JP2013241961A - Gear manufacturing method and gear - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gear manufacturing method capable of obtaining strength improvement effect of a tooth flank, and a gear.SOLUTION: A gear manufacturing method includes a gear forming process for forming a gear and a surface modification treatment process for applying surface modification treatment to a tooth flank of the gear. In the surface modification treatment process, surface modification treatment is applied so that magnitude of compression residual stress on an outermost surface of the tooth flank is 800 MPa or more, maximum height roughness Rz of the tooth flank is 4 μm or below, and a protruded peak part height Rpk of the tooth flank is 0.40 μm or more. According to the gear manufacturing method, a gear with teeth flanks having excellent conformability without causing micro cracks on the surface in a process of adaptive wear can be manufactured while properly wearing in an operation initial stage, and thus enhancing effect of the tooth flank strength can be obtained which is sufficiently higher compared with conventional ones.

Description

  The present invention relates to a gear manufacturing method and a gear.

  In general, the contribution of materials, heat treatment, and surface modification is great for the strength of gears. For this reason, gears that transmit large power are often subjected to surface hardening treatment such as carburizing and induction hardening using alloy steel, and various surface modifications are often employed. As a surface modification of the tooth surface, shot peening is widely used (see, for example, Patent Document 1).

  Shot peening has been put into practical use as a surface modification method that economically improves gear strength. In recent years, shot peening with high hardness is projected using an air nozzle type shot peening machine, etc. Fine particle peening for projecting particles (for example, particles having a diameter of 20 to 300 μm) has been used.

JP 2002-121644 A

  By the way, when performing shot peening or other surface modification, the processing conditions must be selected so as to obtain a target effect. For example, since the effect of shot peening changes depending on conditions such as the size and hardness of shot grains, the projection speed, and the projection time, it is important to select processing conditions according to the purpose.

  As a specific selection method, a method of estimating whether tooth surface strength improvement can be obtained from measurement results of tooth surface properties after processing, that is, hardness, residual stress and roughness, is common. Finally, the effect is confirmed by performing an endurance test using the gear processed under the conditions. When estimating the effect of improving the tooth surface strength from the tooth surface properties, it is generally used that a high compressive residual stress is applied and the tooth surface roughness is low. However, despite the fact that high compressive residual stress is applied to the tooth surface and the tooth surface roughness is low, the expected effect of improving the tooth surface strength cannot be obtained even if the gear is actually processed under the conditions. was there.

  Then, an object of this invention is to provide the gear manufacturing method and gear which can acquire the effect of a tooth surface strength improvement.

  As a result of the study by the present inventor, it was found that the conformity of the tooth surface greatly affects the tooth surface strength. Familiarity refers to a phenomenon in which the unused tooth surfaces after machining are brought into sliding contact at the beginning of operation and the surface roughness protrusions are worn or crushed and become smooth. It has been discovered that the conformability of the tooth surface, that is, the ease of conforming the tooth surface, greatly contributes to the tooth surface strength of the gear, and is an important factor in improving the tooth surface strength.

The gear manufacturing method according to the present invention includes a gear forming step for forming a gear and a surface modification treatment step for subjecting the tooth surface of the gear to a surface modification treatment. Surface modification so that the compressive residual stress of the outermost surface of the surface is 800 MPa or more, the maximum height roughness Rz of the tooth surface is 4 μm or less, and the protruding peak height Rpk of the tooth surface is 0.40 μm or more. It is characterized by processing.
In the measurement of the tooth surface roughness in the present invention, the roughness curve is extracted from the surface shape curve obtained with the tooth profile direction of the gear as the measurement direction using a Gaussian filter with a cutoff wavelength of 0.25 mm, and the inclination correction is performed. As described above, the minimum square curve correction is applied to obtain the maximum height roughness Rz of the tooth surface and the protruding peak height Rpk of the tooth surface.

  The gear manufacturing method according to the present invention improves the conformability of the tooth surface by optimizing the roughness and residual stress of the tooth surface of the power transmission gear represented by the transmission gear, and the tooth called pitching or spalling. The strength against damage that peels off the surface (hereinafter, the strength against damage is referred to as tooth surface strength) is improved.

