Axle case structure

Abstract

To provide an axle case structure 1 that can improve the fatigue strength of a weld bead 14 for joining a cover 4 to an axle case body 2, without increasing the weight, and can suppress crack initiation from a root portion 14a of the weld bead 14 over the entire circumference of the cover 4.SOLUTION: An axle case structure includes an opening 3 formed in a central part of an axle case body 2 for receiving a differential gear, a ring plate 13 attached to the axle case body 2 to include the opening 3, and a cover 4 attached to the ring plate 13 to cover the opening 3. The cover 4 has a bowl portion 4a formed in a bowl shape and a flange portion 4b on an outer periphery of the bowl portion. The flange portion 4b is laid over the ring plate 13 such that the outer peripheral end surface 4c of the flange portion 4b is aligned with the outer peripheral end surface 13c of the ring plate 13, and the outer peripheral end surface 4c of the flange portion 4b and the outer peripheral end surface 13c of the ring plate 13 are joined integrally to the axle case body 2 with a single weld bead 14.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an axle case structure used in trucks, buses, etc., and in particular to an axle case structure that improves the fatigue strength of the welded portion (weld bead) between the axle case body and a cover that covers an opening formed in the center of the axle case body to accommodate a differential gear. [Background technology]

[0002] FIG. 1(a) shows a perspective view of a conventional axle case structure 1J seen from diagonally above the rear, FIG. 1(b) shows a rear view of the center of the axle case structure 1J seen from the rear, and FIG. 1(c) shows a cross-sectional view taken along the line cc in FIG. 1(b). The axle case structure 1J includes an axle case body 2 arranged in the vehicle width direction, an opening 3 formed in the center of the axle case body 2 to accommodate a differential gear (differential mechanism), and a cover 4 welded to the axle case body 2 so as to cover the opening 3. The cover 4 has a bowl-shaped bowl portion 4a and a flange portion 4b formed on the outer periphery of the bowl portion 4a, and the outer peripheral end face of the flange portion 4b is welded to the axle case body 2 along the entire circumferential direction. 5 is a weld bead.

[0003] 1(a), a bracket 6 to which a torque rod (not shown) is attached is provided on the top of the axle case body 2, a spring seat 7 to which a suspension spring (not shown) is attached is provided in the middle of the left and right sides of the axle case body 2, and spindles 8 to support the wheels are provided at the left and right ends of the axle case body 2. A force is applied to the bracket 6 in the fore-and-aft direction of the vehicle body as indicated by arrow A due to acceleration and deceleration while the vehicle is traveling, a downward force is applied to the spring seat 7 as indicated by arrow B due to the vehicle weight and cargo load, and an upward force is applied to the spindle 8 as indicated by arrow C due to road surface reaction force.

[0004] These forces (loads) can cause fatigue cracks to occur in the weld bead 5 that joins the flange portion 4b of the cover 4 to the axle case body 2, starting from the root portion 5a of the weld bead 5 along the throat thickness 5b, as shown in Figure 1(c). The location where fatigue cracks occur in the weld bead 5 along the circumferential direction of the cover 4 correlates with the manner in which the load is applied, and it is known that fatigue cracks occur at six locations K1, K2, K3, K4, K5, and K6 approximately every 60 degrees along the circumferential direction of the weld bead 5 that joins the flange portion 4b of the cover 4 to the axle case body 2, as shown in Figure 1(b) (see Patent Document 1).

[0005] The cracks are believed to be caused by the deformation of the bowl portion 4a of the cover 4 under load. The deformation of the cover 4 will be explained using the vertical load applied to the axle case structure 1J as an example. Figure 2(a) shows the deformation state of the axle case body 2 when the vertical load (arrows B and C) acts, with the amount of deformation magnified. The vertical loads of arrows B and C cause the axle case body 2 to deform so that the central axis X becomes convex downward, and the flange portion 4b of the cover 4 and the opening 3 of the axle case body 2 deform from their original perfect circle to an approximately rice ball shape. At this time, as shown in Figure 2(b), which is a cross-sectional view of line bb in Figure 2(a), a force F acts on the flange portion 4b of the cover 4, which corresponds to the corner of the approximately rice ball shape, in the direction of lifting the cover 4, and a large tensile stress is generated in the root portion 5a of the weld bead 5 that joins the outer peripheral end face of the flange portion 4b and the axle case body 2.

