WO2020104383A1

WO2020104383A1 - Method of manufacturing a strut

Info

Publication number: WO2020104383A1
Application number: PCT/EP2019/081676
Authority: WO
Inventors: Edvin List Clausen; Carsten Pedersen; Eugen Rudolph
Original assignee: Hydro Extruded Solutions As; Bayerische Motoren Werke Aktiengesellschaft
Priority date: 2018-11-20
Filing date: 2019-11-18
Publication date: 2020-05-28
Also published as: DE102018129231A1

Abstract

A method of manufacturing a strut comprising end portions and an intermediate portion, said method comprising the steps of providing a tubular member and forming a connecting portion in the tubular member, characterised in that the tubular member has an average initial first wall thickness (t1) before forming the connecting portion, and the connecting portion is formed by axial compression of a portion of the tubular member until the portion has a second average wall thickness (t2), where t2 > t1; followed by at least partially flattening the thus compressed portion of the tubular member, so as to form a two-layer connecting portion.

Description

METHOD OF MANUFACTURING A STRUT

TECHNICAL FIELD

The present disclosure relates to method of manufacturing a strut suitable for use in automotive vehicles.

BACKGROUND ART

NVH (Noise, Vibration and Harshness) requirements to automotive vehicles require ridged car bodies. The use of tubular struts is a very efficient way to trim body stiffness and the use of such components has increased strongly over the last years. Struts are often produced from steel, but can also be made from aluminium, and should preferably be straight and loaded in a push-pull mode to obtain maximum effect in a body. The stiffness of the connection area is of course important to the function of the strut, and due to the fact that the strut is normally squeezed in this area during use, there is a risk that its stiffness may decrease. In order to increase the stiffness of the connection area in a strut, a local stiffener can be inserted at the end. GB287023 shows a tube assembly for frame structures where the strength of the flattened ends is increased by means of inserts.

SUMMARY OF THE INVENTION

The present disclosure aims at providing a method for manufacturing struts with increased bending stiffness in the connection area, without the need of using inserts.

Accordingly, the present disclosure relates to a method of manufacturing a strut comprising end portions and an intermediate portion. The method comprises the steps of providing a tubular member and forming a connecting portion in the tubular member, wherein the tubular member has an average initial first wall thickness (tl) before forming the connecting portion, and the connecting portion is formed by axial compression of a portion of the tubular member until the portion has a second average wall thickness (t2), where t2 > tl; followed by at least partially flattening the thus compressed portion of the tubular member, so as to form a two-layer connecting portion. The forming by axial compression may further involve forming a transition portion by axially compressing a portion of the tubular member, said transition portion being located directly adjacent to the connecting portion.

The tubular member may have an initial outer circumference, and the forming by axial compression may comprise arranging the axial compression so as to increase the outer circumference of the tubular member in the portion. The forming by axial compression may then be followed by a step of forming the axially compressed portion so as to decrease the outer circumference of the tubular member in the axially compressed portion.

The tubular member may have an initial outer circumference, and the forming by axial compression may comprises arranging the axial compression so as to increase an inner circumference of the tubular member in the portion.

The forming by axial compression may comprise arranging the axial compression so as to decrease the outer circumference of the tubular member in the portion, said portion being located at an end of the tubular member, such that a part of the axially compressed portion attains a conical shape having a circumference, which is smallest at the end of the tubular member, and gradually increases in a direction toward the intermediate portion of the strut.

The method may further comprise a step of forming an opening configured to receive a fastener in the connecting end portion. The opening may be cold-formed after forming of the connecting portion. The axially compressed portion (10a) of the tubular member can be located at an end of the tubular member, so that the connecting portion formed is an end connecting portion.

The axially compressed portion of the tubular member may alternatively be located at a distance from the ends of the tubular member, so that the connecting portion formed is located within the intermediate portion between a first and a second tubular beam portion and between the ends of the strut.

The method may further comprise a step of forming the tubular member in the axially compressed portion, so that the intermediate portion has a first longitudinal central axis (Al), and the connecting end portion (3) has a second longitudinal central axis (A2), wherein the second longitudinal central axis (A2) is offset or at an angle with respect to the first longitudinal central axis (Al).

