Gear pair

Abstract

To provide a low-cost technology capable of suppressing damage caused by trochoidal interference.SOLUTION: A gear pair 1 includes a driving gear 2 and a driven gear 4 that are mutually engaged. The driving gear 2 and the driven gear 4 each have a surface-hardened layer on the respective surfaces. The surface-hardened layer of the driving gear 2 has a surface hardness of 700 HV or more and 780 HV or less. The surface-hardened layer of the driving gear 2 has a carbon concentration ranging from 0.80 to 0.99 mass%. The amount of retained austenite on the surface of the surface-hardened layer of the driving gear 2 is 25 volume% or more and 40 volume% or less. The surface-hardened layer of the driven gear 4 has a surface hardness of 720 HV or more and 800 HV or less. The surface-hardened layer of the driven gear 4 has a carbon concentration ranging from 0.60 to 0.74 mass%. The amount of retained austenite on the surface of the surface-hardened layer of the driven gear 4 is 5 volume% or more and 19 volume% or less. The surface-hardened layer is either a carburized layer or a carbonitrided layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to gear pairs. [Background technology]

[0002] Patent Document 1 listed below discloses a technology that prevents damage to the tooth tip edge and tooth surface even if trochoidal interference occurs in a gear pair by softening the tooth tip edge of the gear more than the tooth base tooth surface (see, for example, Patent Document 1). This method involves subjecting the gear to a normal heat treatment, then reheating only the tooth tip edge portion to locally temper it, resulting in a gear with a softened tooth tip edge portion.

[0003] One way to prevent damage caused by trochoidal interference is to apply three-dimensional tooth surface modification to the gear tooth surface, thereby reducing the local surface pressure at the location on the tooth surface where trochoidal interference occurs.

[0004] Patent Document 2 listed below discloses a gear in which an inner ring raceway of a rolling bearing with the gear shaft as its central axis is provided on the outer periphery of the gear shaft (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2021 / 024418 [Patent Document 2] JP 2004-019823 A Summary of the Invention [Problem to be solved by the invention]

[0006] The method disclosed in Patent Document 1 is a method in which gears are subjected to a normal heat treatment, and then only the tooth tip edge portion is further heat treated, which increases the number of steps and therefore the cost. Furthermore, even in the method of applying three-dimensional tooth surface modification to the gear tooth surface, a processing machine is required for the three-dimensional tooth surface modification, and there are concerns that costs will increase due to the increased labor required for the tooth surface modification.

[0007] Therefore, an object of the present disclosure is to provide a technology that can suppress damage caused by trochoidal interference at low cost. [Means for solving the problem]

[0008] The gear pair according to an embodiment of the present disclosure is a gear pair having a drive gear and a driven gear that mesh with each other. The material composition of the drive gear and the driven gear is as follows: C: 0.12% to 0.23% by mass; Si: 0.15% to 0.35% by mass; Mn: 0.4% to 1.0% by mass; Cr: 0.4% to 1.2% by mass; Mo: zero or 0.15% to 0.30% by mass; Ni: zero or 0.4% to 2.0% by mass; the remainder being unavoidable impurities and Fe. The drive gear and the driven gear each have a surface-hardened layer. The surface hardness of the surface-hardened layer of the drive gear is 700 HV to 780 HV. The carbon concentration of the surface of the surface-hardened layer of the drive gear is 0.80% to 0.99% by mass. The amount of retained austenite on the surface of the surface-hardened layer of the drive gear is 25% by volume or more and 40% by volume or less. The surface hardness of the surface-hardened layer of the driven gear is 720 HV or more and 800 HV or less. The carbon concentration on the surface of the surface-hardened layer of the driven gear is 0.60% by mass or more and 0.74% by mass or less. The amount of retained austenite on the surface of the surface-hardened layer of the driven gear is 5% by volume or more and 19% by volume or less. The surface-hardened layer is either a carburized layer or a carbonitrided layer. [Effects of the Invention]

