Stabilizer bar and process of producing a stabilizer bar

Abstract

A stabilizer bar for a chassis of a motor vehicle comprises a torsion spring portion and two arms bent away from the torsion spring portion; wherein the arms each comprise a formed end portion with a through-opening and a tubular portion, wherein the torsion spring portion comprises a hardened structure with a strength of at least 1000 MPa; and wherein the formed end portions comprise a hardened structure with a strength of at least 800 MPa. A process of producing a corresponding stabilizer is further disclosed.

Claims

1. A tubular stabilizer bar for a chassis of a motor vehicle, comprising: a torsion spring portion and two arms bent away from the torsion spring portion, wherein the arms each comprise a formed end portion with a through-opening and a tubular portion, wherein the torsion spring portion comprises a hardened structure with a strength of at least 1000 megapascals (MPa), wherein the formed end portions comprise a hardened structure with a strength of at least 800 MPa, and wherein the arms, in the tubular portion, each comprise a partial region with a low strength that is smaller than 0.7 times the strength of the torsion spring portion.

2. The tubular stabilizer bar according to claim 1, wherein the formed end portions comprise a strength which amounts to at least 0.8 times the strength of the torsion spring portion.

3. The tubular stabilizer bar according to claim 1, wherein the torsion spring portion comprises a hardness of at least 310 HV (Vickers hardness number), and that the formed end portion comprises a hardness of at least 250 HV.

4. The tubular stabilizer bar according to claim 1, wherein the formed end portion comprises a flattened end region with an approximately constant cross-section along a length of said end region, and a transition region with a variable cross-section along a length of said transition region.

5. The tubular stabilizer bar according to claim 1, wherein the strength of the partial region of the tubular portion is greater than 0.5 times the strength of the torsion spring portion.

6. The tubular stabilizer bar according to claim 5, wherein the partial region with the low strength comprises a distance from the formed end portion which distance corresponds at least to half a diameter of the tubular portion.

7. The tubular stabilizer bar according to claim 1, wherein the formed end portions of the arms are closed so as to be at least one of liquid-proof and gas-proof.

8. The tubular stabilizer bar according to claim 1, wherein there is a variable wall thickness along a length of the stabilizer, wherein a wall thickness of the end portions is smaller than a maximum wall thickness of the stabilizer bar.

9. The tubular stabilizer bar according to claim 8, wherein the wall thickness of the end portions is at least 20% smaller than the maximum wall thickness of the stabilizer, and the maximum wall thickness is formed in a bent portion between the torsion spring portion and a respective one of the arms.

10. The tubular stabilizer bar according to claim 1, wherein the stabilizer bar is produced from a steel material with a carbon content of less than 0.5% and a manganese content of 0.5% to 3.0%.

Description

SUMMARY OF THE DRAWINGS

(1) Examples will be described below with reference to the drawings.

(2) FIG. 1A shows an example stabilizer bar in a plan view with the hardness curve as a function of the length.

(3) FIG. 1B shows an example stabilizer bar in a plan view with the wall thickness curve as a function of the length.

(4) FIG. 2A shows a detail of the stabilizer bar according to FIGS. 1A and 1B in a plan view.

(5) FIG. 2B shows a detail of the stabilizer bar according to FIGS. 1A and 1B in a side view.

(6) FIG. 2C shows a detail of the stabilizer bar according to FIGS. 1A and 1B with the hardness curve.

(7) FIG. 3A shows an example device for producing a stabilizer according to FIG. 1A with a first arm inserted into the device for working purposes.

(8) FIG. 3B shows an example device for producing a stabilizer according to FIG. 1A after an end-sided partial portion of the first arm has been formed.

(9) FIG. 3C shows an example device for producing a stabilizer according to FIG. 1A after the formed partial portion of the first arm has been cut and perforated.

(10) FIG. 4A shows the device according to FIGS. 3A-3C with the second arm inserted into the device for working purposes.

