Torsion beam of coupled torsion beam axle

Abstract

The present invention relates to a torsion beam of a coupled torsion beam axle, which may include: an outer beam having a first cross-section in which an outer protrusion at a middle portion of an end portion of the outer beam is formed to protrude upward in the longitudinal direction of the outer beam, and an outer skirt extending vertically in the longitudinal direction at both end portions of the outer protrusion; and an inner beam having a second cross-section in which the inner protrusion at the middle portion of the end portion of the inner beam is inserted into the outer beam to face the outer protrusion of the outer beam, and an inner skirt extending vertically at both end portions of the inner protrusion, and each outer surface of the inner skirt and each inner surface of the outer skirt being surface-bonded in a mutually mating state. A gap is formed between the outer surface of the inner protrusion and the inner surface of the outer protrusion to form a closed cross-section.

Description

Technical Field

[0001] The present invention relates to a torsion beam of a coupled torsion beam axle, which satisfies durability performance without increasing the weight of components by improving the structure of the torsion beam. Background Art

[0002] The suspension of a vehicle is a device that prevents vibration or impact received from the road surface from being directly transmitted to the vehicle body during driving by connecting the axle and the vehicle body, thereby preventing damage to the vehicle body or cargo and improving ride comfort. The suspension is divided into front suspension and rear suspension.

[0003] As rear suspension for light and medium-sized passenger cars, coupled torsion beam axles (CTBA) are used, which exhibit high driving stability in comparison to low unit costs and weight.

[0004] When the vehicle body pitches during driving, the coupled torsion beam axle absorbs the pitch through the torsional elastic force of the components.

[0005] Furthermore, when a vehicle turns, a displacement difference occurs between the left and right wheels due to the vehicle's rolling characteristics, causing the torsion bar, the intermediate member of the axle, to twist, thereby increasing roll stiffness. Therefore, coupled torsion beam axles are designed to ensure rolling stability.

[0006] High-performance vehicles have high roll stiffness characteristics, which ensures quick response in handling. In small and medium-sized high-performance vehicles, a high-rigidity coupled torsion beam axle is indispensable because the coupled torsion beam axle is used for the rear wheels.

[0007] However, high-rigidity coupled torsion beam axles have the following issues: As input load increases, durability and strength performance decrease, and when components are added to increase rigidity, there is a side effect of increased component weight. Therefore, there is a need for a lightweight, high-rigidity coupled torsion beam axle structure that can meet durability requirements without significantly increasing component weight.

[0008] The information disclosed in the background section of the present invention is only intended to deepen the understanding of the general background technology of the present invention and should not be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to those skilled in the art. Summary of the Invention

[0009] Various aspects of the present invention are directed to providing a torsion beam of a coupled torsion beam axle that satisfies durability performance without increasing the weight of components through structural improvement of the torsion beam.

[0010] According to various exemplary embodiments of the present invention, a torsion beam of a coupled torsion beam axle includes: an outer beam having a first cross-section, in which an outer protrusion at a middle portion of an end portion of the outer beam is formed to protrude upward in a longitudinal direction of the outer beam, and an outer skirt extends vertically in the longitudinal direction at both end portions of the outer protrusion; and an inner beam having a second cross-section, in which an inner protrusion at a middle portion of an end portion in the inner beam is inserted into the outer beam to face the outer protrusion of the outer beam, and an inner skirt extends in the vertical direction at both end portions of the inner protrusion, and each outer surface of the inner skirt and each inner surface of the outer skirt are surface-bonded in a mutually matching state, wherein a gap is formed between the outer surface of the inner protrusion and the inner surface of the outer protrusion to form a closed cross-section.

[0011] The end portion of the inner skirt and the inner surface of the outer skirt in contact with the end portion of the inner skirt may be welded linearly, and the outer surface of the inner skirt and the inner surface of the outer skirt may be partially welded in a longitudinal direction thereof.

[0012] The outer surface of the inner skirt and the inner surface of the outer skirt may be welded and bonded at regular intervals in a longitudinal direction of the inner skirt and the outer skirt.

[0013] The outer surface of the inner skirt and the inner surface of the outer skirt may be welded and bonded at upper and lower portions at regular intervals in the longitudinal direction of the inner and outer skirts, and bonded such that welding positions of the upper and lower portions are staggered from each other.