  In this gear manufacturing method, the tooth surface is moderately worn in the initial stage of operation, and the tooth surface that does not generate a microcrack (or does not propagate even if a microcrack is generated) on the surface during the familiar wear process. Focusing on the good tooth surface, the maximum tooth surface roughness Rz is 4 μm or less and the protruding ridge height Rpk is 0.40 μm or more, so that the tooth surface can be reasonably moderate at the beginning of operation. Wearing gears can be produced. In addition, by setting the compressive residual stress of the outermost surface of the tooth surface to 800 MPa or more, a tooth surface that does not generate a minute crack on the tooth surface in the process of conforming wear or does not propagate even if a micro crack occurs. Can be obtained. Therefore, according to this gear manufacturing method, it is possible to manufacture a gear having a tooth surface with good conformability, so that the effect of improving the tooth surface strength can be obtained more reliably and higher than the conventional one.

  In the gear manufacturing method according to the present invention, a surface modification treatment by shot peening may be performed in the surface modification treatment step. In this case, the gear having the effect of improving the tooth surface strength can be efficiently manufactured by selecting the shot peening processing conditions so as to improve the tooth surface satisfying the above-described conditions.

The gear according to the present invention has a compressive residual stress of 800 MPa or more on the outermost surface of the tooth surface, a maximum height roughness Rz of the tooth surface of 4 μm or less, and a protruding peak height Rpk of the tooth surface of 0.40 μm. It is the above.
This gear has a well-familiar tooth surface that is moderately worn in the initial stage of operation and does not generate a microcrack on the surface during the process of running-in (or does not progress even if a microcrack is produced). As compared with the above, it is possible to obtain a more reliable effect of improving the tooth surface strength.

  ADVANTAGE OF THE INVENTION According to this invention, the gear manufacturing method and gear which can acquire the effect of tooth surface strength improvement can be provided.

It is a graph which shows the relationship between the kind of surface modification of a tooth surface, and a tooth surface strength ratio. It is a graph which shows the relationship between a tooth surface strength ratio and a roughness parameter. It is a graph which shows the relationship between a tooth surface strength ratio and the tooth surface roughness change rate before and after a conforming operation. It is a graph which shows the relationship between the roughness parameter and the tooth surface roughness change rate before and after the conforming operation. It is a graph which shows the relationship between a tooth surface strength ratio and the maximum roughness height of a tooth surface. It is a graph which shows a tooth surface strength ratio and the value of the compressive residual stress of the outermost surface of a tooth surface.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

  An example of the gear according to the present embodiment is a helical gear made of alloy steel (for example, SCM420H) that constitutes a transmission of a vehicle. However, the type of gear according to the present invention is not particularly limited, and may be any of a spur gear, an internal gear, a rack, a straight bevel gear, a curved bevel gear, a screw gear, a worm gear, a hypoid gear, and the like. . Further, the purpose of use of the gear is not limited to that for vehicles.

  In such a gear manufacturing method, first, a gear forming step of forming a gear from an alloy steel by gear cutting is performed. The formation of the gear is not limited to gear cutting, and may be formed by rolling, casting, or other methods. Thereafter, gas carburizing and quenching is applied to the gear to polish the tooth surface, and a surface modification process for improving the strength of the tooth surface is performed.

  Types of surface modification treatment include shot peening in which particles (mainly small-diameter steel balls) collide with the tooth surface at high speed, coating with MoS2 (molybdenum dioxide) applied to the tooth surface, and polishing such as barrel polishing. Processing.

  The types of shot peening are fine particle peening that projects fine particles compared to normal shot peening, hard shot peening that projects particles harder than normal shot peening, and fine particle peening after hard shot peening. There are two-stage peening and so on.

  Hereinafter, with reference to FIGS. 1 to 6, conditions for the surface modification treatment that provides the effect of improving the tooth surface strength will be described.

FIG. 1 is a graph showing the relationship between the types of surface modification of tooth surfaces and the tooth surface strength ratio. Table 1 shows the relationship between the types of surface modification shown in FIG. Table 2 shows details of processing conditions for shot peening and fine particle peening shown in Table 1. In FPBd shown in Tables 1 and 2, the surface modification of the two-stage fine particle peening was performed. In the upper and lower two columns of FPBd shown in Table 2, the upper column indicates the conditions for the first fine particle peening, and the lower column indicates the conditions for the second fine particle peening.

