Abstract:
There is provided a steering wheel. The steering wheel includes: a rim formed in an annular shape, which is to be gripped by a driver; a hub disposed at the center of the rim in a radial direction of the rim; a spoke connecting the hub to the rim in the radial direction; and a metal core member provided inside the rim, the hub, and the spoke. The metal core member includes: a hub core member provided inside the hub; a spoke core member provided inside the spoke; and a rim core member provided inside the rim. The thickness of the hub core member at the center portion thereof is larger than the thickness of the hub core member at the outer circumferential end portion thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 USC §119 from Japanese Patent Application No. 2011-238419, filed on Oct. 31, 2011, the entire contents of which are hereby incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a steering wheel which is held by a steering shaft of a vehicle body and allows a driver to perform drive manipulations such as steering. 
       BACKGROUND OF THE INVENTION 
       [0003]    Conventionally, an annular steering wheel is attached to a sheering shaft of a vehicle to allow a driver to perform drive manipulations such as steering. This steering wheel is configured such that an electrical devices manipulation unit including various switches such as a wiper switch and a turn signal switch, an airbag unit, a cover, etc. are attached to a metal core member (see JP-A-2009-214734, for example). 
         [0004]    However, in the metal core member of the above conventional steering wheel, a hub core member which engages with the steering wheel is generally shaped like a rectangular parallelepiped, as a result of which the material cost is high and a steering manipulation requires a strong force. 
         [0005]    On the other hand, in recent years, steering wheels have been developed which incorporate an airbag device for a driver which serves to confine the driver. However, the incorporation of the airbag tends to increase the weight of the entire steering wheel. Furthermore, since the metal core member needs to strong enough to endure a load that is exerted on the steering wheel when the airbag is developed, the hub core member is made thicker and heavier, which is also a factor in increasing the weight of the entire steering wheel. 
       SUMMARY OF THE INVENTION 
       [0006]    An object of the present invention is to provide a metal core member which can be reduced in weight while securing necessary strength. 
         [0007]    According to one or more illustrative aspects of the present invention, there is provided a steering wheel. The steering wheel includes: a rim formed in an annular shape, which is to be gripped by a driver; a hub disposed at the center of the rim in a radial direction of the rim; a spoke connecting the hub to the rim in the radial direction; and a metal core member provided inside the rim, the hub, and the spoke. The metal core member includes: a hub core member provided inside the hub; a spoke core member provided inside the spoke; and a rim core member provided inside the rim. the thickness of the hub core member at the center portion thereof is larger than the thickness of the hub core member at the outer circumferential end portion thereof 
         [0008]    According to one or more illustrative aspects of the present invention, the metal core member further comprises: a cylindrical boss formed in a cylindrical shape and projecting from the center of the hub core member and configured to engage with the steering shaft. A rear surface of the hub core member, which faces the steering shaft, is inclined. 
         [0009]    According to one or more illustrative aspects of the present invention, the rear surface of the hub core member is curved. 
         [0010]    According to one or more illustrative aspects of the present invention, the hub core member has generally flat uniform thickness portions which are arranged in a left-right direction in a state that the steering wheel is at a reference rotation position where it causes a vehicle provided with the steering wheel to run straight, and uniform thickness portions have penetration holes through which harness lead pipes which project from an electrical devices manipulation unit having a wiper switch are inserted, respectively. 
         [0011]    According to one or more illustrative aspects of the present invention, the thickness of the hub core member at an end portion thereof is larger than or equal to 55% of the thickness of the hub core member at the center thereof. 
         [0012]    According to one or more illustrative aspects of the present invention, the thickness of the hub core member at the center thereof is 14 mm or more, and the thickness of the hub core member at the end portion thereof is in a range of 8.5 mm to 10 mm. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a front view showing the overall structure of a steering wheel according to an embodiment of the present invention; 
           [0014]      FIG. 2  is a front view of a metal core member; 
           [0015]      FIG. 3  is a rear view of the metal core member; 
           [0016]      FIG. 4  is a side view of the metal core member; 
           [0017]      FIG. 5  is a side view of the metal core member which is placed with its front surface down; 
           [0018]      FIG. 6  is a rear perspective view of the metal core member; 
           [0019]      FIG. 7  is an enlarged view, corresponding to  FIG. 6 , of an important part of the metal core member; and 
           [0020]      FIG. 8  is a perspective view showing a relationship between the metal core member and a switch unit. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    Exemplary embodiments of the present invention will be hereinafter described with reference to the drawings. 