[0009] According to the present disclosure, damage caused by trochoidal interference can be suppressed at low cost. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view of a main part showing an example of a gear pair according to an embodiment. [Figure 2] FIG. 2 is an enlarged view of the main part of the gear pair. [Figure 3] FIG. 3 is a schematic diagram showing the roller pitching test. DETAILED DESCRIPTION OF THE INVENTION

[0011] First, the contents of the embodiment will be listed and explained. [Outline of the embodiment]

[0012] (1) A gear pair according to an embodiment of the present disclosure includes a drive gear and a driven gear that mesh with each other. The drive gear and the driven gear are made of a material having a C content of 0.12% by mass or more and 0.23% by mass or less, a Si content of 0.15% by mass or more and 0.35% by mass or less, a Mn content of 0.4% by mass or more and 1.0% by mass or less, a Cr content of 0.4% by mass or more and 1.2% by mass or less, an Mo content of 0.15% by mass or more and 0.30% by mass or less, an Ni content of 0.4% by mass or more and 2.0% by mass or less, and the remainder being unavoidable impurities and Fe. The drive gear and the driven gear each have a surface-hardened layer. The surface hardness of the surface-hardened layer of the drive gear is 700 HV or more and 780 HV or less. The carbon concentration of the surface of the surface-hardened layer of the drive gear is 0.80% by mass or more and 0.99% by mass or less. The amount of retained austenite on the surface of the surface-hardened layer of the drive gear is 25% by volume or more and 40% by volume or less. The surface hardness of the surface-hardened layer of the driven gear is 720 HV or more and 800 HV or less. The carbon concentration on the surface of the surface-hardened layer of the driven gear is 0.60% by mass or more and 0.74% by mass or less. The amount of retained austenite on the surface of the surface-hardened layer of the driven gear is 5% by volume or more and 19% by volume or less. The surface-hardened layer is either a carburized layer or a carbonitrided layer.

[0013] When the tooth tips of the driven gear and the tooth flank of the drive gear come into contact due to trochoidal interference, the tooth flank of the drive gear is pressed with high surface pressure by the tooth tips of the driven gear, which causes the transformation of retained austenite to produce strain-induced martensite on the tooth flank. According to the above configuration, the surface of the surface-hardened layer of the drive gear contains a relatively large amount of retained austenite. Therefore, the amount of work-induced martensite generated on the tooth flank of the drive gear due to contact with the tip of the driven gear is also relatively large. This effectively hardens the portion of the tooth flank of the drive gear that comes into contact with the driven gear, thereby suppressing damage such as pitting that occurs on the tooth flank of the drive gear. Furthermore, when the tooth tips of the driven gear and the tooth flanks of the drive gear come into contact due to trochoid interference, high surface pressure acts on the tooth tips of the driven gear as well as on the tooth flanks of the drive gear. In this regard, since the carbon concentration of the surface-hardened layer of the driven gear is lower than that of the surface-hardened layer of the drive gear, it is possible to suppress the precipitation of coarse carbides in the surface-hardened layer of the driven gear, and to suppress a decrease in tooth tip strength of the driven gear due to the coarse carbides, thereby suppressing damage to the tooth tips of the driven gear.

[0014] (2) In the gear pair of (1) above, it is preferable that the steel type of the material for the drive gear and the steel type of the material for the driven gear are the same. In this case, there is no need to prepare a variety of steel types when manufacturing the gear pair.

[0015] (3) In the gear pair of (1) or (2) above, the drive gear may have a gear body having teeth on its outer periphery, and a bearing raceway for a rolling bearing that rotatably supports the gear body. In this case, the amount of retained austenite on the bearing raceway surface is also relatively large, so that the fatigue strength of the bearing raceway can be increased while suppressing damage to the drive gear.

