JP2026011298A - Sintered Gears - Google Patents

Abstract

The objective of the present invention is to provide a sintered gear with high quietness. [Solution] The sintered gear comprises a main body and teeth formed on the outer peripheral surface of the main body, with the tooth width direction inclined relative to the axial direction, the diameter of the tooth tip circle being 30 to 200 mm, a densified layer being formed on at least a portion of the tooth flank of the teeth, the thickness of the densified layer being 10 to 1000 μm, and the overall density according to Archimedes' method being 6.4 to 7.4 g/cm3. [Selected Figure] Figure 1

Description

This disclosure relates to sintered gears.

Sintered gears are generally sintered bodies made by sintering a gear-shaped green compact obtained by pressing a metal powder-filled mold from above and below with a punch. Such sintered gears can be mass-produced inexpensively, and are therefore 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 properties 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.

WO 2004/030852

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 and improve driving force, and also to improve quietness through smooth meshing. To date, large-sized sintered helical gears have not yet been put to practical use. It has been thought that large sintered bodies tend to vibrate more when driven, resulting in driving noise.

One of the objectives of this disclosure is to provide a sintered gear that is highly quiet.

The present disclosure includes the following embodiments:

[1] A sintered gear comprising: 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 diameter of the tooth tip circle being 30 to 200 mm; a densified layer being formed on at least a portion of the tooth flanks of the teeth; the thickness of the densified layer being 10 to 1000 μm; and the overall density according to Archimedes' method being 6.4 to 7.4 g/cm ³ .

[2] The sintered gear according to [1], wherein the density Dc of the tooth portion in the central 80% portion in the tooth width direction and the density De of the tooth portion in the tooth width direction from at least one end to 10% satisfy the relation De - Dc > 0.1 g/ ^cm3 , and the densities Dc and De are determined according to Archimedes' law.

[3] A sintered gear according to [1] or [2], wherein the density Dc of the central 80% of the tooth portion in the tooth width direction and the density De 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] A sintered gear according to any one of [1] to [3], wherein the porosity Pc of the central 80% of the tooth portion in the tooth width direction and the porosity Pe 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 porosity Pe are calculated according to Archimedes' law.

[5] A sintered gear according to any one of [1] to [4], wherein the porosity Pc of the central 80% of the tooth portion in the tooth width direction and the porosity Pe 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 porosity Pe are calculated according to Archimedes' law.

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

[7] The sintered gear according to [6], wherein the density Da and the density Db satisfy Da - Db > 0.1 g/cm ³ .

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

[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 according to an image analysis method.

[10] A sintered gear according to [9], in which the porosity Pa and the porosity Pb satisfy Pa - Pb < -1%.

[11] A sintered gear according to [9] or [10], wherein the porosity Pa and the porosity Pb satisfy Pa/Pb<1.

[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 diametrically outward from the inner circumferential end of the shaft hole and the matrix hardness Ho of a portion 0.5 mm diametrically inward from the tooth bottom satisfy the relationship Ho - Hi > Hv50.

[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 diametrically outward from the inner circumferential end of the shaft hole and the matrix hardness Ho of a portion 0.5 mm diametrically inward from the tooth bottom satisfy Ho/Hi > 1.

[14] A sintered gear according to any one of [1] to [13], wherein the overall composition contains 0.1 to 1.4 mass% C, with the remainder being Fe and unavoidable impurities.

[15] A sintered gear according to any one of [1] to [13], which is an Fe-Ni-Mo-C alloy or an Fe-Cu-C alloy.

Embodiments of the present disclosure can provide sintered gears that are highly quiet.

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

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

In this disclosure, numerical ranges indicated using "to" indicate ranges that include the numerical values before and after "to" as the minimum and maximum values, respectively. In numerical ranges described in stages in this 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 numerical ranges described in this disclosure, the upper or lower limit of that numerical range may be replaced with a value shown in the examples. In this 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 that step is achieved. Unless otherwise specified, the term "element" refers to either a single element or multiple elements.

<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: 60 kPa (vacuum degree)
Decompression time: 30 min (until no more bubbles appear)
After releasing the vacuum: 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 can be calculated from the density of the sintered body, assuming that the true density of iron is 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.

