Patent Application
20170122395
The present invention relates to a torsion spring, in particular in the form of torsion bar or helical spring which is made of fiber-reinforced plastic and which can be produced in cost-efficient manner, and which has improved elastic energy storage capability, in particular in comparison with springs which are composed only of carbon-fiber-reinforced plastic, and to a method for the design of this type of spring.
Springs are frequently used in the chassis of motor vehicles. They are therefore likewise involved in the attempts to achieve lightweight construction which especially apply to the unsprung masses of these chassis. In this context there have already been many proposals for use of fiber-composite materials (FCM). These relate in particular to torsion springs made of carbon-fiber-reinforced plastics (CRP) and of glass fiber-reinforced plastics (GRP). It is particularly difficult here to achieve low-cost manufacture of these components in a manner that provides the correct loading capability.
The difficulty is further increased in that the respective fibers can transmit only tensile or compressive forces, and that therefore the macroscopic shear loading in the spring wire has to be divided into a tension component and a compression component (with respect to the principal stress axes, +−45° to the longitudinal axis in accordance with Mohr's theory of stress).
Known springs made of FCM are intended to accommodate the tensile and compressive force distribution in the material in the most advantageous manner possible by using windings of the fibers at an angle of +/−45° to the longitudinal axis.
Tension fiber windings exclusively at +45° are also known, the shear stress components here being borne by the matrix material or by compressive stresses in the core.
Suitable springs are designed with the intention of achieving homogeneous loading of all of the spring material used. The intention is therefore that there are no defined points of weakness in the material, but that instead application of uniform maximal stress causes the entire material to reach its loading limit. This maximizes utilization of the material, and thus represents the best achievable level of lightweight construction.
EP 0637700 describes a spring design using carbon fibers wound at an angle of from +−30° to +−60° around the longitudinal axis. A characterizing feature is that the number of tension fibers used differs from the number of compression fibers. In particular, the number of compression fibers is increased in relation to the number of tension fibers. More uniform loading of the fibers is intended here, with resultant improved specific utilization of the material used. Although this gave improved utilization of the material, because the fibers in the direction of tension and of compression are used in different quantitative proportions and, respectively, different layer thicknesses, the dependency of material utilization on spring wire diameter is not eliminated.
U.S. Pat. No. 5,603,490 proposes using fibers in the direction of tension only, and using no compression-loaded fibers. The fibers are to be wound in such a way that they are subject only to tension loading. In the case of a hollow spring this would radically lead to failure caused by shear stresses, and for this reason a pressure-resistant core which accommodates the stresses is required here. However, the long-term hydrostatic stress in the core and the shear stress in the wound fiber shell lead to disadvantageous creep of the plastics matrix (epoxy). This solution cannot therefore be used by way of example for an application in vehicle construction (long-term loading caused by the weight of the vehicle). Although use of only one fiber direction optimizes use of fiber potential in respect of tension loading, long-term loading results in severe creep due to the shear stresses, most of which have to be transmitted through the plastics matrix because of the lack of compressive fiber support.
WO 2014/014481 A1 proposes a fiber structure where the number of the fibers in layers and core is a multiple of a shared base number. Use of a plurality of different materials in a spring (e.g. glass, carbon, or a mixture) is moreover disclosed. It is moreover disclosed that the angles of the individual fibers of the fiber plies in relation to the longitudinal axis can alternate (in particular between a positive and negative angle). The core of the spring can be composed of unidirectional fibers, but there is also disclosure of a solid core or a hollow core. A core made of material with shape memory is also proposed. Although it is mentioned that the spring material can be composed of mixed materials, no relevant practical information is provided, and the procedure for, and effect of, a mixed structure remain unclear. The number of fibers arranged in the layers is to be a whole-number multiple of a shared reference base, but the effect of this likewise remains unclear. This arrangement has the disadvantage that the fibers are present in the layers only in numbers derived from whole-number factors, and there is therefore a lack of optimized appropriate adjustment of layer thicknesses.
The spring designs of the prior art do not achieve an optimized level of lightweight construction, because they do not achieve effective utilization of the material used.
The problem that therefore arises is to produce an arrangement of the fibers within a torsion-loaded spring wire where the loading of the compression- and tension-loaded fibers is maximized in accordance with their loading limit, in order to achieve improved mass-based energy storage density. A particular object of the present invention is as far as possible to use only a limited number of different fiber materials, and thus to achieve a low-cost design in relation to the use of materials, and to propose a method for the design of this type of spring. The spring wire in a helical spring takes the form of a wound helix. The spring, specifically the helical spring, has a spring axis around which the spring wire takes the form of a helix. The cross section of the spring wire is preferably a circular annulus, but can also be elliptical or polygonal.
