WO2025247620A1

WO2025247620A1 - Jounce bumper assembly for a suspension system comprising a support ring and corresponding method

Info

Publication number: WO2025247620A1
Application number: PCT/EP2025/062885
Authority: WO
Inventors: Josef Heidemann; Holger Jahn
Original assignee: BASF Polyurethanes GmbH
Current assignee: BASF Polyurethanes GmbH
Priority date: 2024-05-28
Filing date: 2025-05-12
Publication date: 2025-12-04
Anticipated expiration: 2026-11-28

Abstract

The invention relates to a jounce bumper assembly (100) for a suspension system, preferably a vehicle suspension system, in particular for a vehicle shock absorber, the assembly (100) comprising: a jounce bumper (102) comprising a first end portion (104), a second end portion (106), a longitudinal axis (L) extending from the first end portion (104) to the second end portion (106), wherein the jounce bumper (102) is configured to resiliently deform between an uncompressed state and a compressed state, wherein in the compressed state, the jounce bumper (102) has a smaller length in the direction of the longitudinal axis (L) than in the uncompressed state, and comprises an outer circumferential groove (108) disposed between and spaced apart from the first and second end portion (104, 106), and a support ring (2) that is arranged in the outer circumferential groove (108) and abuts against the jounce bumper (102), said support ring (2) comprising: an inner circumferential surface (4) that defines a through-opening (6) extending along a longitudinal axis (L), said through-opening (6) being configured to fit around the jounce bumper (102), a first axial end face (8), and a second axial end face (10) arranged opposite from the first axial end face (8). The invention further relates to a method. According to the invention, the support ring (2) is formed of a sheet metal material.

Description

Jounce bumper assembly for a suspension system comprising a support ring and corresponding method

The invention relates to a jounce bumper assembly for a suspension system, preferably a vehicle suspension system, in particular for a vehicle shock absorber, the assembly comprising: a jounce bumper comprising a first end portion, a second end portion, a longitudinal axis extending from the first end portion to the second end portion, wherein the jounce bumper is configured to resiliently deform between an uncompressed state and a compressed state, wherein in the compressed state, the jounce bumper has a smaller length in the direction of the longitudinal axis than in the uncompressed state, and comprises an outer circumferential groove disposed between and spaced apart from the first and second end portion, and a support ring that is arranged in the outer circumferential groove and abuts against the jounce bumper, said support ring comprising: an inner circumferential surface that defines a through-opening extending along a longitudinal axis, said through- opening being configured to fit around the jounce bumper, a first axial end face and a second axial end face arranged opposite from the first axial end phase.

Support rings of the aforementioned type are generally known in the industry, in particular in the automotive industry. Jounce bumpers are commonly used as additional spring elements that are mounted to the rods of vehicle shock absorbers. Jounce bumpers are intended to absorb kinetic energy when the vehicle suspension system, typically the shock absorber, is subjected to large forces, and the suspension travel approaches the maximum available travel. Jounce bumpers are thus used to limit or prevent damage to the vehicle (or shock absorber) under excessive loads in the direction of the longitudinal axis.

These jounce bumpers have to meet to distinct criteria that are difficult to combine: One the one hand side, it is desired that the jounce bumper reacts very softly to initial deformation in order to cause little distraction to the suspension dynamics of the vehicle. On the other hand side, it is desired that the jounce bumper can withstand considerable forces and has a progressive spring behaviour to withstand increasing loads and increasing deformation in the direction of the longitudinal axis.

In order to meet these criteria, conventional jounce bumpers have in the past inter alia been equipped with support rings that are placed on the outer periphery of the support ring, often in a correspondingly shaped groove on the jounce bumper. Since the support rings are stiff in the radial direction (at least when compared to the resiliency deformable jounce bumper), they progressively stiffen the jounce bumper the higher its deformation becomes.

The loads which jounce bumpers have to deal with, have been increased over time due to higher vehicle weights. This has resulted for example from an increasing use of battery electric vehicles. It has been found that conventional support rings that are mass-produced for example from plastic are not capable of performing properly for vehicles having a higher vehicle weight. Furthermore, in case these plastic support rings are overloaded, they tend to break and chip of the jounce bumper. Hence, these rings will not provide a failsafe running function.