[0014] The outer surface of the inner skirt and the inner surface of the outer skirt may be welded together in a longitudinal direction of the inner skirt and the outer skirt, and may be welded together so that a welding range of a central portion of the torsion beam is longer than other welding ranges on both sides of the central portion of the torsion beam.

[0015] The outer surface of the inner skirt and the inner surface of the outer skirt may be welded together in the longitudinal direction of the inner skirt and the outer skirt, and may be welded only at both sides of the central portion of the torsion beam instead of at the central portion of the torsion beam.

[0016] The gap may be formed by offsetting a cross section of the inner protrusion into a shape corresponding to a cross section of the outer protrusion.

[0017] The gap may be formed between an entire outer surface portion of the inner protrusion and an entire inner surface portion of the outer protrusion.

[0018] A front surface of the outer surface of the inner skirt may be in surface contact with the inner surface of the outer skirt.

[0019] A concave curved portion, the curved direction of which changes toward the inner side of the inner beam, may be formed at a position where an outer surface of the inner protrusion and an outer surface of the inner skirt are connected.

[0020] A same cross-section portion formed by maintaining a closed cross-section of the same shape may be formed at a central portion of the outer beam and the inner beam, and a variable cross-section portion formed by changing a shape of the closed cross-section may be formed at both sides of the same cross-section portion.

[0021] The variable cross-section portion may include: a first variable portion configured such that one side of the first variable portion is connected to a side portion of the same cross-section portion, and an upper end portion of the inner beam is gradually inclined downward to gradually increase a vertical height of a closed cross-section; and a second variable portion configured such that one side of the second variable portion is connected to the other side of the first variable portion, upper end portions of the outer beam and the inner beam are formed in a horizontal cross-sectional shape having a width of a predetermined length or longer, and the other side of the second variable portion is connected to a side surface of a trailing arm.

[0022] The end portions of the outer beam and the inner beam may be formed in a shape surrounding a portion of a side surface of the trailing arm, an upper end portion of the end portion of the outer beam may be combined with an upper surface of the trailing arm, and an upper end portion of the end portion of the inner beam is combined with a bottom surface of the trailing arm.

[0023] A clamp hole may be formed in a variable cross-section portion adjacent to the uniform cross-section portion of the upper end portion of the outer beam.

[0024] A receiving surface may be formed on an upper end portion of the inner beam facing the clamp hole in a direction perpendicular to an axial direction of the clamp hole.

[0025] By incorporation into this document of the accompanying drawings and Figure 1 The detailed description is intended to illustrate certain principles of the present invention, and other features and advantages of the method and apparatus of the present invention will become more clearly apparent or elucidated. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 2 is a view exemplarily illustrating the shape of a coupled torsion beam axle according to various exemplary embodiments of the present invention.

[0027] Figure 2 are views separately illustrating shapes of outer beams and inner beams according to various exemplary embodiments of the present invention.

[0028] Figure 31 and 2 are views exemplarily illustrating cross-sectional shapes in which outer beams and inner beams are coupled to each other according to various exemplary embodiments of the present invention.

[0029] Figure 4 、 Figure 5 、 Figure 6 as well as Figure 7 are views illustrating examples of different patterns of a portion where an outer beam and an inner beam are coupled according to various exemplary embodiments of the present invention.

[0030] Figure 8 are views for describing a state in which a shape of a closed cross-section formed between an outer beam and an inner beam is changed according to various exemplary embodiments of the present invention.

[0031] It should be understood that the accompanying drawings are not drawn to scale and show a somewhat simplified representation of various features illustrating the basic principles of the invention. The specific design features of the present invention disclosed herein (including, for example, specific dimensions, orientations, locations, and shapes) will be determined in part by the specific intended application and use environment.

[0032] In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. DETAILED DESCRIPTION

[0033] Reference will now be made in detail to various embodiments of the present invention, examples of which are shown in the accompanying drawings and described below. Although the present invention will be described in conjunction with the exemplary embodiments, it should be appreciated that this description is not intended to limit the present invention to those exemplary embodiments. On the other hand, the present invention is intended to cover not only these exemplary embodiments, but also various alternative forms, modifications, equivalent forms, and other embodiments that may be included within the spirit and scope of the present invention as defined by the appended claims.

