Patent Publication Number: US-2020290666-A1

Title: Joint for torque transmission and electric power steering device

Description:
TECHNICAL FIELD 
     A torque-transmission joint of the present invention is to be incorporated into a variety of mechanical apparatuses and is to be used for transmitting torque between a drive shaft and a driven shaft, for example. An electric power steering device of the present invention is to be used as a steering device of an automobile and is configured to use an electric motor as an auxiliary power source, thereby reducing a force necessary for a driver to operate a steering wheel, for example. 
     RELATED ART 
     When applying a steering angle to steered wheels (generally, front wheels except a special vehicle such as a forklift), a power steering device has been widely used as a device for reducing a force necessary for a driver to operate a steering wheel. Also, regarding the power steering device, an electric power steering device configured to use an electric motor as an auxiliary power source has also been recently spread. A variety of structures have been known as a structure of the electric power steering device. In any structure, auxiliary power of the electric motor is applied to a rotary shaft, which is configured to rotate in accordance with an operation of the steering wheel and to apply the steering angle to the steered wheels in association with the rotation, via a decelerator. As the decelerator, a worm decelerator is generally used. According to the electric power steering device using the worm decelerator, a worm configured to be rotatively driven by the electric motor and a worm wheel configured to rotate together with the rotary shaft are meshed with each other so that the auxiliary power of the electric motor can be transmitted to the rotary shaft. In the case of the worm decelerator, when reversing a rotating direction of the rotary shaft, an uncomfortable abnormal noise referred to as gear-tooth striking sound may be generated due to a backlash existing at the meshed part between the worm and the worm wheel, if any measures are not taken. 
     In order to suppress the gear-tooth striking sound, it has been considered to elastically press the worm towards the worm wheel by an elastic member such as a spring.  FIGS. 23 and 24  depict an example of the electric power steering device disclosed in Patent Document 1. A steering wheel  1  is mounted to a rear end portion of a steering shaft  2 . A front end portion of the steering shaft  2  is rotatably supported in a housing  3 . A worm wheel  4  is fixed to a part that is to be rotatively driven by the steering shaft  2 . Worm teeth  5  configured to mesh with the worm wheel  4  are provided on an axially intermediate part of a worm shaft  6 . Both axial end portions of a worm  8  configured to be rotatively driven by an electric motor  7  are rotatably supported in the housing  3  by a pair of rolling bearings  9   a ,  9   b  such as deep groove ball bearings. A pressing piece  10  is externally fitted to a leading end portion of the worm shaft  6 , which protrudes beyond the rolling bearing  9   a . An elastic member such as a coil spring  11  is provided between the pressing piece  10  and the housing  3 . By the coil spring  11 , the worm teeth  5  of the worm shaft  6  are pressed towards the worm wheel  4  via the pressing piece  10 . By this configuration, the backlash between the worm teeth  5  and the worm wheel  4  is suppressed, so that the gear-tooth striking sound is suppressed. 
     According to the above structure of the related art, it is possible to suppress the gear-tooth striking sound from being generated at the meshed part between the worm teeth  5  and the worm wheel  4 . However, it is not possible to suppress an abnormal noise, which is to be generated at a joined part between a leading end portion of an output shaft  12  of the electric motor  7  and a base end portion of the worm shaft  6 . This situation is described as follows. In the shown structure, in order to join the leading end portion of the output shaft  12  of the electric motor  7  and the base end portion of the worm shaft  6  so that torque can be transmitted, the base end portion of the worm shaft  6  is formed with an opened spline hole  13 . In the meantime, the leading end portion of the output shaft  12  is formed with a spline shaft part  14 . The spline shaft part  14  and the spline hole  13  are spline-engaged, so that the output shaft  12  and the worm shaft  6  are joined so that the torque can be transmitted. 
     When the spline shaft part  14  and the spline hole  13  are spline-engaged without a circumferential gap (without the backlash), the abnormal noise does not occur at the joined part (the spline engagement part) between the leading end portion of the output shaft  12  and the base end portion of the worm shaft  6 . However, the backlash actually exists at the spline engagement part. In particular, according to the structure where the backlash between the worm teeth  5  and the worm wheel  4  is suppressed by the structure shown in  FIG. 24 , since it is necessary to cause the worm shaft  6  to oscillate and be displaced, it is not possible to completely remove the backlash of the spline engagement part, so that it is difficult to prevent the abnormal noise. 
     Patent Document 2 discloses a structure where the output shaft of the electric motor and the worm shaft are joined via a metallic cylindrical power transmission member for smooth oscillation and displacement of the worm shaft. Also in the structure of Patent Document 2, in order to cause the worm shaft to oscillate and be displaced, the backlashes exist at the spline engagement parts between the spline shaft parts (male splines) provided at both end portions of the power transmission member and the spline holes (female splines) provided at respective end portions of the worm shaft and the output shaft of the electric motor, respectively. For this reason, the abnormal noise may be generated when reversing a rotating direction of the rotary shaft. 
     CITATION LIST 
     Patent Documents 
     Patent Document 1: JP-A-2004-306898 
     Patent Document 2: JP-A-2012-131249 
     SUMMARY OF THE INVENTION 
     Problems To Be Solved by the Invention 
     The present invention has been made in view of the above situations, and an object of the present invention is to implement a structure of a torque-transmission joint capable of suppressing an abnormal noise when reversing a rotating direction of a drive shaft. 
     Means For Solving the Problems 
     A torque-transmission joint is configured to transmit torque between end portions of a drive shaft and a driven shaft disposed in series in an axial direction, and includes a drive-side transmission member, a driven-side transmission member, and a coupling. 
     The drive-side transmission member is supported to an end portion of the drive shaft, and has a drive-side concave-convex part configured by concave parts and convex parts arranged on an outer peripheral surface. 
     Also, the driven-side transmission member is supported to an end portion of the driven shaft and has a driven-side concave-convex part configured by concave parts and convex parts arranged on an outer peripheral surface. 