In addition, support rings are known that are made of cast material by inserting moldable material into a die or mold and curing the material to assume the desired shape of the support ring. While these support rings may provide the required mechanical properties to withstand even higher vehicle weights, they comprise a considerable material use. This is particularly the case for solid aluminium rings manufactured by cold forging or by casting processes. Solid steel rings comprise a considerable weight.

In light thereof, it was an object of the invention to provide a jounce bumper assembly having a support ring of the aforementioned type, which comprises a lower weight, requires less material and provides a sufficient mechanical rigidity to cover increasing loads due to higher vehicle weights. In addition, it was an object to provide a support ring providing a failsafe running function. The invention attains this object by suggesting a jounce bumper assembly at which the support ring is formed from a sheet metal material. The support ring is preferably formed of a single piece of sheet metal.

The invention is based upon the realisation that by providing a support ring that is formed from a sheet metal material, a high strength of the support ring can be achieved so that the support ring and the corresponding jounce bumper assembly are suitable to be used with battery electric vehicles having a higher vehicle weight. In particular, said support ring has a higher strength compared to plastic rings. In addition, less material is needed compared to solid metal rings. This also results in a lower weight compared to known solutions, in particular steel rings. Another advantage of the proposed solution is that the support ring formed of a sheet metal material provides a failsafe running function. This means that when high loads are applied to the jounce bumper assembly and thus to the support ring, it will not break but rather be deformed. As a consequence, the support ring will stay at the jounce bumper and may thus provide a failsafe running function, even if the support ring has been overloaded. In addition, the sheet metal support ring according to the invention can be manufactured cheaper compared to solid rings, in particular aluminium rings.

According to one embodiment, the support ring comprises a wall, wherein the wall forms the inner circumferential surface, the first axial end face and the second axial end face. Preferably, the sheet metal forms the wall which in turn forms the inner circumferential surface, the first axial end face and the second axial end face. Thus, the support ring is formed in one piece. This allows that the support ring can be conveniently manufactured with only a limited number of forming steps. Moreover, the design ensures that the support ring has a high strength.

According to one embodiment, the wall has a wall thickness that is substantially constant. According to one embodiment, the wall thickness ranges from 0.5 mm to 4 mm. By providing a wall thickness that is substantially constant ranging in the mentioned thickness range, the material use can be reduced and the weight of the support ring is considerably smaller compared to solid support rings. This also ensures that the price for the support ring can be kept lower compared to solid support rings known from the prior art.

According to one embodiment, the inner circumferential surface comprises a first maximum radial extension from the longitudinal axis and the axial end faces comprise a second maximum radial extension from the longitudinal axis, wherein the maximum radial extension of the axial end faces is larger than the maximum radial extension of the inner circumferential surface. In this way, the support ring fits into the circumferential groove of the jounce bumper. Preferably, the support ring has a substantially c-shaped cross section. Preferably, the support ring has an opening extending radially outwards. The opening preferably extends completely around the support ring in the circumferential direction.

According to one embodiment, the axial end faces comprise a contact surface, wherein the contact surface extends in a radial direction with respect to the longitudinal axis. Therewith, the interaction with the jounce bumper is improved.

According to one embodiment, the support ring comprises a stiffening corrugation formed at the inner circumferential surface. Preferably, the stiffening corrugation is a first stiffening corrugation and wherein the support ring comprises at least one additional stiffening corrugation. The stiffening corrugations are preferably equidistantly spaced in a circumferential direction around the longitudinal axis. In particular, the support ring comprises 3-8 stiffening corrugations, in particular four stiffening corrugations.

By providing the stiffening corrugations, the dimensional stability of the support ring is improved. By providing the corrugations equidistantly spaced in a circumferential direction around the longitudinal axis, it is ensured that the dimensional stability is provided evenly in a circumferential direction around the support ring. Furthermore, it is ensured that the corrugations comply with the form of the recess of the jounce bumper.

According to one embodiment, the inner circumferential surface comprises at least two adjacent indentations spaced from one another, and wherein between the adjacent indentations the stiffening corrugation is established. In other words, the stiffening corrugation is established according to this embodiment by providing two adjacent indentations that are spaced from one another. The area between these two indentations is not formed and thus establishes the stiffening corrugation. Preferably, all stiffening corrugations are established by forming two adjacent indentations on each side of the stiffening corrugation. The at least one indentation may comprise a convex or concave shape.

