WO2021024418A1

WO2021024418A1 - Gear and method for manufacturing gear

Info

Publication number: WO2021024418A1
Application number: PCT/JP2019/031131
Authority: WO
Inventors: 久保　愛三; 裕明松岡; 竜太出野
Original assignee: 公益財団法人応用科学研究所
Priority date: 2019-08-07
Filing date: 2019-08-07
Publication date: 2021-02-11

Abstract

In order to prevent separation damage to a tooth-base tooth surface and damage to a tooth-tip edge portion which are caused by trochoid interference at the beginning of engagement and the end of engagement with a counterpart gear, and thus to increase the life of a gear, a gear device, and a mechanical device in which these are incorporated, a tooth-tip edge portion 10a of each tooth 10 of a gear 1 is made softer than a tooth-base tooth surface 10b, so that from the start of the operation of engaging the counterpart gear the tooth-tip edge portion 10a naturally undergoes plastic deformation due to contact with the tooth-base tooth surface 10, thereby preventing damage to the tooth-base tooth surface 10b and the tooth-tip edge portion 10a.

Description

Gears, gear manufacturing methods

The present invention relates to a gear capable of avoiding peeling damage to the tooth surface due to meshing of the gears and a method for manufacturing the gear.

In recent years, gear devices have come to be operated with a high load, and along with this, peeling damage to the tooth surface of the gear has come to occur frequently. Such damage is a serious problem that makes it impossible to operate the gears any more and may lead to a serious accident.

Specifically, in a normal gear operated with a large load, the tooth surface of the drive gear is cut by the tooth tip edge of the mating gear (driven gear). At this time, a lot of frictional debris is generated, and these are bitten into the tooth surface and damage the tooth surface. In addition, the friction debris bitten into the stepped portion of the tooth base tooth surface generated by the notch of the tooth tip collides with each other, and microcracks that trigger the tooth surface peeling occur. On the other hand, the tooth tip edge of the drive gear strongly rubs the tooth root tooth surface of the mating gear when it is disengaged, and chipping of the tooth tip edge occurs due to the large contact stress and high heat generated at that time. Then, the peeling dust and the chipping dust are bitten into the tooth surface of the gear and the mechanical parts such as the bearing of the gear device. As a result, deterioration and damage of the entire device are accelerated, which eventually leads to the occurrence of an accident (see Non-Patent Document 1). FIG. 1 is a photograph showing an actual example of damage to the gear. FIG. 1A shows two examples of flaking damage on the tooth surface on the left and right, and FIG. 1B shows an example of partial tooth defect. FIG. 1 (c) shows an example of pitching damage at the start of meshing of the helical gear and an example of tooth surface peeling starting from there, and FIG. 1 (d) shows an example of chipping damage at the tooth tip edge of the drive gear. Is shown.

Such problems have been conventionally recognized in the present technical field, and in Non-Patent Document 1, Japanese gear devices are described as "matters necessary for strengthening the competitiveness of gear devices and problems to be solved". In order to maintain strong international competitiveness in the future, 1) improvement of durability and reliability, 2) improvement of vibration and noise performance during operation, 3) improvement of power transmission efficiency, 4) prevention of troubles, 5 ) It is stated that it is necessary to carry out technological innovations related to gear devices in all aspects of cost reduction. And, it is said that the basis for improving the overall design / manufacturing technology and building overwhelming competitiveness is "design / manufacturing technology that makes the gear stronger than the gears designed by the conventional method". That is, as a measure against peeling damage as described above for gears, the purpose is "prevention of damage that cannot be dealt with by conventional design methods", but in the current situation where the damage phenomenon has not been fully elucidated, accurate damage prevention measures. The trend in this technical field is that the gears are being researched and developed with the aim of increasing the strength of the gears as a practical countermeasure when the gears have not been constructed.

For example, in Patent Document 1, in order to achieve both high strength of the tooth surface of the gear and toughness of the tooth root, charcoal substances are formed on the tooth surface and the tooth root by treatment such as cooling and heating after the carburizing treatment. By making the dispersion precipitation rate, nitrogen concentration distribution, and austenite amount distribution different, and making the dispersion-precipitated carbide spherical or pseudo-spherical, the tooth surface is made harder than the tooth root, and the tooth surface has pitching resistance. It is said that it provides high-strength gears that have both wear resistance, bending fatigue strength at the tooth root, and impact resistance. The technology described in this document can be said to be an example of efforts aimed at increasing the strength of gears as described above.

JP 2000-18369 JP 2015-187479

Although the direction of research and development to make the gear itself ultra-high strength is one of the measures to prevent damage to the gear and improve its performance, after all, the tooth edge of both the drive gear and the driven gear It is hard to say that it will lead to a fundamental solution to the phenomenon that tooth surface damage and partial loss of teeth as a result of its progress occur due to strong contact. As a measure to avoid such damage, a method of smoothly rounding the edges of the teeth can be considered. However, since the spatial existence of the edges of all the teeth of the gear is not simple, it is not easy to process for smooth rounding, and the absolute performance of the gear is more than the cost required for processing such as for aircraft and competitive automobile drive. It can be said that it can be applied only to special applications that need to be pursued, and it cannot be said that it is a technology that can be used for all gears that are required to be manufactured at low cost.

Therefore, the present inventor has studied in detail the mechanism of occurrence of damage peeling in gears, which was described in Non-Patent Document 1 as "the damage phenomenon has not been sufficiently elucidated at present", and has been carried out in the present technical field as follows. An approach that has not been studied has led to the invention of gears and methods of manufacturing them that can significantly reduce damage during operation with little increase in manufacturing costs.

Prior to the description of the present invention, the mechanism of gear peeling damage clarified by the research of the present inventor will be described. First, FIG. 2 shows a common tangent line (also referred to as an action line) of the basic circle of the driven / driven gear in which the contact of the tooth profile of the involute curve occurs. In the mechanical theory of gear geometry (tooth meshing theory), there is a pair of teeth that are already in contact at the middle position of the line segment AB in the figure, and the contact point is due to the rotational movement of the gear. It teaches that it moves upward (in the direction of A) on the line, and along with that, the next tooth surface comes to point B, where new teeth enter the meshing state. However, if the pair of teeth that are already engaged on the upper side in the figure is greatly deflected by the load, the driven gear naturally causes a rotation delay with respect to the drive gear by the amount of the deflection. As a result, the newly engaged teeth start contacting at point C, which is earlier than point B, which is the starting point of meshing taught in the gear meshing theory. This phenomenon is caused by the phase difference of the actual involute tooth profile with respect to the theoretical normal position in the cross section perpendicular to the gear axis, and also occurs when the tooth has tooth muscle crowning, one-sided contact, pitch error, and the like.