(11) FIG. 4B shows the device according to FIGS. 3A-3C after the end-sided partial portion of the second arm has been formed.

(12) FIG. 4C shows the device according to FIGS. 3A-3C after the formed partial portion of the second arm has been cut and perforated.

DETAILED DESCRIPTION

(13) FIGS. 1 and 2 which will be described jointly below show an example stabilizer 2 for use in the chassis of a motor vehicle. In particular, the stabilizer 2 is configured as a generally tubular, elongate member which is formed to the required shape to be used in the vehicle. The stabilizer 2 comprises a torsion spring portion 3 which can also be referred to as torsion bar spring portion or stabilizer back, as well as two arms 4, 4 which are bent away at an angle from the torsion spring portion 3, which arms can also be referred to as lever arms. Between the torsion spring portion 3 and the arms 4, 4 there is formed a respective bent portion 5, 5. It can be seen that the present stabilizer 2 is mirror-symmetric relative to a central plane E and comprises an arm 4, 4 at both ends. However, it is understood that the stabilizer can also be asymmetric and, depending on the space conditions, it can also comprise non-identical arms 4, 4. If, within the framework of the present disclosure, reference is made to only one of the arms 4, 4, it is understood that this may also apply to the other arm 4 as well.

(14) The torsion spring portion 3 extends in a substantially straight line and can be fixed, for example by two bearings (not illustrated) to the vehicle chassis, for example in the engine chamber or at the underbody of the motor vehicle. The arms 4, 4 serve to connect the stabilizer 2 to the two wheel suspensions (not illustrated) of an axle of the motor vehicle. In the present embodiment, the arms 4, 4 extend substantially straight and, in a plan view of the stabilizer 2, they enclose right angles respectively with the torsion spring portion 3. However, it is to be understood that, depending on the installation conditions and the type of application, the arms can also have different designs, i.e., forms. For example, the arms can also be cropped or bent and/or extend at an angle which deviates from 90 relative to the torsion spring portion 3.

(15) By using the arms 4, 4 of the stabilizer 2 in connection with an associated wheel suspension of the motor vehicle, the two suspensions are coupled to one another in a resilient way. When cornering, i.e., when curves are negotiated, a compression stroke of the vehicle wheel on the outside of the curve and a rebound stroke of the wheel on the inside of the curve cause vehicle roll. Such compression stroke is transferred via the respectively deflected arm 4, 4 and the torsion spring portion 3 to the opposed arm 4, 4. In this way vehicle roll, i.e., a rotational movement around the longitudinal vehicle axle, is counteracted.

(16) The stabilizer 2 is produced from a closed, one-part tubular member, from which end portions are bent away, which end portions form the arms 4, 4. A hardenable steel material is used as starting material which can comprise a carbon content of less than 0.5 percent by weight and a manganese content of 0.5 to 3.0 percent by weight. Apart from iron, it is possible to use other common alloying elements such as chromium and/or boron. The stabilizer 2, respectively the tubular member used for producing the stabilizer, is heat-treated, i.e. hardened and tempered; the tubular member can be hardened and tempered before or after bending the arms 4, 4.

(17) Starting from the bent portion 5, 5, each arm 4, 4 comprises a tubular portion 6, 6 with an at least substantially constant cross-section along the length and a formed portion 7, 7. The formed portions 7, 7 each comprise a flattened region 8, 8 and a transition region 9, 9. The transition region 9, 9 connects the flattened region 8, 8 to the tubular portion 6, 6 and accordingly comprises a variable cross-section along the length of the respective transition zone 9, 9.

