Abstract:
In order to solve a problem of an electric power steering device of an automobile that the steering feeling is deteriorated by dragging the inactive electric motor if a steering member is operated at a small angle to leave an electric motor inactive while the automobile is running straight, the present invention provides a joint for jointing the rotary shaft of a steering assisting electric motor and a worm shaft of a reduction mechanism, comprising a first member rotating integrally with the rotary shaft; a second member rotating integrally with the worm shaft; and an elastic member for coupling the two members in a torque transmittable manner. The elastic member has a relatively small elastic modulus according to the state that the torsion angle between the first and second members is small. Accordingly, the structure and characteristics thereof lead to a better operationality of the steering member.

Description:
BACKGROUND OF THE INVENTION  
   1. Field of the Invention 
   The present invention relates to an electric power steering device of an automobile and to a joint to be used in the electric power steering device. 
   2. Description of the Related Art 
   In the electric power steering device, a steering assisting force is obtained by an electric motor, the vibrations of which are transmitted through a steering shaft to the compartment of the automobile thereby to cause noises. These noises are serious especially in a hybrid car or an electric car, which intrinsically has a quiet compartment. 
   In an ordinary construction, the rotary shaft of the electric motor is so rigidly connected to a worm shaft by a serration joint or the like as to rotate together, and is connected in a torque transmittable manner to the steering shaft through the worm shaft and a worm wheel. 
   In order to prevent the steering assisting force of the electric motor from being unnecessarily fluctuated by a kickback applied from the road surface through the wheels to the steering shaft, there has been provided an electric power steering device (e.g., Patent Publication 1), in which a torque transmitting elastic member is interposed between the rotary shaft of the electric motor and one end of the worm shaft. 
   [Patent Publication 1] 
   JP-A-2002-145083 
   Here, the aforementioned steering assisting force of the electric motor is generated on the basis of a detection signal from a torque sensor for detecting the torsion of a torsion bar leading to the steering shaft. 
   Therefore, the steering assisting force is not generated in case the steering member such as a steering wheel is manipulated (or steered) by such an extremely small steering angle as hardly steers the steering shaft. 
   In this case, therefore, the steering wheel is steered while dragging (or turning as a load) the rotary shaft of the inactive electric motor. As a result, the starting torque at the time of starting the steering operation of the steering member thereby to deteriorate the steering feeling. 
   This defect likewise occurs in case the rotary shaft of the electric motor and the worm shaft are rigidly connected through the serration joint or elastically connected through the elastic member, as in Patent Publication 1. 
   This is because, even in the electric power steering device of Patent Publication 1, it is necessary for transmitting the torque to use an elastic member having considerably hard characteristics, which is hardly deformed at a small steering angle. 
   SUMMARY OF THE INVENTION  
   The present invention has been conceived in view of the problems thus far described and has an object to provide an electric power steering device capable of improving the steering feeling of a small-angle steering at a straight running time, and a joint to be used in the electric power steering device. 
   According to the invention of claim  1 , there is provided an electric power steering device for transmitting the rotation of a steering assisting electric motor to a steering mechanism through a small gear and a large gear, comprising: a first member disposed on the rotary shaft of said electric motor; a second member disposed at one end of the small gear; and an elastic member interposed between the first and second members for transmitting a torque between the two members, wherein said elastic member has a first elastic modulus corresponding to the case, in which the torsion angle between the first and second members is less than a predetermined angle, and a second elastic modulus corresponding to the case, in which said torsion angle is not less than the predetermined angle, and wherein said first elastic modulus is smaller than said second elastic modulus. 
   In the invention, in case the steering member is steered at a small angle while the vehicle is running straight, the elastic member can be easily deformed at the smaller first elastic modulus. Within this small angle range, the steering member can be steered without dragging the rotary shaft of the inactive electric motor. As a result, the starting torque of the steering member at the time of starting the steering operation from the straight running state does not rise so that the steering feeling is improved. 
