Jade Axle

Abstract

The present invention provides a bearing that can extend its lifespan by suppressing the occurrence of indentations on the raceway surface even if foreign matter becomes lodged, and by suppressing the expansion of their size even if indentations do occur. [Solution] A ball bearing 11 having an outer ring 12 with an outer ring raceway surface 12a formed on its inner circumference, an inner ring 13 with an inner ring raceway surface 13a formed on its outer circumference, a plurality of balls 15 incorporated between the outer ring raceway surface 12a and the inner ring raceway surface 13a, and a cage 16 that holds the plurality of balls 15, wherein the balls 15 are made of ceramics, and there is a predetermined relationship between the minimum thickness between the outer circumferential surface of the outer ring 12 and the outer ring raceway surface 12a, the minimum thickness between the inner circumferential surface of the inner ring 13 and the inner ring raceway surface 13a, and the diameter of the balls 15.

Description

Technical Field

[0001] This invention relates to a ball bearing, and more particularly to a ball bearing used in a driving device (e-Axle) for running mounted on an electric vehicle.

Background Art

[0002] In recent years, in electric vehicles, a drive motor, a transmission, and a speed increaser / decreaser are integrated and widely popularized as an e-Axle. The drive motor used in the e-Axle needs to increase the frequency of inverter control at a high voltage for higher efficiency. However, when it becomes high voltage and high frequency, current easily flows inside the bearing that supports the motor shaft, and electric corrosion may occur.

[0003] As technologies for preventing electric corrosion, Patent Document 1 that forms an insulating coating on the outer diameter surface and the axial end surface of the outer ring, and Patent Document 2 that gives electrical conductivity to the lubricant have been studied.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, even when using the methods of Patent Document 1 and Patent Document 2, when exceeding a certain voltage and current, electricity may conduct between the balls and the raceway surfaces, and there is a risk of electric corrosion.

[0006] In response to this, in some cases, ceramic balls are used as insulators to prevent electric current from passing through the inside of the bearing. However, because ceramic balls are harder and less tough than steel balls, there is a risk that cracks may form in the ceramic balls if foreign matter in the oil of the unit becomes lodged in them. Furthermore, because ceramic balls have a small amount of elastic deformation, even if the ceramic balls themselves are not damaged when foreign matter gets caught, the risk of damage to the raceway surface, such as indentations, is higher compared to when steel balls are rolling. Moreover, if indentations form on the raceway surface and their size increases, there is a risk of bulging around the indentations on the raceway surface. If this bulging occurs, the rolling of the ceramic balls will further increase the damage to the raceway surface. For these reasons, under conditions where foreign matter is present, the lifespan of bearings using ceramic balls may be reduced compared to bearings using steel balls.

[0007] The inventors of this invention focused on the fact that when ceramic balls are used as bearings, if foreign matter becomes lodged, indentations are generated on the raceway surface, and the increase in the size of these indentations affects the reduction of bearing life. They then focused on the fact that it is possible to extend the lifespan by suppressing the generation of indentations on the raceway surface even if foreign matter becomes lodged, and by suppressing the expansion of the size of indentations even if they do occur. [Means for solving the problem]

[0008] Based on this observation, this invention provides a ball bearing with the following configuration in order to solve the aforementioned problems. [Configuration 1] A ball bearing comprising an outer ring with an outer ring raceway surface formed on its inner circumference, an inner ring with an inner ring raceway surface formed on its outer circumference, a plurality of balls incorporated between the outer ring and the inner ring, and a cage that holds the plurality of balls, wherein the balls are made of ceramics, and when the minimum thickness between the outer circumference surface of the outer ring and the outer ring raceway surface is Ho, the minimum thickness between the inner circumference surface of the inner ring and the inner ring raceway surface is Hi, and the diameter of the balls is Dw, the relationship between these is satisfied by the following equation (1). 1.7 ≤ Dw / (Hi+Ho) ≤ 2.4 (1)

[0009] This configuration, by using ceramics for the balls, provides an insulating effect. Furthermore, by setting the ball diameter within a predetermined range relative to the bearing width and cross-sectional height, the surface pressure when foreign objects become jammed can be reduced. Therefore, the occurrence of indentations on the raceway surface can be suppressed, and even if indentations do occur, their size can be reduced, thus extending the lifespan of the bearing.

