Ring gear and method for manufacturing ring gear

A ring gear that, together with a sun gear and a pinion gear that is disposed radially outward of the sun gear and meshes with the sun gear, forms a planetary gear unit, the ring gear includes: a body, internal teeth that are configured to mesh with the pinion gear and that are formed in at least a part of an inner peripheral surface of the body, and external teeth that have a lower hardness than the internal teeth and that are formed in at least a part of an outer peripheral surface of the body, wherein a surface nitrogen concentration in the internal teeth is higher than that in at least tooth surfaces of the external teeth.

BACKGROUND

The present disclosure relates to ring gears and methods for manufacturing a ring gear.

Conventionally, planetary gear units including a ring gear, which are used for automatic transmissions for vehicles etc., are known in the art. For example, Japanese Patent Application Publication No. H04-285346 (JP H04-285346 A) discloses such a planetary gear unit.

Japanese Patent Application Publication No. H04-285346 (JP H04-285346 A) discloses a planetary gear unit for use in automatic transmissions for vehicles, which includes a ring gear, a pinion gear that meshes with internal teeth of the ring gear, and a sun gear that is disposed inward of the pinon gear and meshes with the pinion gear. Since the internal teeth of the ring gear are required to have predetermined hardness, the internal teeth of the ring gear in the planetary gear unit of Japanese Patent Application Publication No. H04-285346 (JP H04-285346 A) have been subjected to nitrocarburizing in order to improve hardness of the internal teeth. Induction hardening may be used to increase hardness of the internal teeth. However, since the processing temperature of nitrocarburizing is lower than that of induction hardening, distortion due to nitrocarburizing is restrained and the internal teeth can be formed with high accuracy. Gear noise that is generated by meshing between the pinion gear and the internal teeth of the ring gear is therefore reduced as compared to induction hardening.

In a planetary gear unit disclosed in Japanese Patent No. 4,867,430, a clutch that is a friction engagement element is disposed radially outward of a ring gear. The clutch includes outer thin plates (clutch plates)43that are first friction materials, inner thin plates (clutch discs)44that are second friction materials, etc., and splines formed in the outer peripheral surface of the ring gear R1are engaged with splines formed in the inner peripheral surface of the clutch discs44. The ring gear R1is thus configured to serve also as a hub of a clutch Ca. Similarly, a ring gear R2also serves as a hub of a clutch Cb.

SUMMARY

However, in the case where such a conventional ring gear having internal teeth and external teeth as described in Japanese Patent No. 4,867,430 is subjected to nitrocarburizing described in Japanese Patent Application Publication No. H04-285346 (JP H04-285346 A), hardness of both the internal teeth and the external teeth is increased by nitrocarburizing because the ring gear is placed in a furnace during nitrocarburizing. As this time, surface hardness of the external teeth of the ring gear is significantly increased by nitrocarburizing to a value higher than surface hardness of the clutch discs that mesh with the external teeth. This is disadvantageous in that the clutch discs tend to wear due to contact with the external teeth of the ring gear. Accordingly, it is difficult to restrain wear of a member that meshes with the external teeth. In the case of automatic transmissions for vehicles, if the clutch discs wear and the wear of the clutch discs progresses, not only gear noise may be generated but also a shift shock may occur.

An exemplary aspect of the disclosure provides a ring gear whose internal teeth can be formed with high hardness and with high accuracy and which restrains wear of a member that meshes with external teeth, and a method for manufacturing such a ring gear.

A ring gear according to a first aspect of the present disclosure is a ring gear that, together with a sun gear and a pinion gear that is disposed radially outward of the sun gear and meshes with the sun gear, forms a planetary gear unit, the ring gear including a body, internal teeth that are configured to mesh with the pinion gear and that are formed in at least a part of an inner peripheral surface of the body, and external teeth that have a lower hardness than the internal teeth and that are formed in at least a part of an outer peripheral surface of the body, wherein and surface nitrogen concentration in the internal teeth is higher than that in at least tooth surfaces of the external teeth.