In the state immediately before the contact between the teeth newly engaged at point C starts, the teeth that are already engaged are in charge of the total load, so that the teeth are bent by that amount, and the rotation of the driven gear with respect to the drive gear is delayed. ing. Since the teeth of the driven gear that are newly engaged at point C are in a state where the edges of the tooth tips are in contact with each other, the force shared by this contact portion is extremely small compared to the force that the already engaged teeth have. .. This interference occurs during the tooth contact from point C to point B, that is, when the angle of the tip of the tooth strongly contacts the mating tooth root due to the rotation delay of the driven gear with respect to the driving gear (trochoid interference). The reaction force received by the raised tooth is sufficiently smaller than the force received by the pair of teeth whose tooth surfaces are already engaged, and the duration of the state or the advance of the gear rotation angle is small. Therefore, the movement of the contact point from the point C to the point B during the trochoid interference proceeds in a substantially displacement-forced state in which the amount of rotation delay of the driven gear with respect to the drive gear hardly changes. Such a state is shown in the upper part of FIG. 3 as a schematic view of an attack on the tooth surface of the tooth root by the tooth tip edge of the mating gear.

As shown in the figure, as the rotation of the gear progresses, the tip of the mating tooth approaches the tooth surface of the tooth by drawing a trochoid curve. Here, for convenience, the state at which the gears start to mesh with each other will be limited, but even at the end of meshing, the same state as in the upper part of the figure occurs, and the moving direction of the contact point is opposite to the state at which the gears start to mesh. By replacing the term "beginning of meshing" with "end of meshing" in the following explanation, the state of the end of meshing can be explained in the same way as the state of the beginning of meshing. Now, when this trochoid curve bites into the tooth surface, the trochoid interference state in which the tooth tip edge strongly contacts the tooth surface. Micro teeth called crowning (dental muscle crowning) so that the tooth profile is slightly forward at the center of the tooth width and posterior at both ends of the tooth so that the tooth surface can be applied well to the teeth of the gear over the entire tooth width. Surface shape modification processing is applied. In the figure, the tooth profile of the drive gear is shown by three lines. The center line connected to the adjacent tooth on the right side of the figure is the reference front tooth profile, and the outer line is the tooth profile at the center of the tooth width, and the inside. Lines indicate the tooth profile at both ends of the tooth. In this case, the trochoidal interference becomes large at the central portion of the tooth width. The photograph shown in the lower part of FIG. 3A shows a state in which the tooth surface at the center of the tooth width is strongly attacked by the tooth tip edge of the mating gear. In this way, in the gear to which the tooth muscle crowning is given, a large trochoidal interference occurs in the central part of the tooth width, and as a result, the tooth surface is attacked by the mating tooth tip that meshes according to the shape of the tooth muscle crowning. , It is recognized that the tooth surface at the root of the tooth is beginning to be strongly damaged. On the other hand, it can be seen that trochoidal interference does not occur at both ends of the tooth. Such a situation always occurs as the gear tooth meshing progresses. The photograph of FIG. 3B shows a state in which tooth surface peeling occurs from the trochoidal interference portion. Comparing the photographs of FIGS. 3A and 3B, it is clear at a glance the mechanism by which the tooth surface peels off after the trochoidal interference portion on the tooth root tooth surface begins to be damaged.

FIG. 4 schematically shows the state of change in contact stress on the tooth surface of the drive gear in such a contact state. In the figure, a large contact stress is generated between the tooth tip and the tooth base, heat is generated due to the large relative slip of the tooth surface at the contact point, and the shearing force acting on the surface is large, so that the damage is extremely caused. The state that it becomes easy to do is drawn. Specifically, as shown in the curve schematically showing the state of Hertz stress of tooth surface contact in the figure, the edge and the surface in the trochoid interference region from point C to point B where the contact with the mating tooth surface is newly started. The contact stress increases sharply due to contact with the tooth (indicated by a thick single-point chain line in the figure), and the contact area is large from point B to the tooth tip direction because the contact is between the surface and the mating tooth surface. Therefore, the contact stress becomes small, but when the tooth tip reaches the tooth tip, the tooth tip edge, which is a free end, becomes in a state of engaging with the mating tooth surface, and the contact stress increases in accordance with the decrease in the area of the contact ellipse. Due to such a state, the gears as shown in FIG. 1 are damaged. On the other hand, in the tooth surface durability evaluation method currently used in all the countries of the world in this technical field, the contact between the tooth surfaces is the rationale, and the trocolloid interference is not considered at all. The effective range is substantially the area indicated by the thick arrow in the figure from the point C at the root of the tooth to the front of the tooth tip. That is, the tooth root and tooth tip between points C and B are out of the effective range. That is, the destructive peeling damage experienced in many practical gears often occurs outside the effective range of the tooth surface durability evaluation method, that is, the contact between the tooth tip edge and the tooth root tooth surface, and the conventional tooth surface durability Studies prior to the present invention have revealed that the trigger (trigger damage) is caused by the unexpected contact stress generated in the tooth root and the tooth tip in the force evaluation method.

Here, an example of damage due to the generation of unexpected contact stress at the tooth root and the tooth tip of the gear is shown. FIG. 5A is a photograph showing the tooth surface condition in the intermediate stage in which the helical gear is endurance-operated under a heavy load and the gear life has not yet been reached. Since this helical gear was given crowning to the tooth muscle, trochoidal interference occurred at the tooth root in the center of the tooth width of the mating gear due to contact with the tooth tip edge, causing severe adhesive wear. It shows that gear damage is in progress. FIG. 5B is a photograph of an example in which a passenger car driving gear is endurance-operated, in which the tooth surface state at an intermediate stage where the gear life has not yet been reached is observed by a replica that transfers the tooth surface to acetyl cellulose. The tooth surface at the start of meshing between the tooth tip edge of the helical gear and the tooth root is white, but this is because the surface roughness is crushed and smoothed due to the contact with the tooth tip edge of the mating gear. The state is shown. An enlarged view of this meshing start point is shown in FIG. 5 (c). As a result of high-pressure, high-slip contact with the tooth tip edge of the mating gear, it is observed that the stress state at the contact point exceeds the yield strength of the material, and fine pits and cracks begin to occur. These trigger injuries then develop into injuries as shown in FIG. 1 (c), which is the pre-stage in FIG. 5 (c). FIG. 5D is a photograph showing the tooth surface of the root of the drive gear, which generates a large amount of heat due to the attack of the tip of the mating tooth and causes adhesive wear. The part where the adhesive wear is caused appears white in the photograph. Due to the high temperature of the tooth surface below it, the lubricating oil is sludged and adhered. The tooth surface that has undergone adhesive wear is burned back due to high temperature, its hardness decreases, and it becomes extremely easy to cause damage, and the behavior of the positive feedback system for gear damage is progressing. FIG. 5 (e) is a replica image of the tooth surface with flaking damage shown in FIG. 1 (a) left, and it is clear that the damage was generated from the meshing portion with the tooth tip edge of the mating gear. You can check.