The invention achieves the object with a spring design as claimed in claim 1. The dependent claims disclose advantageous embodiments.
In particular, said object is achieved by achieving the following three sub-objects:
The scope of the torsion-loaded elongate component comprises only the spring support structure, and not the load-introducing elements, for example appropriately designed regions on the spring plate or of the spring restraint.
The following expressions are moreover used in the definitions below:
The considerations below relating to the design method or the fiber arrangement in the spring wire are based on ideal coincidence of the calculated or defined fiber angles αj in the manufactured component. Possible angular displacements, respectively in + and − direction from the calculated values, due to forming operations (for example: winding of the stretched spring wire along the spring axis to give a helix), plant-specific manufacture tolerances (for example: variations in the rotational velocity of the winding plant), or displacements due to handling operations (for example: manual transport of intermediate products) in the manufactured component are not relevant for the purposes of the design method proposed, as long as the absolute angular difference is less than 20°, preferably less than 10°, and very particularly preferably less than 5°.
The preferred method for the design of the spring of the invention provides the following:
Pre-design of the spring on the basis of values derived from experience and in accordance with the prior art. The parameters of this spring are used as starting parameters for the optimization of the spring by the method of the invention.
All of the design steps listed below must be carried out iteratively on their own or repeatedly as an entirety, in order to comply with all of the required design criteria in a loop-based process.
In a first step, the structure of the spring wire is designed in a manner that uses a very stiff fiber material, for example carbon fibers, in at least one tension-loaded group. It is therefore possible to identify, from the design process, a tension-loaded group which has the highest group stiffness. It is then necessary to design at least one compression-loaded group in such a way that it has lower group stiffness. The classification “low” is applied to the compression-loaded group if its group stiffness is at most 90%, preferably at most 80%, and particularly at most 60%, that of the tension-loaded group. For at least one compression-loaded group, therefore, the group stiffness has decreased by at least 10%, preferably at least 20%, and particularly preferably by 40%, in relation to the tension-loaded group with the highest group stiffness. The lower group stiffness is preferably achieved by using, for example, glassfiber material. It is preferable that there are a plurality of tension-loaded groups made of carbon fibers with compression-loaded groups of low group stiffness in contact with one another. It is particularly preferable that all of the tension-loaded groups are composed of carbon fibers and that all of the compression-loaded groups have lower group stiffness.
Pairs are then formed, from the inside toward the outside. The problem of creep of matrix material under long-term loading is generally caused by very high shear stress between the groups in the spring. In order to avoid, or greatly to reduce, shear stresses between the groups, the groups of a pair are to have comparable group extensional stiffness values; these can be influenced by way of example via the cross-sectional area, the fiber content by volume, the fiber angle, or the selection of material. The group ratio is calculated from the two group extensional stiffness values of a pair. The group ratio must lie within a prescribed range. The design method provides that the group ratio GV is in the range 0.2<=GV<=5, preferably 0.5<=GV<=2, and very particularly preferably 0.75<=GV<=1.33.
In order to achieve acceptable group ratios it is necessary by way of example to use appropriately adjusted cross-sectional areas to compensate different material stiffness values of different fiber types. For this, a preferred procedure selects the fiber stiffness in accordance with the loading capability of the fibers. By way of example, carbon fibers (HTCF) have low energy storage density under compressive load, and can preferably therefore be used efficiently for tension-loaded layers. Glass fibers have good compressibility, and can therefore also be used successfully for compression-loaded layers. By way of example, a tension-loaded group made of HTCF fibers and a compression-loaded group made of glass fibers, there being just one layer of each, then form a pair. The tension-loaded group (HTCF) then has higher group stiffness than the compression-loaded group (GF). A preferred group ratio can be established by appropriate adjustment of the layer wall thickness and, with this, the cross-sectional area. The product of group stiffness and relevant cross-sectional area provides the group extensional stiffness. In the case of the pair composed of HTCF and glass fibers, the layer wall thicknesses can by way of example be 1 mm for the tension-loaded group and 2.5 mm for the compression-loaded group. It is thus possible, by way of the relevant cross-sectional areas, to achieve similar group extensional stiffness values for the tension-loaded and the compression-loaded group, and the value of the group ratio is within the desired range. There are therefore almost no shear stresses between the groups, and the spring wire therefore has advantageous creep behavior.
Other preferred embodiments provide the use of one or more intermediate or external layers that are very thin and therefore make hardly any contribution to the support behavior of the component, but which by virtue of their fiber orientation by way of example make a contribution to the transverse stiffness of the spring, or are a terminating layer to counter adverse effects of environmental media. However, the invention intends that only at most 25%, preferably 15%, particularly preferably 5%, of the mass of the spring wire (without core) is composed of layers classified as non-loadbearing.