(18) It can be seen in particular in FIG. 2B that the flattened end regions 8, 8 of the formed end portions 7, 7 are perforated, i.e., punched, with the through-openings 10, 10 serving to connect the stabilizer 2 to a chassis component of the motor vehicle. The flattened end regions 8, 8 are closed so as to be liquid-proof and/or gas-proof, e.g., up to at least 2 bar internal pressure, thus preventing dirt and water from penetrating. The gas-proof sealing operation takes place in connection with the forming operation. In this connection it is advantageous if the stabilizer ends are gas-proof in such a way that after 5 minutes the internal pressure amounts to at least 0.75 times the maximum pressure applied.

(19) Furthermore, it can be seen in FIG. 2A that the transition zones 9, 9 between the adjoining flattened end region 8, 8 and the tubular portion 6, 6 of the arm 4, 4 comprise a variable cross-section along the length, so that a cross-section changing transition between the flat end region 8, 8 and the tubular arm portion 6, 6 is provided.

(20) As already mentioned above, the stabilizer 2 is made of a hardened and tempered steel material. Depending on the starting material, the basic strength of the hardened and tempered steel material amounts to at least 1000 MPa, sometimes 1200 MPa, and optionally at least 1400 MPa. For example, a stabilizer tubular member made of a hardened and tempered steel material 26MnB5 can comprise a basic strength of approximately 1400 MPa to 1585 MPa. A tube member made of hardened and tempered steel material 34MnB5 can comprise an even higher basic strength of approximately 1585 to 1680 MPa.

(21) In the finish-produced condition of the stabilizer, the formed end portions 7, 7 comprise a strength of at least 800 MPa, more particularly at least 1000 MPa. The strength of the formed end portions 7, 7 can be lower than the basic strength of the hardened and tempered steel material and, for example, it can amount to at least 0.8 times to 0.9 times the strength of the hardened and tempered steel material. If the hardened and tempered steel material has a strength of 1200 MPa, the strength of the formed end portions 7, 7 accordingly could thus range between 980 MPa and 1080 MPa, for example.

(22) The strength of the stabilizer is related to the hardness for which an exemplary curve is shown in FIG. 1A, and an enlarged detail for one of the arms 4, 4 of the stabilizer 2 is shown in FIG. 2C. The basic hardness of the stabilizer 2 is referred to as H2 and the hardness of the end portions 7, 7 as H7. A mean basic hardness is designated as 0.5H2. It can be seen that the hardness H7 of the end portions 7, 7 is slightly lower than the basic hardness H2, more particularly corresponding to approximately 0.9 times the basic hardness H2. Example values for the hardness of the stabilizer 2 can be given as follows: the basic hardness H2 can amount to at least 410 HV, more particularly 500 HV. The formed end portions 7, 7 can have a hardness H7 of at least 310 HV, more particularly of at least 400 HV.

(23) Furthermore, it can be seen that the end portions 7, 7, starting from an end face 12, 12 along the respective end region 8, 8 and the adjoining transition region 9, 9, comprise a substantially constant hardness H7 and strength, respectively. More particularly, the respective hardness and strength is greater than 0.8 times the basic hardness H2 and the basic strength of the stabilizer 2, respectively. It is not until the tubular portion 6, 6 of the arm 4, 4 which, for geometric reasons, comprises a greater surface inertia moment than the formed end portion 7, 7, that a partial region 13, 13 with a reduced hardness Hmin is provided. In this case, the reduced hardness Hmin is greater than the mean basic hardness 0.5H2 of the stabilizer 2 and smaller than 0.7 times the basic hardness H2, i.e. 0.7H2>Hmin>0.5H2. The soft partial region 13, 13 is clearly spaced from the transition region 9, 9, more particularly by a length which is greater than half the diameter D6 of the tubular arm portion 6, 6.

