Patent Application
20090071220
The field of the invention is that of motor vehicles. More specifically, the invention relates to non-rigid axles for motor vehicles.
It will be recalled that a non-rigid axle is the term generally used to denote an axle designed to form a torsion element between two wheels.
Conventionally, a non-rigid axle comprises two longitudinal arms each carrying a support for mounting a wheel, and which are connected by a transverse connecting element known as a cross-member or profile.
During axle design there are two parameters which, amongst others, are considered in order to assess the quality of the axle. These are bending and torsion.
The principle of non-rigid axles allows a great deal of flexural stiffness to be combined with a relative torsional flexibility. In general, it is through the geometry of the cross section of the cross-member, via its bending and torsional moments of inertia, that the desired compromise between flexural stiffness and (relative) torsional flexibility is reached.
The last few years have seen a significant evolution of non-rigid axle techniques to the lower- and mid-range sectors of the motor manufacturing market, by virtue of the numerous advantages they have to offer, these including an excellent compromise between behavior and design and the fact that they are economic to produce chiefly using assemblies of the all-welded construction.
These advantages are leading ground-contact system designers consistently to push the technique to its absolute limits. Indeed non-rigid axles have actually run up against a certain number of limitations including a delicate compromise between the longitudinal and transverse stiffness and a life that is governed by the durability of each of their component parts which are subjected to significant elastic deformation.
The ever increasing demand for better comfort and drivability has in particular directed non-rigid axle design toward solutions that involve introducing a torsional stiffness between the two trailing arms, this commonly being known as an “anti-roll bar” or “ARB”, with a view to limiting vehicle body roll under cornering while at the same time maintaining good vertical flexibility in the axle assembly as this is guaranteed to filter out the transmission to the body of irregularities in the road surface.
However, the expansion of non-rigid axle technology to heavier vehicles (large saloon cars, monospace vehicles or even utility vehicles) without in any way coming to terms with the quality of the behavior, is leading the most heavily stressed components ever closer to their absolute operating limits, whether this be static resistance to incidental forces, or fatigue strength in respect of a loading cycle representative of the life of the axle assembly.
The connecting element, or cross-member, therefore forms one of the components that is the trickiest to develop, particularly from the endurance and behavioral standpoints.
At the present time, in non-rigid axles, the cross-member that connects the longitudinal arms is produced using two different technologies.
With a first technology, the cross-member is produced from a sheet metal element that is bent (or pressed) to give it a U-shaped, V-shaped or L-shaped cross section. These cross-members generally have to be combined with an anti-roll bar to provide the axle with torsional stiffness.
The second technology involves incorporating the anti-roll stiffness function into the cross-member.
In this case, the cross-member is manufactured from a tube generally of circular cross section, the tube being subjected at least in its central part to a deformation step (at the outcome of which a portion of the wall is crushed against another portion of the wall) in order to obtain the desired torsional and flexural stiffnesses (examples: Peugeot 806 (trade name) or Opel Zafira (trade name)).
The wide variety of anti-roll stiffnesses needed to suit the requirements is provided by altering the cross-sectional shape of the cross-member and/or by changing the thickness of the tube.
The invention applies to cross-members produced using this second technology, these correspondingly being known by the names of “closed-profile cross-members” or “closed-section cross-members”.
In general, all non-rigid axles that use a cross-member made up of a tube have in their central region a concavity of U-shaped or V-shaped cross section.
The cross-member tube is deformed only in a transverse portion in order to reduce its torsional stiffness and maintains cross sections of higher torsional inertia (for example circular cross sections) at the ends, to make it easier to weld to the suspension arms.
In either one of the technologies just mentioned, the cross-member of a non-rigid axle is characterized by a high flexural stiffness and a low torsional stiffness, it being necessary for the latter to be gaged precisely (associated with the anti-roll stiffness of the axle).
As far as the closed-section cross-members are concerned, the entirety of the stiffness is afforded by the cross-member, the torsional inertia of which has been carefully chosen to suit the anti-roll stiffness required in the technical specification. As a result, the tolerance on the anti-roll stiffness of the axle is entirely dependent on that of the cross-member itself.
During the manufacture of closed-section axle cross-members (by deforming a tube), the manufacturer is generally faced with the difficulty of complying with the anti-roll stiffness tolerance specified by the motor vehicle manufacturer through the technical specification.
One of the factors that influence the anti-roll stiffness is the spread on tube thickness.
The problem is that of reducing the sensitivity of axle anti-roll stiffness to spread on tube thickness.
The orders of magnitude are such that a spread of +/−0.1 mm on the tube thickness may give rise to something like three times the axle anti-roll stiffness tolerance band in the technical specification.
Several solutions have been applied to this problem of keeping control over the anti-roll stiffness of a closed-section twist-beam axle.
A first solution is to work with tube suppliers to gain control over tube thickness spread (which results from a rolling and welding process), using a sorting method.
In practice though, the potential for such a solution is relatively limited.
A second solution is to use tubes from a rolling and welding process which are then re-drawn in order to improve the tolerance on the thickness.
This solution proves effective but the associated cost is high. It therefore increases the overall cost of the axle, this being something which is usually incompatible with the demands of motor vehicle manufacturers.
According to another solution, recourse is had to a hot pressing process which provides better control over the geometry of the sections formed.
However, it has been found in practice that this technique is unable to reduce the sensitivity to spread on the thickness.
According to yet another solution, the sheet metal of the tube, at each end of the cross-member, is connected using a method of “clinching” (in which one metal sheet is driven into the other by punching).