         [0022]    A steering wheel  101  according to an embodiment of the present invention will be hereinafter described with reference to the drawings.  FIG. 1  is a front view showing the overall structure of the steering wheel  101  according to the embodiment. As shown in  FIG. 1 , the steering wheel  101  is equipped with a annular-like rim  102 , a hub  103  which is disposed approximately at the center of the rim  102 , and plural (in this example, three) spokes  104  which link the hub  103  to the rim  102  in the radial direction. A front cover  105  is attached to a front portion of the hub  103 . The rim  102  is formed with a grip  107  which is made of a resin material, for example. 
         [0023]    The steering wheel  101  has a metal core member  10  inside. The structure of the metal core member  10 , which is an essential element of the embodiment, will be described below with reference to  FIGS. 2-8 .  FIGS. 2-4  are a front view, a rear view, and a side view of the metal core member  10 , respectively.  FIG. 5  is a side view of the metal core member  10  which is placed with its front surface down.  FIG. 6  is a rear perspective view of the metal core member  10 .  FIG. 7  is an enlarged view, corresponding to  FIG. 6 , of an important part of the metal core member  10 .  FIG. 8  is a perspective view showing a relationship between the metal core member  10  and a switch unit. 
         [0024]    As shown in  FIGS. 2-7 , the metal core member  10 , which is an integral member made of, for example, a metal or an alloy containing magnesium, is composed of a hub core member  20  which is provided inside the hub  103  and held through engagement with a tip portion of a steering shaft (not shown), a rim core member  40  which is provided inside the rim  102 , supported by plural (in this example, three) spoke core members  30 , and shaped like a ring so as to surround the hub core member  20 , and the spoke core members  30  which are provided inside the respective spokes  104  and link the hub core member  20  to the rim core member  40  in the radial direction. 
         [0025]    In the hub core member  20  which is generally rectangular in a front view, a generally cylindrical boss portion  22  is formed at the center (the rotation center as defined by the spoke core members  30 ) so as to be integral with the other portion and to project rearward. An insertion hole  22   a,  into which a tip portion of the steering shaft is to be inserted, penetrates through the boss portion  22 . Seat portions  24  are formed on both sides of the boss portion  22 . The seat portions  24  are flat (i.e., uniform in thickness) and extend in the left-right direction of the vehicle body (in the vehicle width direction) in a state that the steering wheel  101  is at a reference rotation position where it causes the vehicle to run straight (see  FIGS. 5-8 ). A fixing hole  24   a  and a harness penetration hole  24   b  are formed through each seat portion  24 . Hub surfaces  26  are formed as back surfaces of the hub core member  20  so that the hub core member  20  is thick near the boss portion  22  which is a central portion to engage with the steering shaft and decreases gradually in thickness as the position goes outward in the radial direction to the outside edge. A front surface  20   a  of the hub core member  20  is flat to enable attachment of an airbag device (not shown) or the like. 
         [0026]    As shown in  FIG. 5 , each hub surface  26  is inclined from a thick portion  26   a  which is adjacent to the boss portion  22  to a thin portion  26   b.  It is desirable that each hub surface  26  have a curved surface extending from the thick portion  26   a  and the thin portion  26   b.  In this case, where the metal core member  10  is made of magnesium, it is desirable that the thickness T2 of the thin portion  26   b  be greater than or equal to 55% of the thickness T1 of the thick portion  26   a  (see  FIG. 5 ). More specifically, it is preferable that the thickness T1 of the maximum thickness portion of the thick portion  26   a  be greater than or equal to 14 mm and the thickness T2 of the minimum thickness portion of the thin portion  26   b  be in the range of 8.5 mm to 10 mm. In this case, the thickness T1 of the thick portion  26   a  is not decreased starting from the boundary with the boss portion  22  and, instead, the thickness T1 of the thick portion  26   a  is kept approximately equal to the maximum thickness. As a result, each hub surface  26  is generally shaped like a bowl. 
         [0027]    The spoke core members  30  are left and right core members  32  which extend in the left-right direction of the vehicle body (in the vehicle width direction) in a state that the steering wheel  101  is at a reference rotation position where it causes the vehicle to run straight and a front-rear core member  34  which extends in the front-rear direction of the vehicle body in the same state. 
         [0028]    The rim core member  40  is circular and connected to tip portions of the respective spoke core members  30 . The rim core member  40  is circular or rectangular, for example, in cross section. 
         [0029]    Incidentally, in tests which are carried out in a state that the airbag mounted in the steering wheel  101  has not been activated (i.e., it is not developed), a heavy load is imposed on the steering wheel  101 . To increase the durability of the metal core member  10 , the sub surfaces  26  which are peripheral portions of the hub core member  20  need to be thick. Conventionally, each hub surface  26  including its portion adjacent to the boss portion  22  had a constant thickness of about 15 mm. This increased the weight of the hub core member  20 . Thus, it was difficult to reduce the weight of the hub core member  20 . 