[0016] [Details of the embodiment] Preferred embodiments will now be described with reference to the drawings. At least some of the embodiments described below may be combined in any manner. [Gear Pairs] Fig. 1 is a cross-sectional view of a main part of an example of a gear pair according to an embodiment, and Fig. 2 is an enlarged view of a main part of the gear pair. In FIG. 1, a gear pair 1 is a gear pair used in a power transmission device of a vehicle, including an electric vehicle, an internal combustion engine vehicle, a hybrid vehicle, and the like.

[0017] The gear pair 1 has a drive gear 2 and a driven gear 4 that mesh with each other. The drive gear 2 has a gear body 6, a plurality of teeth 7, and a shaft portion 8. The gear body 6, the plurality of teeth 7, and the shaft portion 8 are integrally formed. The gear body 6 is disk-shaped. As shown in Figure 2, a plurality of teeth 7 are provided on the outer periphery of the gear body 6. The shaft portion 8 is aligned along the central axis C of the gear body 6. Therefore, the central axis C is also the central axis of the shaft portion 8. A bearing portion 10 is provided at a first end 8a of the shaft portion 8. The bearing portion 10 has an outer ring 12 and a plurality of tapered rollers 14. The outer ring 12 has an outer ring raceway 12a on its inner circumferential side. An inner ring raceway 8b is provided at the first end 8a of the shaft portion 8. The plurality of tapered rollers 14 are interposed between the outer ring raceway 12a and the inner ring raceway 8b so as to be able to roll freely. Therefore, the bearing portion 10, together with the inner ring raceway 8b of the shaft portion 8, constitutes a tapered roller bearing. The bearing portion 10 rotatably supports the shaft portion 8 and the gear body 6. Furthermore, a rotational force output by a motor or an internal combustion engine is applied to the second end 8c of the shaft portion 8. Thus, the shaft portion 8 and the gear body 6 are rotationally driven by the motor or the internal combustion engine.

[0018] As shown in Figures 1 and 2, the driven gear 4 has a gear body 16 and a plurality of teeth 17. The gear body 16 is disk-shaped. The gear body 16 is supported by a bearing (not shown) so as to be rotatable around an axis parallel to the central axis C. The plurality of teeth 17 are provided on the outer periphery of the gear body 16. The plurality of teeth 7 of the drive gear 2 and the plurality of teeth 17 of the driven gear 4 mesh with each other. The drive gear 2 rotates in the direction of the arrow in the figure. The driven gear 4 is driven by the drive gear 2 and rotates in the direction of the arrow in the figure. As a result, the driven gear 4 is rotationally driven by the drive gear 2.

[0019] In Figure 2, when the drive gear 2 rotates, the tooth portion 7 of the drive gear 2 that meshes with the tooth portion 17 is deflected in the direction opposite to the rotation direction of the drive gear 2. This causes the rotation of the drive gear 2 to relatively advance compared to the rotation of the driven gear 4, and as shown in Figure 2, the tooth tip 17a of the tooth portion 17 may come into contact with the tooth flank 7a of the tooth portion 7. This contact between the tooth tip 17a and the tooth flank 7a is trochoid interference.

[0020] [Materials for the driving gear 2 and driven gear 4] The driving gear 2 and the driven gear 4 are formed using alloy steel for mechanical structures. The preferred compositions of the materials for the driving gear 2 and the driven gear 4 are as follows: C is 0.12% by mass or more and 0.23% by mass or less, Si is 0.15 mass% or more and 0.35 mass% or less, Mn is 0.4 mass% or more and 1.0 mass% or less, Cr is 0.4% by mass or more and 1.2% by mass or less, Mo is not contained or is contained in an amount of 0.15 mass% or more and 0.30 mass% or less, Ni is not contained or is 0.4 mass% or more and 2.0 mass% or less, The balance is unavoidable impurities and Fe.

