JP7728672B2 - Rotary power transmission device and method of manufacturing the same - Google Patents

Description

The present invention relates to a rotary power transmission device in which a ring gear is welded to a case member, and a method for manufacturing the same.

Conventionally, rotary power transmission devices in which a ring gear is welded to a flange portion of a case member are used in, for example, the drive force transmission system of an automobile, which transmits the driving force of a drive source to the wheels. One example of such a rotary power transmission device is a differential gear, which outputs the driving force input via a propeller shaft while allowing differential motion between a pair of output shafts, and is used in four-wheel drive vehicles and rear-wheel drive vehicles (FR vehicles).

The differential device has a differential gear mechanism consisting of a pair of side gears meshed with multiple pinion gears, a differential case that houses the differential gear mechanism, and an annular ring gear attached to the outer periphery of the differential case. The ring gear is fixed to a flange portion provided on the differential case and meshes with the gear portion of the pinion gear shaft connected to the propeller shaft. Traditionally, the ring gear was typically fastened to the flange portion of the differential case using bolts, but in recent years, advances in dissimilar metal welding technology have led to widespread use of welding the ring gear to the flange portion of the differential case.

The differential device described in Patent Document 1 has an annular press-fit surface formed on the outer peripheral surface of the differential case, and a circular plate-shaped flange portion provided on one axial end of the differential case, with the radially outer end of this flange portion forming an abutment portion on the case side. The ring gear has press-fit holes that are press-fitted into the press-fit surface of the differential case, and a gear-side abutment portion that abuts against the abutment portion on the case side. When fixing the ring gear to the differential case, the ring gear is press-fitted into the press-fit surface of the differential case, and the abutment portions on the case side and gear side are welded together with the abutment portion on the gear side abutting against the abutment portion on the case side.

In the differential device described in Patent Document 2, the ring gear is fixed to the differential case using a welding structure similar to that of Patent Document 1. However, to prevent the heat generated during welding from increasing the pressure inside the space formed between the differential case and the ring gear, causing gas to escape and resulting in defects such as holes in the weld, the differential case is provided with a communication hole that connects this space to the outside, which is open to the atmosphere. Furthermore, in the differential device of Patent Document 2, one end of the communication hole is open to the inner surface of the shaft support hole into which the pinion shaft that supports the multiple pinion gears of the differential gear mechanism is inserted, or the pin insertion hole into which the pin that prevents the pinion shaft from coming out and rotating is inserted, and this opening is closed by the pinion shaft or pin, so that foreign matter such as press-fit powder generated when the ring gear is pressed in and spatter generated during welding is not discharged to the outside through the communication hole.

Patent Document 3 describes a welding method for laser welding a ring gear to a flange portion of a differential case. In this welding method, to prevent blowholes from occurring in the weld (porous defects caused by gas generated within the weld not being released upon solidification but being trapped within the weld), through-holes that run axially through the flange portion are formed as external opening holes that open the internal space between the differential case and the ring gear to the outside.

Differential gears in which the ring gear is fixed to the flange portion by welding include those equipped with a multi-plate clutch to limit the differential movement of a pair of side gears, such as that described in Patent Document 4, and those in which multiple pinion gears are arranged in parallel with a pair of side gears, such as that described in Patent Document 5.

Patent Document 6 also describes measuring the external shape of the flange after butt-welding the flange and ring gear of a case (differential case or transfer case), and determining the quality of the weld based on the measured shape. Figures 8A and 8B of Patent Document 6 show a transfer device in which a ring gear is fixed by welding to the flange of a transfer case, which has a through-hole formed in the center to accommodate a drive shaft or other driving axle.

JP 2016-65582 A (see paragraphs [0017] and [0018] of the specification) JP 2017-62003 A (see paragraphs [0003]-[0006] of the specification) Japanese Patent Application Laid-Open No. 2020-175418 (see paragraphs [0058] and [0063], and Figures 2A and 2B) JP 2010-180976 A (see paragraphs [0042]-[0044] and FIG. 2 of the specification) JP 2013-160259 A (see paragraph [0044] of the specification, and FIGS. 1 and 3) International Publication No. WO2012/143986 (see paragraph [0070] of the specification, and FIGS. 8A and 8B)

As described in Patent Document 2, if communication holes connecting the space formed between the differential case and ring gear to the outside are formed on the inner surface of the shaft support holes or pin insertion holes, burrs may form around these openings, making it impossible to insert the pinion shafts or pins into the holes, which could interfere with the assembly of the differential gear mechanism. This increases the time and cost required for manufacturing. Furthermore, as described in Patent Document 3, forming axial through holes in the flange portion of the differential case reduces the strength of the flange portion.

The present invention aims to provide a rotary power transmission device and manufacturing method that prevents defects in the weld due to pressure buildup in the space between the case member and the ring gear during welding, while also preventing problems with component assembly or a reduction in the strength of the flange due to the formation of an air hole to release this pressure.