<Density and porosity by image analysis>
The density and porosity of the 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 the 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.

<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, and more specifically, the Hv when a load of 100 g is applied to the surface of the matrix in the case of a high hardness, and a load of 10 g is applied to the surface of the matrix in the case of a low hardness. Hv is measured by the method specified in JIS Z 2244.

This disclosure relates to a sintered gear comprising 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 this disclosure will be described below using the drawings. All drawings used in the following description are schematic representations of sintered gears, and the present invention is not limited to the specific examples shown in the drawings.

Figure 1 is a perspective view of an example of a sintered gear. Figure 2 is a side view of the sintered gear shown in Figure 1. Figure 3 is a front view of the sintered gear shown in Figure 1 from the axial direction. Figure 4 is a diametric end view of the axial center of the sintered gear shown in Figure 1. Figure 5 is a cross-sectional view of the teeth of an example of a sintered gear.

In this disclosure, referring to Figure 5, the tooth portion 20 comprises 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 root circle, and the bottom surface of the tooth located on the tooth root 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 is the diameter of the circle connecting the tips of the tooth portions.

In FIG. 1, the sintered gear 100 has a main body 10 and a toothed portion 20. The main body 10 is cylindrical, with the toothed portion 20 formed on its outer circumferential surface. The main body 10 may have an axial hole 30 in its center. A shaft supporting the sintered gear 100 can be inserted into the axial 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 portions 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 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.

The sintered gear 100 is preferably a metal sintered body, and 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 they may also be made of separate members and joined together. If they are separate members, it is preferable that at least the teeth 20 is a metal sintered body.

According to some embodiments, a sintered gear is provided, comprising 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 tooth tip diameter being 30 to 200 mm, a densified layer formed on at least a portion of the tooth flanks of the teeth, the densified layer having a thickness of 10 to 1000 μm, and an overall density according to Archimedes' law of 6.4 to 7.4 g/ ^cm3 . Compared to typical spur gears, such sintered gears have a higher meshing ratio between the gears, which makes the teeth less susceptible to wear and increases their relative strength. Furthermore, the continuous meshing of the teeth results in smoother rotation and improved quietness. Furthermore, sintered gears with a tooth tip diameter exceeding 30 mm are difficult to mold, which can lead to problems with vibration damping. Therefore, large-sized sintered gears have been difficult to produce, and quietness has also been considered problematic. However, although sintered gears have pores due to the manufacturing process, by controlling the overall density within the range of 6.4 to 7.4 g/ ^cm3 , the pores can absorb vibrations while providing strength, making even large-sized sintered gears quiet. In this case, by making the thickness of the densified layer on at least a portion of the tooth flank of the tooth portion 10 to 1000 μm, vibrations transmitted from the tooth flank can be easily absorbed, making it possible to provide quietness while maintaining the strength of the tooth flank.

From this perspective, the 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 greater driving force. A diameter of 200 mm or less can provide a gear with a greater number of teeth to increase the reduction ratio.

The sintered gear may have an overall density of 6.4 to 7.4 g/cm ^3. This level of overall density provides strength and excellent vibration absorption, thereby improving quietness. From this perspective, the overall density of the sintered gear is more preferably 6.7 to 7.3 g/cm ³ , 6.5 to 7.3 g/cm ³ , or 6.6 to 7.2 g/cm ^3. When the sintered gear is an iron-based sintered body, it is even better if the overall density falls within these ranges. In the present disclosure, the overall density of the sintered gear can be measured by the Archimedes method as described above, using the sintered gear itself as a measurement sample.

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

A sintered gear has a densified layer formed on at least a portion of the tooth surface, 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 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 where 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.

The thickness of the densified layer of a sintered gear may be 10 μm or more, 50 μm or more, or 100 μm or more. The thickness of the densified layer of a 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 even better. Within these ranges, vibrations transmitted from the tooth surface are more easily absorbed, allowing the tooth surface to maintain strength while also being quiet. A thickness of 10 μm to 500 μm is more preferable, and a thickness of 10 μm to 300 μm is even more preferable.