   In claim  1 , according to the invention of claim  2 : said first and second members have protrusions for meshing in a torque transmittable manner while clamping the elastic member in the rotational direction of the rotary shaft; the corresponding protrusions of the first and second members have clamping faces acting as torque transmission faces capable of clamping the elastic member inbetween; and at least either said clamping faces and the clamped faces as the torque transmission faces of the elastic member confronting the former are provided with projections for establishing the first elastic modulus by compressing the elastic member locally when said torsion angle is less than the predetermined angle. In the invention, the two-stage characteristics of the elastic modulus of the elastic member can be easily achieved by the simple structure, in which the projections are merely formed on the elastic member or the protrusions of the first and second members. 
   In claim  1 , according to the invention of claim  3 : said first and second members have a plurality of projections for meshing in a torque transmittable manner while clamping the elastic member in the rotating direction of the rotary shaft; said elastic member has a cavity portion; and the elastic member is compressed to bury the cavity portion substantially when said torsion angle is not less than the predetermined angle. In the invention, the two-stage characteristics of the elastic modulus of the elastic member can be easily achieved by the simple structure, in which the cavity portion is merely formed in the elastic member. For example, the elastic member is provided with a portion, which is bifurcated into two branches across a slit to form the cavity portion. 
   According to claim  4 , there is provided a joint for use in an electric power steering device according to claim  1 ,  2  or  3 , comprising: a first member; a second member; and an elastic member interposed between the first and second members for transmitting a torque between the two members, wherein said elastic member has a first elastic modulus corresponding to the case, in which the torsion angle between the first and second members is less than a predetermined angle, and a second elastic modulus corresponding to the case, in which said torsion angle is not less than the predetermined angle, and wherein said first elastic modulus is smaller than said second elastic modulus. 
   According to the invention, it is possible to achieve the aforementioned effects of claim  1 ,  2  or  3  by the elastic member. Moreover, the rotary shaft and the small gear can be exemplified by those of the prior art having the same standard sizes so that the electric motor and the small gear can be made inexpensive. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic diagram showing a schematic construction of an electric power steering device according to one embodiment of the invention; 
       FIG. 2  is a sectional view of an essential portion of the electric power steering device of  FIG. 1 ; 
       FIG. 3  is an exploded perspective view of a joint; 
       FIG. 4  is a sectional view taken along line IV—IV of  FIG. 2 ; 
       FIG. 5  is a graphic diagram illustrating a relation between a torsion angle θ between the first and second members of the joint, and a transmission torque T by an elastic member; 
       FIG. 6  is a sectional view of a joint according to another embodiment of the invention; 
       FIG. 7  is a sectional view of a joint according to still another embodiment of the invention; and 
       FIGS. 8A and 8B  are schematic sectional views showing elastic members individually according to further embodiments of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the invention will be described with reference to the accompanying drawings. 
     FIG. 1  is a schematic diagram showing a schematic construction of an electric power steering device according to one embodiment of the invention. Referring to  FIG. 1 , this electric power steering device  1  is provided with: a first steering shaft  3  connected integrally and rotatably to a steering member  2  such as a steering wheel; a second steering shaft  5  connected coaxially to the first steering shaft  3  through a torsion bar  4 ; and a steering mechanism  6  having a rack-and-pinion mechanism leading to the second steering shaft  5 . 
   This steering mechanism  6  is provided with a steering axle  7  arranged to extend transversely of a vehicle, and knuckle arms  10  jointed to the two ends of the steering axle  7  through tie rods  8  thereby to support steering wheels  9 . The steering axle  7  is so supported by the not-shown housing as to slide in the axial direction. At a portion of the steering axle  7 , there is formed a rack  7   a  meshing with a pinion  11 , which is so disposed at the end portion of the second steering shaft  5  as to rotate together with the second steering shaft  5 . 
   When the steering member  2  is operated to rotate the first and second steering shafts  3  and  5 , this rotation is converted by the pinion  11  and the rack  7   a  into linear motions of the steering axle  7  in the transverse directions of the vehicle. As a result, the steering wheels  9  are steered. 