     Also, the coupling is configured by combining a high-stiffness body and a low-stiffness body having stiffness lower than the high-stiffness body, and has a first coupling-side concave-convex part configured by concave parts and convex parts arranged at one axial end portion of an inner peripheral surface, a second coupling-side concave-convex part configured by concave parts and convex parts arranged at the other axial end portion of the inner peripheral surface, a third coupling-side concave-convex part configured by concave parts and convex parts alternately arranged at an axially intermediate part of the inner peripheral surface, and a partition wall part joined to an axially intermediate portion of the third coupling-side concave-convex part, at least the third coupling-side concave-convex part is formed at the high-stiffness body, and concave parts configuring the first coupling-side concave-convex part and the second coupling-side concave-convex part are formed at the low-stiffness body. In the meantime, the partition wall part may be formed at any one of the high-stiffness body and the low-stiffness body. 
     Also, the drive-side concave-convex part is engaged with the first coupling-side concave-convex part, and the drive-side concave-convex part is engaged to a part, which is located further toward one axial side than the partition wall part, of the third coupling-side concave-convex part in a state where a circumferential gap, which is greater than a circumferential gap (including a zero gap) provided to an engagement part between the drive-side concave-convex part and the first coupling-side concave-convex part (provided between circumferential side surfaces of convex parts configuring the drive-side concave-convex part and convex parts configuring the first coupling-side concave-convex part), is interposed therebetween (a state where the circumferential gap is provided between circumferential side surfaces of the convex parts configuring the drive-side concave-convex part and convex parts configuring the third coupling-side concave-convex part of the one axial side). 
     Also, the driven-side concave-convex part is engaged to the second coupling-side concave-convex part, and the driven-side concave-convex part is engaged to a part, which is located further toward the other axial side than the partition wall part, of the third coupling-side concave-convex part in a state where a circumferential gap, which is greater than a circumferential gap (including a zero gap) provided to an engagement part between the driven-side concave-convex part and the second coupling-side concave-convex part (provided between circumferential side surfaces of convex parts configuring the driven-side concave-convex part and convex parts configuring the second coupling-side concave-convex past), is interposed therebetween (a state where the circumferential gap is provided between circumferential side surfaces of the convex parts configuring the driven-side concave-convex part and convex parts configuring the third coupling-side concave-convex part of the other axial side). 
     Also, at least a part of the partition wall part is arranged between axial end faces of the drive-side transmission member and the driven-side transmission member. 
     When implementing the torque-transmission joint, for example, the high-stiffness body may have a main part having the third coupling-side concave-convex part and the partition wall part provided on an inner peripheral surface thereof, and a pair of sub-parts provided at positions at which the main part is sandwiched from both axial sides. Also, the low-stiffness body may have a drive-side low-stiffness body and a driven-side low-stiffness body. The drive-side low-stiffness body may have the concave parts configuring the first coupling-side concave-convex part at a plurality of circumferential places on an inner peripheral surface thereof, and may be joined to the sub-part of one axial side of the pair of sub-parts. The driven-side low-stiffness body may have the concave parts configuring the second coupling-side concave-convex part at a plurality of circumferential places on an inner peripheral surface thereof, and may be joined to the sub-part of the other axial side of the pair of sub-parts. 
     When implementing the torque-transmission joint, for example, a radially inner side of the partition wall part may be formed with a support hole, and a preload member made of an elastic material for applying an axial preload to the driven shaft in a use state may be internally fitted and supported to the support hole. 
     When implementing the torque-transmission joint, for example, a radial gap may be interposed in at least one engagement part of an engagement part between the drive-side concave-convex part and a part, which is located further toward one axial side than the partition wall part, of the first coupling-side concave-convex part and the third coupling-side concave-convex part and an engagement part between the driven-side concave-convex part and a part, which is located further toward the other axial side than the partition wall part, of the second coupling-side concave-convex part and the third coupling-side concave-convex part (radial gaps may be respectively provided between a radially outer end face of a convex part configuring a radially inner concave-convex part of a pair of concave-convex parts to be engaged with each other and a bottom surface of a concave part configuring a radially outer concave-convex part and between a bottom surface of a concave part configuring the radially inner concave-convex part and a radially inner end face of a convex part configuring the radially outer concave-convex). 
     When implementing the torque-transmission joint, for example, the partition wall part may be formed to extend radially inward from ab axially intermediate part of an inner peripheral surface of the third coupling-side concave-convex part, parts, which are located at both axial sides with the partition wall part being interposed therebetween, of concave parts configuring the third coupling-side concave-convex part are formed to be connected with each other by through-holes formed at a plurality of circumferential places on the partition wall part, and a diameter of an inscribed circle of the respective through-holes may be smaller than a diameter of an inscribed circle (tooth tip circle) of the convex parts configuring the third coupling-side concave-convex part. 
     When implementing the torque-transmission joint, for example, guide concave parts, which are concave in the axial direction, may be provided at at least one portions of portions corresponding to one axial end opening peripheral edge portions of the concave parts configuring the first coupling-side concave-convex part at a plurality of circumferential places on one axial side surface of the coupling and portions corresponding to the other axial end opening peripheral edge portions of the concave parts configuring the second coupling-side concave-convex part at a plurality of circumferential places on the other axial side surface of the coupling. 
     When implementing the torque-transmission joint, for example, a circumferential width dimension of each convex part configuring at least one of the drive-side concave-convex part and the driven-side concave-convex part may vary (without being constant) in the axial direction. 
     In this case, for example, a configuration where the circumferential width dimension of the convex part decreases toward the partition wall part with respect to the axial direction may be adopted. 
     Also, for example, the circumferential width dimension of the convex part may decrease from an axially intermediate part toward both end edge portions. 
     An electric power steering device includes a housing, a rotary shaft, a worm wheel, a worm, and an electric motor. 
     The rotary shaft is rotatably supported to the housing. 
     Also, the worm wheel is coaxially supported to the rotary shaft and is configured to rotate together with the rotary shaft. 
     Also, the worm has worm teeth provided on an axially intermediate part of a worm shaft, and both axial end portions of the worm shaft are rotatably supported to the housing by bearings with the worm teeth being meshed with the worm wheel. 