Preference is further given to the use of a spring wire in which the mass of the compression-loaded groups with lower group stiffness makes up a proportion of at least 20%, preferably 50%, and particularly preferably 95%, based on the mass of all of the compression-loaded groups of the spring wire. A very particularly preferred embodiment of the torsion spring of the invention provides that the tension-loaded groups, from the inside to the outside, have the same group stiffness (within the balance of manufacturing tolerances). In an embodiment to which further particular preference is given, the compression-loaded groups also have, or only these have, the same group stiffness from the inside to the outside (within the balance of manufacturing tolerances). It is further preferable that at least one internally situated compression-loaded group has lower group stiffness than a tension-loaded group situated further outward.
Preference is further given to the use of a spring wire in which at least a proportion of 50% of the total mass of the spring wire (without core), preferably 75%, and particularly preferably 95%, of the groups of the spring wire have been successfully allocated to a pair.
Preference is further given to the application of an exterior plastics ply or ply made of matrix material without fiber reinforcement. However, the method of the invention provides that when loading occurs at least 75%, preferably 85%, particularly preferably 95%, of the elastic energy is stored by the fiber-composite material and not by the exterior plastics ply. Another possibility included here is that the exterior plastics ply fractures, but the fiber-reinforced plies and the optionally present core remain undamaged.
It is preferable to use computer assistance for the computational design of the spring of the invention. After design in accordance with the method of the invention, the spring is manufactured by processes of the prior art.
The invention designs the selection of materials for the load-bearing cross section of the spring (ignoring coatings, etc. which are non-loadbearing) in such a way that the compression- and tension-loaded groups are composed of a small number of different fiber materials. It is preferable to use a low-cost material for the compression-loaded groups, an example being glass fiber (GF). In the region of the tension-loaded fibers it is possible to also use carbon fiber (CF) in order to increase mass-specific energy storage density.
It is preferable that the spring wire of the spring of the invention has a circular cross section. However, elliptical or polygonal cross sections are also possible. The spring optionally has a core. This is preferably composed of fiber-composite material in which the fibers run unidirectionally, parallel to the longitudinal axis. Other preferred embodiments provide a hollow core in which a fiber-composite material or an unreinforced plastics shell surrounds an axial cavity. Preference is further given to a core which is composed entirely of plastic, or a core which is formed exclusively by a cavity.
It is further preferable that the pair ratios of adjacent pairs differ only slightly from one another, so that shear stresses occurring between the pairs are also minimized, and therefore the tendency of the spring wire toward creep is minimized. Said pair ratio is calculated from the stiffness values of the groups of the two pairs.
The spring of the invention is preferably used in vehicle construction, in motor vehicles, and in rail vehicles. However, use is possible in any of the application sectors of helical springs or more generally of torsion springs where the environmental conditions do not attack the materials used in the spring to an unacceptable extent.
Table 1 shows the inventive example 1 of the design method of the invention with a wound textile, where the layers are always arranged in alternation in the form of glassfiber ply (compression-loaded) and carbon-fiber ply (tension-loaded). The table has two parts, and to improve legibility the first four columns containing characterizing information are repeated in the second part.
Table 2 shows the fiber materials used for the inventive example 1, with their properties. The properties are known from the prior art, and have merely been collated here.
Table 3 shows the inventive example 2 of the design method of the invention with a wound textile, where the arrangement has the compression-loaded layers as glassfiber ply or as basalt-fiber ply, and has the tension-loaded layers as carbon-fiber ply. In the fourth ply, the inventive example 2 has fibers oriented along the longitudinal axis. The table likewise has two parts, and to improve legibility the first four columns containing characterizing information are repeated in the second part.
Table 4 shows the fiber materials used for the inventive example 2, with their properties. The properties are known from the prior art, and have merely been collated here.
In all of the inventive examples, the cross-sectional area is calculated by way of the formula for the cross section of a circular annulus. For each inventive example, the specific factual situation is described by using a sectional depiction of the spring wire, a table to describe the properties of the spring wire, and a table to show the relevant properties of the materials.
The inventive example 1 shows a spring wire arrangement of the invention composed of wound textile plies and of a hollow core (
The glassfiber layers and carbon-fiber layers in each inventive example form groups. All of the groups are successfully allocated to pairs. In accordance with the invention, all of the compression-loaded groups have lower group stiffness, and the inventive example 1 therefore provides a preferred variant of a torsion spring of the invention.
The inventive example 2 shows a spring wire arrangement of the invention composed of wound textile plies and of a hollow core (
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2014 211 096.3 | Jun 2014 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/058031 | 4/14/2015 | WO | 00 |
