Geared shaft and method for manufacturing geared shaft

Abstract

A geared shaft comprising: a first sintered gear having an outer peripheral surface provided with a plurality of first teeth, and an inner peripheral surface provided with a first inner spline; a second sintered gear having an outer peripheral surface provided with a plurality of second teeth, and an inner peripheral surface provided with a second inner spline; and a shaft having an outer peripheral surface provided with a first outer spline and a second outer spline, wherein the first sintered gear and the shaft are coupled together by a press-fit structure between the first inner spline and the first outer spline, the second sintered gear and the shaft are coupled together by a press-fit structure between the second inner spline and the second outer spline, and the distance between the center of a meshing pitch circle of the first teeth and the center of a meshing pitch circle of the second teeth is 50 μm or less.

Description

Geared shaft and method for manufacturing geared shaft

[0001] This disclosure relates to a geared shaft and a method for manufacturing the geared shaft. This application claims priority to Japanese Application No. 2023-184997, filed October 27, 2023, and incorporates by reference all of the contents of the above-mentioned Japanese application.

[0002] Patent Document 1 discloses a method for manufacturing a sintered body having an outer circumferential surface provided with a plurality of helical teeth. The method for manufacturing a sintered body in Patent Document 1 includes a step of preparing a raw material powder, a step of uniaxially pressing the raw material powder using a die to produce a cylindrical green compact, a step of machining the outer circumferential surface of the green compact to produce a pressed compact having an outer circumferential surface provided with a plurality of helical teeth, and a step of sintering the pressed compact.

[0003] Japanese Patent Application Laid-Open No. 2017-186625

[0004] The geared shaft of the present disclosure includes a first sintered gear having an outer peripheral surface on which a plurality of first teeth are provided and an inner peripheral surface on which a first internal spline is provided, a second sintered gear having an outer peripheral surface on which a plurality of second teeth are provided and an inner peripheral surface on which a second internal spline is provided, and a shaft having an outer peripheral surface on which a first external spline and a second external spline are provided. The first sintered gear and the shaft are coupled together by a press-fit structure between the first internal spline and the first external spline. The second sintered gear and the shaft are coupled together by a press-fit structure between the second internal spline and the second external spline. The distance between the center of a meshing pitch circle of the first teeth and the center of a meshing pitch circle of the second teeth is 50 μm or less.

[0005] Fig. 1 is a schematic perspective view showing a geared shaft of an embodiment. Fig. 2 is a longitudinal sectional view showing a geared shaft of an embodiment. Fig. 3 is a partial transverse sectional view showing a geared shaft of an embodiment. Fig. 4 is another partial transverse sectional view showing a geared shaft of an embodiment. Fig. 5 is a schematic view showing the center of a meshing pitch circle of a first helical tooth of a first sintered gear and the center of a meshing pitch circle of a second helical tooth of a second sintered gear in a geared shaft of an embodiment.

[0006] [Problem to be Solved by the Present Disclosure] The helical gear of Patent Document 1 is sometimes used in combination with a shaft. However, Patent Document 1 does not disclose how to combine the helical gear with the shaft. Therefore, there is a need for an efficient combination of the helical gear with the shaft. In particular, when combining multiple helical gears with one shaft, there is a need for more efficient combination. One of the objects of the present disclosure is to provide a geared shaft that is highly productive.

[0007] [Advantages of the Present Disclosure] The geared shaft of the present disclosure has excellent productivity.

[0008] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described.

[0009] (1) A geared shaft according to one embodiment of the present disclosure includes a first sintered gear having an outer peripheral surface on which a plurality of first teeth are provided and an inner peripheral surface on which a first internal spline is provided, a second sintered gear having an outer peripheral surface on which a plurality of second teeth are provided and an inner peripheral surface on which a second internal spline is provided, and a shaft having an outer peripheral surface on which a first external spline and a second external spline are provided. The first sintered gear and the shaft are coupled together by a press-fit structure between the first internal spline and the first external spline. The second sintered gear and the shaft are coupled together by a press-fit structure between the second internal spline and the second external spline. The distance between the center of the meshing pitch circle of the first teeth and the center of the meshing pitch circle of the second teeth is 50 μm or less. This means that the coaxiality of the first sintered gear and the second sintered gear is 50 μm or less.

[0010] The geared shaft of (1) above can be manufactured simply by press-fitting the shaft into the first sintered gear and then press-fitting the shaft into the second sintered gear, which has excellent productivity. The geared shaft of (1) above, which has excellent productivity, is low cost.

[0011] The geared shaft (1) is used in a traction motor system (eAxle) of a vehicle primarily powered by a motor, such as an electric vehicle (BEV: Battery Electric Vehicle) or a fuel cell electric vehicle (FCEV: Fuel Cell Electric Vehicle). In this application, high torque acts on the geared shaft (1). The geared shaft (1) is capable of withstanding high torque because the first sintered gear and the shaft are connected by a press-fit structure between the first inner spline and the first outer spline, and the second sintered gear and the shaft are connected by a press-fit structure between the second inner spline and the second outer spline. Furthermore, the geared shaft (1) is unlikely to experience large runout of the first sintered gear and the second sintered gear even under conditions where high torque acts, because the coaxiality of the first sintered gear and the second sintered gear is 50 μm or less. Therefore, the geared shaft of (1) above is easy to suppress vibration and noise. Furthermore, the geared shaft of (1) above can prevent a load from being unevenly applied to either the first sintered gear or the second sintered gear, so deformation of either the first sintered gear or the second sintered gear is easy to suppress and the gear is excellent in durability. Therefore, the geared shaft of (1) above is suitable for applications where high torque is applied.

[0012] (2) In the geared shaft of (1) above, the first teeth and the second teeth are both helical teeth.

[0013] The geared shaft of (2) above is highly productive even when it has helical teeth.