In the ring gear according to the first aspect of the present disclosure, as described above, the ring gear is formed so that the surface nitrogen concentration in the internal teeth is higher than that in at least the tooth surfaces of the external teeth. Accordingly, in the internal teeth, a nitride layer is formed at surfaces of the internal teeth, whereby the internal teeth can be formed with high hardness. Since the nitride layer is formed by nitriding at a lower processing temperature than induction hardening, distortion due to nitriding is restrained and the internal teeth can be formed with high accuracy. This allows the internal teeth to have sufficient wear resistance and reduces gear noise that is generated as a gear meshes with an inner periphery of the ring gear. In the external teeth, hardness of the external teeth can be reduced as compared to the case where the nitride layer is present at surfaces of the external teeth. This restrains wear of a member that meshes with the external teeth.

A method for manufacturing a ring gear according to a second aspect of the present disclosure is a method for manufacturing a ring gear having internal teeth in an inner peripheral surface and external teeth in an outer peripheral surface from an annular raw blank, including the steps of: forming an intermediate body by forming the internal teeth in an inner peripheral surface of the annular raw blank; forming a nitride layer at a surface of the intermediate body by nitriding an entirety of the intermediate body; removing the nitride layer formed in a part of an outer peripheral surface of the intermediate body resulting from the nitriding, which is a part where formation of the external teeth is desired; and forming the external teeth in a part of the intermediate body from which the nitride layer has been removed and where formation of the external teeth is desired.

In the method for manufacturing a ring gear according to the second aspect of the present disclosure, as described above, the nitride layer is formed at surfaces of the internal teeth by nitriding, and the nitride layer at surfaces of the external teeth is removed. Accordingly, in the internal teeth, the nitride layer is formed at the surfaces of the internal teeth, whereby the internal teeth can be formed with high hardness. Since the nitride layer is formed by nitriding at a lower processing temperature than induction hardening, distortion due to nitriding is restrained and the internal teeth can be formed with high accuracy. This allows the internal teeth to have sufficient wear resistance and reduces gear noise that is generated as a gear meshes with an inner periphery of the ring gear. In the external teeth, hardness of the external teeth can be reduced as compared to the case where the nitride layer is present at the surfaces of the external teeth. This restrains wear of a member that meshes with the external teeth.

According to the present disclosure, as described above, internal teeth are formed with high hardness and with high accuracy, and wear of a member that meshes with external teeth is restrained.

DETAILED DESCRIPTION OF EMBODIMENTS

A planetary gear unit according to an embodiment of the present disclosure will be described with reference toFIGS. 1 to 3.

A planetary gear unit100is configured to be used as a part for automatic transmissions of vehicles. For example, the planetary gear unit100is configured to be used for automatic transmissions of automobiles. As shown inFIG. 1, the planetary gear unit100includes a ring gear1, a sun gear2, and pinion gears3a,3b. The ring gear1is configured so that clutch discs4a,4bmesh with its outer periphery. The planetary gear unit100includes a double-pinion planetary gear including the pinion gears3a,3b.

As shown inFIG. 1, the sun gear2is disposed radially inward of the ring gear1. The sun gear2is configured to mesh with a plurality of the pinion gears3adisposed radially outward of the sun gear2. That is, the sun gear2is disposed approximately in the center of the ring gear1in the radial direction. The sun gear2is also disposed approximately in the middle of the plurality of pinion gears3a. The sun gear2is rotatable about a rotary shaft21.

The pinion gears3a,3bare disposed radially inward of the ring gear1. The pinion gears3a,3bare disposed radially outward of the sun gear2. That is, the pinion gears3a,3bare disposed between the sun gear2and the ring gear1. The pinion gear3ais configured to mesh with the sun gear2and the pinion gear3b. The pinion gear3bis configured to mesh with the pinion gear3aand the ring gear1. A plurality of the pinion gears3bare coupled by a carrier31. The pinion gear3ais rotatable about a rotary shaft32a. The pinion gear3bis rotatable about a rotary shaft32b. The plurality of pinion gears3a,3bare rotatable relative to the sun gear2around the sun gear2.

