Sintered gear

Abstract

To provide a sintered gear having high quietness.SOLUTION: An addendum circle is 30 to 200mm, a densified layer is formed on at least a part of a toothed surface of the toothed portion, a thickness of the densified layer is 10 to 1000 μ m, and an overall density according to the Archimedes method is 6.4 to 7. 4g / cm3.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to sintered gears. [Background technology]

[0002] Sintered gears are generally sintered bodies obtained by sintering a gear-shaped green compact obtained by pressing a metal powder-filled die from above and below with a punch. Because these sintered gears can be mass-produced inexpensively, they are offered as a wide variety of products for a variety of applications. Sintered gears have pores remaining in the metal matrix due to their manufacturing method. Because the pores absorb vibrations, sintered gears are characterized by superior vibration damping compared to gears made from melt-cast steel. Patent Document 1 discloses a sintered gear with controlled porosity in the densified layer of the tooth surface of the tooth portion, as a highly quiet sintered gear. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2004 / 030852 Summary of the Invention [Problem to be solved by the invention]

[0004] Helical gears offer superior driving force and quietness compared to conventional spur gears. Helical gears are used in drive motors for electric vehicles (EVs) and other vehicles to increase the meshing ratio between the gears to improve driving force and also to improve quietness through smooth meshing. To date, large sintered helical gears have not been put to practical use. It has been thought that large sintered bodies tend to vibrate more when sintered gears are driven, which can easily cause driving noise.

[0005] An object of the present disclosure is to provide a sintered gear that is highly quiet. [Means for solving the problem]

[0006] The present disclosure includes the following embodiments.

[0007] [1] A toothed wheel having a main body and teeth formed on the outer peripheral surface of the main body, the tooth width direction of which is inclined with respect to the axial direction, the tooth tip diameter being 30 to 200 mm, a densified layer being formed on at least a part of the tooth surface of the tooth, the thickness of the densified layer being 10 to 1000 μm, and the overall density according to Archimedes' law being 6.4 to 7.4 g / cm 3 That is, sintered gears.

[0008] [2] The density Dc of the central 80% portion of the tooth in the tooth width direction and the density De of the portion from at least one end to 10% in the tooth width direction are De-Dc>0.1 g / cm 3 and the densities Dc and De are determined according to Archimedes' law.

[0009] [3] A sintered gear according to [1] or [2], wherein the density Dc of the tooth portion in the central 80% of the tooth width direction and the density De of the tooth portion in the tooth width direction from at least one end to 10% of the tooth width satisfy De / Dc>1, and the densities Dc and De are calculated according to Archimedes' law.

[0010] [4] A sintered gear according to any one of [1] to [3], wherein the porosity Pc of the tooth portion in the central 80% of the tooth width direction and the porosity Pe of the tooth portion in the tooth width direction from at least one end to 10% of the tooth width satisfy Pe-Pc<-1%, and the porosity Pc and the porosity Pe are calculated according to Archimedes' law.

[0011] [5] A sintered gear according to any one of [1] to [4], wherein the porosity Pc of the tooth portion in the central 80% portion in the tooth width direction and the porosity Pe of the tooth portion in the tooth width direction from at least one end to 10% satisfy Pe / Pc<1, and the porosity Pc and the porosity Pe are calculated according to Archimedes' law.

[0012] [6] A sintered gear according to any one of [1] to [5], wherein the density Da of the tooth surface on one side in the outer circumferential direction of the tooth portion is different from the density Db of the tooth surface on the other side in the outer circumferential direction of the tooth portion, and the densities Da and Db are determined by an image analysis method.

[0013] [7] Density Da and density Db are Da-Db>0.1g / cm 3 [6] A sintered gear according to [6], which satisfies the above.

[0014] [8] A sintered gear according to [6] or [7], wherein the density Da and the density Db satisfy Da / Db>1.

[0015] [9] A sintered gear according to any one of [1] to [8], wherein the porosity Pa of one tooth surface in the circumferential direction of the tooth portion is different from the porosity Pb ​​of the other tooth surface in the circumferential direction of the tooth portion, and the porosity Pa and the porosity Pb ​​are determined by an image analysis method.

[0016]

[10] The sintered gear according to [9], wherein the porosity Pa and the porosity Pb ​​satisfy Pa-Pb<-1%.

[0017]

[11] The sintered gear according to [9] or

[10] , wherein the porosity Pa and the porosity Pb ​​satisfy Pa / Pb<1.

[0018]

[12] A sintered gear according to any one of [1] to

[11] , wherein the main body has a shaft hole, and in a diametric cross section at the axial center of the main body, the matrix hardness Hi of a portion 5 mm from the inner peripheral end of the shaft hole to the outer periphery in the diametric direction and the matrix hardness Ho of a portion 0.5 mm from the tooth bottom to the inner periphery in the diametric direction satisfy Ho-Hi>Hv50.

[0019]

[13] A sintered gear according to any one of [1] to

[11] , wherein the main body has a shaft hole, and in a diametric cross section at the axial center of the main body, the matrix hardness Hi of a portion 5 mm from the inner peripheral end of the shaft hole to the outer periphery in the diametric direction and the matrix hardness Ho of a portion 0.5 mm from the tooth bottom to the inner periphery in the diametric direction satisfy Ho / Hi>1.

[0020]

[14] The sintered gear according to any one of [1] to

[13] , wherein the overall composition contains 0.1 to 1.4 mass % of C, with the remainder being Fe and unavoidable impurities.

[0021]

[15] The sintered gear according to any one of [1] to

[13] , which is an Fe—Ni—Mo—C alloy or an Fe—Cu—C alloy. [Effects of the Invention]

[0022] According to the embodiments of the present disclosure, a sintered gear having high quietness can be provided. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 1 is a perspective view showing an example of a sintered gear. [Figure 2] FIG. 2 is a side view of the sintered gear shown in FIG. [Figure 3] FIG. 3 is a front view of the sintered gear shown in FIG. 1 as viewed from the axial direction. [Figure 4] FIG. 4 is a diametrical end view of the sintered gear shown in FIG. 1 at the axial center thereof. [Figure 5] FIG. 5 is a diametric end view of the teeth of the sintered gear shown in FIG. [Figure 6] FIG. 6 shows the specified range of face widths for the sintered gears in FIG. [Figure 7] FIG. 7 shows the designated lengths in an enlarged view of the sintered gear in FIG. [Figure 8] FIG. 8 is a graph showing the matrix hardness of the sintered gears measured in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0024] Several embodiments of the present disclosure will be described in detail below, but these are merely examples and the present invention is not limited to these examples.

[0025] In the present disclosure, a numerical range indicated using "to" indicates a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in the present disclosure, the upper or lower limit of a numerical range in a certain stage can be arbitrarily combined with the upper or lower limit of a numerical range in another stage. In the numerical ranges described in the present disclosure, the upper or lower limit of the numerical range may be replaced with a value shown in the examples. In the present disclosure, the term "step" includes not only an independent step, but also a step that cannot be clearly distinguished from other steps, as long as the intended effect of the step is achieved. Unless otherwise specified, an element refers to either a single element or multiple elements.