(24) FIG. 1B shows the wall thickness curve of the stabilizer 2 along the length L. It can be seen that the stabilizer 2 comprises a variable wall thickness d along the length L and it is understood that the wall thickness along the length can also be constant. In the present embodiment, the wall thickness curve is symmetric relative to the central plane of the stabilizer. It is proposed more particularly that the wall thickness d4 of the arms 4, 4 is reduced relative to the wall thickness d5 of the bent portions 5, 5. In the bent portions 5, 5 the stabilizer has the greatest thickness d5. Starting from the two bent portions 5, 5 and looking towards the central plane, the stabilizer 2 again comprises a reduced wall thickness d3 in the torsion spring portion 3, which, in the present embodiment, comprises two partial regions with different thicknesses. It is to be understood, however that, depending on the load conditions of the stabilizer, a different number of partial regions with variable thickness can also be provided which deviates from two, i.e. one, three or more than three partial regions with different thickness. The transition regions d3, d4, d5 are constant, i.e. continuous, so that the most uniform load conditions possible is achieved, with stress peaks being avoided. It is proposed more particularly that the formed end portions 7, 7 comprise a wall thickness d7 which is reduced by at least 20% relative to the maximum wall thickness d5 of the stabilizer.

(25) For producing a tubular member with a variable thickness along the length, it is possible, for example, to pull a tube initially having a constant wall thickness over a stepped mandrel, so that portions with different wall thicknesses are produced in the tube (a Tailor Drawn Tube). Alternatively, the tube can be produced from flexibly produced strip material as a starting material, a process which will be explained in greater detail below. Such flexibly produced strip material is also referred to as a Tailor Rolled Blank.

(26) An exemplary process for producing the stabilizer 2 comprises the following steps: producing a tube; bending the arms 8, 8 of the tube, hardening and tempering the tube before or after the arms 8, 8 have been bent so that the hardened and tempered tube comprises a hardness structure with a strength of at least 1000 MPa; heating an end-sided partial portion of a first one of the arms 8, 8 after the tube has been hardened and tempered, respectively, with the strength of the end-sided partial portion being reduced to below 800 MPa; hot-forming the end-sided partial portion after having been heated for producing the formed end portion 7, 7; punching the formed end portion 7, 7 to produce a through-opening 10, 10; hardening the formed end portion such that the formed end portion comprises a hardened structure with a strength of at least 800 MPa. Subsequently, the second arm 8, 8 will be treated in the same way.

(27) The tube can be produced by the sub-steps: rolling a steel strip; forming the steel strip into a slotted tubular member; and longitudinally welding the slotted tubular member to form a closed tube. For producing a variable material thickness along the length of the stabilizer 2, the steel strip can be flexibly rolled prior to being formed into a tube or a tube with an initially constant wall thickness can be flexibly drawn after having been welded. In the case of flexible rolling, the strip material obtains a variable thickness along the length by changing the rolling gap during said process. The operation of bending the arms 8, 8 takes place in a suitable bending device. Hardening and tempering takes place via the sub-steps: heating the tube to an austenitising temperature; quenching the heated tube; and tempering the quenched tube. Hardening and tempering preferably take place after the arms 8, 8 have been bent, but in principle, it can also take place before hardening and tempering. After hardening and tempering the hardened and tempered tubular member 2 is existent having a substantially constant cross-section along the length L and having the above-mentioned basic strength of at least 1000 MPa.

(28) Based on this, the end-sided partial portions of the arms 8, 8 are produced as explained below with reference to FIGS. 3A to 3C.

(29) FIGS. 3A to 3C show a combined hot-forming, punching and hardening tool 14 which, below, can also simply be referred to as the tool. Before the hardened and tempered tubular member 2 is placed into the tool 14, the end-sided partial portion 15 of the first arm 8 is heated, which can be effected by induction heating. The length of the heated partial portion 15 at least corresponds to the length of the formed end portion 7 of the arm 8. After the partial portion 15 has been heated, the tubular member 2 is quickly placed into the tool 14, as shown in FIG. 3A.