[0014] (3) In the geared shaft of (1) or (2) above, each tooth tip of the first internal spline may be press-fitted into each tooth root of the first external spline, and each tooth tip of the second internal spline may be press-fitted into each tooth root of the second external spline. A press-fit allowance between each tooth tip of the first internal spline and each tooth root of the first external spline may be 5 μm or more and 50 μm or less, and a press-fit allowance between each tooth tip of the second internal spline and each tooth root of the second external spline may be 5 μm or more and 50 μm or less.

[0015] In the geared shaft of (3) above, the press-fit interference is 5 μm or more, so that the first sintered gear and the second sintered gear are less likely to become misaligned relative to the shaft even under conditions where high torque is applied. Therefore, the geared shaft of (3) above is less likely to experience large runout of the first sintered gear and the second sintered gear. In the geared shaft of (3) above, the press-fit interference is 50 μm or less, so that the distance between the centers, i.e., the coaxiality of the first sintered gear and the second sintered gear, is easily kept to 50 μm or less. Furthermore, in the geared shaft of (3) above, the press-fit interference is 50 μm or less, so that deformation of the first teeth and the second teeth due to press-fitting of the shaft into the first sintered gear and the second sintered gear can be suppressed.

[0016] (4) In any of the geared shafts described in (1) to (3) above, the number of teeth of the first internal spline may be a multiple or a submultiple of the number of the plurality of first teeth, and the number of teeth of the second internal spline may be a multiple or a submultiple of the number of the plurality of second teeth.

[0017] The geared shaft of (4) is less likely to experience uneven loads on the first internal spline, the first external spline, the second internal spline, and the second external spline caused by the first teeth and the second teeth meshing with the teeth of the mating gear. Therefore, the geared shaft of (4) can suppress uneven deformation of the first internal spline, the first external spline, the second internal spline, and the second external spline, and can improve durability.

[0018] (5) In the geared shaft of any one of (1) to (4) above, the length of the shaft may be 100 mm or more.

[0019] Sintered parts in which a first sintered gear and a second sintered gear are integrally formed on a long shaft cannot be manufactured by near-net-shape molding. The geared shaft of (5) above can be manufactured simply by press-fitting a shaft into the first sintered gear and then press-fitting the shaft into the second sintered gear. Therefore, even geared shafts of (5) above with a shaft length of 100 mm or more can be easily manufactured. Therefore, the geared shaft of (5) above has excellent productivity.

[0020] (6) In the geared shaft according to any one of (1) to (5) above, the shortest distance between the first sintered gear and the second sintered gear may be 1 mm or more and 150 mm or less.

[0021] Sintered parts in which the first sintered gear and the second sintered gear are integrally formed on a long shaft and the shortest distance between the first sintered gear and the second sintered gear is long cannot be manufactured by near-net-shape molding. The geared shaft of (6) above can be manufactured simply by press-fitting a shaft into the first sintered gear and then press-fitting a shaft into the second sintered gear. Therefore, even geared shafts of (6) above in which the shortest distance is 1 mm or more can be easily manufactured. Therefore, geared shafts of (6) above in which the shortest distance is 1 mm or more have excellent productivity. Furthermore, geared shafts of (6) above in which the shortest distance is 1 mm or more are suitable for a wide range of applications. The geared shaft of (5) above in which the shortest distance is 150 mm or less does not become too large, and therefore has excellent productivity.

[0022] (7) In any of the geared shafts described in (1) to (6) above, the thickness of the first sintered gear may be 3 mm or more and 150 mm or less, and the thickness of the second sintered gear may be 3 mm or more and 150 mm or less.

[0023] In the geared shaft of (7) above, the thickness of the first sintered gear and the second sintered gear is 3 mm or more, which increases the tooth trace length and increases the strength of the first teeth and the second teeth. In the geared shaft of (7) above, the thickness of the first sintered gear and the second sintered gear is 150 mm or less, which does not increase the tooth trace length and therefore provides excellent productivity.

[0024] (8) In any of the geared shafts described in (1) to (7) above, the outer diameter of the first sintered gear may be 30 mm or more and 150 mm or less, and the outer diameter of the second sintered gear may be 30 mm or more and 150 mm or less.

[0025] In the geared shaft of (8), the outer diameters of the first sintered gear and the second sintered gear are 30 mm or more, which makes it easy to reduce the load acting on the first teeth and the second teeth. In the geared shaft of (8), the outer diameters of the first sintered gear and the second sintered gear are 150 mm or less, which makes the first sintered gear and the second sintered gear not too large, which results in excellent productivity.

[0026] (9) A method for manufacturing a geared shaft according to one embodiment of the present disclosure includes: Step A: preparing a first sintered gear having an outer peripheral surface on which a plurality of first teeth are provided and an inner peripheral surface on which a first internal spline is provided; and Step B: preparing a shaft having an outer peripheral surface on which a plurality of second teeth are provided and an inner peripheral surface on which a second internal spline is provided; and Step C: coupling the first sintered gear to the shaft by press-fitting the first internal spline to the first external spline, and coupling the second sintered gear to the shaft by press-fitting the second internal spline to the second external spline. Step A includes a step of cutting or grinding each tooth tip of the first internal spline and each tooth tip of the second internal spline. Step B includes a step of cutting or grinding each tooth root of the first external spline and each tooth root of the second external spline.

[0027] In the geared shaft manufacturing method of (9) above, step A includes cutting or grinding each tooth tip of the first internal spline and each tooth tip of the second internal spline, and step B includes cutting or grinding each tooth root of the first external spline and each tooth root of the second external spline. Therefore, the geared shaft manufacturing method of (9) above can manufacture a shaft in which the distance between the center of the tooth root circle of the first external spline and the center of the tooth root circle of the second external spline is 50 μm or less. Therefore, the geared shaft manufacturing method of (9) above can manufacture a geared shaft in which the distance between the center of the meshing pitch circle of the first teeth and the center of the meshing pitch circle of the second teeth is 50 μm or less. The geared shaft manufacturing method of (9) above can couple the first sintered gear and the shaft by press-fitting the shaft into the first sintered gear and then press-fitting the shaft into the second sintered gear, thereby coupling the first sintered gear and the shaft, and then coupling the second sintered gear and the shaft. Therefore, the geared shaft manufacturing method of (9) above can manufacture geared shafts with high productivity.