   A torque sensor  12  is provided for detecting the steering torque with the relative rotation displacement is through the torsion bar  4  between the first and second steering shafts  3  and  5 . The torque detection result of this torque sensor  12  is given to a control unit  13 . On the basis of the torque detection result, the control unit  13  controls a voltage to be applied to a steering assisting electric motor  15  through a driver  14 . The rotation of the rotary shaft  16  of the electric motor  15  is transmitted through a joint  17  to a reduction mechanism  18 , by which it is reduced in speed and transmitted to the second steering shaft  5  thereby to assist the steering operation. The reduction mechanism  18  is provided with a worm shaft  19  as a small gear and a worm wheel  20  as a large gear, which mesh with each other. 
   With reference to  FIG. 2 , the worm shaft  19  is coaxially jointed through the joint  17  to the rotary shaft  16  of the electric motor  15  and has worm teeth  24  formed integrally at its axial intermediate portion. 
   The worm wheel  20  is constructed, although not shown, to include: an annular mandrel so jointed to the second steering shaft  5  as to rotate together, and an annular synthetic resin member having teeth formed on its outer circumference and fitted on the mandrel. This mandrel is inserted into a mold, for example, when the synthetic resin member is molded of the resin. 
   The worm shaft  19  is arranged to intersect the core of the second steering shaft  5 . The worm shaft  19  has first and second end portions  21  and  22  opposed to each other in the axial direction. Worm teeth  24  are formed at an intermediate portion  23  between those end portions  21  and  22 . These first and second end portions  21  and  22  are radially reduced from the intermediate portion  23 , and positioning step portions  25  and  26  are formed between the intermediate portion  23  and the end portions  21  and  22 , respectively. 
   The first and second end portions  21  and  22  of the worm shaft  19  are rotatably supported by respectively corresponding first and second bearings  27  and  28 . 
   The first bearing  27  is made of a rolling bearing such as a ball bearing and is provided with an inner ring  29 , an outer ring  30  and rolling elements  31 . The inner ring  29  of the first bearing  29  is fitted and fixed on the first end portion of the worm shaft  19 . One end portion of the inner ring  29  of the first bearing  27  abuts against the positioning step portion  25  of the worm shaft  19  thereby to regulate the movement of the worm shaft  19  in a first axial direction X (to the motor in the axial direction). 
   On the other hand, the outer ring  30  of the first bearing  27  is fitted in a bearing hole  33  of a housing  32  for housing the reduction mechanism  18 . One end portion of the outer ring  30  of the first bearing  27  is positioned in abutment against a positioning step portion  34  of the housing  32  thereby to regulate the movement of the outer ring  30  in the first axial direction X. 
   The second bearing  28  is made of a rolling bearing such as a ball bearing and is provided with an inner ring  35 , an outer ring  36  and rolling elements  37 . The inner ring  35  of the second bearing  28  is fitted and fixed on the second end portion of the worm shaft  19 . One end portion of the inner ring  35  of the second bearing  28  abuts against the positioning step portion  26  of the worm shaft  19  thereby to regulate the movement of the worm shaft  19  in a second axial direction Y (opposed to the first axial direction X). As a result, the intermediate portion  23  of the worm shaft  19  is axially clamped between the inner rings  29  and  35  of the first and second bearings  27  and  28 . 
   On the other hand, the outer ring  36  of the second bearing  28  is fitted in a bearing hole  38  of the housing  32 . This bearing hole  38  leads to a threaded hole  39  opened to the outside, and a pre-load adjusting member  41  having a threaded portion on its outer circumference is screwed into that threaded hole  39 . 
   This adjusting member  41  abuts against one end portion of the outer ring  36  of the second bearing  28  thereby to bias the outer ring  36  in the first axial direction X. This biasing force by the adjusting member  41  is applied to the outer ring  30  through the rolling elements  37  and the inner ring  35  of the second bearing  28 , the worm shaft  19 , and the inner ring  29  and the rolling elements  31  of the first bearing  27 , and the movement of the outer ring  30  in the first axial direction is regulated by the positioning step portion  34 . Therefore, the pre-load according to the screwed position of the adjusting member  41  is applied to the first and second bearings  27  and  28 . Here in  FIG. 2 , reference numeral  42  designates a lock nut for locking the screwed position of the adjusting member  41 . This lock nut  42  is screwed into the threaded portion of the adjusting member  41 . 
