Rolling bearings and rolling elements for rolling bearings

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] This invention relates to rolling bearings and rolling elements for rolling bearings.

Background Art

[0002] Generally, rolling bearings are required to have durability, that is, a long service life. However, for the long service life of rolling bearings that rotate at high speed in a specific lubrication environment, depending on such usage conditions, the outer ring, inner ring, shaft, or rolling elements that make up the rolling parts need to overcome specific problems.

[0003] For example, e-Axles that integrate major components such as motors used in automobiles, etc., and rolling bearings used in multi-stage automatic transmissions (ATs) are assumed to be used in a high-speed rotation state. In order to keep torque loss low, it is required to use as little low-viscosity lubricating oil as possible. Moreover, there are cases where the entire bearing is in a high-temperature state or where it is used with foreign matter mixed in the lubricating oil.

[0004] On the surface of the steel rolling parts of rolling bearings used under such severe usage conditions, there is a possibility of a peeling phenomenon occurring. This peeling phenomenon does not occur due to internal-origin peeling starting from inclusions contained in the steel parts, but rather occurs from peeling starting from minute spooling that occurs on the surface. Minute spooling often occurs on the rolling parts of rolling bearings or the shaft that rotatably supports the shaft of a planetary gear when the inner diameter surface roughness of the planetary gear in a planetary gear mechanism widely used in e-Axles, automatic transmissions, etc., is smooth.

[0005] Japanese Patent No. 6211051 (Patent Document 1) describes a rolling bearing component in which the hardness at a depth of 40 μm from the component surface is limited to 870 HV0.3 to 1000 HV0.3 by nitriding treatment so that mechanical elements used in aircraft can be safely used and can withstand a certain degree of damage, and the damage when a rolling load is applied is not enlarged as much as possible.

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

[0007] [Patent Document 1] Patent No. 6211051 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, in the prior art described in Patent Document 1, by increasing the hardness of the rolling elements of a rolling bearing to a predetermined range at a depth of 40 μm from the surface, damage such as minute spalling can be avoided for the rolling elements themselves, but this has an aggressive nature that can damage the mating parts that come into contact with them. In particular, when the part in question is a rolling element, increasing the hardness of its surface to the predetermined range increases the aggressiveness towards the mating parts that come into contact with the rolling elements, making the mating parts more susceptible to damage such as minute spalling.

[0009] Therefore, the object of this invention is to solve the problem described above, that when the surface hardness of the rolling elements of a rolling bearing is increased, they tend to damage the parts they come into contact with, and to prevent rolling elements with increased surface hardness from inducing minute spalling on the outer ring, inner ring, shaft, or cage that they come into contact with, and furthermore, to provide rolling elements and long-life rolling bearings using them that do not induce minute spalling on the parts they come into contact with even when the rolling bearing is used with a dilute lubricant. [Means for solving the problem]

[0010] To solve the above problems, this invention relates to a rolling bearing in which rolling elements are interposed by a cage to rotatably hold the rolling elements between an inner ring or shaft and an outer ring, wherein the micro-Vickers hardness of the surface of the rolling element is 820 to 930 Hv 0.1, the surface roughness (Ra) of the rolling element is 0.01 to 0.05 μm, and the skewness (Rsk) is -4.0 to 0. 0 The rolling elements for rolling bearings are such that they have a maximum cross-sectional height (Rt) of 0.7 μm or less, and the absolute value of the ratio of surface roughness (Ra) to skewness (Rsk) (Ra / Rsk) is 5.0 or less. (Ra / Rsk) is a parameter that indicates the degree of unevenness, and the smaller this value, the smaller the average value of the absolute values ​​of the unevenness, and the more valleys there are.

[0011] As described above, the rolling elements for rolling bearings of this invention have a micro-Vickers hardness of 820-930 Hv0.1 on the surface of the rolling elements, and the surface roughness (Ra) is prepared to be extremely smooth at 0.01-0.05 μm, thus suppressing the occurrence of minute spalling on the surface.

[0012] Furthermore, the skewness (Rsk), a parameter for the surface roughness of the rolling element, ranges from -4.0 to 0. 0 and The value is 0 or negative (0 or less), and the maximum cross-sectional height (Rt) is 0.7 μm or less. As a result, the ratio of minute peaks and valleys observed from the cross-sectional shape of the rolling element is such that the proportion of valleys is greater than or equal to the proportion of peaks.