The carrier31has a carrier plate31aand a carrier plate31b, and the carrier plates31a,31bsupport the rotary shafts32a,32b. The rotary shaft32arotatably supports the pinion gear3a, and the rotary shaft32brotatably supports the pinion gear3b.

The clutch discs4a,4bare disposed radially outward of the ring gear1. A plurality of the clutch discs4a,4bare provided in the direction of the axis of rotation of the ring gear1. Specifically, the plurality of clutch discs4aare arranged on one side of the ring gear1in the axial direction. The plurality of clutch discs4bare arranged on the other side of the ring gear1in the axial direction. A clutch hub10is formed integrally on the outer periphery of the ring gear1. The clutch hub10has splines (external teeth) formed in its outer peripheral surface. The clutch discs4a,4bare engaged with the splines of the clutch hub10. The clutch discs4aare engaged with external teeth12bof the ring gear1with tooth surfaces of the clutch discs4ain contact with tooth surfaces of the external teeth12bof the ring gear1. The clutch discs4bare engaged with external teeth12aof the ring gear1with tooth surfaces of the clutch discs4bin contact with tooth surfaces of the external teeth12aof the ring gear1.

As shown inFIG. 3, each clutch disc4a,4bhas a plurality of teeth formed in its inner periphery and including top lands41, bottom lands42, and tooth surfaces43. Each clutch disc4a,4bhas a plurality of friction materials40bonded to its surface.

A hydraulic servo50has a clutch drum5, a piston member52, a cancel plate53, and a return spring54, and these components form a hydraulic oil chamber55and a cancel oil chamber56.

The clutch drum5is located radially outward of the ring gear1, and a clutch plates51are engaged with splines formed in the inner peripheral surface of a drum portion5aof the clutch drum5. The plurality of the clutch plates51are provided so as to face the plurality of clutch discs4a. That is, the clutch plates51and the clutch discs4aare alternately arranged along the axis of rotation.

The piston member52together with the clutch drum5forms the oiltight hydraulic oil chamber55therebetween which is sealed by a seal ring521and a sealing member522. The piston member52has an extended pressing portion52ain its outer periphery. The pressing portion52ais disposed such that its front end faces the leftmost clutch plate51inFIG. 1.

The return spring54is disposed in a compressed state between the cancel plate53and the piston member52. The cancel plate53, the piston member52, and a sealing member disposed on the outer peripheral portion of the piston member52form the oiltight cancel oil chamber56.

With the above configuration, the piston member52is moved in the axial direction by the balance between oil pressures produced in the hydraulic oil chamber55and the cancel oil chamber56, so that the clutch plates51and the clutch discs4aare pressed against or separated from each other. A clutch is thus engaged or disengaged.

A hydraulic servo60has a clutch drum6, a piston member62, a cancel plate63, and a return spring64, and these components form a hydraulic oil chamber65and a cancel oil chamber66.

The clutch drum6is located radially outward of the ring gear1, and a clutch plates61are engaged with splines formed in the inner peripheral surface of a drum portion6aof the clutch drum6. The plurality of the clutch plates61are provided so as to face the plurality of clutch discs4b. That is, the clutch plates61and the clutch discs4bare alternately arranged along the axis of rotation.

The piston member62together with the clutch drum6forms the oiltight hydraulic oil chamber65therebetween which is sealed by a seal ring621and a sealing member622. The piston member62has an extended pressing portion62ain its outer periphery. The pressing portion62ais disposed such that its front end faces the rightmost clutch plate61inFIG. 1.

The return spring64is disposed in a compressed state between the cancel plate63and the piston member62. The cancel plate63, the piston member62, and a sealing member disposed on the outer peripheral portion of the piston member62form the oiltight cancel oil chamber66.

With the above configuration, the piston member62is moved in the axial direction by the balance between oil pressures produced in the hydraulic oil chamber65and the cancel oil chamber66, so that the clutch plates61and the clutch discs4bare pressed against or separated from each other. A clutch is thus engaged or disengaged.