[0010] [Configuration 2] The ball bearing according to [Configuration 1], wherein the surface nitrogen concentration of the outer ring raceway and the inner ring raceway is 0.02% by mass or more and 0.6% by mass or less. This configuration improves the surface hardness of both the outer and inner ring raceways. As a result, the occurrence of indentations on the raceway surfaces can be further suppressed, and even if indentations do occur, their size can be further reduced.

[0011] [Configuration 3] The ball bearing described in [Configuration 2], wherein the steel material of the outer ring and the inner ring is SUJ3. By adopting this configuration, even if foreign matter gets into the ball bearing, indentations are less likely to occur, and even if indentations do occur, their size can be suppressed, thereby extending the lifespan of the ball bearing.

[0012] [Configuration 4] A ball bearing according to any one of [Configuration 1] to [Configuration 3], which does not have a sealing member. In this configuration, even if foreign matter enters the bearing space and becomes jammed, the surface pressure at the time of jamming can be reduced, thereby suppressing the occurrence of indentations on the raceway surface. Even if indentations do occur, their size can be reduced, making it possible to extend the lifespan while maintaining an insulating effect.

[0013] [Configuration 5] A ball bearing according to any one of [Configuration 1] to [Configuration 3], used as a ball bearing to support the rotating shaft of an electric motor for driving an electric vehicle, or a ball bearing to support the rotating shaft of a reduction gear that reduces the rotation of the electric motor. A ball bearing having the configuration described in any one of [Configuration 1] to [Configuration 3] can be used as a ball bearing to support the rotating shaft of an electric motor for driving an electric vehicle, or as a ball bearing to support the rotating shaft of a reduction gear that reduces the rotation of the electric motor. [Effects of the Invention]

[0014] The ball bearing of this invention uses ceramic balls, and sets the ball diameter within a predetermined range relative to the bearing width and cross-sectional height, that is, sets the ball diameter larger than usual. This provides an insulating effect, improves resistance to electrolytic corrosion, and reduces surface pressure when foreign matter becomes lodged. As a result, the occurrence of indentations on the raceway surface can be suppressed, and even if indentations do occur, their size can be reduced, and the amount of protrusions that occur around the indentations can be suppressed, making it possible to extend the lifespan while maintaining an insulating effect.

[0015] Furthermore, by setting the surface nitrogen concentration of the outer and inner ring raceways of the ball bearing of this invention within a predetermined range, the hardness of the raceway surface can be increased, further suppressing the occurrence of indentations on the raceway surface even if foreign matter becomes lodged. Moreover, even if indentations do occur, their size can be further reduced, thereby further suppressing the amount of protrusions that occur around the indentations. As a result, damage to the balls and raceway surface can be further reduced, leading to an even longer lifespan for the ball bearing. [Brief explanation of the drawing]

[0016] [Figure 1] Cross-sectional view showing an example of a ball bearing according to an embodiment of this invention. [Figure 2] Figure 1 schematically illustrates an example of the use of a ball bearing incorporated into the drive system for electric vehicles. [Figure 3] Figure showing the results of the examples and comparative examples. [Modes for carrying out the invention]

[0017] Figure 1 shows a ball bearing 11 according to an embodiment of the present invention. This ball bearing 11 includes an outer ring 12, an inner ring 13 coaxially provided radially inward of the outer ring 12, a plurality of balls 15 incorporated at regular intervals in the circumferential direction in an annular bearing space 14 formed between the outer ring 12 and the inner ring 13, and a cage 16 that holds the plurality of balls 15.

[0018] The axial direction is a direction parallel to the central axis of the outer ring 12 (the central axis of the bearing), the radial direction is a direction perpendicular to the central axis of the outer ring 12, and the circumferential direction is a direction along the circumference that revolves around the central axis of the outer ring 12.

[0019] As shown in Figure 1, on the inner circumference of the outer ring 12, there are formed an outer ring raceway surface 12a where the balls 15 rollingly contact, and a pair of outer ring shoulder surfaces 12b that extend in the circumferential direction and are axially adjacent to both sides of the outer ring raceway surface 12a. The outer ring raceway surface 12a is the inner surface of a groove with a circular arc-shaped cross-section that extends in the circumferential direction on the inner circumference of the outer ring 12, and the outer ring shoulder surface 12b is a cylindrical surface with a constant inner diameter along the axial direction.