     Also, the electric motor is configured to rotatively drive the worm with being supported to the housing, for example. 
     An output shaft of the electric motor, which is a drive shaft, and the worm shaft, which is a driven shaft, are connected to each other by a torque-transmission joint so that torque can be transmitted. 
     In the electric power steering device, the above-described torque-transmission joint may be applied. 
     When implementing the electric power steering device, for example, a preload applying mechanism configured to elastically press the worm toward the worm wheel may be provided between a leading end portion of the worm shaft (an end portion opposite to a side joined to the output shaft of the electric motor via the torque-transmission joint) and the housing. 
     Effects of the Invention 
     According to the torque-transmission joint and the electric power steering device configured as described above, it is possible to suppress an abnormal sound when reversing a rotating direction of the drive shaft. 
     That is, when the torque that is to be transmitted between the drive shaft and the driven shaft is small and an amount of circumferential elastic deformation of the low-stiffness body is small, the circumferential side surfaces of the convex parts configuring the drive-side concave-convex part and driven-side concave-convex part are not in contact with the circumferential side surfaces of the convex parts configuring the pair of third coupling-side concave-convex parts (high-stiffness body). Accordingly, it is possible to suppress the abnormal sound even when reversing the rotating direction of the drive shaft. Meanwhile, in this state, the torque is transmitted from the drive-side transmission member to the coupling via the engagement part between the drive-side concave-convex part and the first coupling-side concave-convex part (low-stiffness body), and the torque transmitted to the coupling is transmitted to the driven-side transmission member via the engagement part between the second coupling-side concave-convex part (low-stiffness body) and the driven-side concave-convex part. 
     Also, when the torque that is to be transmitted between the drive shaft and the driven shaft is large, the amount of circumferential elastic deformation of the low-stiffness body increases and the circumferential side surfaces of the convex parts configuring the drive-side concave-convex part and driven-side concave-convex part are in contact with the circumferential side surfaces of the convex parts configuring the pair of third coupling-side concave-convex parts (high-stiffness body). Since the force of the contact is lowered in association with the circumferential elastic deformation of the low-stiffness body, it is possible to suppress the abnormal sound accompanied by the contact. Accordingly, even though the torque increases when reversing the rotating direction of the drive shaft, it is possible to suppress the abnormal sound, Meanwhile, in this state, most of the torque is transmitted from the drive-side transmission member to the coupling via the engagement part between the drive-side concave-convex part and the third coupling-side concave-convex part of one axial side, and most of the torque transmitted to the coupling is transmitted to the driven-side transmission member via the engagement part between the third coupling-side concave-convex part (high-stiffness body) of the other axial side and the driven-side concave-convex part. 
     Also, the partition wall part is provided, so that it is possible to restrain an axial position of the coupling relative to the drive-side transmission member and the driven-side transmission member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view similar to  FIG. 24 , depicting a first example of an embodiment. 
         FIG. 2  is a perspective view of a torque-transmission joint. 
         FIG. 3  is an exploded perspective view of the torque-transmission joint. 
         FIG. 4  is an enlarged view of an A part of  FIG. 1 . 
         FIG. 5  is a sectional view taken along a line B-B of  FIG. 4  (a sectional view taken along a line D-D). 
         FIG. 6  is a sectional view taken along a line C-C of  FIG. 4  (a sectional view taken along a line E-E). 
         FIGS. 7A and 7B  depict a drive-side transmission member, in which  FIG. 7A  is a sectional view taken along a line F-F of  FIG. 7B , and  FIG. 7B  is a view, as seen from the right of  FIG. 7A . 
         FIGS. 8A and 8B  depict a driven-side transmission member, in which 
         FIG. 8A  is a view seen from the left of  FIG. 8B , and  FIG. 8B  is a sectional view taken along a line G-G of  FIG. 8A . 
         FIG. 9A to 9C  depict three examples of a convex part configuring a drive-side concave-convex part (driven-side concave-convex part), as seen from a radially outer side. 
         FIG. 10  depicts a high-stiffness body, as seen from an axial direction. 
         FIG. 11A  is a sectional view taken along a line H-H of  FIG. 10 , and  FIG. 11B  is a sectional view taken along a line I-I of  FIG. 10 . 
         FIG. 12  depicts a drive-side low-stiffness body (driven-side low-stiffness body), as seen from the axial direction. 
         FIG. 13A  is a sectional view taken along a line J-J of  FIG. 12 , and  FIG. 13B  is a sectional view taken along a line K-K of  FIG. 12 . 
         FIG. 14A  depicts a preload member, as seen from the axial direction, and  FIG. 14B  is a view, as seen from the right of  FIG. 14A . 
         FIG. 15  is a perspective view of a torque-transmission joint, depicting a second example of the embodiment. 
         FIG. 16  is an exploded perspective view of the torque-transmission joint. 
         FIG. 17  is an enlarged view equivalent to the A part of  FIG. 1 . 
         FIG. 18  depicts the torque-transmission joint, as seen from one axial side. 
         FIGS. 19A and 19B  depict a drive-side transmission member, in which  FIG. 19A  is a view, as seen from the left of  FIG. 19B  and  FIG. 19B  is a sectional view taken along a line L-L of  FIG. 19A . 
         FIGS. 20A and 20B  depict a driven-side transmission member, in which  FIG. 20A  is a sectional view taken along a line M-M of  FIG. 20B , and  FIG. 20B  is a view, as seen from the right of  FIG. 20A . 
         FIG. 21  depicts a third example of the embodiment, as seen from the axial direction of the high-stiffness body. 
         FIG. 22A  is a sectional view taken along a line N-N of  FIG. 21 , and  FIG. 22B  is a sectional view taken along a line O-O of  FIG. 21 . 
         FIG. 23  is a partially cut side view depicting an example of the conventional structure of an electric power steering device. 
         FIG. 24  is an enlarged sectional view taken along a line P-P of  FIG. 23 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     First Example of Embodiment 
     A first example of an embodiment of the present invention will be described with reference to  FIGS. 1 to 14 . 