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 tooth in the central 80% of the tooth in the tooth width direction and the density De of the tooth in the tooth from at least one end to 10% 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 tooth in the central 80% of the tooth in the tooth width direction and the porosity Pe of the tooth in the tooth from at least one end to 10% of the tooth width direction relate to the quietness of a sintered gear.

The sintered gear preferably satisfies the following conditions:
(1a) The density Dc and the density De satisfy the relationship De-Dc>0.1 g/ ^cm3 .
(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) At least two combinations of (1a) to (1d) are satisfied.

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

The sample 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 cutting out the central 80% portion in the tooth width direction. Similarly, the samples for measuring physical properties of the portions e1 and e2 extending from the end 10% are obtained by cutting the tooth portion from the tooth root surface of the sintered gear and cutting out the portion extending from the end 10% in the tooth width direction. Using the obtained sample, the density and porosity can be measured using the Archimedes method as described above.

In the sintered gear of this embodiment, the density is low in the center of the tooth and high at the ends. This ensures high strength due to the high density at the ends, while the low density at the center creates a difference in density. If the density of the gear were uniform overall, vibrations generated on the tooth surfaces as the gears mesh would be transmitted through the sintered matrix to the shaft onto which the gear is inserted. In contrast, as mentioned above, when there is a difference in density between the center and ends of the tooth, vibrations tend to be selectively transmitted to the high-density areas, so vibrations propagate from the vibration-generating position diffuse along the gear surface and gradually attenuate as they propagate. In this way, creating a difference in density between the center and ends of the tooth improves vibration damping, leading to improved quietness.

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.
In the above (1b), De/Dc may be greater than 1. For example, 1.0<De/Dc<1.2 may be satisfied.
In the above (1c), Pe-Pc may be less than -1.0, less than -2.5, or less than -3.8. For example, Pe-Pc may be less than -10 and less than -1.0.
In the above (1d), Pe/Pc may be less than 1, less than 0.8, or less than 0.6. For example, Pe/Pc may be less than 0.05 and less than 1.0.

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

The density Dc of the central 80% portion c of the sintered gear is not particularly limited and may be 6.7 to 6.9 g/ ^mm³ . The density De of the end 10% portions e1 and e2 of the sintered gear is not particularly limited and may be 7.1 to 7.3 g/ ^mm³ . At least one of the end portions e1 and e2 of the sintered gear should be in this range, and it is even better if both are in this range.

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

For example, in the metal powder compression molding process, methods include 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 compressed, filling the metal powder into a die and warm- or cold-forming it so that the temperature at the center of the die is relatively lower than at the edges, compressing it so that the density of the compact is lower than that expected from the calculated value of the metal powder, and compressing it by setting the density of the compact to a low value in advance.

Other methods include reducing the amount of molding lubricant used in the metal powder preparation process to increase the compression ratio, reducing the amount of molding lubricant used in the die and punch in the metal powder compression molding process, and filling the die with metal powder that is more compressible in the center of the die than at the ends in the metal powder compression molding process, in which the metal powder filling stepwise varies in the axial direction. Other methods include making the rolling allowance of the teeth of the green compact thinner in the center than at the ends in the metal powder compression molding process.

Second Embodiment
A second embodiment will now be described. 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.

The sintered gear preferably satisfies the following conditions:
(2a) The density Da and the density Db satisfy Da-Db>0.1 g/ ^cm3 .
(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) At least two combinations of (2a) to (2d) are satisfied.

The tooth flank on one side of the tooth portion in the circumferential direction is the surface indicated by 23a in Figure 5. The tooth flank on the other side of the tooth portion in the circumferential direction is the surface indicated by 23b in Figure 5. In a sintered gear, it is preferable that at least one tooth portion of the multiple tooth portions meets the above condition, but it is more preferable that more than half, 80% or more, or 90% or more of the multiple tooth portions meet the above condition, and it is even more preferable that all of the multiple tooth portions meet the above condition.

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

In the sintered gear of this embodiment, one tooth flank has a high density and the other tooth flank has a low density. Because the gear is unidirectional, the high density on one side that meshes with the gear ensures greater strength. In addition, the low density on the other side allows vibrations to be absorbed, further improving vibration damping.