[0006] This tensile stress causes fatigue cracks to occur in the weld bead 5 starting from the root 5a. For example, when the flange 4b ​​of the cover 4 is deformed into a roughly rice-ball shape as shown in FIG. 2(a), the areas where cracks are likely to occur due to vertical loads are the upper K1 and the left and right diagonal lower K3 and K5 of the cover 4 shown in FIG. 1(b), where deformation is prominent. In addition, when a front-rear load (arrow A) from the torque rod acts on the bracket 6 on the upper part of the axle case main body 2 shown in FIG. 1(a), the upper K1, lower K4, and the left and right diagonal upper K2 and K6 of the cover 4 shown in FIG. 1(b) are areas where cracks are likely to occur. As shown in FIG. 1(c) and FIG. 2(b), cracks in the weld bead 5 start from the root 5a of the weld bead 5 and progress along the throat thickness 5b. This is because the tensile stress generated by the force F pulling up the cover 4 reaches a maximum value at the throat thickness 5b, which is the smallest cross section. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 8-67108 Summary of the Invention [Problem to be solved by the invention]

[0008] As explained using Figures 1 and 2, when the axle case body 2 is deformed by various loads, cracks may occur starting from the root portion 5a in the weld bead 5 that joins the flange portion 4b of the cover 4 to the axle case body 2. Several methods have been used so far to prevent cracks in the weld bead 5. These measures will be explained using Figures 3(a), 3(b), and 3(c).

[0009] FIG. 3(a) shows an example of a measure to increase the plate thickness t of the cover 4. As is clear from the non-measured product in FIG. 1(c), the plate thickness t of the cover 4 is increased by about two times. In the case of fillet welding, the stress calculation of the welded part (weld bead 5) generally uses the value of the throat thickness 5b. The stress at the throat thickness 5b is calculated by dividing the load (force F) by the leg length 5a, assuming that the theoretical throat thickness 5b is 70% of the leg length L. Therefore, increasing the plate thickness of the member and increasing the leg length L and throat thickness 5b leads to a reduction in the stress at the throat thickness 5b, and is effective in suppressing the occurrence of cracks. However, this method is problematic in that the weight increase due to the increase in the plate thickness t of the cover 4 becomes a problem. If the plate thickness t of the cover 4 is doubled to double the throat thickness 5b, the mass increases by about 8 kg in a standard axle case structure 1J for large vehicles. This weight increase is an element that should be avoided in the design of products that strongly require weight reduction.

[0010] FIG. 3(b) shows an example of a measure in which a reinforcing ring 9 is added. The reinforcing ring 9 is welded (10 is a weld bead) to the axle case body 2 so as to surround the opening 3, and the flange portion 4b of the cover 4 is welded (5 is a weld bead) to the reinforcing ring 9. With this configuration, the rigidity of the axle case body 2 is increased by the reinforcing ring 9, so the deformation of the approximately rice ball shape of the flange portion 4b of the cover 4 and the opening 3 described above with reference to FIG. 2(a) is reduced, and the stress of the root portion 10a of the weld bead 10 is reduced. However, the effect of reducing stress is smaller than that of the above-mentioned case in which the plate thickness t of the cover 4 is increased (FIG. 3(a)), and in actual applications, it is often necessary to combine this with an increase in the plate thickness t of the cover 4. Naturally, even with this method, the increase in weight due to the addition of the reinforcing ring 9 is a problem. In a standard axle case for a large vehicle, the weight increases by about 11 kg (7 kg for the ring + 4 kg for the cover), and this is a measure that should be avoided from the perspective of weight reduction.