     Like the conventional structure shown in  FIGS. 23 and 24 , in an electric power steering device of the first example, a steering wheel  1  is mounted to a rear end portion of a steering shaft  2 , a front end portion of the steering shaft  2  is rotatably supported in a housing  3 , and a worm wheel  4  is fixed to a part that is to be rotatively driven by the steering shaft  2 . Worm teeth  5  configured to mesh with the worm wheel  4  are provided on an axially intermediate part of a worm shaft  6   a . Both axial end portions of a worm  8  configured to be rotatively driven by an electric motor  7  are rotatably supported in the housing  3  by a pair of rolling bearings (ball bearings, in the shown example)  9   a ,  9   b . Also, a preload applying mechanism  15  is provided between the rolling bearing  9   a  externally fitted to a leading end portion of the worm shaft  6   a  and the housing  3 , so that the worm teeth  5  provided on the worm shaft  6   a  are pressed toward the worm wheel  4 . By this configuration, a backlash between the worm teeth  5  and the worm wheel  4  is suppressed, so that a gear-tooth striking sound is suppressed. 
     In the first example, a leading end portion of an output shaft  12   a  of the electric motor  7 , which corresponds to the drive shaft of the claims, and a base end portion of the worm shaft  6   a , which corresponds to the driven shaft of the claims, are joined so that torque can be transmitted via a torque-transmission joint  16 . 
     The torque-transmission joint  16  includes a drive-side transmission member  17 , a driven-side transmission member  18 , a coupling  19 , and a preload member  20 . 
     Meanwhile, in the specification and the claims, regarding the torque-transmission joint  16 , “one axial side” refers to a right side in  FIGS. 2 to 4 , and “the other axial side” refers to a left side of  FIGS. 2 to 4 . 
     The drive-side transmission member  17  has a circular ring shape as a whole, and is externally fitted and fixed (supported so that torque can be transmitted) to the leading end portion of the output shaft  12   a  with relative rotation and axial relative displacement being held back by interference fitting, spline fitting, swaging or the like. An outer peripheral surface of the drive-side transmission member  17  is provided with a drive-side concave-convex part  21  configured by concave parts  41  and convex parts  42  alternately arranged in a circumferential direction. In the first example, a circumferential width dimension W A  (refer to  FIG. 7B ) of the convex part  42  does not change in a radial direction, and does not also change in an axial direction, as shown in  FIG. 9A . The drive-side transmission member  17  is made of synthetic resin in which reinforced fibers are mixed, as necessary, or metal such as iron alloy, copper alloy and aluminum alloy (cast product, forged product, sintered metal product and the like). 
     The driven-side transmission member  18  has a circular ring shape as a whole, and is externally fitted and fixed (supported so that torque can be transmitted) to the base end portion of the worm shaft  6   a  with relative rotation and axial relative displacement being held back by interference fitting, spline fitting, swaging or the like. An outer peripheral surface of the driven-side transmission member  18  is provided with a driven-side concave-convex part  22  configured by concave parts  43  and convex parts  44  alternately arranged in a circumferential direction. In the first example, a circumferential width dimension W B  (refer to  FIG. 9A ) of the convex part  44  does not change in the radial direction, and does not also change in the axial direction, as shown in  FIG. 9A . The driven-side transmission member  18  is made of synthetic resin in which reinforced fibers are mixed, as necessary, or metal such as iron alloy, copper alloy and aluminum alloy (cast product, forged product, sintered metal product and the like). 
     In the first example, the drive-side transmission member  17  and the driven-side transmission member  18  are made to have the same shape and dimension, and axial directions thereof are opposite to each other in an assembled state of  FIG. 4 . For this reason, in the first example, the components can be commonly used for the drive-side transmission member  17  and the driven-side transmission member  18 . 
     The coupling  19  is formed to have a cylindrical shape by a combination of a high-stiffness body  23 , a drive-side low-stiffness body  24 , which is a low-stiffness body, and a driven-side low-stiffness body  25 , which is a low-stiffness body. Specifically, the drive-side low-stiffness body  24  is joined (externally fitted) to one axial end portion of the high-stiffness body  23 , and the driven-side low-stiffness body  25  is joined (externally fitted) to the other axial end portion of the high-stiffness body  23 . 
     The coupling  19  has a first coupling-side concave-convex part  27 , a second coupling-side concave-convex part  28 , a third coupling-side concave-convex part  29 , and a partition wall part  26 . The first coupling-side concave-convex part  27  is configured by concave parts  45  and convex parts  46  alternately arranged in the circumferential direction at one axial end portion of an inner peripheral surface of the coupling  19 . The second coupling-side concave-convex part  28  is configured by concave parts  47  and convex parts  48  alternately arranged in the circumferential direction at the other axial end portion of the inner peripheral surface of the coupling  19 . The third coupling-side concave-convex part  29  is configured by concave parts  49  and convex parts  50  alternately arranged in the circumferential direction at an axially intermediate part of the inner peripheral surface of the coupling  19 . The partition wall part  26  is joined to an axially central part of the third coupling-side concave-convex part  29 . 