In order to solve the above-mentioned problems, the present invention provides a method for manufacturing a bearing comprising: an annular ring gear; a cylindrical portion having an annular fitting portion formed with a fitting surface into which an inner peripheral surface of the ring gear is fitted, and a flange portion provided so as to protrude radially outward from the fitting surface, the cylindrical portion having an accommodation space formed therein for accommodating an accommodation member ; and a case member having first and second side walls that sandwich the accommodation member, the flange portion being provided on an outer periphery of the cylindrical portion closer to the second side wall than an axial center portion of the cylindrical portion, wherein at least a portion of the ring gear and the flange portion are aligned in an axial direction parallel to a rotation axis of the case member, the first and second side walls sandwich the accommodation member in the axial direction, and the ring gear is provided with a housing space formed therein; a ring gear is welded to the flange portion around its entire circumference, and an annular space is formed between the case member and the ring gear from the welded portion to the fitting portion, and the case member has a rounded corner formed on the fitting portion side at the base end of the flange portion, and an air hole is formed to communicate between the inside and the outside of the annular space, and one end of the air hole opens on the outer surface of the case member between the flange portion and the fitting surface in the axial direction, which is closer to the fitting surface than the center of the range where the rounded corner is formed in the axial direction , and the other end opens on a non-contact surface of the case member that does not come into contact with the accommodating member.

In order to solve the above-mentioned problems, the present invention also provides a rotary power transmission device in which the housing member includes a pair of side gears, a plurality of pinion gears that mesh with the pair of side gears, a pinion shaft that supports the plurality of pinion gears, and a pin that prevents the pinion shaft from rotating relative to the case member, and the case member is formed with a shaft insertion hole into which the pinion shaft is inserted and a pin insertion hole into which the pin is inserted as the housing space, and the air hole is formed from the annular space toward the radially inward direction of the case member so as not to communicate with the shaft insertion hole and the pin insertion hole.

In order to solve the above-mentioned problems, the present invention also provides a method for manufacturing the above-mentioned rotary power transmission device, comprising: a case member fabrication process for forming the shaft insertion hole, the pin insertion hole, and the fitting surface in a casting that will become the case member, and drilling the air hole to fabricate the case member; a fitting process for fitting the ring gear into the fitting portion of the case member; a welding process for welding the flange portion of the case member to the ring gear; an inspection process for ultrasonically detecting flaws in the welded portion between the flange portion and the ring gear from the surface of the flange portion opposite the ring gear side; and an assembly process for assembling the pair of side gears, the multiple pinion gears, the pinion shaft, and the pin to the case member.

The rotary power transmission device and manufacturing method of the present invention prevent defects in the weld due to pressure buildup in the space between the case member and the ring gear during welding, while also preventing problems with component assembly and a reduction in the strength of the flange due to the formation of an air hole to release this pressure.

1 is a cross-sectional view showing a differential gear according to an embodiment of the present invention. 4A and 4B are perspective views showing a differential case. 4A and 4B are diagrams illustrating the configuration of a differential case as viewed from the axial direction. 5A to 5C are explanatory diagrams showing a method for manufacturing a differential gear. FIG. 4 is a cross-sectional view showing a cross section of the differential case in the periphery of an air hole. FIG. 10 is a stress analysis diagram showing the stress generated in the differential case by the intensity of the color. FIG. 10 is a cross-sectional view showing a part of a differential case according to a first modified example. FIG. 10 is a cross-sectional view showing a part of a differential case according to a second modified example.

[Embodiment]
An embodiment of the present invention will be described with reference to FIGS. 1 to 6 . The embodiment described below is a preferred example of how the present invention can be implemented. While some parts specifically exemplify various technically preferable aspects, the technical scope of the present invention is not limited to this specific embodiment. Furthermore, in this embodiment, a differential gear mounted on the rear wheels of a rear-wheel drive vehicle or a four-wheel drive vehicle with a drive source such as an engine or an electric motor mounted in front will be described as an example of the rotary power transmission device of the present invention. However, the present invention is not limited to this, and can also be applied to a differential gear for a front-wheel drive vehicle. It is also possible to apply the present invention to rotary power transmission devices other than differentials, such as a transfer case.

Figure 1 is a cross-sectional view showing a differential gear according to an embodiment of the present invention. This differential gear 1 outputs driving force (rotational power) input via a propeller shaft extending in the fore-and-aft direction of the vehicle to a pair of output shafts (drive shafts) while allowing differential rotation. Figure 1 also shows portions of outer races 81, 82 of constant velocity joints provided at the ends of the left and right drive shafts.

The differential device 1 includes an annular ring gear 2, a differential case 3 to which the ring gear 2 is fixed, and a differential gear mechanism 4 housed in the differential case 3. The differential case 3 is formed by machining or other processing on a hollow casting made of, for example, spheroidal graphite cast iron. The ring gear 2 is made of a metal that is harder than the differential case 3, for example, carburized steel. In the differential device 1, the differential case 3 rotates integrally with the ring gear ₂ about a rotation axis O1 to transmit rotational power.

