Abstract:
A carcass-free steering wheel for motor vehicles consists of a pot-shaped lower part ( 1 ) and an upper part ( 2 ) connected thereto along its edge side, both parts ( 1, 2 ) having an outer shell ( 3, 4 ) of a high-performance plastic material, which at least in the case of the lower part ( 1 ) is reinforced by webs ( 5, 6, 7 ) pointing inwards and being molded on integrally.

Description:
FIELD OF THE INVENTION 
     The invention relates to a steering wheel for motor vehicles, consisting of a pot-shaped lower part and an upper part connected thereto along its edge side, both parts having an outer shell of high-performance plastic material. Such a steering wheel is known from DE-A-21 29 286, for example. 
     BACKGROUND OF THE INVENTION 
     Steering wheels for motor vehicles consist generally of a metallic steering wheel carcass, a casing of foamed plastic material having a leather-like outer skin and an airbag housed in the steering wheel and having a cover visually and haptically adapted to the outer skin of the steering wheel. The carcass may consist of steel, aluminum, magnesium or combinations of these metals and is designed such that it can transmit all forces occurring while driving. Furthermore, today&#39;s steering wheels are often still designed in such a way that they can deform specifically during an accident when the drier strikes the steering wheel. Together with the abruptly inflating airbag the forces acting upon the casualty shall be reduced by this and injuries shall be reduced and prevented in the optimum case. 
     The production of such steering wheels is relatively complicated because in addition to the line for producing the metallic carcasses a second production line must be established for sheathing the carcasses with a suitable, foamable plastic material forming a tight, abrasion-resistant outer skin. If the steering wheel carcasses are made by casting, they will have to be freed from nipples or flashes before they can be inserted in the mold to be sheathed with plastics. Steering wheels which consist of prefabricated hubs, spokes and steering wheel rims require additional riveting, bolting or welding steps until a carcass ready for sheathing exists. 
     In addition to the expenses regarding product engineering it is considered disadvantageous in connection with conventional steering wheels that the case as such already offers a certain impact protection because the supporting steering wheel carcass is covered at least in all impact areas but does not make a contribution to the structural strength of the steering wheel. This strength has to be achieved by a corresponding cross-sectional design of the steering wheel carcass so that narrow limits are set to the general striving for weight reduction in motor vehicles even if aluminum or magnesium alloys are used. 
     U.S. Pat. No. 5,649,455 discloses an energy-absorbing element which is attached to a steering wheel hub and has honeycomb webs. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a steering wheel which enables further savings of weight without limiting its functions as regards the transmission of forces occurring while driving and as regards the impact protection. 
     In order to achieve this object it is proposed according to the invention that in a steering wheel of the initially mentioned kind at least the outer shell of the lower part is reinforced by webs pointing inwards and being molded on integrally, which webs plastically deform in an impact with taking up strain energy. 
     The invention is based on the consideration that a steering wheel carcass can be dispensed with when suitable high-performance plastic materials are used and by corresponding fashioning care is taken that said main functions can be met sufficiently. In addition, the change from carcass to shell design has the further advantage that less material expense is required to achieve certain strength values with equal materials because the supporting material can be utilized much better in terms of strength when it is disposed in the outer regions of a component. impact protection can also be obtained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Expedient embodiments of the inventive concept are described in the subclaims 2 to 12. Further particulars are explained in more detail by means of the embodiments shown in FIGS. 1 a  to  7 , in which: 
     FIG. 1 a  shows a longitudinal section through a steering wheel according to the invention, 
     FIGS. 1 b - 1   d  show various stages of steering wheel deformation in the case of a crash, 
     FIG. 2 shows a partial section through a steering wheel with airbag installation volume, 
     FIG. 3 shows a top view of half a lower shell of a steering wheel and also shows the plane upon which FIG. 2 is taken, 
     FIG. 4 shows a top view of part of the manufacturing mold, 
     FIG. 5 shows a perspective view of part of a manufacturing mold, 
     FIG. 6 shows a schematic illustration of the plane upon which the steering wheel of FIGS. 1 a - 2  is taken, and 
     FIG. 7 shows a schematic illustration of a part of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The steering wheel shown in section in FIG. 1 a  consists of a lower part  1  and an upper part  2 , both parts  1 ,  2  having an outer shell  3 ,  4  of high-performance plastic material. Both parts  1 ,  2  are connected to each other along their edges. The lower part  1  is reinforced by integrally molded-on webs which protrude inwardly, i.e. into the space between the outer shells  3 ,  4 . As shown, the webs  5 ,  6 ,  7  may have differing wall thicknesses and heights. The webs are preferably produced with a gradually changing deformation resistance. The upper webs  5  have a smaller wall-thickness and are arranged at a greater lateral distance than the webs  6  in the step lying underneath. The webs  7  in the lowermost step are arranged in even more confined fashion and have an even greater wall thickness. It is self-evident that instead of the three steps shown it is also possible to provide two or more than three steps and that webs can be provided which have a wall thickness constantly tapering from bottom to top. It is common practice to realize in a steering wheel impact protection a deformation resistance increasing in the impact direction. 