When meshing a sintered gear with an opposing gear, it is preferable for the tooth surface 23a of the toothed portion of the sintered gear to serve as the meshing surface. The high surface strength of the tooth surface 23a of the toothed portion, along with the low density or porosity of the tooth surface 23b on the opposite side of the toothed portion, allows vibrations generated on the tooth surface 23a to propagate to the tooth surface 23b side, absorbing the vibrations throughout the sintered gear and further improving quietness.

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.
In the above (2b), Da/Db may be greater than 1. For example, 1.0<Da/Db<1.2.
In the above (2c), Pa−Pb may be less than −1.0, less than −1.5, or less than −2.0. For example, −10<Pa−Pb<−1.0.
In the above (2d), Pa/Pb may be less than 1, less than 0.95, or less than 0.9. For example, 0.5<Pa/Pb<0.9.

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

The density Da of the tooth surface 23a of the sintered gear is not particularly limited, and may be 6.7 to 7.3 g/ ^mm3 .
The density Db of the tooth surface 23b of the sintered gear is not particularly limited and may be 6.7 to 7.3 g/ ^mm3 .

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

For example, in the metal powder compression molding process, examples include a method in which metal powder is filled into a die and a punch is used to apply an axial load to the die 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 metal powder is filled into a die and a punch is used to apply an axial load to the die while rotating the punch and moving the die in the direction of the punch load so that both components apply loads in opposite directions; a method in which the metal powder is compressed at an increased speed; a method in which metal powder is compressed in two or more stages; a method in which the metal powder is compressed so that the green density is lower than that expected from the calculated value; and a method in which the green density is set low in advance and then compressed.

Other methods include reducing the solid lubricant content to increase the compression ratio in the metal powder preparation process, and reducing the amount of molding lubricant used in the die and punch in the metal powder compression molding process. Other methods include making the rolling allowance of the tooth portion of the green compact relatively thinner on tooth surface 23a than on tooth surface 23b in the metal powder compression molding process.

Third Embodiment
A 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 extending 5 mm diametrically outward from the inner peripheral end of the shaft hole and the matrix hardness Ho of a portion extending 0.5 mm diametrically inward from the tooth bottom of the tooth portion relate to the quietness of the sintered gear.

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).

In a diametric cross section at the axial center of the main body, the portion 5 mm diametrically outward from the inner circumferential 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 is the portion indicated by o in Figure 7. The matrix hardness of portions i and o are compared between portions on the same diametric line.

In the sintered gear of this embodiment, the matrix hardness increases from the axial center of the main body 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 quietness.

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

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 metal powder, pressurizing the metal powder to form a green compact, and sintering the green compact to obtain a sintered body.

In order to precisely control the matrix hardness Hi and base hardness Ho, the composition of the metal powder and the conditions of the sintering process are particularly important. For example, one method is to add alloy elements with high hardenability to the metal powder, which makes it easier to form a hard structure in the surface layer by quenching. Other methods include carburizing and quenching after sintering, which makes it easier to form a hard structure in the surface layer, and reducing the density of the powder compact so that carbon penetrates deep into the surface layer by carburizing and quenching. Still other methods include using a low-carbon metal powder and carburizing and quenching after sintering, which makes it easier to form a hard structure in the surface layer by making the bulk structure of the sintered gear a soft structure, thereby improving vibration damping.

The sintered gear of this embodiment preferably has a gentler gradient in matrix hardness from the surface layer compared to that of ingot-cast material. This indicates that the relatively low overall porosity of the sintered gear allows carburizing and quenching to progress deep into the surface layer, particularly to the surface layer at the bottom of the teeth. This increases the strength of the entire tooth. On the other hand, if the overall porosity of the sintered gear is too 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 overall porosity of the sintered gear as described above. Furthermore, controlling the overall density and overall porosity of the sintered gear as described above ensures sufficient vibration absorption throughout the sintered gear, further improving quietness.

