Gear

Abstract

To provide a gear design method that can easily determine an appropriate effective carburizing depth according to the gear module, and a gear that can suppress pitching and flaking by increasing residual compressive stress while suppressing damage caused by shear stress.SOLUTION: A design method for a gear which is subjected to a carburizing heat treatment and rotates while generating frictional resistance force by sliding of the tooth tip surface, includes a first step to obtain a proper range, which is a range of average hardness for obtaining a desired residual compressive stress and tensile strength, by referring to first characteristic information 8 indicating the relationship between average hardness and residual compressive stress and tensile strength from the surface to a specified depth, and a second step to obtain a range of effective carburizing depth corresponding to the proper range obtained in the first step, by referring to second characteristic information 9 indicating the relationship between average hardness and effective carburizing depth for each right-angled tooth module obtained by dividing a pitch circle diameter by the number of teeth. In addition, a value of the residual compressive stress at the tooth tip divided by the residual compressive stress at the tooth base is 0.8 or more and 1.3 or less for the gear.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention , tooth relates to a vehicle.

Background Art

[0002] Conventionally, a gear that rotates while generating frictional resistance by sliding on the tooth tip surface has been used, for example, as a pinion gear of a differential device of a vehicle (for example, see Patent Document 1).

[0003] The differential device described in Patent Document 1 includes a differential case that rotates receiving the driving force of a vehicle driving source, a pair of side gears rotatably supported relative to the differential case, and a plurality (for example, eight) of pinion gears slidably and rotatably supported in a storage hole formed in the differential case. The plurality of pinion gears constitute a plurality of sets (for example, four sets) of pinion gear sets each consisting of two meshed pinion gears. In the storage hole, the pinion gears of each set mesh with each other, and the two pinion gears of each set mesh separately with a pair of side gears. During power transmission, due to the driving force transmitted from the differential case to the pinion gears and the meshing reaction force between the side gears and the pinion gears, the tooth tip surfaces of the respective pinion gears are pressed against the inner surface of the storage hole and rotate. At this time, differential restriction of the pair of side gears is achieved by the frictional resistance generated between the tooth tip surface of each pinion gear and the inner surface of the storage hole. Each pinion gear is subjected to carburizing heat treatment (carburizing, quenching, tempering) in order to improve strength and wear resistance and enhance durability.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The case carburizing depth of a gear is generally represented by the effective case depth, which is the distance (depth) from the surface to the part where the Vickers hardness becomes 550 HV. In addition, gears are required to have strength to prevent damage caused by repeated high surface pressure such as pitting (local erosion or corrosion) and flaking (the phenomenon where the surface peels off in scales), and to prevent damage caused by shear stress such as cracks. To prevent damage caused by shear stress, it is effective to increase the tensile strength by increasing the effective case depth. Also, to prevent pitting and flaking, it is desirable to increase the residual compressive stress by case carburizing heat treatment. Furthermore, this residual compressive stress is also effective in suppressing damage caused by shear stress.

[0006] Since the residual compressive stress may decrease if the effective case depth is made too deep, in order to increase the residual compressive stress while suppressing damage caused by shear stress, for example, it is necessary to trial-produce a plurality of gears with different effective case depths, conduct experiments and measurements, and find an appropriate effective case depth. However, since the required driving force transmission capacity of the differential device of a vehicle varies depending on the vehicle type, the module (pitch diameter / number of teeth), number, and size of the pinion gears and the size of the side gears are various. Therefore, in order to find the appropriate range of effective case depth for the pinion gears and side gears of a newly developed differential device, a lot of cost and time are required.

[0007] The present invention has been made in view of the above circumstances, and its object is , scissors To provide a gear capable of suppressing pitting and flaking by increasing the residual compressive stress while suppressing damage caused by shear stress.

Means for Solving the Problems

[0009] , thisIn order to achieve the above object, the invention provides a carburized and heat-treated gear that rotates while generating frictional resistance by the sliding of the tooth tip surface, wherein the value obtained by dividing the residual compressive stress at the tooth tip by the residual compressive stress at the tooth root is 0.8 or more and 1.3 or less, and the residual compressive stress at the tooth tip and the residual compressive stress at the tooth root are each 400 MPa or more.