[0011] FIG. 3(c) shows an example of a countermeasure in which the back side of the cover 4 is added to the opening 3 of the axle case 2 and welded (11 is a weld bead). The weld bead 11 added to the back side of the cover 4 suppresses the opening deformation of the flange portion 4b of the cover 4 caused by the force F in the direction of pulling up the cover 4 shown in FIG. 2(b) described above, and in addition, by increasing the number of welds (weld beads 5, 11 in FIG. 3(c)) that serve as a force transmission member from the axle case main body 2 that deforms as shown in FIG. 2(a) to the cover 4, the stress of the root portion 5a of the weld bead 5 of the flange portion 4b of the cover 4 is reduced. However, in this example of a countermeasure, in order to expose the weld line for the construction of the back welding, it is necessary to cut out the inner diameter of the opening 3 of the axle case main body 2 as shown in FIG. 3(c) (12 is a cutout portion). However, as shown in Fig. 1(b), four locations on the inner diameter of the opening 3 of the axle case main body 2, namely, the left and right upper diagonal portions D1 and D2 and the left and right lower diagonal portions D3 and D4, are locations that cannot be cut out because they are locations where a part of the differential device (bearing cap) abuts as a positioning portion of the differential device (differential mechanism) (not shown) housed in the axle case main body 2 abuts. If these locations D1, D2, D3, and D4 are not cut out, welding on the back side (weld bead 11) shown in Fig. 3(c) cannot be performed, and as shown in Fig. 1(b), the weld beads 5 on the radial outside of these locations D1, D2, D3, and D4 are locations K2, K3, K5, and K6 where cracks are likely to occur when the cover 4 is pulled up as shown in Fig. 2(b) due to deformation of the axle case main body 2, so not welding on the back side creates the possibility of cracks occurring in the weld beads 5.

[0012] Patent Document 1 discloses an invention in which a cutout is provided in a portion of the opening 3 of the axle case body 2 that avoids the positioning portion of the differential gear, and the back side of the cover 4 is welded to the axle housing body 2 at the cutout. This invention is effective in suppressing cracks in the weld beads 5 of the diagonal lower parts K3 and K5 of the cover 4 due to vertical loads. However, it cannot address cracks in the weld beads 5 of the diagonal upper parts K2 and K6 of the cover 4 due to the load of the torque rod applied to the bracket 6. The reason is that the axle case body 2 is constantly subjected to vertical loads (arrows B and C), and tensile forces are generated along the circumference of the end face of the opening 3 near the positioning portion of the differential gear at the diagonal upper parts D1 and D2 of the inner diameter part of the opening 3. For this reason, if the opening 3 is locally cut out at the diagonal upper parts K2 and K6 of the cover for backside welding, new cracks may occur in the housing body 2 starting from the cutout due to circumferential tensile forces. Therefore, the measure of adding backside welding as described in Patent Document 1 cannot address the cracks in the weld beads 5 at the diagonal upper portions K2 and K6 of the cover 4, and issues remain.

[0013] As described above, as a measure to suppress cracks occurring in the weld bead 5 of the cover 4 due to deformation of the axle case body 2 under various loads, increasing the plate thickness t of the cover 4 shown in FIG. 3(a) and adding the reinforcing ring 9 shown in FIG. 3(b) can suppress cracks in the weld bead 5 over the entire circumference of the cover 4 in the circumferential direction because there is no need to cut out the inner circumference of the opening 3, but the weight increases. On the other hand, backside welding (weld bead 11) shown in FIG. 3(c) can avoid an increase in weight, but it is necessary to cut out the opening 3 to expose the weld line, and the positioning of the differential gear cannot be performed. If backside welding is not performed without cutting out the four points of the left and right diagonal upper parts D1 and D2 and the left and right diagonal lower parts D3 and D4 of the opening 3 in order to position the differential gear, cracks may occur in the weld beads 5 (K2, K3, K5, K6) on the radial outside of those parts.

[0014] Therefore, an object of the present invention is to provide an axle case structure that can improve the fatigue strength of the weld bead that joins the cover to the axle case body without increasing the weight, and can suppress the occurrence of cracks from the root of the weld bead around the entire circumference of the cover. [Means for solving the problem]

[0015] According to the axle case structure of the present invention devised to achieve the above-mentioned object, there is provided an axle case structure comprising: an opening formed in the center of the axle case body to accommodate a differential gear; a thin ring-shaped ring plate attached to the axle case body so as to encompass the opening; and a cover attached to the ring plate so as to cover the opening, wherein the cover has a bowl portion formed in a bowl shape and a flange portion formed on the outer periphery of the bowl portion, the flange portion is overlapped on the ring plate so that the outer periphery end face of the flange portion is aligned with the outer periphery end face of the ring plate, and the outer periphery end face of the flange portion and the outer periphery end face of the ring plate are joined integrally to the axle case body by a single weld bead around the entire circumference of the flange portion.

