tapered roller bearing

Abstract

To provide a tapered roller bearing enabling prolongation of the life thereof.SOLUTION: A tapered roller bearing comprises an inner ring, an outer ring having an outer ring raceway surface, and tapered rollers having rolling surfaces. An average value of a carbon concentration of steel constituting the inner ring between an inner ring raceway surface and a position in which distance in vertical depth direction from the inner ring raceway surface becomes 10 μm, an average value of a carbon concentration of steel constituting the outer ring between the outer ring raceway surface and a position in which distance in a vertical depth from the outer ring raceway surface becomes 10 μm, and an average value of a carbon concentration of steel constituting the tapered rollers between the rolling surfaces and a position in which a distance in a vertical depth direction from the rolling surfaces becomes 10 μm are equal to or lower than 1.0 mass%. An average value of a nitrogen concentration of steel constituting the inner ring between the inner ring raceway surface and a position in which a distance in a vertical depth direction from the inner ring raceway surface reaches 10 μm, and an average value of a nitrogen concentration of steel constituting the outer ring between the outer ring raceway surface and a position in which a distance in a vertical depth direction from the outer ring raceway surface reaches 10 μm are equal to or higher than 0.15 mass%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a tapered roller bearing. [Background technology]

[0002] Japanese Patent Laid-Open Publication No. 6-341441 (Patent Document 1) describes a rolling bearing, in which the carbon and nitrogen concentrations in the steel on the raceway surfaces and rolling surfaces are specified in order to ensure grindability. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 6-341441 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, in order to improve fuel efficiency, there has been a trend toward using low-viscosity lubricants and reducing the amount of lubricant in automobile transmissions and differentials. Furthermore, as units become more compact, bearing loads tend to increase. Furthermore, the use of aluminum housings tends to increase bearing misalignment. Despite being used in such harsh environments, tapered roller bearings are still required to have a long life.

[0005] However, the nitrogen concentration in the steel on the raceway surfaces and rolling surfaces of the rolling bearing described in Patent Document 1 is set to a low upper and lower limit in consideration of grindability. Furthermore, the carbon concentration in the steel on the raceway surfaces and rolling surfaces of the rolling bearing described in Patent Document 1 is set to a high value in consideration of hardness, making it difficult for nitrogen to penetrate into the steel on the raceway surfaces and rolling surfaces. As a result, the nitrogen and carbon concentrations in the raceway surfaces and rolling surfaces of the rolling bearing described in Patent Document 1 may result in an insufficient bearing life.

[0006] The present invention has been made in view of the above-mentioned problems of the prior art. More specifically, the present invention provides a tapered roller bearing that can improve the lifespan. [Means for solving the problem]

[0007] The tapered roller bearing of the present invention comprises an inner ring having an inner ring raceway surface, an outer ring having an outer ring raceway surface opposing the inner ring raceway surface, and tapered rollers having rolling surfaces in contact with the inner ring raceway surface and the outer ring raceway surface. The inner ring, outer ring, and tapered rollers are formed from hardened steel. The average carbon concentration of the steel constituting the inner ring between the inner ring raceway surface and a position 10 μm vertically from the inner ring raceway surface in the depth direction, the average carbon concentration of the steel constituting the outer ring between the outer ring raceway surface and a position 10 μm vertically from the outer ring raceway surface in the depth direction, and the average carbon concentration of the steel constituting the tapered rollers between the rolling surfaces and a position 10 μm vertically from the rolling surface in the depth direction are all 1.0 mass percent or less. The average nitrogen concentration in the steel making up the inner ring between the inner ring raceway surface and a position 10 μm away from the inner ring raceway surface in the vertical depth direction, and the average nitrogen concentration in the steel making up the outer ring between the outer ring raceway surface and a position 10 μm away from the outer ring raceway surface in the vertical depth direction, are 0.15 mass percent or more. The average nitrogen concentration in the steel making up the tapered roller between the rolling surface and a position 10 μm away from the rolling surface in the vertical depth direction is 0.05 mass percent or more, and is 0.15 mass percent or more at at least one-third of multiple measurement points equally spaced around the circumferential direction.

[0008] In the tapered roller bearing, the arithmetic mean roughness of the inner ring raceway surface and the arithmetic mean roughness of the outer ring raceway surface may be 0.15 μm Ra or less, and the arithmetic mean roughness of the rolling surfaces may be 0.10 μm Ra or less.

[0009] In the above-mentioned tapered roller bearing, the inner ring raceway surface may be provided with a single arc crowning having a drop ratio of 70 percent or more, or cut crowning may be provided at both ends in the axial direction. The outer ring raceway surface may be provided with a single arc crowning having a drop ratio of 70 percent or more, or cut crowning may be provided at both ends in the axial direction. The rolling surfaces may be provided with logarithmic crowning.

[0010] In the above-mentioned tapered roller bearing, the length of martensite crystal grains in the steel making up the inner ring at a position where the vertical depth distance from the inner ring raceway surface is equal to 5 percent of the average diameter of the tapered rollers, the length of martensite crystal grains in the steel making up the outer ring at a position where the vertical depth distance from the outer ring raceway surface is equal to 5 percent of the average diameter of the tapered rollers, and the length of martensite crystal grains in the steel making up the tapered rollers at a position where the vertical depth distance from the rolling surface is equal to 5 percent of the average diameter of the tapered rollers may be 300 μm or less.

[0011] In the above-mentioned tapered roller bearing, the length of martensite crystal grains in the steel constituting the inner ring at a position where the vertical depth distance from the inner ring raceway surface is equal to 5 percent of the average diameter of the tapered rollers may be 50 μm or less.