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

[0014] Therefore, the rolling elements for rolling bearings of this invention can contact the outer ring, inner ring, or cage via a lubricating oil film on the surface, even in a thin lubrication environment.

[0015] In this way, the friction and stress distribution due to contact are minimized on the parts that the rolling elements come into contact with, resulting in a rolling bearing that is less prone to causing minute spalling on the parts even in a thin lubrication environment.

[0016] Furthermore, in order to prevent minute spalling from occurring in the rolling elements of the rolling bearing of this invention, it is preferable that the amount of retained austenite in the surface layer up to 50 μm from the surface of the rolling elements is 15 to 35 volume percent.

[0017] When the amount of retained austenite in the surface layer up to 50 μm from the surface of the rolling element is 15 to 35 volume%, the stress in a predetermined region up to 25 μm from the surface (contact surface) is relaxed, and the effect of suppressing the occurrence of minute spalling is sufficiently enhanced.

[0018] Furthermore, if minute spalling occurs, it is preferable to work harden the surface such that the grain size number (JIS G0551) of the prior austenite grain boundaries in the surface layer up to 50 μm from the surface of the rolling element is 9 to 11, in order to slow the propagation of cracks from the minute spalling to the surrounding area.

[0019] The rolling bearing provided with rolling elements that exhibit the above-described operation can sufficiently prevent the generation of minute spalling with respect to the outer ring, inner ring, and cage that may come into contact, and thus can sufficiently extend the life of a rolling bearing that rotates at high speed. Even when it is applied to a rolling bearing that supports a planetary gear (planetary pinion) of a planetary gear mechanism widely used in, for example, an e-Axle or an automatic transmission, the above-described intended effects are achieved.

Advantages of the Invention

[0020] In this invention, since the rolling elements are balls or rollers having a micro-Vickers hardness, surface roughness (Ra), skewness (Rsk), maximum cross-sectional height (Rt), and absolute value of the ratio (Ra / Rsk) of the surface roughness (Ra) to the skewness (Rsk) on the surface of the rolling elements within a predetermined range, it becomes difficult to damage the components consisting of the outer ring, inner ring, shaft, or cage that come into contact with the rolling elements. In particular, the rolling elements prepared to have increased surface hardness do not induce minute spalling with respect to the components that come into contact with them. Furthermore, when the rolling bearing is used with a dilute lubricating oil, the rolling elements do not damage the components such as minute spalling, and there is an advantage that it becomes a rolling bearing with an ultra-long life when using such rolling elements.

Brief Description of the Drawings

[0021] [Figure 1] Schematic diagram of a planetary gear mechanism [Figure 2] Perspective view showing a rolling bearing and its rolling elements of an embodiment with a part of the planetary gear cut away [Figure 3] Optical microscope photograph of the surface of the rolling element of Example 1 taken with a microscope [Figure 4] Chart showing the roughness curve of the surface of the rolling element of Example 1 measured with a surface roughness measuring instrument [Figure 5] Optical microscope photograph of the surface of the rolling element of Comparative Example 1 taken with a microscope [Figure 6] Chart showing the roughness curve of the surface of the rolling element of Comparative Example 1 measured with a surface roughness measuring instrument

Modes for Carrying Out the Invention

[0022] Embodiments of this invention will be described below with reference to the accompanying drawings. As shown in Figures 1 and 2, the embodiment is a rolling bearing and its rolling element (needle roller) 3 that rotatably supports the shaft 2 of a planetary gear 1 incorporated into a planetary gear mechanism A. The micro-Vickers hardness of the surface of the rolling element 3 is 820-930 Hv0.1, its surface roughness parameter (Ra) is 0.01-0.05 μm, and its skewness (Rsk) is -4.0-0. 0 Yes, 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 measurement method conforms to 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 measurement method conforms to 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 comprises a ring gear 4 having internal teeth and surrounding the outer circumference, a 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, and assembled 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 typical rolling bearing, and is rotatably supported relative to the outer ring of the planetary gear 1 by the rolling elements 3 which are rotatably held in the squirrel-cage type cage 6. The end of shaft 2 is connected to a carrier (not shown in the figure), and the rotational force necessary for the revolution of 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 consist of needle-shaped rollers, are made of high-carbon chromium bearing steel (SUJ material). For example, SUJ2 is a steel material that has high wear resistance and is also readily available.