[0026] <Density and porosity by Archimedes method> The density and porosity of the sintered body can be determined by measuring the dry weight, oil-soaked weight, and water weight of the sintered body according to the Archimedes method specified in JIS Z2501. The detailed conditions for measuring the density are as follows: Testing machine: Electronic balance (A&D Co., Ltd. "GR-202") Temperature: Room temperature (25℃) The conditions for measuring the oil-immersed mass are as follows. Oil: Killer spindle oil (specific gravity: 0.856) Pressure: 60kPa (vacuum degree) Decompression time: 30 minutes (until no more bubbles appear) After decompression release: Keep in oil for 5 minutes Wipe off the oil on the surface and measure the mass to four decimal places using an electronic balance. When the sintered body is an iron-based sintered body, the porosity is calculated from the density of the sintered body by multiplying the true density of iron by 7.87 g / cm 3 When the sintered body is a sintered body of another metal, the porosity can be calculated using the true density of the appropriate bulk metal.

[0027] <Density and porosity by image analysis> The density and porosity of a sintered body can also be determined by image analysis of a microscope image. More specifically, five or more images of multiple observation fields are randomly taken at 200x magnification on the surface or cross section of a sintered body. It is preferable that the images of each observation field are taken from positions as evenly distributed as possible on the surface. Next, the porosity can be determined and the density can be calculated by analyzing the images using image analysis software.

[0028] <Base hardness> The matrix hardness of a sintered body is the Vickers hardness (Hv) of the surface of the matrix excluding the pores of the sintered body. Specifically, it is the Hv when a load of 100 g is applied to the surface of the matrix for high hardness and a load of 10 g is applied to the surface of the matrix for low hardness. Hv is measured by the method specified in JIS Z 2244.

[0029] The present disclosure relates to a sintered gear including a main body and teeth formed on the outer peripheral surface of the main body, the teeth being inclined in the tooth width direction relative to the axial direction. The sintered gear of the present disclosure will be described below with reference to the drawings. The drawings used in the following description all show schematic views of sintered gears, and the present invention is not limited to the specific examples shown in the drawings.

[0030] Fig. 1 is a perspective view of an example of a sintered gear. Fig. 2 is a side view of the sintered gear shown in Fig. 1. Fig. 3 is a front view of the sintered gear shown in Fig. 1 from the axial direction. Fig. 4 is a diametric end view of the axial center of the sintered gear shown in Fig. 1. Fig. 5 is a cross-sectional view of a tooth portion of an example of a sintered gear.

[0031] In this disclosure, referring to FIG. 5, the tooth portion 20 includes a tooth tip 21, which is the tip surface of the tooth portion, a tooth base 22, which is the root of the tooth portion, a tooth bottom 24 between adjacent tooth bases, and tooth surfaces 23a and 23b, which are the meshing surfaces of the teeth. The circle connecting the tooth bases is called the tooth bottom circle, and the bottom surface of the tooth located on the tooth bottom circle is called the tooth bottom. The tooth depth of the tooth portion 20 is the distance from the tooth bottom to the tooth tip, and is the sum of the tooth base and tooth addendum. The thickness of the tooth portion at the tooth base in the outer circumferential direction is the tooth thickness. The tooth tip circle diameter means the diameter of the circle connecting the tips of the tooth portions.

[0032] In FIG. 1, the sintered gear 100 has a main body 10 and a toothed portion 20. The main body 10 is cylindrical, and the toothed portion 20 is formed on its outer circumferential surface. The main body 10 may have a shaft hole 30 in its center. A shaft supporting the sintered gear 100 can be inserted into the shaft hole 30. The toothed portion 20 has a tooth width direction that is inclined with respect to the axial direction of the main body 10. The number and pitch of the toothed portion 20 formed on the outer circumferential surface of the main body 10 are not limited and can be set appropriately depending on the desired sintered gear. The toothed portion 20 also has a tooth depth and a tooth thickness that can be set appropriately depending on the desired sintered gear. Such a sintered gear 100 can be provided as a helical gear. In another example, the main body 10 may be cylindrical, and the main body 10 may be integrally molded with the shaft.

[0033] The sintered gear 100 is preferably a metal sintered body, more preferably an iron-based sintered body. A metal sintered body is a sintered body obtained by sintering a compact of metal powder. The main body 10 and the teeth 20 are preferably an integrally molded body, but may also be made of separate members and joined together. If they are separate members, at least the teeth 20 is preferably a metal sintered body.

[0034] According to some embodiments, the tooth tip diameter is 30 to 200 mm, a densified layer is formed on at least a part of the tooth surface of the tooth portion, the densified layer has a thickness of 10 to 1000 μm, and the overall density according to Archimedes' law is 6.4 to 7.4 g / cm 3 This provides a sintered gear that is rated at 1000 W. Compared to ordinary spur gears, this type of sintered gear has a higher meshing ratio between the gears, which makes the teeth less susceptible to damage and increases its relative strength. Furthermore, the continuous meshing of the teeth results in smoother rotation and improved quietness. Furthermore, when the diameter of the tooth tip circle of a sintered gear exceeds 30 mm, it becomes difficult to mold the gear, which can lead to problems with vibration damping. Therefore, it has been difficult to produce large-sized sintered gears, and it has also been considered that quietness is also an issue. However, although sintered gears have pores due to the manufacturing process, when the overall density is 6.4 to 7.4 g / cm3, 3By controlling the thickness within this range, the pores can absorb vibrations while maintaining strength, making it possible to achieve quietness even in large-sized sintered gears. In this case, by setting the thickness of the densified layer on at least a portion of the tooth flank of the tooth portion to 10 to 1000 μm, vibrations transmitted from the tooth flank can be easily absorbed, making it possible to achieve quietness while maintaining the strength of the tooth flank.

[0035] From this perspective, the tooth tip diameter of the sintered gear may be 30 to 200 mm, 32 to 150 mm, or 34 to 120 mm. A diameter of 30 mm or more can provide a sintered gear with a greater driving force. A diameter of 200 mm or less can provide a gear with a larger number of teeth to increase the reduction ratio.

[0036] Sintered gears have an overall density of 6.4 to 7.4 g / cm 3 With an overall density of this level, strength is provided and the gear has excellent vibration absorption properties, resulting in improved quietness. From this perspective, the overall density of the sintered gear is 6.7 to 7.3 g / cm. 3 , 6.5~7.3g / cm 3 , or 6.6 to 7.2 g / cm 3 It is more preferable that the total density of the sintered gear satisfies these ranges when the sintered gear is an iron-based sintered body. In the present disclosure, the total density of the sintered gear can be measured by the Archimedes method as described above, using the sintered gear itself as a measurement sample.

[0037] The sintered gear preferably has a total porosity of 6.0 to 18.7%. This level of total porosity provides strength and excellent vibration absorption, further improving quietness. From this perspective, the total porosity of the sintered gear is more preferably 7.2 to 17.4%, or 8.5 to 16.1%. In the present disclosure, the total porosity can be measured from the density by the Archimedes method, as described above, using the sintered gear itself as a measurement sample.