[0016] In the axle case structure according to the present invention, the inner peripheral end face of the ring plate may be joined to the inner face of the cover from the back side of the cover by another weld bead.

[0017] In the axle case structure of the present invention, the inner diameter of the opening in the axle case body may be smaller than the inner diameter of the inner end face of the ring plate, and a part of the opening may serve as a positioning portion against which the differential gear abuts to determine the position of the differential gear.

[0018] In the axle case structure of the present invention, the throat thickness of a single weld bead that welds the outer peripheral end face of the flange portion and the outer peripheral end face of the ring plate integrally to the axle case body may be greater than or equal to the plate thickness of the cover.

[0019] In the axle case structure according to the present invention, the plate thickness of the ring plate may be equal to the plate thickness of the flange portion of the cover. Effect of the Invention

[0020] The axle case structure according to the present invention can provide the following effects. (1) A ring plate is placed over the flange of the cover, and the outer peripheral end face of the flange and the outer peripheral end face of the ring plate are integrally joined to the axle case body with a single weld bead, so that the ring plate functions as a spacer to increase the leg length of the weld bead, and the throat thickness of the weld bead is increased compared to when there is no ring plate. As a result, the fatigue strength of the weld bead is improved, and cracks that start at the root of the weld bead due to deformation of the axle case body can be suppressed over the entire circumference of the cover. (2) The ring plate is used as a spacer to increase the leg length of the weld bead and to increase the throat thickness of the weld bead, so there is no need to increase the plate thickness of the cover to increase the leg length of the weld bead as in the conventional example shown in Fig. 3(a), and the weight of the cover does not increase. Also, because the ring plate is a spacer that increases the leg length of the weld bead in cooperation with the plate thickness of the flange part of the cover, even with a plate thickness thinner than that of the reinforcing ring of the conventional example shown in Fig. 3(b), the leg length and throat thickness of the weld bead can be secured at the same level, resulting in a lighter weight. (3) As described above, according to the present invention, the fatigue strength of the weld bead joining the cover to the axle case body can be improved around the entire circumference of the cover while minimizing the increase in weight, and the occurrence of cracks at the root of the weld bead due to deformation of the axle case body under various loads can be suppressed around the entire circumference of the cover. [Brief description of the drawings]

[0021] [Figure 1] 1A is an explanatory diagram of a conventional axle case structure, FIG. 1A is an overall perspective view of the axle case structure showing the position and direction of the load applied to the axle case body, FIG. 1B is a partial rear view of the central part of the axle case structure as seen from the rear, and FIG. 1C is a cross-sectional view taken along line cc in FIG. [Diagram 2](a) is a rear view showing how a conventional axle case structure is deformed by loads applied from above and below, and (b) is a cross-sectional view taken along line bb in (a), showing how the cover is deformed in a pulled manner as the axle case structure deforms. [Diagram 3] FIG. 1 shows a conventional example of preventing cracks in the weld bead that joins the cover and the axle case body, where (a) is a cross-sectional view of a conventional example in which the plate thickness of the cover is increased, (b) is a cross-sectional view of a conventional example in which a reinforcing ring is added, and (c) is a cross-sectional view of a conventional example in which back welding is added. [Figure 4] 4(a) is a cross-sectional view showing the cover and ring plate of the axle case structure according to one embodiment of the present invention in contact with the axle case body, and (b) is a partial cross-sectional view showing the axle case structure according to one embodiment of the present invention, taken along line IVb-IVb in FIG. 5. [Diagram 5] FIG. 2 is a partially cutaway rear view showing the center portion of the axle case structure according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] A preferred embodiment of the present invention will be described in detail below with reference to the accompanying drawings. The dimensions, materials, and other specific numerical values ​​shown in the embodiment are merely examples for facilitating understanding of the invention, and do not limit the present invention unless otherwise specified. In this specification and drawings, elements having substantially the same functions and configurations are given the same reference numerals to avoid repeated explanations, and elements not directly related to the present invention are not shown.