[0012] In the above tapered roller bearing, the hardness of the inner ring at the inner ring raceway surface, the hardness of the outer ring at the outer ring raceway surface, and the hardness of the tapered rollers at the rolling surfaces may be 58 HRC or more and 64 HRC or more. The hardness of the inner ring at a position where the distance in the vertical depth direction from the inner ring raceway surface is equal to 5 percent of the average diameter of the tapered rollers, the hardness of the outer ring at a position where the distance in the vertical depth direction from the outer ring raceway surface is equal to 5 percent of the average diameter of the tapered rollers, and the hardness of the tapered roller at a position where the distance in the vertical depth direction from the rolling surfaces is equal to 5 percent of the average diameter of the tapered rollers may be 58 HRC or more.

[0013] In the above-mentioned tapered roller bearing, the vertical depth direction distance that nitrogen penetrates from the inner ring raceway surface into the steel constituting the inner ring, the vertical depth direction distance that nitrogen penetrates from the outer ring raceway surface into the steel constituting the outer ring, and the vertical depth direction distance that nitrogen penetrates from the rolling surface into the steel constituting the tapered rollers may be 0.3 mm or more.

[0014] In the above-mentioned tapered roller bearing, the amount of retained austenite in the steel making up the inner ring at a position 50 μm away from the inner ring raceway surface in the vertical depth direction, the amount of retained austenite in the steel making up the outer ring at a position 50 μm away from the outer ring raceway surface in the vertical depth direction, and the amount of retained austenite in the steel making up the tapered rollers at a position 50 μm away from the rolling surface in the vertical depth direction may be 20 volume percent or more and 45 volume percent or less.

[0015] In the tapered roller bearing described above, the steel constituting the inner ring, the steel constituting the outer ring, and the steel constituting the tapered rollers may be either SCr435 as specified in the JIS standard or SCM435 as specified in the JIS standard. [Effects of the Invention]

[0016] According to the tapered roller bearing of the present invention, the life can be improved. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a cross-sectional view of a tapered roller bearing 10. [Figure 2] FIG. 10 is a cross-sectional view of a tapered roller bearing 10 according to a modified example. [Figure 3] FIG. 10 is an explanatory diagram for specifying the drop amount ratio of the single arc crowning applied to the inner ring raceway surface 1da. [Figure 4] 10 is an explanatory diagram for specifying the drop amount ratio of the single arc crowning applied to the outer ring raceway surface 2ca. FIG. [Figure 5]10 is an explanatory diagram for specifying the drop amount ratio of the single arc crowning applied to the rolling surface 3c. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0018] The details of the embodiment will be described with reference to the drawings. In the following drawings, the same or corresponding parts are designated by the same reference numerals, and redundant description will not be repeated.

[0019] (Configuration of tapered roller bearing according to embodiment) The configuration of a tapered roller bearing according to an embodiment (hereinafter referred to as "tapered roller bearing 10") will be described below.

[0020] Fig. 1 is a cross-sectional view of a tapered roller bearing 10. As shown in Fig. 1, tapered roller bearing 10 has a center axis A. Tapered roller bearing 10 has an inner ring 1, an outer ring 2, a plurality of tapered rollers 3, and a cage 4.

[0021] The inner ring 1 has an annular (ring-shaped) shape and has a first end face 1a, a second end face 1b, an inner peripheral surface 1c, and an outer peripheral surface 1d.

[0022] The first end face 1a and the second end face 1b are end faces of the inner ring 1 in the direction along the central axis A (hereinafter referred to as the "axial direction"). The second end face 1b is the face opposite the first end face 1a in the axial direction. The outer diameter of the inner ring 1 on the side of the second end face 1b is smaller than the outer diameter of the inner ring 1 on the side of the first end face 1a. The inner ring 1 has a large flange 1e on the side of the first end face 1a and a small flange 1f on the side of the second end face 1b.

[0023] The inner peripheral surface 1c extends along the circumferential direction. The circumferential direction is a direction along the circumference of a circle centered on the central axis A. The inner peripheral surface 1c faces the central axis A. The inner ring 1 is attached to a shaft (not shown) at the inner peripheral surface 1c.

[0024] The outer peripheral surface 1d extends in the circumferential direction. The outer peripheral surface 1d faces away from the central axis A. In other words, the outer peripheral surface 1d is the opposite surface of the inner peripheral surface 1c in the radial direction (the direction perpendicular to the central axis A). The outer peripheral surface 1d has an inner ring raceway surface 1da. The inner ring raceway surface 1da is located between the large flange 1e and the small flange 1f in the axial direction.

[0025] The inner ring raceway surface 1da is the portion of the outer peripheral surface 1d that comes into contact with the tapered roller 3 (more specifically, the rolling surface 3c). The inner ring raceway surface 1da is inclined so that the distance from the center axis A decreases from one side in the axial direction (the side closer to the first end face 1a) to the other side in the axial direction (the side closer to the second end face 1b).

[0026] The arithmetic mean roughness (Ra) of the inner ring raceway surface 1da is preferably 0.15 μmRa or less. The arithmetic mean roughness of the inner ring raceway surface 1da is measured in accordance with the JIS standard (JIS B 0601:2001).

[0027] The outer ring 2 has an annular shape and has a first end face 2a, a second end face 2b, an inner peripheral surface 2c, and an outer peripheral surface 2d.

[0028] The first end face 2a and the second end face 2b are axial end faces of the outer ring 2. The second end face 2b is the opposite face to the first end face 2a in the axial direction. The first end face 2a is on one side in the axial direction, and the second end face 2b is on the other side in the axial direction.

[0029] The inner peripheral surface 2c extends in the circumferential direction and faces the central axis A. The inner peripheral surface 2c has an outer ring raceway surface 2ca. The outer ring raceway surface 2ca is the portion of the inner peripheral surface 2c that comes into contact with the tapered rollers 3 (more specifically, the rolling surfaces 3c).

[0030] The outer ring raceway surface 2ca is inclined so that the distance from the center axis A decreases from one axial side to the other axial side. The outer ring raceway surface 2ca faces the inner ring raceway surface 1da.

[0031] The arithmetic mean roughness of the outer ring raceway surface 2ca is preferably 0.15 μmRa or less. The arithmetic mean roughness of the outer ring raceway surface 2ca, like the arithmetic mean roughness of the inner ring raceway surface 1da, is measured in accordance with JIS standard (JIS B 0601:2001).