[0020] On the outer circumference of the inner ring 13, there are formed an inner ring raceway surface 13a where the balls 15 rollingly contact, and a pair of inner ring shoulder surfaces 13b that extend in the circumferential direction and are axially adjacent to both sides of the inner ring raceway surface 13a. The inner ring raceway surface 13a is the outer surface of a groove with a circular arc-shaped cross-section that extends in the circumferential direction on the outer circumference of the inner ring 13, and the inner ring shoulder surface 13b is a cylindrical surface with a constant outer diameter along the axial direction.

[0021] The balls 15 are in rolling contact with the outer ring raceway surface 12a and the inner ring raceway surface 13a. The outer ring raceway surface 12a is formed symmetrically with respect to the axial center of the outer ring 12, and the inner ring raceway surface 13a is also formed symmetrically with respect to the axial center of the inner ring 13. Examples of this ball bearing 11 include a deep groove ball bearing. In the present invention, these balls 15 are made of ceramics. Since ceramics have insulating properties, it is possible to prevent current from passing through the inside of the bearing. Note that the material of the ceramics may be an oxide-based material such as alumina or zirconia, or silicon nitride (Si3N4), etc.

[0022] The retainer 16 is a component that partially surrounds the ball 15 and maintains it at a constant distance in the circumferential direction. This retainer 16 is made of a metal plate such as an iron plate or resin. Furthermore, lubricant is provided in the bearing space 14 between the outer ring 12 and the inner ring 13, as needed. Furthermore, both axial sides of the bearing space 14 between the outer ring 12 and the inner ring 13 may be sealed with a sealing member or the like, or they may be left open without a sealing member or the like. As will be described later, even if foreign matter gets caught between the outer ring raceway surface 12a, the inner ring raceway surface 13a (hereinafter sometimes collectively referred to as the "raceway surface") and the ball 15 in the ball bearing 11 according to this invention, the surface pressure at the time of foreign matter entrapment can be reduced, thereby suppressing the occurrence of indentations on the raceway surface. In addition, even if indentations do occur, their size can be reduced. For this reason, damage to the raceway surface can be reduced even without a sealing member, and the lifespan of the ball bearing can be extended.

[0023] When the minimum thickness between the outer circumferential surface of the outer ring 12 and the outer ring raceway surface 12a is Ho, the minimum thickness between the inner circumferential surface of the inner ring 13 and the inner ring raceway surface 13a is Hi, and the diameter of the ball is Dw, the relationship between these satisfies equation (1) below. 1.7 ≤ Dw / (Hi+Ho) ≤ 2.4 (1) The Dw / (Hi+Ho) of a typical ball bearing is 1.5. In contrast, in this invention, Dw / (Hi+Ho) is 1.7 or higher, so the ball bearing 11 according to this invention uses balls with a larger diameter than a typical ball bearing. Therefore, if foreign matter gets caught between the raceway surface and the ball 15, the surface pressure between the foreign matter and the raceway surface can be reduced compared to a typical ball bearing, thereby suppressing the occurrence of indentations on the raceway surface. Furthermore, even if indentations do occur, their size can be reduced. As a result, damage to the raceway surface can be reduced, making it possible to extend the lifespan of the ball bearing. Furthermore, Dw / (Hi+Ho) is preferably 2.4 or less. By setting it to 2.4 or less, sufficient thickness can be ensured between the outer circumferential surface of the outer ring 12 and the outer ring raceway surface 12a, and between the inner circumferential surface of the inner ring 13 and the inner ring raceway surface 13a, thereby ensuring sufficient raceway strength.

[0024] Incidentally, surface nitriding treatment can be applied to the raceway surface to increase its hardness. In this case, the surface nitrogen concentration of the raceway surface (outer ring raceway surface 12a and inner ring raceway surface 13a) is preferably 0.02 mass% or more and 0.6 mass% or less. More preferably, the surface nitrogen concentration is 0.4 mass% or less. This improves the balance with the carbon concentration and stabilizes the hardness. By keeping it within this range, it is possible to further suppress the occurrence of indentations that may occur on the raceway surface when foreign matter is caught, and even if indentations do occur, their size can be further suppressed.