The differential case 3 is housed in the differential carrier 5 and is rotatably supported relative to the differential carrier 5 by a pair of tapered roller bearings 71, 72. The differential carrier 5 has a carrier body 51 and a lid body 52, which are fastened to each other with bolts (not shown). The carrier body 51 and the lid body 52 are formed with insertion holes 510, 520, respectively, through which the outer races 81, 82 of the constant velocity joints are inserted. Lubricating oil (differential oil) is sealed inside the differential carrier 5, and seal members 73, 74 that prevent leakage of this lubricating oil are attached to the inside of the insertion holes 510, 520.

The differential carrier 5 houses a pinion gear shaft 6 connected to the propeller shaft. The pinion gear shaft 6 integrally includes a shaft portion 61 rotatably supported relative to the differential carrier 5 by a tapered roller bearing 75 and a gear portion 62 having a larger diameter than the shaft portion 61, and the gear portion 62 meshes with the ring gear 2. When the differential device 1 mounted on a vehicle is viewed vertically from above, the rotation axis _O1 of the differential case 3 and the rotation axis O2 of the pinion gear shaft 6 are perpendicular to each other. Driving force from a vehicle drive source such as an engine is input to the ring gear 2 via the propeller shaft and the pinion gear shaft _6. Hereinafter, the direction parallel to the rotation axis _O1 of the differential case 3 will be referred to as the axial direction, and the direction perpendicular to the rotation axis _O1 will be referred to as the radial direction.

The differential gear mechanism 4 includes a pair of left and right side gears 41, 42, multiple pinion gears 43, 44 that mesh with the side gears 41, 42, a pinion shaft 45 that supports the multiple pinion gears 43, 44, and a pin 46 that prevents the pinion shaft 45 from coming loose and rotating relative to the differential case 3. The pinion shaft 45 is inserted through the shaft holes 430, 440 of the pinion gears 43, 44, respectively.

The side gears 41, 42 each have a cylindrical portion 411, 421 in the center of which the stem portions 811, 821 of the outer races 81, 82 are spline-fitted to prevent relative rotation, and a gear portion 412, 422 that meshes with the pinion gears 43, 44. Washers 11 to 14 are respectively disposed between the differential case 3 and the gear portions 412, 422 of the side gears 41, 42 and the back surfaces of the pinion gears 43, 44.

The differential case 3 is one aspect of the "case member" of the present invention. The side gears 41, 42, pinion gears 43, 44, pinion shaft 45, pin 46, and washers 11-14 are one aspect of the "accommodated member" of the present invention housed in the differential case 3.

2(a) and (b) are perspective views showing the differential case 3. The differential case 3 integrally includes a cylindrical portion 31 having an internal storage space 310 for accommodating the side gears 41, 42 and the pinion gears 43, 44, first and second side walls 32, 33 that axially sandwich the side gears 41, 42 and the pinion gears 43, 44, first and second cylindrical boss portions 34, 35 that extend axially from the centers of the first and second side walls 32, 33, respectively, an annular fitting portion 36 having a fitting surface 36a with which the inner peripheral surface 2a of the ring gear 2 fits, an annular flange portion 37 that protrudes radially outward from the fitting surface 36a, and a rounded corner portion 38 that is provided between the fitting portion 36 and the flange portion 37. Insertion holes 340 and 350 are formed in the centers of the first and second boss portions 34 and 35, respectively, through which the stem portions 811 and 821 of the outer races 81 and 82 are inserted.

The cylindrical portion 31 is formed with a pair of windows 311, 312 for introducing the side gears 41, 42, pinion gears 43, 44, and washers 11-14 into the storage space 310. The flange portion 37 is provided on the outer periphery of the end of the cylindrical portion 31 closer to the second side wall 33 than the axial center. The rounded corner 38 is formed on the mating portion 36 side of the base end 371 (see Figure 1), which is the inner diameter end of the flange portion 37. Figure 2(a) shows the window portion 311 side of one of the pair of windows 311, 312, and Figure 2(b) shows the other window portion 312 side after the differential case 3 is rotated 180° from the state shown in Figure 2(a).

In addition to the aforementioned accommodation space 310, the differential case 3 is formed with a shaft insertion hole 313 as an accommodation space for accommodating the pinion shaft 45, a pin insertion hole 314 as an accommodation space for accommodating the pin 46, and an air hole 30, which will be described later. The shaft insertion holes 313 are formed in two locations on the cylindrical portion 31, penetrating radially from the inside to the outside of the cylindrical portion 31. The pin insertion hole 314 is formed in the cylindrical portion 31 along the axial direction, intersecting one of the two shaft insertion holes 313. The pin insertion hole 314 opens toward the first side wall 32 and is closed on the second side wall 33 side of the shaft insertion hole 313.