[0038] The sintered gear has a densified layer formed on at least a portion of the tooth surface of the tooth portion, and the thickness of the densified layer may be 10 to 1000 μm. The densified layer can be formed by applying pressure to the surface of the tooth portion during the manufacturing process of the sintered gear. One form of the densified layer is observed on the surface of the sintered gear, where the surface is smoothed and porosity is reduced. The densified layer can be observed by observing the density difference between the surface layer and the center in the depth direction from the surface of the sintered gear on a cross section of the sintered gear in the diameter direction. The densified layer observed in the surface layer is observed as a layer in which porosity is reduced compared to the center and crystal particles are densely packed. The thickness of the densified layer is the distance from the surface of the sintered gear in the depth direction when the sintered gear is cut in the diameter direction. The thickness of the densified layer is determined by measuring the tooth surface at three or more random locations and taking the arithmetic average of the measurements.

[0039] The thickness of the densified layer of the sintered gear may be 10 μm or more, 50 μm or more, or 100 μm or more. The thickness of the sintered gear may be 1000 μm or less, 900 μm or less, or 800 μm or less. For example, a thickness of 10 to 1000 μm, 50 to 900 μm, or 100 to 800 μm is more preferable. Within these ranges, vibrations transmitted from the tooth surface are easily absorbed, and the tooth surface can be made strong while also being quiet. A thickness of 10 μm to 500 μm is more preferable, and even more preferable is a thickness of 10 μm to 300 μm.

[0040] First Embodiment The first embodiment will be described below. One aspect of this embodiment is based on the fact that the density Dc of the central 80% of the tooth in the tooth width direction and the density De of the tooth in at least one end portion extending from 10% to 15% on either side of the tooth width direction relate to the quietness of a sintered gear. Another aspect of this embodiment is based on the fact that the porosity Pc of the central 80% of the tooth in the tooth width direction and the porosity Pe of the tooth in at least one end portion extending from 10% to 15% on either side of the tooth width direction relate to the quietness of a sintered gear.

[0041] The sintered gear preferably satisfies the following conditions: (1a) Density Dc and density De are greater than or equal to 0.1 g / cm 3 Meet the following. (1b) The density Dc and the density De satisfy De / Dc>1. (1c) The porosity Pc and the porosity Pe satisfy Pe-Pc<-1%. (1d) The porosity Pc and the porosity Pe satisfy Pe / Pc<1. (1e) Satisfy at least two combinations of (1a) to (1d).

[0042] The central 80% portion of the tooth portion in the tooth width direction is the portion indicated by c in FIG. 6. The portion of the tooth portion extending from at least one end in the tooth width direction to 10% is the portion indicated by e1 or e2 in FIG. 6. It is sufficient that at least one of the portions e1 and e2 extending from the end to 10% satisfies the above condition, but it is more preferable that both satisfy the above condition. In a sintered gear, it is preferable that at least one tooth portion of the multiple tooth portions satisfies the above condition, but it is more preferable that more than half, 80% or more, or 90% or more of the multiple tooth portions satisfy the above condition, and it is even more preferable that all of the multiple tooth portions satisfy the above condition.

[0043] The specimen for measuring physical properties of the central 80% portion c is obtained by cutting the tooth portion from the tooth root surface of the sintered gear and then cutting out the central 80% portion in the face width direction. Similarly, the specimens for measuring physical properties of the end portions e1 and e2 are obtained by cutting the tooth portion from the tooth root surface of the sintered gear and then cutting out the end portions in the face width direction. The density and porosity of the obtained specimen can be measured by the Archimedes method as described above.

[0044] In the sintered gear of this embodiment, the density is low in the center of the tooth and high in the end. This ensures high strength due to the high density at the end, while the low density at the center creates a density difference. If the gear density were uniform overall, vibrations generated on the tooth surfaces by meshing would be transmitted to the shaft on which the gear is inserted through the sintered matrix. In contrast, as mentioned above, if there is a density difference between the center and end of the tooth, vibrations tend to be selectively transmitted to the high-density areas, so vibrations propagating from the vibration-generating position are diffused along the gear surface and gradually attenuated as they propagate. In this way, creating a density difference between the center and end of the tooth improves vibration damping, leading to improved quietness.

[0045] From this perspective, it is more preferable that: In the above (1a), De-Dc may be greater than 0.1, greater than 0.2, or greater than 0.3. For example, 0.1 <De-Dc<0.8であってよい。 In the above (1b), De / Dc may be greater than 1. For example, 1.0 <De / Dc<1.2であってよい。 In the above (1c), Pe-Pc may be less than -1.0, less than -2.5, or less than -3.8. For example, -10 <Pe-Pc<-1.0であってよい。 In the above (1d), Pe / Pc<1, Pe / Pc<0.8, or Pe / Pc<0.6 may be satisfied. For example, 0.05 <Pe / Pc<1.0であってよい。

[0046] Each of (1a) to (1d) may be satisfied alone, but two, three, or all four may also be satisfied.

[0047] The density Dc of the central 80% portion c of the sintered gear is not particularly limited, and is 6.7 to 6.9 g / mm 3 The density De of the portions e1 and e2 of the sintered gear that are 10% from the end is not particularly limited, and may be 7.1 to 7.3 g / mm 3At least one of the portion e1 and the portion e2 of the end of the sintered gear may be within this range, and it is even better if both are within this range.

[0048] The sintered gear according to this embodiment is not particularly limited and can be obtained by appropriately controlling each step in a method of preparing a metal powder, press-molding the metal powder into a powder compact, and sintering the powder compact to obtain a sintered body.

[0049] For example, in the metal powder pressure molding process, there are methods such as filling the metal powder into a die and applying a load to the die in both axial directions using upper and lower punches, increasing the speed at which the metal powder is pressed, filling the metal powder into a die and warm-molding or cold-molding so that the temperature at the center of the die is relatively lower than at the ends, pressing so that the density of the molded body is lower than that expected from the calculated value of the metal powder, and pressing after setting the density of the molded body to be low in advance.

[0050] Other examples include a method of increasing the compression ratio by reducing the content of molding lubricant in the metal powder preparation step, a method of reducing the amount of molding lubricant used in the die and punch in the metal powder compression molding step, a method of varying the metal powder filled in the die in stages in the axial direction in the metal powder compression molding step, and filling the center of the die with metal powder that has relatively higher compressibility than the end portions, etc. Also, other examples include a method of making the rolling allowance of the teeth of the green compact relatively thinner in the center than in the end portions in the metal powder compression molding step.

[0051] <Second embodiment> A second embodiment will be described below. One aspect of this embodiment is based on the fact that the difference between the density Da of the tooth flank on one side of the outer periphery of the tooth portion and the density Db of the tooth flank on the other side of the outer periphery of the tooth portion relates to the quietness of a sintered gear. Another aspect of this embodiment is based on the fact that the difference between the porosity Pa of the tooth flank on one side of the outer periphery of the tooth portion and the porosity Pb ​​of the tooth flank on the other side of the outer periphery of the tooth portion relates to the quietness of a sintered gear.