     The high-stiffness body  23  has a circular ring shape as a whole, and both axial half parts thereof are symmetric with respect to the axial direction. The high-stiffness body  23  has a main part  30 , a plurality of core parts  31 ,  31 , the third coupling-side concave-convex part  29  and the partition wall part  26 . The main part  30  has a cylindrical shape. The core parts  31 ,  31  protrude axially from a plurality of places equidistantly spaced in a circumferential direction on both axial side surfaces of the main part  30 , and have a fan shape, respectively, as seen from the axial direction. The third coupling-side concave-convex part  29  is configured by the concave parts  49  and the convex parts  50  alternately arranged in the circumferential direction on an inner peripheral surface of the main part  30 . The partition wall part  26  extends radially inward from an axially central part of an inner peripheral surface of the third coupling-side concave-convex part  29 . Meanwhile, in the first example, an aggregate of the respective core parts  31 ,  31  provided at one axial side of the main part  30  and an aggregate of the respective core parts  31 ,  31  provided at the other axial side of the main part  30  correspond to the sub-parts defined in the claims, respectively. Also, the partition wall part  26  has a circular ring shape and is formed at a radially inner side (radially central part) with a circular support hole  32 . Also, a radially outer half part of the partition wall part  26  is formed with through-holes  33 ,  33  having a substantially rectangular (elliptical) shape, which is long in the radial direction, at a plurality of places in alignment with the respective concave parts  49 ,  49 , which configure the third coupling-side concave-convex part  29 , with respect to the circumferential direction. Parts, which are located at both axial sides with the partition wall part being interposed therebetween, of the respective concave parts  49 ,  49  are formed to be connected with each other by the respective through-holes  33 ,  33 . That is, the respective concave parts  49 ,  49  continuously extend over an entire length while longitudinally central portions thereof are not partitioned by the partition wall part  26 . Also, a diameter d A  of an inscribed circle of the respective through-holes  33 ,  33  is smaller than a diameter d B  of an inscribed circle (an inscribed circle of the respective core parts  31 ,  31 ) of the convex parts  50 ,  50  (d A &lt;d B , refer to  FIG. 10 ). The high-stiffness body  23  is made of a material, which is more difficult to be elastically deformed (has higher stiffness) than an elastic material such as rubber-like elastomer configuring the drive-side low-stiffness body  24  and the driven-side low-stiffness body  25 . As the material of the high-stiffness body  23 , an elastic material such as rubber-like elastomer satisfying the above condition, a synthetic resin in which reinforced fibers are mixed, as necessary, and metal such as iron alloy, copper alloy, aluminum alloy, sintered metal and the like may be exemplified. In the meantime, when implementing the present invention, the respective through-holes  33 ,  33  may be omitted (the respective through-holes  33 ,  33  are blocked) to partition the longitudinally (axially) central portions of the respective concave parts  49 ,  49  by the partition wall part  26 . 
     The drive-side low-stiffness body  24  is made of an elastic material such as rubber-like elastomer having stiffness lower than the high-stiffness body  23 , and has a circular ring shape as a whole, and both axial half parts thereof are symmetric with respect to the axial direction. The drive-side low-stiffness body  24  has a coupling cylinder part  34   a  having a short cylinder shape and a plurality of pairs of flat plate-shaped clamped pieces  35   a ,  35   a  extending radially inward and in parallel with each other from a plurality of places equidistantly spaced in the circumferential direction on an inner peripheral surface of the coupling cylinder part  34   a . In the first example, a part of which three surfaces are surrounded by each pair of the clamped pieces  35   a ,  35   a  and the coupling cylinder part  34   a  is the concave part  45  configuring the first coupling-side concave-convex part  27 . That is, the drive-side low-stiffness body  24  has the concave parts  45  provided at the plurality of circumferential places on the inner peripheral surface. Also, guide concave parts  36   a ,  36   a , which are concave in the axial direction, are provided at portions corresponding to both axial end opening peripheral edge portions of the concave parts  45  at the plurality of circumferential places on both axial end faces of the drive-side low-stiffness body  24 . An inner surface shape of each of the guide concave parts  36   a ,  36   a  is not limited to a partially cylindrical concave surface extending in the radial direction, as shown. For example, the inner surface may have a concave surface shape extending radially and having a rectangular section (as seen from the radial direction). 
     Each of the core parts  31 ,  31  provided at one axial end portion of the high-stiffness body  23  is inserted one by one into an inner side of a part of which three surfaces are surrounded by the two clamped pieces  35   a ,  35   a , which are adjacent to each other in the circumferential direction and have a circumferential gap therebetween gradually decreasing toward the radially inner side, and the coupling cylinder part  34   a  of the drive-side low-stiffness body  24  configured as described above, without circumferential and radial rattling. The other axial end face of the coupling cylinder part  34   a  of the drive-side low-stiffness body  24  is contacted to one axial end face of the main part  30  of the high-stiffness body  23 . In this state, the drive-side low-stiffness body  24  is joined and fixed to the high-stiffness body  23  by adhesion or the like. In the first example, in this state, a combination of each of the core parts  31 ,  31  provided at one axial end portion of the high-stiffness body  23  and the pair of clamped pieces  35   a ,  35   a  adjacent to both circumferential sides of each of the core parts  31 ,  31  is the convex part  46  configuring the first coupling-side concave-convex part  27 . In the meantime, a circumferential distance X (refer to  FIG. 10 ) between the core parts  31 ,  31  adjacent to each other in the circumferential direction is preferably equal to or smaller than a circumferential distance Y (refer to  FIG. 12 ) between circumferentially outer surfaces of the pair of clamped pieces  35   a ,  35   a    31  adjacent to each other in the circumferential direction (X≤Y). 
     The driven-side low-stiffness body  25  is made of an elastic material such as rubber-like elastomer having stiffness lower than the high-stiffness body  23 , and has a circular ring shape as a whole, and both axial half parts thereof are symmetric with respect to the axial direction. The driven-side low-stiffness body  25  has a coupling cylinder part  34   b  having a short cylinder shape and a plurality of pairs of flat plate-shaped clamped pieces  35   b ,  35   b  extending radially inward and in parallel with each other from a plurality of places equidistantly spaced in the circumferential direction on an inner peripheral surface of the coupling cylinder part  34   b . In the first example, a part of which three surfaces are surrounded by each pair of the clamped pieces  35   b ,  35   b  and the coupling cylinder part  34   b  is the concave part  47  configuring the second coupling-side concave-convex part  28 . That is, the driven-side low-stiffness body  25  has the concave parts  47  provided at the plurality of circumferential places on the inner peripheral surface. Also, guide concave parts  36   b ,  36   b , which are concave in the axial direction, are provided at portions corresponding to both axial end opening peripheral edge portions of the concave parts  47  at the plurality of circumferential places on both axial end faces of the driven-side low-stiffness body  25 . An inner surface shape of each of the guide concave parts  36   b ,  36   b  is not limited to a partially cylindrical concave surface extending in the radial direction, as shown. For example, the inner surface may have a concave surface shape extending radially and having a rectangular section (as seen from the radial direction). 
