Rolling bearing and rolling element for rolling bearing

Abstract

To provide a rolling element and a long-life rolling bearing using the same, in which the rolling element having enhanced surface hardness does not induce micro-spalling to an outer ring, an inner ring or a shaft, and a cage in contact with the rolling element, and the micro-spalling is not induced to a component of a contact partner even when the rolling bearing is used with thin lubricating oil.SOLUTION: The micro Vickers hardness of the surfaces of the rolling elements (3) is 820 to 930Hv0. 1, the roughness (Ra) of the surfaces of the rolling elements (3) is 0.01 to 0.05 μ m, the skewness (Rsk) is - 4.0 to 0.0 μ m, the maximum cross-sectional height (Rt) is 0.7 μm or less, and the ratio (Ra / Rsk) of the roughness (Ra) to the skewness (Rsk) is 5.0 or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a rolling bearing and a rolling element for the rolling bearing. [Background technology]

[0002] Generally, durability, i.e., a long life, is required of rolling bearings. However, to extend the life of rolling bearings that rotate at high speeds under specific lubrication environments, it is necessary to overcome problems specific to the rolling parts, which consist of the outer ring, inner ring, shaft, or rolling elements, depending on the usage conditions.

[0003] For example, rolling bearings used in e-axles, which combine major components such as motors used in automobiles, and in multi-stage automatic transmissions (ATs), are expected to be used at high speeds, and in order to keep torque loss low, they require the use of as little low-viscosity lubricating oil as possible.Furthermore, they may be used in conditions where the entire bearing is hot or where foreign matter has been mixed into the lubricating oil.

[0004] Peeling may occur on the surfaces of the steel rolling components of rolling bearings used under such severe conditions. This spalling phenomenon is not caused by internal spalling originating from inclusions contained in the steel part, but rather by spalling originating from micro-spalling on the surface. When the inner diameter surface roughness of the planetary gears in planetary gear mechanisms commonly used in e-axles, automatic transmissions, etc. is smooth, micro-spalling often occurs on the rolling components of the rolling bearings that rotatably support the planetary gear shafts, or on the shafts themselves.

[0005] Japanese Patent No. 6211051 (Patent Document 1) describes a rolling bearing part in which the hardness at a depth of 40 μm from the part surface is limited to 870 HV0.3 to 1000 HV0.3 by nitriding treatment, so that the mechanical elements used in aircraft can be used safely and the part will have a long life and be able to withstand a certain degree of damage, thereby minimizing the extent of damage when a rolling load is applied.

[0006] Patent Document 1 also describes that the rolling bearing component has an "edge zone" in which the nitrogen content decreases from the outer portion near the surface toward the inner portion, and a "core zone" with a nearly constant hardness, and that the hardness at a depth of 40 μm is 870 to 1000 HV0.3, and the hardness at a depth of 300 μm is at most 250 HV0.3 lower than the hardness at a depth of 40 μm, and that the absolute value of the compressive residual stress at the surface is 500 to 1000 MPa, and that the compressive residual stress decreases from the outside to the inside in the edge zone. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6211051 Summary of the Invention [Problem to be solved by the invention]

[0008] However, in the conventional technology described in Patent Document 1, by increasing the hardness of the rolling elements of a rolling bearing within a predetermined range at a depth of 40 μm from the surface, damage such as micro-spalling can be avoided for the rolling elements themselves, but the rolling elements are aggressive and can cause damage to the mating parts that come into contact with them. In particular, when the parts in question are rolling elements, increasing the hardness of the surface to within the predetermined range increases the aggressiveness of the rolling elements toward the mating parts that come into contact with them, posing the problem that the mating parts are more likely to suffer damage such as micro-spalling.