[0028] (10) In the method for manufacturing a geared shaft according to (9) above, the first teeth and the second teeth are both helical teeth.

[0029] The manufacturing method of the geared shaft in (10) above allows for the production of geared shafts having helical teeth with good productivity.

[0030] (11) In the method for manufacturing a geared shaft according to (9) or (10), each tooth tip of the first internal spline is press-fitted into each tooth root of the first external spline, and each tooth tip of the second internal spline is press-fitted into each tooth root of the second external spline, wherein the press-fit allowance between each tooth tip of the first internal spline and each tooth root of the first external spline is 5 μm or more and 50 μm or less, and the press-fit allowance between each tooth tip of the second internal spline and each tooth root of the second external spline is 5 μm or more and 50 μm or less.

[0031] In the geared shaft manufacturing method of (11) above, by setting the press-fitting allowance to 5 μm or more, it is possible to manufacture a geared shaft in which the runout of the first sintered gear and the second sintered gear is unlikely to become large. In the geared shaft manufacturing method of (11) above, by setting the press-fitting allowance to 50 μm or less, it is possible to easily achieve a coaxiality of the first sintered gear and the second sintered gear of 50 μm or less, and it is also possible to suppress deformation of the first teeth and the second teeth due to press-fitting of the shaft into the first sintered gear and the second sintered gear.

[0032] (12) In the manufacturing method of any of the geared shafts described in (9) to (11) above, the number of teeth of the first internal spline is a multiple or a submultiple of the number of the plurality of first teeth, and the number of teeth of the second internal spline is a multiple or a submultiple of the number of the plurality of second teeth.

[0033] The manufacturing method of a geared shaft described above in (12) can suppress uneven deformation of the first internal spline, the first external spline, the second internal spline, and the second external spline, and can manufacture a geared shaft with improved durability.

[0034] <Details of the Embodiments of the Present Disclosure> The embodiments of the present disclosure will be described below with reference to the drawings. The same reference numerals in the drawings indicate the same objects. The sizes of the components shown in each drawing are expressed for the purpose of clarity of explanation and do not necessarily represent the actual dimensional relationships.

[0035] <<Embodiment>> [Geared Shaft] A geared shaft 100 according to an embodiment will be described with reference to FIGS. 1 to 5. An example in which the gear teeth are helical will be described below. As shown in FIG. 1, the geared shaft 100 according to the embodiment includes a first sintered gear 1, a second sintered gear 2, and a shaft 3. One of the features of the geared shaft 100 according to the embodiment is that the first sintered gear 1 and the second sintered gear 2 are coupled to the shaft 3 by a specific press-fit structure. The longitudinal cross-sectional view of FIG. 2 is a cross-sectional view of the geared shaft 100 taken along a plane along the central axis of the shaft 3. In FIG. 2, the first helical teeth 13 and the second helical teeth 23, which will be described later, are shown in simplified form. The partial cross-sectional view of FIG. 3 is a portion of a cross-sectional view of the first sintered gear 1 and the shaft 3 taken along a plane perpendicular to the central axis of the shaft 3. The partial cross-sectional view of FIG. 4 is a portion of a cross-sectional view of the second sintered gear 2 and the shaft 3 taken along a plane perpendicular to the central axis of the shaft 3. Fig. 5 shows, in a simplified manner, the center C1 of the meshing pitch circle P1 of the first helical tooth 13 of the first sintered gear 1 shown in Fig. 1 and the center C2 of the meshing pitch circle P2 of the second helical tooth 23 of the second sintered gear 2 shown in Fig. 1. For convenience of explanation, the centers C1 and C2 are shown spaced apart.

[0036] [First Sintered Gear and Second Sintered Gear] The first sintered gear 1 has an outer peripheral surface 11 provided with a plurality of first helical teeth 13 as shown in Fig. 1 and an inner peripheral surface 12 provided with a first internal spline 14 as shown in Fig. 3. The second sintered gear 2 has an outer peripheral surface 21 provided with a plurality of second helical teeth 23 as shown in Fig. 1 and an inner peripheral surface 22 provided with a second internal spline 24 as shown in Fig. 4. Hereinafter, in the common description of the first sintered gear 1 and the second sintered gear 2, they will be simply referred to as sintered gears.

[0037] (Material) Sintered gears are made of a plurality of metal particles bonded together. The metal is, for example, an iron-based material or a non-ferrous metal. The iron-based material is, for example, pure iron or an iron alloy. The non-ferrous metal is, for example, copper, copper alloy, titanium, titanium alloy, aluminum, aluminum alloy, magnesium, magnesium alloy, or pure tungsten. An iron-based material is suitable as the material for sintered gears.

[0038] In the present disclosure, pure iron refers to iron with a purity of 99% or more. That is, in the present disclosure, pure iron refers to iron with an iron (Fe) content of 99% by mass or more.

[0039] An iron alloy contains additive elements, with the remainder consisting of iron (Fe) and inevitable impurities. Iron alloys contain the most Fe. The additive elements contained in the iron alloy are, for example, one or more elements selected from the group consisting of copper (Cu), nickel (Ni), tin (Sn), chromium (Cr), molybdenum (Mo), manganese (Mn), and carbon (C). The total content of Cu, Ni, Sn, Cr, Mo, and Mn, when the entire sintered gear is taken as 100% by mass, is, for example, 0.5% by mass to 5.0% by mass, or 1.0% by mass to 3.0% by mass. The total content of C, when the entire sintered gear is taken as 100% by mass, is, for example, 0.2% by mass to 2.0% by mass, or 0.4% by mass to 1.0% by mass.