     Each of the core parts  31 ,  31  provided at the other axial end portion of the high-stiffness body  23  is inserted one by one into an inner side of a part of which three surfaces are surrounded by the two clamped pieces  35   b ,  35   b , which are adjacent to each other in the circumferential direction and have a circumferential gap therebetween gradually decreasing toward the radially inner side, and the coupling cylinder part  34   b  of the driven-side low-stiffness body  25  configured as described above, without circumferential and radial rattling. One axial end face of the coupling cylinder part  34   b  of the driven-side low-stiffness body  25  is contacted to the other axial end face of the main part  30  of the high-stiffness body  23 . In this state, the driven-side low-stiffness body  25  is joined and fixed to the high-stiffness body  23  by adhesion or the like. In the first example, in this state, a combination of each of the core parts  31 ,  31  provided at the other axial end portion of the high-stiffness body  23  and the pair of clamped pieces  35   b ,  35   b  adjacent to both circumferential sides of each of the core parts  31 ,  31  is the convex part  48  configuring the second coupling-side concave-convex part  28 . In the meantime, the circumferential distance X (refer to  FIG. 10 ) between the core parts  31 ,  31  adjacent to each other in the circumferential direction is preferably equal to or smaller than the circumferential distance Y (refer to  FIG. 12 ) between the circumferentially outer surfaces of the pair of clamped pieces  35   a ,  35   a    31  adjacent to each other in the circumferential direction (X≤Y), 
     In the first example, the drive-side low-stiffness body  24  and the driven-side low-stiffness body  25  are made to have the same shape and dimension. For this reason, in the first example, the components can be commonly used for the drive-side low-stiffness body  24  and the driven-side low-stiffness body  25 . 
     As clearly seen from the above description, in the first example, the third coupling-side concave-convex part  29  and the partition wall part  26  are formed at the high-stiffness body  23  of the coupling  19 , the concave parts  45  configuring the first coupling-side concave-convex part  27  are formed at the drive-side low-stiffness body  24  of the coupling  19 , and the concave parts  47  configuring the second coupling-side concave-convex part  28  are formed at the driven-side low-stiffness body  25  of the coupling  19 . Also, the guide concave parts  36   a ,  36   a  are provided at the portions corresponding to one axial end opening peripheral edge portions of the concave parts  45  configuring the first coupling-side concave-convex part  27  at the plurality of circumferential places on one axial end face of the coupling  19 . The guide concave parts  36   b ,  36   b  are provided at the portions corresponding to the other axial end opening peripheral edge portions of the concave parts  47  configuring the second coupling-side concave-convex part  28  at the plurality of circumferential places on the other axial side surface of the coupling  19 . The respective guide concave parts  36   a ,  36   a  are respectively provided on both axial side surfaces of the drive-side low-stiffness body  24 , and the respective guide concave part  36   b ,  36   b  are respectively provided on both axial side surfaces of the driven-side low-stiffness body  25 . Therefore, it is not necessary to consider the mounting directionality of the drive-side low-stiffness body  24  and the driven-side low-stiffness body  25  to the high-stiffness body  23 , so that it is possible to easily assemble the coupling  19 . 
     The preload member  20  is made of an elastic material such as rubber-like elastomer, and has a circular plate shape. An outer peripheral surface of the preload member  20  is formed at an axially central part with an engaging groove  37  over an entire circumference. Continuous parts between the outer peripheral surface and both axial side surfaces of the preload member  20  are respectively provided with chamfered portions  38 ,  38 . Each of the chamfered portions  38 ,  38  is not limited to a C-chamfered portion as shown, and may also be an R-chamfered portion. The preload member  20  is internally fitted and. supported to the support hole  32  formed at the radially inner side of the partition wall part  26  in a state where the inner peripheral edge portion of the partition wall part  26  of the coupling  19  is engaged to the engaging groove  37 . In the meantime, when pushing the preload member  20  to the inner side of the support hole  32  from one axial side (or the other axial side) so as to mount the preload member  20  to the support hole  32 , the chamfered portion  38  is guided to an inner peripheral edge portion of the support hole  32 , so that a diameter of the preload member  20  can be efficiently elastically reduced. Accordingly, it is possible to easily perform the pushing operation. 
     One axial side half part of the drive-side concave-convex part  21  of the drive-side transmission member  17  is engaged to the first coupling-side concave-convex part  27  of the coupling  19  without a circumferential gap (without providing a circumferential gap between the circumferential side surfaces of the convex parts  42  configuring the drive-side concave-convex part  21  and the convex parts  46  configuring the first coupling-side concave-convex part  27 ). Also, the other axial side half part of the drive-side concave-convex part  21  is engaged to one axial side half part (part located further toward one axial side than the partition wall part  26 ) of the third coupling-side concave-convex part  29  of the coupling  19  with a circumferential gap being interposed therebetween (with circumferential gaps α, α being provided between the circumferential side surfaces of the convex parts  42  configuring the drive-side concave-convex part  21  and the convex parts  50  configuring the third coupling-side concave-convex part  29 ). 
     Accordingly, in an initial mounted state (a state in which torque is not transmitted), both the circumferential side surfaces of one axial side half parts of the convex parts  42  configuring the drive-side concave-convex part  21  are in contact with the circumferential side surfaces of the convex parts  46  configuring the first coupling-side concave-convex part  27  facing each other in the circumferential direction. In contrast, in the initial mounted state, both the circumferential side surfaces of the other axial side half parts of the convex parts  42  configuring the drive-side concave-convex part  21  are not in contact with the circumferential side surfaces of the convex parts  50  configuring the third coupling-side concave-convex part  29  facing each other in the circumferential direction. That is, as shown in  FIG. 6 . the circumferential width dimension W A  (refer to  FIG. 7B ) of the convex part  42  configuring the drive-side concave-convex part  21  is smaller than a circumferential width dimension W C  of the concave part  49  configuring the third coupling-side concave-convex part  29  (W A &lt;W C ). 