[0009] Therefore, the object of this invention is to solve the problem that, as mentioned above, when the rolling elements of a rolling bearing have increased surface hardness, they are likely to damage parts that come into contact with them, and to provide a rolling element with increased surface hardness that does not induce micro-spalling or the like in the outer ring, inner ring, shaft, or cage that comes into contact with it, and further to provide a rolling element that does not induce micro-spalling in parts that come into contact with it even when the rolling bearing is used with a dilute lubricating oil, and a rolling bearing that uses such a rolling element with a long life. [Means for solving the problem]

[0010] In order to solve the above problems, the present invention provides a rolling element for a rolling bearing in which the rolling element is rotatably held by a cage between an inner ring or a shaft and an outer ring, the rolling element having a micro Vickers hardness of 820 to 930 Hv0.1 on the surface of the rolling element, a surface roughness (Ra) of 0.01 to 0.05 μm, a skewness (Rsk) of -4.0 to 0.0 μm, a maximum cross-sectional height (Rt) of 0.7 μm or less, and an absolute value of the ratio (Ra / Rsk) of the surface roughness (Ra) to the skewness (Rsk) of 5.0 or less. Note that (Ra / Rsk) is a parameter that indicates the degree of unevenness, and the smaller this value, the smaller the average absolute value of the unevenness and the more valleys there are.

[0011] The rolling elements for rolling bearings of the present invention configured as described above have a micro-Vickers hardness of the rolling element surface increased to 820 to 930 Hv0.1, and are prepared to have an extremely smooth surface with a surface roughness (Ra) of 0.01 to 0.05 μm, so that the occurrence of micro-spalling is suppressed on the surface.

[0012] Furthermore, since the skewness (Rsk), a parameter of the surface roughness of the rolling element, is 0 or a negative value (less than 0), that is, -4.0 to 0.0 μm, and the maximum cross-sectional height (Rt) is 0.7 μm or less, the proportion of tiny peaks and valleys observed from the cross-sectional shape of the rolling element is greater than or equal to the proportion of valleys compared to peaks.

[0013] Furthermore, the maximum depth of the valleys is specified to be relatively shallow at Rt 0.7 μm or less, and the rolling element has an appropriate surface roughness such that the ratio of the absolute value of Ra / Rsk is 5.0 or less. As a result, the surface of the rolling element is extremely smooth, and the extremely small amount of low-kinematic viscosity lubricating oil is held evenly on the surface without being unevenly distributed in the valleys of appropriate depth, so that even with an extremely small amount of low-viscosity lubricating oil, an oil film can be formed efficiently and evenly on the surface of the rolling element.

[0014] Therefore, the rolling elements for a rolling bearing of the present invention can contact the outer ring, inner ring or cage with a film of lubricating oil on the surface thereof even in a lean lubrication environment.

[0015] In this way, the parts that the rolling elements come into contact with experience minimal friction due to contact and uneven distribution of load stress, resulting in a rolling bearing that is less likely to cause micro-spalling in the parts even in a lean lubrication environment.

[0016] Furthermore, in order to prevent micro-spalling from occurring in the rolling elements of the rolling bearing of the present invention, the amount of retained austenite in the surface layer portion up to 50 μm from the surface of the rolling elements is preferably 15 to 35% by volume.

[0017] By setting the amount of retained austenite in the surface layer from the surface of the rolling element to 50 μm to 15 to 35 volume %, stress is alleviated in a specified region from the surface (contact surface) to 25 μm, thereby sufficiently enhancing the effect of suppressing the occurrence of micro-spalling.

[0018] Furthermore, if micro-spalling occurs, it is preferable to work-harden the surface so that the grain size number (JIS G 0551) of the prior austenite grain boundary in the surface layer portion up to 50 μm from the surface of the rolling element is 9 to 11, in order to retard the propagation of cracks from the micro-spalling to the periphery.

[0019] A rolling bearing equipped with rolling elements that achieve the above-mentioned effect can sufficiently prevent the occurrence of micro-spalling in the outer ring, inner ring, and cage, which may come into contact with each other, and therefore it is possible to sufficiently extend the life of a rolling bearing that rotates at high speeds.The above-mentioned expected effect can also be achieved when applied to a rolling bearing that supports the planetary gear (planetary pinion) of a planetary gear mechanism that is commonly used in e-axles, automatic transmissions, etc. [Effects of the Invention]

[0020] In this invention, the rolling elements of a rolling bearing are made up of balls or rollers whose absolute values ​​of micro Vickers hardness, surface roughness (Ra), skewness (Rsk), maximum cross-sectional height (Rt), and the ratio of surface roughness (Ra) to skewness (Rsk) (Ra / Rsk) are within specified ranges. This makes it difficult for the rolling elements to cause damage to parts that come into contact with the rolling elements, such as the outer ring, inner ring, shaft, or cage. In particular, rolling elements that have been prepared to have high surface hardness do not induce micro-spalling in the parts that come into contact with them. Furthermore, when a rolling bearing is used with a dilute lubricating oil, the rolling elements do not cause damage such as micro-spalling to the parts, and this has the advantage of allowing a rolling bearing using such rolling elements to have an extremely long life. [Brief explanation of the drawings]