The indentations may be formed by applying a forming body radially inwards, in particular wherein one forming body may be assigned to each indentation. According to one embodiment, the support ring may be rotated so that one forming body may be utilized to form the indentations of the support ring one after the other. According to one embodiment, the support ring comprises a coating, in particular an anticorrosion coating. The anti-corrosion coating is applied to ensure that the support ring made of sheet metal is prevented against corrosion.

According to one embodiment, the jounce bumper is partially or completely made of a volume-compressible material. Preferably, the volume-compressible material is a cellular polyisocyanate polyaddition product.

The jounce bumper can be composed of an elastomer, but it can also be composed of a plurality of elastomers which are present in layers, in shell form or in another form or also in a mixture with one another. The polyisocyanate polyaddition products are preferably constructed on the basis of microcellular polyurethane elastomers, on the basis of thermoplastic polyurethane or from combinations of said two materials which may optionally comprise polyurea structures.

Microcellular polyurethane elastomers which, in a preferred embodiment, have a density according to DIN 53420 of 200 kg/m3 to 1 100 kg/m3, preferably 300 kg/m3 to 800 kg/m3, a tensile strength according to DIN 53571 of 2 N/mm2, preferably 2 N/mm2 to 8 N/mm2, an elongation according to DIN 53571 of 300%, preferably 300% to 700%, and a tear strength according to DIN 53515 of preferably 8 N/mm to 25 N/mm are particularly preferred.

The elastomers are preferably microcellular elastomers on the basis of polyisocyanate polyaddition products, preferably having cells with a diameter of 0.01 mm to 0.5 mm, particularly preferably 0.01 to 0.15 mm.

Elastomers on the basis of polyisocyanate polyaddition products and the production thereof are known in general and described numerously, for example in EP A 62 835, EP A 36 994, EP A 250 969, DE A 195 48 770 and DE A 195 48 771 .

Production customarily takes place by reacting isocyanates with compounds which are reactive to isocyanates.

The elastomers on the basis of cellular polyisocyanate polyaddition products are customarily produced in a mold in which the reactive starting components are reacted with one another. Suitable molds here are generally customary molds, for example metal molds, which, on the basis of their shape, ensure the three dimensional shape according to the invention of the spring element. In one embodiment, the contour elements are integrated directly in the casting mold; in a further embodiment, they are retrospectively incorporated into the basic body. In a preferred embodiment, the spring element is cooled for this purpose until it solidifies, preferably with liquid nitrogen, and processed in this state.

The polyisocyanate polyaddition products can be produced according to generally known methods, for example by the following starting substances being used in a single or two stage process:

(a) isocyanate,

(b) compounds reactive to isocyanates,

(c) water and optionally

(d) catalysts,

(e) blowing agents and/or

(f) auxiliary and/or additional substances, for example polysiloxanes and/or fatty acid sulfonates.

The surface temperature of the inner wall of the mold is customarily 40°C to 95°C, preferably 50°C to 90°C. The production of the molded parts is advantageously carried out at an NCO/OH ratio of 0.85 to 1 .20, wherein the heated starting components are mixed and brought in a quantity corresponding to the desired molded part density into a heated, preferably tightly closing molding tool. The molded parts are cured for 5 minutes to 60 minutes and then can be removed from the mold. The quantity of the reaction mixture introduced into the molding tool is customarily dimensioned in such a manner that the molded bodies obtained have the density already presented. The starting components are customarily introduced into the molding tool at a temperature of 15°C to 120°C, preferably of 30°C to 110°C. The degrees of compression for producing the molded bodies lie between 1 .1 and 8, preferably between 2 and 6. The cellular polyisocyanate polyaddition products are expediently produced according to the “one shot” method with the aid of high pressure technology, low pressure technology or in particular reaction injection molding technology (RIM) in open or preferably closed molding tools. The reaction is carried out in particular by compression in a closed molding tool. The reaction injection molding technology is described, for example, by H. Piechota and H. Rohr in "Integralschaumstoffe", Carl Hanser- Verlag, Munich, Vienna 1975; D.J. Prepelka and J.L. Wharton in Journal of Cellular Plastics, March/April 1975, pages 87 to 98 and U. Knipp in Journal of Cellular Plastics, March/April 1973, pages 76-84.

Alternatively, the jounce bumper is partially or completely made of rubber.

