Rolling bearing and rolling element for rolling bearing

Abstract

In the present invention, a rolling element for a rolling bearing and a rolling bearing comprising the same are provided. A rolling element (3) has a surface micro-Vickers hardness of 820 Hv0.1 to 930 Hv0.1, a surface roughness (Ra) of 0.01 µm to 0.05 µm, a skewness (Rsk) of -4.0 µm to 0.0 µm, a maximum profile 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.

Description

Rolling bearings and rolling elements for rolling bearings

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

[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 are required to use as little low-viscosity lubricating oil as possible.Moreover, there are cases in which the bearings are used in conditions where the entire bearing is at a high temperature or where foreign matter has been mixed into the lubricating oil.

[0004] Flaking may occur on the surface of the steel rolling components of rolling bearings used under such harsh conditions. This flake-off phenomenon is not caused by internal flake-off originating from inclusions contained in the steel components, but rather by flake-off originating from micro-spalling on the surface. When the inner diameter surface roughness of a planetary gear in a planetary gear mechanism commonly used in e-axles, automatic transmissions, etc. is smooth, micro-spalling often occurs on the rolling components of the rolling bearing that rotatably supports the planetary gear shaft, or on the planetary gear shaft.

[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 surface of the part 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 spread of damage when a rolling load is applied.

[0006] Patent Document 1 also describes that there is an "edge zone" in which the nitrogen content decreases from the outer portion near the surface of the rolling bearing component toward the inner portion, and a "core zone" having a nearly constant hardness, and that the hardness at a depth of 40 μm is 870 to 1000 HV0.3, and that the hardness at a depth of 300 μm is at most 250 HV0.3 less 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.

[0007] Patent No. 6211051

[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 themselves 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, which poses the problem of making the mating parts more susceptible to 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.

[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), which is a parameter of the surface roughness of the rolling element, is 0 or a negative value (0 or less), i.e., -4.0 to 0.0 μm, and the maximum cross-sectional height (Rt) is 0.7 μm or less, the proportion of minute 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. Therefore, 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] In addition, 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 portion up to 50 μm from the surface of the rolling element to 15 to 35 volume %, stress in a predetermined region up to 25 μm from the surface (contact surface) is alleviated, thereby sufficiently enhancing the effect of suppressing the occurrence of micro-spalling.

[0018] Furthermore, when micro-spalling occurs, it is preferable to work-harden the surface so that the grain size number (JIS G0551) 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.For example, when applied to a rolling bearing that supports the planetary gear (planetary pinion) of a planetary gear mechanism commonly used in e-axles, automatic transmissions, etc., the above-mentioned desired effect is also achieved.

[0020] In this invention, the rolling elements of a rolling bearing are made 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 means that the rolling elements are less likely to cause damage to parts that come into contact with the rolling elements, such as outer rings, inner rings, shafts, or cages. 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 that a rolling bearing using such rolling elements can be made to have an extremely long life.

[0021] Schematic diagram of a planetary gear mechanism. A perspective view showing a rolling bearing and its rolling elements according to an embodiment, with a portion of the planetary gear cut away. An optical microscope photograph of the rolling element surface of Example 1, taken with a microscope. A diagram showing a roughness curve of the rolling element surface of Example 1, measured with a surface roughness measuring instrument. An optical microscope photograph of the rolling element surface of Comparative Example 1, taken with a microscope. A diagram showing a roughness curve of the rolling element surface of Comparative Example 1, measured with a surface roughness measuring instrument.

[0022] An embodiment of the present invention will be described below with reference to the accompanying drawings. As shown in Figures 1 and 2, this 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 surface of the rolling elements 3 has a micro Vickers hardness of 820 to 930 Hv0.1, a surface roughness parameter (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 hardness is measured in accordance with JIS Z2244-1 or ISO6507-1, and the hardness is defined as the average value of measurements taken at three locations on the surface of the rolling elements. The surface roughness is measured in accordance with JIS B0601, JIS B0633, ISO3274 or ISO4288, and the average value of measurements taken at three points on the surface of the rolling element is defined as the surface roughness.

[0023] As shown in FIG. 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 disposed at the center of the ring gear 4, and a plurality of planetary gears 1 having external teeth and disposed between the ring gear 4 and the sun gear 5 and 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 made of steel, and the rolling elements 3, which are 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 having a composition similar to that of SUJ materials specified in global standards (ASTM, GB, DIN, etc.) can also be used. Incidentally, cages, which are not included in the rolling components of this invention, are 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 balance 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] That is, by setting the carbon content of the 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 the steel to 1.10 mass% or less, the risk of large carbides being formed during the steel manufacturing stage can be reduced.

[0030] Furthermore, by setting 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, and by setting the silicon content to 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 mass% or less, the risk of 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 mass% or less, the material hardness before heat treatment can be controlled low, improving workability in the cold process. Furthermore, by limiting the chromium content to 0.90 to 1.65 mass%, the hardenability of the steel is improved.