[0026] Furthermore, for the rolling elements, steel materials with a composition similar to SUJ material as described in global standards (ASTM, GB, DIN, etc.) can also be used. Incidentally, the retainers, 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 steel used for 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] As shown in the above composition example, by setting the carbon content of the steel material to 0.93 to 1.10 mass%, the hardness and carbide content of the rolling member after quenching and hardening can be significantly affected.

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

[0030] Furthermore, by limiting the silicon content of the steel to 0.35 mass% or less, the increase in hydrogen storage in the steel is suppressed, reducing the risk of delamination due to hydrogen embrittlement. By limiting it to 0.15 mass% or more, nitrides are more easily precipitated during heat treatment, resulting in higher hardness.

[0031] Furthermore, by keeping the sulfur content below 0.025% by mass, the risk of chemical bonding with Mn and other elements to form nonmetallic inclusions such as manganese sulfide is reduced. By keeping the Mn content below 0.50% by mass, the hardness of the material before heat treatment can be controlled to a low level, improving machinability in cold working processes. Furthermore, setting the chromium content to 0.90-1.65% by mass contributes to the hardenability of the steel.

[0032] Other steel compositions may include a rolling element steel composition comprising 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 remainder being iron and impurities.

[0033] The rolling elements, made of steel with the above composition, have other components such as the outer ring, inner ring, or shaft, and the contact surfaces (contact surfaces) that come into contact with the cage, all of which are prepared to have a micro-Vicas hardness of 820 to 930 HV 0.1. Of the other components mentioned above, the outer ring, inner ring, and cage are made of the same composition as the rolling elements, or of well-known steel materials.

[0034] Suitable steel materials for shaft members used in planetary gear mechanisms include those with the following compositions (contents): Specifically, it is a steel material with a composition consisting of 0.10-0.40 volume% carbon, 0.10-2.50 volume% silicon, 0.30-1.20 volume% manganese, 0.40-3.00 volume% chromium, 1.00 volume% or less molybdenum, 2.00 volume% or less nickel, and the remainder being iron and unavoidable impurities.

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

[0036] By maintaining a retained austenite content of 15% by volume or more, even when foreign matter becomes lodged in a rolling bearing under conditions of foreign matter-contaminated lubrication, the propagation of cracks from spalling can be suppressed. This is because the stress at the location where relatively soft retained austenite is present in rolling components at a distance of 50 μm from the surface (contact surface) is relieved. Furthermore, it is appropriate to prevent excessive hardness reduction in the surface layer by keeping the amount of retained austenite at the above-mentioned location to 35 volume% or less. The amount of retained austenite is determined by measuring a single 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 (all regions) be between 15 volume% and 35 volume% so that the occurrence of micro-spalling is suppressed by relaxing the stress in a predetermined region. However, the average value of the amount of retained austenite in the entire surface layer may be between 15 volume% and 35 volume%.

[0038] The grain size number of the prior austenite crystals in the surface layer up to 50 μm from the surface of the rolling component is preferably in the range of 9 to 11 (JIS G0551). By densifying the crystal grains of the needle rollers to within the above grain size number range, the propagation of cracks in the event of micro-spalling can be slowed.

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

[0040] During measurement, the grain size number of the prior austenite crystal is measured and determined at a depth of 50 μm from the exposed surface (contact surface). However, if the predetermined grain size number is measured at any position shallower than 50 μm, it will not affect the determination.

[0041] The rolling elements have a surface roughness (Ra) of 0.01 to 0.05 μm and a skewness (Rsk) of -4.0 to 0. 0 Yes, under the conditions that 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 surface shape is extremely smooth and has a surface roughness that improves the oil film formation properties for lubricating oils with low kinematic viscosity.

[0042] This makes it possible to avoid contact with mating components (such as shafts in planetary gear mechanisms) due to irregularities (peaks and valleys) in the surface roughness shape, even in a dilute lubrication environment, and to suppress the occurrence of minute spalling of the mating component.

[0043] If the surface roughness (Ra) exceeds the upper limit of the above numerical range, the difference in height of the roughness shape becomes too large, which is undesirable because it easily imparts stress to the material in contact with the rolling element. From the viewpoint of resistance to minute spalling, 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 rolling element surface is a uniformly polished wear surface. However, a negative Rsk value of less than -4.0 is undesirable because, although the necessary lipophilic effect is obtained, the strength of the rolling element surface may decrease, making it more susceptible to micro-sparing. From the viewpoint of resistance to micro-sparing, an Rsk of -2.0 to 0. 0 If available, it would be preferable.