Advantages of the Invention

[0010] According to the present invention, it is possible to provide a gear design method capable of easily obtaining an appropriate effective carburized depth according to the normal module of the gear teeth, and a gear capable of increasing the residual compressive stress while suppressing damage caused by shear stress to suppress pitching and flaking.

Brief Description of the Drawings

[0011] , [Figure 1] The figure is a cross-sectional view showing a configuration example of a differential device in which a gear according to an embodiment of the present invention is used as a pinion gear. , [Figure 2] It is an exploded perspective view of the differential device. , [Figure 3] It is a cross-sectional view taken along line A-A of FIG. 1. , [Figure 4] (a) and (b) are a cross-sectional view and a side view of a first pinion gear. , [Figure 5] (a) and (b) are micrographs of the structure in cross-sections near the tooth tip and near the tooth root of the first pinion gear. , [Figure 6] It is a graph showing first characteristic information and second characteristic information used in the gear design method. , [Figure 7] (a) is a graph showing the relationship between the value obtained by dividing the effective carburized depth by the normal module of the gear teeth and the residual compressive stress at the tooth tip and the tooth root. (b) is a graph showing the relationship between the value obtained by dividing the effective carburized depth by the normal module of the gear teeth and the value obtained by dividing the residual compressive stress at the tooth tip by the residual compressive stress at the tooth root.

Embodiments for Carrying Out the Invention

[0012] [Embodiment] Embodiments of the present invention will be described with reference to FIGS. 1 to 7. Note that the embodiments described below are shown as preferred specific examples for implementing the present invention, and although there are parts that specifically illustrate various technically preferable technical matters, the technical scope of the present invention is not limited to this specific embodiment.

[0013] FIG. 1 is a cross-sectional view showing a configuration example of a differential device in which a gear according to an embodiment of the present invention is used as a pinion gear (a first pinion gear 3 and a second pinion gear 4 described later). FIG. 2 is an exploded perspective view of the differential device. FIG. 3 is a cross-sectional view taken along line A-A of FIG. 1. FIGS. 4(a) and (b) are a cross-sectional view and a side view of the first pinion gear 3.

[0014] The differential device 1 allows differential of the driving force of the vehicle input to the housing 2 and outputs it from a pair of side gears 5 and 6. The drive source is, for example, an engine or an electric motor, and the pair of output shafts are, for example, left and right drive shafts. Further, the differential device 1 is arranged in an oil bath so that a part thereof is immersed in the lubricating oil enclosed in the transmission case.

[0015] The differential device 1 includes a housing 2 in which a plurality of bores (hollows) 20 are formed, a plurality of first and second pinion gears 3 and 4 respectively housed in the plurality of bores 20, a first side gear 5 meshed with the plurality of first pinion gears 3, a second side gear 6 meshed with the plurality of second pinion gears 4, a center washer 70 disposed between the first side gear 5 and the second side gear 6, and end washers 71 and 72 disposed between the first and second side gears 5 and 6 and the housing 2.

[0016] The first pinion gear 3 and the second pinion gear 4 are helical gears with tooth flanks inclined in the axial direction, and generate an axial thrust force by meshing with the first side gear 5 and the second side gear 6. Also, the first pinion gear 3 and the second pinion gear 4 are manufactured in the same process and have the same normal module, but the inclination directions of the tooth flanks are opposite to each other, and the axial arrangements within the bore 20 of the housing 2 are also opposite to each other.

[0017] The housing 2 and the first and second side gears 5, 6 are relatively rotatable about a common axis of rotation O. The housing 2 is composed of a housing body 21 and a housing cover 22. The housing body 21 is in the shape of a bottomed cylinder, and integrally has a cylindrical portion 211 formed in a cylindrical shape, a disk-shaped bottom portion 212 that closes one end of the cylindrical portion 211, and a flange portion 213 provided on the outer periphery of the other end of the cylindrical portion 211. The housing cover 22 integrally has a disk-shaped disk portion 221 and a flange portion 222 provided on the outer periphery of the disk portion 221.

[0018] The flange portion 213 of the housing body 21 and the flange portion 222 of the housing cover 22 are attached to each other, so that the bolt insertion holes 213a, 222a communicate with each other. Bolts for fixing a ring gear (not shown) are inserted into these bolt insertion holes 213a, 222a. The driving force of the driving source is input to the housing 2 from this ring gear, and the housing 2 rotates about the axis of rotation O. Note that the ring gear may be fixed to the housing 2 by welding.