[0025] This surface nitriding treatment can be performed using a general method, such as holding the outer ring 12 and inner ring 13 in an ammonia gas atmosphere at 700°C to 900°C for a certain period of time. The nitrogen concentration on the surface of the orbital plane can be measured using an electron probe microanalyzer (EPMA) by performing a line analysis in the depth direction from the orbital plane in a cross section perpendicular to the orbital plane, and the average nitrogen concentration up to a position where the depth from the orbital plane is 10 μm.

[0026] Furthermore, the outer ring 12 and inner ring 13 used in this invention can be made of high-carbon chromium bearing steel such as SUJ2 or SUJ3, and by using these, high wear resistance can be achieved. SUJ3 is preferable because it has a higher manganese content than SUJ2, resulting in better hardenability and increased hardness.

[0027] Furthermore, while the size of the ball bearing used in this invention is not particularly limited, when used in a ball bearing with an inner diameter of 10 mm or more and 50 mm or less, the minimum thickness between the outer circumferential surface of the outer ring 12 and the outer ring raceway surface 12a, and the minimum thickness between the inner circumferential surface of the inner ring 13 and the inner ring raceway surface 13a become thinner, which may lead to cracking of the raceway rings. However, using SUJ3, which has good hardenability, is preferable because its high hardness makes it less likely for cracking of the raceway rings to occur.

[0028] Figure 2 shows an example of a drive unit (e-Axle) for electric vehicles using the ball bearing 11 according to this invention. This drive unit is an integrated unit of an electric motor 20 for driving electric vehicles such as EVs (battery electric vehicles) and HEVs (hybrid electric vehicles), and a reduction gear 21 that reduces the rotation of the electric motor 20.

[0029] This drive unit includes an electric motor 20 having a rotor shaft 22 which is a rotating shaft, an input shaft 23 of a reduction gear 21 to which the rotation of the rotor shaft 22 is input, an output shaft 24 of the reduction gear 21, an input gear 25 fixed to the input shaft 23, an output gear 26 fixed to the output shaft 24, an intermediate gear 27 that reduces the rotation of the input gear 25 and transmits it to the output gear 26, and an intermediate shaft 28 fixed to the intermediate gear 27 so as to rotate together with the intermediate gear 27.

[0030] The rotor shaft 22 is a drive shaft that is rotationally driven by the electric motor 20. The output shaft 24 of the reduction gear 21 is connected to a wheel (not shown) of the electric vehicle. The input shaft 23, intermediate shaft 28, and output shaft 24 are arranged in parallel with a gap between them. The input gear 25, intermediate gear 27, and output gear 26 are helical gears. This drive device reduces the rotation of the electric motor 20 by sequentially transmitting it to the input gear 25, intermediate gear 27, and output gear 26, and outputs the reduced rotation to the wheel (not shown) from the output shaft 24.

[0031] Furthermore, the ball bearing of this invention can be used as a ball bearing 11 that supports the rotor shaft 22 of the electric motor 20, and as a ball bearing 11 that supports the rotating shaft (input shaft 23, intermediate shaft 28) of the reduction gear 21 that reduces the rotation of the electric motor 20. [Examples]

[0032] The present invention will be explained below using practical examples. The results of the following examples and comparative examples are shown in Figure 3.

[0033] (Examples 1 and 2) Deep groove ball bearings 6207 (bearing inner diameter: 35 mm, ball diameter: 7 / 16 inch, balls: ceramic) were used as ball bearings 11 in the input shaft 23 of the drive unit (e-Axle) for the electric vehicle shown in Figure 2. Two ball bearings were used: one set to have a Dw / (Hi+Ho) of 1.8 (Example 1), and the other set to have a Dw / (Hi+Ho) of 2.2 (Example 2). Assuming that the maximum load was applied, a load of 0.57C was added to the basic dynamic load rating C, and the bearings were driven in a foreign matter environment for durability testing. After 110 hours of operation, the condition of the deep groove ball bearings in Examples 1 and 2 was examined. No abnormalities were found in the balls, and although slight indentations were observed on the raceway surface, no delamination was observed.

[0034] The evaluation of the durability test results is as follows: • ◎: There were no abnormalities in the ball, and no delamination was observed on the track surface. • ○: There were no abnormalities in the ball, and although slight indentations were observed on the track surface, no delamination was seen. • ×: Delamination was observed on the ball and the track surface.