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

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

[0046] Barrel polishing can be performed using a well-known barrel polishing machine and general methods. Typically, media (abrasive stones or abrasives), liquid compound, water, and the workpiece are placed in the barrel polishing machine's tank. However, this invention eliminates the use of abrasives and other media, resulting in a surface roughness (Ra) of 0.01-0.05 μm and a skewness (Rsk) of -4.0-0. 0、 The surface roughness shape can be adjusted to meet extremely smooth and specific conditions, such as a maximum cross-sectional height (Rt) of 0.7 μm or less and an absolute value of Ra / Rsk of 5.0 or less. Furthermore, this can be achieved not only by whether or not media (abrasive stones or abrasives) are used, but also by adjusting the type of media used or the mixing ratio.

[0047] Furthermore, in barrel polishing, by employing a method that polishes while applying centrifugal force, the surface hardness of the workpiece can be efficiently and sufficiently increased. To this end, a barrel polishing machine can be designed to rotate each of its multiple tanks while the entire machine revolves, and centrifugal barrel polishing can be performed by applying centrifugal force through high-speed rotation to each tank.

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

[0049] Furthermore, by not using abrasives in the barrel finishing process, a secondary effect is that the power consumption required to manufacture abrasives in advance can be reduced, thereby reducing carbon dioxide emissions.

[0050] A method for manufacturing rolling components, using needle-shaped rollers (diameter 1.5 to 5.5 mm) as a representative example of this embodiment of the invention, is described below. First, high-carbon chromium bearing steel (SUJ material) is used as the steel material for forming the needle rollers, and wire rods are prepared by drawing them multiple times. These wire rods are then subjected to processes such as cutting, forging, and turning to form the approximate shape of the needle rollers, and then heat treatment is applied to the steel material.

[0051] The heat treatment process for steel materials to be used as needle rollers involves first a preparatory heating step in which the steel material is heated to 850°C to 940°C, and then, as the overall heat treatment step, heating and holding the material above the A1 transformation point in a heat treatment gas.

[0052] Specifically, an endothermic modification gas (RX gas) is used as the base, and enrichment gases such as propane and butane, which serve as carbon sources, are added. In this atmosphere, the steel material is heated to a temperature between 850°C and 940°C. The entire heat treatment process may also be a nitrification process.

[0053] Next, a quenching process is performed. In the quenching process, the steel, which has been kept above the A1 transformation point, is rapidly cooled to a temperature below the Ms point (martensitic transformation initiation point) by immersion in oil (oil cooling). Alternatively, cooling by immersion in water (water cooling) may be performed instead of oil cooling. After that, a tempering process is carried out.

[0054] In the tempering process, the steel material, which has been hardened by quenching in the quenching process, is heat-treated at a temperature below the A1 transformation point (160-200°C). By holding it at this temperature for a predetermined time and then cooling it in room temperature air, toughness can be improved. Subsequently, a grinding process is performed, which yields needle-shaped rollers with the final dimensions and shape.

[0055] Furthermore, the surface roughness is adjusted by performing processing treatments such as barrel finishing and super-finishing grinding.

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

[0057] Rolling bearings incorporating such rolling elements can be used under harsh operating conditions, including rotational speeds of 9000 rpm or higher and lubricating oil kinematic viscosity of 13 cSt (40°C) and 4 cSt (100°C) or lower, as well as under operating conditions where rapid acceleration and deceleration are expected. These rolling bearing components, or rolling bearings using them, can be used in e-Axle reducers and the like. The inner diameter of the rolling bearings supporting the planetary gears of a planetary gear mechanism is φ7 to 50 mm.

[0058] Although the rolling bearings in the embodiments described above are exemplified by needle bearings (needle-shaped roller bearings), they are not limited to these and may also be cylindrical roller bearings, tapered roller bearings, self-aligning roller bearings, ball bearings, etc.

[0059] The rolling elements of the embodiment configured as described above have high spalling resistance and low aggressiveness towards contacting objects such as shaft members, allowing them to be used with low-viscosity lubricants.