This solution provides better control over the spread on the actual length of the working section (the section with the lowest torsional inertia), but does not in any way compensate for spread on the tube thickness.
It is a particular objective of the invention to alleviate the disadvantages of the prior art.
More specifically, it is an objective of the invention to propose a technique for the manufacture of a closed-section cross-member for a non-rigid axle which allows better control over the spread on the torsional stiffness of the cross-member by comparison with the solutions of the prior art.
Another objective of the invention is to provide a technique such as this which is simple in design and easy to implement.
Another objective of the invention is to propose a method of manufacture corresponding to such a technique.
These objectives, together with others which will become apparent later on, are achieved by virtue of the invention the subject of which is tooling for manufacturing a closed-section cross-member intended to connect two longitudinal arms of a motor vehicle non-rigid axle, said tooling comprising at least one die intended to collaborate with a punch to form, on said closed section of said cross-member, a length of torsion zone, and means for holding said cross-member in position, characterized in that said die and/or said punch are length-adjustable so that said length of said torsion zone can be adapted.
This then yields an adaptive system allowing the length of the torsion zone (also known as the “working zone”) of the cross-member, and therefore its torsional stiffness, to be varied.
Such variation in the length of the torsion zone can be obtained simply by adjusting the dimensions of the die and of the punch of the shaping tooling, and to do so in a way integrated into the tooling as will become more clearly apparent later.
According to a preferred solution, said die and said punch each comprise at least two parts that can be moved away from/toward each other.
This then yields a particularly simple and effective way to adapt the tooling to suit the desired torsional stiffness of the cross-member.
According to a first embodiment, said two parts of said die and/or of said punch can be actuated by at least one actuating cylinder.
According to a second embodiment, said two parts of said die and/or of said punch are held in the continuation of one another using screwing means.
In this case, the tooling preferably comprises a set of shims which can be interposed between said two parts of said die and/or of said punch.
The set of shims may then comprise a variety of shims of different thicknesses capable of covering a given range with the desired precision.
According to an advantageous solution, said holding means comprise at least one variable-travel clamp.
Clamps such as this contribute to the modular nature of the tooling, allowing it to be adapted to suit both the thickness of the cross-member and the length thereof.
The invention also relates to a method for manufacturing a closed-section cross-member intended to connect two longitudinal arms of a motor vehicle non-rigid axle, using tooling comprising at least one die intended to collaborate with a punch to form, on said closed section of said cross-member, a length of torsion zone, and means for holding said cross-member in position, characterized in that it comprises a step of adjusting the length of said die and/or of said punch so as to adapt said length of said torsion zone.
As a preference, the method comprises a prior step of calculating said length of said torsion zone according to the desired torsional stiffness of said cross-member and according to the wall thickness of said section.
According to an advantageous solution, the method comprises a step of adjusting the travel of two clamps that form said holding means.
Other features and advantages of the invention will become more clearly apparent from reading the following description of a preferred embodiment of the invention which is given by way of entirely nonlimiting illustrative embodiment and from the attached drawings among which:
a, 4b, 5 and 6 are views illustrating the influence of the torsion zone of a cross-member on the torsional stiffness thereof;
With reference to
The conventional tooling comprises a punch 2 and a die 3 which are actuated by a press and are intended to collaborate with one another to form the torsion zone, and clamps 4 intended to press against the ends of the tube 1.
In an initial phase, the tube is placed in the press, and the clamps are brought up against the tube in order to immobilize it, then the press is closed.
At the end of the forming step, the press is opened (the die 3 and the punch 2 therefore being moved away from each other) and the clamps are retracted from the tube (
With reference to
For such a cross-member, the geometric profile of the cross section of the tube, the tube thickness and the length of the torsion zone are key factors in obtaining the torsional stiffness of the cross-member.
In addition, for a given section profile and tube thickness, the variation in the length ΔL of the torsion zone varies the torsional stiffness R of the cross-member in the following way: if L increases, P decreases, and vice versa.
Thus, when the length of the torsion zone L is minimal, the cross-member has maximum torsional stiffness (
As mentioned previously, the principle of the invention illustrated by
In order to do this, according to the embodiment illustrated by
The separation between the parts 2a, 2b on the one hand, and the parts 3a, 3b on the other hand, is obtained by interposing one or more shims 6 between them.
Of course, the number and the thickness of the shims are chosen to suit the desired ΔL.
The parts 2a, 2b of the punch are held together with the shims 6 using threaded rods 5. The same is true of the parts 3a, 3b of the die and the shims 6.
Furthermore, the travel of the clamps 4 is controlled by hydraulic cylinder actuators (not depicted) that perform the translational movement of the clamps.
It will be noted that the lengths of the punch and of the die can be altered hydraulically, for example using on-board cylinder actuators, according to another conceivable embodiment.
The adaptive adjustment of the tooling which has just been described is performed between each production run (a run being defined by a batch of tubes that are characterized by a particular mean tube thickness), on the press, with or without partial disassembly of the tooling.
Prior to setting up the tooling, the length of the torsion zone is calculated according to the tube thickness, this being for a stiffness laid down in the technical specification.
Such tooling therefore, in production terms allows components to be formed in runs.
The graph of
The graph of
The graph of
In the above example, the variation in working length (denoted L in
Without altering the length of the working zone (that is to say using techniques of the prior art) the spread on the tube thickness leads to a spread of ±7.25 m.daN/° (±8%) on torsional stiffness.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0510131 | Oct 2005 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR06/50990 | 10/4/2006 | WO | 00 | 8/22/2008 |