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

[0033] The rolling elements made of steel of the above composition have their surfaces (contact surfaces) in contact with the outer ring, inner ring, shaft, and cage as other parts adjusted to a micro-Vickers hardness of 820 to 930 HV0.1. Of the above other parts, the outer ring, inner ring, and cage are either made of the same composition as the rolling elements or are made of known steel materials, etc.

[0034] The steel composition of the shaft member used in the planetary gear mechanism may be one having the following composition (content): 0.10 to 0.40 volume percent carbon, 0.10 to 2.50 volume percent silicon, 0.30 to 1.20 volume percent manganese, 0.40 to 3.00 volume percent chromium, 1.00 volume percent or less molybdenum, 2.00 volume percent or less nickel, with the balance being iron and unavoidable impurities.

[0035] The amount of retained austenite in the surface layer portion of the rolling element or other component up to 50 μm from the surface 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 progression 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 at a position 50 μm from the surface (contact surface) in a rolling component having relatively soft retained austenite is alleviated. Furthermore, by setting the amount of retained austenite at this position to 35% by volume or less, it is appropriate to prevent excessive reduction in hardness of the surface layer. 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, the amount of retained austenite in the entire surface layer portion (all regions) is preferably 15 vol% or more and 35 vol% or less so that the occurrence of micro-spalling can be suppressed by relieving stress in a predetermined region, but the average amount of retained austenite in the entire surface layer portion may also be 15 vol% or more and 35 vol% 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 G0551). 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] In the measurement, 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 predetermined grain size number is measured at any position shallower than the depth of 50 μm, this does not affect the judgment.

[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 kinetic viscosity.

[0042] This makes it possible to avoid 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 to suppress 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, an Ra of 0.01 to 0.02 μm is more preferable. 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, if the Rsk is a negative value less than -4.0, although the necessary lipophilic effect is obtained, 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, 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 described 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 microspalling 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, and 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. This can be achieved not only by using or not using media (grinding stones and abrasives), but also by adjusting the type of media used or the blending 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 component, using a needle roller (diameter 1.5 to 5.5 mm) according to an embodiment of the present invention as a representative example, will be described below. First, a wire rod is prepared by drawing multiple times using high-carbon chromium bearing steel (SUJ material) for forming the needle roller. This wire rod is then subjected to processes such as cutting, forging, and turning, 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 maintained at the A1 transformation point or higher in a heat treatment gas.

[0052] Specifically, an enriched gas such as propane gas or butane gas serving as a carbon source is added to an endothermic converted gas (RX gas) as a base, 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] Then, a quenching process is performed. In the quenching process, the steel material 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 (martensitic transformation start point). Alternatively, instead of oil quenching, the steel material may be immersed in water to be cooled (water quenching). Then, a tempering process is performed.

[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 to 200°C). The steel is held at this temperature for a predetermined 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 in almost the same process as described above, including cutting of steel material, forging and 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 a reducer for an e-Axle, or a rolling bearing using the same. The inner diameter of the rolling bearing that supports the planet gears of the planetary gear mechanism is φ7 to 50 mm.

[0058] The type of rolling bearing in the embodiment described above is 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.

[0061] [Example 1] Needle rollers were manufactured by the following process and incorporated into rolling bearings that support planetary gears of a planetary gear mechanism. High carbon chromium bearing steel (SUJ2) was used as the steel material for forming the needle rollers, and chromium molybdenum steel (SCM material) was used as the shaft member. Wire rod was prepared by multiple wire drawing processes, and was then cut, forged, and turned to form the general 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 an endothermic converted gas (RX gas) as a base, and the steel material was heated to 850°C or higher and 940°C or lower.

[0063] In the subsequent quenching process, the steel material maintained 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 within 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 a centrifugal barrel processing was carried out using an abrasive in the manufacturing process of Example 1 described above.

[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 the surface hardness and surface roughness thereof were measured as follows.

[0068] <Hardness Measurement Test> The surfaces of the needle rollers were measured using 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 surfaces of the rolling elements. The micro Vickers hardness of the rolling element surfaces of Example 1 was 860 Hv0.1, and the hardness of the rolling element surfaces of Comparative Example 1 was 860 Hv0.1.

[0069] <Measurement of surface roughness> Measurement was made using a well-known contact type (stylus type) 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, and 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 addition, in Comparative Example 1, Ra was 0.08 μm, skewness (Rsk): −3.48 μm, maximum cross-sectional height (Rt): 1.41 μm, and 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: 6670 N Moment load: 13.5 N m Outer ring rotation speed: 9000 rpm Lubricating oil: clean oil (kinematic viscosity at 100°C: 3 cSt) Lubrication conditions: circulating oil supply

[0073] 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.

[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 generation of 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.

[0075] A Planetary gear mechanism 1 Planetary gear 2 Shaft 3 Rolling element 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. 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 up to 50 μm from the surface is 15 to 35% by volume.

3. A rolling element for a rolling bearing according to claim 1 or 2, wherein the grain size number (JIS G0551) of the prior austenite grain boundary in the surface layer portion of said 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. A rolling bearing according to claim 4, wherein said rolling bearing is a rolling bearing that supports a planet gear of a planetary gear mechanism.

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