[0019] A plurality of through holes 211a penetrating the cylindrical portion 211 in the radial direction are formed in the cylindrical portion 211 of the housing body 21, and the lubricating oil supplied into the cylindrical portion 211 from these through holes 211a lubricates the meshing between the first and second pinion gears 3, 4 with each other, and between the first and second pinion gears 3, 4 and the first and second side gears 5, 6.

[0020] In the cylindrical portion 211 of the housing body 21, a plurality of bores 20 extending parallel to the rotation axis O are formed from the end on the housing lid 22 side toward the bottom 212. In the present embodiment, five bores 20 are formed in the circumferential direction at equal intervals in the cylindrical portion 211, and one first pinion gear 3 and one second pinion gear 4 are respectively accommodated in each bore 20. The first and second pinion gears 3 and 4 are held rotatably within the bore 20 about the rotation axis O of the housing 2 as the revolution axis and about the rotation axis parallel to the rotation axis O as the rotation axis.

[0021] The first pinion gear 3 integrally has a columnar shaft portion 30, a long gear portion 31 provided at one end of the shaft portion 30, and a short gear portion 32 provided at the other end of the shaft portion 30. Similarly, the second pinion gear 4 integrally has a columnar shaft portion 40, a long gear portion 41 provided at one end of the shaft portion 40, and a short gear portion 42 provided at the other end of the shaft portion 40. The first and second pinion gears 3 and 4 are formed such that the axial length of the long gear portions 31 and 41 is longer than the axial length of the short gear portions 32 and 42. The long gear portion 31 of the first pinion gear 3 is meshed with the short gear portion 42 of the second pinion gear 4, and the short gear portion 32 of the first pinion gear 3 is meshed with the long gear portion 41 of the second pinion gear 4.

[0022] The first side gear 5 has a gear portion 51 composed of a plurality of external teeth 511 (shown in FIG. 3), and this gear portion 51 is meshed with the long gear portion 31 of the first pinion gear 3. Also, a spline fitting hole 52 is provided at the center of the first side gear 5, and one output shaft (for example, the drive shaft of the left wheel) is non-rotatably connected to this spline fitting hole 52. Similarly, the second side gear 6 has a gear portion 61 composed of a plurality of external teeth, and this gear portion 61 is meshed with the long gear portion 41 of the second pinion gear 4. A spline fitting hole 62 is provided at the center of the second side gear 6, and the other output shaft (for example, the drive shaft of the right wheel) is non-rotatably connected to this spline fitting hole 62.

[0023] As shown in Fig. 3, the bore 20 opens radially inwardly of the cylindrical portion 211 of the housing body 21. A part of the long gear portion 31 of the first pinion gear 3 protruding from the bore 20 meshes with the gear portion 51 of the first side gear 5, and a part of the long gear portion 41 of the second pinion gear 4 protruding from the bore 20 meshes with the gear portion 61 of the second side gear 6. A part of the axial direction of the long gear portion 31 of the first pinion gear 3 meshes with the gear portion 51 of the first side gear 5, and another part of the axial direction meshes with the short gear portion 42 of the second pinion gear 4. Similarly, a part of the axial direction of the long gear portion 41 of the second pinion gear 4 meshes with the gear portion 61 of the second side gear 6, and another part of the axial direction meshes with the short gear portion 32 of the first pinion gear 3.

[0024] Fig. 3 shows the periphery of the meshing portion between the long gear portion 31 of the first pinion gear 3 and the gear portion 51 of the first side gear 5. Fig. 4(a) shows a cross-section of the long gear portion 31 of the first pinion gear 3 together with the pitch circle PC, and Fig. 4(b) shows a side surface of a part of the long gear portion 31 together with the tooth width S. This tooth width S is the tooth width on the pitch circle PC in the direction perpendicular to the tooth flanks (width at right angles to the teeth). Fig. 4(a) corresponds to the cross-section taken along line B-B of Fig. 4(b). Although detailed illustration is omitted, the meshing portions between the long gear portion 41 of the second pinion gear 4 and the gear portion 61 of the second side gear 6 and the short gear portion 32 of the first pinion gear 3 are similarly configured.