As shown inFIG. 2, the ring gear1includes internal teeth11and external teeth12. The external teeth12are formed in two tiers and have the external teeth12aand the external teeth12b. The ring gear1is disposed radially outward of the sun gear2. That is, the ring gear1, together with the sun gear2, the pinion gears3adisposed radially outward of the sun gear2and meshing with the sun gear2, and the pinion gears3bdisposed radially outward of the pinion gears3aand meshing with the pinion gears3a, forms the planetary gear unit100. The ring gear1meshes with the pinion gears3bby the internal teeth11.

As shown inFIG. 1, the ring gear1is configured so that its radially outer portion meshes with the plurality of clutch discs4a,4b. That is, the ring gear1is configured to be used as the clutch hub10. The ring gear1is also configured so that its radially inner portion meshes with the plurality of pinion gears3b.

The ring gear1is formed in an annular shape. A steel material is used as a base material for the ring gear1. For example, the steel material that is used as the base material contains iron and carbon as elements. The base material may further contain silicon, manganese, phosphorus, sulfur, chromium, aluminum, nitrogen, etc. as elements as necessary.

As shown inFIG. 2, the internal teeth11are formed in the inner periphery of the ring gear1. The internal teeth11are configured to mesh with the pinion gears3b. In the present embodiment, a nitride layer13is formed at the surfaces of the internal teeth11by nitriding the base material of the ring gear1. Specifically, the nitride layer13having a higher nitrogen concentration than the base material is formed at the surfaces of the internal teeth11by nitriding the base material of the ring gear1.

As shown inFIG. 2, the external teeth12are formed in the outer periphery of the ring gear1. The external teeth12include top lands121, bottom lands122, and tooth surfaces123and are configured so that the clutch discs4a,4bdisposed radially outward of the ring gear1mesh with the external teeth12. In the present embodiment, the base material of the ring gear1is exposed at the surfaces of the external teeth12. That is, no nitride layer13is present in at least the tooth surfaces123out of the surfaces of the external teeth12. Specifically, the surface nitrogen concentration in at least the tooth surfaces123of the external teeth12is the nitrogen concentration of the base material.

The nitride layer13is formed by introducing nitrogen into the base material located near the surface of the ring gear1. Specifically, the nitride layer13is formed by gas nitrocarburizing. The nitride layer13is harder than the base material of the ring gear1. For example, the base material has Vickers hardness of about 250 HV. The nitride layer13has Vickers hardness of about 600 HV or more and about 900 HV or less. That is, the hardness of the nitride layer13is twice or more that of the base material.

(Manufacturing Method of Ring Gear)

Next, a method for manufacturing the ring gear1will be described with reference toFIGS. 4A to 4E.

First, as shown inFIG. 4A, a raw blank101for the ring gear1is formed by hot forging from a steel material (base material) for forming the ring gear1. Specifically, the base material is formed into an annular shape. The inside diameter of the annular shape is adjusted in view of a machining allowance for forming internal teeth11. The outside diameter of the annular shape is also adjusted in view of a machining allowance for cutting away a nitride layer13that is to be formed later and a machining allowance for forming external teeth12. The inside diameter and the outside diameter of the raw blank101and the end faces of both ends in the direction of the axis of rotation of the raw blank101are also adjusted as necessary.

Next, as shown inFIG. 4B, the internal teeth11are formed in the inner periphery of the raw blank101to produce an intermediate body102. The internal teeth11are formed by, e.g., broaching and chamfering of tooth ends. As shown inFIG. 4C, the intermediate body102is then nitrided to produce an intermediate body103. The intermediate body103thus has the nitride layer13at its surface. This nitriding is performed by gas nitrocarburizing.

Then, as shown inFIG. 4D, the nitride layer13in the outer periphery of the intermediate body103resulting from the nitriding is removed to produce an intermediate body104. At this time, the nitride layer13is removed until the base material in the outer periphery of the intermediate body103is exposed. That is, the nitride layer13formed in a part of the outer peripheral surface of the intermediate body103resulting from the nitriding, namely in a part where formation of the external teeth12is desired, is removed. It is preferable that the outer peripheral surface of the intermediate body103be a cylindrical shape having a smooth surface that is even in the circumferential direction, because this facilitates removal of the nitride layer13formed in the outer periphery by turning.