[0040] The composition of the sintered gear can be confirmed by component analysis using ICP optical emission spectrometry (ICP-OES).

[0041] (Relative Density) The relative density of the sintered gear is, for example, 95% or more. Sintered gears with a relative density of 95% or more have excellent mechanical properties such as strength. The relative density of the sintered gear may be 96% or more, particularly 97% or more. The upper limit of the relative density of the sintered gear is not particularly limited and can be appropriately selected within the range that allows production. The relative density of the sintered gear may be, for example, 99.9% or less. The relative density of the sintered gear may be 95% or more and 99.9% or less, further 96% or more and 99.9% or less, particularly 97% or more and 99.9% or less.

[0042] The relative density of a sintered gear refers to the ratio (%) of the actual density of the sintered gear to the true density of the sintered gear. The relative density of a sintered gear is calculated by [(actual sintered gear density / true density of sintered gear) × 100]. The actual density of a sintered gear can be determined by immersing the sintered gear in oil to impregnate the sintered gear with oil and calculating [oil-impregnated density × (mass of sintered gear before oil-impregnation / mass of sintered gear after oil-impregnation)]. The oil-impregnated density is (mass of sintered gear after oil-impregnation / volume of sintered gear after oil-impregnation). The actual density of a sintered gear can be calculated by (mass of sintered gear before oil-impregnation / volume of sintered gear after oil-impregnation). The volume of a sintered gear after oil-impregnation can typically be measured by the liquid displacement method. The true density of a sintered gear is the theoretical density calculated from the composition of the sintered gear when it is assumed that there are no voids inside.

[0043] (Outer peripheral surface) As shown in FIG. 1 , the first helical teeth 13 of the first sintered gear 1 are arranged in parallel on the outer peripheral surface 11 of the first sintered gear 1 around the central axis of the first sintered gear 1. Each of the first helical teeth 13 is inclined at a predetermined helix angle with respect to the central axis of the first sintered gear 1. The tooth trace of each of the first helical teeth 13 intersects with the central axis of the first sintered gear 1. The second helical teeth 23 of the second sintered gear 2 are arranged in parallel on the outer peripheral surface 21 of the second sintered gear 2 around the central axis of the second sintered gear 2. Each of the second helical teeth 23 is inclined at a predetermined helix angle with respect to the central axis of the second sintered gear 2. The tooth trace of each of the second helical teeth 23 intersects with the central axis of the second sintered gear 2. The helix angles of the first helical teeth 13 and the second helical teeth 23 are different from each other.

[0044] (Inner peripheral surface) As shown in FIG. 3 , the first internal spline 14 of the first sintered member is provided on the inner peripheral surface 12 of the first sintered gear 1. The first internal spline 14 has teeth and tooth spaces alternately arranged around the central axis of the first sintered gear 1. The number of teeth of the first internal spline 14 may be the same as the number of first helical teeth 13 shown in FIG. 1 , or may be more or less than the number of first helical teeth 13. The number of teeth of the first internal spline 14 may be a multiple or a divisor of the number of first helical teeth 13. A multiple is a number obtained by multiplying a positive integer by a positive integer. A divisor is a positive integer that divides a positive integer. For example, if the number of first helical teeth 13 is 32, multiples of 32 are 32, 64, 128, etc., and divisors of 32 are 1, 2, 4, 8, 16, and 32. Therefore, the number of teeth of the first internal spline 14 is 128, 64, 32, 16, 8, 4, 2, etc. The second internal spline 24 of the second sintered member is provided on the inner circumferential surface 22 of the second sintered gear 2. The second internal spline 24 has teeth and tooth spaces alternately arranged around the central axis of the second sintered gear 2. The number of teeth of the second internal spline 24 may be the same as the number of second helical teeth 23 shown in FIG. 1 , or may be more or less than the number of second helical teeth 23. The number of teeth of the second internal spline 24 may be a multiple or a submultiple of the number of second helical teeth 23.

[0045] As shown in Figures 3 and 4, the first internal spline 14 and the second internal spline 24 are mainly composed of a plurality of tooth roots 141, 241, a plurality of tooth tips 142, 242, and a plurality of tooth flanks 143, 243. Each tooth root 141, 241 is a surface that forms the space between adjacent teeth, i.e., the bottom of the tooth gap. Each tooth tip 142, 242 is a surface that forms the tip region of each tooth. Each tooth flank 143, 243 is a surface that connects each tooth root 141, 241 and each tooth tip 142, 242. Each tooth trace of the first internal spline 14 is aligned along the central axis of the first sintered gear 1. Each tooth trace of the second internal spline 24 is aligned along the central axis of the second sintered gear 2.

[0046] (Thickness) The thickness T1 of the first sintered gear 1 and the thickness T2 of the second sintered gear 2 shown in FIG. 2 can be selected appropriately. The thickness T1 is the length along the central axis of the first sintered gear 1, and the thickness T2 is the length along the central axis of the second sintered gear 2. The thicknesses T1 and T2 may be the same or different from each other. The thicknesses T1 and T2 are, for example, 3 mm or more and 150 mm or less. The thicknesses T1 and T2 may be 5 mm or more and 100 mm or less, or 10 mm or more and 70 mm or less.

[0047] (Outer Diameter) The outer diameter D1 of the first sintered gear 1 and the outer diameter D2 of the second sintered gear 2 can be selected appropriately. The outer diameters D1 and D2 are tooth tip diameters. The outer diameters D1 and D2 may be the same or different. In this example, the outer diameter D1 is larger than the outer diameter D2. The outer diameters D1 and D2 are, for example, 30 mm or more and 150 mm or less. The outer diameters D1 and D2 may be 40 mm or more and 120 mm or less, or 50 mm or more and 100 mm or less.

