the bottom of the main body 22.

[0066] The drive unit 23 includes a rotor blade 24 and a motor 25 that rotates the rotor blade 24. In the drive unit 23, a pair of rotor blades 24 are provided on both sides of the motor 25 in the axial direction. Each rotor blade 24 has two blades that extend radially outward.

[0067] The main body 22 is equipped with a battery (not shown) and a control device (not shown). The control device is also called a flight controller. The control of the electric vertical take-off and landing aircraft 21 is performed by the control device, for example, as follows: The control device outputs a command to change the rotational speed of the motor 25, which should adjust the lift based on the difference between the current attitude and the target attitude. Based on this command, the amplifier provided in the motor 25 adjusts the amount of power sent from the battery to the motor 25, and the rotational speed of the motor 25 (and rotor blades 24) is changed. Furthermore, the adjustment of the rotational speed of the motor 25 is performed simultaneously for multiple motors 25, thereby determining the attitude of the aircraft.

[0068] Figure 7 shows a partial cross-sectional view of the motor in the drive unit. In Figure 7, the aforementioned rotor blades are attached to one end (upper side of the figure) of the rotating shaft 27 of the motor 25, and a rotor is attached to the other end (lower side of the figure). The rotor is positioned opposite a stator fixed to the housing and is rotatable relative to the stator. The motor 25 can be configured as an outer rotor type brushless motor or an inner rotor type brushless motor.

[0069] In Figure 7, the motor 25 comprises a housing (device housing) 26, a rotor (not shown), a stator (not shown), an amplifier (not shown), and two rolling bearings (deep groove ball bearings) 31, 31. The housing 26 has an outer cylinder 26a and an inner cylinder 26b, with a cooling medium passage 26c provided between them. By flowing a cooling medium through this passage 26c, excessive temperature rise can be prevented. The rolling bearings 31, 31 rotatably support the rotating shaft 27 within the inner cylinder 26b. In Figure 7, the rolling bearing 31 has hard-coated rolling elements 341 and standard rolling elements 342 as rolling elements 34. The rolling bearing 31 corresponds to the rolling bearing of the present invention.

[0070] In the rolling bearing 31, the outer diameter shape of the outer ring 33 is substantially the same as the shape of the fitting portion on the inner circumference of the housing, and it is directly fitted to the housing 26 without using a bearing housing or the like. An inner ring spacer 28 and an outer ring spacer 29 are inserted between the rolling bearings 31 and 31, and a preload is applied. The outer ring spacer 29 is provided with nozzle members 30, 30 for injecting lubricating oil for cooling and lubricating the rolling bearings 31, 31. The nozzle member 30 has a lubricating oil flow path inside that guides the air-oil supplied from an external lubricating oil supply device (not shown) to the bearing space.

[0071] In an electric vertical takeoff and landing aircraft, when taking off and landing in a desert area or a coastal area, etc., there is a possibility that foreign matter may enter the inside of the bearing in the drive unit. As a result, peeling occurs on the rolling surface, etc., starting from the indentation generated accordingly, which may lead to a short life of the bearing. By applying the rolling bearing of the present invention, peeling due to indentation can be prevented even during flight in a desert area or a coastal area, etc., leading to an improvement in the long-term durability of the bearing. Thereby, the restrictions on the takeoff and landing locations and flight locations of the electric vertical takeoff and landing aircraft are relaxed.

[0072] Note that the bearing configuration in the drive unit is not limited to the configuration in FIG. 7. In FIG. 7, the rotation axis of the motor and the rotation axis of the rotor blade are the same rotation axis, but a configuration in which the rotation axis of the motor and the rotation axis of the rotor blade are connected via a transmission mechanism may also be used. In this case, the rolling bearing that supports the rotation axis in the drive unit may be the rolling bearing that supports the rotation axis of the motor, or the rolling bearing that supports the rotation axis of the rotor blade.

Example

[0073] Regarding the tapered roller bearing, the influence on the load due to the relationship between the diameter D1 of the rolling element with the hard film and the diameter D2 of the standard rolling element, etc., was examined by CAE (Computer Aided Engineering) analysis.