The method may further comprise a step of forming the tubular member in the axially compressed portion, so that the first tubular beam portion has a first longitudinal central axis (Al), and the second tubular beam portion has a third longitudinal central axis (A3), wherein the third longitudinal central axis (A3) is offset or at an angle with respect to the first longitudinal central axis (Al).

The flattening of the axially compressed portion of the tubular member may suitable comprise compressing the compressed portion in a flattening direction, which is transverse to the longitudinal direction of the tubular member, and compressing said portion sideways in a direction transverse to the flattening direction. The method may further comprise a step of forming at least one longitudinal edge portion of the flattened connecting portion in a direction at an angle to the longitudinal direction of the flattened connecting portion.

The tubular member may suitably be a tubular aluminium member, preferably an extruded aluminium tubular member.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the method of the present disclosure, and struts obtained thereby, are illustrated by the appended non-limiting schematic drawings, wherein

Figure la is a schematic perspective view of a strut obtainable by the method of the present disclosure;

Figure lb is a schematic perspective view of a cross section in the longitudinal direction of the strut in Fig. la;

Figure 2 is a schematic perspective view of another strut obtainable by the method of the present disclosure;

Figure 3 is a schematic illustration of the different portions of a tubular member before axial compression; Figure 4 schematically illustrates the steps of the method of the present disclosure;

Figure 5 schematically illustrates a step of axial compression;

Figure 6 is an enlarged view of a cross-section in the longitudinal direction of an end of the strut shown in Figs, la and lb;

Figures 7a-7c schematically illustrate the connecting end portion and a part of the

intermediate portion of another example of a strut obtainable by the method of the present disclosure. Fig. 7a is a cross-sectional perspective view, Fig. 7b is a perspective view and Fig. 7c is a cross-sectional side view;

Figures 8a-8c schematically illustrate the connecting end portion and a part of the

intermediate portion of another example of a strut obtainable by the method of the present disclosure. Fig. 8a is a cross-sectional perspective view, Fig. 8b is a perspective view and Fig. 8c is a cross-sectional side view;

Figures 9a-9b schematically illustrate the connecting end portion and a part of the

intermediate portion of another example of a strut obtainable by the method of the present disclosure. Figs. 9a is view of a cross-section in the longitudinal direction , and Fig. 9b shows a perspective view of the end connecting portion shown in Fig 9a;

Figures lOa-c schematically illustrate the stepwise forming of a connecting portion located in the intermediate portion of the tubular member;

Figures lla-llb schematically illustrate an example of an axially compressed end portion and a part of the intermediate portion of a tubular member before flattening;

Figures 12a-12b schematically illustrate an example of an axially compressed and shrunk end portion and a part of the intermediate portion of a tubular member before flattening;

Figures 13a-13b schematically illustrate examples of axially compressed end portions and parts of the intermediate portion of tubular members before flattening;

Figures 14a-14d schematically illustrate examples of cross-sectional areas in a direction transverse to the longitudinal axis of the connecting portion. DETAILED DESCRIPTION

The automotive industry continuously strives toward weight saving solutions, and it is therefore of great interest to find solutions, which allow for light-weight materials, such as aluminium, to be used. However, when using aluminium, an increased focus must be directed toward the design requirements in order to obtain load bearing members that can fulfil the requirements put thereon. In struts mounted in automotive structures, the connection areas are subject to the highest local stresses. This is particularly pronounced when the axis of connection area is not in line with the axis of loading.

Struts typically have connections areas for attachment to an automotive structure, where the connection area is a flattened part of the strut. In order to improve stiffness in the connection area, inserts can be used, or the connection area can be formed with for example bent side edges to better take up kinetic forces. These ways are often costly or not sufficiently efficient.

The present disclosure aims at providing a method for manufacturing struts having increased bending stiffness in the connection area, without the need of using inserts. The struts obtained by this method comprise end portions and an intermediate portion, and have one or more connection portions, which can be located at the end portions and/or within the intermediate portion. In order to achieve the desired performance, the struts have an increased wall thickness at the connection portions, which are the areas that are subject the highest stress/stiffness requirements during use. The method comprises forming a tubular member into a strut, where the forming includes increasing the wall thickness of the tubular member at the portions which are to form the connecting portions by means of axial compression, followed by at least partly flattening the axially compressed portion into a two-layer connecting portion. By at least partly flattening is meant that a first part of the axially compressed portion is flattened into the two-layer connecting portion, whereas a second part of the axially compressed portion forms a widening portion, which gradually opens toward the intermediate portion. The flattening can be performed by means of a press forming tool.