[0048] [Shaft] As shown in Figures 3 and 4, the shaft 3 has an outer circumferential surface 31 on which a first external spline 33 and a second external spline 34 are provided. The shaft 3 may be made of a sintered material or an ingot-cast material. The material of the shaft 3 is, for example, the material described in the section on the materials of the first sintered gear 1 and the second sintered gear 2. The material of the shaft 3 may be the same as or different from the material of the first sintered gear 1 and the second sintered gear 2, as long as it is the material described in the section on the materials of the first sintered gear 1 and the second sintered gear 2.

[0049] As shown in FIG. 2 , the shaft 3 of this example has a first small diameter portion 35, a large diameter portion 36, and a second small diameter portion 37, which are arranged in this order from the first end to the second end. A first step is provided at the connection between the first small diameter portion 35 and the large diameter portion 36. A second step is provided at the connection between the large diameter portion 36 and the second small diameter portion 37. A first external spline 33 is provided on the outer circumferential surface of the first small diameter portion 35. The first external spline 33 has teeth and tooth spaces alternately arranged around the central axis of the shaft 3 on the outer circumferential surface of the first small diameter portion 35. A second external spline 34 is provided on the outer circumferential surface of the second small diameter portion 37. The second external spline 34 has teeth and tooth spaces alternately arranged around the central axis of the shaft 3 on the outer circumferential surface of the second small diameter portion 37.

[0050] The first internal spline 14 is press-fitted into the first external spline 33. The shaft 3 and the first sintered gear 1 are connected by the press-fit structure between the first external spline 33 and the first internal spline 14. In this example, the first step prevents the first sintered gear 1 from approaching the second sintered gear 2. The second internal spline 24 is press-fitted into the second external spline 34. The shaft 3 and the second sintered gear 2 are connected by the press-fit structure between the second external spline 34 and the second internal spline 24. In this example, the second step prevents the second sintered gear 2 from approaching the first sintered gear 1. The geared shaft 100 can withstand high torque because the first sintered gear 1 and the shaft 3 are connected by the press-fit structure between the first internal spline 14 and the first external spline 33, and the second sintered gear 2 and the shaft 3 are connected by the press-fit structure between the second internal spline 24 and the second external spline 34. The number of first external splines 33 is the same as the number of first internal splines 14. The number of second external splines 34 is the same as the number of second internal splines 24 .

[0051] As shown in Figures 3 and 4, the first external spline 33 and the second external spline 34 are mainly composed of a plurality of tooth roots 331, 341, a plurality of tooth tips 332, 342, and a plurality of tooth flanks 333, 343. Each tooth root 331, 341 is a surface that forms the space between adjacent teeth, i.e., the bottom of the tooth gap. Each tooth tip 332, 342 is a surface that forms the tip region of each tooth. Each tooth flank 333, 343 is a surface that connects each tooth root 331, 341 and each tooth tip 332, 342.

[0052] In this example, as shown in FIG. 3 , each tooth bottom 331 of the first external spline 33 is press-fitted into each tooth tip 142 of the first internal spline 14, and as shown in FIG. 4 , each tooth bottom 341 of the second external spline 34 is press-fitted into each tooth tip 242 of the second internal spline 24. The first press-fit allowance α1 between each tooth bottom 331 of the first external spline 33 and each tooth tip 142 of the first internal spline 14 shown in FIG. 3 is, for example, 5 μm or more and 50 μm or less. The second press-fit allowance α2 between each tooth bottom 341 of the second external spline 34 and each tooth tip 242 of the second internal spline 24 shown in FIG. 4 is, for example, 5 μm or more and 50 μm or less. By setting the first press-fit allowance α1 and the second press-fit allowance α2 to 5 μm or more, the first sintered gear 1 and the second sintered gear 2 are less likely to be misaligned with respect to the shaft 3 even under conditions where high torque is applied. Therefore, the runout of the first sintered gear 1 and the second sintered gear 2 is less likely to increase. By setting the first press-fit allowance α1 and the second press-fit allowance α2 to 50 μm or less, it is easy to set the distance Dc between the center C1 of the meshing pitch circle P1 of the first helical tooth 13 and the center C2 of the meshing pitch circle P2 of the second helical tooth 23 shown in FIG. 5 to 50 μm or less. Here, the distance Dc represents the coaxiality of the first sintered gear 1 and the second sintered gear 2. Furthermore, deformation of the first helical tooth 13 and the second helical tooth 23 due to press-fitting of the shaft 3 into the first sintered gear 1 and the second sintered gear 2 can be suppressed. The first press-fit allowance α1 and the second press-fit allowance α2 shown in FIGS. 3 and 4 are not limited to 5 μm or more and 50 μm or less, and may be 0 μm or more and 80 μm or less, or 10 μm or more and 50 μm or less.

[0053] The first press-fitting allowance α1 can be calculated from the materials of the first sintered gear 1 and the shaft 3, the specifications of the first internal spline 14 and the first external spline 33, and the disengagement load of the first sintered gear 1 from the shaft 3. The second press-fitting allowance α2 can be calculated from the materials of the second sintered gear 2 and the shaft 3, the specifications of the second internal spline 24 and the second external spline 34, and the disengagement load of the second sintered gear 2 from the shaft 3.