[0034] Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0035] Figure 1 2 is a view schematically illustrating a coupled torsion beam axle according to various exemplary embodiments of the present invention.

[0036] Reference Figure 1 The torsion beam 1 is formed into a "U"-shaped cross-section that opens downward, so that both end portions of the torsion beam 1 are configured to be arranged in the left-right direction of the vehicle body, a pair of trailing arms 2 are respectively connected to the both end portions of the torsion beam 1, and both end portions of the trailing arms 2 are configured to be arranged in the front-rear direction of the vehicle body.

[0037] In addition, although not Figure 1, the rear wheel is coupled to the outer portion of the rear end portion of the trailing arm 2 , and the lower end portion of the shock absorber is coupled to the inner portion of the rear end portion of the trailing arm 2 .

[0038] Meanwhile, the torsion beam 1 of the coupled torsion beam axle of the present invention is constructed to include an outer beam 10 and an inner beam 20 .

[0039] Figure 2 are views respectively showing shapes of an outer beam 10 and an inner beam 20 according to various exemplary embodiments of the present invention, and Figure 3 1 and 2 are views exemplarily illustrating a cross-sectional shape in which an outer beam 10 and an inner beam 20 are coupled to each other according to various exemplary embodiments of the present invention.

[0040] Reference Figure 2 and Figure 3 First, the outer beam 10 is formed such that the outer protrusion 12 at the middle portion of its end portion is formed into a cross-sectional shape protruding upward in the longitudinal direction of the outer beam, and the outer skirt 14 extends longitudinally in the vertical direction at both end portions of the outer protrusion 12.

[0041] For example, the outer protrusion 12 is formed into a cup shape whose upper end portion is horizontal and both horizontal end portions are inclined downward in their outward direction, and is formed into a cross-sectional structure in which the outer skirt 14 is formed in a straight line in the downward direction at the front end portion and the rear end portion of the outer protrusion 12.

[0042] In addition, the inner beam 20 is formed such that the inner protrusion 22 of the middle portion of the end portion thereof is inserted into the outer beam 10 to face the cross-sectional shape of the outer protrusion 12, and the inner skirt 24 extends in the vertical direction at both end portions of the inner protrusion 22. Therefore, the outer surface of the inner skirt 24 and the inner surface of the outer skirt 14 are surface-bonded in a mutually matched state.

[0043] For example, the inner protrusion 22 is also formed in a cup shape protruding upward in cross section, and is formed in a structure in which the inner skirt 24 is formed straightly in the lower direction at the front and rear end portions of the inner protrusion 22 .

[0044] A gap g is formed between the outer surface of the inner protrusion 22 and the inner surface of the outer protrusion 12 to form a space of a closed cross-section between the outer beam 10 and the inner beam 20 .

[0045] That is, the outer beam 10 and the inner beam 20 are respectively provided by press-forming a plate material, and the inner protrusion 22 of the inner beam 20 is coupled in such a manner as to be fitted in the outer protrusion 12 of the outer beam 10 .

[0046] However, a gap g is formed along the outer surface of the inner protrusion 22 and the inner surface of the outer protrusion 12 to form a space of closed cross-section between the outer beam 10 and the inner beam 20. In addition, the outer surface of the inner skirt 24 is bonded to the inner surface of the outer skirt in a surface contact state, thereby increasing the bonding area between the outer beam 10 and the inner beam 20.

[0047] Therefore, by implementing a closed cross-section structure on the section of the beam where torsion occurs, the rigidity of the torsion beam 1 can be increased to ensure high roll rigidity equivalent to that of a multi-link mechanism. In addition, by increasing the joint area between the outer beam 10 and the inner beam 20, the durability of the torsion beam 1 can be ensured.

[0048] Furthermore, since the structure of increasing durability and rigidity is not due to a structure of increasing weight such as increasing the torsion beam 1 or increasing the thickness of the torsion beam 1 , a lightweight design of the CTBA is also possible.

[0049] In addition, the inner beam 20 and the outer beam 10 may be joined by the linear welding W1 or by using the plug welding W2 method together with the linear welding W1.

[0050] Reference Figure 3 , the weld bond W1 is linearly performed along the end portion of the inner skirt 24 and the inner surface of the outer skirt 14 in contact with the end portion of the inner skirt 24 .