Gear damage as shown above occurs because the hard tooth tip edge attacks the tooth surface of the mating gear, which is softer than that. Conventionally, in order to increase the strength of gears, only increasing the tooth surface hardness has been aimed at. The most common carburizing and quenching method used to increase the strength of gears is to diffuse carbon from the tooth surface to increase its hardness, which is a gear heat treatment method along this direction. At the time of this treatment, the tooth edge has surfaces in many directions as compared with the tooth surface, and carbon invades from all the surfaces, so that it is inevitably harder than the tooth surface in which carbon invades only from one surface, and The tendency to become brittle is inevitable. Therefore, it is probable that the hard edge attacked the tooth surface of the mating gear, so that the above-mentioned damage was likely to occur. That is, even if the gear is manufactured so as to have a constant and uniform hardness, there is a hardness difference that the tooth tip edge is harder than the tooth root tooth surface when each part is microscopically grasped. Can be said to be particularly remarkable when carburizing and quenching is performed to increase the hardness.

On the other hand, logically considering the state of occurrence of such damage, it is expected that such damage will be significantly reduced if the hardness of the tooth tip edge is lower than the tooth surface of the mating gear that meshes with it. Tooth. As one of the methods for manufacturing a gear that lowers the hardness of the tooth tip edge, for example, a method of coating the tooth surface with a material having a hardness lower than that of the base material, as described in Patent Document 2, is used. Conceivable. However, the purpose of the technique described in the same document is to apply compressive residual stress to the tooth surface by improving the adhesion between the base material and the coating layer whose surface hardness is lower than the hardness of the base material. Even if shot peening is performed, the base material and the coating layer do not easily peel off, and compressive residual stress is applied to the tooth surface, which is caused by the generation of unexpected contact stress between the tooth root and the tooth tip of the gear. There is no suggestion in the document to prevent damage. Then, in order to achieve the purpose of improving the durability of the tooth surface, a coating layer coated with a material having a hardness lower than that of the base material is applied to the central part of the tooth surface (intermediate part between the tooth tip side and the tooth root side of the tooth surface). In this coating layer, the hardness is inclined (changed) so that the hardness becomes closer to that of the base material from the outer surface side to the base material side, and the thickness and surface thickness of the coating layer at the center of the tooth surface are formed. A uniform, low-hardness (softer than the base metal) coating layer is formed on the tooth tip side and the tooth root side in order to equalize. However, assuming that a coating layer softer than the base material is formed only on the tooth surface on the tooth tip side for the purpose of making the hardness of the tooth tip edge lower than the hardness of the tooth root tooth surface, such a gear When the teeth are engaged and operated, the coating layer is easily peeled off at the tooth tip edge, and the hard base material is exposed, which leads to the result of causing damage to the gear.

In view of the above problems, the present invention can significantly reduce such damage in order to fundamentally solve the cause of damage on the tooth tip edge and the tooth surface of the tooth roots of the meshed gears. The main purpose is to provide gears based on new ideas and manufacturing methods capable of manufacturing such gears with almost no cost increase.

That is, the present invention is a gear constituting the gear device, wherein the hardness of the tooth tip edge portion in a softened state in each tooth is softer than the hardness of the tooth surface of the mating gear. ..

Further, the present invention is characterized in that, in a gear constituting a gear device, the hardness of the tooth tip edge portion in a softened state in each tooth is softer than the hardness of the tooth surface of the same tooth. Is.

That is, in the present invention, the hardness of the tooth tip edge portion of each tooth may be softer than the hardness of the tooth root tooth surface of the mating gear, and the tooth root tooth of the same gear as the tooth tip edge portion. It may be soft in comparison with the hardness of the surface, or may satisfy both of them. Hereinafter, the term "tooth surface" is not particularly limited, and is assumed to be common to both the tooth surface of the mating gear and the tooth surface of the gear itself. The gears subject to the present invention include all types of gears such as spur gears, helical gears, bevel gears, screw gears, hypoid gears, internal gears, rack pinions, worms and worm wheels, and are driven. Both gears and driven gears are included. Further, even in a gear in which trochoidal interference does not occur, the present invention is effective for all gears in which the tooth tip edge portion contacts the tooth root tooth surface of the mating gear. In the present invention, the tooth tip edge portion includes a tooth tip edge of each tooth and is a limited fixed region from the tooth tip surface to the tooth surface in the tooth profile direction (particularly within about 0.7 mm from the tooth tip edge in the tooth profile direction). The range of). Since the hardness (hardness) of each part of the gear varies slightly for each measurement point, the hardness (or softness) of the tooth tip edge portion and the tooth root tooth surface is a region included in them. The evaluation is made based on the average value of the hardness measured at a plurality of points of the above, and the same applies to the description of each invention described below.

In such a gear of the present invention, when the tooth tip edge of the driven gear, which is softer than the tooth surface of the driving gear, starts to come into contact with the tooth surface of the driving gear at the beginning of meshing between the gears, the hardness of the driven gear is relatively low. Since the edge of the tooth tip is naturally crushed (plastically deformed) and rounded, the tooth surface of the drive gear, especially the tooth surface endurance evaluation method of the conventional method, occurred outside the effective range, so no countermeasure was taken. Peeling damage at the meshing limit point can be easily prevented. On the other hand, at the end of meshing between the gears, when the tooth tip edge of the drive gear, which is softer than that, starts to come into contact with the tooth surface of the driven gear, the tooth tip edge of the drive gear having a relatively low hardness naturally becomes. Since it is crushed (plastically deformed) and rounded, it is possible to easily prevent chipping damage at the tooth surface of the driven gear, particularly at the meshing limit point. That is, contrary to the conventional tendency of increasing hardness in the present technical field, the tooth tip edge portion that is not directly related to power transmission is positively softened, and the tooth tip edge portion is immediately after the start of gear operation. By making it possible to lose the hardness of the tooth surface of the mating gear and automatically roll it, trochoid interference can be suppressed, so that damage to the tooth surface can be significantly reduced. It becomes. Of course, damage such as a chipped edge of the rounded tooth tip is less likely to occur. According to such a technique, the life of the gear can be extended and the cause of failure of the device incorporating the gear device can be reduced. Since the tooth tip edges and tooth tip surfaces of the drive gear and the driven gear are hardly involved in power transmission in the gear device, even if deformation such as being rounded due to contact with the tooth root tooth surface of the mating gear occurs. The fact that the operation of the gear device is not affected and that the present invention can prevent damage or loss of the tooth tip edge portion greatly contributes to prolonging the life of the gear device. Further, in the gear of the present invention, since the metal base material is exposed, it can be manufactured at a lower cost than the gear whose tooth surface is coated with a material softer than the base material. It is also possible to avoid the problem of peeling of the coating layer due to the operation of the gear device.