In a further aspect, the invention relates to a jounce bumper assembly for a suspension system, preferably a vehicle suspension system, in particular for a vehicle shock absorber, the assembly comprising: a jounce bumper comprising a first end portion, a second end portion, a longitudinal axis extending from the first end portion to the second end portion, wherein the jounce bumper is configured to resiliently deform between an uncompressed state and a compressed state, wherein in the compressed state, the jounce bumper has a smaller length in the direction of the longitudinal axis than in the uncompressed state, and comprises an outer circumferential groove disposed between and spaced apart from the first and second end portion, and a support ring that is arranged in the outer circumferential groove and abuts against the jounce bumper, said support ring comprising: an inner circumferential surface that defines a through-opening extending along a longitudinal axis, said through-opening being configured to fit around the jounce bumper, a first axial end face and a second axial end face arranged opposite from the first axial end phase.

The invention according to the further aspect suggests that the support ring may be formed from a metal pipe section. The metal pipe section may be preferably cut off from a pipe and formed thereafter. The pipe may be a precision pipe. The support ring may be formed from the metal pipe section by applying at least one forming body radially inwards towards the pipe section. Forming the support ring from a metal pipe section provides an alternative way of manufacturing the support ring.

The jounce bumper assembly according to the further aspect of the invention, in particular the support ring formed from a metal pipe section, may take advantage of the same benefits and preferred embodiments as the jounce bumper assembly according to the first aspect of the invention. In this regard and in order to avoid unnecessary repetition, reference is made to the above explanations.

The invention has been described so far with regard to jounce bumper assemblies. In a further aspect, the invention relates to a method for manufacturing a support ring of a jounce bumper assembly of any one of the preceding embodiment. The invention suggests a method having the following steps: providing a sheet metal or a metal pipe section, forming the sheet metal material orthe metal pipe section and receiving a support ring according to any one of the preceding embodiments.

According to one embodiment, the method further comprises the step: forming at least two indentations by radially applying a forming body, wherein the indentations are spaced from one another, and wherein in between the adjacent indentations a stiffening corrugation is established. Preferably, four stiffening corrugations are established by the proposed method, wherein the stiffening corrugations are equidistantly spaced around the circumferential direction.

It has been found that by forming the support ring of sheet metal, a support ring can be provided that has a high strength, a small and equal wall thickness, requires less material compared to solid material rings and therefore has a lower weight. Further, the sheet metal ring can be deformed but will not break at misuse loads, thereby providing failsafe running conditions. Further, based on the proposed manufacturing method, the support ring can be conveniently manufactured at low costs.

In one embodiment, the method comprises the steps: providing a jounce bumper, in particular comprising a first end portion, a second end portion, a longitudinal axis extending from the first end portion to the second end portion, wherein the jounce bumper is configured to resiliently deform between an uncompressed state and a compressed state, wherein in the compressed state, the jounce bumper has a smaller length in the direction of the longitudinal axis than in the uncompressed state, and comprises an outer circumferential groove disposed between and spaced apart from the first and second end portion, attaching the support ring to the circumferential groove of the jounce bumper.

The method comprises the same advantages and preferred embodiments as the jounce bumper assembly of the present invention and vice versa. In order to avoid unnecessary repetition, reference is made to the above explanations.

In a further aspect, the invention relates to a support ring for a jounce bumper assembly, said support ring comprising an inner circumferential surface that defines a through-opening extending along a longitudinal axis, said through-opening being configured to fit around the jounce bumper, a first axial end face, and a second axial end face arranged opposite from the first axial end face. With regard to this further aspect, the invention suggests that the support ring is formed from a sheet metal material. The support ring comprises the same advantages and preferred embodiments as the jounce bumper assembly of the present invention and vice versa. In order to avoid unnecessary repetition, reference is made to the above explanations.

The invention will hereinafter be described in more detail with respect to a preferred embodiment with reference to the accompanying figures, wherein:

Fig. 1 shows a schematic three-dimensional view of a jounce bumper assembly according to a preferred embodiment having a support ring,

Fig. 2 shows a sectional view of a jounce bumper assembly according to Fig. 1 ,

Fig. 3 shows the support ring of Figs. 1 and 2 in a schematic three-dimensional view,

Fig. 4 shows the support ring of Figs. 1-3 in a schematic top view, and

Fig. 5a, b show sectional views of the support ring according to Figs. 1-4.