[0023] (Axle case structure 1) Fig. 5 is a partially cutaway view of an axle case structure 1 according to one embodiment of the present invention as seen from the rear, and Fig. 4(b) is a cross-sectional view taken along line IVb-IVb of Fig. 5, which shows a main part of the axle case structure 1. As shown in Fig. 5 and Fig. 4(b), the axle case structure 1 according to this embodiment includes an opening 3 formed in the center of an axle case main body 2 to accommodate a differential gear (differential mechanism), a thin ring-shaped ring plate 13 attached to the axle case main body 2 so as to enclose the opening 3, and a cover 4 attached to the ring plate 13 so as to cover the opening 3.

[0024] As shown in Figures 4(a) and 5, the cover 4 has a bowl portion 4a formed in a bowl shape and a flange portion 4b formed on the outer periphery of the bowl portion 4a. The outer diameter of the outer peripheral end face 4c of the flange portion 4b of the cover 4 is the same as the outer diameter of the outer peripheral end face 13c of the ring plate 13, and as shown in Figure 4(a), the flange portion 4b is overlapped on the ring plate 13 so that the outer peripheral end face 4c of the flange portion 4b is aligned with the outer peripheral end face 13c of the ring plate 13, and as shown in Figure 4(b), the outer peripheral end face 4c of the flange portion 4b and the outer peripheral end face 13c of the ring plate 13 are joined (welded) to the axle case main body 2 integrally with one weld bead 14 over the entire circumference of the flange portion 4b.

[0025] Other configurations of the axle case structure 1 of this embodiment (bracket for torque rod, spring seat, spindle, etc.) are the same as those of the conventional axle case structure 1J shown in Figure 1, and descriptions of parts to which load is applied, such as bracket 6 for torque rod, spring seat 7, spindle 8, etc., will be omitted.

[0026] (another weld bead 15) As shown in Figures 4(a) and 4(b), the inner peripheral end face 13d of the ring plate 13 is also joined (fillet welded) to the inner surface of the cover 4 from the back side of the cover 4 with another weld bead 15. As shown in Figure 4(a), the ring plate 13 and the cover 4 are overlapped so that the outer peripheral end face 13c of the ring plate 13 and the outer peripheral end face 4c of the flange portion 4b of the cover 4 are aligned, and while held by a jig (not shown), they are joined from the back side of the cover 4 with another weld bead 15 to form a sub-assembly before being welded to the axle case body 2. The welding range of the back weld where the separate weld bead 15 is applied is sufficiently within the area (areas K1, K2, K3, K4, K5, K6 in Figure 1(b)) where cracks are likely to occur in one weld 14 that welds the flange portion 4b and the ring plate 13 integrally to the axle case body 2 as the axle case body 2 deforms under various loads (arrows A, B, C, etc.), but it may also extend over the entire circumference of the inner end face of the ring plate 13.

[0027] As shown in Fig. 4(a), the radial width of the ring plate 13 is preferably such that, when the ring plate 13 is placed on the flange portion 4b of the cover 4 so that the outer peripheral end face 13c of the ring plate 13 is aligned with the outer peripheral end face 4c of the flange portion 4b of the cover 4, the upper end of the inner peripheral end face 13d of the ring plate 13 is located at the radius stop portion 4d of the flange portion 4b that smoothly connects the flange portion 4b of the cover 4 and the bowl portion 4a in a radius shape. This makes it easier to weld the inner peripheral end face 13d of the ring plate 13 to the inner surface of the cover 4 by fillet welding with another weld bead 15 from the back side of the cover 4, and an appropriate penetration state is obtained. In this embodiment, the radial width W of the ring plate 13 is set to 15 mm.

[0028] As shown in Figure 4(a), the subassembly of the ring plate 13 and cover 4, which are integrated with another weld bead 15, is placed at a predetermined position on the axle case body 2 so as to surround the opening 3. Then, as shown in Figure 4(b), the outer peripheral end face 4c of the flange portion 4b and the outer peripheral end face 13c of the ring plate 13 are integrally joined (welded) to the axle case body 2 with a single weld bead 14 around the entire circumference of the flange portion 4b.

[0029] As shown in Figure 4(b), by welding the back side of the cover 4 to the inner end surface 13d of the ring plate 13 with another weld bead 15, when the axle case main body 2 is deformed by load B, load C, etc. as shown in Figure 2(a) and a force is generated that pulls up the cover 4 as shown in Figure 2(b), the flange portion 4b of the cover 4 shown in Figure 4(b) is prevented from being lifted away from the ring plate 13, and the occurrence of cracks originating from the portion 14x between the ring plate 13 and the flange portion 4b of one weld bead 14 can be suppressed.