The forming of connecting portions in struts by means of axial compression and flattening provides for manufacture of struts with improved cost efficiency and enhanced flexibility, as compared with other methods of changing the wall thickness in the connecting portions, such as variable extrusion or tailored rolling. The tubular member has an average initial first wall thickness tl before forming. The connecting portion is formed by axial compression of a portion of the tubular member until it has a second average wall thickness t2, which is greater than the initial first average wall thickness, i.e. t2>tl. The wall thickness in the connecting portion is preferably increased by at least 20 % by the axial compression (t2 > 1.2 x tl), more preferably at least 30%, most preferably at least 40%. The tubular member is preferably provided as an extruded member, which is preferably cut to a desired length from an extruded tubular raw material before axial compression. As said above the connecting portion can be located at an end of the strut or within the intermediate portion between the end portions. When the connecting portion is an end connecting portion, the flattening of the axially compressed portion into a two-layer is preferably performed on an outermost end of the axially compressed portion, so that it comprises a flattened two layer portion in which the inner surfaces are brought into contact with each other, and a portion which is still partially tubular with a diameter increasing toward the intermediate portion. The flattening of the axially compressed portion can comprise compressing said portion in a flattening direction, which is transverse to the longitudinal direction of the tubular member, and compressing said portion sideways in a direction transverse to the flattening direction.

The axial compression can be arranged so as to increase the outer circumference of the tubular member during compression, which can be followed by a step of shrinking or forming the axially compressed portion so as to decrease the outer circumference of the tubular member in the axially compressed portion. The outer circumference can thus be decreased to the initial circumference before axial compression, or to a smaller circumference. The outer circumference can be increased by axial compression by using an inner supporting tool in the intermediate tubular member, where the supporting tool has the same inner diameter as the intermediate tubular member, and then letting the outer diameter grow during the compression. The shrinking can be obtained by sliding a die, having an inner diameter corresponding to the initial outer diameter of the intermediate tubular member, over the axially compressed area and thus decreasing the diameter of the compressed area to the initial circumference Figs, lla-b shows an example of an end portion of a tubular member, in which the outer circumference has been increased during axial compression. Figs. 12a-b shows the end portion of Figs, lla-b after shrinking the outer circumference back to the initial size. The forming by axial compression can also be arranged so as to increase an inner circumference of the tubular member in the portion, for example, by letting an inner tool expand the inner diameter, while holding an axial counter-pressure on the thus expanded end portion of the tubular member. An example of this is shown in Fig. 13a.

Figs. 7a-c shows an example of an end connection portion, wherein the wall thickness in the widening portion 9 gradually changes and decreases from the flattened area toward the intermediate portion 2, and a part of the axially compressed portion forms a widening portion, which gradually opens from the flattened end toward the intermediate portion 2. This way of forming the end connecting portion allows for improving the weight to stiffness ratio. Figs. 8a- c shows an example of an end connection portion, where the outer circumference has been increased by axial compression, but has not been shrunk before flattening.

The forming by axial compression can be arranged so as to decrease the outer circumference of the tubular member in the portion, which is to form an end connecting portion, such that the axially compressed portion attains a conical shape having a circumference, which is smallest at the end of the tubular member, and gradually increases in a direction toward the intermediate portion of the strut. An example of this is shown in Figs. 9a-b, and the resulting cross-section is shown in Fig. 14b.

The axial compression is a cold-forming operation that can be performed by means of a mandrel supporting the interior surface of the tubular member, a die supporting the outer surface of the tubular member and a compressing tool which can apply an axial compression force on an cross- sectional surface of the tubular member, or by clamping sections of the tubular member with clamping tools, which are moved toward each other to axially compress the member. The mandrel and/or the supporting die will not always be needed, depending on the desired end design requirement. The outer surface of the axially compressed portion can be formed or shrunk by sliding a cold die over it, to decrease the outer diameter or otherwise form the outer surface.