Figures 1 and 2 show a jounce bumper assembly 100 for a suspension system (not shown). The suspension system is preferably a vehicle suspension system, in particular for a vehicle shock absorber. The assembly 100 comprises a jounce bumper 102 and a support ring 2.

The jounce bumper 102 comprises a first end portion 104 and a second end portion 106. The jounce bumper 102 furthermore comprises a longitudinal axis L which extends from the first end portion 104 to the second end portion 106. The jounce bumper 102 is configured to resiliently deform between an uncompressed state as shown in Figures 1 and 2 and a compressed state (not shown). In the compressed state, the jounce bumper 102 has a smaller length in the direction of the longitudinal axis L than in the uncompressed state. The jounce bumper 102 furthermore comprises an outer circumferential groove 108 which is disposed between and spaced apart from the first and second end portion 104, 106. The jounce bumper 102 is partially or completely made of a volume-compressible material, in particular of a cellular polyisocyanate polyaddition product.

The support ring 2 is arranged on the outer circumferential groove 108 and abuts against the jounce bumper 102. The support ring 2 comprises an inner circumferential surface 4 that defines a through-opening 6 as shown in Figure 3. The inner circumferential surface 4 is configured to fit around the jounce bumper 102. The support ring 2 furthermore comprises a first axial end face 8 and a second axial end face 10, which is arranged opposite from the first axial end face 8.

The support ring 2 is formed from a sheet metal material. In this way, it is ensured that the support ring 2 has a high strength. Compared to solid support rings known from the prior art, less material is needed resulting in a lower weight. Furthermore, the support ring 2 can be manufactured at a cheaper price compared to solid rings, in particular aluminium rings.

The support ring 2 is illustrated in more detail in Figures 3 - 5. The support ring 2 comprises a wall 12 made of the sheet material. The wall 12 forms the inner circumferential surface 4, the first axial end face 8 and the second axial end face 10. The wall 12 has a wall thickness T. The wall thickness T is substantially constant. In particular, the wall thickness T may range from 0,5m to 4mm.

The inner circumferential surface 4 comprises a first radial extension 16 from the longitudinal axis L as shown in Figure 4. Further, the axial end faces 8, 10 comprise a second radial extension 18 from the longitudinal axis L. The first radial extension 16 of the axial end faces 8, 10 is larger than the second radial extension 18 of the inner circumferential surface 4. The axial end faces 8, 10 comprise a contact surface 20. The contact surfaces 20 extend in a radial direction R with respect to the longitudinal axis. In particular, the contact surfaces 20 extend perpendicular to the longitudinal axis L.

The support ring 2 comprises four stiffening corrugations 22a - 22d. The stiffening corrugations 22a - 22d are formed at the inner circumferential surface 4. The stiffening corrugations 22a - 22d are equidistantly spaced in a circumferential direction C around the longitudinal axis L. In the Figure, four stiffening corrugations 22a - 22d are shown. It needs to be noted that the support ring 2 may also comprise less or more stiffening corrugations. Furthermore, the inner circumferential surface 4 comprises four indentations 24a - 24d. The indentations 24a - 24d are spaced from one another. Between two neighbouring indentations 24a - 24d a stiffening corrugation 22a - 22d is established. In other words, when forming the indentations 24a - 24d, the space between said indentations 24a - 24d establishes the stiffening corrugations 22a - 22d. The indentations 24a - 24d have a convex shape in a radially inward perspective. Optionally, the support ring 2 comprises a coating 14, in particular an anti-corrosion coating. The coating 14 prevents the support ring 2 made of sheet metal from corroding. The support ring 2 may be manufactured by providing a sheet metal and thereafter forming the sheet metal in order to receive a support ring 2 as described hereinabove. The indentations 24a - 24d may be formed by applying a forming body radially inwardly. The space between two neighbouring indentations 24a - 24d defines the respective stiffening corrugation 22a - 22d.

The jounce bumper 102 having the support ring 2 according to the invention provides a failsafe running function. This means that under load the support ring 2 may be deformed but will not break and therefor remains at the jounce bumper 102.