[0021] [Figure 1] Schematic diagram of a planetary gear mechanism [Figure 2] FIG. 1 is a perspective view showing a rolling bearing and its rolling elements according to an embodiment, with a part of a planetary gear cut away. [Figure 3] Optical microscope photograph of the rolling element surface of Example 1 taken with a microscope [Figure 4] 1 is a diagram showing a roughness curve obtained by measuring the surface of the rolling element of Example 1 using a surface roughness measuring instrument. [Figure 5] Optical microscope photograph of the rolling element surface of Comparative Example 1 taken with a microscope [Figure 6] Graph showing the roughness curve of the rolling element surface of Comparative Example 1 measured with a surface roughness measuring instrument DETAILED DESCRIPTION OF THE INVENTION

[0022] An embodiment of the present invention will now be described with reference to the accompanying drawings. As shown in Figures 1 and 2, the embodiment is a rolling bearing that rotatably supports a shaft 2 of a planetary gear 1 incorporated in a planetary gear mechanism A, and its rolling elements (needle rollers) 3, in which the micro Vickers hardness on the surface of the rolling elements 3 is 820 to 930 Hv0.1, the surface roughness parameter (Ra) is 0.01 to 0.05 μm, the skewness (Rsk) is -4.0 to 0.0 μm, the maximum cross-sectional height (Rt) is 0.7 μm or less, and the absolute value of Ra / Rsk is 5.0 or less. The hardness is measured in accordance with JIS Z2244-1 or ISO6507-1, and the average value of measurements taken at three locations on the rolling element surface is defined as the hardness. The surface roughness is measured in accordance with JIS B0601, JIS B0633, ISO3274, or ISO4288, and the average value of measurements taken at three locations on the rolling element surface is defined as the surface roughness.

[0023] As shown in Figure 1, the planetary gear mechanism A includes a ring gear 4 having internal teeth surrounding the outer periphery, a sun gear (sun gear) 5 having external teeth and positioned at the center of the ring gear 4, and a plurality of planetary gears 1 having external teeth and positioned between the ring gear 4 and the sun gear 5, assembled so as to mesh with each other.

[0024] As shown in Figure 2, the shaft 2 of the planetary gear 1 corresponds to the inner ring of a general rolling bearing, and is rotatably supported relative to the planetary gear 1, which corresponds to the outer ring, by the planetary gear 1 and rolling elements 3 rotatably held in a cage-type retainer 6. The end of the shaft 2 is connected to a carrier (not shown), and the rotational force required for the revolution of the planetary gear mechanism A is input and output from this carrier, thereby driving the planetary gear mechanism.

[0025] The rolling elements in this embodiment are steel rolling elements, and the rolling elements 3 consisting of needle rollers are made of high carbon chromium bearing steel (SUJ material). For example, SUJ2 is a steel material that is highly wear resistant and easily available.

[0026] As the steel material for the rolling elements, steel materials with a composition similar to the SUJ material specified in global standards (ASTM, GB, DIN, etc.) can also be used. Incidentally, the cage, which is not included in the rolling components of this invention, is generally made of carbon steel for machine structures.

[0027] A specific example of the composition of the steel used for the steel rolling elements is steel containing 0.93 to 1.10 mass% carbon, 0.5 mass% or less manganese, 0.025 mass% or less sulfur, 0.15 to 0.35 mass% silicon, 0.90 to 1.65 mass% chromium, and 0.30 mass% or less nickel, with the remainder being iron and impurities. The elements and composition of the steel can be detected by spectroscopic analysis such as EPMA.

[0028] By setting the carbon content of the steel material to 0.93 to 1.10 mass % as in the above composition example, it is possible to greatly affect the hardness and carbide amount of the rolling contact member after quench hardening.