   The joint  17  is provided with: a first member  43  so connected to the rotary shaft  16  of the electric motor  15  as to rotate together; a second member  44  so connected to the first end portion  21  of the worm shaft  19  as to rotate together and confronting the first member  43 ; and an elastic member  45  interposed between the first and second members  43  and  44  thereby to transmit the torque between the two members  43  and  44 . The first and second members are made of a metal, for example, and the elastic member  45  is made of synthetic resin or synthetic rubber having elasticity. 
   The first member  43  is formed of an annular member having a connection hole  46  such as a press-fit hole at its center. The rotary shaft  16  of the electric motor  15  is press-fitted in the connection hole  46  so that the first member  43  and the rotary shaft  16  are so jointed as to rotate together. 
   The second member  44  is formed of an annular member having a joint hole  47  such as a serrated hole at its center. In this joint hole  47 , there is inserted the protruding portion of the first end portion of the worm shaft  19  from the first bearing  27 , so that the second member  44  and the worm shaft  19  are jointed integrally rotatably by the serration joint, for example. 
   With reference to  FIG. 3  presenting an exploded perspective view of the joint  17  and  FIG. 4  presenting a sectional diagram taken along line IV—IV of  FIG. 2 , the elastic member  45  is formed in a star shape having a plurality of plate-shaped clamped portions  49  extending radially from the body portion  48  at its center. 
   With reference to  FIG. 3 , the axial opposed portions  43   a  and  44   a  of the first and second members  43  and  44  have a plurality of protrusions  50  and  51 , respectively, at a circumferentially equal spacing, and these protrusions  50  and  51  are made to mesh with each other in a staggered relation to clamp the corresponding clamped portions  49  of the elastic member  45 , as shown in  FIG. 4 . 
   With reference to  FIG. 4 , the individual protrusions  50  and  51  are provided with clamping faces  52  and  53  acting as torque transmission faces, which are formed of flat faces opposed in the circumferential direction when the protrusions  50  and  51  are in meshing engagement with each other. Between these clamping faces  52  and  53 , there are clamped the corresponding clamped portions  49  of the elastic member  45 . 
   On the other hand, the clamped portions  49  of the elastic member  45  have clamped faces  54  and  55  as the torque transmission faces, which are opposed to the clamping faces  52  and  53  as the toque transmission faces of the protrusions  50  and  51 . The clamped faces  54  and  55  have projections  56  formed at portions close to the leading ends of the clamped portions  49 . Specifically, the clamped portions  49  are gradually narrowed from the root end portions to the leading end portions but are widened at the portions of the projections  56 . 
   When the angle of torsion between the first and second members  43  and  44  is small, the projections  56  of the clamped faces  54  or  55  of the elastic member  45  come exclusively into abutment against the corresponding clamping faces  52  or  53  in accordance with the torsional direction so that the elastic member  45  is locally compressed. When the torsion angle is large, on the other hand, the clamped faces  54  or  55  of the elastic member  45  come generally wholly into abutment of the corresponding clamped faces  52  or  53  so that the elastic member  45  is wholly compressed. As a result, the elastic member  45  has an elastic modulus of two-stage characteristics with respect to the torsion angle. 
   Specifically, reference is made to  FIG. 5  illustrating a relation between the torsion angle θ between the first and second members  43  and  44  and the transmission torque T by the elastic member  45 . When the torsion angle θ is smaller than a predetermined angle θ 1  (θ&lt;θ 1 ), a smaller first elastic modulus K 1  is achieved as the changing rate of the transmission torque T to the torsion angle θ. When the torsion angle θ is not smaller than the predetermined angle θ 1  (θ≧θ 1 ), a larger second elastic modulus K 2  is achieved as the changing rate of the transmission torque T. The first elastic modulus K 1  is set smaller by 1/20 to 1/50, for example, than the second elastic modulus K 2  (K 1 &lt;K 2 ). 