[0051] Furthermore, the outer surface of the inner skirt 24 and the inner surface of the outer skirt 14 may be applied with a pattern in the longitudinal direction, and may be additionally locally welded and bonded W2.

[0052] Since the outer beam 10 and the inner beam 20 are press-formed, welding may be performed in a state where surface matching between the outer skirt 14 and the inner skirt 24 cannot be properly performed due to press-forming tolerances depending on manufacturing conditions.

[0053] In this case, in addition to the linear welding, plug welding that is locally combined is added to ensure surface matching between the outer skirt 14 and the inner skirt 24. Therefore, the quality of the welded portion can be ensured, and since the vertical lengths of the outer skirt 14 and the inner skirt 24 are increased to form the welding surface, additional rigidity can be ensured.

[0054] Figure 4 、 Figure 5 、 Figure 6 as well as Figure 7 1 and 2. The diagrams illustrate examples of different styles of plug welding at a portion where an outer beam 10 and an inner beam 20 are joined according to various exemplary embodiments of the present invention, and the presence, position, and length of the plug welding may be variously set according to target rigidity and durability.

[0055] Figure 4 1 is a diagram for explaining a first example of plug welding, in which the inner skirt 24 and the outer skirt 14 are welded together W2 at equal intervals in the longitudinal direction of the inner skirt 24 and the outer skirt 14 .

[0056] This may be a basic plug welding method of the outer beam 10 and the inner beam 20 .

[0057] Figure 5 This is a diagram for explaining a second example of plug welding, in which the inner skirt 24 and the outer skirt 14 are welded together W2 at equal intervals at the upper and lower parts in the longitudinal direction of the inner skirt 24 and the outer skirt 14, and can be joined in a form in which the welding position of the upper part and the welding position of the lower part are staggered from each other.

[0058] This is a type for increasing the rigidity of the torsion beam 1. When the lengths of the outer skirt and the inner skirt 24 in the vertical direction increase and the contact area thereof increases, by forming a Z-shaped welding form at the upper and lower positions of the outer skirt and the inner skirt 24, the surface contact between the outer skirt and the inner skirt 24 can be more reliably formed, and the rigidity of the torsion beam 1 can be further improved by increasing the amount of material to ensure the contact area.

[0059] Figure 6 This is a diagram for explaining a third example of plug welding, in which the inner skirt 24 and the outer skirt 14 are welded together W2 in the longitudinal direction of the inner skirt 24 and the outer skirt 14, and can be combined so that the welding range of the center portion is longer than the other welding ranges on both sides of the center portion of the torsion beam.

[0060] Since the torsion of the torsion beam 1 becomes excessively large due to twisting when the wheel stroke is large, the present invention reinforces the portion where durability may be weakened by configuring the central portion of the torsion beam 1 to have greater torsion resistance.

[0061] Figure 7 This is a diagram for explaining a fourth example of plug welding, in which the inner skirt 24 and the outer skirt 14 are welded together W2 in the longitudinal direction of the inner skirt 24 and the outer skirt 14, and welding is not performed at the center portion of the torsion beam, but welding is performed only on both sides of the center portion of the torsion beam.

[0062] That is, the degree of deformation caused by plug welding heat varies depending on the thickness of the torsion beam 1. When the weight of the torsion beam 1 is reduced by forming the inner beam 20 and the outer beam 10 using thin plate materials, thermal deformation can be minimized because welding is not performed at the center portion of the torsion beam 1.

[0063] At the same time, if Figure 3As shown, according to various exemplary embodiments of the present invention, the gap g may be formed by offsetting the cross-section of the inner protrusion 22 into a shape corresponding to the cross-section of the outer protrusion 12 .

[0064] Here, a gap g is formed between the entire outer surface portion of the inner protrusion 22 and the entire inner surface portion of the outer protrusion 12, and this can be applied to the middle portion of the outer beam 10 and the inner beam 20 based on the left-right direction of the vehicle body (the same cross section will be described later).

[0065] That is, when the torsion beam 1 is twisted, since the deformation of the center portion is the largest, the roll rigidity can be improved by adjusting the offset of the gap g formed in the center portion of the torsion beam.

[0066] Furthermore, a structure is formed in which the front surface of the outer surface of the inner skirt portion 24 is in surface contact with the inner surface of the outer skirt portion.