In the gear of the present invention, it is desirable that the tooth tip edge portion softer than the tooth root tooth surface is the tooth tip edge portion in a tempered state. Since the entire gear is usually hardened and tempered in the gear manufacturing process, the tooth tip in the tempered state is further tempered by further tempering only the tooth tip edge portion after the process. An edge portion is obtained. Tempering is commonly used as a process of adjusting the hardness by reheating a steel structure that has become hard and brittle due to martensite formation by quenching, and giving the structure tenacity and toughness while softening it. It is a heat treatment applied to the whole (gear in the present invention) or a part of the part in the case of a shaft, and only a small part (local) of the part such as the edge of the tooth tip is hardened. Returning is not usually done. In the present invention, only the tooth tip edge portion is in a tempered state, so that only the tooth tip edge portion is selectively softened and has tenacity and toughness. It can be a gear having a soft tooth tip edge portion.

In the present invention, the hardness of the tooth tip edge portion with respect to the tooth base tooth surface needs to be 90% or less, preferably 50% or more, and 70% or less and 50% or more. It is desirable because the tooth tip edge portion is easily rounded by contact with the tooth surface. The lower limit of the hardness of the tooth tip edge with respect to the tooth surface is set to 50% because it is difficult in the manufacturing process to reduce the hardness to less than 50%.

In the present invention, it is desirable that the most appropriate portion of the tooth root tooth surface where the hardness is compared with the tooth tip edge portion is the tooth surface near the meshing limit point with the mating gear. In the present invention, the meshing limit point with the mating gear is that the tooth tip of the mating gear newly entering the meshing state comes into contact with the tooth root of each tooth closest to the tooth bottom direction in the gear meshing theory. It shall refer to the position of the tooth base to be obtained.

In the method for manufacturing a gear of the present invention, the hardness of the tooth tip edge portion is reduced to a tooth by undergoing a tooth tip edge softening treatment step by tempering the tooth tip edge portion of each tooth with respect to the gear after quenching. A method of manufacturing a gear that is softer than the hardness of the original tooth surface is suitable.

In the tooth tip edge softening treatment step, it is preferable to perform heat treatment on the tooth tip of each tooth as a specific target, and one of the suitable tempering methods is high frequency induction on the tooth tip edge. A method of tempering by heating can be mentioned. In this case, a high-frequency induction heating coil is placed near the tooth tip of the gear, and the tooth tip edge is heated by energizing this coil under the temperature and time conditions according to the material of the gear, and then naturally cooled. It is possible to adopt a method of tempering and softening the tooth tip edge portion.

Since the tooth tip edge portion of the gear of the present invention is softer than the tooth root tooth surface of the mating gear or the tooth root tooth surface of the same tooth of the gear, the tooth tip edge portion is formed by operating the gear device. Will come into contact with the tooth surface of the tooth of the mating gear and cause plastic deformation and be rounded, so that trocolloid interference will be significantly suppressed at the beginning and end of meshing between the teeth, and conventional teeth. As a result of preventing peeling damage on the tooth root tooth surface that occurred outside the effective range in the surface durability evaluation method and at the same time preventing damage to the tooth tip edge, the gear life is greatly extended, and the gear device and its incorporation. However, it is possible to prevent equipment failure. Further, by passing through a step of locally heating and softening the tooth tip edge portion by using a high frequency tempering method or the like, a gear in which only the tooth tip edge portion of the present invention is softened can be manufactured at low cost. Can be done.

The figure which shows the example of the tooth damage caused by the conventional gear by a photograph. Explanatory drawing which shows the contact state of the tooth tip edge which does not follow the conventional gear meshing theory. The schematic diagram which shows the attack state to the tooth root tooth surface by the tooth tip edge of the conventional mating gear, and the figure which shows the state of the tooth root tooth surface attacked by the tooth muscle crowning by a photograph. The schematic diagram which shows the generation state of the unexpected contact stress by the contact of a tooth root and a tooth tip edge part in a conventional gear. The figure which shows the damage example which was made by attacking the tooth surface of a tooth root by the tooth tip edge part in the conventional gear with a photograph. FIG. 5 is a cross-sectional view showing a gear device to which one embodiment of the present invention is applied. A partially enlarged view of the gear of the same embodiment. The figure which shows the 1st example of the tooth tip edge softening process in the manufacturing method of the gear of the same embodiment. The figure which shows the 2nd example of the tooth tip edge softening process in the manufacturing method of the gear of the same embodiment. The figure which shows the 3rd example of the tooth tip edge softening process in the manufacturing method of the gear of the same embodiment. The figure which shows the 4th example of the tooth tip edge softening process in the manufacturing method of the gear of the same embodiment. In one embodiment of the present invention, as a principle explanation of the tooth tip edge softening step, a diagram showing a temperature distribution state in a tempering test using a disc spring as a sample steel piece. As an explanation of the same principle, a diagram showing a continuous cooling transformation diagram of eutectoid carbon steel together with a constant temperature transformation diagram and a constant-speed cooling curve. The schematic diagram which shows the disc spring and the circular coil used in the same Example. The figure explaining the tempering condition in the same Example. The schematic diagram which shows the hardness measurement point of the disc spring used in the cross-sectional hardness test after high frequency tempering in the same Example. The figure which shows the cross-sectional hardness change of the disc spring in the same test as a graph. The figure which shows the hardness measurement point of the disc spring used in the cross-sectional hardness distribution test of the inner diameter edge part in the same Example. The figure which shows the cross-sectional hardness distribution of the inner diameter edge part in the same test. The figure which shows the state of the tempered structure of the inner diameter edge part by the photograph together with the cross-sectional hardness distribution in the same test.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
In the present embodiment, an example of a gear device X including a drive gear 11 and a driven gear 12 to which the gear 1 of the present invention is applied, and the gear 1 (hereinafter, the drive gear 11 and the driven gear 12 are collectively referred to as “gear X”. A few examples of the manufacturing method of) will be described. Here, for the sake of brevity, a pair of gears 1 as the gear device X (for the sake of brevity, the drive gear 11 and the driven gear 12 are both spur gears. However, this description applies to all gears. (Effective) is taken as an example, and FIG. 6 shows an enlarged cross-sectional view of the meshed portion.