   According to this embodiment, in case the steering member  2  is steered at a small angle while the vehicle is running straight, and when the electric motor  15  is in an inactive state establishing no steering assisting force, the steering member  2  can be lightly operated by deforming the elastic member  45  easily at the smaller first elastic modulus K 1  without dragging the inactive electric motor  15  (i.e., without turning it as a load). Therefore, the starting torque of the steering member  2  at the time of starting the steering operation from the straight running state does not rise so that the steering feeling is improved. 
   Especially, the elastic member  45  can be given the two-stage characteristics of the first and second elastic moduli K 1  and K 2  by the simple structure, in which the projections  56  are merely formed on the clamped faces  54  and  55  as the torque transmission faces of the elastic member  45 , as opposed to the clamping faces  52  and  53  as the torque transmission faces of the protrusions  50  and  51  of the first and second members  43  and  44 . Moreover, the elastic member  45  may be a homogeneous member so that the manufacturing cost can be lowered. 
   Here, each of the clamped faces  54  and  55  may have a plurality of projections  56 . 
   Next,  FIG. 6  shows another embodiment of the invention. With reference to  FIG. 6 , this embodiment is different from that of  FIG. 4  in the following point. In the embodiment of  FIG. 4 , the projections  56  are formed on the clamped faces  54  and  55  as the torque transmission faces of the elastic member  45 . In this embodiment, on the contrary, the two-stage characteristics of an elastic member  45 A is achieved by forming projections  56 A on the clamping faces  52  and  53  as the torque transmission faces of the protrusions  50  and  51  of the first and second members  43  and  44 . Each of the clamping faces  52  and  53  may have a plurality of projections  56 A. Moreover, the description of the components of  FIG. 6  similar to those of the embodiment of  FIG. 4  is omitted by designating them by the common reference numerals. 
   Next,  FIG. 7  shows still another embodiment. With reference to  FIG. 7 , this embodiment is different from that of  FIG. 4  in the following point. In the embodiment of  FIG. 4 , the projections  56  are formed on the clamped faces  54  and  55  as the torque transmission faces of the elastic member  45 . In this embodiment, those projections are omitted, and a slit  57  is formed as a cavity portion extending from the leading end of the clamped portion  49  toward the body portion  48  of an elastic member  45 B, thereby to form a pair of branch portions  58  and  59  bifurcated across the slit  57 . The outer side portions of the branch portions  58  and  59  are diverged the more to the leading end. The description of the components of  FIG. 7  similar to those of the embodiment of  FIG. 4  is omitted by designating them by the common reference numerals. 
   In this embodiment, when the torsion angle between the first and second members  43  and  44  is small, the paired branch portions  58  and  59  approach each other while reducing the slit  57 , so that they are not seriously compressed. After the torsion angle became so large that the slit  57  substantially disappeared, however, the individual branch portions  58  and  59  are subject to the ordinary compression. In this embodiment, too, the two-stage characteristics of the elastic member  45 B can be achieved by the simple structure having the slit  57 . 
   Here in this embodiment, the cavity portion can be exemplified by a foam portion  60 , which is formed by foaming a portion of an elastic member  45 C, as shown in  FIG. 8A . 
   The invention should not be limited to the foregoing individual embodiments but is also enabled to achieve the two-stage characteristics of the elastic modulus by using an elastic member  45 D of a laminated structure, in which first layers  61  of a smaller elastic modulus are arranged individually on the clamped faces  54  and  55  and in which a second layer  62  of a larger elastic modulus is arranged as a core layer between the first layers  61 . 
   It is also arbitrary to give the second elastic modulus either characteristics of a plurality stages or characteristics, in which the elastic modulus increases continuously and progressively. 
   Moreover, the protrusions  56  of each of the first and second members  43  and  44  may be single or three or more. In addition, the small gear and the large gear thus far described may be variously modified within the scope of the claims of the invention into parallel axis gears such as spur gears or helical gears, gears having intersecting axes such as bevel gears, or skew gears such as hypoid gears.