[0032] The outer peripheral surface 2d extends in the circumferential direction and faces away from the central axis A. In other words, the outer peripheral surface 2d is the radially opposite surface of the inner peripheral surface 2c. The outer ring 2 is attached to a housing (not shown) at the outer peripheral surface 2d.

[0033] The tapered roller 3 has a truncated cone shape. Note that this truncated cone shape does not have to be a mathematically strict truncated cone shape. The tapered roller 3 has a small diameter surface 3a, a large diameter surface 3b, and a rolling surface 3c. The small diameter surface 3a, the large diameter surface 3b, and the rolling surface 3c correspond to the top surface, bottom surface, and side surface of the above-mentioned truncated cone shape, respectively.

[0034] The diameter of the small diameter surface 3a is smaller than the diameter of the large diameter surface 3b. The small diameter surface 3a faces the small flange 1f, and the large diameter surface 3b faces the large flange 1e. The rolling surface 3c contacts the inner ring raceway surface 1da and the outer ring raceway surface 2ca. The arithmetic mean roughness of the rolling surface 3c is preferably 0.10 μmRa or less. The arithmetic mean roughness of the rolling surface 3c, like the arithmetic mean roughness of the inner ring raceway surface 1da, is measured in accordance with JIS standard (JIS B 0601:2001).

[0035] The outer diameter of the tapered roller 3 increases from the small diameter surface 3a side toward the large diameter surface 3b side. The average value of the outer diameter of the tapered roller 3 from the small diameter surface 3a side to the large diameter surface 3b side will be referred to as the average diameter of the tapered roller 3 hereinafter.

[0036] The inner ring 1, outer ring 2, and tapered rollers 3 are formed from hardened steel. The steel constituting the inner ring 1, the steel constituting the outer ring 2, and the steel constituting the tapered rollers 3 is preferably either SCr435 or SCM435, as specified in the JIS standard (JIS G 4053:2016). The steel constituting the inner ring 1, the steel constituting the outer ring 2, and the steel constituting the tapered rollers 3 may be the same or different.

[0037] Nitriding treatment is performed on the surfaces of the steel constituting the inner ring 1, the steel constituting the outer ring 2, and the steel constituting the tapered rollers 3. The surfaces of the steel constituting the inner ring 1, the steel constituting the outer ring 2, and the steel constituting the tapered rollers 3 may also be subjected to carbo-nitriding treatment.

[0038] The carbon concentration in the steel making up the inner ring 1 at the inner ring raceway surface 1da is 1.0 mass percent or less. The carbon concentration in the steel making up the inner ring 1 at the inner ring raceway surface 1da is the average value between the inner ring raceway surface 1da and a position 10 μm away from the inner ring raceway surface 1da in the vertical depth direction, and is measured using an EPMA (Electron Probe Micro Analyzer) or a tabletop solid metal emission spectrometer. The vertical depth direction distance from a certain surface is the distance from that surface measured along a straight line perpendicular to that surface. The carbon concentration in the steel making up the inner ring 1 at the inner ring raceway surface 1da is measured in a cross section cut along a plane including the center axis A.

[0039] The carbon concentration in the steel making up the outer ring 2 at the outer ring raceway surface 2ca is 1.0 mass percent or less, and the carbon concentration in the steel making up the tapered rollers 3 at the rolling surface 3c is 1.0 mass percent or less. The carbon concentrations in the steel making up the outer ring 2 at the outer ring raceway surface 2ca and the carbon concentration in the steel making up the tapered rollers 3 at the rolling surface 3c are measured by the same method as the carbon concentration in the steel making up the inner ring 1 at the inner ring raceway surface 1da. The carbon concentration in the steel making up the outer ring 2 at the outer ring raceway surface 2ca is measured in a cross section cut by a plane including the central axis A, and the carbon concentration in the steel making up the tapered rollers 3 at the rolling surface 3c is measured in a cross section perpendicular to the central axis of the tapered rollers 3.

[0040] The carbon concentration in the steel constituting the inner ring 1 at the inner ring raceway surface 1da, the carbon concentration in the steel constituting the outer ring 2 at the outer ring raceway surface 2ca, and the carbon concentration in the steel constituting the tapered roller 3 at the rolling surface 3c are preferably 0.5 mass percent or more.

[0041] The nitrogen concentration in the steel making up the inner ring 1 at the inner ring raceway surface 1da is 0.15 mass percent or more. The nitrogen concentration in the steel making up the inner ring 1 at the inner ring raceway surface 1da is the average value between the inner ring raceway surface 1da and a position 10 μm away from the inner ring raceway surface 1da in the vertical depth direction, and is measured using an EPMA or a tabletop solid metal emission spectrometer. The nitrogen concentration in the steel making up the inner ring 1 at the inner ring raceway surface 1da is measured in a cross section cut along a plane including the center axis A.

[0042] The nitrogen concentration in the steel making up the outer ring 2 at the outer ring raceway surface 2ca is 0.15 mass percent or more. The nitrogen concentration in the steel making up the outer ring 2 at the outer ring raceway surface 2ca is measured using the same method as the nitrogen concentration in the steel making up the inner ring 1 at the inner ring raceway surface 1da. The nitrogen concentration in the steel making up the outer ring 2 at the outer ring raceway surface 2ca is measured in a cross section cut along a plane including the center axis A.

[0043] The nitrogen concentration in the steel making up the tapered rollers 3 on the rolling surface 3c is 0.05 mass percent or more. The nitrogen concentration in the steel making up the tapered rollers 3 on the rolling surface 3c is an average value measured using the same method as for the nitrogen concentration in the steel making up the inner ring 1 on the inner ring raceway surface 1da. The nitrogen concentration in the steel making up the tapered rollers 3 on the rolling surface 3c is measured at multiple measurement points equally spaced along the circumferential direction on a cross section perpendicular to the central axis of the tapered rollers 3, and at one-third or more of the multiple measurement points, the nitrogen concentration in the steel making up the tapered rollers 3 on the rolling surface 3c is 0.15 mass percent or more. The multiple measurement points from which the average value is calculated may be, for example, 12, taking into consideration the existence of locations with low nitrogen concentration along the circumferential direction where the tapered rollers 3 contact each other during heat treatment and the economic viability of the measurement.