[0021] Examples of steel types that satisfy the above composition include chromium steel, chromium molybdenum steel, and nickel chromium molybdenum steel, which are specified in JIS G 4053 Alloy Steel for Machine Structures. Examples of chrome steel include SCr415 and SCr420. Examples of chrome molybdenum steel include SCM415 and SCM420. Examples of nickel chrome molybdenum steel include SNCM220 and SNCM420.

[0022] The drive gear 2 and the driven gear 4 each have a surface-hardened layer on their surface. The surface-hardened layer is either a carburized layer or a carbonitrided layer. The following description is for the case where the surface-hardened layer is a carburized layer. The surface hardness, carbon concentration, retained austenite, and coarse carbides of the carbonitrided layer are the same as those of the carburized layer.

[0023] [Carburized layer on driving gear 2] The surface hardness of the carburized layer of the drive gear 2 is 700 HV or more and 780 HV or less. From the viewpoint of ensuring the strength of the surface of the drive gear 2, the surface hardness of the carburized layer is preferably 700 HV or more. If the surface hardness of the carburized layer is lower than 700 HV, sufficient strength may not be ensured. As will be explained later, the carbon concentration in the carburized layer of the drive gear 2 is relatively high, and the hardness after quenching tends to be low. For this reason, the surface hardness of the carburized layer is preferably 780 HV or less.

[0024] The carbon concentration in the surface of the carburized layer of the driving gear 2 is preferably 0.80 mass % or more and 0.99 mass % or less. The carbon concentration at the surface of the carburized layer affects the hardness and strength of the carburized layer, as well as the amount of retained austenite in the carburized layer and the carbides that precipitate within the layer. If the carbon concentration on the surface of the carburized layer of the drive gear 2 is higher than 0.99 mass %, coarse carbides precipitate in the layer, which may result in a decrease in strength. Therefore, the carbon concentration at the surface of the carburized layer of the driving gear 2 is preferably 0.99 mass % or less. Furthermore, if the carbon concentration on the surface of the carburized layer of the drive gear 2 is lower than 0.80 mass %, the amount of retained austenite in the carburized layer will be small, and there is a risk that the necessary amount of retained austenite will not be secured. By setting the carbon concentration on the surface of the carburized layer of the drive gear 2 to 0.80 mass % or more and 0.99 mass % or less, the amount of retained austenite in the carburized layer is made appropriate, and strength is maintained.

[0025] The amount of retained austenite on the surface of the carburized layer of the driving gear 2 is preferably 25% by volume or more and 40% by volume or less. The amount of retained austenite on the surface of the carburized layer of the drive gear 2 affects the amount of deformation-induced martensite that forms on the tooth surface 7a when the tooth tip 17a of the driven gear 4 comes into contact with the tooth surface 7a of the drive gear 2 due to trochoidal interference. If the amount of retained austenite on the surface of the carburized layer of the drive gear 2 is less than 25% by volume, the amount of deformation-induced martensite that forms on the tooth surface 7a may be insufficient, and the damage suppression effect of the drive gear 2 may not be fully achieved. If the amount of retained austenite on the surface of the carburized layer of the drive gear 2 is more than 40% by volume, the surface hardness of the carburized layer may be reduced more than necessary. Therefore, the amount of retained austenite on the surface of the carburized layer of the drive gear 2 is preferably 25% by volume or more and 40% by volume or less. More preferably, the amount of retained austenite on the surface of the carburized layer of the driving gear 2 is 30% by volume or more and 39% by volume or less.

[0026] When the tooth tip 17a of the driven gear 4 and the tooth flank 7a of the drive gear 2 come into contact with each other due to trochoidal interference, the tooth flank 7a is pressed with high surface pressure by the tooth tip 17a. As a result, deformation-induced martensite is generated on the tooth flank 7a due to the transformation of retained austenite. According to the above configuration, a relatively large amount of retained austenite is contained on the surface of the carburized layer of the drive gear 2. Therefore, the amount of deformation-induced martensite generated on the tooth flank 7a of the drive gear 2 due to contact with the tooth tip 17a of the driven gear 4 is also relatively large. As a result, the portion of the tooth flank 7a that comes into contact with the driven gear 4 is effectively hardened, and damage such as pitting that occurs on the tooth flank 7a of the drive gear 2 is suppressed.