  In FIG. 1, the relative evaluation of the gear strength ratio of each type of surface modification was performed on the basis of Std that was not subjected to surface modification treatment. In SHPa to SHPc subjected to shot peening, only SHPa exceeded Std, and SHPb and SHPc fell below Std. Further, FPBa to FPBd (FPBd is two-stage fine particle peeling) subjected to fine particle peening all exceeded Std. MoS2 coated with MoS2 also exceeded Std. On the other hand, Barrel subjected to barrel polishing was lower than Std. Thus, the kind and conditions of surface modification have a great influence on the tooth surface strength.

  For the measurement of tooth surface strength, for example, a constant torque is applied to the test gear and endurance operation is performed until separation damage (pitching, spalling) occurs on the tooth surface, and the total number of gear rotations until the strength limit is obtained. Can be done. The strength limit can be determined by observing the tooth surface at regular intervals during the durability operation. That is, the area of the peeled portion generated on the tooth surface is obtained, and when this exceeds a certain value, it can be determined as the strength limit.

  This test is performed for several kinds of load torques (for example, 4 to 6 types), and the obtained results are plotted on an SN diagram (stress-repetitive number diagram) to improve the surface under the target conditions. The tooth surface strength of the treated test gear can be obtained. The tooth surface strength of the test gears with different processing conditions was determined in the same procedure, and the difference on the SN diagram was evaluated as the strength difference.

  FIG. 2 is a graph showing the relationship between the tooth surface strength ratio and the tooth surface roughness change rate ξ before and after the conforming operation. The tooth surface roughness change rate ξ before and after the conforming operation is obtained by dividing Rz after the conforming operation by Rz before the conforming operation with respect to the maximum roughness height Rz of the tooth surface defined in JIS B0601,2001. . The tooth surface roughness change rate ξ is one of the indexes that quantitatively represents the conformability of the tooth surface, that is, the ease of conformation of the tooth surface. The smaller the value of ξ, the better the conformability.

  As shown in FIG. 2, there is a good correlation between the tooth surface roughness change rate ξ and the tooth surface strength ratio, and the smaller the tooth surface roughness change rate ξ, the better the tooth surface adaptability. It can be seen that the surface strength is high. When the conformability is good, the tooth surface strength is improved by the fact that the tooth surfaces that mesh with each other are rubbed together, resulting in an increase in the true contact area, a reduction in local contact pressure, and a lubricating oil film on the tooth surface. This is considered to be due to the effect of forming better.

  Here, it should be noted that even with the same type of surface modification, the conformity of the tooth surface may be improved or worsened depending on the processing conditions, and in the latter case, the tooth surface strength is not improved. Conventionally, in order to improve the tooth surface strength, it has been recognized that it is desirable that the value (absolute value) of the compressive residual stress applied to the tooth surface is high and the tooth surface roughness is low. According to this research, it has been found that even if the surface modification treatment is performed according to the conventional criteria, the conformity of the tooth surface may not be improved, and thus the tooth surface strength is not necessarily improved.

  Here, the tooth surface having good conformability is a first condition that the tooth surface is appropriately worn in the initial stage of operation, and a micro crack is not generated on the tooth surface in the process of conforming wear (or a micro crack is not generated). The tooth surface satisfies the second condition that the crack does not propagate even if it occurs. In order to improve the conformability of the tooth surface and increase the tooth surface strength, it is desirable to satisfy both conditions. The tooth surface strength may not be improved only by the first condition. This is because a microcrack is generated on the tooth surface in the process of familiar wear, and when the crack progresses, it results in peeling damage to the tooth surface. It is considered that the factors affecting the first condition are tooth surface properties and tooth surface roughness, and the factors affecting the second condition are compressive residual stresses on the outermost surface of the tooth surface.

  FIG. 3 is a graph showing the relationship between the tooth surface roughness change rate ξ before and after the conforming operation, that is, the conformability of the tooth surface and the tooth surface roughness parameter Rpk before the operation. The roughness parameter Rpk is one of the characteristics of the height characteristic based on the linear load curve, that is, the roughness shape. The roughness parameter Rpk is called the protruding peak height (JIS B0671-2 2002) and indicates the average height of the protruding peak on the core of the roughness curve defined in JIS B0671-2 2002. Value.