[0025] The long gear portion 31 of the first pinion gear 3 has a base portion 311 inside the root circle and a plurality of teeth 312 provided to protrude radially outward from the base portion 311. Similarly, the short gear portion 42 of the second pinion gear 4 has a base portion 421 inside the root circle and a plurality of teeth 422 provided to protrude radially outward from the base portion 421.

[0026] In this embodiment, five teeth 312 are provided on the long gear portion 31 of the first pinion gear 3, and the same number of teeth 422 are also provided on the short gear portion 42 of the second pinion gear 4. Also, the pitch circle diameter (PCD) of the long gear portion 31 of the first pinion gear 3 is the same as the pitch circle diameter of the short gear portion 42 of the second pinion gear 4, and their respective modules (pitch circle diameter / number of teeth) are also the same. Hereinafter, the teeth 312 of the long gear portion 31 of the first pinion gear 3 are referred to as first teeth 312, and the teeth 422 of the short gear portion 42 of the second pinion gear 4 are referred to as second teeth 422.

[0027] The inner surface 200 of the bore 20 includes a first sliding surface 200a on which the tip surface 312a of the first tooth 312 slides, and a second sliding surface 200b on which the tip surface 422a of the second tooth 422 slides. The first sliding surface 200a and the second sliding surface 200b are each formed in an arc shape when viewed in the axial direction of the housing body 21.

[0028] Due to the driving force transmitted from the housing 2 to the first pinion gear 3, as well as the meshing reaction force with the gear portion 51 of the first side gear 5 and the short gear portion 42 of the second pinion gear 4, the tip surface 312a of the first tooth 312 of the long gear portion 31 of the first pinion gear 3 is pressed against the first sliding surface 200a of the bore 20. Also, due to the driving force transmitted from the housing 2 to the second pinion gear 4, as well as the meshing reaction force with the long gear portion 31 of the first pinion gear 3, the tip surface 422a of the second tooth 422 of the short gear portion 42 of the second pinion gear 4 is pressed against the second sliding surface 200b of the bore 20.

[0029] When a rotational difference occurs between the left and right wheels of the vehicle and differential (relative rotation) occurs between the first side gear 5 and the second side gear 6, while the first tooth 312 and the second tooth 422 are engaged, the first pinion gear 3 and the second pinion gear 4 rotate within the bore 20. At this time, the first pinion gear 3 rotates while generating a frictional resistance force with the housing 2 due to the sliding of the tooth tip surface 312a, and the second pinion gear 4 rotates while generating a frictional resistance force with the housing 2 due to the sliding of the tooth tip surface 422a. Also, similarly in the short gear portion 32 of the first pinion gear 3 and the long gear portion 41 of the second pinion gear 4, a frictional resistance force is generated with the housing 2 due to the sliding of the tooth tip surface. These frictional resistance forces become differential limiting forces that suppress the differential of the first side gear 5 and the second side gear 6.

[0030] The first pinion gear 3 and the second pinion gear 4 are subjected to carburizing heat treatment in order to improve wear resistance and enhance durability. Carburizing heat treatment is a process in which carbon is infiltrated and diffused into the surface of the steel material, followed by quenching and tempering. The effective case depth (ECD: Effective Case Depth), which is the distance from the surface of the portion where the Vickers hardness becomes 550HV, varies depending on the carburizing temperature and carburizing time. Note that the effective case depth at the tooth tip and tooth root is the depth in the radial direction perpendicular to the central axis. The effective case depth on the tooth surface is the depth in the direction perpendicular to the tooth surface.

[0031] Figure 5(a) is a micrograph of the cross-section near the tooth tip 31a of the first tooth 312 in the first pinion gear 3 (part A in Figure 4(a)). Figure 5(b) is a micrograph of the cross-section near the tooth root 31b of the long gear portion 31 of the first pinion gear 3 (part B in Figure 4(a)). In Figures 5(a) and (b), the higher the proportion of infiltrated carbon, the lighter the color. Also, in Figures 5(a) and (b), the effective case depth (ECD), which is the distance between the point P1 on the surface of the steel material and the point P2 where the hardness becomes 550HV in the depth direction perpendicular to the surface at point P1, is shown.