[0054] As shown in FIG. 5 , the distance Dc between the center C1 of the meshing pitch circle P1 of the first helical tooth 13 and the center C2 of the meshing pitch circle P2 of the second helical tooth 23 is 50 μm or less. The centers C1 and C2 of the meshing pitch circles P1 and P2 can be derived by measuring the dimensions of the first external spline 33 and the second external spline 34. In this example, each tooth tip 142 of the first internal spline 14 is press-fitted into each tooth root 331 of the first external spline 33, and each tooth tip 242 of the second internal spline 24 is press-fitted into each tooth root 341 of the second external spline 34. Therefore, the distance between the center of the tooth root circle of the first external spline 33 and the center of the tooth root circle of the second external spline 34 is 50 μm or less. By having the distance Dc be 50 μm or less, the runout of the first sintered gear 1 and the second sintered gear 2 is less likely to increase in the geared shaft 100 even under conditions where high torque is applied. Therefore, the geared shaft 100 is easy to suppress vibration and noise. Furthermore, since the geared shaft 100 can prevent a load from being unevenly applied to either the first sintered gear 1 or the second sintered gear 2, deformation of either the first sintered gear 1 or the second sintered gear 2 is easy to suppress, and the geared shaft 100 has excellent durability. Therefore, the geared shaft 100 is suitable for applications where high torque is applied. If the distance Dc is 30 μm or less, the above-mentioned runout is even easier to suppress.

[0055] The length L of the shaft 3 shown in FIG. 2 can be selected appropriately depending on the application of the geared shaft 100. The length L of the shaft 3 is the length along the central axis of the shaft 3. The length L is, for example, 100 mm or more. The length L may be 110 mm or more, or 120 mm or more. The length L is, for example, 150 mm or less. The length L may be 100 mm or more and 150 mm or less, 110 mm or more and 150 mm or less, or 120 mm or more and 150 mm or less.

[0056] The shortest length between the first external spline 33 and the second external spline 34 of the shaft 3 can be selected appropriately depending on the application of the geared shaft 100. The shortest length is the shortest length between the first external spline 33 and the second external spline 34 along the central axis of the shaft 3. The shortest distance Dm between the first sintered gear 1 and the second sintered gear 2 is determined by the shortest length. The shortest distance Dm is the shortest distance between the first sintered gear 1 and the second sintered gear 2 along the central axis of the shaft 3. The shortest distance Dm is, for example, 1 mm or more and 150 mm or less. The shortest distance Dm may be 3 mm or more and 100 mm or less, or 5 mm or more and 80 mm or less.

[0057] [Method for manufacturing a geared shaft] The method for manufacturing a geared shaft of the embodiment includes steps A to C. Step A produces a first sintered gear 1 and a second sintered gear 2. Step B prepares a shaft 3. Step C joins the first sintered gear 1 and the second sintered gear 2 to the shaft 3 by press-fitting the shaft 3 into the first sintered gear 1 and the second sintered gear 2. The method for manufacturing a geared shaft of the embodiment can manufacture the geared shaft 100 of the embodiment described above.

[0058] [Step A: Fabrication of Sintered Gears] The first sintered gear 1 and the second sintered gear 2 are fabricated, for example, by sequentially performing steps a1 to a8. In step a1, raw material powder is prepared. In step a2, the raw material powder is pressure-molded to fabricate a green compact having a cylindrical outer peripheral surface without helical teeth and an inner peripheral surface provided with internal splines. In step a3, the green compact is gear-cut to fabricate a first processed body having an outer peripheral surface provided with multiple helical teeth. In step a4, the first processed body is sintered to fabricate a sintered body. In step a5, each helical tooth of the sintered body is chamfered to fabricate a second processed body. In step a6, the second processed body is heat-treated to fabricate a heat-treated body. In step a7, each internal spline and end face of the heat-treated body is machined or polished. In step a8, each helical tooth of the heat-treated body is machined or polished. The order of steps a7 and a8 may be reversed.

[0059] [Step a1: Preparation of Raw Material Powder] In step a1, raw material powder containing iron-based powder or non-ferrous powder is prepared. The raw material powder may further contain a lubricant. When a lubricant is applied to the mold, the raw material powder does not need to contain a lubricant. The raw material powder does not contain an organic binder.

[0060] (Iron-based powder) The iron-based powder is one type of powder selected from the group consisting of pure iron powder, ferrous alloy powder, first mixed powder, second mixed powder, third mixed powder, and fourth mixed powder. As described above, the pure iron constituting the pure iron powder has a purity of 99% or more. The type of ferrous alloy constituting the ferrous alloy powder is the iron alloy described above. The first mixed powder is composed of pure iron powder and powder of an alloying element. The powder of the alloying element is powder of an element that diffuses into the pure iron powder to produce the iron alloy described above when step a4 is performed. The alloying element is an additive element of the iron alloy described above. When the sintered gear to be manufactured is composed of an iron alloy containing multiple additive elements, the powder of the alloying element includes powder of multiple additive elements. The second mixed powder is composed of ferric alloy powder and carbon powder. Examples of ferric alloys constituting the ferric alloy powder include Fe-Cu alloys, Fe-Ni-Mo-Cu alloys, Fe-Ni-Mo alloys, Fe-Cr alloys, Fe-Ni alloys, and Fe-Mo-Mn-Cr alloys. The third mixed powder is composed of pure iron powder, powders of the alloying elements, and ferrous alloy powder. The fourth mixed powder is composed of pure iron powder, powders of the alloying elements, ferric alloy powder, and carbon powder.

[0061] (Non-ferrous powder) The non-ferrous powder is a powder of the non-ferrous metals described above.

[0062] (Lubricant) The lubricant enhances the lubricity of the raw material powder when it is pressed in step a2, thereby improving moldability. Examples of the lubricant include higher fatty acids, metal soaps, fatty acid amides, and higher fatty acid amides. Known lubricants can be used. The lubricant may be in any form, such as a solid, powder, or liquid. At least one of these lubricants can be used alone or in combination. The lubricant content in the raw material powder, when the total raw material powder is 100% by mass, is, for example, 0.1% by mass to 2.0% by mass, 0.3% by mass to 1.5% by mass, or 0.5% by mass to 1.0% by mass. This content within the above range facilitates the production of a dense powder compact in step a2. Furthermore, volumetric shrinkage due to loss of lubricant during sintering of the processed body in step a4 can be reduced, facilitating the production of a sintered gear with high dimensional accuracy and high density.