[0027] [Carburized layer on driven gear 4] The preferred surface hardness of the carburized layer of the driven gear 4 is 720 HV or more and 800 HV or less. The surface hardness of the carburized layer is preferably 720 HV or more from the viewpoint of ensuring the strength of the surface of the driven gear 4. If the surface hardness of the carburized layer of the driven gear 4 is lower than 720 HV, sufficient strength may not be ensured. Furthermore, if the surface hardness of the carburized layer of the driven gear 4 is higher than 800 HV, the toughness will decrease, and the mechanical properties of the driven gear 4 may deteriorate.

[0028] The carbon concentration in the surface of the carburized layer of the driven gear 4 is preferably 0.60 mass % or more and 0.74 mass % or less. As mentioned above, the carbon concentration at the surface of the carburized layer not only affects the hardness and strength of the carburized layer, but also the amount of retained austenite in the carburized layer and the carbides that precipitate within the layer. If the carbon concentration on the surface of the carburized layer of the driven gear 4 is lower than 0.60 mass %, it becomes difficult to ensure the hardness and strength required for the carburized layer. Furthermore, if the carbon concentration on the surface of the carburized layer of the driven gear 4 is higher than 0.74 mass %, coarse carbides will precipitate on the tooth tip, which may reduce strength. Therefore, the carbon concentration in the surface of the carburized layer of the driven gear 4 is preferably 0.60 mass % or more and 0.74 mass % or less.

[0029] The amount of retained austenite on the surface of the carburized layer of the driven gear 4 is preferably 5% by volume or more and 19% by volume or less. The amount of retained austenite on the surface of the carburized layer of the driven gear 4 affects the mechanical properties of the driven gear 4. If the amount of retained austenite on the surface of the carburized layer of the driven gear 4 is less than 5% by volume, the toughness may decrease, and the mechanical properties of the driven gear 4 may also decrease. If the amount of retained austenite on the surface of the carburized layer of the driven gear 4 is more than 19% by volume, the surface hardness of the carburized layer may decrease more than necessary. Therefore, the amount of retained austenite on the surface of the carburized layer of the driven gear 4 is preferably 5% by volume or more and 19% by volume or less. It is more preferable that the amount of retained austenite on the surface of the carburized layer of the driven gear 4 is 5% by volume or more and 11% by volume or less.

[0030] When the tooth tip 17a of the driven gear 4 and the tooth flank 7a of the drive gear 2 come into contact due to trochoid interference, high surface pressure acts on the tooth tip 17a of the driven gear 4 as well as on the tooth flank 7a. In this regard, the carbon concentration of the carburized layer of the driven gear 4 is lower than that of the drive gear 2, so precipitation of coarse carbides in the carburized layer of the driven gear 4 is suppressed, and a decrease in tooth tip strength of the driven gear 4 due to coarse carbides is suppressed. As a result, damage to the tooth tips of the driven gear 4 is suppressed.

[0031] Thus, according to the above configuration, by making the amount of retained austenite in the drive gear 2 greater than the amount of retained austenite in the driven gear 4, processing-induced martensite is effectively generated in the drive gear 2, thereby suppressing damage such as pitting caused by trochoidal interference, suppressing the precipitation of coarse carbides on the driven gear 4, and suppressing damage to the tooth tips of the driven gear 4.

[0032] The preferred combination of the steel type of the material for the drive gear 2 and the steel type of the material for the driven gear 4 is the same. For example, a preferred combination is that the steel type of the material for the drive gear 2 is SCM420 and the steel type of the material for the driven gear 4 is SCM420. In this case, there is an advantage that it is not necessary to prepare a variety of steel types when manufacturing the gear pair 1.