As shown in FIG. 7A by partially enlarging the front tooth profile of some of the teeth of the gear 1 (driving gear 11 and driven gear 12) immediately after manufacturing, this gear 1 has the tooth tips of each tooth 10. The edge portion 10a is manufactured to be softer than the tooth surface 10b. Although the figure shows only a part of the teeth 10 of the gear 1, the relationship between the hardness of the tooth tip edge portion 10a and the tooth root tooth surface 10b is the same for all the teeth 10. Further, the drive gear 11 and the driven gear 12 have a common relationship between the hardness of the tooth tip edge portion 10a and the tooth root tooth surface 10b of each tooth 10. Here, in the present embodiment, the tooth tip edge portion 10a is defined as a localized constant region from the tooth tip surface 102 to the tooth surface 103 in the tooth profile direction including the tooth tip edge 101 of each tooth 10. In particular, it is assumed that the tooth profile direction indicates a certain range (within about 0.7 mm) from the tooth tip edge 101. The tooth tip edge portion 10a is outside the effective range on the tooth tip side of the tooth surface endurance evaluation method that is usually used.

As described above, the tooth tip edge portion 10a of each tooth 10 is softer than the tooth root tooth surface 10b, that is, the tooth tip edge portion 10a of the gear 10 is the mating gear 10 (the driven gear 12 with respect to the driving gear 11 and the driven gear 12). Since it is softer than the tooth surface 10b of the drive gear 11), when the drive gear 11 and the working gear 12 are engaged with each other and the gear device X is started to operate, the tooth surface 10b of the mating gear becomes hard immediately after that. The tooth tip edge portion 10a that comes into contact without biting is naturally plastically deformed and rounded as shown in FIG. 7 (b), and thereafter, the tooth surface 10b of the mating gear is not peeled and damaged. At the same time, the tooth tip edge portion 10a is not damaged.

Next, the method for manufacturing the gear 1 of the present embodiment, particularly the tooth tip edge softening step for softening only the tooth tip edge portion 10a, which is characteristic of the present invention, after gear cutting and heat treatment according to a conventional method. Some types will be described. In addition, this step may be performed before or after the final finishing process of the tooth surface. In the tooth tip edge softening step described in the present embodiment, the object is induced and heated by using a high frequency power source in the same manner as general induction hardening, and tempering is performed. However, ordinary induction hardening or induction hardening is performed. A significant difference from the tempering method is that not the entire tooth 10 of the gear 1 to be processed is heated, but only the tooth tip edge portion 10a is locally heated.

First, as an first example, a tooth tip edge softening step of softening only the tooth tip edge portion 10a of each tooth 10 of the gear 1 by the circular coil 20 will be described. As shown in FIG. 8 as a cross-sectional view, a circular coil 20 having a diameter slightly larger than the tooth tip circle 10c is substantially concentric with the gear shaft 1z in the vicinity of the outer side of the tooth tip surface 102 of the gear 1 to be processed. Arrange as follows. The circular coil 20 is connected to a high-frequency power source (not shown), and by energizing the circular coil 20, the tooth tips are heated. At that time, in order to even out the state of electromagnetic induction heating for the tooth tips of each tooth 10 as much as possible, the circular coil 20 is energized while rotating the gear 10 around the gear shaft 1z (in the direction of the arrow in the figure). Is desirable. In this case, as a function of the high frequency frequency, the applied voltage / current, and the gap difference between the tooth tip circle 10c and the inner circumference 20a of the circular coil 20 due to the rotation of the gear 1, the tooth tip edge portion 10a has a substantially sinusoidal time. It will be heated according to the circumstances. After some time has passed, when the energization is stopped when the temperature distribution near the tooth tip edge portion 10a becomes appropriate, the tooth tip is naturally cooled by heat dissipation into the atmosphere and heat conduction to the inside of the gear 10, and the tooth is cooled. The tip edge portion is tempered and softened, and the tooth tip edge softening step is completed.

In this example, the step of softening the tooth tip edge portion of the spur gear as the gear 1 has been described, but in the case of the gear 1 having a wide tooth width and the circular coil 20 cannot heat the entire gear 10 at the same time and all at once, The circular coil 20 may be heated while being moved in parallel with the gear shaft 1z, or this operation may be repeated a plurality of times. Further, instead of the circular coil 20, the coil may have a shape deviated from a perfect circular cross section or a polygonal shape. In that case, it is preferable to heat the gear 10 while rotating it. Further, although the circular coil 20 can have a circular cross section without a break, in order to facilitate the installation of the circular coil 20 on the gear 10, a circular coil with a part cut out, a cut or a hinge in a part, and the like. It is also possible to use a circular coil that can be opened and closed by providing a.

The second example of the tooth tip edge softening step is an example in which the tooth tip of the gear 1 is tempered using a plate coil. In this example, as shown in FIG. 9, one plate-shaped coil 21 connected to a high-frequency power supply is used, and the plate-shaped coil 21 is attached to the tooth tip surface 102 of the gear 1 as in the circular coil 20 of the first example. In the vicinity of the outside, it is slightly separated from the tooth tip circle 10c. When the plate coil 21 is energized at high frequency while rotating the gear 1 (in the direction of the arrow in the figure) in this state, the tooth tip edge portion 10a is heated when the tooth tip passes near the magnetic field generated by the plate coil 21. The state in which the temperature changes in a substantially pulsed manner, that is, it is naturally cooled immediately, is repeated. In the case of this example, the heating time can be adjusted by changing the rotation speed of the gear 1, and it becomes easy to set the optimum tempering condition of the tooth tip edge portion 10a. Then, when the energization is stopped when the temperature distribution near the tooth tip edge portion 10a becomes appropriate after the lapse of a predetermined rotation speed or rotation time, the tooth tip dissipates heat into the atmosphere and heats the inside of the gear 10. It is naturally cooled by conduction, and the tooth tip edge portion is tempered and softened, and the tooth tip edge softening step is completed. The advantage of the tooth tip edge softening step in this example is that the gear 1 is extremely easy to install on the plate coil pair 21.