[0029] In other words, by setting the carbon content of steel to 0.93 mass% or more, sufficient hardness and carbide amount can be ensured without introducing a large amount of carbon into the steel during heat treatment. Also, by setting the carbon content of steel to 1.10 mass% or less, the risk of large carbides forming during the steel manufacturing stage is reduced.

[0030] Furthermore, by keeping the silicon content of the steel to 0.35 mass% or less, the increase in the amount of hydrogen absorbed in the steel is suppressed, reducing the risk of peeling due to hydrogen embrittlement, while by making it 0.15 mass% or more, nitrides are more likely to precipitate during heat treatment, increasing hardness.

[0031] Furthermore, by limiting the sulfur content to 0.025% by mass or less, the risk of sulfur chemically bonding with Mn and other elements to form non-metallic inclusions such as manganese sulfide is reduced.By limiting the Mn content to 0.50% by mass or less, the material hardness before heat treatment can be kept low, improving workability in the cold process. Furthermore, the chromium content of 0.90 to 1.65 mass % contributes to the hardenability of the steel.

[0032] Another example of the composition of the steel material for rolling elements may be a steel material composition containing 0.95 to 1.10 mass% carbon, 0.90 to 1.15 mass% manganese, 0.025 mass% or less of sulfur, 0.40 to 0.70 mass% of silicon, 0.90 to 1.20 mass% of chromium, and 0.25 mass% or less of nickel, with the remainder being iron and impurities.

[0033] The rolling elements made of steel having the above composition have a micro-Vickers hardness of 820 to 930 HV0.1 on the surfaces (contact surfaces) that come into contact with the outer ring, inner ring, shaft, or cage as other components. Of the other components, the outer ring, inner ring and cage are made of the same composition as the rolling elements or are made of known steel materials or the like.

[0034] As the steel composition of the shaft member used in the planetary gear mechanism, steel having the following composition (content) can be mentioned as an appropriate one. That is, the steel has a composition of 0.10 to 0.40 volume % carbon, 0.10 to 2.50 volume % silicon, 0.30 to 1.20 volume % manganese, 0.40 to 3.00 volume % chromium, 1.00 volume % or less molybdenum, 2.00 volume % or less nickel, and the balance being iron and unavoidable impurities.

[0035] The amount of retained austenite in the surface layer portion up to 50 μm from the surface of the rolling element or other component is preferably 15 to 35% by volume The amount of retained austenite can be measured using an X-ray stress measurement device.

[0036] By setting the amount of retained austenite to 15% by volume or more, it is possible to suppress the propagation of cracks caused by spalling, even when a rolling bearing becomes entrapped in foreign matter under operating conditions with contaminated lubrication. This is because stress is alleviated at the position 50 μm from the surface (contact surface) of a rolling component that has relatively soft retained austenite. Furthermore, it is appropriate to prevent excessive reduction in hardness of the surface layer by limiting the amount of retained austenite at the above-mentioned position to 35% by volume or less. The amount of retained austenite is determined by measuring a cross section with an arbitrary surface (contact surface) as the reference position.

[0037] As described above, it is preferable that the amount of retained austenite in the entire surface layer portion (all regions) be 15 volume % or more and 35 volume % or less so that the occurrence of micro-spalling is suppressed by relieving stress in specified regions, but the average amount of retained austenite in the entire surface layer portion may also be 15 volume % or more and 35 volume % or less.

[0038] The grain size number of the prior austenite crystals in the surface layer portion up to 50 μm from the surface of the rolling component is preferably within the range of 9 to 11 (JIS G 0551). By densifying the crystal grains of the needle roller within the above grain size number range, it is possible to delay the propagation of cracks when micro-spalling occurs.

[0039] The grain size number of the prior austenite crystals can be determined by corroding the rolling parts with a nitric acid ethanol solution or a picric acid ethanol solution to reveal the crystal grains, observing them with an optical microscope, etc., and measuring the grain size.

[0040] When measuring, the grain size number of the prior austenite crystals is measured at a position 50 μm deep from the exposed surface (contact surface) and judged. However, even if the specified grain size number is measured at any position shallower than the depth of 50 μm, this does not affect the judgement.

[0041] If the rolling elements have a surface roughness (Ra) of 0.01 to 0.05 μm, a skewness (Rsk) of −4.0 to 0.0 μm, a maximum cross-sectional height (Rt) of 0.7 μm or less, and an absolute value of Ra / Rsk of 5.0 or less, the surface shape is extremely smooth and has a surface roughness shape that improves oil film formability for lubricating oils with low kinematic viscosity.