[0030] (Differential gear positioning part) 5 and 4(b), the inner diameter of the opening 3 of the axle case main body 2 is set smaller than the inner diameter of the inner peripheral end face 13d of the ring plate 13, and parts of the opening 3 (left and right upper parts D1, D2 and left and right lower parts D3, D4) serve as positioning parts with which the differential gear abuts to determine the position of the differential gear. That is, the left and right upper parts D1, D2 and left and right lower parts D3, D4 of the opening 3 of the axle case main body 2 shown in FIG. 5 abut against a part of the differential gear (bearing cap) (not shown), thereby determining the position of the differential gear.

[0031] (Through thickness of one weld bead 14) As shown in Fig. 4(b), the throat thickness 14b of one weld bead 14 that integrally welds the outer peripheral end surface 4c of the flange portion 4b of the cover 4 and the outer peripheral end surface 13c of the ring plate 13 to the axle case body 2 is equal to or greater than the plate thickness of the flange portion 4b of the cover 4. As a result, as shown in Fig. 2(a), when the axle case body 2 is deformed by loads B, C, etc. and a force F is generated that pulls the cover 4 up as shown in Fig. 2(b), the portion where the stress of the force transmitted from the cover 4 to the axle case body 2 is maximum is no longer the throat thickness 14b of one weld bead 14. This makes it possible to avoid cracks in the weld bead 14 caused by deformation of the axle case body 2.

[0032] (The plate thickness t1 of the ring plate 13 and the plate thickness t2 of the flange portion 4b of the cover 13) As shown in FIG. 4(b), consider the case where the plate thickness t1 of the ring plate 13 is equal to the plate thickness t2 of the flange portion 4b of the cover 4, and one weld bead 14 is formed so as to cover the entire outer peripheral end surface 13c of the ring plate 13 and the outer peripheral end surface 4c of the flange portion 4b. If the plate thickness t1 of the cover is a and the plate thickness t2 of the flange portion 4b is a, the leg length L of one weld bead 14 is 2a, and the throat thickness 14b (theoretical throat thickness) is 70% of the leg length L, so that it is 2a×0.7=1.4a. Therefore, the throat thickness 14b (=1.4a) is sufficiently (1.4 times) thicker than the plate thickness t2 (=a) of the flange portion 4b. Even if one weld bead 14 does not completely cover the upper end of the outer peripheral surface 4c of the flange portion 4b, it is highly likely that the throat thickness 14b can be secured to be thicker than the plate thickness t2 of the flange portion 4b.

[0033] Specifically, if the plate thickness t2 of the flange portion 4b and bowl portion 4a of the cover 4 is 4.5 mm and the throat thickness 14b of one weld bead 14 is 6 mm, then the throat thickness 14b is 70% of the leg length L, so the leg length L is about 8.5 mm, and the plate thickness t1 of the ring plate 13 is sufficient at about 4.5 mm, which is equivalent to the plate thickness t2 of the flange portion 4b of the cover 4. If the plate thickness t1 of the ring plate 13 is 4.5 mm, when the ring plate 13 and the flange portion 4b are overlapped and welded up to the upper edge of the outer circumferential end face 4c of the flange portion 4b, the leg length is 9 mm and the throat thickness 14b is 9 x 0.7 = 6.3 mm, so the required throat thickness 14b can be ensured. Even if one weld bead 14 does not completely cover the upper end of the outer peripheral end face 4c of the flange portion 4b, it is believed that the throat thickness 14b will be smaller than 6.3 mm, but will be thicker than the plate thickness t2 of the flange portion 4b (=4.5 mm).

[0034] (Action and effect) The axle case structure 1 according to this embodiment can provide the following functions and effects.

[0035] 4(b), ring plate 13 is placed on flange portion 4b of cover 4, and outer peripheral end face 4c of flange portion 4b and outer peripheral end face 13c of ring plate 13 are integrally joined to axle case main body 2 over the entire circumference of flange portion 4b by one weld bead 14, so that ring plate 13 functions as a spacer for increasing leg length L of weld bead 14, and throat thickness 14b of weld bead 14 is increased compared to when there is no ring plate 13. As a result, fatigue strength of weld bead 14 is improved, and cracks that would occur from root portion 14a in weld bead 14 as a result of deformation of axle case main body 2 can be suppressed over the entire circumference of cover 4.