[0044] The vertical depth direction distance that nitrogen penetrates from the inner ring raceway surface 1da into the steel that constitutes the inner ring 1 is preferably 0.3 mm or more. The vertical depth direction distance that nitrogen penetrates from the inner ring raceway surface 1da into the steel that constitutes the inner ring 1 is the vertical depth direction distance between the inner ring raceway surface 1da and the position closest to the inner ring raceway surface 1da among positions where the nitrogen concentration is below the detection limit by EPMA.

[0045] The vertical depth direction distance that nitrogen penetrates from outer ring raceway surface 2ca into the steel that constitutes outer ring 2 and the vertical depth direction distance that nitrogen penetrates from rolling surface 3c into the steel that constitutes tapered roller 3 are also preferably 0.3 mm or more. The vertical depth direction distance that nitrogen penetrates from outer ring raceway surface 2ca into the steel that constitutes outer ring 2 and the vertical depth direction distance that nitrogen penetrates from rolling surface 3c into the steel that constitutes tapered roller 3 are measured using the same method as for the vertical depth direction distance that nitrogen penetrates from inner ring raceway surface 1da into the steel that constitutes inner ring 1.

[0046] The length of martensite grains in the steel constituting the inner ring 1 at a position where the distance in the vertical depth direction from the inner ring raceway surface 1da is equal to 5 percent of the average diameter of the tapered rollers 3 is preferably 300 μm or less. The length of the martensite grains is the longest width of the martensite grains and is determined directly by observing a cross section. It is even more preferable that the length of martensite grains in the steel constituting the inner ring 1 at a position where the distance in the vertical depth direction from the inner ring raceway surface 1da is equal to 5 percent of the average diameter of the tapered rollers 3 is 50 μm or less.

[0047] The length of martensite grains in the steel constituting the inner ring 1 at a position where the vertical depth distance from the inner ring raceway surface 1da is equal to 5 percent of the average diameter of the tapered rollers 3 is measured by the following method. First, a cross-sectional image is taken using a microscope at a position where the vertical depth distance from the inner ring raceway surface 1da is equal to 5 percent of the average diameter of the tapered rollers 3. Prior to taking this cross-sectional image, the cross-section is mirror-polished and etched with nital or the like. The magnification for microscope observation is appropriately selected so that a sufficient number of martensite grains are included in the observation field.

[0048] Second, based on the cross-sectional image, the length of each martensite grain in the longitudinal direction is measured. The maximum value of these measurements is regarded as the length of the martensite grain in the steel constituting the inner ring 1 at a position where the distance in the vertical depth direction from the inner ring raceway surface 1da is equal to 5 percent of the average diameter of the tapered roller 3.

[0049] The length of the martensite crystal grains in the steel constituting the outer ring 2 at a position where the vertical depth distance from the outer ring raceway surface 2ca is equal to 5 percent of the average diameter of the tapered rollers 3, and the length of the martensite crystal grains in the steel constituting the tapered rollers 3 at a position where the vertical depth distance from the rolling surface 3c is equal to 5 percent of the average diameter of the tapered rollers 3, are preferably 300 μm or less.

[0050] The length of the martensite crystal grains in the steel constituting the outer ring 2 at a position where the vertical depth distance from the outer ring raceway surface 2ca is equal to 5 percent of the average diameter of the tapered rollers 3 and the length of the martensite crystal grains in the steel constituting the tapered rollers 3 at a position where the vertical depth distance from the rolling surface 3c is equal to 5 percent of the average diameter of the tapered rollers 3 are measured in the same manner as the length of the martensite crystal grains in the steel constituting the inner ring 1 at a position where the vertical depth distance from the inner ring raceway surface 1da is equal to 5 percent of the average diameter of the tapered rollers 3.

[0051] Furthermore, the position where the vertical depth distance from the inner ring raceway surface 1da (outer ring raceway surface 2ca, rolling surface 3c) is equal to 5 percent of the average diameter of the tapered roller 3 is the position where a shear stress of 90 percent of the maximum shear stress acting on the inner ring raceway surface 1da (outer ring raceway surface 2ca, rolling surface 3c) acts.

[0052] The amount of retained austenite in the steel constituting inner ring 1 at a position 50 μm away from inner ring raceway surface 1da in the vertical depth direction is preferably 20 volume percent or more and 45 volume percent or less. The amount of retained austenite in the steel constituting inner ring 1 at a position 50 μm away from inner ring raceway surface 1da in the vertical depth direction is measured by X-ray diffraction. More specifically, the amount of retained austenite in the steel constituting inner ring 1 at a position 50 μm away from inner ring raceway surface 1da in the vertical depth direction is measured by the ratio of the intensity of the diffraction peak of the austenite phase to the sum of the intensities of the diffraction peaks of other phases when irradiated with X-rays.

[0053] The amount of retained austenite in the steel constituting the outer ring 2 at a position 50 μm away from the outer ring raceway surface 2ca in the vertical depth direction and the amount of retained austenite in the steel constituting the tapered rollers 3 at a position 50 μm away from the rolling surface 3c in the vertical depth direction are preferably 20 volume percent or more and 45 volume percent or less.

[0054] The amount of retained austenite in the steel constituting the outer ring 2 at a position 50 μm vertically in depth from the outer ring raceway surface 2ca and the amount of retained austenite in the steel constituting the tapered roller 3 at a position 50 μm vertically in depth from the rolling surface 3c are measured using the same method as that for the amount of retained austenite in the steel constituting the inner ring 1 at a position 50 μm vertically in depth from the inner ring raceway surface 1da.