[0033] In addition, the drive gear of the gear pair disclosed herein has a gear body 6 in which the drive gear 2 has a tooth portion 7 on its outer periphery, and an inner ring raceway 8b (bearing raceway) for a bearing portion 10 that supports the gear body 6 so that it can rotate freely, so that the amount of retained austenite on the surface of the inner ring raceway 8b is relatively large, which suppresses damage to the drive gear 2 while also increasing the fatigue strength of the inner ring raceway 8b.

[0034] [Method for manufacturing the driving gear 2 and the driven gear 4] The drive gear 2 and the driven gear 4 are manufactured by processing the above-mentioned raw material into a predetermined shape, heat treating the obtained rough product to form a carburized layer, and then finishing it.

[0035] The heat treatment includes carburizing and quenching and tempering. In carburizing and quenching, the raw material is held in a carburizing atmosphere at the carburizing temperature for a certain period of time. Carburizing causes carbon to diffuse and penetrate into the surface of the raw material. The raw material is then held at a temperature at which it turns into austenite, and then rapidly cooled to the quenching temperature. Through these heat treatments, a carburized layer is formed on the surfaces of the drive gear 2 and the driven gear 4. The carburized and quenched rough product is then tempered. The surface of the tempered rough product is then finished to obtain the driving gear 2 and the driven gear 4.

[0036] The carburizing temperature, carburizing atmosphere, holding time during carburizing, temperature at which austenite is formed (holding temperature), holding time at the temperature at which austenite is formed, quenching temperature, and tempering temperature are adjusted as appropriate according to the specifications of the carburized layer. Furthermore, the heat treatment may be carbonitriding instead of carburizing. In carbonitriding, the raw material is maintained in a carbonitriding atmosphere at a predetermined temperature for a certain period of time. Carbonitriding causes carbon to diffuse and penetrate into the surface of the raw material, while simultaneously causing nitrogen to diffuse and penetrate. Therefore, the surface hardened layers of the driving gear 2 and the driven gear 4 may be carbonitrided layers instead of carburized layers.

[0037] [About the evaluation test] Next, the evaluation test of the present disclosure will be described. The test specimens used in the evaluation test are examples that correspond to the gear pair of the present disclosure. Therefore, the present disclosure is not limited to these test specimens. In this evaluation test, five types of test specimens were manufactured, and the pitting life and the state of coarse carbide precipitation of each test specimen were evaluated.

[0038] [Production of test samples] Test pieces were formed using SCM420 material, carburized, quenched, and tempered, and then the surface was finish-machined to obtain test pieces 1 to 5 shown in Table 1. The heat treatment conditions for test pieces 1 to 5 were individually adjusted so that the carbon concentration on the surface of the carburized layer, the surface hardness of the carburized layer, and the amount of retained austenite on the surface of the carburized layer would be predetermined values. Two types of test specimens were manufactured for each of test pieces 1 to 5. One of the two types of test specimens was a roller-shaped specimen (φ26 mm, width 28 mm). The other was a rectangular column-shaped specimen (20 mm × 20 mm × 50 mm). The carbon concentration of the surface of the carburized layer, the surface hardness of the carburized layer, and the amount of retained austenite in the carburized layer of test samples 1 to 5 are as shown in Table 1 below.

[0039] [Table 1]

[0040] The carbon concentration on the surface of the carburized layer was measured using an EPMA (Electron Probe Micro Analyzer). The rectangular columnar test piece was cut using a high-speed cutter in a direction perpendicular to the flat surface toward the interior, and the cut specimen was embedded in resin and the cut surface was mirror-polished. The carbon concentration of the mirror-polished cut surface was continuously measured in a direction perpendicular to the surface of the test piece toward the interior. The carbon concentration measured at a position 0.05 mm from the surface of the test piece was defined as the carbon concentration on the surface of the carburized layer.