Figure 8 shows a plot of the matrix hardness (HV(0.1)) in the depth direction from the tooth bottom to the inner circumferential direction in the diametric direction for a sintered gear with a gentle gradient in matrix hardness from the surface layer. 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 0.1 mm to 2 mm from the surface layer is preferably -180 Hv/mm or less, more preferably -160 Hv/mm or less, and even more preferably -140 Hv/mm or less.

In order to obtain sufficient vibration absorption within the sintered gear, it is preferable for the interior of the sintered gear to have a sufficient soft structure. Therefore, the C content of the overall composition of the sintered gear should be 0.1 to 1.4 mass%, 0.1 to 1 mass%, or 0.1 to 0.4 mass%. Furthermore, in order to obtain sufficient hardenability during hardening, it is preferable for the overall composition of the sintered gear to contain 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.

<Metal sintered body>
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.

As the metal sintered body, iron-based, titanium-based, nickel-based, aluminum-based, copper-based, magnesium-based, alumina-based, or a mixture of these materials can be used, but iron-based sintered bodies are preferred.

The iron-based sintered body preferably has a composition in which the largest amount of iron is contained among the constituent elements, and may, for example, contain one or more elements selected from the group consisting of Ni, Mo, Cu, Mn, Cr, and C, with the remainder consisting of Fe and unavoidable impurities.

As an example, the iron-based sintered body preferably contains 0.1 to 1.4 mass% C, with the remainder being Fe and unavoidable impurities. A low-carbon iron-based sintered body has a structure that contains a large amount of soft phase, making it 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 become quieter, while also ensuring better wear resistance due to meshing in the teeth of the sintered gear. In this example, C: 0.1 to 1 mass% is more preferable.

In another example, the iron-based sintered body preferably has a composition containing, by mass, one or more elements selected from the group consisting of Ni: 0.1-5%, Mo: 0.1-5%, Cu: 0.1-3%, Mn: 0.1-1%, and Cr: 0.1-5%, C: 0.1-1.4%, with the remainder consisting of 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 hard phase in the surface layer, thereby achieving more sufficient wear resistance in the teeth. In this example, C: 0.1-1 mass% is more preferable.

As raw material powders, various types of powders can be used: mixed powders in which pure iron powder is blended with powders of each alloying element; pre-alloyed steel powders in which each element is fully alloyed; and partially diffused alloyed steel powders (also called composite alloyed steel powders) in which powders of each alloying element are partially attached and diffused onto the surface of pure iron powder or pre-alloyed steel powder.

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 cumulative value reaches 50% in the volume-based particle size distribution. 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.

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 is 0.1% or more, preferably 0.3% or more, and more preferably 0.5% or more, so that the material strength can be increased. Ni is preferably 5% or less, more preferably 4% or less, and may be 3% or less, 2% or less, or 1% or less.

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 is 0.1% or more, preferably 0.3% or more, and more preferably 0.5% or more, so that the strength of the material can be increased. Mo is preferably 5% or less, and may be 3% or less, 2% or less, or 1% or less.

Cu: 0.1 to 3%
Cu has the effect of diffusing into Fe to increase the strength of the material. By making Cu 0.1% or more, preferably 0.5% or more, and more preferably 1% or more, diffusion into Fe can be promoted. Cu is preferably 3% or less, which can suppress the generation of a soft Cu phase and prevent a decrease in material strength, and also suppress the generation of a Cu liquid phase during sintering, thereby improving the dimensional accuracy of the entire product. Cu is preferably 3% or less, and may be 2% or less, 1% or less, or 0.5% or less.

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, 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.

Cr: 0.1 to 5%
Cr 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. Cr is 0.1% or more, preferably 0.5% or more, and more preferably 1.0% or more, so that the material strength can be increased. Cr 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.

C: 0.1-1.4%
C has the effect of improving strength by partially dissolving in Fe. By setting the C content to 0.1% or more, preferably 0.2% or more, and more preferably 0.3% or more, a metal structure with high matrix hardness is generated, thereby increasing material strength. By setting the C content to 1.4% or less, the amount of carburization during carburizing and quenching can be increased, making it easier to generate a hard structure in the surface layer. 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, which allows it to absorb vibrations. 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 increase the compressibility of the compact.