[0063] [Step a2: Pressurizing of Raw Powder] In step a2, the raw powder is pressurized to produce a powder compact having an outer cylindrical surface without helical teeth and an inner surface provided with internal splines. The powder compact is produced using an appropriate mold capable of producing the outer and inner surfaces. That is, the internal splines on the inner surface are formed by transferring the shape of the mold. To prevent the metal powder from sticking to the mold, the above-mentioned lubricant may be applied to the inner surface of the die or the pressing surface of the punch.

[0064] The higher the molding pressure, the higher the relative density of the green compact produced. The molding pressure is, for example, 1560 MPa or higher. A molding pressure of 1560 MPa or higher makes it easier to produce a green compact with a relative density of 95% or higher. The relative density of the green compact is calculated by [(density of actual green compact / true density of green compact) x 100]. The method for calculating the density of an actual green compact is the same as the method for calculating the density of an actual sintered gear described above. The method for calculating the true density of a green compact is the same as the method for calculating the true density of a sintered gear described above. The molding pressure may further be 1660 MPa or higher, 1760 MPa or higher, particularly 1860 MPa or higher, or 1960 MPa or higher. There is no particular upper limit to the molding pressure.

[0065] [Step a3: Gear Cutting] In step a3, the outer peripheral surface of the powder compact is gear-cut to produce a first processed body having an outer peripheral surface provided with a plurality of helical teeth. The gear-cutting tool is, for example, a hob, a broach, or a pinion cutter. Before being subjected to gear cutting, the outer peripheral surface of the powder compact may be coated with or immersed in a volatile or plastic solution containing an organic binder. This coating or immersion tends to reduce cracking or chipping of the surface of the powder compact during gear cutting.

[0066] [Step a4: Sintering] In step a4, the first processed body is sintered to produce a sintered body. The first processed body shrinks due to sintering. Therefore, the relative density of the sintered body is equal to or higher than the relative density of the first processed body. If the relative density of the powder compact is 95% or higher, the relative density of the sintered body will be 95% or higher. Sintering conditions can be appropriately selected depending on the composition of the raw material powder. The sintering temperature may be, for example, 1100°C or higher and 1400°C or lower, or 1200°C or higher and 1300°C or lower. The sintering time may be, for example, 15 minutes or higher and 150 minutes or lower, or 20 minutes or higher and 60 minutes or lower. Known sintering conditions can be applied.

[0067] [Step a5: Chamfering] In step a5, each helical tooth is chamfered to produce a second workpiece. Chamfering is performed, for example, on the root edge, flank edge, and tip edge of the helical tooth. The root edge is the ridge line between the end face and root of the sintered gear. The tooth flank edge is the ridge line between the end face and tooth flank of the sintered gear. The tip edge is the ridge line between the end face and tip of the sintered gear. The chamfering tool is, for example, a brush.

[0068] [Step a6: Heat Treatment] In step a6, the second processed body is heat-treated to produce a heat-treated body. The heat treatment is a quenching treatment and a tempering treatment. This heat treatment tends to improve the mechanical properties of the heat-treated body, particularly the hardness and toughness. The quenching treatment may be a carburizing quenching treatment. Known conditions can be applied to the quenching treatment and the tempering treatment.

[0069] [Step a7: Cutting or Grinding Internal Splines and End Faces] In step a7, the internal splines and end faces of the heat-treated body are cut or polished. In this example, the target of machining for each internal spline is the tooth tip. This cutting or polishing process tends to reduce the distance Dc between the center C1 of the meshing pitch circle P1 of the first helical tooth 13 and the center C2 of the meshing pitch circle P2 of the second helical tooth 23 to 50 μm or less. Furthermore, the first press-fit allowance α1 and the second press-fit allowance α2 can be set within the above-mentioned ranges.

[0070] [Step a8: Cutting or Polishing of Helical Teeth] In step a8, each bevel tooth is cut or polished. In this example, the tip of each bevel tooth is cut or polished. This cutting or polishing allows the tips of the multiple bevel teeth to be adjusted to the designed dimensions.

[0071] [Step B] The shaft 3 can be prepared, for example, by sequentially performing steps b1 to b4. In step b1, a bar stock is prepared. The material of the bar stock is the same as that of the shaft 3 described above. In step b2, the bar stock is gear-cut to produce a processed body having an outer peripheral surface on which a first external spline and a second external spline are provided. In step b3, the processed body is heat-treated to produce a heat-treated body. The heat treatment conditions are the same as those of step a6 described above. In step b4, the first external spline and the second external spline of the heat-treated body are cut or polished. In this example, the tooth roots of the first external spline and the second external spline are cut or polished. This cutting or polishing process tends to reduce the distance between the center of the tooth root circle of the first external spline 33 and the center of the tooth root circle of the second external spline 34 to 50 μm or less. Therefore, the distance Dc between the center C1 of the meshing pitch circle P1 of the first helical tooth 13 and the center C2 of the meshing pitch circle P2 of the second helical tooth 23 is likely to be 50 μm or less. In addition, the first press-fit allowance α1 and the second press-fit allowance α2 can be set within the above-mentioned ranges.

[0072] [Step C] In step C, the first sintered gear 1 and the shaft 3 are joined by press-fitting the first external spline 33 into the first internal spline 14, and the second external spline 34 is press-fitted into the second internal spline 24, thereby joining the second sintered gear 2 and the shaft 3. In this example, each tooth bottom 331 of the first external spline 33 is press-fitted into each tooth tip 142 of the first internal spline 14, and each tooth bottom 341 of the second external spline 34 is press-fitted into each tooth tip 242 of the second internal spline 24. By these joining steps, the geared shaft 100 of the embodiment is manufactured.