As shown in FIG. 1, the pin 46 is inserted into a through-hole 450 that passes radially through the pinion shaft 45, and is press-fit into the pin insertion hole 314. The shaft insertion hole 313 and the pin insertion hole 314 are drilled in the differential case 3 using a cutting tool such as a drill.

3(a) is a structural diagram of the differential case 3 and ring gear 2 viewed axially from the flange portion 37 side. FIG. 3(b) is a structural diagram of the differential case 3 and ring gear 2 viewed axially from the opposite side of the flange portion 37. A recess 330 is formed in the second side wall 33 of the differential case 3, which is closer to the flange portion 37, to reduce the weight of the differential case 3. The recess 330 is recessed axially from the axial end face 33a, which is the outer surface of the second side wall 33, and is formed in an annular shape on the outer periphery of the second boss portion 35. Furthermore, a plurality of reinforcing ribs 331 are formed on the second side wall 33 between the outer circumferential surface 330a and inner circumferential surface 330b of the recess 330. In this embodiment, as an example, eight ribs 331 are formed radially at equal intervals in the circumferential direction. The flange portion 37 is formed in a portion corresponding to the outer periphery of the recess 330.

The ring gear 2 is arranged axially aligned with the flange portion 37 of the differential case 3. In this embodiment, the entire outer diameter side of the ring gear 2, excluding the peripheral edge portion, is axially aligned with the flange portion 37. However, the entire radial length of the ring gear 2 may be axially aligned with the flange portion 37. In other words, it is sufficient that at least a portion of the ring gear 2, including the inner diameter side end, is axially aligned with the flange portion 37.

In addition, in this embodiment, the ring gear 2 is a hypoid gear, a type of bevel gear, with its tooth surface facing away from the flange portion 37 and its back surface fixed to the differential case 3 by welding to the flange portion 37. The welded portion W (see Figure 1), where the ring gear 2 and the flange portion 37 are welded, is formed around the entire circumference of the ring gear 2 and the flange portion 37. Next, with reference to Figures 4(a) to (c), a method for manufacturing the differential device 1, including the process of welding the ring gear 2 and the flange portion 37, will be described.

The differential device 1 is manufactured through the following steps: a differential case fabrication process in which the differential case 3 is fabricated by casting and machining; a fitting process in which the ring gear 2 is fitted into the fitting portion 36 of the differential case 3; a welding process in which the flange portion 37 of the differential case 3 is welded to the ring gear 2; an inspection process in which the welded portion W is ultrasonically inspected from the surface of the flange portion 37 opposite the ring gear 2 side; and an assembly process in which the differential gear mechanism 4 is assembled inside the differential case 3.

In the differential case manufacturing process, a casting that will become the differential case 3 is cast, and the shaft insertion holes 313, pin insertion holes 314, and mating surface 36a are formed in this casting. The air holes 30 are also drilled to manufacture the differential case 3. The air holes 30 are circular holes that communicate with the inside and outside of the annular space S (see Figure 4(b)) formed between the differential case 3 and the ring gear 2 when the ring gear 2 is fitted into the fitting portion 36 of the differential case 3. The air holes 30 are also formed in a position that is not axially aligned with the pair of windows 311, 312 formed in the cylindrical portion 31 of the differential case 3.

Figure 4(a) shows the state in which the ring gear 2 is positioned on the outer periphery of the cylindrical portion 31 of the differential case 3 as a preliminary step to the fitting process. The outer diameter of the cylindrical portion 31 is smaller than the inner diameter of the ring gear 2, and a gap is formed between the outer periphery 31a of the cylindrical portion 31 and the inner periphery 2a of the ring gear 2. The surface of the flange portion 37 of the differential case 3 facing the ring gear 2 is formed with a case-side opposing surface 37a and a case-side abutment surface 37b located closer to the ring gear 2 than the case-side opposing surface 37a in the axial direction of the differential case 3. The case-side abutment surface 37b is formed closer to the tip end 372 of the flange portion 37 than the case-side opposing surface 37a.

The surface of the ring gear 2 facing the flange portion 37 is formed with a gear-side opposing surface 2b that faces the case-side opposing surface 37a of the flange portion 37 in the axial direction, and a gear-side abutment surface 2c that faces the case-side abutment surface 37b of the flange portion 37 in the axial direction. Furthermore, a tapered surface 2d is formed between the inner peripheral surface 2a of the ring gear 2 and the gear-side opposing surface 2b, with an inner diameter that increases toward the gear-side opposing surface 2b.