[0067] That is, by forming the vertical length of the outer skirt 14 to cover the length of the inner skirt 24 to increase the bonding area while ensuring that the vertical length of the inner skirt 24 is as long as possible, when the torsion beam 1 is torsioned, the torsional stress concentration phenomenon is prevented, thereby improving the durability of the torsion beam 1.

[0068] Meanwhile, according to various exemplary embodiments of the present invention, a concave curved portion R whose curved direction changes toward the inside of the inner beam 20 may be formed at a location where the outer surface of the inner protrusion 22 is connected to the outer surface of the inner skirt 24 .

[0069] For example, the upper end portion of the curved portion R is connected to the lower end portion of the inner protrusion 22 , the lower end portion of the curved portion R is connected to the upper end portion of the inner skirt 24 , and the middle portion of the curved portion R is formed in a shape turned inward toward the inside of the inner beam 20 .

[0070] That is, in a case where the bent portion R is not formed at the portion where the inner protrusion 22 and the inner skirt 24 are connected, the gap g formed between the portion where the inner protrusion 22 and the inner skirt 24 are connected and the inner surface of the outer beam 10 becomes narrower, and therefore, not only is it limited in ensuring the rigidity of the corresponding portion, but also the coating performance is deteriorated.

[0071] Therefore, according to various exemplary embodiments of the present invention, since the bent portion R is formed at the position where the inner protrusion 22 is connected to the inner skirt 24, the gap g between the bent portion R and the inner surface of the outer beam 10 is increased to additionally ensure the rigidity of the torsion beam 1 and ensure the fluidity of the paint, thereby improving the coating performance.

[0072] In an exemplary embodiment of the present invention, the curved portion R may be formed to have a predetermined curvature.

[0073] at the same time, Figure 8 1 and 2 are views for describing a state in which a shape of a closed cross-section formed between an outer beam 10 and an inner beam 20 is changed according to various exemplary embodiments of the present invention.

[0074] Reference Figure 8 A uniform cross-section portion L1 formed by maintaining a closed cross-section of the same shape is formed at the center portion of the outer beam 10 and the inner beam 20 formed along the left-right direction of the vehicle body, and a variable cross-section portion formed by changing the shape of the closed cross-section is formed at both sides of the uniform cross-section portion L1.

[0075] That is, when torsion occurs in the torsion beam 1, the central portion where deformation is most likely to occur forms and maintains the same cross-sectional shape as the closed cross-sectional structure, and forms a variable cross-sectional shape of the closed cross-sectional structure to improve the coupling with the trailing arm 2 on both sides of the central portion of the torsion beam.

[0076] The variable cross-section portion is a first variable portion L2 and a second variable portion L3. The first variable portion L2 is configured so that one side thereof is connected to the side of the same cross-section portion L1, and the upper end portion of the inner beam 20 is gradually inclined downward to gradually increase the vertical height of the closed cross-section.

[0077] The second variable portion L3 is configured such that one side thereof is connected to the other side of the first variable portion L2, the upper end portions of the outer beam 10 and the inner beam 20 are formed in a horizontal cross-sectional shape having a width of a predetermined length or longer, and the other side is connected to the side surface of the trailing arm 2.

[0078] Furthermore, in the case of the second variable portion L3 , the end portions of the outer beam 10 and the inner beam 20 are formed in a shape surrounding a portion of the side surface of the trailing arm 2 , and the upper end portion of the end portion of the outer beam 10 is joined to the upper surface of the trailing arm 2 .

[0079] Furthermore, an upper end portion of the end portion of the inner beam 20 is joined to the bottom surface of the trailing arm 2 .

[0080] That is, the uniform cross-section portion L1 is the maximum torsion point of the torsion beam 1 , and the torsional rigidity of the torsion beam 1 is ensured by keeping the shape and area of ​​the closed cross section between the inner beam 20 and the outer beam 10 uniform.

[0081] However, in the first variable portion L2, the lengths of the upper and lower portions of the closed cross-sectional shape can be increased, so that the end portions of the inner beam 20 and outer beam 10 surround the side periphery of the trailing arm 2 and also increase its width in the front-to-rear direction. To this end, in the first variable portion L2, the length of the lower end portion of the outer skirt 14 can be formed to extend downward while the upper end portion of the outer protrusion 12 is formed upward, and the length of the lower end portion of the inner skirt 24 can be formed to extend downward while the upper end portion of the inner protrusion 22 is formed downward.