[0074] <CAE Analysis Setting Conditions> Bearing: Tapered roller bearing (HM265049 / HM265010) Inner diameter: φ368.249 mm Outer diameter: φ523.875mm Overall width: 101.6mm Inner ring width: 79.375mm Outer rim width: 101.6 mm Standard rolling element diameter D2 (nominal diameter): 36.732 mm Number of rolls: 32 Contact angle α: 12°30′00″ (α is defined in JIS B1534) Outer ring small end diameter E: φ464.00 (E is defined in JIS B1534) Basic static radial load rating (C0r): 305,000 kgf (2,990 kN) Radial load (Fr): 30,500 kgf Displacement of the axis center position toward the maximum load position: 0.045 mm C0r ratio ((Fr / C0r)×100): 10% Hardness of the hard coating on the rolling element: 20 GPa or higher

[0075] In the example, the C0r of the tapered roller bearing used was 305,000 kgf, but it is not typically used under load conditions where the ratio of Fr to C0r is 100%. It is also not used in the region where the ratio of Fr to C0r is 4% or less. In the present invention, the ratio of Fr to C0r is within a range other than these, preferably 10% or more and less than 35%.

[0076] The values ​​of equation (1) under the above conditions are as follows: (D2-A)-(B+C)≦D1≦D2-A (1) A = 0.045 mm, B = 0.001 mm, and C = 0.002 mm. Therefore, the diameter D1 of the hard-coated rolling element is within the range of equation (2) below. D2-0.048≦D1≦D2-0.045 (2)

[0077] First, based on equation (2) above, the following three patterns were established as the relationship between the diameter D1 of the hard-coated rolling element and the diameter D2 of the standard rolling element. Pattern 1: The diameter of the rolling element is not changed (no hard-coated rolling elements) Pattern 2: D1 = D2 - 0.045 (upper limit) Pattern 3: D1 = D2 - 0.048 (lower limit) Figures 8 to 10 show the load distribution of the rolling elements when a radial load (Fr) is applied to the rolling elements directly above the axis center of the rolling bearing for each pattern. In Figures 9 and 10, the number of hard-coated rolling elements is assumed to be one. Also, in each figure, the position of maximum load is rolling element number 1.

[0078] Figure 8 shows the load distribution when all 32 rolling elements have the same diameter (Pattern 1). When there is no difference in the diameter of the rolling elements, it can be seen that the rolling element placed at rolling element number 1 is subjected to approximately 40,000 N, which is the largest load among the conditions set in Figures 8 to 10.

[0079] Figure 9 shows the load distribution when there is one hard-coated rolling element (rolling element number 1) and its diameter is pattern 2. The load on the hard-coated rolling element is significantly lower than in Figure 8, at approximately 20,000 N. Figure 10 shows the load distribution for pattern 3. The load on the hard-coated rolling element (rolling element number 1) was slightly lower compared to Figure 9. This is likely because the diameter of the hard-coated rolling element was smaller compared to Figure 9, resulting in a smaller load on that rolling element.

[0080] Next, we investigated the changes in load caused by changing the arrangement of multiple hard-coated rolling elements.

[0081] Figure 11 is a load distribution diagram for Pattern 2, in which 16 hard-coated rolling elements are arranged alternately with standard rolling elements, starting from rolling element number 1. The load on the hard-coated rolling element placed at rolling element number 1 is significantly larger than in the case where there is only one hard-coated rolling element (rolling element number 1 only) (Figure 9), and the loads on the standard rolling elements on either side of it are also significantly larger. In particular, the loads on the standard rolling elements at rolling element numbers 2 and 32 were higher than the loads on the rolling elements at the corresponding positions in Figure 8.

[0082] Figure 12 is a load distribution diagram for Pattern 2, in which three hard-coated rolling elements are arranged consecutively at rolling element numbers 1, 2, and 32. The load on the hard-coated rolling elements at rolling element numbers 1, 2, and 32 was significantly reduced compared to the load on the rolling elements at the corresponding positions in Figure 8.

[0083] Figure 13 is a load distribution diagram for Pattern 2, in which five hard-coated rolling elements are arranged consecutively at rolling element numbers 1, 2, 3, 31, and 32. The load on the hard-coated rolling elements at rolling element numbers 1, 2, 3, 31, and 32 was significantly reduced compared to the load on the rolling elements at the corresponding positions in Figure 8.