4(b) shows a state in which the ring gear 2 is fitted into the fitting portion 36 of the differential case 3. In the fitting process, the ring gear 2 moves axially relative to the differential case 3 along the direction of arrow _D1 shown in FIG. 4(a) until the case-side contact surface 37b and the gear-side contact surface 2c come into contact, and the ring gear 2 is press-fitted into the fitting portion 36 of the differential case 3. The outer diameter of the fitting portion 36 (diameter of the fitting surface 36a) is slightly larger than the inner diameter of the ring gear 2 (diameter of the inner peripheral surface 2a), and the ring gear 2 is fitted externally into the fitting portion 36 with a predetermined interference. The fitting surface 36a and the inner peripheral surface 2a of the ring gear 2 are cylindrical surfaces formed to be parallel to the rotation axis _O1 and are machined with high precision by, for example, grinding. When the ring gear 2 is fitted into the fitting portion 36, the fitting surface 36a and the inner peripheral surface 2a of the ring gear 2 come into close contact with each other without any gaps around the entire circumference. The outer diameter of the fitting portion 36 and the inner diameter of the ring gear 2 may be the same. In this case, too, the fitting surface 36a and the inner peripheral surface 2a of the ring gear 2 come into close contact with each other without any gaps around the entire circumference.

As shown enlarged in Figure 4(b), the fitting portion 36 is trapezoidal in an axial cross section of the differential case 3, with the flange-side inclined surface 36b and the anti-flange-side inclined surface 36c, which form a pair of opposite sides when the fitting surface 36a is the upper base, being tapered. The flange-side inclined surface 36b is formed continuous with the outer surface 38a of the rounded corner 38. The anti-flange-side inclined surface 36c functions as a guide surface that abuts against the tapered surface 2d of the ring gear 2 to align the ring gear 2 and the differential case 3 when the ring gear 2 is fitted into the fitting portion 36 of the differential case 3.

In the welding process, the ring gear 2 is pressed axially toward the flange portion 37 with a predetermined pressure, and welding is performed with the case-side contact surface 37b and the gear-side contact surface 2c in close contact. During this welding, a linear filler material 91 is supplied to the groove 10 formed on the radially outer side of the case-side contact surface 37b and the gear-side contact surface 2c. The ring gear 2 and differential case 3 are rotated while this filler 91 is heated, for example, with a laser beam or electron beam, to form a weld W around the entire circumference.

As shown enlarged in Figure 4(b), an annular space S is formed around the entire circumference between the differential case 3 and the ring gear 2, from the weld zone W to the mating portion 36. The annular space S is a space surrounded by the gear-side opposing surface 2b, tapered surface 2d, and part of the inner circumferential surface 2a of the ring gear 2, as well as the case-side opposing surface 37a of the flange portion 37 of the differential case 3, the outer surface 38a of the rounded corner portion 38, and the flange-side inclined surface 36b of the mating portion 36. In the welding process, full-penetration welding is performed so that part of the weld zone W reaches the annular space S.

The annular space S is open to the outside through the air holes 30 in the differential case 3. When gas is generated by heating the filler 91 or when the air in the annular space S expands due to the heat generated during welding, the air in the annular space S is discharged through the air holes 30. This prevents the pressure in the annular space S from rising significantly above atmospheric pressure, thereby preventing defects in the welded joint W caused by pressure increases in the annular space S.

Figure 4(c) shows the inspection process. During the inspection process, the ring gear 2 and differential case 3 are positioned so that the ring gear 2 is vertically above the flange portion 37. The entire circumference of the weld W is inspected for defects, such as porosity, using an ultrasonic flaw detector 92. The ultrasonic flaw detector 92 has a nozzle 921 that sprays water 93 as a contact medium and an ultrasonic transmitter/receiver 922 located within the nozzle 921. The ultrasonic transmitter/receiver 922 generates ultrasonic waves and receives ultrasonic waves reflected by the weld W being inspected. The water 93 is sprayed vertically upward from the nozzle 921 toward the axial end face 37c of the flange portion 37 opposite the ring gear 2 side, and the ultrasonic waves are transmitted between the ultrasonic transmitter/receiver 922 and the axial end face 37c. If no defects that would affect the strength of the weld W are found during this ultrasonic flaw detector, the inspection result is considered a pass.

In the assembly process, the side gears 41, 42 and the washers 11, 12 are inserted into the differential case 3 through the windows 311, 312, and the pinion gears 43, 44 are meshed with the side gears 41, 42 near the windows 311, 312. The side gears 41, 42 and the pinion gears 43, 44 are then rotated about the rotation axis _O1 to position the pinion gears 43, 44 so that the shaft holes 430, 440 are radially aligned with the shaft insertion hole 313. Washers 13, 14 are then inserted between the pinion gears 43, 44 and the inner surface of the differential case 3, and the pinion shaft 45 is then inserted into the shaft insertion hole 313 as shown in FIG. 1 . Then, a pin 46 is press-fitted into the pin insertion hole 314 to prevent the pinion shaft 45 from coming out.