The third example of the tooth tip edge softening step is an example in which the tooth tip of the gear 1 is tempered using two plate-shaped coils. In this example, as shown in FIG. 9, a set of plate-shaped coil pairs 22 composed of two plate-shaped

coils

22a and 22a connected to a high-frequency power supply, respectively, is used, and each plate-shaped coil 22a is used as a gear 1 for example. They are arranged so as to face each other in the radial direction so that each plate-shaped coil 22a is slightly separated from the tooth tip circle 10c in the vicinity of the outer side of the tooth tip surface 102 of the gear 1 as in the case of the first example and the second example. When the plate-shaped coil 21a is energized at high frequency while rotating the gear 1 (in the direction of the arrow in the figure) in this state, the tooth tip edge portion 10a heats up when the tooth tip passes near the magnetic field generated by each plate-shaped coil 22a. Then, the temperature changes in a substantially pulsed manner, in which the temperature is naturally cooled immediately. In this example, by arranging the two plate-shaped

coils

22a and 22a at positions corresponding to 180 degrees of the diameter of the gear to be processed (gear 1), the eccentricity of the gear 1 or the like causes the heated state of each tooth 10 of the gear 1 to be heated. The effect of non-uniformity can be eliminated. Further, high frequency power supplies having different frequencies can be connected to each plate coil 22a. As a result, by changing the frequency of the voltage and current when energized for each of the plate-shaped

coils

22a and 22a, the depth of the eddy current heat source in the material of the tooth tip (inside the gear 1) and the tooth edge of heat generation It is possible to more appropriately adjust the temperature distribution in the vicinity of the tooth tip edge portion 10a by changing the degree of concentration on the 10a. On the other hand, both

plate coils

22a and 22a should be used with the same high frequency power supply having the same voltage and the same frequency so that the magnetic fields generated by the two

plate coils

22a and 22a arranged at both sides of the diameter of the gear 1 are equal. (It is preferable to connect both plate-shaped

coils

22a and 22a to a common high-frequency power supply). As a result, the influence of the fluctuation of the distance between each plate-shaped coil 22a and the tooth tip circle 1c of the gear 1 on the heat generation of the tooth tip is canceled, and the eccentricity of the gear 1 and the inaccuracies in the installation position of the gear 1 are canceled. Regardless, it is possible to create a situation where all tooth tips are heated evenly. In the case of this example, the heating time can be adjusted by changing the rotation speed of the gear 1, and it becomes easy to set the optimum tempering condition of the tooth tip edge portion 10a. Then, when the energization is stopped when the temperature distribution near the tooth tip edge portion 10a becomes appropriate after the lapse of a predetermined rotation speed or rotation time, the tooth tip dissipates heat into the atmosphere and heats the inside of the gear 10. It is naturally cooled by conduction, and the tooth tip edge portion is tempered and softened, and the tooth tip edge softening step is completed. The advantage of the tooth tip edge softening step in this example is that the gear 1 is extremely easy to install on the plate coil pair 21.

As in the second and third examples, in the tooth tip edge softening step using the plate-shaped coil, since the number of plate-shaped coils can be one or more, any plurality of plate-shaped coils can be used. Can be applied. As an example, the fourth example of the tooth tip edge softening step is also an example in which the tooth tip of the gear 1 is tempered using a plate coil. In this example, as shown in FIG. 11, high frequencies are used. It differs from the third example described above in that it uses two sets of plate coil pairs 23 and 24 connected to the power supply. In this fourth example, the pair of

plate coils

23a and 23a constituting the plate coil pair 23 and the pair of

plate coils

24a and 24a constituting the plate coil pair 24 are 90 in the diameter direction of the gear 10, respectively. The plate-shaped

coils

23a and 24a are arranged so as to face each other with different degrees of angles and phases, and the plate-shaped

coils

23a and 24a are slightly separated from the tooth tip circle 10c near the outer side of the tooth tip surface 102 of the gear 1 as in the first to third examples. I am doing it. In this case, by changing the frequency of the voltage and the current when the plate coil pair 23 and the plate coil pair 24 are energized, the depth of the eddy current heat source in the material of the tooth tip (inside the gear 1) and the heat generation are generated. It is possible to more appropriately adjust the temperature distribution in the vicinity of the tooth tip edge portion 10a by changing the degree of concentration of the tooth tip edge portion 10a and the like. Similar to this example, the number of coil pairs consisting of a pair of plate-shaped coils can be further increased.

In the tooth tip edge softening step as described above, as described in the next embodiment, the tooth tip edge can be tempered by heating for a very short time, and the method is also simple. Only the tooth tip edge softening step is added to the manufacturing process of the gear 1 of the above, and the gear 1 with less damage can be manufactured without causing a large cost increase. In the tooth tip edge softening step described above, the softening treatment of the tooth tip edge 10a for the spur gear has been described, but in these examples, the gear having teeth formed along the circumferential direction of the cross section. It can be applied in general. Further, in the case of an internal tooth gear, various methods by high-frequency tempering such as heating with a coil arranged on the inner peripheral side of the gear on which the teeth are formed and heating with a plate-shaped coil in the case of a rack are performed. The tooth tip edge softening step can be carried out. In addition to the high-frequency tempering method, it is also possible to carry out a local tooth tip edge softening process by heating by irradiating only the tooth tip edge of the gear with a laser and then naturally cooling it. is there. Regardless of which method is used for the tooth tip edge softening step, the energizing time, voltage, frequency, heating temperature, etc. may be appropriately set according to the material of the gear.

Here, among the methods for manufacturing gears in the present invention, the principle of the tooth tip edge softening step by the high-frequency tempering method in the above-described embodiment will be described first, and then a verification test using a sample steel piece will be described.

In the tooth tip edge softening step in the above-described embodiment, the target gear, particularly the tooth tip edge of the gear, is heated by induction by induction, as in the case of general induction hardening. When an alternating magnetic field is applied to the tip of the gear by a high-frequency AC current coil, an eddy current is generated in the material near the tip of the tooth of the gear, but this eddy current is concentrated on the edge due to the skin effect. As shown in FIG. 12, which is the result of the electromagnetic induction heating simulation, the temperature of the edge portion is raised. In the figure, the temperature distribution of the right half of the disc spring 2 by the circular coil 20 under two tempering conditions (hereinafter referred to as condition I and condition II) in the tempering test using the disc spring 2 as a sample steel piece described later. Is shown. The degree of temperature rise is determined by the size and shape of the object, the distance to the coil, the power frequency, the energizing time, and the steel grade of the gear. When heating is stopped in such a temperature distribution state, the heated portion is naturally cooled and the temperature gradually decreases. The hardness distribution inside the tooth tip is a function of how the temperature at each point inside the gear tooth changes over time and the metallurgical properties of the gear material. It will be decided.