List of references

2 support ring

4 inner circumferential surface

6 through-opening

8 first axial end face

10 second axial end face

12 wall section

14 coating

16 first radial extension

18 second radial extension

20 contact surface

22a-d stiffening corrugation

24a-d indentations

100 jounce bumper assembly

102 jounce bumper

104 first end portion

106 second end portion

108 outer circumferential groove

C circumferential direction

L longitudinal axis

R radial direction

T wall thickness

Claims

1 . A jounce bumper assembly (100) for a suspension system, preferably a vehicle suspension system, in particular for a vehicle shock absorber, the assembly (100) comprising: a jounce bumper (102) comprising a first end portion (104), a second end portion (106), a longitudinal axis (L) extending from the first end portion (104) to the second end portion (106), wherein the jounce bumper (102) is configured to resiliency deform between an uncompressed state and a compressed state, wherein in the compressed state, the jounce bumper (102) has a smaller length in the direction of the longitudinal axis (L) than in the uncompressed state, and comprises an outer circumferential groove (108) disposed between and spaced apart from the first and second end portion (104, 106), and a support ring (2) that is arranged in the outer circumferential groove (108) and abuts against the jounce bumper (102), said support ring (2) comprising: an inner circumferential surface (4) that defines a through-opening (6) extending along a longitudinal axis (L), said through-opening (6) being configured to fit around the jounce bumper (102), a first axial end face (8), and a second axial end face (10) arranged opposite from the first axial end face (8), characterized in that the support ring (2) is formed from a sheet metal material.

2. The jounce bumper assembly (100) of claim 1 , wherein the support ring (2) comprises a wall (12), wherein the wall (12) forms the inner circumferential surface (4), the first axial end face (8) and the second axial end face (10).

3. The jounce bumper assembly (100) of claim 2, wherein the wall (12) has a wall thickness (T) that is substantially constant, in particular wherein the wall thickness (T) ranges from 0.5 mm to 4 mm.

4. The jounce bumper assembly (100) of anyone of the preceding claims, wherein the inner circumferential surface (4) comprises a first maximum radial extension (16) from the longitudinal axis (L) and the axial end faces (8, 10) comprise a second maximum radial extension (18) from the longitudinal axis (L), wherein the maximum radial extension (16) of the axial end faces (8, 10) is larger than the maximum radial extension (18) of the inner circumferential surface (4).

5. The jounce bumper assembly (100) of anyone of the preceding claims, wherein the axial end faces (8, 10) comprise a contact surface (20), and wherein the contact surface (20) extends in a radial direction (R) with respect to the longitudinal axis (L).

6. The jounce bumper assembly (100) of anyone of the preceding claims, wherein the support ring (2) comprises a stiffening corrugation (22a-d) formed at the inner circumferential surface (4).

7. The jounce bumper assembly (100) of claim 6, wherein the stiffening corrugation (22a-d) is a first stiffening corrugation (22a) and wherein the support ring (2) comprises at least one additional stiffening corrugation (22b-d).

8. The jounce bumper assembly (100) of claim 7, wherein the stiffening corrugations (22a-d) are equidistantly spaced in a circumferential direction (C) around the longitudinal axis (L).

9. The jounce bumper assembly (100) of claim 7 or 8, wherein the support ring (2) comprises 3-8 stiffening corrugations (22a-d), in particular 4 stiffening corrugations (22a-d).

10. The jounce bumper assembly (100) of claim 6 to 9, wherein the inner circumferential surface (4) comprises at least two adjacent indentations (24a-d) spaced from one another, and wherein between the adjacent indentations (24a-d) the stiffening corrugation (22a-d) is established.

11 . The jounce bumper assembly (100) of anyone of the preceding claims, wherein the support ring (2) comprises a coating (14), in particular an anti-corrosion coating.

12. The jounce bumper assembly (100) of anyone of the preceding claims, wherein the jounce bumper (102) is partially or completely made of a volume-compressible material.

13. The jounce bumper assembly (100) of claim 12, wherein the volume-compressible material is a cellular polyisocyanate polyaddition product.

14. Jounce bumper assembly (100) according to the preamble of claim 1 , characterized in that the support ring (2) is formed from a metal pipe section.

15. A method for manufacturing a support ring (2) of a jounce bumper assembly (100) of anyone of claims 1 to 13, the method comprising: - providing a sheet metal, forming the sheet metal and receiving a support ring (2) according to anyone of claims 1 to 13, in particular wherein the method further comprises: forming at least two indentations (24a-d) by radially applying a forming body, wherein the indentations (24a-d) are spaced from one another, and wherein between the adjacent indentations (24a-d) a stiffening corrugation (22a-d) is established.