[0032] Incidentally, even if the differential devices mounted on large vehicles such as trucks and those mounted on ordinary passenger cars have the same structure, the tooth specifications may differ to ensure strength. Therefore, even if the effective case depth of the first and second pinion gears 3 and 4 of the differential device 1 mounted on a certain vehicle type is directly applied to the pinion gears of other differential devices, the required strength and durability may not be satisfied. In particular, the pinion gear rotates while generating frictional resistance on the tooth tip surface to transmit the driving force to the side gear, so it is a component with a large load and is likely to be damaged. For this reason, conventionally, every time a new differential device was developed, it was necessary to trial-produce and test a plurality of pinion gears with different effective carburizing depths to find an appropriate effective carburizing depth, but this required a great deal of cost and time.

[0033] The gear design method described below makes it possible to easily obtain an appropriate effective case depth according to the module of the gear in view of such circumstances. This design method is based on the findings that there is a certain common correlation between the residual compressive stress, tensile strength, and average hardness of the gear after carburizing heat treatment, even if the modules are different, and that there is a predetermined proportional relationship between the average hardness and the effective case depth for each module. Here, the average hardness refers to the average hardness in the range from the surface to a predetermined depth. In the present embodiment, this predetermined depth is set to half the tooth width S (S / 2) at a tooth right angle in the direction perpendicular to the tooth flanks.

[0034] Next, this gear design method will be described in detail based on a specific example. In the following description, a helical gear (helical gear) with tooth flanks inclined in the axial direction is the object of design, and the normal module is used as the value of the module. The normal module m n is obtained by the following formula (1). m n = D P ·cosβ / n …(1) However, D P is the pitch diameter, β is the helix angle, and n is the number of teeth. Note that the normal module mn and the module m perpendicular to the axis t There is the following relationship between them and the helical angle β. For spur gears with tooth flanks parallel to the axial direction, β becomes zero, so the module perpendicular to the tooth and the module perpendicular to the axis have the same value. m t =m n / cosβ …(2)

[0035] (Gear design method) The gear design method according to the present embodiment refers to the first characteristic information showing the relationship between the average hardness, the residual compressive stress, and the tensile strength, and obtains an appropriate range (hardness range) of the average hardness at which the desired residual compressive stress and tensile strength can be obtained in a first step. In a second step, referring to the second characteristic information showing the relationship between the average hardness and the effective case depth for each module perpendicular to the tooth of the gear, a range (depth range) of the effective case depth corresponding to the hardness range obtained in the first step is obtained. Further, in the present embodiment, as the first characteristic information, information showing the relationship between the average hardness, the residual compressive stress at the tooth tip, the residual compressive stress at the tooth root, and the tensile strength is used, and in the first step, a range of the average hardness at which the desired residual compressive stress at the tooth tip, the residual compressive stress at the tooth root, and the tensile strength can be obtained is obtained as the appropriate range.

[0036] FIG. 6 shows graphs representing the first characteristic information 8 and the second characteristic information 9 arranged vertically. In the upper graph of the first characteristic information 8, the horizontal axis represents the average hardness, the left vertical axis represents the residual compressive stress, and the right vertical axis represents the tensile strength. In the lower graph of the second characteristic information 9, the horizontal axis represents the average hardness, and the vertical axis represents the effective case depth.

[0037] The first characteristic information 8 is obtained based on the experimental results of variously changing the carburizing and heat treatment conditions for a plurality of differential device pinion gears with a normal module of 2.0 to 2.5, and shows the relationship between the average hardness from 550 to 700 HV, the residual compressive stress at the tooth tip and tooth root, and the tensile strength. The average hardness may be obtained by averaging the hardness at a plurality of measurement points between the depth from the tooth surface (depth zero) on the pitch diameter to half of the tooth width S on the normal plane perpendicular to the tooth flanks. For example, an approximate function representing the hardness corresponding to the depth is generated based on the depths at the positions of Vickers hardness of 550 HV and 700 HV, and the integral value from depth 0 to half of the tooth width in this approximate function is divided by half of the tooth width to obtain it. The residual compressive stress is obtained from the measurement results of the interatomic distance by X-ray diffraction method. The tensile strength can be obtained by applying the average hardness obtained as described above to this relationship based on the general relationship between the hardness and tensile strength of the metal material. Note that the tensile strength of the metal material is measured by the metal material tensile test method specified in JIS Z2241.