[0052] The sintered gear preferably satisfies the following conditions: (2a) Density Da and density Db are Da-Db>0.1 g / cm 3 Meet the following. (2b) The density Da and the density Db satisfy Da / Db>1. (2c) The porosity Pa and the porosity Pb ​​satisfy Pa-Pb<-1%. (2d) The porosity Pa and the porosity Pb ​​satisfy Pa / Pb<1. (2e) Satisfy at least two combinations of (2a) to (2d).

[0053] The tooth flank on one side of the tooth portion in the outer circumferential direction is the surface indicated by 23a in Fig. 5. The tooth flank on the other side of the tooth portion in the outer circumferential direction is the surface indicated by 23b in Fig. 5. In a sintered gear, it is preferable that at least one tooth portion out of the plurality of tooth portions satisfies the above condition, but it is more preferable that at least half, 80% or more, or 90% or more of the plurality of tooth portions satisfies the above condition, and it is even more preferable that all of the plurality of tooth portions satisfies the above condition.

[0054] The density and porosity of the tooth surfaces of the sintered gears were calculated by analyzing images of the metal structure of the tooth surface taken at a magnification of 200 times using image analysis software to determine the porosity. Measurements were taken at three or more random locations on each of the tooth surfaces 23a and 23b, and the average values ​​of these measurements were used as the density and porosity of the tooth surfaces.

[0055] In the sintered gear of this embodiment, the density is high on one tooth surface and low on the other. Because the gear is unidirectional, the high density on one surface that meshes with the gear ensures greater strength. Furthermore, the low density on the other surface allows for better absorption of vibrations, resulting in improved vibration damping.

[0056] When meshing a sintered gear with a counter gear, it is preferable that the tooth surface 23a of the toothed portion of the sintered gear be the meshing surface. The high surface strength of the tooth surface 23a of the toothed portion and the low density or porosity of the tooth surface 23b on the opposite side of the toothed portion allow vibrations generated on the tooth surface 23a to propagate to the tooth surface 23b side, and the vibrations are absorbed by the entire sintered gear, further improving quietness.

[0057] From this perspective, it is more preferable that: In the above (2a), Da-Db may be greater than 0.1, greater than 0.13, or greater than 0.16. For example, 0.1 <Da-Db<0.5であってよい。 In the above (2b), Da / Db may be greater than 1. For example, 1.0 <Da / Db<1.2であってよい。 In the above (2c), Pa-Pb<-1.0, Pa-Pb<-1.5, or Pa-Pb<-2.0 may be satisfied. For example, -10 <Pa-Pb<-1.0であってよい。 In the above (2d), Pa / Pb<1, Pa / Pb<0.95, or Pa / Pb<0.9 may be satisfied. For example, 0.5 <Pa / Pb<0.9であってよい。

[0058] Each of (2a) to (2d) may be satisfied alone, but two, three, or all four may also be satisfied.

[0059] The density Da of the tooth surface 23a of the sintered gear is not particularly limited, and is 6.7 to 7.3 g / mm 3 It may be. The density Db of the tooth surface 23b of the sintered gear is not particularly limited and is 6.7 to 7.3 g / mm 3 It may be.

[0060] The sintered gear according to this embodiment is not particularly limited and can be obtained by appropriately controlling each step in a method of preparing a metal powder, press-molding the metal powder into a powder compact, and sintering the powder compact to obtain a sintered body.

[0061] For example, in the metal powder compression molding process, examples include a method in which the metal powder is filled into a die, and a load is applied to the die in the axial direction using a punch while both the die and punch are rotated in opposite directions to apply the load in a direction that brings the two components closer together; a method in which the metal powder is filled into a die, and a load is applied to the die in the axial direction using a punch while the punch is rotated and the die is moved in the load direction of the punch so that the two components apply the load in opposite directions to each other; a method in which the metal powder is compressed at an increased speed; a method in which the metal powder is compressed in two or more stages; a method in which the metal powder is compressed so that the compact density is lower than that expected from the calculated value; and a method in which the compact density is set low in advance and then compressed.

[0062] Other examples include a method of increasing the compression ratio by reducing the content of solid lubricant in the metal powder preparation process, a method of reducing the amount of molding lubricant used in the die and punch in the metal powder pressure molding process, etc. Also, other examples include a method of making the rolling allowance of the tooth portion of the powder compact relatively thinner on tooth surface 23a than on tooth surface 23b in the metal powder pressure molding process.

[0063] <Third embodiment> The third embodiment will be described below. One aspect of this embodiment is based on the fact that the main body has a shaft hole, and in a diametric cross section at the axial center of the main body, the matrix hardness Hi of a portion 5 mm diametrically outward from the inner peripheral end of the shaft hole and the matrix hardness Ho of a portion 0.5 mm diametrically inward from the tooth bottom of the tooth portion relate to the noise reduction of the sintered gear.

[0064] The sintered gear preferably satisfies the following conditions: (3a) The matrix hardness Hi and the matrix hardness Ho satisfy Ho-Hi>Hv50. (3b) The matrix hardness Hi and the matrix hardness Ho satisfy Ho / Hi>1. (3c) Satisfy the combination of (3a) and (3b).

[0065] In a diametric cross section at the axial center of the main body, the portion 5 mm diametrically outward from the inner peripheral end of the axial hole is the portion indicated by i in Figure 7. Furthermore, in a diametric cross section at the axial center of the main body, the portion 0.5 mm diametrically inward from the tooth bottom of the tooth portion is the portion indicated by o in Figure 7. The matrix hardnesses of portions i and o are compared between portions on the same diametric line.

[0066] In the sintered gear of this embodiment, the matrix hardness of the main body increases from the axial center toward the surface. Vibrations are more likely to be selectively transmitted to high-density areas, and are therefore selectively transmitted to the surface and attenuated. In other words, vibration damping is improved, resulting in improved noise reduction.

[0067] From this perspective, it is more preferable that: In the above (3a), Ho-Hi > 150, Ho-Hi > 160, or Ho-Hi > 170 may be satisfied. For example, 100 <Ho-Hi<200であってよい。 In the above (3b), Ho / Hi may be greater than 1. For example, 1.0 <Ho / Hi<1.5であってよい。

[0068] The sintered gear of this embodiment is not particularly limited and can be obtained by appropriately controlling each step in a method that involves preparing a metal powder, press-molding the metal powder into a green compact, and sintering the green compact to obtain a sintered body.

[0069] The composition of the metal powder and the conditions of the sintering process are particularly important for precisely controlling the matrix hardness Hi and Ho. For example, one method is to add alloy elements with high hardenability to the metal powder, which facilitates the formation of a hard structure in the surface layer by quenching. Other methods include carburizing and quenching after sintering to facilitate the formation of a hard structure in the surface layer, or reducing the density of the green compact to allow carbon to penetrate deep into the surface layer by carburizing and quenching. Another method is to use a low-carbon metal powder and then perform carburizing and quenching after sintering, which increases the vibration damping capacity of the bulk structure of the sintered gear by making it soft, thereby facilitating the formation of a hard structure in the surface layer of the sintered gear.