[0082] Furthermore, since the inner beam 20 and the outer beam 10 in the second variable portion L3 have a horizontal cross-sectional shape of the upper end portion that is wider than that of the inner beam 20 and the outer beam 10 in the first variable portion L2, the areas bonded to the upper and lower surfaces of the trailing arm 2 are widened, thereby improving the welding strength to the trailing arm 2.

[0083] Furthermore, according to various exemplary embodiments of the present invention, the clamp hole 16 may be formed in the variable cross-section portion adjacent to the same cross-section portion L1 of the upper end portion of the outer beam 10 , and the clamp hole 16 may be formed at a position where the first variable portion L2 starts in the same cross-section portion L1 .

[0084] Furthermore, a receiving surface 26 may be formed on an upper end portion of the inner beam 20 facing the clamp hole 16 in a direction perpendicular to the axial direction of the clamp hole 16 .

[0085] Therefore, through the jig hole 16 , not only fixing during welding but also measurement of the gap between the outer beam 10 and the inner beam 20 is possible.

[0086] However, when torsion beam 1 is twisted, deformation in uniform cross-sectional portion L1 is greatest. Therefore, when jig hole 16 is located in uniform cross-sectional portion L1, durability cracks may occur in jig hole 16. Therefore, by forming jig hole 16 while avoiding uniform cross-sectional portion L1, the gap between outer beam 10 and inner beam 20 can be measured while preventing cracking of torsion beam 1.

[0087] Therefore, according to various exemplary embodiments of the present invention, by forming a wide closed cross-section structure in the cross-section of the torsion beam 1 where torsion occurs, the rigidity of the CTBA can be improved to ensure high roll rigidity equivalent to that of a multi-link mechanism, and further, by increasing the bonding area between the outer beam 10 and the inner beam 20, the manufacturability and durability performance of the torsion beam 1 can be ensured.

[0088] Furthermore, since a structure with increased durability and rigidity is not achieved due to a structure with increased weight such as adding the torsion beam 1 or increasing the thickness of the torsion beam 1 , a lightweight design of the CTBA is also possible.

[0089] For ease of explanation and precise definition in the appended claims, the terms "above," "below," "inside," "outside," "up," "down," "upward," "downward," "front," "back," "inside," "outside," "inwardly," "outwardly," "interior," "exterior," "inner," "outwardly," "forward," and "rearward" are used to describe features of the exemplary embodiments with reference to the positions of such features as shown in the accompanying drawings. It will be further understood that the term "connect" or its derivatives refers to both direct and indirect connections.

[0090] The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. The foregoing descriptions are not intended to be exhaustive, nor are they intended to limit the invention to the precise forms disclosed, and it is apparent that many variations and modifications are possible in light of the foregoing teachings. The exemplary embodiments have been selected and described in order to explain the specific principles of the invention and its practical application, thereby enabling others skilled in the art to realize and utilize the various exemplary embodiments of the invention and their various alternative forms and modifications. The scope of the invention is intended to be defined by the appended claims and their equivalents.

Claims

1. A torsion beam of a coupled torsion beam axle, the torsion beam comprising: an outer beam having a first cross-section in which an outer protrusion at a middle portion of an end portion of the outer beam is formed into a first cross-section protruding upward in a longitudinal direction of the outer beam, and an outer skirt extends vertically in the longitudinal direction at first and second end portions of the outer protrusion; as well as an inner beam having a second cross-section in which an inner protrusion at a middle portion of an end portion in the inner beam is inserted into the outer beam to face the outer protrusion of the outer beam, and an inner skirt vertically extends between a first end portion and a second end portion of the inner protrusion, each outer surface of the inner skirt and each inner surface of the outer skirt being surface-bonded in a mutually mating state, wherein a gap is formed between the outer surface of the inner protrusion and the inner surface of the outer protrusion to form a closed cross-section between the outer surface of the inner protrusion and the inner surface of the outer protrusion; wherein a concave curved portion whose curvature direction changes toward the inner side of the inner beam is formed at a position where the outer surface of the inner protrusion is connected to each outer surface of the inner skirt; The upper end portion of the concave curved portion is connected to the lower end portion of the inner protrusion, the lower end portion of the concave curved portion is connected to the upper end portion of the inner skirt, and the middle portion of the concave curved portion is formed in an inwardly turned shape toward the inner side of the inner beam.