[0038] As shown in the graph of the first characteristic information 8, the residual compressive stress at the tooth root decreases as the average hardness increases. The residual compressive stress at the tooth tip reaches a maximum value (about 525 MPa) at around 625 HV of the average hardness, and when the average hardness increases or decreases from 625 HV, the residual compressive stress at the tooth tip gradually decreases from the maximum value. Also, the tensile strength increases as the average hardness increases.

[0039] In the second characteristic information 9 shown in FIG. 6, for the cases where the normal module is 2.2 and 2.4, the relationship between the average hardness and the effective carburized depth is shown. The average hardness and the effective carburized depth are in a substantially proportional relationship, and as the normal module increases, the effective carburized depth with respect to the average hardness becomes larger (deeper). Note that the number of teeth of the pinion gear of the differential device is generally 5 to 7.

[0040] For example, when the required residual compressive stress at the tooth tip and tooth root is 400 MPa or more and the tensile strength is 1900 MPa or more, as shown in FIG. 6, the appropriate range of the average hardness obtained by referring to the first characteristic information 8 is about 585 to 635 HV. Further, the range of the effective carburized depth corresponding to the appropriate range of the average hardness obtained by referring to the second characteristic information 9 is about 0.9 to 1.3 mm as shown in FIG. 6 when the normal module of the tooth is 2.4. Therefore, if carburizing heat treatment is performed so that the effective carburized depth is, for example, 1.1 ± 0.2 mm, the desired residual compressive stress and tensile strength can be obtained. Thus, according to the gear design method according to the present embodiment, it is possible to easily obtain an appropriate effective carburized depth corresponding to the normal module of the tooth.

[0041] As shown by the dashed-dotted line in FIG. 6, the upper limit value of the appropriate range of the average hardness may be extended to, for example, 660 HV. This is because increasing the tensile strength is particularly effective for improving the durability against shear stress. When the average hardness is increased, the residual compressive stress at the tooth root particularly decreases. However, in a pinion gear for a differential gear device having 5 to 7 teeth, since the normal module of the tooth is larger and the pressure angle is larger compared to a general gear, pitching and flaking near the tooth root due to the decrease in the residual compressive stress tend not to occur easily. For this reason, the adverse effect due to the decrease in the residual compressive stress at the tooth root hardly occurs, and by increasing the average hardness, a pinion gear with high durability against shear stress can be obtained while ensuring a high residual compressive stress at the tooth tip. When the average hardness is 660 HV in FIG. 6, the effective carburized depth is about 1.55 mm.

[0042] (Appropriate residual compressive stress ratio at the tooth tip and tooth root of the gear) FIG. 7(a) is a graph showing the relationship between the value obtained by dividing the effective carburized depth by the normal module of the tooth (horizontal axis) and the residual compressive stress at the tooth tip and tooth root (vertical axis). FIG. 7(b) is a graph showing the relationship between the value obtained by dividing the effective carburized depth by the normal module of the tooth (horizontal axis) and the value obtained by dividing the residual compressive stress at the tooth tip by the residual compressive stress at the tooth root (vertical axis).

[0043] As shown in Fig. 6, the residual compressive stress at the tooth tip and the residual compressive stress at the tooth root become equal when the average hardness is about 585 HV, and in the ranges before and after that, both the residual compressive stress at the tooth tip and the residual compressive stress at the tooth root become high values. In other words, in the range where the difference between the residual compressive stress at the tooth tip and the residual compressive stress at the tooth root is large, either the residual compressive stress at the tooth tip or the residual compressive stress at the tooth root becomes small, which becomes the weakest part of the gear (for example, the pinion gear for a differential device), and damage is likely to occur.

[0044] Therefore, the ratio of the residual compressive stress at the tooth tip to the residual compressive stress at the tooth root shown in Fig. 7(b) (hereinafter, this ratio (tooth tip residual compressive stress / tooth root residual compressive stress) is referred to as the residual compressive stress ratio) is desirably 0.8 or more and 1.3 or less. Also, within this range, it is possible to ensure a tensile strength that can prevent damage caused by shear stress. Therefore, according to a gear with a residual compressive stress ratio of 0.8 or more and 1.3 or less, it is possible to suppress damage caused by shear stress while increasing the residual compressive stress to suppress pitting and flaking.