     Also, in this state, radial gaps are respectively interposed in the engagement parts between the drive-side concave-convex part  21  and one axial side half parts of the first coupling-side concave-convex part  27  and third coupling-side concave-convex part  29 . That is, the radial gaps β, γ are respectively provided between a radially outer end face of the convex part  42  configuring the drive-side concave-convex part  21  and bottoms surfaces of the concave part  45  configuring the first coupling-side concave-convex part  27  and the concave part  49  configuring the third coupling-side concave-convex part  29  and between a bottom surface of the concave part  41  configuring the drive-side concave-convex part  21  and radially inner end faces of the convex part  46  configuring the first coupling-side concave-convex part  27  and the convex part  50  configuring the third coupling-side concave-convex part  29 . In order to form the radial gaps β, γ, a distance ϕS between the bottom surfaces of the concave parts  49  radially facing each other is set greater than a distance ϕT between radially outer surfaces of the convex parts  42  radially facing each other (ϕS&gt;ϕT), and a distance ϕU between the convex parts  50  radially facing each other is set greater than a distance ϕV between the bottom surfaces of the concave parts  41  radially facing each other (ϕU&gt;ϕV). 
     Also, the other axial side half part of the driven-side concave-convex part  22  of the driven-side transmission member  18  is engaged to the second coupling-side concave-convex part  28  of the coupling  19  without a circumferential gap (without providing a circumferential gap between the circumferential side surfaces of the convex parts  44  configuring the driven-side concave-convex part  22  and the convex parts  48  configuring the second coupling-side concave-convex part  28 ). Also, one axial side half part of the driven-side concave-convex part  22  is engaged to the other axial side half part (part located further toward the other axial side than the partition wall part  26 ) of the third coupling-side concave-convex part  29  of the coupling  19  with a circumferential gap being interposed therebetween (with the circumferential gaps α, α being provided between the circumferential side surfaces of the convex parts  44  configuring the driven-side concave-convex part  22  and the convex parts  50  configuring the third coupling-side concave-convex part  29 ). 
     Accordingly, in the initial mounted state (a state in which torque is not transmitted), both the circumferential side surfaces of the other axial side half parts of the convex parts  44  configuring the driven-side concave-convex part  22  are in contact with the circumferential side surfaces of the convex parts  48  configuring the second coupling-side concave-convex part  28  facing each other in the circumferential direction. In contrast, in the initial mounted state, both the circumferential side surfaces of one axial side half parts of the convex parts  44  configuring the driven-side concave-convex part  22  are not in contact with the circumferential side surfaces of the convex parts  50  configuring the third coupling-side concave-convex part  29  facing each other in the circumferential direction. That is, as shown in  FIG. 6 , the circumferential width dimension W B  (refer to  FIG. 8A ) of the convex part  44  configuring the driven-side concave-convex part  22  is smaller than the circumferential width dimension W C  of the concave part  49  configuring the third coupling-side concave-convex part  29  (W B &lt;W C ). 
     Also, in this state, radial gaps are respectively interposed in the engagement parts between the driven-side concave-convex part  22  and the other axial side half parts of the second coupling-side concave-convex part  28  and third coupling-side concave-convex part  29 . That is, the radial gaps β, γ are respectively provided between a radially outer end face of the convex part  44  configuring the driven-side concave-convex part  22  and bottoms surfaces of the concave part  47  configuring the second coupling-side concave-convex part  28  and the concave part  49  configuring the third coupling-side concave-convex part  29  and between a bottom surface of the concave part  43  configuring the driven-side concave-convex part  22  and radially inner end faces of the convex part  48  configuring the second coupling-side concave-convex part  28  and the convex part  50  configuring the third coupling-side concave-convex part  29 . In order to form the radial gaps β, γ the distance ϕS between the bottom surfaces of the concave parts  49  radially facing each other is set greater than the distance ϕT between the radially outer surfaces of the convex parts  42  radially facing each other (ϕS&gt;ϕT), and the distance ϕU between the convex parts  50  radially facing each other is set greater than the distance ϕV between the bottom surfaces of the concave parts  43  radially facing each other (ϕU&gt;ϕV). 
     Also, in this state, the partition wall part  26  of the coupling  19  is disposed between the other axial end face of the drive-side transmission member  17  and one axial end face of the driven-side transmission member  18 , which face each other in the axial direction. Also, the preload member  20  is elastically compressed and clamped between the leading end face of the output shaft  12   a  of the electric motor  7  and the base end face of the worm shaft  6   a , which face each other in the axial direction. Thereby, the worm shaft  6   a  is urged toward the other axial side on the basis of the elastic force of the preload member  20 , so that an axial preload is applied to the rolling bearing  9   b  configured to rotatably support the worm shaft  6   a  to the housing  3 . On the other hand, the preload member  20  may be elastically compressed and clamped between an end face of the driven-side transmission member  18  and an end face of the drive-side transmission member  17 . 
     According to the torque-transmission joint and the electric power steering device of the first example configured as described above, it is possible to suppress an abnormal sound from being generated when reversing a rotating direction of the output shaft  12   a  of the electric motor  7 . 
     That is, when the torque that is to be transmitted between the output shaft  12   a  of the electric motor  7  and the worm shaft  6   a  is small and an amount of circumferential elastic compression of each of the clamped pieces  35   a ,  35   b  configuring the drive-side low-stiffness body  24  and the driven-side low-stiffness body  25  is small, the circumferential side surfaces of the convex parts  42  configuring the drive-side concave-convex part  21  and the convex parts  44  configuring the driven-side concave-convex part  22  are not in contact with the circumferential side surfaces of the convex parts  50  configuring the third coupling-side concave-convex part  29  (the circumferential gap α does not disappear). Accordingly, it is possible to suppress the abnormal sound even when reversing the rotating direction of the output shaft  12   a  of the electric motor  7 . Meanwhile, in this state, the torque is transmitted from the drive-side transmission member  17  to the coupling  19  via the engagement part between the drive-side concave-convex part  21  and the first coupling-side concave-convex part  27 , and the torque transmitted to the coupling  19  is transmitted to the driven-side transmission member  18  via the engagement part between the second coupling-side concave-convex part  28  and the driven-side concave-convex part  22 . 