[0042] This prevents contact with the mating member (such as the shaft of a planetary gear mechanism) due to unevenness (peaks and valleys) in the surface roughness profile, even in a lean lubrication environment, and also suppresses the occurrence of micro-spalling of the mating member.

[0043] If the surface roughness (Ra) exceeds the upper limit of the above numerical range, the difference in height of the roughness profile becomes too large, which is undesirable because it tends to impart stress to the surface with which the rolling element comes into contact. From the viewpoint of microspalling resistance, it is more preferable if Ra is 0.01 to 0.02 μm. Furthermore, the skewness (Rsk) must be 0 or a negative value, assuming that the surface of the rolling element is a uniformly polished wear surface. However, a negative Rsk value of less than -4.0 provides the necessary lipophilic effect, but is undesirable because it may reduce the strength of the rolling element surface and make microspalling more likely to occur. From the perspective of microspalling resistance, an Rsk of -2.0 to 0.0 μm is more preferable.

[0044] If the maximum cross-sectional height (Rt) exceeds 0.7 μm, as mentioned above, the strength of the surface of the rolling element may decrease, making microspalling more likely to occur, which is undesirable. From the viewpoint of microspalling resistance, it is more preferable if Rt is 0.3 μm or less. Furthermore, if the absolute value of Ra / Rsk exceeds 5.0, the ability to efficiently form an oil film on the surface of the rolling element using a very small amount of low-viscosity lubricating oil decreases, which is undesirable.From the viewpoint of micro-spalling resistance, it is more preferable if the absolute value of Ra / Rsk is 2.0 or less.

[0045] The parameters of surface roughness (Ra), skewness (Rsk), and maximum cross-sectional height (Rt) can be adjusted by work hardening treatment (barrel processing) near the surface or grinding (ultra-finishing processing).

[0046] Barrel finishing can be performed using a conventional method using a well-known barrel finishing machine. The tank of the barrel finishing machine is usually filled with media (grinding stones and abrasives), liquid compound, water, and rollers for the workpiece. However, in this invention, by not using media such as abrasives, the surface roughness profile can be adjusted to achieve extremely smooth and specific conditions, such as a surface roughness (Ra) of 0.01 to 0.05 μm, a skewness (Rsk) of -4.0 to 0.0 μm, a maximum cross-sectional height (Rt) of 0.7 μm or less, and an absolute value of Ra / Rsk of 5.0 or less. It is possible to achieve this not only by using or not using media (grinding stones or abrasives), but also by adjusting the type of media used or the mixing ratio.

[0047] Furthermore, in barrel finishing, if a method of polishing while applying centrifugal force is adopted, the hardness of the surface of the workpiece can be efficiently and sufficiently increased. For this purpose, a barrel finishing machine can be used in which multiple tanks each rotate on their own axis while the entire machine revolves, and centrifugal barrel finishing can be performed by applying centrifugal force to each tank due to high-speed rotation.

[0048] By performing barrel processing in this manner, the efficiency of work hardening of the surface layer of the rolling component is improved, the hardness and densification of crystal grains near the surface of the component are efficiently improved, and the surface roughness can also be adjusted to meet the desired conditions.

[0049] Furthermore, as a secondary effect of not using abrasives in barrel processing, it is possible to reduce the power consumption required to prepare the abrasives in advance, thereby reducing carbon dioxide emissions accordingly.

[0050] A method for manufacturing a rolling element, taking a needle roller (diameter 1.5 to 5.5 mm) as a representative example of an embodiment of the present invention, will be described below. First, a high carbon chromium bearing steel (SUJ material) for forming needle rollers is used as the steel material, and wire is prepared by drawing it multiple times. This wire is then processed by cutting, forging, turning, etc., and the steel formed into the general shape of the needle roller is then heat treated.

[0051] The heat treatment process for steel material to be made into needle rollers begins with a preparatory heating step in which the steel material is heated to 850°C or higher and 940°C or lower, and then, as the overall heat treatment step, the steel material is heated to and held at the A1 transformation point or higher in a heat treatment gas.