[0045] In addition, for the pinion gear for a differential device, due to the inclination within the bore generated by the differential torque (the difference between the torque transmitted to one side gear and the torque transmitted to the other side gear) and the frictional sliding with the inner surface of the bore of the housing in a state where the differential torque is generated, especially the tooth tip part is under high load. Therefore, it is desirable that the residual compressive stress at the tooth tip is higher than the residual compressive stress at the tooth root, that is, it is desirable that the residual compressive stress ratio is greater than 1.0. The residual compressive stress ratio can be increased or decreased, for example, by adjusting the effective case depth. For example, in the example shown in Fig. 6, the effective case depth corresponding to an appropriate range where the residual compressive stresses at the tooth tip and the tooth root are both 400 MPa or more and the residual compressive stress at the tooth tip is higher than the residual compressive stress at the tooth root (about 595 - 635 HV) is obtained, so that a gear with the residual compressive stress at the tooth tip higher than the residual compressive stress at the tooth root can be obtained.

[0046] In the above gear design method, although the description mainly focuses on the case of the pinion gear of the differential device, the range of the effective carburized depth obtained for the pinion gear may be directly applied to the side gear of the differential device. Since the side gear has a larger pitch diameter and more teeth than the pinion gear, it is easier to ensure strength and wear resistance than the pinion gear. If the required strength and wear resistance are obtained for the pinion gear, even if the same carburizing treatment is applied to the side gear, the strength and wear resistance of the entire differential gear will not be impaired. In addition, by making the carburizing treatments for the pinion gear and the side gear common, it becomes possible to reduce the cost associated with the carburizing treatment.

[0047] (Supplementary Note) As described above, the present invention has been explained based on the embodiments, but these embodiments do not limit the invention according to the claims. It should also be noted that not all combinations of the features described in the embodiments are essential means for solving the problems of the invention. In addition, the present invention can be appropriately modified by omitting some configurations, adding or replacing configurations, within the scope not departing from its gist. Furthermore, it is possible to combine some configurations of the above-described multiple embodiments with each other, and it is also possible to make modifications as follows, for example.

[0048] In the above-described embodiment, the case where the first and second pinion gears 3 and 4 of the differential device 1 each have a long gear portion 31, 41 and a short gear portion 32, 42 has been described. However, the configuration of the differential device is not limited to this, and the present invention can be applied to the pinion gears of differential devices having various configurations and their designs. For example, a plurality of pinion gears having different axial lengths are accommodated in each of a plurality of bores of the housing, and one of the pinion gears having a long axial length is meshed with a large-diameter side gear, and the other pinion gear having a short axial length is meshed with one of the pinion gears and a small-diameter side gear. The present invention may be applied to the pinion gears of the differential device having such a configuration. Further, the present invention may be applied to the pinion gears of a differential device configured to accommodate pinion gears in a plurality of accommodation holes of an annular cage disposed between an internal gear and an external gear, and mesh these pinion gears with the internal gear and the external gear. In this case, the tooth tip surface of the pinion gear slides on the inner surface of the accommodation hole of the cage to generate frictional resistance.

Explanation of Signs

[0049] 1... Differential device 2... Housing 20... Bore 200... Inner surface 3... First pinion gear (gear) 312a... Tooth tip surface 4... Second pinion gear (gear) 422a... Tooth tip surface 5... First side gear 6... Second side gear

Claims

1. A carburized and heat-treated gear that rotates while generating frictional resistance by the sliding of the tooth tip surface, wherein the value obtained by dividing the residual compressive stress at the tooth tip by the residual compressive stress at the tooth root is 0.8 or more and 1.3 or less, and the residual compressive stress at the tooth tip and the residual compressive stress at the tooth root are each 400 MPa or more, a gear.

2. In a differential device that inputs the driving force of a vehicle into a housing and outputs it from a pair of side gears while allowing differential, it is used as a pinion gear that meshes with one of the pair of side gears, and the tooth tip surface generates frictional resistance by sliding on the inner surface of a bore formed in the housing, the gear according to claim 1.

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