[0073] In the manufacturing method of the geared shaft of the embodiment, in step a7, each tooth tip 142 of the first internal spline 14 and each tooth tip 242 of the second internal spline 24 are cut or polished, and in step b4, each tooth bottom 331 of the first external spline 33 and each tooth bottom 341 of the second external spline 34 are cut or polished. Therefore, the manufacturing method of the geared shaft of the embodiment can manufacture the geared shaft 100 of the above-mentioned embodiment. The manufacturing method of the geared shaft of the embodiment can connect the first sintered gear 1 and the shaft 3 by press-fitting the shaft 3 into the first sintered gear 1 and press-fitting the shaft 3 into the second sintered gear 2, thereby connecting the first sintered gear 1 and the shaft 3 and connecting the second sintered gear 2 and the shaft 3, and therefore the geared shaft 100 can be manufactured with high productivity.

[0074] The present invention is not limited to these examples, but is defined by the scope of the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0075] 100 Geared shaft 1 First sintered gear 11 Outer peripheral surface 12 Inner peripheral surface 13 First helical tooth (first tooth) 14 First internal spline 141 Tooth root 142 Tooth tip 143 Tooth surface 2 Second sintered gear 21 Outer peripheral surface 22 Inner peripheral surface 23 Second helical tooth (second tooth) 24 Second internal spline 241 Tooth root 242 Tooth tip 243 Tooth surface 3 Shaft 31 Outer peripheral surface 33 First external spline 331 Tooth root 332 Tooth tip 333 Tooth surface 34 Second external spline 341 Tooth root 342 Tooth tip 343 Tooth surface 35 First small diameter portion 36 Large diameter portion 37 Second small diameter portion T1, T2 Thickness D1, D2 Outer diameter Dm Shortest distance L Length α1 First press-fit allowance α2 Second press-fit allowance P1, P2 Meshing pitch circle C1, C2 Center Dc Distance

Claims

1. A geared shaft comprising: a first sintered gear having an outer peripheral surface on which a plurality of first teeth are provided and an inner peripheral surface on which a first internal spline is provided; a second sintered gear having an outer peripheral surface on which a plurality of second teeth are provided and an inner peripheral surface on which a second internal spline is provided; and a shaft having an outer peripheral surface on which a first external spline and a second external spline are provided, wherein the first sintered gear and the shaft are coupled by a press-fit structure between the first internal spline and the first external spline, and the second sintered gear and the shaft are coupled by a press-fit structure between the second internal spline and the second external spline, and the distance between the center of the meshing pitch circle of the first teeth and the center of the meshing pitch circle of the second teeth is 50 μm or less.

2. The geared shaft according to claim 1, wherein said first plurality of teeth and said second plurality of teeth are both helical teeth.

3. A geared shaft as described in claim 1 or claim 2, wherein each tooth tip of the first internal spline and each tooth bottom of the first external spline are press-fitted, each tooth tip of the second internal spline and each tooth bottom of the second external spline are press-fitted, a press-fitting allowance between each tooth tip of the first internal spline and each tooth bottom of the first external spline is 5 μm or more and 50 μm or less, and a press-fitting allowance between each tooth tip of the second internal spline and each tooth bottom of the second external spline is 5 μm or more and 50 μm or less.

4. A geared shaft as described in any one of claims 1 to 3, wherein the number of teeth of the first internal spline is a multiple or a submultiple of the number of the first plurality of teeth, and the number of teeth of the second internal spline is a multiple or a submultiple of the number of the second plurality of teeth.

5. A geared shaft as claimed in any one of claims 1 to 4, wherein the length of the shaft is 100 mm or more.

6. A geared shaft according to any one of claims 1 to 5, wherein the shortest distance between the first sintered gear and the second sintered gear is 1 mm or more and 150 mm or less.

7. A geared shaft according to any one of claims 1 to 6, wherein the thickness of the first sintered gear is 3 mm or more and 150 mm or less, and the thickness of the second sintered gear is 3 mm or more and 150 mm or less.

8. A geared shaft according to any one of claims 1 to 7, wherein the outer diameter of the first sintered gear is 30 mm or more and 150 mm or less, and the outer diameter of the second sintered gear is 30 mm or more and 150 mm or less.

9. A method for manufacturing a geared shaft, comprising: step A of producing a first sintered gear having an outer peripheral surface on which a plurality of first teeth are provided and an inner peripheral surface on which a first internal spline is provided, and a second sintered gear having an outer peripheral surface on which a plurality of second teeth are provided and an inner peripheral surface on which a second internal spline is provided; step B of preparing a shaft having an outer peripheral surface on which a first external spline and a second external spline are provided; and step C of joining the first sintered gear and the shaft by pressing in the first internal spline and the first external spline, and joining the second sintered gear and the shaft by pressing in the second internal spline and the second external spline, wherein step A includes a step of cutting or grinding each of the tooth tips of the first internal spline and each of the tooth tips of the second internal spline, and step B includes a step of cutting or grinding each of the tooth bottoms of the first external spline and each of the tooth bottoms of the second external spline.

10. The method of manufacturing a geared shaft according to claim 9, wherein the first plurality of teeth and the second plurality of teeth are both helical teeth.

11. A method for manufacturing a geared shaft as described in claim 9 or claim 10, comprising: pressing each tooth tip of the first internal spline into each tooth bottom of the first external spline; pressing each tooth tip of the second internal spline into each tooth bottom of the second external spline; a press-fitting allowance between each tooth tip of the first internal spline and each tooth bottom of the first external spline is 5 μm or more and 50 μm or less; and a press-fitting allowance between each tooth tip of the second internal spline and each tooth bottom of the second external spline is 5 μm or more and 50 μm or less.

12. A method for manufacturing a geared shaft as described in any one of claims 9 to 11, wherein the number of teeth of the first internal spline is a multiple or a submultiple of the number of the first plurality of teeth, and the number of teeth of the second internal spline is a multiple or a submultiple of the number of the second plurality of teeth.

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