2. The torsion beam of the coupled torsion beam axle according to claim 1, wherein: Welding is performed linearly along each end portion of the inner skirt and each inner surface of the outer skirt that contacts each end portion of the inner skirt.

3. The torsion beam of the coupled torsion beam axle according to claim 1, wherein: Each outer surface of the inner skirt and each inner surface of the outer skirt are partially welded in the longitudinal direction.

4. The torsion beam of the coupled torsion beam axle according to claim 3, wherein: Each outer surface of the inner skirt and each inner surface of the outer skirt are welded at predetermined intervals in a longitudinal direction of the inner skirt and the outer skirt.

5. The torsion beam of the coupled torsion beam axle according to claim 3, wherein: Each outer surface of the inner skirt and each inner surface of the outer skirt are welded at upper and lower portions of the outer skirt and the inner skirt at predetermined intervals in the longitudinal direction of the inner skirt and the longitudinal direction of the outer skirt, and are combined so that welding positions of the upper and lower portions are staggered from each other.

6. The torsion beam of the coupled torsion beam axle according to claim 5, wherein: The upper and lower portions of the outer skirt and the inner skirt are sequentially combined in a zigzag shape in the upper and lower portions.

7. The torsion beam of the coupled torsion beam axle according to claim 3, wherein: Each outer surface of the inner skirt and each inner surface of the outer skirt are welded in the longitudinal direction of the inner skirt and the outer skirt, and are combined so that a welding range of the central portion of the torsion beam is longer than other welding ranges on the first end side and the second end side of the central portion of the torsion beam.

8. The torsion beam of the coupled torsion beam axle according to claim 3, wherein: Each outer surface of the inner skirt and each inner surface of the outer skirt are welded in the longitudinal direction of the inner skirt and the outer skirt, and are welded not at the center portion of the torsion beam but at the first end side and the second end side of the center portion of the torsion beam.

9. The torsion beam of the coupled torsion beam axle according to claim 1, wherein: The gap is formed by offsetting the cross section of the inner protrusion into a shape corresponding to the cross section of the outer protrusion.

10. The torsion beam of the coupled torsion beam axle according to claim 1, wherein: The gap is formed between the entire outer surface portion of the inner protrusion and the entire inner surface portion of the outer protrusion.

11. The torsion beam of the coupled torsion beam axle according to claim 1, wherein: A front surface of each outer surface of the inner skirt is in contact with each inner surface of the outer skirt.

12. The torsion beam of the coupled torsion beam axle according to claim 1, in, The outer beam and the inner beam have a same cross-section formed at their center portions by maintaining closed cross sections of the same shape. A variable cross-section portion formed by changing the shape of a closed cross-section is formed at a first end side and a second end side of the uniform cross-section portion.

13. The torsion beam of the coupled torsion beam axle according to claim 12, wherein: The variable cross-section portion includes: a first variable portion, wherein a first side of the first variable portion is connected to a side portion of the same-section portion, and an upper end portion of the inner beam is inclined downward to increase a vertical height of the closed section; a second variable portion, wherein a first side of the second variable portion is connected to a second side of the first variable portion, an upper end portion of the outer beam and an upper end portion of the inner beam are formed in a horizontal cross-sectional shape having a width of a predetermined length or longer, and the second side of the second variable portion is connected to a side surface of the trailing arm.

14. The torsion beam of the coupled torsion beam axle according to claim 13, in, The end portions of the outer beam and the inner beam are formed in a shape surrounding a portion of the side surface of the trailing arm, The upper end portion of the end portion of the outer beam is combined with the upper surface of the trailing arm, An upper end portion of the end portion of the inner beam is joined to a bottom surface of the trailing arm.

15. The torsion beam of the coupled torsion beam axle according to claim 12, wherein: A clamp hole is formed in a variable cross-section portion adjacent to the same cross-section portion of the upper end portion of the outer beam among the variable cross-section portions.

16. The torsion beam of the coupled torsion beam axle according to claim 15, wherein: A receiving surface is formed on an upper end portion of the inner beam facing the clamp hole in a direction perpendicular to the axial direction of the clamp hole.

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