[0055] The hardness of the inner ring 1 at the inner ring raceway surface 1da, the hardness of the outer ring 2 at the outer ring raceway surface 2ca, and the hardness of the tapered rollers 3 at the rolling surface 3c are preferably 58 HRC or more and 64 HRC or less. The hardness of the inner ring 1 at a position where the vertical depth distance from the inner ring raceway surface 1da is equal to 5 percent of the average diameter of the tapered rollers 3, the hardness of the outer ring 2 at a position where the vertical depth distance from the outer ring raceway surface 2ca is equal to 5 percent of the average diameter of the tapered rollers 3, and the hardness of the tapered rollers 3 at a position where the vertical depth distance from the rolling surface 3c is equal to 5 percent of the average diameter of the tapered rollers 3 are preferably 58 HRC or more.

[0056] The hardness of the inner ring 1 at the inner ring raceway surface 1da, the hardness of the outer ring 2 at the outer ring raceway surface 2ca, and the hardness of the tapered roller 3 at the rolling surface 3c, as well as the hardness of the tapered roller 3 at a position where the vertical depth distance from the rolling surface 3c is equal to 5% of the average diameter of the tapered roller 3, are measured in accordance with the Rockwell hardness test method specified in the JIS standard (JIS Z 2245:2016).

[0057] (Variation) Figure 2 is a cross-sectional view of a tapered roller bearing 10 according to a modified example. The cage 4 is not shown in Figure 2. As shown in Figure 2, the inner ring raceway surface 1da may be provided with a single arc crowning. That is, in a cross-sectional view parallel to the central axis A, the curve representing the inner ring raceway surface 1da may be formed by a single arc.

[0058] The drop ratio of the single-arc crowning applied to the inner ring raceway surface 1da is 70 percent or more. FIG. 3 is an explanatory diagram for determining the drop ratio of the single-arc crowning applied to the inner ring raceway surface 1da. As shown in FIG. 3, in a cross-sectional view parallel to the center axis A, a line connecting both ends of the inner ring raceway surface 1da in the axial direction is defined as line L1 (shown by a dotted line in FIG. 3), the apex of the curve representing the inner ring raceway surface 1da is defined as apex P1, and the distance between apex P1 and line L1 is defined as distance DIS1. The value obtained by dividing distance DIS1 (unit: μm) by the length of line L1 (unit: mm) and multiplying this value by 100 (expressed mathematically as 100 × DIS1 (μm) ÷ L1 (mm)) is the drop ratio of the single-arc crowning applied to the inner ring raceway surface 1da. The two axial ends of the inner ring raceway surface 1da are identified by the boundary with the recessed portion 1db and the boundary with the recessed portion 1dc, respectively.

[0059] The outer ring raceway surface 2ca may be provided with a single arc crowning. The drop ratio of the single arc crowning provided on the outer ring raceway surface 2ca is 70 percent or more. The drop ratio of the single arc crowning provided on the outer ring raceway surface 2ca is calculated in the same manner as the drop ratio of the single arc crowning provided on the inner ring raceway surface 1da.

[0060] FIG. 4 is an explanatory diagram for determining the drop ratio of the single circular-arc crowning applied to the outer ring raceway surface 2ca. As shown in FIG. 4, in a cross-sectional view parallel to the center axis A, the line connecting both axial ends of the outer ring raceway surface 2ca is called line L2 (shown by a dotted line in FIG. 4), the apex of the curve representing the outer ring raceway surface 2ca is called apex P2, and the distance between apex P2 and line L2 is called distance DIS2. The value obtained by dividing distance DIS2 (unit: μm) by the length of line L2 (unit: mm) and multiplying this value by 100 (expressed mathematically as 100 × DIS2 (μm) ÷ L2 (mm)) is the drop ratio of the single circular-arc crowning applied to the outer ring raceway surface 2ca. Note that both axial ends of the outer ring raceway surface 2ca are determined by the boundary with inclined surface 2cb and the boundary with inclined surface 2cc, respectively.

[0061] The rolling surface 3c may be logarithmically crowned. That is, in a cross-sectional view parallel to the central axis A, the curve representing the rolling surface 3c may be a logarithmic curve. This logarithmic curve is expressed, for example, by the following equation 1.

[0062]

number

[0063] A single arc crowning may be applied to the rolling surface 3c. The drop ratio of the single arc crowning applied to the rolling surface 3c is 70 percent or more. The drop ratio of the single arc crowning applied to the rolling surface 3c is calculated in the same way as the drop ratio of the single arc crowning applied to the inner ring raceway surface 1da.

[0064] FIG. 5 is an explanatory diagram for determining the drop amount ratio of the single circular-arc crowning applied to the rolling surface 3c. As shown in FIG. 5, in a cross-sectional view parallel to the central axis A, a line connecting both ends of the rolling surface 3c in the axial direction is defined as line L3 (shown by a dotted line in FIG. 5), the apex of the curve representing the rolling surface 3c is defined as apex P3, and the distance between apex P3 and line L3 is defined as distance DIS3. The value obtained by dividing the distance DIS3 (unit: μm) by the length of line L3 (unit: mm) and multiplying this value by 100 (expressed mathematically as 100 × DIS3 (μm) ÷ L3 (mm)) is the drop amount ratio of the single circular-arc crowning applied to the rolling surface 3c. Note that both ends of the rolling surface 3c in the axial direction are defined by the boundary with inclined surface 3da and the boundary with inclined surface 3db, respectively.

[0065] In the tapered roller bearing 10 according to the above-described modified example, a single arc crowning is applied to the inner ring raceway surface 1da and the outer ring raceway surface 2ca, but instead of a single arc crowning, a cut crowning may be applied to both axial ends of the inner ring raceway surface 1da (outer ring raceway surface 2ca).

[0066] (Effects of tapered roller bearing 10 according to embodiment) The effects of the tapered roller bearing 10 will be described below.