[0070] The sintered gear of this embodiment preferably has a gentler gradient in matrix hardness from the surface layer compared to that of a melt-cast material. This indicates that the sintered gear has a relatively low overall porosity, allowing carburizing and quenching to progress deep into the surface layer, particularly to the surface layer at the tooth roots. This increases the strength of the entire tooth. On the other hand, if the overall porosity of the sintered gear is high, the hard tissue generated by carburizing and quenching may expand excessively, resulting in dimensional errors. Therefore, it is preferable to control the overall density and porosity of the sintered gear as described above. Furthermore, controlling the overall density and porosity of the sintered gear as described above ensures sufficient vibration absorption throughout the sintered gear, further improving noise reduction.

[0071] For a sintered gear having such a gentle gradient in matrix hardness from the surface layer, a plot of matrix hardness (HV(0.1)) in the depth direction from the tooth bottom to the radially inner side is shown in FIG. 8. It can be seen that the sintered gear of this embodiment has a gentler gradient than both ingot-cast material and conventional sintered material. For the sintered gear of this embodiment, the gradient in matrix hardness from the surface layer to 0.1 mm to 2 mm is preferably −180 Hv / mm or less, more preferably −160 Hv / mm or less, and even more preferably −140 Hv / mm or less.

[0072] To obtain sufficient vibration absorption within the sintered gear, it is preferable that the interior of the sintered gear have a sufficient soft structure. Therefore, the C content of the entire sintered gear is preferably 0.1 to 1.4 mass%, 0.1 to 1 mass%, or 0.1 to 0.4 mass%. Furthermore, to obtain sufficient hardenability during hardening, the entire sintered gear preferably contains at least one element selected from the group consisting of Si, Mn, Cr, Ni, Mo, and Cu. For example, the sintered gear may be an Fe-Ni-Mo-C alloy or an Fe-Cu-C alloy.

[0073] <Sintered metal> The metal sintered body will be described below. Metal sintered bodies can be produced by powder metallurgy and may contain porosity due to the raw material powder. Because metal sintered bodies are porous, they have the advantage of absorbing vibrations and being quieter than metal materials formed through a melting process.

[0074] As the metal sintered body, an iron-based, titanium-based, nickel-based, aluminum-based, copper-based, magnesium-based, alumina-based or other sintered body, or a mixture of these materials, can be used, but an iron-based sintered body is preferably used.

[0075] The iron-based sintered body preferably has a composition that contains iron in the largest amount among the constituent elements, and may have a composition that contains, for example, one or more elements selected from the group consisting of Ni, Mo, Cu, Mn, Cr, and C, with the balance being Fe and inevitable impurities.

[0076] As an example, the iron-based sintered body preferably contains 0.1 to 1.4 mass % of C, with the remainder being Fe and inevitable impurities. A low-carbon iron-based sintered body has a structure containing a large amount of soft phase, and is therefore superior in vibration absorption. By forming a hard phase in the surface layer of this iron-based sintered body through carburizing and quenching, the sintered gear as a whole can absorb vibrations and improve noise reduction, while the teeth of the sintered gear can be made to have better wear resistance due to meshing. In this example, it is more preferable that the C content be 0.1 to 1 mass %.

[0077] Another example of an iron-based sintered body preferably has a composition containing, in mass %, one or more elements selected from the group consisting of 0.1-5% Ni, 0.1-5% Mo, 0.1-3% Cu, 0.1-1% Mn, and 0.1-5% Cr, and 0.1-1.4% C, with the remainder being Fe and unavoidable impurities. This composition improves hardenability, resulting in an appropriate amount of hard phase being contained throughout the sintered gear, thereby increasing the material strength of the entire sintered gear. Furthermore, carburizing and quenching make it easier to form a hard phase in the surface layer, thereby enabling more sufficient wear resistance of the teeth. In this example, it is more preferable that C be 0.1-1% by mass.

[0078] As the raw material powder, a mixed powder in which each alloying element powder is blended with pure iron powder, a pre-alloyed steel powder in which each element is completely alloyed, and a partially diffused alloyed steel powder (also called a composite alloyed steel powder) in which each alloying element powder is partially adhered and diffused on the surface of pure iron powder or pre-alloyed steel powder can be used.

[0079] The particle size of the raw material powder is preferably 5 μm or more and 200 μm or less in terms of the particle diameter (D50) at which the integrated value in the volume-based particle size distribution is 50%. More preferably, the particle size of the raw material powder made of metal is 5 μm or more and 100 μm or less, or 10 μm or more and 50 μm or less in terms of D50. Here, D50 can be measured using a laser diffraction particle size distribution analyzer or the like.

[0080] An example of the composition of an iron-based sintered body will be described below. In the following description, % of the content ratio indicates % by mass. Ni: 0.1 to 5% Ni improves the hardenability of the iron-based sintered body, and after sintering and cooling, has the effect of including a hardened structure in the iron-based sintered body and the effect of remaining as austenite. Ni content of 0.1% or more, preferably 0.3% or more, and more preferably 0.5% or more can increase the strength of the material. Ni content is preferably 5% or less, more preferably 4% or less, and may be 3% or less, 2% or less, or 1% or less.

[0081] Mo: 0.1 to 5% Mo improves the hardenability of the iron-based sintered body and has the effect of making the iron-based sintered body contain a hardened structure after sintering and cooling. Mo content of 0.1% or more, preferably 0.3% or more, and more preferably 0.5% or more can increase the strength of the material. Mo content is preferably 5% or less, and may be 3% or less, 2% or less, or 1% or less.

[0082] Cu: 0.1 to 3% Cu diffuses into Fe to increase the strength of the material. A Cu content of 0.1% or more, preferably 0.5% or more, and more preferably 1% or more, can promote diffusion into Fe. Cu content is preferably 3% or less, which can suppress the generation of a soft Cu phase, preventing a decrease in material strength, and also suppress the generation of a Cu liquid phase during sintering, improving the dimensional accuracy of the entire product. Cu content is preferably 3% or less, and may be 2% or less, 1% or less, or 0.5% or less.

[0083] Mn: 0.1 to 1% Mn improves the hardenability of the iron-based sintered body and has the effect of making the iron-based sintered body contain a hardened structure after sintering and cooling. Mn content of 0.1% or more, preferably 0.2% or more, and more preferably 0.3% or more can increase the strength of the material. Mn content is preferably 1% or less, and may be 0.9% or less, 0.8% or less, or 0.7% or less.

[0084] Cr: 0.1 to 5% Cr improves the hardenability of the iron-based sintered body and has the effect of imparting a hardened structure to the iron-based sintered body after sintering and cooling. Cr content of 0.1% or more, preferably 0.5% or more, and more preferably 1.0% or more can increase the strength of the material. Cr content is preferably 5% or less, more preferably 4.5% or less, and even more preferably 4% or less, and may be 3.5% or less, 3% or less, 2.5% or less, or 2% or less.