[0052] Specifically, an endothermic converted gas (RX gas) is used as a base, and an enriched gas such as propane gas or butane gas serving as a carbon source is added, and the steel material is heated to 850°C or higher and 940°C or lower in such an atmosphere. The entire heat treatment process may be a nitriding process.

[0053] This is followed by a quenching process. In this quenching process, the steel material, which has been maintained at or above the A1 transformation point, is immersed in oil (oil quenching) to be rapidly cooled to a temperature below the Ms point (the martensitic transformation start point). Alternatively, the steel material may be immersed in water (water quenching) instead of oil quenching. This is followed by a tempering process.

[0054] In the tempering process, the steel material that has been quench-hardened in the quenching process is heat-treated at a temperature below the A1 transformation point (160-200°C). The steel is held at this temperature for a specified time, and then cooled in air at room temperature, thereby improving toughness. This is followed by a grinding process, which yields needle rollers with the final dimensions and shape.

[0055] After that, processing such as barrel processing and super-finish grinding is performed to adjust the surface roughness.

[0056] Furthermore, when manufacturing rolling elements such as rollers or balls in a form other than needle rollers, they can be manufactured using almost the same process as above, including cutting, forging, turning, heat treatment, grinding, and work hardening treatment near the surface (barrel processing, etc.).

[0057] A rolling bearing incorporating such rolling elements can be used under harsh conditions, such as a rotational speed of 9,000 rpm or more and a lubricating oil kinematic viscosity of 13 cSt (40°C) or 4 cSt (100°C) or less, as well as under conditions where sudden acceleration and deceleration are expected, and is a rolling component of a rolling bearing that can be used in e-Axle reducers, etc. The inner diameter of the rolling bearing that supports the planet gears of a planetary gear mechanism is φ7 to 50 mm.

[0058] The type of rolling bearing in the above-described embodiment has been exemplified as a needle bearing (needle roller bearing), but is not limited to this and may be a cylindrical roller bearing, a tapered roller bearing, a self-aligning roller bearing, a ball bearing, etc.

[0059] The rolling elements of the embodiment configured as described above have high spalling resistance themselves, and are less aggressive to contacting members such as shaft members, so they can be used with low-viscosity lubricating oil.

[0060] As a secondary effect, this reduces the agitation resistance caused by the lubricating oil in rolling bearings, contributing to reduced fuel and electricity costs for automobiles using rolling bearings. Another secondary effect is that when the heat treatment of the rollers is full heat treatment (soaking), the nitrogen diffusion time during the conventional nitriding process can be shortened, reducing the amount of electricity used to maintain the temperature inside the furnace and contributing to a reduction in carbon dioxide emissions. Furthermore, by not using abrasives in the surface roughness adjustment process, the electricity used to manufacture the abrasives can be reduced. [Example]

[0061] [Example 1] The needle rollers were manufactured using the following process and incorporated into rolling bearings that support planetary gears of a planetary gear mechanism. The steel materials used were high carbon chromium bearing steel (SUJ2) for forming the needle rollers and chromium molybdenum steel (SCM material) for the shaft components. Wire material was prepared by multiple wire drawing processes, and then cut, forged, and turned to form the approximate shape.

[0062] In the heat treatment process (deep hardening), the steel material was first heated to 850 to 940°C, and then, in the overall heat treatment process, enriched gas was added to endothermic converted gas (RX gas) as the base, and the material was heated to 850°C or higher and 940°C or lower.

[0063] In the subsequent quenching process, the steel material held at or above the A1 transformation point was immersed in oil (oil cooling) to be rapidly cooled to a temperature below the Ms point (martensitic transformation starting point).

[0064] In the tempering process, the steel material that had been quench-hardened in the quenching process was heat-treated at a temperature below the A1 transformation point (160 to 200°C), held for a predetermined time, and then cooled in air at room temperature.

[0065] Subsequently, a centrifugal barrel processing process was performed without using a grinding process or abrasives to increase the hardness near the surface (the region from the surface to a depth of 25 μm), and the surface roughness parameters Ra, Rsk, and Rt were adjusted to the desired range.

[0066] [Comparative Example 1] A needle roller (Comparative Example 1) was manufactured in exactly the same manner as in Example 1, except that in the manufacturing process of Example 1 described above, centrifugal barrel processing was carried out using an abrasive.