The differential case fabrication process, fitting process, welding process, and inspection process are performed in this order, but the assembly process may be performed between the differential case fabrication process and the fitting process, or after the inspection process. Performing the assembly process after the inspection process prevents water 93, which acts as a contact medium, from getting on the housing components, namely, the side gears 41, 42, pinion gears 43, 44, pinion shaft 45, pin 46, and washers 11-14. Furthermore, by assembling the washers 11-14, side gears 41, 42, pinion gears 43, 44, pinion shaft 45, and pin 46 only to differential cases 3 that pass the inspection process, it is possible to prevent these parts from being assembled to cases with defective welds W, improving work efficiency.

Figure 5 is a cross-sectional view of the differential case 3 along the axial direction, including the air hole 30. One end 301 of the air hole 30 opens to the outer peripheral surface 3a of the differential case 3 between the flange portion 37 and the mating surface 36a in the axial direction of the differential case 3 (the axial range indicated by the double-headed arrow A in Figure 5), and communicates with the annular space S. The other end 302 of the air hole 30 opens to the inner surface of the recess 330 of the differential case 3. More specifically, the other end 302 of the air hole 30 opens to a curved surface 330d between the bottom surface 330c of the recess 330 and the outer peripheral surface 330a. The inner surface (curved surface 330d) of the recess 330 where the other end 302 of the air hole 30 opens is a non-contact surface that does not come into contact with the housing members housed in the differential case 3.

Furthermore, the air holes 30 are formed linearly from the annular space S toward the radially inward direction of the differential case 3 at an angle that intersects with the axial direction of the differential case 3 so as not to communicate with either the shaft insertion holes 313 or the pin insertion holes 314. In this embodiment, the air holes 30 are formed at an angle relative to the axial and radial directions of the differential case 3 and perpendicular to the circumferential direction.

In the cross section shown in Figure 5, the outer surface 38a of the rounded corner 38 is a curved surface formed in an arc shape with a radius of curvature R centered on the center point C of the curve. The air hole 30 has its end (one end 301) on the annular space S side open closer to the mating surface 36a than the center of the area where the rounded corner 38 is formed in the axial direction of the differential case 3 (left-right direction in Figure 5). In other words, the air hole 30 opens at a location on the mating surface 36a side of the perpendicular bisector L of the radius of curvature R in the axial direction of the differential case 3.

As shown in FIG. 5 , when the diameter of air hole 30 is D and the axial distance from the center point of curvature C to end 301 a of one end 301 of air hole 30 on the flange portion 37 side of the opening periphery, M, the radius of curvature R of outer surface 38 a, hole diameter D, and distance M are set to dimensions that satisfy the following inequalities (1) and (2).
M/R≦0.3…(1)
D/R≦0.5…(2)
The strength of the differential device 1 is ensured by forming the rounded corners 38 and the air holes 30 with dimensions that satisfy the inequalities (1) and (2).

6 is a stress analysis diagram in which the stress generated in the differential case 3 is indicated by the intensity of the color when a load is applied in the direction of arrow _D2 to a portion of the circumferential direction of the case-side abutment surface 37b of the flange portion 37 of the differential case 3 before the formation of the air holes 30 shown by the dashed lines. In this stress analysis diagram, the darker the color, the higher the stress generated. The load direction indicated by arrow _D2 is the direction in which the flange portion 37 receives a pressing force from the ring gear 2 during the fitting process and welding process in manufacturing the differential device 1, and is also the direction in which the flange portion 37 receives a meshing reaction force between the ring gear 2 and the pinion gear shaft 6.

As shown in Figure 6, high stress occurs near the case-facing surface 37a of the flange portion 37 and the case-facing surface 37a of the outer surface 38a of the rounded corner portion 38. For this reason, in this embodiment, the air hole 30 is opened closer to the mating surface 36a than the perpendicular bisector L shown in Figure 5. Furthermore, by forming the air hole 30 so that the above inequality (1) is satisfied, the area where the air hole 30 opens can be further away from the area where high stress occurs.

The size of the hole diameter D depends on the size of the vehicle on which the differential gear 1 is mounted, but it is desirable that it be between 2 mm and 4 mm. If the hole diameter D is smaller than 2 mm, the drill is more likely to break when forming the air holes 30, and if the hole diameter D exceeds 4 mm, it will affect the strength of the differential case 3.

(Effects of the embodiment)
According to the embodiment described above, one end 301 of the air hole 30 opens into the annular space S, and air within the annular space S is discharged from the air hole 30 during welding. This prevents defects from occurring in the welded portion W due to an increase in pressure inside the annular space S during welding. Furthermore, since the air hole 30 is formed avoiding a portion where high stress occurs when an axial load is applied to the flange portion 37 from the ring gear 2, a decrease in the strength of the differential case 3 due to the formation of the air hole 30 can be suppressed. Furthermore, the air hole 30 does not communicate with the shaft insertion hole 313 and the pin insertion hole 314, and the opening periphery of the other end 302 does not contact the components (washers 11-14, side gears 41, 42, pinion gears 43, 44, pinion shaft 45, and pin 46) of the differential gear mechanism 4 housed in the differential case 3. Therefore, even if burrs are generated when forming the air hole 30, the burrs do not interfere with the assembly of the differential gear mechanism 4. Furthermore, since the other end 302 of the air hole 30 opens to the inner surface (curved surface 330d) of the recess 330 that is recessed in the axial direction from the outer surface (axial end surface 33a) of the second side wall 33, the length of the air hole 30 can be shortened to prevent a decrease in strength, and water 93 as a contact medium will not enter the annular space S from the air hole 30 during the inspection process.