(30) The combined hot-forming, punching and hardening tool 14 comprises a rack 16, an upper tool part 17 and a lower tool part 18. At the upper tool part 17, an upper forming die 19 is provided which cooperates with a corresponding lower forming die 20 of the lower tool part 18. For forming the arm 8, the upper tool part 17 is moved towards the lower tool part 18, with the end-sided partial portion 15 of the tubular member 2 positioned therebetween and formed by the forming dies 19, 20, i.e., being flattened and pressed. To that extent, the two forming dies form parts of the hot-forming device and can also be referred to as pressing- or flattening dies. For moving the upper tool part 17 a driving unit 21 is provided which, for example, can comprise two hydraulic cylinders 22, 22.

(31) FIG. 3B shows the tool 14 with moved-out hydraulic cylinders 22, 22 and correspondingly lowered upper tool part 17 as well as the formed condition of the tubular member 2. It can be seen that the previously tubular partial portion 15 of the arm 4 has been formed into the formed portion 7 with a flattened end region 8 and a transition region 9.

(32) Subsequently, the projecting end of the formed portion 7 is cut off and the flattened end region 8 is perforated, respectively punched. For this purpose, the tool 14 has at the upper tool part 17 a cutting device 23 for cutting the projecting end and a punching device 24 for perforating the flattened end region 8. The cutting device 23 and the punching device 24, which can also be referred to as perforating device 24, comprise a common drive unit 25 which typically comprises a hydraulic cylinder. While the pressing force of the forming device 19, 20 for forming the arm 8 is maintained, a cutters and a punch move through the forming dies 19, 20 and perforate the flattened region 7 and cut the end.

(33) FIG. 3C shows the tool 14 with a moved-out hydraulic cylinder 25 and a correspondingly lowered cutting and punching device 23, 24. It can be seen that a projecting end of the formed portion 7 is cut off and that the through-opening 10 has been produced in the flattened end region 8.

(34) Hardening of the formed end portions 8 is achieved by rapidly cooling the end portion by a planar contact of the workpiece with the forming and pressing dies 19, 20. The rapid cooling process takes place so quickly that an at least partial martensitic structure is obtained in the formed end portion 8. More particularly, a diffusion of iron and carbon is avoided, so that the cubic surface-centred austenite, which is present after the partial portion 15 has been heated, is transformed into the tetragonally distorted, space-centred grid of the martensite as the result of a displacement or folding process. In respect of the density of the flattened end region 8, it is advantageous that martensite, due to its grid structure, comprises a slightly larger volume than austenite, so that the wall portions of the flattened end regions, which wall portions lying on one another, are particularly solid and densely layered.

(35) After producing the formed and hardened portion 7 of the first arm 8, the stabilizer 2 is taken out of the tool 14. Subsequently, the second end 15 and the second arm 8 respectively are treated analogously. The respective process shown in FIGS. 4A to 4C takes place identically to the process of producing the first arm 8. Thus, for the sake of brevity, reference is hereby made to the above description of the production of the first arm 8.

(36) It is understood that the process as described is exemplary only and that it can be modified. For example, the stabilizer, can also be removed directly from the forming tool after having been hot formed, and can be hardened and rapidly cooled respectively by separate means. For example, after the stabilizer has been removed from the forming tool, the formed end portions 8, 8 can be cooled in a separate cooling device by means of a liquid medium.

(37) Optionally, the tubular member can be internally blasted prior to forming and hardening the leg ends 7, 7. After the formed and hardened portions 7, 7 have been produced, the stabilizer 2 can be externally blasted, coated and optionally provided with attaching parts.

(38) An advantage of the stabilizer 2 and of the production process, is that the stabilizer 2 can withstand high loads and comprises a long service life due to the high strength of the end portions 7, 7. The stabilizer ends 7, 7 can be produced in a favourable manner simply, rapidly and with high process reliability in one single tool 14, i.e., in the described combined hot-forming, punching and hardening tool 14. Hardening the end portions 7, 7 does not only achieve a higher strength, but also has a positive effect on the density of the stabilizer 2, which, overall, contributes to a long service life.