[0036] As shown in Fig. 4(b), ring plate 13 is used as a spacer for increasing leg length L of weld bead 14 to increase throat thickness 14b of weld bead 14, so there is no need to increase leg length L of weld bead 5 by increasing plate thickness t of cover 4 as in the conventional example shown in Fig. 3(a), and there is no increase in the weight of cover 4. Also, because ring plate 13 is a spacer that increases leg length L of weld bead 14 in cooperation with plate thickness t2 of flange portion 4b of cover 4, even with a plate thickness thinner than reinforcing ring 9 of the conventional example shown in Fig. 3(b), the same or greater leg length L and throat thickness 14b of weld bead 14 can be ensured, and both weight reduction and improved fatigue strength of the welded portion (weld bead 14) can be achieved. Specifically, the axle case structure 1 of this embodiment shown in Figure 4(b) is only about 0.8 kg heavier than the conventional type with no crack prevention measures shown in Figures 1(c) and 2(b), and is significantly lighter than the weight increase (+8kg to +11kg) of the conventional type with crack prevention measures shown in Figures 3(a), 3(b), and 3(c).

[0037] 4(b) and 5, with the axle case structure 1 according to this embodiment, the fatigue strength of the weld beads 14 joining the cover 4 to the axle case body 2 can be improved over the entire circumference of the cover 4 while minimizing an increase in weight, and the occurrence of cracks from the root portions 14a of the weld beads 14 caused by deformation of the axle case body 2 due to various loads can be suppressed over the entire circumference of the cover 4. Also, since there is no need to change the shape of the cover 4, which is a large press-molded product, there is no need to change the press die for the cover 4, and costs can be suppressed.

[0038] (About another weld bead 15) As shown in Fig. 4(b), the inner peripheral end surface 13d of the ring plate 13 is also joined to the inner surface of the cover 4 from the back side of the cover 4 by another weld bead 15. This prevents the flange portion 4b of the cover 4 shown in Fig. 4(b) from being pulled up and separated from the ring plate 13 when the axle case body 2 is deformed by loads B, C, etc. as shown in Fig. 2(a) and a force F is generated that pulls the cover 4 up as shown in Fig. 2(b). As a result, it is possible to prevent the occurrence of cracks originating from a portion 14x between the ring plate 13 and the flange portion 4b of one weld bead 14.

[0039] The welding range of the back weld where the separate weld bead 15 shown in Fig. 4(b) is applied does not necessarily have to cover the entire circumference of the inner peripheral end surface 13d of the ring plate 13, but may include at least the portion (portions K1, K2, K3, K4, K5, and K6 shown in Fig. 1(b)) where a crack occurs in one weld bead 14 that integrally welds the flange portion 4b and the ring plate 13 to the axle case main body 2. In these portions K1, K2, K3, K4, K5, and K6, a force F that pulls up the cover 4 as shown in Fig. 2(b) occurs significantly as the axle case main body 2 deforms, so that the separate weld bead 15 shown in Fig. 4(b) should be used to prevent the flange portion 4b of the cover 4 from being lifted away from the ring plate 13.

[0040] (Regarding the positioning part of the differential) As shown in Fig. 5, the inner diameter of the opening 3 of the axle case main body 2 is set smaller than the inner diameter of the inner peripheral end surface 13d of the ring plate 13, and parts of the opening 3 (left and right upper diagonal parts D1, D2, left and right lower diagonal parts D3, D4) serve as positioning parts with which the differential gear abuts to determine the position of the differential gear. That is, the left and right upper diagonal parts D1, D2, left and right lower diagonal parts D3, D4 of the opening 3 of the axle case main body 2 abut a part of the differential gear (bearing cap) (not shown) to determine the position of the differential gear. Therefore, it is possible to easily achieve both the positioning of the differential gear and the prevention of cracks in the weld bead 14, and there is no problem that the positioning of the differential gear cannot be determined due to the notch part 12 of the opening 3 as in the conventional example shown in Fig. 3(c).