The sintered gear is preferably an Fe-Ni-Mo-C alloy or an Fe-Cu-C alloy. It is even better if the proportions of each element in these compositions are within the ranges mentioned above.

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 high density. Reducing the amount of molding lubricant makes it possible to obtain a high-density powder compact. Metal soaps such as lithium stearate, zinc stearate, barium stearate, calcium stearate, and magnesium stearate can be used as molding lubricants. Other examples 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.

<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.

A raw material powder having the above-mentioned composition can be used for powder compaction. A powder compact can be produced using a press machine equipped with a mold capable of uniaxial pressure. A typical mold includes a die with a through hole, and an upper punch and a lower punch 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. A powder compact can be produced by compressing the raw material powder in the cavity with the upper and lower punches at a predetermined compacting pressure. A powder compact may also be produced using a press machine equipped with a mold capable of biaxial pressure.

An example of a method for manufacturing a powder compact of a sintered gear according to this embodiment is described below. A die is prepared that forms a cavity in the shape of the sintered gear, and a punch that fits into the inner peripheral surface of the die. The inner peripheral surface of the die is formed with teeth and tooth grooves that match the shape of the teeth of the sintered gear. In other words, the inner peripheral surface of the die is formed with the longitudinal direction of the teeth and tooth grooves inclined relative to the axial direction. The outer peripheral surface of the punch is formed with a shape that fits into the inner peripheral surface of the die. The die and punch are pressed together in the axial direction and rotated in the outer peripheral direction along the shape of the teeth, causing the punch to fit into and be inserted into the die.

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. In another configuration, 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, resulting in a green compact. Furthermore, since control of the density distribution of the green compact is also dependent 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, a load is applied to the tooth portion not only linearly from the axial direction but also in the rotational direction, making it possible to appropriately change the density and porosity of the tooth portion in the tooth width direction, the tooth surface of the tooth portion, or a combination of these.

The powder compact may be larger than the final product, taking into account processing allowances. Also, the teeth of the powder compact may be larger than the teeth of the final product, taking into account rolling allowances.

The pressure (surface pressure) for uniaxial pressing can be 600 MPa or more. Increasing the surface pressure can increase the relative density of the powder compact. A preferred surface pressure is 700 MPa or more, and a more preferred surface pressure is 800 MPa or more. It is also preferable that the surface pressure be 1000 MPa or less. This forms a non-densified layer within the powder compact, improving vibration absorption and noise reduction.

To prevent the metal powder from sticking to the mold, an external lubricant may be applied to the inner surface of the mold (the inner 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.

The powder compact is preferably fired 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, and more preferably 1000°C or higher. This promotes the diffusion of Ni, Mo, Cu, Mn, Cr, etc. into the Fe, producing a metal structure with high matrix hardness and further increasing tensile strength. Furthermore, this maximum holding temperature is preferably 1250°C or lower, and more preferably 1200°C or lower. This prevents excessive diffusion of other elements into the Fe and prevents a decrease in material strength. The powder compact is preferably held at the maximum holding temperature for 10 to 90 minutes.

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

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 base structure to contain an appropriate amount of ferrite phase, pearlite phase, bainite phase, martensite phase, or a combination thereof, thereby increasing the strength of the material. 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 base structure from containing an excessive amount of martensite phase, increasing the vibration absorption ability of the soft structure and further improving quietness.

In sintered bodies, the metal material does not completely melt, creating gaps between the particles, which become pores. These pores absorb vibrations in sintered gears, which can contribute to improved quietness.

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. Additionally, additional structures such as steps, grooves, and screw holes can be machined. This post-processing can be performed more easily if the sintered gear is made of a low-carbon material.

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 used, processing pressure, and 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.

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 being pressed 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.

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 of the tooth portion during the rolling process, allowing for more precise control of the density and porosity of the surface layer of the tooth portion.

Sintered gears may optionally be hardened after rolling. Hardening is preferred to strengthen the metal structure. Hardening is preferably performed by a process that involves rapid cooling. For example, strengthening processes such as carburizing and hardening, bright hardening, induction hardening, and carbonitriding heat treatment can be used, with carburizing and hardening being particularly preferred.