For example, the axial compression can be obtained as shown schematically in Fig. 5 by using a mandrel 22 having a forward section 23 with a cross-sectional shape and size corresponding to the initial interior of the tubular member, and compressing section 24 with a cross-sectional shape and size corresponding to or larger than the exterior of the tubular member, where the transition between the forward section 23 and the compressing section 24 is immediate, so that the compressing section comprises a contact surface 25 which is substantially perpendicular to the longitudinal axis of the mandrel. When inserted into the tubular member 10, the contact surface 25 will abut with the end surface 21 of the tubular member and an end section 10a will be axially compressed due to the force exerted on the tubular member by the mandrel 22, and the wall thickness will consequently increase. The tubular member can typically be fixed by clamping during the axial compression.

The axial compression can further involve forming a transition portion located directly adjacent to the connecting portion. In the transition portion, the wall thickness gradually changes from the average initial first wall thickness tl in the intermediate portion to the average wall thickness t2 of the connecting portion. When the strut is subjected to bending and or torsional loads the highest tension will be created around the connection portion, and the loads will decrease in direction away from the connection portion. The shape of the transition area can be controlled by an outer supporting tool, having an inner geometry with the required shape of the transition area. The pressure created by the axial force on the end portion, will cause the metal to be moved/flow into the outer supporting tool having the transition geometry in the inside part of the tool.

The method can further comprise a step of forming the tubular member in the axially compressed end portion, so that the intermediate portion has a first longitudinal central axis Al, and the connecting end portion has a second longitudinal central axis A2, wherein the second longitudinal central axis A2 is offset or at an angle with respect to the first longitudinal central axis Al, in order to align the connecting portion of the strut with the connection portion of the part of the structure to which the strut is to be attached. This step can involve bending or otherwise forming the end portion of the tubular member into a desired shape, with the longitudinal central axis A2 offset or at an angle with respect to the first longitudinal central axis Al, and can be performed by means of a press form tool arrangement.

The connecting portion can also be located at a distance from the ends of the tubular member, so that the connecting portion formed is located within the intermediate portion between a first and a second tubular beam portion and between the ends of the strut. This can be performed in a similar way as the described for forming the end connecting portion. When inner supporting or forming tools are used, they need just be extended to reach into the needed location, and outer tool(s) are placed at the required location.

The method can further or alternatively comprise a step of bending or forming the tubular member in the axially compressed portion which is to form a connecting portion within the intermediate portion, which step involves bending or forming the tubular member so that the first tubular beam portion has a first longitudinal central axis Al, and the second tubular beam portion has a third longitudinal central axis A3, wherein the third longitudinal central axis A3 is offset or at an angle with respect to the first longitudinal central axis Al (Fig. 10c). This can suitably be performed by means of a press- or roll form tool.

The method can also comprise a step of bending or forming at least one longitudinal edge portion of the flattened connecting portion in a direction at an angle to the longitudinal direction of the flattened connecting portion, and is suitably done by means of a press form tool, where the shape is given by the press tool shape.

The method may suitably comprise a step of forming an opening configured to receive a fastener in the connecting end portion, in order to facilitate mounting of the strut to an automotive structure. The opening can be obtained by punching or drilling a hole in the formed end connection portion, but is preferably cold-formed after forming of the connecting portion, so that all material which was originally present in the tubular element from which the end connecting portion is formed is maintained in the end connecting area and can be used to increase the width and/or thickness of the end connecting portion.

Figs, la and lb illustrates an example of a strut 1 that can be manufactured by the above described method. The strut 1 comprises end portions 3 and an intermediate portion 2, and connecting portions 4 are formed at the ends and openings 8 for mounting the strut are formed in the connecting portions 4. Fig. 2 illustrates another example of a strut that can be manufactured by the above described method. In this case, connection portions 4 are located at the ends portions 3 and an additional connection portion 5 is located within the intermediate portion 2, between a first and a second tubular beam portion 12a, and 12b, respectively. Fig. 2 also show where transition portions 6 and 7 are located. Figure 6 is an enlarged cross-sectional side view of an end of the strut shown in Figs, la and lb, illustrating how the wall thickness t2 in the axially compressed end connecting portion is greater than the initial wall thickness tl in the intermediate portion 2. In this example, a transition portion 6 has been formed between the end connecting portion 4 and the intermediate portion, in which the wall thickness of the tubular member decreases from t2 in the axially compressed portion to tl in the intermediate portion. The part of the axially compressed portion closest to the end 11 has been flattened, so that a two-layer connecting portion 4 is formed closest to the end, and a second part of the axially compressed portion forms a widening portion 9, which gradually opens up toward the transition portion. In this example, the end connecting portion has also been bent so that a longitudinal axis A2 of the end connecting portion 4 is offset with respect to the longitudinal axis A1 of the intermediate portion 2. An opening 8 has been formed in the end connecting portion 4.