Subsequently, as shown inFIG. 4E, the external teeth12are formed in the outer periphery of the intermediate body104from which the nitride layer13has been removed, thereby producing the ring gear1. That is, the external teeth12are formed in the outer periphery of the intermediate body104where the base material is exposed. Specifically, the external teeth12are formed in the part of the intermediate body104from which the nitride layer13has been removed, namely in the part where formation of the external teeth12is desired. The external teeth12are formed by, e.g., cutting with a hob cutter. Since there is no nitride layer13in the outer periphery of the intermediate body104and the intermediate body104thus has lower hardness in its outer periphery, the external teeth12can be easily formed in the outer periphery of the intermediate body104. Thereafter, the ring gear1is deburred by shot blasting and cleaned as necessary. The ring gear1is manufactured in this manner.

In the present embodiment, as described above, the nitride layer13is formed at the surfaces of the internal teeth11by nitriding the base material for the ring gear1, and the surfaces of the external teeth12are formed so that the base material is exposed. This allows the internal teeth11to have high hardness. Since the nitride layer13is formed by nitriding at a lower processing temperature than induction hardening, distortion due to nitriding is restrained and the internal teeth11can be formed with high accuracy. This allows the internal teeth11to have sufficient wear resistance and reduces gear noise that is generated as the pinion gears3bmesh with the inner periphery of the ring gear1. The surfaces of the external teeth12are formed so that the base material is exposed. This allows the external teeth12to have lower hardness than in the case where the nitride layer13is present at the surfaces of the external teeth12. This restrains wear of the clutch discs4a,4bthat mesh with the external teeth12. The clutch discs4a,4band the external teeth12of the ring gear1contact each other by the tooth surfaces of core plates forming the clutch discs4a,4band the tooth surfaces of the external teeth12of the ring gear1. It is therefore important to form the external teeth12of the ring gear1so that the base material is exposed at the surfaces of the tooth surfaces of the external teeth12.

In the present embodiment, as described above, the external teeth12of the ring gear1are configured so that the clutch discs4a,4bdisposed radially outward of the ring gear1mesh with the external teeth12, and the nitride layer13is configured to be harder than the base material for the ring gear1. Since the hardness of the external teeth12of the ring gear1which mesh with the clutch discs4a,4bcan be made smaller than that of the nitride layer13, wear of the clutch discs4a,4bis effectively restrained without increasing the hardness of the clutch discs4a,4b.

In the present embodiment, as described above, the nitride layer13in the outer periphery of the intermediate body103resulting from the nitriding is removed until the base material in the outer periphery of the ring gear1is exposed, and the external teeth12are formed in the outer periphery of the intermediate body104where the base material has been exposed. This allows the external teeth12of the ring gear1to be formed so as not to include the nitride layer13and thus more reliably restrains wear of the clutch discs4a,4bthat mesh with the external teeth12.

In the present embodiment, as described above, the nitride layer13of the intermediate body103is formed by gas nitrocarburizing. This allows the nitride layer13having a substantially uniform thickness to be formed in both the tooth surfaces (protruding portions) and the bottom lands (recessed portions) of the internal teeth11.

In the present embodiment, as described above, the outer peripheral surface of the intermediate body103resulting from the nitriding is a smooth surface that is even in the circumferential direction. This facilitates removal of the nitride layer13in the outer peripheral surface of the intermediate body103.

In the present embodiment, as described above, the outer periphery of the intermediate body103is formed into a smooth cylindrical shape that is even in the circumferential direction, and the nitride layer13in the outer periphery is removed by turning. The nitride layer13can thus be cut away by turning with a chip that is harder than a cutter, which facilitates removal of the nitride layer13in the outer periphery of the intermediate body103.

Next, an example will be described in which the inventors actually formed the nitride layer13by nitriding the surface of the ring gear1by gas nitrocarburizing and measured the thicknesses of the nitride layer13at the tooth surfaces (protruding portions) and the bottom lands (recessed portions) in order to verify the effects of gas nitrocarburizing.