[0060] As a secondary effect, this method can suppress the stirring resistance caused by lubricating oil in rolling bearings, contributing to reduced fuel consumption and electricity consumption in automobiles using rolling bearings. A further secondary effect is that, when the rollers are heat-treated as a whole (dip galvanizing), the nitrogen diffusion time during the conventional nitriding process can be shortened, reducing the amount of electricity used to maintain the furnace temperature and contributing to a reduction in carbon dioxide emissions. In addition, by not using abrasives in the surface roughness adjustment process, the electricity used to manufacture abrasives can be reduced. [Examples]

[0061] [Example 1] Needle-shaped rollers were manufactured using the following process and incorporated into rolling bearings that support the planetary gears of the planetary gear mechanism. High-carbon chromium bearing steel (SUJ2) was used to form the needle rollers, and chromium-molybdenum steel (SCM material) was used as the shaft member. Wire rods were prepared by drawing them multiple times, and the approximate shape was formed by cutting, forging, and turning.

[0062] As a heat treatment process (soaking), the steel material was first heated to 850-940°C, and then, as an overall heat treatment process, an enrichment gas was added to an endothermic modified gas (RX gas) as a base, and the material was heated to between 850°C and 940°C.

[0063] In the subsequent quenching process, the steel, which was maintained above the A1 transformation point, was rapidly cooled to a temperature below the Ms point (martensitic transformation initiation point) by immersion in oil (oil cooling).

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

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

[0066] [Comparative Example 1] The needle rollers of Comparative Example 1 were manufactured in exactly the same manner as in Example 1, except that a centrifugal barrel machining process was performed using an abrasive in the manufacturing process of Example 1 described above.

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

[0068] <Hardness Measurement Test> The surface of the needle-shaped rollers was measured using a Vickers hardness tester. A load of 100g was applied, and the diagonal length of the indentation was measured to determine the micro-Vickers hardness (Hv0.1) of the rolling element's surface. 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 also 860Hv0.1.

[0069] <Measurement of surface roughness> The surface roughness was measured 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 advanced along the axial direction to measure the surface roughness. 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): -0.1 3、 The maximum cross-sectional height (Rt) was 0.17 μm, and the absolute value of Ra / Rsk was 0.15.

[0071] Furthermore, in Comparative Example 1, Ra: 0.08 μm, skewness (Rsk): -3.4 8、 The maximum cross-sectional height (Rt) was 1.41 μm, and the absolute value of Ra / Rsk was 0.02.

[0072] Assuming the usage conditions of planetary gear mechanisms (Figures 1 and 2) incorporating the rolling elements (rollers) of Example 1 and Comparative Example 1, the spalling resistance was evaluated using a radial load testing machine under the following test conditions. Radial load: 6670N • Moment load: 13.5 N·m Outer wheel rotation speed: 9000 rpm • Lubricating oil: Cleaning 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 planetary gears incorporated into a planetary gear mechanism, provided a longer lifespan for the mating component compared to Comparative Example 1. [Industrial applicability]

[0074] This invention is applicable to rolling bearings that are lubricated with insufficient amounts of lubricant or low-viscosity liquid lubricant, particularly high-speed rotating rolling bearings, and rolling bearings where it is necessary to minimize the rotational torque and heat generation of the bearing. It can be used in reduction and transmission mechanisms such as e-axles, automatic transmissions (AT), and continuously variable transmissions (CVT) in automobiles, as well as in rolling bearings incorporated into various industrial machines such as high-speed rotating machine tools, and thus has broad industrial applications. [Explanation of Symbols]

[0075] A Planetary gear mechanism 1 Planetary gear 2 axes 3 Rolling element 4 Ring Gear 5 Sun Gear 6 Cage

Claims

1. A rolling element for a rolling bearing, wherein the micro-Vickers hardness of the rolling element surface is 820 to 930 Hv 0.1, the surface roughness (Ra) is 0.01 to 0.05 μm, the skewness (Rsk) is -4.0 to 0.0, the maximum cross-sectional height (Rt) is 0.7 μm or less, and the absolute value of the ratio of the surface roughness (Ra) to the skewness (Rsk) (Ra / Rsk) is 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 up to 50 μm from the surface of the rolling element is 15 to 35 volume percent.

3. A rolling bearing rolling element according to claim 1 or 2, wherein the grain size number (JIS G0551) of the prior austenite grain boundaries in the surface layer up to 50 μm from the surface of the rolling element is 9 to 11.

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

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

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