Fig. 3 shows schematically a tubular member 10 before axial compression with the different portions which are to form different parts of the final strut, where the portions 10a, which are located at the ends 11 of the tubular member will be formed into the connecting end portions 4, as shown in Figs, la and 2, and the portion 10b will be formed into the intermediate connecting portions 5, as shown in Fig. 2. As can be seen in Fig. 3, the intermediate portion 2 comprises the first and a second tubular beam portion 12a, 12b and the portion 10b.

Figure 4 schematically illustrates the steps of the method of manufacturing a strut comprising the steps of providing 101 a tubular member and forming 102 a connecting portion in the tubular member by axially compressing 103 a portion of the member so that the average wall thickness increases from an average initial first wall thickness tl to a second average wall thickness t2, followed by at least partially flattening 105 the thus compressed portion, so that a two-layer connecting portion is obtained. The flattening typically comprises compressing at least a part of the axially compressed portion in a flattening direction, which is transverse to the longitudinal direction of the tubular member, to obtain a cross-sectional area as shown in for example Fig. 14a. If desired, the flattening can also comprise compressing the axially compressed portion sideways in a direction transverse to the flattening direction, so that the longitudinal edges on the connecting portions become flattened. An example of the cross- section of a flattened connection portion having such flattened longitudinal edges 30 is shown in Fig. 14d. The forming by axial compression 103 can further be arranged to involve forming a transition portion located directly adjacent to the connecting portion, or increasing the outer circumference of the tubular member in the connecting portion, which can be followed by decreasing it again, or increasing an inner circumference of the tubular member in the portion so as to obtain a wider connecting portion. The forming by axial compression 103 can also be arranged to involve forming an end portion of the tubular member into a conical shape having a smallest circumference at the end of the tubular member, which gradually increases in a direction toward the intermediate portion of the strut.

The forming of the connecting portion 4 can also include a step of bending 107a the axially compressed and flattened connecting end portion, so that the intermediate portion 2 has a first longitudinal central axis Al, and the connecting end portion 3 has a second longitudinal central axis (A2), which is offset or at an angle with respect to the first longitudinal central axis (Al). This is shown in e.g. Figs, la and 2. When a connecting portion 5 is formed located within the intermediate portion 2 the forming can include a step of bending 107b the tubular member in the axially compressed portion 10b, so that the first tubular beam portion 12a has a first longitudinal central axis Al, and the second tubular beam portion 12b has a third longitudinal central axis (A3), which is offset or at an angle with respect to the first longitudinal central axis (Al), as illustrated in Fig. 2. Figs. lOa-c illustrate the tubular member 10 during the forming of a connecting portion 5 within the intermediate portion 2, where Fig. 10a shows the tubular member 10 after axial compression, Fig. 10b shows how the member has been bent in the axially compressed portion 10b, and Fig. 10c shows how the connecting portion 5 has been flattened and an opening 8 has been provided therein. In this example, end connecting portions 4 have also been formed.

Further, the forming of the connecting portion 4, 5 can comprise a step of bending 108 at least one longitudinal edge portion 13 of the flattened connecting portion in a direction at an angle to the longitudinal direction of the flattened connecting portion. Fig. 14c shows an example of the cross-section of the connecting portion having such a bent edge portion 13. If desired both longitudinal edge portions 13 can be bent, in the same or different directions. This forming can be obtained by means of a press form tool. As mentioned above, the method can further comprise a step 106 of forming an opening 8 configured to receive a fastener in the connecting end portion, and this can be obtained by cold forming, or punching or drilling a hole in the axially compressed and flattened connecting portion 4, 5.