[0085] C: 0.1 to 1.4% C has the effect of improving strength by partially dissolving in Fe. A C content of 0.1% or more, preferably 0.2% or more, and more preferably 0.3% or more, generates a metal structure with high matrix hardness and enhances material strength. A C content of 1.4% or less increases the amount of carburization during carburizing and quenching, facilitating the generation of a hard surface structure. It is even better if the C content is 1.3% or less, 1.2% or less, 1.1% or less, or 0.5% or less. A low-carbon sintered gear has a soft phase throughout its structure, providing vibration absorption. The strength of the teeth can be improved by subjecting the sintered gear to carburizing and quenching. From this perspective, the C content may be 1% or less, 0.5% or less, or 0.4% or less. C can be added in the form of graphite powder to enhance the compressibility of the compact.

[0086] The sintered gear is preferably an Fe-Ni-Mo-C alloy or an Fe-Cu-C alloy, and it is even better if the proportions of the elements in these compositions are within the above-mentioned ranges.

[0087] The raw powder of the iron-based sintered body may contain a molding lubricant. The inclusion of a molding lubricant prevents seizure when the powder compact is removed from the die. On the other hand, the inclusion of a molding lubricant hinders densification. By reducing the amount of molding lubricant, a high-density powder compact can be obtained. As the molding lubricant, metal soaps such as lithium stearate, zinc stearate, barium stearate, calcium stearate, and magnesium stearate can be used. Other examples of the molding lubricant that can be used include fatty acid amides such as lauric acid amide, stearic acid amide, and palmitic acid amide, and higher fatty acid amides such as ethylene bisstearic acid amide.

[0088] <Method of manufacturing a metal sintered body> Hereinafter, a method for manufacturing an iron-based sintered body will be described as one embodiment of a metal sintered body. An iron-based sintered body can be obtained by mixing raw material powders to obtain a target composition, pressurizing the mixture to produce a powder compact, and then firing the powder compact. Other metal sintered bodies can also be manufactured in the same way.

[0089] A raw material powder having the above-described composition can be used for powder compaction. The powder compact can be typically produced by using a press machine having a mold capable of uniaxial pressure. A typical example of the mold is one that includes a die having a through hole, and an upper punch and a lower punch that are fitted into the upper and lower openings of the through hole, respectively. The inner peripheral surface of the die and the end face of the lower punch form a cavity. The raw material powder is filled into the cavity. The powder compact can be produced by compressing the raw material powder in the cavity with the upper and lower punches at a predetermined molding pressure. The powder compact may also be molded by using a press machine having a mold capable of biaxial pressure.

[0090] An example of a method for manufacturing a powder compact of a sintered gear according to this embodiment will be described. A die with a cavity in the shape of the sintered gear and a punch that fits into the inner peripheral surface of the die are prepared. Teeth and tooth grooves are formed on the inner peripheral surface of the die in a shape that matches the shape of the teeth of the sintered gear. That is, the teeth and tooth grooves on the inner peripheral surface of the die are formed so that the longitudinal direction of the teeth and tooth grooves is inclined relative to the axial direction. The outer peripheral surface of the punch is formed in a shape that fits into the inner peripheral surface of the die. The die and punch are pressed in the axial direction and rotated in the outer peripheral direction along the shape of the teeth, so that the punch fits into and is inserted into the die.

[0091] In this example, the punch at one axial end of the die may be fixed to the bottom surface of the die, and the punch at the other end may be inserted by being pressed and rotated against the die. Alternatively, punches at both axial ends of the die may be inserted by being pressed and rotated against the die. In this configuration, the metal powder filled in the die is compressed from both axial ends, allowing for more precise control of the gradient of the compression ratio from the center of the green compact to both axial ends, thereby providing a green compact. Furthermore, since control of the density distribution of the green compact also depends on the relative movement of the die and punch, being able to insert punches from both axial ends into the die allows for more precise control of the compression ratio of the green compact. Furthermore, in this configuration, since a load is applied to the tooth portion not only linearly from the axial direction but also in the rotational direction, it is possible to appropriately change the density and porosity of the tooth portion in the tooth width direction, the tooth surface, or a combination thereof.

[0092] The powder compact may be larger than the final product in consideration of processing allowances, and the teeth of the powder compact may be larger than the teeth of the final product in consideration of rolling allowances.

[0093] The pressure (surface pressure) of the uniaxial pressing can be 600 MPa or more. By increasing the surface pressure, the relative density of the powder compact can be increased. A preferred surface pressure is 700 MPa or more, and a more preferred surface pressure is 800 MPa or more. Furthermore, the surface pressure is preferably 1000 MPa or less. This forms a non-densified layer in the powder compact, which improves vibration absorption and leads to improved noise reduction.

[0094] To prevent the metal powder from sticking to the mold, an external lubricant may be applied to the inner peripheral surface of the mold (the inner peripheral surface of the die or the pressing surface of the punch). Examples of external lubricants that can be used include metal soaps such as lithium stearate and zinc stearate. Other examples include fatty acid amides such as lauric acid amide, stearic acid amide, and palmitic acid amide, and higher fatty acid amides such as ethylene bisstearic acid amide.

[0095] The powder compact is preferably sintered in a non-oxidizing atmosphere at a maximum holding temperature of 900°C to 1250°C. This maximum holding temperature is preferably 900°C or higher, more preferably 1000°C or higher. This promotes the diffusion of Ni, Mo, Cu, Mn, Cr, etc. into Fe, resulting in the formation of a metal structure with high matrix hardness and increased tensile strength. Furthermore, this maximum holding temperature is preferably 1250°C or lower, more preferably 1200°C or lower. This suppresses excessive diffusion of other elements into Fe, preventing a decrease in material strength. The powder compact is preferably held at the maximum holding temperature for 10 to 90 minutes.

[0096] After firing, the sintered body is preferably cooled at a cooling rate of 2°C / min to 150°C / min. At this cooling rate, it is preferable to cool the body from the maximum holding temperature to a temperature range of 900 to 200°C.

[0097] This cooling rate may be 2°C / min or more, more preferably 5°C / min or more, and even more preferably 10°C / min or more. This allows the matrix structure to contain an appropriate amount of ferrite phase, pearlite phase, bainite phase, martensite phase, or a combination thereof, thereby increasing the material strength. This cooling rate may be 400°C / min or less, preferably 300°C / min or less, and more preferably 200°C / min or less. This prevents the martensite phase from being excessively contained in the matrix structure, increasing the vibration absorption ability of the soft structure and further improving quietness.

[0098] In sintered compacts, the metal material does not completely melt, so gaps form between the particles, which become pores. The pores absorb vibrations in the sintered gear, which can contribute to improving noise reduction.

[0099] The sintered body obtained as described above may be optionally post-processed to a shape similar to that of the final product. This optional post-processing can eliminate the effects of dimensional errors, thermal expansion, burrs, etc., and can also process additional structures such as steps, grooves, and screw holes. This post-processing can be performed more easily if the sintered gear is made of a low-carbon material.