Carburizing and quenching can be performed by heat treating the material 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 main 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 soft structure remains inside, the vibration-damping properties of the sintered gear as a whole are maintained, improving noise reduction.

In carburizing and quenching, after heat treatment, it is preferable to rapidly cool the gear in oil at 30 to 150°C, and even more preferable to rapidly cool it in oil at 50 to 120°C. This prevents the hard phase from being excessively incorporated into the matrix, improving the vibration absorption ability of the soft structure and further improving quietness. If the oil temperature is higher than 200°C, the cooling rate will be insufficient, and the surface will not harden into a hardened structure, remaining a soft phase, and the internal matrix hardness will also be low. 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 destruction.

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

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. Of these, EVs are rapidly gaining popularity as a product with the potential to bring about major changes in global energy issues. As EVs gain more attention, electric powertrains are becoming increasingly important. Electric powertrains consist of a drive motor, which serves as the drive source, an inverter that drives it, and power transmission mechanisms such as a reduction gear and a differential gear unit (diff). Furthermore, electric axles, which integrate a drive motor, power transmission mechanism, and in some cases, an inverter, have become increasingly common within electric powertrains in recent years. The sintered gear of this embodiment can be used in these power transmission mechanisms.

The electric axle holds the 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, it is possible to make the motor smaller while maintaining the same output. On the other hand, in EVs that do not have engines, the gear noise of the reducer and differential becomes more 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 1
Graphite powder was added to iron alloy powder (particle size: 80 mesh) having 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) containing 0.3% by mass of graphite powder. This mixed powder was used as a raw material powder to carry out the following operations.

[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 die was filled with raw material powder, and the upper and lower punches were pressed against the die while rotating from above and below to obtain a helical gear-shaped green compact. The green compact was prepared at a molding pressure of 700 MPa. The green compact was prepared so that the final sintered gear would have a tooth tip circle diameter of 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 rolling was 10 μm.

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 perform carburizing and quenching. It was then maintained in air at 150°C for 100 minutes to perform tempering, yielding the sintered gear of Sample 1.

[Sintered material 2]
For sintered material 2, the same raw material powder as that for sample 1 was used, and a cylindrical die and upper and lower punches were prepared. The cylindrical body was formed by conventional uniaxial upper and lower pressure without rotating the punch. The forming pressure was 700 MPa.

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 loaded into the die again and pressed at 1500 MPa with upper and lower punches to obtain a forged body. A gear of the same dimensions as above was then machined, and carburized, quenched, and tempered according to the above conditions to obtain Sintered Material 2.

[Melted 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 the ingot material 1.

[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 of Sample 1 was measured by the Archimedes method specified in JIS Z2501. A 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 extending from the ends to 10% in the tooth width direction (e1 or e2 in FIG. 6) were measured by the Archimedes method. The true density of iron was ^set to 7.87 g/cm3 from the measured densities, and the porosities Pc, Pe1, and Pe2 were calculated.

Images of the tooth surfaces 23a and 23b of the tooth portion of the sintered gear of sample 1 were taken at 200x magnification. The density distribution was determined by analyzing the images using image analysis software, and the densities Da and Db and porosities Pa and Pb were calculated. Photographs were taken at three locations on the tooth surface, with multiple photographic locations evenly distributed across the tooth surface.

The sintered gear of sample 1 was cut diametrically at the center in the axial direction. In this cross section, the matrix hardness Hi was measured at a portion i 5 mm diametrically outward from the inner peripheral end of the shaft hole, and the matrix hardness Ho was measured at a portion o 0.5 mm diametrically inward from the tooth bottom. Measurements were taken at five points each, evenly spaced in the direction of rotation, and the average value was calculated as the matrix 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. More specifically, it is the Hv when a load of 100 g is applied to the surface of the matrix in the case of high hardness, and a load of 10 g is applied to the surface of the matrix in the case of 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

A sintered gear of sample 1 (density 7.0 g/cm ³ ) was cut in the diametric direction at the center in the axial direction. In the cross section, the matrix hardness was measured from the surface 0.1 mm to 5 mm deep, with the depth direction being from the bottom of the tooth to the inner circumferential side in the diametric direction. The measurement conditions were as described above. The matrix hardness was similarly measured using ingot material 1 and sintered material 2 (density 7.6 g/cm ³ ). The results are shown in Figure 8. It can be seen that sintered material 1 has a gentler gradient of matrix hardness from the surface compared to ingot material 1 and sintered material 2.