Claims

1. A method (100) of manufacturing a strut (1) comprising end portions (3) and an intermediate portion (2), said method comprising the steps of providing (101) a tubular member (10) and forming (102) a connecting portion (4, 5) in the tubular member (10), characterised in that the tubular member (10) has an average initial first wall thickness (tl) before forming the connecting portion (4, 5), and the connecting portion (4, 5) is formed by axial compression (103) of a portion (10a, 10b) of the tubular member (10) until the portion (10a, 10b) has a second average wall thickness (t2), where t2 > tl; followed by at least partially flattening (105) the thus compressed portion (10a, 10b) of the tubular member (10), so as to form a two-layer connecting portion (4, 5).

2. The method of claim 1, wherein the forming by axial compression (103) further involves forming a transition portion (6, 7) by axially compressing a portion (10a, 10b) of the tubular member, said transition portion (6, 7) being located directly adjacent to the connecting portion (4, 5).

3. The method of claim 1 or 2, wherein tubular member (10) has an initial outer circumference, and the forming by axial compression (103) comprises arranging the axial compression so as to increase the outer circumference of the tubular member in the portion (10a, 10b).

4. The method of claim 3, wherein the forming by axial compression (103) is followed by a step of forming (104) the axially compressed portion (10a, 10b) so as to decrease the outer circumference of the tubular member in the axially compressed portion (10a, 10b).

5. The method of any one of claims 1-3, wherein tubular member (10) has an initial outer circumference, and the forming by axial compression (103) comprises arranging the axial compression so as to increase an inner circumference of the tubular member in the portion (10a).

6. The method of claim 1 or 2, wherein tubular member (10) has an initial outer circumference, and the forming by axial compression (103) comprises arranging the axial compression so as to decrease the outer circumference of the tubular member in the portion (10a), said portion (10a) being located at an end (11) of the tubular member (10), such that a part of the axially compressed portion (10a) attains a conical shape having a circumference, which is smallest at the end (11) of the tubular member (10), and gradually increases in a direction toward the intermediate portion (2) of the strut.

7. The method of any one of claims 1-6, further comprising a step (106) of forming an opening (8) configured to receive a fastener in the connecting end portion (4, 5).

8. The method of claim 7, wherein the opening (8) is cold-formed (106) after forming of the connecting portion (4, 5).

9. The method of any one of claims 1-8, wherein the axially compressed portion (10a) of the tubular member (10) is located at an end (11) of the tubular member (10), so that the connecting portion formed (102) is an end connecting portion (4).

10. The method of any one of claims 1-9, wherein the axially compressed portion (10b) of the tubular member (10) is located at a distance from the ends (11) of the tubular member (10), so that the connecting portion formed (102) is located within the intermediate portion (2) between a first and a second tubular beam portion (12a, 12b) and between the ends (11) of the strut.

11. The method of claim 9, further comprising a step of forming (107a) the tubular member in the axially compressed portion (10a), so that the intermediate portion (2) has a first longitudinal central axis (Al), and the connecting end portion (3) has a second longitudinal central axis (A2), wherein the second longitudinal central axis (A2) is offset or at an angle with respect to the first longitudinal central axis (Al).

12. The method of claim 10, further comprising a step of forming (107b) the tubular member in the axially compressed portion (10b), so that the first tubular beam portion (12a) has a first longitudinal central axis (Al), and the second tubular beam portion (12b) has a third longitudinal central axis (A3), wherein the third longitudinal central axis (A3) is offset or at an angle with respect to the first longitudinal central axis (Al).

13. The method of any one of claims 1-12, wherein the flattening (105) of the axially compressed portion (10a, 10b) of the tubular member (10) comprises compressing said portion (10a, 10b) in a flattening direction, which is transverse to the longitudinal direction of the tubular member (10), and compressing said portion (10a, 10b) sideways in a direction transverse to the flattening direction.

14. The method of any one of claims 1-13, further comprising a step of forming (108) at least one longitudinal edge portion (13) of the flattened connecting portion (4, 5) in a direction at an angle to the longitudinal direction of the flattened connecting portion (4, 5).

15. The method of any one of claims 1-14, wherein the tubular member 10 is a tubular aluminium member, preferably an extruded aluminium tubular member.