FIG. 13 shows an example of a continuous cooling transformation diagram (CCT diagram, solid line in the diagram) of eutectoid carbon steel. Here, a constant temperature transformation diagram (TTT diagram) showing the hardenability of the steel material of the gear targeted in the tooth tip edge softening step is shown in the right half of the diagram with dotted lines and solid lines Ps (start line of pearlite transformation). It is assumed that it is an S-shaped curve of Pf (the completion line of pearlite transformation). Further, the temperature change at each point in the material of the tooth tip is represented by a constant-velocity cooling curve shown by a alternate long and short dash line that decreases from a position slightly above 700 ° C. in a parabolic manner. When the temperature at a certain point inside the tooth tip changes rapidly like the leftmost constant-velocity cooling curve, this point means that it is hardened and the hardness increases. On the other hand, when the temperature gradually decreases as shown by the two constant-velocity cooling curves on the right side, this point means that the temperature is tempered and the hardness decreases. The symbol Ac ₁ shown in the figure indicates the temperature at which austenite begins to form during heating, Ms indicates the temperature at which austenite begins to transform into martensite during cooling, and M50% and M90% indicate the temperature during cooling. It means the temperature at which 50% and 90% of austenite transform into martensite, respectively.

Next, a sample steel piece having a hardness of about 650 (Hv650), which imitates the tooth tip edge of a gear, is subjected to high frequency induction heating as shown in FIG. 12, and a pseudo tempering experiment of the tooth tip edge is performed. It was. The sample steel piece employs a disc spring 2 as shown in a plan view (FIG. 14A) and a longitudinal sectional view (FIG. 14B) in FIG. 14, and the upper surface (concave surface) of the disc spring 2 is adopted. The inner diameter edge portion 2a on the) side is regarded as the tooth tip edge portion 10a of the tooth 10 on the gear 1. In the experiment, the inner diameter edge portion 2a was tempered by intensively high-frequency induction heating of the inner diameter edge portion 2a while rotating the disc spring 2 by a circular coil 20 having an outer diameter slightly smaller than the inner diameter of the disc spring 2. It is tempered and the degree of hardness and tempered range is examined. There are two types of tempering conditions, condition I and condition II as shown in FIG. 15, and the frequency, anode voltage, and anode current are equalized under both conditions, and only the heating time is 0.9 seconds under condition I. , Condition II provides a difference of 1.0 second. As a result, although not shown, a temper color was observed at the inner diameter edge portion 2a on the concave surface side from the appearance observation of the disc spring 2 subjected to the high frequency tempering treatment under the condition I, and the range was 1. It reached up to 4 mm.

First, the change in cross-sectional hardness of the disc spring 2 due to high-frequency tempering was investigated. Three types of disc springs 2 before tempering and disc springs 2 after tempering under conditions I and II (all equivalent products) were cut in the radial direction, and as shown in FIG. 16, each disc spring was cut. For each of the left half 2A and the right half 2B of 2, 0.1 mm inside from the concave surface, 0.2 mm interval in the range of 0.2 to 0.6 mm in the radial direction from the inner diameter edge, 0.6 to 3.0 mm In the range of, measurement points (indicated by black circles in the figure) are set at positions at intervals of 0.3 mm, and the cross-sectional hardness at each measurement point is measured with a Micro Vickers hardness tester (load 2.9 N, 5 seconds). , FIG. 17 is shown as a cross-sectional hardness change diagram. The cross-sectional hardness of the disc spring 2 before tempering showed a value of 600 to 650 Hv. After tempering by high-frequency heating under Condition I, the hardness could be reduced to about 400 Hv in the vicinity of the inner diameter edge portion 2a. On the other hand, after about reversion by high-frequency heating under Condition II in which the heating time was longer than that under Condition I by 0.1 seconds, a result slightly harder than that under Condition I was obtained at most of the measurement points. In the case of condition I, an extreme increase in hardness occurred in the vicinity of the inner diameter edge portion 2a of the left half portion 2A, but this was due to the positional relationship between the disc spring 2 and the circular coil 20, rotation and heating (energization). It is presumed that quenching occurred depending on the timing and tempering conditions. Further, in both condition I and condition II, the hardness at the measurement point distal to the circular coil 20 was 600 to 650 Hv, which was close to the hardness before tempering. Therefore, at these measurement points, the high frequency due to the circular coil 20 was obtained. The result was that no tempering was done. Similar to this experiment, as a result of experiments under several other tempering conditions, the inner diameter edge portion 2a having a hardness of about 650 Hv before tempering was about 400 Hv (baked) by the tempering process using the circular coil 20. It was found that the hardness can be locally reduced to about 60% of the hardness before tempering, and the hardness at a place away from the inner diameter edge can not be changed at all.

Next, the distribution of the cross-sectional hardness of the inner diameter edge portion 2a of the disc spring 2 was measured in detail. Similar to the measurement test of the change in the cross-sectional hardness of the countersunk spring 2 due to the high frequency tempering described above, the countersunk spring 2 tempered under the condition I was cut in the radial direction, and as shown in FIG. Only the measurement point of the inner diameter edge portion in the cross section of the right half portion 2B is shown, but the measurement point is actually set for the left half portion 2A as well), the diameter direction and the inner diameter from the inner diameter edge on the concave surface side. With the position of 0.2 mm each in the internal direction (vertical direction in the figure) as the reference point for the measurement point, the measurement points are set at intervals of 0.2 mm in the vertical and horizontal directions in the figure based on this reference point, and the inner diameter from the reference point is further set. Measurement points were also set at positions of 0.1 mm in length and width on the edge side, and the cross-sectional hardness at each measurement point was measured with a Micro Vickers hardness tester (load 2.9 N, 5 seconds). Shown as a distribution map. In the figure, along with the numerical value (Vickers hardness) of the measurement result at each measurement point, the boundary of hardness 450Hv is shown by a solid line, the boundary of 500Hv is shown by a chain line, and the boundary of hardness 550Hv is shown by a broken line. From this result, the boundary of 450 Hv is in the range of 0.4 to 0.7 mm from the inner diameter edge portion, and the boundary of 500 Hv is in the range of 0.9 to 1.0 mm from the inner diameter edge portion (0.9 mm is left without illustration). (Based on the measurement results in half 2A), it was found that the boundary of 550 Hv was in the range of 1.3 to 1.4 mm from the inner diameter edge. Similar to this experiment, when the cross-sectional hardness distribution was measured by changing the heating time by the circular coil 20, it was found that the tempering range was widened by extending the heating time, and the tempering range was narrowed by shortening the heating time. ..