[0035] (Comparative Example 1) In contrast to the deep groove ball bearing used in Example 1, a ball bearing set to Dw / (Hi+Ho)=1.5 was used, and a durability test was conducted in the same manner as in Example 1. After 110 hours of operation, the condition of the deep groove ball bearing in Comparative Example 1 was examined, and delamination was observed on the balls and raceway surface.

[0036] (Comparative Example 2) A ball bearing (Comparative Example 2) was prepared by nitriding the surface layers of the outer ring 12 and inner ring 13 of the deep groove ball bearing used in Comparative Example 1, resulting in a surface nitrogen concentration of 0.025 mass%. These were used as ball bearings 11 in the input shaft 23 of the drive unit (e-Axle) for the electric vehicle shown in Figure 2. Assuming that the maximum load was applied, a load of 0.57C was further applied to the basic dynamic load rating C, and the bearings were driven in a foreign matter environment to conduct a durability test. After 110 hours of operation, the condition of the deep groove ball bearing in Comparative Example 2 was examined, and while there was no delamination on the raceway surface, delamination was observed on the balls.

[0037] (Examples 3 and 4) In Example 3, a deep groove ball bearing was prepared by changing the ball diameter to 15 / 32 inches (Dw / (Hi+Ho)=1.8) compared to the deep groove ball bearing of Example 1. Furthermore, in Example 4, a deep groove ball bearing was prepared in which the outer and inner rings were subjected to nitriding treatment in the same manner as in Comparative Example 2, resulting in a surface nitrogen concentration of 0.025 mass%. Durability tests were conducted using these bearings in the same manner as in Examples 1 and 2 and Comparative Example 1. After 110 hours of operation, the condition of the deep groove ball bearings in Examples 3 and 4 was examined. In Example 3, there were no abnormalities in the balls, and although slight indentations were observed on the raceway surface, no delamination was seen. In Example 4, there were no abnormalities in the balls, and no delamination was observed on the raceway surface.

[0038] The surface nitridation concentration was measured using an electron probe microanalyzer (EPMA) by performing line analysis in the depth direction from the orbital surface in a cross section perpendicular to the orbital surface. Specifically, it was calculated as the average value of the nitrogen concentration up to a position where the depth from the orbital surface is 10 μm. As a result, the surface nitridation concentrations of Example 4 and Comparative Example 2, which underwent nitridation treatment, were in the range of 0.02 mass% to 0.6 mass%. [Explanation of Symbols]

[0039] 11 Ball bearings 12 Outer ring 12a Outer ring raceway 12b Outer ring shoulder surface 13 Inner circle 13a Inner ring raceway surface 13b Inner shoulder surface 14. Bearing space 15 balls 16 Cage 20 Electric motors 21 Reducer 22 Rotor shaft (rotating shaft) 23 Input axis (rotation axis) 28 Intermediate axis (rotation axis)

Claims

1. Outer ring (12) with outer ring raceway surface (12a) formed on the inner circumference, An inner ring (13) having an inner ring raceway surface (13a) formed on its outer circumference, A ball bearing (11) having a plurality of balls (15) incorporated between the outer ring raceway surface (12a) and the inner ring raceway surface (13a), and a cage (16) that holds the plurality of balls (15), The aforementioned ball (15) is made of ceramics, When Ho is the minimum thickness between the outer circumferential surface of the outer ring (12) and the outer ring raceway surface (12a), Hi is the minimum thickness between the inner circumferential surface of the inner ring (13) and the inner ring raceway surface (13a), and Dw is the diameter of the ball (15), the relationship between these is such that the following equation (1) is satisfied in a ball bearing. 1.7≦Dw / (Hi+Ho)≦2.4 (1)

2. The ball bearing according to claim 1, wherein the surface nitrogen concentration of the outer ring raceway surface (12a) and the inner ring raceway surface (13a) is 0.02% by mass or more and 0.6% by mass or less.

3. The ball bearing according to claim 2, wherein the steel material of the outer ring (12) and the inner ring (13) is SUJ3.

4. A ball bearing according to any one of claims 1 to 3, which does not have a sealing member.

5. A ball bearing according to any one of claims 1 to 3, used as a ball bearing for supporting the rotating shaft (22) of an electric motor (20) for driving an electric vehicle, or a ball bearing (11) for supporting the rotating shafts (23, 28) of a reduction gear (21) that reduces the rotation of the electric motor (20).

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