[0067] In tapered roller bearing 10, the nitrogen concentration in the steel constituting inner ring 1 at inner ring raceway surface 1da and the nitrogen concentration in the steel constituting outer ring 2 at outer ring raceway surface 2ca are 0.15 mass percent or more, improving temper softening resistance and increasing the amount of retained austenite. As a result, hardness and toughness are ensured at inner ring raceway surface 1da and outer ring raceway surface 2ca.

[0068] When the tapered rollers 3 are subjected to heat treatment (nitriding treatment), the tapered rollers 3 are loaded into a heat treatment furnace while stacked flat on a tray, and so there are areas with low nitrogen concentration where the tapered rollers 3 come into contact with each other. However, since the nitrogen concentration is 0.15 mass percent or more at one-third or more of the multiple measurement points on the tapered rollers 3, it is possible to ensure hardness and toughness on the rolling surfaces 3c. The multiple measurement points from which the average value is obtained may be, for example, 12 points, taking into consideration the existence of areas with low nitrogen concentration that vary along the circumferential direction where the tapered rollers 3 come into contact with each other and the economic viability of the measurement.

[0069] As the carbon content in steel increases, it becomes more difficult for nitrogen to penetrate into the steel. However, because the carbon concentration in the steel making up inner ring 1 at inner ring raceway surface 1da, the carbon concentration in the steel making up outer ring 2 at outer ring raceway surface 2ca, and the carbon concentration in the steel making up tapered roller 3 at rolling surface 3c are all 1 mass percent or less, it is possible to ensure the amount of nitrogen penetrating from inner ring raceway surface 1da, outer ring raceway surface 2ca, and rolling surface 3c. As described above, tapered roller bearing 10 can achieve a longer bearing life.

[0070] In a tapered roller bearing 10, if the arithmetic mean roughness of the inner ring raceway surface 1da and the arithmetic mean roughness of the outer ring raceway surface 2ca are 0.15 μmRa or less, and the arithmetic mean roughness of the rolling surface 3c is 0.10 μmRa or less, a good lubrication condition (oil film thickness) can be ensured on the inner ring raceway surface 1da, the outer ring raceway surface 2ca, and the rolling surface 3c, thereby further extending the life of the bearing.

[0071] If the inner ring raceway surface 1da is provided with a single arc crowning or cut crowning with a drop ratio of 70 percent or more, the outer ring raceway surface 2ca is provided with a single arc crowning or cut crowning with a drop ratio of 70 percent or more, or the rolling surface 3c is provided with a logarithmic crowning, the edge stress is alleviated, making it possible to tolerate a greater axial tilt. Therefore, in this case, even when the tapered roller bearing 10 is mounted in a low-rigidity housing such as an aluminum housing, edge peeling can be suppressed and the life can be further extended.

[0072] When the length of the martensite crystal grains in the steel making up the inner ring 1 at a position where the vertical depth distance from the inner ring raceway surface 1da is equal to 5 percent of the average diameter of the tapered rollers 3, the length of the martensite crystal grains in the steel making up the outer ring 2 at a position where the vertical depth distance from the outer ring raceway surface 2ca is equal to 5 percent of the average diameter of the tapered rollers 3, and the length of the martensite crystal grains in the steel making up the tapered rollers 3 at a position where the vertical depth distance from the rolling surface 3c is equal to 5 percent of the average diameter of the tapered rollers 3 are 300 μm or less, the rolling fatigue strength of the inner ring raceway surface 1da, outer ring raceway surface 2ca and rolling surface 3c is improved by the refinement of the martensite crystal grains, thereby further extending the life of the bearing.

[0073] Since the rotational load count of the inner ring raceway surface 1da is greater than the rotational load count of the outer ring raceway surface 2ca and the rotational load count of the rolling surface 3c, the bearing life can be further extended by setting the length of the martensite crystal grains in the steel constituting the inner ring 1 at a position where the vertical depth distance from the inner ring raceway surface 1da is equal to 5 percent of the average diameter of the tapered roller 3 to 50 μm or less.

[0074] When the hardness of the inner ring 1 at the inner ring raceway surface 1da, the hardness of the outer ring 2 at the outer ring raceway surface 2ca, and the hardness of the tapered roller 3 at the rolling surface 3c are 58 HRC or more and 64 HRC or less, wear resistance of the inner ring raceway surface 1da, the outer ring raceway surface 2ca, and the rolling surface 3c can be ensured. When the hardness of the inner ring 1 at a position where the vertical depth distance from the inner ring raceway surface 1da is equal to 5 percent of the average diameter of the tapered rollers 3, the hardness of the outer ring 2 at a position where the vertical depth distance from the outer ring raceway surface 2ca is equal to 5 percent of the average diameter of the tapered rollers 3, and the hardness of the tapered roller 3 at a position where the vertical depth distance from the rolling surface 3c is equal to 5 percent of the average diameter of the tapered rollers 3 are 58 HRC or more, internally initiated flaking can be suppressed, further extending the life of the bearing.

[0075] Furthermore, when the vertical depth direction distance that nitrogen penetrates from the inner ring raceway surface 1da into the steel that constitutes the inner ring 1, the vertical depth direction distance that nitrogen penetrates from the outer ring raceway surface 2ca into the steel that constitutes the outer ring 2, and the vertical depth direction distance that nitrogen penetrates from the rolling surface 3c into the steel that constitutes the tapered roller 3 are 0.3 mm or more, a sufficient nitrogen concentration can be obtained on the inner ring raceway surface 1da, the outer ring raceway surface 2ca, and the rolling surface 3c, thereby further increasing the lifespan of the bearing.

[0076] When the amount of retained austenite in the steel making up the inner ring 1 at a position 50 μm vertically in depth from the inner ring raceway surface 1da, the amount of retained austenite in the steel making up the outer ring 2 at a position 50 μm vertically in depth from the outer ring raceway surface 2ca, and the amount of retained austenite in the steel making up the tapered roller 3 at a position 50 μm vertically in depth from the rolling surface 3c are 25 to 40 volume percent, the hardness of the inner ring raceway surface 1da, outer ring raceway surface 2ca, and rolling surface 3c can be maintained, while the bearing life can be extended in situations where foreign matter is mixed into the lubricating oil.