[0041] The surface hardness of the carburized layer was measured using a micro-Vickers hardness tester. Measurements were performed on the cross-section of the rectangular columnar test piece obtained as described above. The cross-sectional hardness measured at a position 0.05 mm from the surface of the test piece on the cut surface was defined as the surface hardness of the carburized layer.

[0042] The amount of retained austenite was measured using an X-ray diffractometer. The rectangular columnar test specimens were electrolytically polished, and X-rays were irradiated at a position 0.05 mm from the surface. Quantitative analysis was performed based on the intensity of the diffraction peaks indicating austenite (γ phase), and the amount of retained austenite on the surface of the carburized layer of the rectangular columnar test specimens was determined.

[0043] [Evaluation of Pitting Life] A roller pitting test was carried out using the roller-shaped test piece, and the pitting life was determined. The pitting life is the number of rotations until pitting occurs in the roller-shaped test piece when the roller pitting test is carried out. FIG. 3 is a schematic diagram showing the roller pitching test. As shown in Fig. 3, the central axis of the roller-shaped test piece 20 was pressed against the outer peripheral surface 22a of the load roller 22, with the central axis of the roller-shaped test piece 20 being pressed toward the central axis of the load roller 22, thereby applying a predetermined surface pressure (test surface pressure) to the outer peripheral surface 20a. In the roller pitting test, the roller-shaped test piece 20 was rotated with a predetermined test surface pressure applied, while being lubricated with lubricating oil between the outer peripheral surface 20a and the outer peripheral surface 22a. The number of rotations until pitting occurred on the outer peripheral surface 20a of the roller-shaped test piece 20 was determined.

[0044] The conditions for the roller pitching test are as follows: Testing machine: Two-cylinder rolling fatigue testing machine Test surface pressure: 2000MPa, 3500MPa Roller test piece rotation speed: 2000 min -1 Load roller rotation speed: 557 min -1 Relationship between the central axis of the roller-shaped test piece and the central axis of the loaded roller: Parallel The direction of the velocity of the roller-shaped test piece and the direction of the velocity of the load roller at the contact point between the roller-shaped test piece and the load roller: the same direction Slip rate: -40% Lubricant: ATF (automatic transmission lubricant) In addition, the slip ratio S ris defined by the following formula: Slip ratio S r =((V S -V L )?V S ) x 100 In the above formula, V S is the peripheral speed (m / s) of the roller-shaped test piece 20, V L is the peripheral speed of the load roller 22 (m / s).

[0045] The specifications of the load roller are as follows: Dimensions: φ130mm, width 18mm, outer surface crowning radius of curvature 150mm Material: SCM420, carburized, quenched and tempered Carbon concentration on the surface of the carburized layer: 0.72 mass% Surface hardness of carburized layer: 742HV Amount of retained austenite on the surface of the carburized layer: 15.9% by volume

[0046] [Evaluation of coarse carbide precipitation state] A cut piece of the rectangular columnar test piece was embedded in resin, and the corners of the test piece on the mirror-polished cut surface were assumed to be simulated gear tooth tips. After etching with nital to reveal the microstructure, the corners on the cut surface were observed under a metallurgical microscope to evaluate the state of coarse carbide precipitation. The number of coarse carbides with a major axis length of 5 μm or more was counted within a 180 μm × 135 μm field of view. If the number of coarse carbides with a major axis length of 5 μm or more within a 180 μm × 135 μm field of view was 1 or more, coarse carbides were present, and if the number was 0, coarse carbides were not present.

[0047] [Evaluation results] The evaluation results of the pitting occurrence life and the state of coarse carbide precipitation are shown in Table 2 below.