  As shown in FIG. 3, the tooth surface roughness change rate ξ and the roughness parameter Rpk have a good correlation. In the range shown in the graph of FIG. 3, the higher the roughness parameter Rpk, the better the tooth surface roughness change rate ξ, that is, the conformability of the tooth surface. In the measurement of the tooth surface roughness, the cut-off wavelength is set to 0.degree. From the curve of the surface shape obtained using a surface roughness shape measuring machine (Surcom 1400A manufactured by Tokyo Seimitsu Co., Ltd.) with the tooth profile direction of the gear as the measurement direction. A roughness curve was extracted using a 25 mm Gaussian filter, and least square curve correction was applied as inclination correction.

  FIG. 4 is a graph showing the relationship between the roughness parameter Rpk and the tooth surface strength ratio. As shown in FIG. 4, it can be seen that the effect of improving the tooth surface strength is obtained when the roughness parameter Rpk is 0.40 μm or more. The roughness parameter Rpk is more preferably 0.50 μm or more, and further preferably 0.60 μm or more.

  As described above, in order to obtain the effect of improving the tooth surface strength, there is a problem with the conventional judgment standard that the tooth surface roughness is low, but if the tooth surface roughness is excessively large, the tooth surface strength is low. It has been known for a long time. FIG. 5 is a graph showing the relationship between the tooth surface strength ratio and the maximum height roughness Rz (JIS B601 2001) of the tooth surface. In addition, the maximum height roughness Rz of the tooth surface was measured by the same method as the roughness parameter Rpk.

  As shown in FIG. 5, the tooth surface strength ratio tends to decrease when the maximum height roughness Rz exceeds 4.00 μm. For this reason, the maximum height roughness Rz is preferably 4.00 μm or less, and more preferably 3.50 μm or less.

FIG. 6 is a graph showing the relationship between the magnitude (absolute value) of the compressive residual stress σ _R on the outermost surface of the tooth surface and the tooth surface strength ratio. The compressive residual stress σ _R can be measured by, for example, an X-ray diffraction method, and is measured by applying X-rays to the outermost surface of the tooth surface without performing electrolytic polishing or the like. As shown in FIG. 6, the higher the compressive residual stress σ _R applied to the outermost surface of the tooth surface, the higher the tooth surface strength. From this result, it can be seen that the effect of improving the tooth surface strength is obtained when the compressive residual stress σ _{R on} the outermost surface of the tooth surface is 800 MPa or more. Incidentally, more preferably magnitude of the compressive residual stress sigma _R ^R is equal to or greater than 1000 MPa, and still more preferably at least 1200 MPa.

  Based on the results described above, in the gear manufacturing method according to the present embodiment, in the surface modification treatment step, the magnitude of the compressive residual stress on the outermost surface of the tooth surface is 800 MPa or more, and the maximum height roughness Rz of the tooth surface is The surface modification treatment is performed so that the tooth surface roughness parameter (projection peak height) Rpk is not less than 4.00 μm and not less than 0.40 μm.

  In the gear manufacturing method according to the present embodiment, the tooth surface is moderately worn in the initial stage of operation, and a minute crack is not generated on the surface in the process of familiar wear (or even if a minute crack is generated). Focusing on the fact that the tooth surface is a well-familiar tooth surface, the maximum height roughness Rz of the tooth surface is 4 μm or less, and the protruding ridge height Rpk is 0.40 μm or more. It is possible to produce a gear whose tooth surface is worn. In addition, by setting the compressive residual stress of the outermost surface of the tooth surface to 800 MPa or more, a tooth surface that does not generate a minute crack on the tooth surface in the process of conforming wear or does not propagate even if a micro crack occurs. Can be obtained. Therefore, according to this gear manufacturing method, it is possible to manufacture a gear having a tooth surface with good conformability, so that the effect of improving the tooth surface strength can be obtained more reliably and higher than the conventional one.

  Further, as shown in FIGS. 1 to 6, the effect of improving the tooth surface strength described above was obtained in FPBa to FPBd subjected to the surface modification treatment by fine particle peening under the current treatment conditions. In this way, by adopting shot peening as the surface modification process and selecting the shot peening process conditions so as to modify the tooth surface satisfying the above conditions, a gear with improved tooth surface strength can be efficiently obtained. Can be manufactured.

  And the gear manufactured by this gear manufacturing method has a well-fitting tooth surface that is moderately worn in the initial stage of operation and does not generate minute cracks on the surface during the running-in process. Thus, a more reliable effect of improving the tooth surface strength can be obtained.