Furthermore, as a macro observation, the peripheral inner diameter of the left half 2A and the right half 2B of the disc spring 2 was nightally etched. In the structure in this state, as shown in the photograph in FIG. 20, it can be seen that the inner diameter edge portion 2a is strongly tempered. In the figure, the same boundaries of hardness 450Hv, 500Hv, and 550Hv as those shown in FIG. 19 are shown, but the darker the color (the part closer to the inner diameter edge portion 2a), the stronger it is burned and softened. You can grasp the state of being. In addition, in micro observation, it was confirmed that the inner diameter edge portion 2a heated in the vicinity of the circular coil 20 was strongly burned back, and the influence of heating became smaller as the distance from the circular coil 20 increased.

From the results of various experiments in the present embodiment described above, when the disc spring 2 as the sample steel piece is replaced with the gear of the present invention, the tooth tip edge portion of the gear is appropriately softened by the tooth tip edge portion softening step. It is necessary to set delicate conditions depending on the size of the object, the distance to the coil, the power frequency, the energization time, and the steel grade of the gear, but it is certain that only the edge of the tooth tip is tempered and softened. Found to exist in. Further, if the same high-frequency tempering as in the experiment for the disc spring 2 is performed on the tooth tip edge portion of the gear, the hardness of the tooth tip edge portion is about 60% with respect to the tooth surface of the tooth root far from the coil. It is assumed that further tempering of the edge portion is possible if the treatment conditions are selected in detail. The problem is that if the temperature drop rate of the edge part becomes too rapid after heating, it will be in a hardened state and the hardness of the edge part will not decrease, so the material, shape, and size of the object should be such that this does not occur. It is necessary to set by adjusting the coil design, the distance between the edge and the coil, the energizing frequency, the voltage, the rotation speed of the object, and the like. In order for the tooth tip edge portion in contact with the tooth root tooth surface of the mating gear to be plastically deformed and rounded at the time of meshing with the mating gear, at least a significant difference in hardness from the tooth root tooth surface must be recognized. Therefore, it can be said that it is desirable that the degree of softening of the tooth tip edge portion by the tooth tip edge softening step is 90% or less. Further, if it is not required to soften the tooth tip edge portion up to 60%, and it is considered that sufficient plastic deformation can be obtained by softening about 70%, the range of the tooth tip edge portion in the tooth surface direction is , It is sufficient to set it to about 0.7 to 1.0 mm from the tooth tip edge. Note that the above dimensions may vary slightly from 0.7 to 1.0 mm depending on the size of the gear teeth and therefore the module. For example, in the case of a large module tooth, this dimension may be slightly increased. Further, since it is difficult to reduce the degree of softening to less than 50% in the manufacturing process, it is appropriate to set 50% as the lower limit value.

It is known that the hardness of the teeth of a hardened gear is generally the hardest at the tip of the tooth, and the hardness gradually decreases toward the tooth root. Except for gears whose tooth profile near the tooth tip edge is extremely specially modified, this hard tooth tip edge attacks the tooth surface of the mating tooth by trochoidal interference and induces gear damage. In the case of a gear in which the tooth tip edge portion is softened from the vicinity of the tooth surface meshing limit point of the mating gear according to the present invention, a special modification process is performed on the tooth surface shape near the tooth tip edge, which is extremely expensive. It is possible to manufacture a gear having high strength and reliability by softening the tooth tip edge portion only by subjecting the gear to high-frequency heat treatment for an extremely short time in a factory atmosphere, which has great industrial significance.

In this way, when a gear is manufactured by softening the hardness of the tooth tip edge portion through a process of softening the tooth tip edge portion by tempering, the hardness of the tooth tip edge portion is higher than the hardness of the tooth root tooth surface of the mating gear. Can be made sufficiently low (soft), and if trochoid interference occurs when the gears are engaged and the tooth tip edge part makes strong contact with the tooth surface of the mating gear, the softened tooth tip edge part makes strong contact. As a result of plastic deformation due to stress and natural rounding from the initial operation of the gear device, the degree of damage to the tooth surface at the root of the tooth can be extremely reduced, and the degree of damage to the tooth tip edge can also be reduced. Become.

The present invention makes it possible to prevent peeling damage and damage to the tooth tip edge portion due to trocolloid interference on the tooth root tooth surface of the gear, which could not be prevented in the past. Therefore, all kinds of gears and gear devices In addition, the product life of the device incorporating the gear can be extended, which can be extremely useful in the product field to which the gear is applied.

1 ... Gear 10 ... Tooth 10a ... Tooth tip edge 10b ... Tooth root tooth surface

Claims

In the gears that make up the gear device
A gear characterized in that the hardness of the tooth tip edge portion in a softened state in each tooth is softer than the hardness of the tooth base tooth surface of the mating gear.
In the gears that make up the gear device
A gear characterized in that the hardness of the tooth tip edge portion in a softened state in each tooth is softer than the hardness of the tooth surface of the tooth.
The gear according to claim 1 or 2, wherein the tooth tip edge portion is in a state of being softened by tempering.
The gear according to any one of claims 1 to 3, wherein the hardness of the tooth tip edge portion with respect to the tooth root tooth surface is 50% or more and 90% or less.
The gear according to any one of claims 1 to 4, wherein the hardness of the tooth tip edge portion is softer than the tooth surface hardness near the meshing limit point on the tooth root tooth surface.
The method for manufacturing a gear according to any one of claims 1 to 5.
The feature is that the hardness of the tooth tip edge is softer than the hardness of the tooth root surface by undergoing the tooth tip edge softening step by tempering the tooth tip edge of each tooth with respect to the gear after quenching. How to manufacture gears.
The method for manufacturing a gear according to claim 6, wherein the tooth tip edge softening step is a heat treatment for the tooth tip of each tooth as a specific target.
The method for manufacturing a gear according to claim 7, wherein the tooth tip edge softening step is based on a high-frequency tempering method in which the tooth tip of each tooth is a specific target.