         [0030]    In view of the above, the present inventors and other persons analyzed stress distributions with application of a load by a known CAE analysis and have found that weight reduction can be attained while the metal core member  10  is prevented from breaking even when the airbag is developed by setting the thickness of the thick portion  26   a  which is adjacent to the boss portion  22  (the central portion of the hub core member  20 ) on which highest stress is exerted greater than or equal to 10 mm and setting the thickness of the thin portion  26   b  which is distant from the boss portion  22  and on which only low stress is exerted smaller than or equal to 10 mm. We have also found that a stress reducing effect can be obtained by forming a curved surface having an R (a gentle mountain shape in a side view) from the thick portion  26   a  to the thin portion  26   b.    
         [0031]    For example, in the above-configured metal core member  10 , as shown in  FIG. 8 , an electrical devices manipulation unit  16  including various switches such as a wiper switch  12  and a turn signal switch  14  is attached to the seat portions  24 . 
         [0032]    In this case, the electrical devices manipulation unit  16  can be attached to the seat portions  24  stably because the seat portions  24  are flat though the hub surfaces  26  have curved surfaces. The electrical deices manipulation unit  16  is attached to the seat portions  24  by inserting bolts or the like into the fixing holes  24   a.  At the same time, projected, circular-rod-shaped harness lead pipes  16   a  are inserted into the harness penetration holes  24   b,  as a result of which a power cable, a wire harness, etc. provided inside each harness lead pipe  16   a  can be connected to the airbag device, a horn switch, etc. (not shown) which are mounted on the front surface  20   a  of the hub core member  20 . 
         [0033]    As described above, in the embodiment, the hub core member  20  of the metal core member  10  has the thick portions  26   a  as portions adjacent to the boss portion  22  which is to engage with the steering shaft. With this measure, the portions, adjacent to the boss portion  22 , of the hub core member  20  are made as thick as in the conventional structure and necessary strength can thereby be secured. More specifically, when a load is imposed on the steering wheel  101  in a state that the airbag has not been activated (i.e., it is not developed), the thick portions  26   a  of the hub core member  20  can absorb the load to prevent durability reduction of the hub core member  20 . Since the thickness of the hub core member  20  is decreased gradually from the thick portions  26   a  which are adjacent to the boss portion  22  to the outside edges past the thin portions  26   b,  respectively, the amount of material used is reduced and the weight can be reduced accordingly. 
         [0034]    In the embodiment, in particular, the generally cylindrical boss portion  22  which projects rearward at the center of the hub core member  20  and is to engage with the steering shaft is integral with the other portion of the hub core member  20 , and each hub surface (back surface)  26  of the hub core member  20  is inclined from the thick portion  26   a  to the thin portion  26   b  to form a curved surface. As stress can be diverted to the back side of the hub core member  20  and the degree of stress concentration on the central portion of the hub core member  20  can be reduced. Thus, the durability reduction of the hub core member  20  can be suppressed even more effectively. 
         [0035]    In the embodiment, in particular, the harness insertion holes  24   b  into which the harness lead pipes  16   a  are inserted are formed through the seat portions  24  of the hub core member  20 . With this measure, whereas weight reduction is attained by forming the thickness portions  26   b  in the above described manner, a necessary length in the penetration direction of the harness insertion holes  24   b  through which the harness lead pipes  16   a  are inserted can be secured by leaving only those portions of the hub core member  20  which are to be penetrated by the harness lead pipes  16   a  thick and uniform in thickness. As a result, the stability of the penetration structure including the harness lead pipes  16   a  can be increased and the work of attaching the electrical devices manipulation unit  16  to the steering wheel  101  to form an integral structure can be made easier. 
         [0036]    Although the embodiment is directed to the case of using magnesium as a material, a magnesium alloy, aluminum, or an aluminum alloy may be used instead. Where any of these alternative materials is used, the above-mentioned specific numerical values of dimensions may be modified as appropriate taking a strength difference between the materials into consideration. 
         [0037]    In the invention, the structures of the spoke core members  30  and the rim core member  40  are not limited to the ones described in the embodiment and their shapes and sizes can be modified as appropriate according to switches and an airbag incorporated. 
         [0038]    In addition, features of the above-described embodiment and modifications may be combined as appropriate. 
         [0039]    Furthermore, although not described specifically in this specification, various other modifications can be made without departing from the spirit and scope of the invention. 
         [0040]    While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the invention. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the sprit of the invention. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and sprit of the invention.