  In addition, minute scaly ridges are formed in random directions on the tooth surfaces of the gear manufactured by this gear manufacturing method. These scaly crests are provided by shot peening and are formed from a texture in which undulations with a pitch of about several μm to about 100 μm are randomly layered. By providing such scaly scissors on the tooth surface, the tooth surface of the other gear is scraped with the scissors to smooth the tooth surface and improve the conformability, thereby suppressing peeling damage. Thus, the strength of the tooth surface can be improved.

  The present invention is not limited to the embodiment described above. The surface modification treatment is not limited to mere shot peening or fine particle peening, and for example, hard shot peening for projecting a shot material having a hardness higher than usual may be used. By applying a high compressive residual stress to the tooth root of the gear by hard shot peening, the fracture strength of the tooth root can be improved.

  However, in hard shot peening, the compressive residual stress applied to the outermost surface of the tooth surface is slightly lower and the maximum height roughness Rz of the tooth surface is higher, so the magnitude of the compressive residual stress on the outermost surface of the tooth surface is large. Is not more than 800 MPa, the maximum height roughness Rz of the tooth surface is 4 μm or less, and the protrusion peak height Rpk of the tooth surface is not more than 0.40 μm. Therefore, by performing two-stage peening that performs fine particle peening after hard shot peening, the tooth surface strength can be sufficiently improved by satisfying the above conditions while improving the fracture strength of the tooth root.

  Further, the surface modification process does not necessarily need to be shot peening and may be any process that can modify the tooth surface so as to satisfy the above conditions.

Next, examples according to the present invention will be described. Table 3 shows the conditions of fine particle peening for Examples 1 and 2 and Comparative Examples 1 and 2.

Example 1
After forming a gear from the alloy material (SCM420H), the tooth surface of the gear was subjected to surface modification treatment by fine particle peening. The conditions for fine particle peening were as follows: arc height 0.24 mmN, coverage 200%, projection pressure 0.4 MPa, shot material particle size 80 μm, and shot material hardness 700-850 Hv.

(Example 2)
The conditions for fine particle peening were the same as in Example 1 except that the arc height was 0.41 mmN, the projection pressure was 0.6 MPa, and the shot material particle size was 130 μm.

(Comparative Example 1)
The conditions for fine particle peening were the same as in Example 1 except that the arc height was 0.19 mmN, the coverage was 300%, the projection pressure was 0.3 MPa, and the shot material particle size was 50 μm.

(Comparative Example 2)
The conditions for fine particle peening were the same as in Comparative Example 1 except that the arc height was 0.22 mmN and the projection pressure was 0.5 MPa.

Table 4 shows the measurement results of the tooth surface properties of Examples 1 and 2 and Comparative Examples 1 and 2 described above.

  As shown in Table 4, in Examples 1 and 2, the magnitude of the compressive residual stress on the outermost surface of the tooth surface is 800 MPa or more, the maximum height roughness Rz of the tooth surface is 4 μm or less, and the protruding peak portion of the tooth surface. The condition that the height Rpk was 0.40 μm or more was satisfied, and the effect of improving the tooth surface strength could be obtained. On the other hand, Comparative Examples 1 and 2 did not satisfy the above conditions and could not obtain a sufficient effect of improving the tooth surface strength.

STd: Gears not subjected to surface modification treatment SHPa to SHPc: Gears subjected to shot peening FPBa-FPBd: Gears subjected to fine particle peening Rpk Roughness parameter (projection crest height) ξ: Rate of change in tooth surface roughness σ _R … Compressive residual stress

Claims (3)

A gear forming process for forming gears;
A surface modification treatment step of performing a surface modification treatment on the tooth surface of the gear;
Including
In the surface modification treatment step, the compressive residual stress of the outermost surface of the tooth surface is 800 MPa or more, the maximum height roughness Rz of the tooth surface is 4 μm or less, and the protruding peak height Rpk of the tooth surface. A gear manufacturing method in which a surface modification treatment is performed so that the current becomes 0.40 μm or more.   The gear manufacturing method according to claim 1, wherein a surface modification treatment by shot peening is performed in the surface modification treatment step. A gear having a compressive residual stress of 800 MPa or more on the outermost surface of the tooth surface, a maximum height roughness Rz of the tooth surface of 4 μm or less, and a protruding peak height Rpk of the tooth surface of 0.40 μm or more. .

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