[0084] Figure 14 is a load distribution diagram for Pattern 2, in which seven hard-coated rolling elements are arranged consecutively as rolling element numbers 1, 2, 3, 4, 30, 31, and 32. The load on the hard-coated rolling element number 1 is approximately the same as the load on the rolling element number 1 in Figure 8, resulting in a high load being placed on the hard-coated rolling element.

[0085] Based on these results, even when hard-coated rolling elements are placed at the maximum load position, the load on these rolling elements can be suppressed by adjusting the number, arrangement, and even the diameter of the hard-coated rolling elements. Furthermore, as the hard-coated rolling elements wear down the raised indentations on the rolling surface in accordance with the bearing's rotation, delamination of the rolling surface caused by these indentations can be suppressed, ultimately leading to a longer bearing lifespan. [Industrial applicability]

[0086] The rolling bearing of the present invention effectively wears down only the raised indentations while suppressing wear on the rolling surfaces of the mating inner and outer rings, as well as peeling of the hard film. As a result, it offers superior indentation removal ability, long-term durability, and manufacturing cost, even when used under conditions where foreign matter is present. Therefore, the rolling bearing of the present invention can be applied to a variety of applications, including those under harsh lubrication conditions. In particular, it can be suitably used in wheel support devices (especially bearings for axles of construction machinery used in construction sites and mines), helicopters, and electric vertical take-off and landing aircraft, where conditions are prone to foreign matter contamination. [Explanation of Symbols]

[0087] 1. Tapered roller bearing (rolling bearing) 2 Inner ring 3 Outer ring 4 Rolling elements 41 Hard-coated rolling elements 42 Standard Rolling Elements 5 Cage 6 Hard membrane 7 axes 8 V-blocks 9 Positioning member 10 Dial Gauge 11. Deep groove ball bearings (rolling bearings) 12 Inner Ring 13 Outer ring 14 Rolling elements 141 Hard-coated rolling element 142 Standard Rolling Elements 15 Cage 16. Sealing member 17 Grease 21 Electric Vertical Takeoff and Landing Aircraft 22 Main body 23 Drive unit 24 rotor blades 25 Motor 26 Housing 27 Rotation axis 28 Inner Wheel Spacer 29 Outer wheel spacer 30 Nozzle component 31 Rolling bearings 32 Inner Ring 33 Outer ring 34 Rolling element 341 Hard-coated rolling element 342 Standard Rolling Elements

Claims

1. A rolling bearing comprising an inner ring, an outer ring, and a plurality of rolling elements interposed between the inner and outer rings, rotatably supporting a shaft inserted through the inner ring, and receiving a radial load from the shaft or an axle box, At least one of the rolling elements is a hard-film-coated rolling element having a hard film formed on its surface, and the remaining rolling elements are standard rolling elements having no hard film formed on their surface, A rolling bearing characterized in that the number of the hard film-coated rolling elements is less than 20% of the total number of the rolling elements, and when there are two or more hard film-coated rolling elements, the hard film-coated rolling elements are arranged adjacent to each other in the circumferential direction.

2. 2. The rolling bearing according to claim 1, wherein the diameter D1 of the hard film-coated rolling element is expressed by the following formula (1): (D2-A)-(B+C)≦D1≦D2-A...(1) D1: diameter of the hard film-coated rolling element D2: diameter of the standard rolling element A: Amount of movement of the shaft center position toward the maximum load position B: Allowable amount of indentation swelling C: Difference in diameter between the standard rolling elements

3. 3. The rolling bearing according to claim 1, wherein the ratio of the radial load acting on said rolling bearing to the basic static rated radial load is 10% or more and less than 35%.

4. 4. The rolling bearing according to claim 1, wherein the hard film is a DLC film.

5. 5. The rolling bearing according to claim 1, wherein the DLC film has an indentation hardness of 20 GPa or more as measured by the ISO 14577 method.

6. 6. The rolling bearing according to claim 1, wherein the rolling elements are tapered rollers.

7. the rolling bearing is mounted on an electric vertical take-off and landing aircraft that flies by rotation of the rotors and includes a plurality of drive units each having a rotor and a motor that rotates the rotors; 7. The rolling bearing according to claim 1, which is a bearing for supporting a rotating shaft in the drive unit.

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