[0048] [Table 2]

[0049] As shown in Table 2, in the roller pitting test, under the condition of a test surface pressure of 2000 MPa, pitting occurred in all of test pieces 1 to 5 at a rotation number of 1 × 10 7 This did not occur even when the number of times reached. On the other hand, under the condition of a test surface pressure of 3500 MPa, pitting occurred in test pieces 1 to 3 at a rate of 1.0 × 10 6 This occurred with fewer rotations than In addition, under the condition of a test surface pressure of 3500 MPa, pitting occurred in test pieces 4 and 5, with a magnitude of 1.0 × 10 6 This occurred at more than one rotation. From these results, the pitting initiation life of test specimens 4 to 5 is longer than that of test specimens 1 to 3. The amount of retained austenite in test specimens 4 to 5 is larger than that of test specimens 1 to 3. This shows that the larger the amount of retained austenite, the longer the pitting initiation life.

[0050] It should be noted that coarse carbides are judged to be present in test samples 4 and 5, while coarse carbides are judged to be absent in test samples 1 to 3. The carbon concentration of the carburized layer of test samples 1 to 3 is lower than that of the carburized layer of test samples 4 and 5.

[0051] These results show that specimens 4 and 5 can effectively suppress damage such as pitting occurring on the outer peripheral surface 20a compared to specimens 1 to 3. Therefore, specimens 4 and 5 are suitable for application to a drive gear 2 where pitting occurs on the tooth surface. Furthermore, it is preferable to apply the test specimens 1 to 3, which were determined to be free of coarse carbides, to the driven gear 4, which requires high tooth tip strength.

[0052] 〔others〕 It should be noted that the embodiments disclosed herein are to be considered as illustrative in all respects and not restrictive. For example, although the drive gear 2 in FIG. 1 has an inner ring raceway 8b, the drive gear 2 may have an inner peripheral surface with an outer ring raceway. Furthermore, although the drive gear 2 in FIG. 1 has the inner ring raceway 8b, the drive gear 2 may be a simple gear without an inner ring raceway. The scope of the present invention is defined by the claims, not by the meaning described above, and is intended to include meanings equivalent to the claims and all modifications within the scope thereof. [Explanation of symbols]

[0053] 1 gear pair 2 Drive gear 4 Driven gear 6 Gear body 7 Teeth 8b Inner raceway 10 Bearing section

Claims

1. A gear pair having a driving gear and a driven gear that mesh with each other, The composition of the material of the drive gear and the composition of the material of the driven gear are: C is 0.12% by mass or more and 0.23% by mass or less, Si is 0.15 mass% or more and 0.35 mass% or less, Mn is 0.4 mass% or more and 1.0 mass% or less, Cr is 0.4% by mass or more and 1.2% by mass or less, Mo is not contained or is contained in an amount of 0.15 mass % or more and 0.30 mass % or less, Ni is not contained or is contained in an amount of 0.4 mass% or more and 2.0 mass% or less; The remainder is unavoidable impurities and Fe, the drive gear and the driven gear each have a surface-hardened layer on a surface thereof; the surface hardness of the surface hardened layer of the drive gear is 700 HV or more and 780 HV or less; a carbon concentration on the surface of the surface-hardened layer of the drive gear is 0.80 mass% or more and 0.99 mass% or less; an amount of retained austenite on the surface of the surface hardened layer of the drive gear is 25% by volume or more and 40% by volume or less; the surface hardness of the surface-hardened layer of the driven gear is 720 HV or more and 800 HV or less; a carbon concentration on the surface of the surface-hardened layer of the driven gear is 0.60 mass% or more and 0.74 mass% or less, an amount of retained austenite on the surface of the surface hardened layer of the driven gear is 5% by volume or more and 19% by volume or less; The surface hardened layer is either a carburized layer or a carbonitrided layer. Gears Vs.

2. The steel type of the material of the drive gear and the steel type of the material of the driven gear are the same.

2. The gear pair according to claim 1.

3. The drive gear is a gear body having teeth on its outer periphery; a bearing raceway for a rolling bearing that rotatably supports the gear body; 3. A gear pair according to claim 1 or 2.

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