[0100] The sintered body obtained as described above may be subjected to pressure processing to densify the surface of the tooth portion. Examples of pressure processing include sizing, rolling, and extrusion. These pressure processing methods reduce the porosity of the surface and form a highly dense densified layer. Pressure processing may be performed in stages in multiple steps, or multiple types of processing may be combined. The shape of the sintered body may be determined taking into account the reduction in processing allowance due to pressure processing in the final gear shape. The reduction in processing allowance that can be achieved by a single pressure processing operation varies depending on the processing conditions, i.e., the material being processed, the processing pressure, and the processing temperature. For example, when rolling is used, the desired densified layer can be formed in a single process by adjusting the rolling allowance, rolling pressure, and temperature according to the thickness of the densified layer to be formed. Preferably, the pressure processing is rolling.

[0101] Rolling can be performed using gear-shaped rolling dies. The gear to be rolled is sandwiched between two rolling dies, and the dies are rotated in the same direction while pushing the gear toward the center, forming a densified layer. Rolling not only makes the shape of the teeth more precise, but also improves the strength of the tooth surfaces.

[0102] The sintered body obtained as described above may have a rolling allowance in the tooth portion. By providing an extra rolling allowance in the shape of the tooth portion of the final product, a densified layer is formed on the surface layer of the tooth portion by the rolling process, and the density and porosity of the surface layer of the tooth portion can be controlled more precisely.

[0103] After rolling, the sintered gear may be optionally quenched. Quenching is preferably performed to strengthen the metal structure. Quenching is preferably performed by a process accompanied by rapid cooling. For example, strengthening treatments such as carburizing and quenching, bright hardening, induction hardening, and carbonitriding heat treatment can be used, with carburizing and quenching being particularly preferred.

[0104] Carburizing and quenching can be performed by heat treating the gear in a carbon atmosphere at 850-1050°C, followed by rapid cooling. During carburizing and quenching, carbon penetrates through the pores on the surface of the teeth and the inner surface of the body, forming a hard structure on the surface. This increases the hardness of the matrix on the surface of the sintered gear, making it resistant to wear caused by gear meshing. Meanwhile, because the soft structure remains inside, the vibration-damping properties of the sintered gear as a whole are maintained, improving noise reduction.

[0105] In carburizing and quenching, after heat treatment, the gear is preferably quenched in oil at 30 to 150°C, and more preferably at 50 to 120°C. This prevents the hard phase from being excessively incorporated into the matrix, improving the vibration absorption of the soft structure and noise reduction. If the oil temperature is higher than 200°C, the cooling rate will be insufficient, and the surface will not harden, remaining a soft phase, and the internal matrix will have a low hardness. On the other hand, if the oil temperature is lower than 30°C, the cooling rate will be too fast, causing quench cracks, which may become the starting point for gear chipping or fracture.

[0106] After carburizing and quenching, the sintered body may be subjected to a treatment to restore impact resistance, such as tempering. The tempering temperature is preferably about 100 to 300° C. The tempering holding time can be, for example, 10 to 180 minutes.

[0107] The sintered gear of this embodiment can be used in vehicles powered by electricity. Examples of vehicles powered by electricity include electric vehicles (EVs), hybrid vehicles, plug-in hybrid vehicles, trains, motorcycles, electric bicycles, mopeds, and construction vehicles. EVs, among other vehicles, are rapidly gaining popularity as a product with the potential to bring about major changes in the world's energy problems. As EVs gain increasing attention, electric powertrains are becoming increasingly important. Electric powertrains consist of a drive motor, which serves as the drive source, an inverter that drives the motor, and power transmission mechanisms such as a reduction gear and a differential gear (diff). Furthermore, electric axles, which integrate a drive motor, a power transmission mechanism, and in some cases, an inverter, have become increasingly common. The sintered gear of this embodiment can be used in these power transmission mechanisms.

[0108] The electric axle is key to differentiating EVs, as it affects the driving range from a full charge and power performance. For example, by increasing the rotation speed of the drive motor in the electric axle, the motor can be made smaller while maintaining the same output. On the other hand, in EVs without engines, the gear noise of the reducer and differential becomes noticeable due to the increased rotation speed, resulting in a loss of quietness. To address this issue, using the sintered gear of this embodiment in the reducer can increase strength by improving the meshing ratio between the gears, and improve quietness through smooth meshing. [Example]

[0109] Example 1 Graphite powder was added to iron alloy powder (particle size: 80 mesh) with a composition of Fe-0.5%Ni-0.5%Mo by mass ratio to prepare a mixed powder (composition: Fe-0.5%Ni-0.5%Mo-0.3%C) with a graphite powder ratio of 0.3 mass%. This mixed powder was used as the raw material powder to carry out the following operations.

[0110] [Sample 1 (sintered material 1)] A die with helical-shaped teeth and tooth spaces was prepared, along with upper and lower punches shaped to fit into the die. The raw material powder was filled into the die, and the upper and lower punches were pressed against the die while rotating from above and below, yielding a green compact with a helical gear shape. The compacting pressure was 700 MPa. The green compact was prepared so that the tooth tip circle diameter of the final sintered gear would be 70 mm. The green compact was also prepared so that the tooth width direction of the final sintered gear would be inclined at 25° relative to the axial direction. The teeth were formed so that the rolling allowance during the rolling process would be 10 μm.

[0111] The resulting powder compact was sintered in a non-oxidizing atmosphere at 1195°C for 30 minutes to obtain a sintered gear. This sintered gear was then subjected to rolling. Rolling was performed by sandwiching the sintered gear between a pair of rolled gears and applying pressure. The rolled sintered gear was then maintained at 900°C for 60 minutes in a carburizing gas atmosphere, and then rapidly cooled at 100°C / second to be carburized and quenched. It was then tempered in air at 150°C for 100 minutes to obtain the sintered gear of Sample 1.

[0112] [Sintered material 2] Sintered material 2 was made using the same raw material powder as sample 1. A cylindrical die and upper and lower punches were prepared, and a cylindrical body was formed using conventional uniaxial upper and lower pressure without rotating the punch. The forming pressure was 700 MPa.

[0113] The resulting powder compact was sintered in a non-oxidizing atmosphere at 1195°C for 30 minutes to obtain a cylindrical body, which was then filled into a die again and pressed at 1500 MPa with upper and lower punches to obtain a forged body. Gears of the same dimensions as above were then machined, and carburized, quenched, and tempered according to the above conditions to obtain sintered material 2.

[0114] [Melting material 1] Gears of the same dimensions as above were produced from the molten steel by cutting, and were subjected to carburizing, quenching, and tempering under the above conditions to obtain ingot material 1.

[0115] [Measurement method] The density, porosity, and matrix hardness were measured according to the following procedure, and the results are shown in Table 1. The overall density of the sintered gear sample 1 was measured by the Archimedes method specified in JIS Z2501. The tooth portion of the sintered gear was cut out, and the density Dc of the central 80% portion in the tooth width direction (c in Fig. 6) and the densities De1 and De2 of the portions from the ends on both sides in the tooth width direction up to 10% (e1 or e2 in Fig. 6) were measured by the Archimedes method. From the measured densities, the true density of iron was determined to be 7.87 g / cm 3 The porosities Pc, Pe1, and Pe2 were calculated.