[0077] (Bearing life test) A bearing life test for confirming the effects of the tapered roller bearing 10 will be described below.

[0078] <Sample and test conditions> For the bearing life test, Samples 1 to 8 (Table 1), Samples 9 to 14 (Table 2), Samples 15 to 18 (Table 3), Samples 19 to 20 (Table 4), and Samples 21 to 22 (Table 5) were prepared. Although not shown in the tables, in Samples 1 to 22, the carbon concentrations on the inner ring raceway surface, outer ring raceway surface, and rolling contact surface were 1.0 mass percent or less. The dimensions of the tapered roller bearings for Samples 1 to 22 were an outer diameter of 92 mm, an inner diameter of 55 mm, and a width of 17 mm. Gear oil SAE75W-85 was used as the lubricant for Samples 1 to 22.

[0079] [Table 1]

[0080] [Table 2]

[0081] [Table 3]

[0082] [Table 4]

[0083] [Table 5]

[0084] Two types of test conditions were used in the bearing life test. The first test conditions (Table 6) were applied to Samples 1 to 8, Samples 9 to 14, Samples 15 to 18, and Samples 19 to 20. The second test conditions (Table 7) were applied to Samples 21 and 22.

[0085] [Table 6]

[0086] [Table 7]

[0087] <Test Results> Samples 1 to 6 satisfied the following conditions: (A) the nitrogen concentration on the inner ring raceway surface was 0.15 mass percent or more, (B) the nitrogen concentration on the outer ring raceway surface was 0.15 mass percent or more, and (C) the nitrogen concentration on the rolling surface was 0.05 mass percent or more and was 0.15 mass percent or more at one-third or more of the 12 measurement points equally spaced around the circumference (hereinafter, these conditions will be referred to as "Condition A," "Condition B," and "Condition C," respectively). On the other hand, Sample 7 did not satisfy Condition B, and Sample 8 did not satisfy Condition C.

[0088] Samples 1 to 6 exceeded the calculated life (life calculated from the basic dynamic load rating). On the other hand, Samples 7 and 8 fell short of the calculated life. From this comparison, it was experimentally demonstrated that the life of tapered roller bearing 10, which satisfies all of Conditions A, B, and C, is improved.

[0089] Sample 4 and Sample 9 satisfy all of Condition A, Condition B, and Condition C. In Sample 4, single arc crowning with a drop ratio of 70 percent or more is applied to the outer ring raceway surface, while in Sample 9, cut crowning is applied to the outer ring raceway surface. Sample 4 and Sample 9 exhibited approximately the same life span. From this, it was experimentally clarified that when Condition A, Condition B, and Condition C are satisfied, approximately the same life span is exhibited when cut crowning is applied to the raceway surface and when single arc crowning with a drop ratio of 70 percent or more is applied to the raceway surface.

[0090] Samples 11 to 13 satisfied all of Conditions A, B, and C. In Sample 11, the drop amount ratio of the single circular arc crowning applied to the outer ring raceway surface and the drop amount ratio of the single circular arc crowning applied to the inner ring raceway surface were 70 percent or more. On the other hand, in Samples 12 and 13, either the drop amount ratio of the single circular arc crowning applied to the outer ring raceway surface or the drop amount ratio of the single circular arc crowning applied to the inner ring raceway surface was less than 30 percent.

[0091] The life of Sample 11 was longer than the life of Sample 12 and the life of Sample 13. From this comparison, it was experimentally clarified that when Conditions A, B, and C are satisfied, the life of the bearing can be further improved by setting the drop amount ratio of the single circular-arc crowning applied to the outer ring raceway surface and the drop amount ratio of the single circular-arc crowning applied to the inner ring raceway surface to 70 percent or more.

[0092] Sample 10 is similar to Sample 14 in that it satisfies conditions A, B, and C, but differs from Sample 14 in that the crowning applied to the rolling surfaces is logarithmic crowning. The life of Sample 10 was longer than that of Sample 14. From this comparison, it was experimentally demonstrated that when conditions A, B, and C are satisfied, applying logarithmic crowning to the rolling surfaces further improves the life of the bearing.

[0093] Samples 15 to 18 satisfied all of Conditions A, B, and C. Samples 15 and 16 satisfied Condition (D) that the arithmetic mean roughness of the inner ring raceway surface and the arithmetic mean roughness of the outer ring raceway surface are 0.15 μmRa or less and the arithmetic mean roughness of the rolling surfaces is 0.10 μmRa or less (hereinafter referred to as "Condition D"). On the other hand, Samples 17 and 18 did not satisfy Condition D.

[0094] The lifespan of Sample 15 and Sample 16 exceeded the lifespan of Sample 17 and Sample 18. This comparison experimentally revealed that when Condition A, Condition B, and Condition C are satisfied, the bearing lifespan can be further improved by also satisfying Condition D.

[0095] Samples 19 and 20 satisfied all of Condition A, Condition B, and Condition C. Sample 19 satisfied Condition (E) that the length of martensite crystal grains on the inner ring raceway surface be 300 μm or less (hereinafter referred to as "Condition E"). On the other hand, Sample 20 did not satisfy Condition E. The life of Sample 19 was longer than that of Sample 20. From this comparison, it was experimentally revealed that when Condition A, Condition B, and Condition C are satisfied, the life of the bearing can be further improved by also satisfying Condition E.

[0096] Samples 21 and 22 satisfied condition E. Sample 21 had shorter martensite grains on the inner ring rolling surface compared to Sample 22. Due to this difference, the life of Sample 21 was longer than that of Sample 22. In this way, it was experimentally demonstrated that the life of a bearing can be further improved by shortening the length of the martensite grains on the inner ring rolling surface (more specifically, by shortening the length of the martensite grains to 50 μm or less).