The sintered gear of Sample 1 was cut diametrically at the center in the axial direction. The cut surface was observed, and the thickness of the densified layer was measured at three locations in the depth direction from the surface of the tooth flank of the sintered gear's tooth portion. The arithmetic average value determined that the densified layer of Sample 1 was 200 μm.

[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 the sound pressure level measurement, 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 "Gear Tester"
Temperature: Room temperature (25℃)
Lubricant: ATF (Automatic Transmission Fluid) dripped Torque: 15 Nm
Rotation speed: Adjustable within the range of 0 to 2,000 rpm Mating material: SCM420H

The sintered gear of sample 1 is large in size but exhibits excellent quietness. It also exhibits excellent wear resistance on the tooth surface. 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.

10 body portion 20 tooth portion 21 tooth tip 22 tooth base 23a tooth surface 23b tooth surface 24 tooth bottom 30 shaft hole portion 100 sintered gear

Claims (15)

A sintered gear comprising: a main body; and teeth formed on an outer peripheral surface of the main body, the teeth having a tooth width direction inclined with respect to an axial direction, the tooth tip circle diameter being 30 to 200 mm, a densified layer being formed on at least a portion of the tooth flanks of the teeth, the densified layer having a thickness of 10 to 1000 μm, and an overall density according to Archimedes' method being 6.4 to 7.4 g/ ^cm3 . 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 > 0.1 g/ ^cm3 , and the densities Dc and De are determined according to Archimedes' law. A sintered gear as described in claim 1, 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 up to 10% satisfy De/Dc > 1, and the densities Dc and De follow Archimedes' law. A sintered gear as described in claim 1, wherein the porosity Pc of the central 80% of the tooth portion in the tooth width direction and the porosity Pe 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 follow Archimedes' law. A sintered gear as described in claim 1, wherein the porosity Pc of the central 80% of the tooth portion in the tooth width direction and the porosity Pe 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 follow Archimedes' law. A sintered gear as described in claim 1, wherein the density Da of the tooth surface on one side of the tooth portion in the circumferential direction is different from the density Db of the tooth surface on the other side of the tooth portion in the circumferential direction, and the densities Da and Db are determined according to an image analysis method. ^7. The sintered gear according to claim 6, wherein the density Da and the density Db satisfy Da-Db>0.1 g/cm. A sintered gear according to claim 6, wherein the density Da and the density Db satisfy Da/Db > 1. A sintered gear as described in claim 1, wherein the porosity Pa of one tooth surface in the circumferential direction of the tooth portion differs 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 according to an image analysis method. The sintered gear according to claim 9, wherein the porosity Pa and the porosity Pb satisfy Pa - Pb < -1%. A sintered gear as described in claim 9, wherein the porosity Pa and the porosity Pb satisfy Pa/Pb < 1. 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 of the main body portion, the matrix hardness Hi of a portion 5 mm diametrically outward from the inner peripheral end of the shaft hole portion and the matrix hardness Ho of a portion 0.5 mm diametrically inward from the tooth bottom of the tooth portion satisfy the relationship Ho - Hi > Hv50. 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 of the main body portion, the matrix hardness Hi of a portion 5 mm diametrically outward from the inner peripheral end of the shaft hole portion and the matrix hardness Ho of a portion 0.5 mm diametrically inward from the tooth bottom of the tooth portion satisfy the relationship Ho/Hi > 1. A sintered gear according to any one of claims 1 to 13, having an overall composition containing 0.1 to 1.4 mass% C, with the remainder being Fe and unavoidable impurities. A sintered gear according to any one of claims 1 to 13, which is an Fe-Ni-Mo-C alloy or an Fe-Cu-C alloy.