[0116] The images of the tooth flanks 23a and 23b of the tooth portion of the sintered gear of Sample 1 were taken at 200x magnification and analyzed using image analysis software to determine the density distribution, and the densities Da and Db and porosities Pa and Pb were calculated. Three locations on the tooth flank were photographed, and the multiple locations were evenly distributed across the tooth flank.

[0117] The sintered gear of Sample 1 was cut in the diametric direction at the center in the axial direction. In the cross section, the matrix hardness Hi was measured at a portion i located 5 mm from the inner peripheral end of the shaft hole toward the outer periphery in the diametric direction, and the matrix hardness Ho was measured at a portion o located 0.5 mm from the tooth bottom toward the inner periphery in the diametric direction. Measurements were taken at five points for each measurement, evenly spaced in the rotational direction, and the average value was calculated as the matrix hardness.

[0118] The matrix hardness of a sintered body is the Vickers hardness (Hv) of the surface of the matrix excluding the pores of the sintered body. More specifically, it is the Hv when a load of 100 g is applied to the surface of the matrix for high hardness, and a load of 10 g is applied to the surface of the matrix for low hardness. Hv is measured by the method specified in JIS Z 2244. The measurement conditions are as follows: Testing machine: Mitutoyo Corporation "HM-200" Test temperature: Room temperature (25°C) Test load: 100g, 10g

[0119] Sample 1 (density 7.0 g / cm 3The sintered gears (100%) were cut in the diametric direction at the center of the axial direction. In the cross section, the matrix hardness was measured from the surface 0.1 mm to 5 mm deep from the tooth bottom to the inner circumferential side in the diametric direction. The measurement conditions were as described above. 3 ) was used to measure the matrix hardness in the same way. The results are shown in Figure 8. It can be seen that Sintered Material 1 has a gentler gradient in the matrix hardness from the surface layer compared to Ingot Material 1 and Sintered Material 2.

[0120] The sintered gear of Sample 1 was cut in the diametric direction at the center of the axial direction. The cut surface was observed, and the thickness of the densified layer was measured at three points in the depth direction from the surface of the tooth flank of the sintered gear. The arithmetic mean value was found to be 200 μm.

[0121] [Table 1]

[0122] [Evaluation method] (Evaluation of sound pressure level and wear amount) The sound pressure level was measured using Sample 1. The shape of the tooth flank of the sintered gear was also measured before and after measuring the sound pressure level, and the amount of wear on the meshing surface of the tooth flank was calculated. The results are shown in Table 2. The sound pressure level measurement conditions were as follows: Testing machine: Ono Sokki Co., Ltd. "Gear Tester" Temperature: Room temperature (25℃) Lubricating oil: ATF (Automatic Transmission Fluid) dripping Torque: 15Nm Rotation speed: Adjustable in the range of 0 to 2,000 rpm Counterpart material: SCM420H

[0123] [Table 2]

[0124] It can be seen that the sintered gear of Sample 1 is large in size but has excellent quietness. It can also be seen that the tooth surface has excellent wear resistance. This sintered gear has a gradient in density and porosity in the tooth width direction, with different densities and porosities on each tooth surface. In the cross section of the sintered gear, the surface layer around the tooth bottom has a higher matrix hardness than the interior around the shaft hole. [Explanation of symbols]

[0125] 10 Main body 20 Tooth 21 Tooth tip 22 Tooth dedendum 23a Tooth surface 23b Tooth surface 24 Root 30 Shaft hole 100 Sintered Gears

Claims

1. The tooth tip diameter is 30 to 200 mm. A densified layer is formed on at least a part of the tooth surface of the tooth portion. The densified layer has a thickness of 10 to 1000 μm. The overall density according to Archimedes' method is 6.4 to 7.4 g / cm. ３ That is, sintered gears.

2. The density Dc of the tooth portion at a central 80% portion in the tooth width direction and the density De of the tooth portion at least at one end portion up to 10% in the tooth width direction are De-Dc>0.1 g / cm ３ 2. The sintered gear according to claim 1, wherein the density Dc and the density De satisfy the Archimedes law.

3. 2. The sintered gear according to claim 1, wherein a density Dc of a central 80% portion in the tooth width direction and a density De of a portion of the tooth portion extending from at least one end to 10% in the tooth width direction satisfy De / Dc>1, and the densities Dc and De are determined according to Archimedes' law.

4. 2. The sintered gear according to claim 1, wherein a porosity Pc of a central 80% portion in the tooth width direction and a porosity Pe of a portion of the tooth portion extending from at least one end to 10% in the tooth width direction satisfy Pe-Pc<-1%, and the porosity Pc and the porosity Pe are determined according to Archimedes' law.

5. 2. The sintered gear according to claim 1, wherein a porosity Pc of a central 80% portion in the tooth width direction and a porosity Pe of a portion of the tooth width direction extending from at least one end to 10% of the tooth satisfy Pe / Pc<1, and the porosity Pc and the porosity Pe satisfy Archimedes' law.

6. 2. The sintered gear according to claim 1, wherein a density Da of a tooth surface on one side in the outer circumferential direction of the tooth portion is different from a density Db of a tooth surface on the other side in the outer circumferential direction of the tooth portion, and the densities Da and Db are determined according to an image analysis method.

7. The density Da and the density Db are Da-Db>0.1 g / cm ３ 7. The sintered gear according to claim 6, which satisfies the following:

8. 7. The sintered gear according to claim 6, wherein the density Da and the density Db satisfy Da / Db>1.

9. 2. The sintered gear according to claim 1, wherein a porosity Pa of one tooth surface in the circumferential direction of the tooth portion is different from a porosity Pb ​​of the other tooth surface in the circumferential direction of the tooth portion, and the porosity Pa and the porosity Pb ​​are determined by an image analysis method.

10. 10. The sintered gear according to claim 9, wherein the porosity Pa and the porosity Pb ​​satisfy Pa - Pb < -1%.

11. 10. The sintered gear according to claim 9, wherein the porosity Pa and the porosity Pb ​​satisfy Pa / Pb<1.

12. 2. The sintered gear according to claim 1, wherein the main body has a shaft hole, and in a diametric cross section at the axial center of the main body, a matrix hardness Hi of a portion extending 5 mm radially outward from an inner peripheral end of the shaft hole and a matrix hardness Ho of a portion extending 0.5 mm radially inward from a tooth bottom of the tooth portion satisfy Ho-Hi>Hv50.

13. 2. The sintered gear according to claim 1, wherein the main body portion has a shaft hole portion, and in a diametric cross section at the axial center portion of the main body portion, a matrix hardness Hi of a portion extending 5 mm radially outward from an inner peripheral end of the shaft hole and a matrix hardness Ho of a portion extending 0.5 mm radially inward from a tooth bottom of the tooth portion satisfy Ho / Hi > 1.

14. 14. The sintered gear according to claim 1, wherein the overall composition contains 0.1 to 1.4 mass % of C, with the remainder being Fe and unavoidable impurities.

15. 14. The sintered gear according to claim 1, which is an Fe—Ni—Mo—C alloy or an Fe—Cu—C alloy.

Images