[0041] (Regarding the throat thickness 14b of one weld bead 14) As shown in Figure 4(b), the throat thickness 14b of one weld bead 14 that integrally welds the outer peripheral end face 4c of the flange portion 4b and the outer peripheral end face 13c of the ring plate 13 to the axle case body 2 is equal to or greater than the plate thickness t2 of the bowl portion 4a and the flange portion 4b of the cover 4. As a result, when the axle case body 2 is deformed by loads B, C, etc. as shown in Figure 2(a) and a force F is generated that pulls it up to the cover 4 as shown in Figure 2(b), the part where the stress of the force transmitted from the cover 4 to the axle case body 2 is maximum is no longer the throat thickness 14b of one weld bead 14, and the occurrence of cracks in one weld bead 14 can be avoided. In other words, since the throat thickness 14b of one weld bead 14 is greater than the plate thickness t2 of the cover 4, the strength of the throat thickness 14b is greater than the strength of the cover 4, and when the axle case body 2 is deformed by load B, load C, etc., cracks from the root portion 14a of one weld bead 14 can be suppressed.

[0042] (Regarding the plate thickness t1 of the ring plate 13 and the plate thickness t2 of the flange portion 4b of the cover 4) 4(b), the plate thickness t1 of the ring plate 13 is equal to the plate thickness t2 of the flange portion 4b of the cover 4. This ensures that the throat thickness 14b of one weld bead 14 that integrally welds the outer peripheral end surface 4c of the flange portion 4b and the outer peripheral end surface 13c of the ring plate 13 to the axle case body 2 is equal to or greater than the plate thickness t2 of the flange portion 4b of the cover 4, thereby reliably preventing the occurrence of cracks in one weld bead 14. In this embodiment, the bowl portion 4a and the flange portion 4b of the cover 4 have the same plate thickness.

[0043] The above describes a preferred embodiment of the present invention with reference to the attached drawings. However, it goes without saying that the present invention is not limited to the above-described embodiment, and various modified or altered examples within the scope of the claims also fall within the technical scope of the present invention. [Industrial Applicability]

[0044] The present invention can be used in an axle case structure 1 used for trucks, buses, etc., which has improved fatigue strength of a weld bead 14 that joins the axle case body 2 to a cover 4 that covers an opening 3 formed in the center of the axle case body 2 to accommodate a differential gear. [Explanation of symbols]

[0045] 1 Axle case structure 2 Axle case body 3 aperture 4 Cover 4a Bowl section 4b Flange part 4c Outer edge 13 Ring plate 13c Outer edge 13d Inner circumference end face 14 One weld bead 14a Root 14b throat thickness 15 Another weld bead D1, D2, D3, D4 Positioning parts t1 Ring plate thickness t2 Plate thickness of the cover flange

Claims

1. an opening formed in a center of an axle case body to accommodate a differential gear; a thin ring plate attached to the axle case body so as to include the opening; and a cover attached to the ring plate so as to cover the opening, The cover has a bowl portion formed in a bowl shape and a flange portion formed on an outer periphery of the bowl portion, The flange portion is overlapped with the ring plate such that an outer peripheral end surface of the flange portion is aligned with an outer peripheral end surface of the ring plate, An outer peripheral end surface of the flange portion and an outer peripheral end surface of the ring plate are integrally joined to the axle case body by a single weld bead over the entire circumference of the flange portion. The axle case structure is characterized by:

2. 2. The axle case structure according to claim 1, wherein the inner peripheral end face of the ring plate is joined to the inner surface of the cover from the back side of the cover by another weld bead.

3. 3. The axle case structure according to claim 2, characterized in that an inner diameter of the opening of the axle case body is smaller than an inner diameter of the inner end face of the ring plate, and a part of the opening serves as a positioning portion against which the differential gear abuts to determine the position of the differential gear.

4. 4. An axle case structure as described in any one of claims 1 to 3, characterized in that the throat thickness of the weld bead that integrally welds the outer peripheral end surface of the flange portion and the outer peripheral end surface of the ring plate to the axle case body is greater than or equal to the plate thickness of the cover.

5. 5. The axle case structure according to claim 4, wherein the ring plate has a thickness equal to a thickness of the flange portion of the cover.

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