(Modification)
Next, modified examples of the differential case 3 will be described with reference to Figures 7 and 8. In Figures 7 and 8, components common to those described in the above embodiment are designated by the same reference numerals as those in Figure 4 and the like, and redundant description will be omitted. In Figures 7 and 8, the ring gear 2 is indicated by a virtual line (two-dot chain line).

Figure 7 is a cross-sectional view showing a portion of a differential case 3A according to Modification 1. In the above embodiment, the air holes 30 are formed at an angle relative to the axial and radial directions of the differential case 3. However, in the differential case 3A according to Modification 1, the air holes 30A are formed perpendicular to the axial direction of the differential case 3A. As in the above embodiment, one end 301 of the air holes 30A opens to the end of the outer surface 38a of the rounded corner 38 on the mating portion 36 side.

The other end 302 of the air hole 30A opens into a portion of the cylindrical inner surface 310a of the storage space 310, which faces the outer peripheral surface 421a (see Figure 1) of the cylindrical portion 421 of the left side gear 42, via a gap. At least a portion of this cylindrical inner surface 310a is a non-contact surface that does not come into contact with storage members such as the side gear 42 housed in the differential case 3. The other end 302 of the air hole 30A opens into this non-contact surface. Note that the outer peripheral surface 421a of the cylindrical portion 421 of the side gear 42 may come into contact with the inner surface 310a of the storage space 310 in the portion where the air hole 30A does not open.

Figure 8(a) is a cross-sectional view showing a portion of the differential case 3B according to Modification 2. Figure 8(b) is an enlarged view of a portion of Figure 8(a). In the above embodiment, the case where the tooth surface of the ring gear 2 faces the cylindrical portion 31 was described. However, in the differential case 3B according to Modification 2, the tooth surface of the ring gear 2 faces away from the cylindrical portion 31. Furthermore, in the differential case 3B according to Modification 2, the air hole 30B is formed perpendicular to the axial direction of the differential case 3A.

One end 301 of the air hole 30A opens to an outer peripheral surface 3Ba of the differential case 3B that is formed parallel to the rotation axis _O1 between the rounded corner 38 and the fitting portion 36. The other end 302 of the air hole 30B, as in the above embodiment, opens to an inner surface of the recess 330 that is a non-contact surface that does not come into contact with the housing members housed in the differential case 3B, more specifically, to a curved surface 330d between a bottom surface 330c and the outer peripheral surface 330a of the recess 330.

These variants 1 and 2 also achieve the same effects as the above-described embodiment. Furthermore, in the differential case 3B according to variant 2, air holes 30B are not formed in the driving force transmission path from the welded portion W to the area surrounding the shaft insertion hole 313 of the differential case 3A, so the strength of the differential case 3B is not reduced by forming the air holes 30B.

(Additional Note)
The present invention has been described above based on embodiments and modifications, but these embodiments and modifications do not limit the scope of the invention as claimed. It should be noted that not all of the combinations of features described in the embodiments and modifications are necessarily essential to the means for solving the problems of the invention. Furthermore, the present invention can be appropriately modified and implemented by omitting some components or adding or substituting components within the scope of the invention, and modifications such as those described below are possible.

In the above embodiment, the air holes 30 are formed at an angle to the radial direction of the differential case 3 and open to the curved surface 330d of the recess 330. However, this is not limiting, and the air holes 30 may open to the outer peripheral surface 330a or bottom surface 330c of the recess 330. Furthermore, depending on the axial depth of the recess 330, the air holes 30 may be formed along the radial direction and open to the inner surface of the recess 330 (the outer peripheral surface 330a or the curved surface 330d).

In addition, in the above embodiment, the entire opening of the air hole 30 on the annular space S side is formed on the outer surface 38a of the rounded corner 38. However, this is not limited to this, and a portion of the opening of the air hole 30 on the annular space S side may be formed on the flange-side inclined surface 36b. Furthermore, the air hole 30 may open on the outer peripheral surface of the differential case 3 between the rounded corner 38 and the fitting portion 36.

In addition, in the above embodiment, the present invention has been described as being applied to a differential device (a so-called open differential) that does not have a differential limiting function that limits the differential rotation of the pair of side gears 41, 42. However, the present invention is not limited to this, and can also be applied to a differential device that has a differential limiting function that limits the differential rotation of the pair of side gears. An example of such a differential device with a differential limiting function is one that has a multi-plate clutch, such as that described in Patent Document 4 above, in which multiple inner clutch plates that rotate integrally with each of the pair of left and right side gears and multiple outer clutch plates that rotate integrally with the differential case are arranged alternately in the axial direction. In this case, air holes are opened in the multiple outer clutch plates and non-contact surfaces that do not come into contact with the multiple outer clutch plates and other housing members.