     In FIGS. 1 b  to  1   d  it is shown how the steering wheel deforms when the outer shell  4  of the upper part  2  is loaded abruptly in an accident. In the case of little load or at the beginning of greater load, the webs  5  of the uppermost layer deform first. Depending on the severeness of the crash and in the course of the crash, respectively, webs  6  and  7  of the middle step and lower step, respectively, deform as well. The outer shells  3  and  4  themselves remain substantially unchanged in this case. In particular the steering wheel carcass is not deformed because for reasons of strength it must be given a relatively high structural reinforcement by webs. FIGS. 1 a  to  1   d  do not give reference numerals because the individual parts are readily clear in synopsis with FIG. 1 a  and here it was only important to explain the step-wise deformation of the webs. 
     FIG. 2 shows how a space  8  free from webs can be provided between the outer shells  3  and  4  to accommodate an airbag module. In such cases, the outer shell  4  of the upper part  2  is, of course, equipped with a cover  9  for the airbag module which is known shall tear open along the predetermined breaking lines  10  and can swing open along hinge lines  11  when the airbag is filled abruptly with gas. 
     It is evident from the top view according to FIG. 3 onto half a lower part  1  for a steering wheel according to the invention how webs  5 ,  6  and  7  are arranged as hexagonal honeycombs. In this case, it was dispensed with to show the differing wall thicknesses of the webs  5 ,  6  and  7 . The outer shell  3  has a first layer of webs  7  all over, which are arranged in a certain narrow distance grid. In some regions, this first layer of webs  7  is superposed by a second layer of webs  6  and a third layer of webs  5 , which are also arranged as hexagonal honeycombs such that the web walls of the superposed layers coincide as much as possible so that the webs having greatest length and greatest diameter of the hexagons fit into the grid of the shorter, narrower webs of the other steps. In the middle there is again provided a space  8  which only has the webs  7  of the lowermost step but not the webs  6  and  5  of the upper steps. 
     It is evident from FIGS. 4 and 5 how the moldings have to be designed to form a hexagonal honeycomb structure of the webs. For forming the webs  5  having the greatest grid size hexagonal moldings  5 ′ are required, the draft required for removing from the mold also having to be taken into consideration. Likewise hexagonal moldings  6 ′ with a corresponding draft are required for the webs  6  having a mean grid distance. Finally, hexagonal moldings  7 ′ are superposed on these moldings  6 ′ for forming webs  7  which are closest together. Both FIG.  4  and FIG. 5 only show part of the molding for forming the honeycomb web structure according to the invention to make comprehension easier. 
     When the inventive concept is realized, it is, of course, also possible to use other web structures. In particular, it is also conceivable to make the draft required in any case much stronger than necessary so as to create webs whose wall thickness changes constantly from bottom to top. In this way, a constantly changing deformation behavior would be obtained. FIG. 7 shows a schematic illustration of a three-dimensional view of the hexagonal webs of FIG. 3 with wall thickness constantly changing from bottom to top.