[0067] Microscope images of the surfaces of the obtained rolling elements (needle rollers) of Example 1 and Comparative Example 1 are shown in FIG. 3 (Example 1) and FIG. 5 (Comparative Example 1), and their surface hardness and surface roughness were measured as follows.

[0068] <Hardness measurement test> The surface of the needle roller was measured with a Vickers hardness tester, and a load of 100 g was applied, and the diagonal length of the indentation was measured to measure the micro Vickers hardness (Hv0.1) of the surface of the rolling element. The micro Vickers hardness of the rolling element surface in Example 1 was 860Hv0.1, and the hardness of the rolling element surface in Comparative Example 1 was 860Hv0.1.

[0069] <Surface roughness measurement> Measurements were made using a well-known contact (stylus) roughness measuring instrument. The stylus of the measuring instrument was brought into contact with the outer diameter surface of the needle roller and moved along the axial direction to measure the surface roughness, and the parameters Ra, Rsk, and Rt were analyzed. These results are shown in Figure 4 (Example 1) and Figure 6 (Comparative Example 1).

[0070] The measured surface roughness in Example 1 was Ra: 0.02 μm, skewness (Rsk): −0.13 μm, maximum cross-sectional height (Rt): 0.17 μm, and the absolute value of Ra / Rsk was 0.15.

[0071] In Comparative Example 1, Ra was 0.08 μm, skewness (Rsk): −3.48 μm, maximum cross-sectional height (Rt): 1.41 μm, and the absolute value of Ra / Rsk: 0.02.

[0072] Assuming the use state of a planetary gear mechanism (FIGS. 1 and 2) incorporating planetary gears equipped with the rolling elements (rollers) of Example 1 and Comparative Example 1, spalling resistance was evaluated using a radial load tester under the following test conditions. Radial load: 6670N Moment load: 13.5N m Outer ring rotation speed: 9000 rpm Lubricating oil: Clean oil (kinematic viscosity at 100°C: 3 cSt) Lubrication conditions: Circulating oil supply

[0073] [Table 1] As a result, in the spalling resistance evaluation test using low viscosity oil with Example 1, good results were obtained, and it was confirmed that Example 1, as a rolling bearing supporting a planetary gear incorporated in a planetary gear mechanism, has a longer life of the mating member than Comparative Example 1. [Industrial Applicability]

[0074] The present invention can be applied to rolling elements used in rolling bearings that are lubricated with an insufficient amount of lubricant or a low-viscosity liquid lubricant, particularly rolling bearings that rotate at high speeds, and rolling bearings in which it is necessary to minimize the rotational torque and heat generated by the bearing, as well as rolling bearings incorporated into reduction mechanisms and transmission mechanisms such as e-axles, automatic transmissions (ATs), and continuously variable transmissions (CVTs) in automobiles, and various types of industrial machinery such as high-speed rotating machine tools, and can be used in a wide range of industrial fields. [Explanation of symbols]

[0075] A. Planetary gear mechanism 1 Planetary gear 2-axis 3 Rolling elements 4 ring gear 5. Sun Gear 6 Cage

Claims

1. A rolling element for a rolling bearing, the rolling element surface having a micro Vickers hardness of 820 to 930 Hv0.1, a surface roughness (Ra) of 0.01 to 0.05 μm, a skewness (Rsk) of −4.0 to 0.0 μm, a maximum cross-sectional height (Rt) of 0.7 μm or less, and an absolute value of the ratio (Ra / Rsk) of the surface roughness (Ra) to the skewness (Rsk) of 5.0 or less.

2. 2. The rolling element for a rolling bearing according to claim 1, wherein the amount of retained austenite in the surface layer portion of the rolling element extending from the surface to 50 μm is 15 to 35% by volume.

3. 3. The rolling element for a rolling bearing according to claim 1, wherein the grain size number (JIS G0551) of the prior austenite grain boundary in the surface layer portion of the rolling element up to 50 μm from the surface is 9 to 11.

4. A rolling bearing comprising the rolling element for a rolling bearing according to claim 1 or 2.

5. 5. The rolling bearing according to claim 4, wherein the rolling bearing is a rolling bearing that supports a planet gear of a planetary gear mechanism.

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