The present invention may also be applied to a differential device, such as that described in Patent Document 5, in which multiple pinion gears are arranged parallel to a pair of side gears and have a differential limiting function that limits the differential rotation of the pair of side gears by friction between the tooth tips of the pinion gears and the inner surface of the differential case. In this case, air holes are opened on the non-contact surfaces of the multiple pinion gears, etc., that do not come into contact with the housing members housed in the differential case.

Furthermore, the present invention may also be applied to a transfer device in which a ring gear is welded to the flange portion of the transfer case, as described in Patent Document 6 above. In this case, the air hole is opened on the non-contact surface that does not come into contact with the drive shaft housed in a through-hole that serves as an accommodation space formed in the center of the transfer case.

REFERENCE SIGNS LIST 1... Differential device (rotational power transmission device) 2... Ring gear 3, 3A, 3B... Differential case (case member) 30, 30A, 30B... Air hole 301... One end 302... Other end 310... Storage space 310a... Inner surface (non-contact surface)
313... Shaft insertion hole (accommodating space) 314... Pin insertion hole (accommodating space)
330: Recessed portion 36: Fitting portion 36a: Fitting surface 37: Flange portion 4: Differential gear mechanism 41, 42: Side gears (accommodating member)
43, 44... Pinion gears (housing members) 45... Pinion shaft (housing member)
46: Pin (accommodating member) S: Annular space W: Welded portion

Claims (5)

an annular ring gear;
a cylindrical portion having an annular fitting portion formed with a fitting surface into which an inner peripheral surface of the ring gear is fitted, and a flange portion provided so as to protrude radially outward from the fitting surface, the cylindrical portion having an accommodation space formed therein for accommodating an accommodation member , and a case member having first and second side walls sandwiching the accommodation member, the flange portion being provided on an outer periphery of the cylindrical portion closer to the second side wall than an axial center portion of the cylindrical portion ,
At least a portion of the ring gear and the flange portion are aligned in an axial direction parallel to a rotation axis of the case member, and first and second side walls sandwich the housing member in the axial direction,
the ring gear is welded to the flange portion along its entire periphery, and an annular space is formed between the case member and the ring gear from the welded portion to the fitting portion,
The case member has a rounded corner formed on the fitting portion side of a base end of the flange portion, and an air hole communicating the inside and outside of the annular space,
the air hole has one end that opens to the outer peripheral surface of the case member between the flange portion and the fitting surface in the axial direction, closer to the fitting surface than the center of the range in which the rounded corner portion in the axial direction is formed , and the other end that opens to a non-contact surface of the case member that does not come into contact with the housing member.
Rotational power transmission device. The housing member includes a pair of side gears, a plurality of pinion gears meshing with the pair of side gears, a pinion shaft supporting the plurality of pinion gears, and a pin that prevents the pinion shaft from rotating relative to the case member,
The case member is formed with a shaft insertion hole into which the pinion shaft is inserted and a pin insertion hole into which the pin is inserted, as the accommodation space,
the air hole is formed from the annular space toward the radially inward direction of the case member so as not to communicate with the shaft insertion hole and the pin insertion hole;
2. The rotary power transmission device of claim 1. The case member has a recess formed in the second side wall,
the other end of the air hole is open to the inner surface of the recess.
3. The rotary power transmission device of claim 2. In a cross section of the case member along the axial direction, including the air hole, the axial distance from the center point of curvature of the outer surface of the rounded corner portion to the end of the opening periphery at the one end of the air hole on the flange portion side is defined as M, and the radius of curvature of the outer surface of the rounded corner portion is defined as R, where M is the axial distance from the center point of curvature of the outer surface of the rounded corner portion to the end of the opening periphery at the one end of the air hole on the flange portion side, and R is the radius of curvature of the outer surface of the rounded corner portion, the value of M/R is 0.3 or less.
4. A rotary power transmission device according to claim 1 . A method for manufacturing a rotary power transmission device according to claim 2 or 3, comprising the steps of:
a case member fabrication step of forming the shaft insertion hole, the pin insertion hole, and the fitting surface in a casting that will become the case member, and also forming the air hole to fabricate the case member;
a fitting step of fitting the ring gear to the fitting portion of the case member;
a welding step of welding the flange portion of the case member and the ring gear;
an inspection step of ultrasonically detecting flaws in the welded portion between the flange portion and the ring gear from a surface of the flange portion opposite to the ring gear side;
an assembling step of assembling the pair of side gears, the plurality of pinion gears, the pinion shaft, and the pin to the case member,
A method for manufacturing a rotary power transmission device.