[0097] Although the embodiments of the present invention have been described above, the above-described embodiments can be modified in various ways. Furthermore, the scope of the present invention is not limited to the above-described embodiments. The scope of the present invention is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims. [Industrial Applicability]

[0098] The above-described embodiment is particularly advantageously applied to tapered roller bearings for automobiles, for example. [Explanation of symbols]

[0099] 10 Tapered roller bearing, 1 Inner ring, 1a 1st end face, 1b 2nd end face, 1c Inner circumferential face, 1d Outer circumferential face, 1da Inner raceway surface, 1db Inner ring part, 1dc Inner ring part, 1e Large tsuba, 1f Small tsuba, 2 Outer ring, 2a 1st end face, 2b 2nd end face, 2c Inner circumferential face, 2ca Outer ring raceway surface, 2cb, 2cc inclined surface, 2d outer peripheral surface, 3a small diameter surface, 3b large diameter surface, 3c rolling surface, 3da, 3db inclined surface, 4 retainer, A central axis, DIS1, DIS2, DIS3 distance, L1, L2, L3 straight line, P vertex.

Claims

1. an inner ring having an inner ring raceway surface; an outer ring having an outer ring raceway surface facing the inner ring raceway surface; tapered rollers having rolling surfaces that contact the inner ring raceway surface and the outer ring raceway surface, the inner ring, the outer ring, and the tapered rollers are formed of hardened steel, an average value of the carbon concentration in the steel constituting the inner ring between the inner ring raceway surface and a position 10 μm away from the inner ring raceway surface in the vertical depth direction, an average value of the carbon concentration in the steel constituting the outer ring between the outer ring raceway surface and a position 10 μm away from the outer ring raceway surface in the vertical depth direction, and an average value of the carbon concentration in the steel constituting the tapered roller between the rolling surface and a position 10 μm away from the rolling surface in the vertical depth direction are 1.0 mass percent or less, an average value of the nitrogen concentration in the steel constituting the inner ring between the inner ring raceway surface and a position 10 μm away from the inner ring raceway surface in a vertical depth direction, and an average value of the nitrogen concentration in the steel constituting the outer ring between the outer ring raceway surface and a position 10 μm away from the outer ring raceway surface in a vertical depth direction, are 0.15 mass percent or more, A tapered roller bearing in which the average nitrogen concentration in the steel constituting the tapered roller between the rolling surface and a position 10 μm away from the rolling surface in the vertical depth direction is 0.05 mass percent or more, and is 0.15 mass percent or more at 1 / 3 to 3 / 4 of 12 measurement points equally spaced circumferentially around the entire circumference of the rolling surface.

2. the arithmetic mean roughness of the inner ring raceway surface and the arithmetic mean roughness of the outer ring raceway surface are 0.15 μm Ra or less, 2. The tapered roller bearing according to claim 1, wherein the arithmetic mean roughness of the rolling surface is 0.10 μm Ra or less.

3. the inner ring raceway surface is provided with a single arc crowning having a drop ratio of 70% or more, or with cut crowning at both ends in the axial direction, The outer ring raceway surface is provided with a single arc crowning having a drop ratio of 70% or more, or with cut crowning at both ends in the axial direction, 3. The tapered roller bearing according to claim 1, wherein the rolling surface is logarithmically crowned.

4. 4. A tapered roller bearing according to claim 1, wherein the length of martensite crystal grains in the steel constituting the inner ring at a position where the distance in the vertical depth direction from the inner ring raceway surface is equal to 5 percent of the average diameter of the tapered rollers, the length of martensite crystal grains in the steel constituting the outer ring at a position where the distance in the vertical depth direction from the outer ring raceway surface is equal to 5 percent of the average diameter of the tapered rollers, and the length of martensite crystal grains in the steel constituting the tapered rollers at a position where the distance in the vertical depth direction from the rolling surface is equal to 5 percent of the average diameter of the tapered rollers are 300 μm or less.

5. 5. A tapered roller bearing according to claim 4, wherein the length of martensite crystal grains in steel constituting said inner ring at a position where the distance in the vertical depth direction from said inner ring raceway surface is equal to 5 percent of the average diameter of said tapered rollers is 50 μm or less.

6. the hardness of the inner ring at the inner ring raceway surface, the hardness of the outer ring at the outer ring raceway surface, and the hardness of the tapered roller at the rolling surface are 58 HRC or more and 64 HRC or more, 6. A tapered roller bearing according to any one of claims 1 to 5, wherein the hardness of the inner ring at a position where the vertical depth distance from the inner ring raceway surface is equal to 5 percent of the average diameter of the tapered rollers, the hardness of the outer ring at a position where the vertical depth distance from the outer ring raceway surface is equal to 5 percent of the average diameter of the tapered rollers, and the hardness of the tapered roller at a position where the vertical depth distance from the rolling surface is equal to 5 percent of the average diameter of the tapered rollers are 58 HRC or more.

7. A tapered roller bearing according to any one of claims 1 to 6, wherein the vertical depth direction distance that nitrogen penetrates from the inner ring raceway surface into the steel constituting the inner ring, the vertical depth direction distance that nitrogen penetrates from the outer ring raceway surface into the steel constituting the outer ring, and the vertical depth direction distance that nitrogen penetrates from the rolling surface into the steel constituting the tapered rollers are 0.3 mm or more.

8. 8. A tapered roller bearing according to any one of claims 1 to 7, wherein the amount of retained austenite in steel constituting the inner ring at a position 50 µm away from the inner ring raceway surface in the vertical depth direction, the amount of retained austenite in steel constituting the outer ring at a position 50 µm away from the outer ring raceway surface in the vertical depth direction, and the amount of retained austenite in steel constituting the tapered rollers at a position 50 µm away from the rolling surface in the vertical depth direction are 20 volume percent or more and 45 volume percent or less.

9. A tapered roller bearing according to any one of claims 1 to 8, wherein the steel constituting the inner ring, the steel constituting the outer ring, and the steel constituting the tapered rollers is either SCr435 as specified in the JIS standard or SCM435 as specified in the JIS standard.

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