In this example, the intermediate body102was subjected to gas nitrocarburizing to form the nitride layer13. This gas nitrocarburizing was performed at about 560° C. for two hours. The thickness of the nitride layer13at the tooth surfaces (protruding portions) of the ring gear1was 0.23 mm. The thickness of the nitride layer13at the bottom lands (recessed portions) of the ring gear1was 0.27 mm. This result shows that the nitride layer13having a substantially uniform thickness can be formed in both the tooth surfaces (protruding portions) and the bottom lands (recessed portions) by gas nitrocarburizing.

The embodiment disclosed herein is merely by way of example in all respects and should not be construed as restrictive. The scope of the present disclosure is defined by the claims rather than by the above description of the embodiment and the example, and the disclosure is intended to cover all alterations (modifications) and equivalents which fall within the scope of the claims.

For example, the above embodiment shows an example in which the planetary gear unit of the present disclosure includes a double-pinion planetary gear. However, it should be understood that the planetary gear unit of the present disclosure may include a single-pinion planetary gear including only one type of pinion gears. In this case, a ring gear is located radially outward of a sun gear, and the planetary gear unit includes a plurality of pinion gears of the same type which mesh with both the sun gear and the ring gear. The above embodiment shows an example in which the planetary gear unit of the present disclosure is used as a part for automatic transmissions for vehicles. However, the present disclosure is not limited to this. The planetary gear unit of the present disclosure may be used in applications other than a part for automatic transmissions for vehicles.

The above embodiment shows an example in which the ring gear of the present disclosure is used for planetary gear units. However, the present disclosure is not limited to this. The ring gear of the present disclosure may be used in applications other than planetary gear units.

The above embodiment shows an example of the configuration in which the nitride layer is formed in the ring gear by gas nitrocarburizing. However, the present disclosure is not limited to this. In the present disclosure, the nitride layer may be formed in the ring gear by a process other than gas nitrocarburizing. For example, the nitride layer may be formed by salt bath nitriding.

The above embodiment shows an example of the configuration in which the nitride layer in the outer periphery of the ring gear is removed by turning. However, the present disclosure is not limited to this. In the present disclosure, the nitride layer in the outer periphery of the ring gear may be removed by a process other than turning. For example, the nitride layer in the outer periphery of the ring gear may be removed by cutting with an end mill, a cutter, etc.

The above embodiment shows an example of the configuration in which the clutch discs are disposed radially outward of the external teeth of the ring gear and mesh with the external teeth of the ring gear. However, the present disclosure is not limited to this. In the present disclosure, a gear other than the clutch discs may be disposed radially outward of the external teeth of the ring gear and mesh with the external teeth of the ring gear.

The above embodiment shows an example of the configuration in which the internal teeth are formed in the entire inner peripheral surface of the ring gear. However, the present disclosure is not limited to this. In the present disclosure, the internal teeth need only to be formed in at least a part of the inner peripheral surface of the ring gear. The area where the internal teeth are to be formed in the inner peripheral surface of the ring gear is determined by which part of the inner peripheral surface of the ring gear is desired to mesh with the pinion gears.

The above embodiment shows an example of the configuration in which the external teeth are formed in the entire outer peripheral surface of the ring gear. However, the present disclosure is not limited to this. In the present disclosure, the external teeth need only to be formed in at least a part of the outer peripheral surface of the ring gear. The area where the external teeth are to be formed in the outer peripheral surface of the ring gear is also determined by which part of the outer peripheral surface of the ring gear is desired to be used as a clutch hub. In the present embodiment, the outer peripheral surface of the ring gear is used as two types of hubs. However, the outer peripheral surface of the ring gear may be used as a single type of hub.

The above embodiment shows an example of the configuration in which the surface nitrogen concentration in the external teeth is the nitrogen concentration of the base material. However, the present disclosure is not limited to this. In the present disclosure, the surface nitrogen concentration in at least the tooth surfaces of the external teeth need only to be the nitrogen concentration of the base material. This is because the external teeth and the clutch discs contact each other by their tooth surfaces.