     Also, when the torque that is to be transmitted between the output shaft  12   a  of the electric motor  7  and the worm shaft  6   a  is large, the amount of circumferential elastic compression of each of the clamped pieces  35   a ,  35   b  configuring the drive-side low-stiffness body  24  and the driven-side low-stiffness body  25  increases and the circumferential side surfaces of the convex parts  42  configuring the drive-side concave-convex part  21  and the convex parts  44  configuring the driven-side concave-convex part  22  are in contact with the circumferential side surfaces of the convex parts  50  configuring the third coupling-side concave-convex part  29  (the circumferential gap α disappears). Since the force of the contact is lowered in association with the circumferential elastic compressive deformation of each of the clamped pieces  35   a ,  35   b , it is possible to suppress the abnormal sound accompanied by the contact. Accordingly, even though the torque increases when reversing the rotating direction of the output shaft  12   a  of the electric motor  7 , it is possible to suppress the abnormal sound. Meanwhile, in this state, most of the torque is transmitted from the drive-side transmission member  17  to the coupling  19  via the engagement part between the drive-side concave-convex part  21  and one axial side half part of the third coupling-side concave-convex part  29 , and most of the torque transmitted to the coupling  19  is transmitted to the driven-side transmission member  18  via the engagement part between the other axial side half part of the third coupling-side concave-convex part  29  and the driven-side concave-convex part  22  (the remaining torque is transmitted from the drive-side transmission member  17  to the driven-side transmission member IS, like the above case where the torque is small). 
     As described above, in the first example, it is possible to divide the torque transmission characteristic between the output shaft  12   a  of the electric motor  7  and the worm shaft  6   a  into two stages, in accordance with the magnitude of the torque to be transmitted. 
     In the meantime, in case of implementing the present invention, when the circumferential gap between the drive-side concave-convex part  21  and one axial side half part of the third coupling-side concave-convex part  29  and the circumferential gap between the driven-side concave-convex part and the other axial side half part of the third coupling-side concave-convex part  29  are made to be different from each other or the circumferential gaps are respectively provided between the drive-side concave-convex part  21  and the first coupling-side concave-convex part  27  and between the driven-side concave-convex part  22  and the second coupling-side concave-convex part  28 , it is possible to more divide the torque transmission characteristic between the output shaft  12   a  and the worm shaft Ca than the two stages. 
     Also, the radial gaps β, γ are respectively interposed in the engagement parts between the drive-side concave-convex part  21  and one axial side half parts of the first coupling-side concave-convex part  27  and the third coupling-side concave-convex part  29 . For this reason, it is possible to naturally permit inclination of central axes of the drive-side transmission member  17  and the coupling  19 , based on the radial gaps β, γ. 
     Also, in the first example, the radial gaps β, γ are respectively interposed in the engagement parts between the driven-side concave-convex part  22  and the other axial side half parts of the second coupling-side concave-convex part  28  and the third coupling-side concave-convex part  29 . For this reason, it is possible to naturally permit inclination of central axes of the driven-side transmission member  18  and the coupling  19 , based on the radial gaps β, γ. 
     Accordingly, in the first example, even when misalignment (axis misalignment or eccentricity of the output shaft  12   a  of the electric motor  7 , and axis misalignment, inclination or eccentricity of the worm shaft  6   a ) occurs, the central axis of the coupling  19  is naturally inclined relative to the central axes of the drive-side transmission member  17  and the driven-side transmission member  18 , so that it is possible to smoothly transmit the torque. 
     Also, as shown in  FIG. 4 , the other axial side surface of the drive-side transmission member  17  is located further toward the other axial side than the other axial side surface of the coupling  19  by an axial dimension δ A , and one axial side surface of the driven-side transmission member  18  is located further toward one axial side than one axial side surface of the coupling  19  by an axial dimension δ B . For this reason, gaps of the axial dimensions δ A , δ B  exist between both axial end faces of the coupling  19  and the electric motor  7  and rolling bearing  9   b , respectively. Accordingly, in the first example, by the gaps, it is possible to prevent a problem that when the central axes of the drive-side transmission member  17  and driven-side transmission member  18  and the coupling  19  are inclined relative to each other, both axial end faces of the coupling  19  interfere with the electric motor  7  and the rolling bearing  9   b  and the inclinations are thus obstructed. When the coupling  19  is inclined relative to the drive-side transmission member  17  and the driven-side transmission member  18 , the inclination is absorbed by the drive-side low-stiffness body  24  and the driven-side low-stiffness body  25 . Accordingly, it is possible to easily incline the coupling  19 . 
     Also, as shown in  FIG. 5 , the radially inner end portion of each of the clamped pieces  35   a ,  35   a  ( 35   b ,  35   b ) configuring the drive-side low-stiffness body  24  (the driven-side low-stiffness body  25 ) is arranged at the more radially inner side than the radially inner end portion of each of the core parts  31 ,  31  configuring the high-stiffness body  23 . For this reason, by the respective radial gaps β, γ, when the central axes of the drive-side transmission member  17  (the driven-side transmission member  18 ) and the coupling  19  are inclined relative to each other, the radially inner end portion of each of the clamped pieces  35   a ,  35   a  ( 35   b ,  35   b ) can be contacted to the outer peripheral surface of the drive-side transmission member  17  (the driven-side transmission member  18 ) before the radially inner end portion of each of the core parts  31 ,  31  is contacted to the outer peripheral surface of the drive-side transmission member  17  (the driven-side transmission member  18 ). Thereby, the force by which the radially inner end portion of each of the core parts  31 ,  31  is contacted to the outer peripheral surface of the drive-side transmission member  17  (the driven-side transmission member  18 ) is lowered, so that it is possible to suppress the abnormal sound. 
     Also, the partition wall part  26  of the coupling  19  is arranged between the other axial end face of the drive-side transmission member  17  and one axial end face of the driven-side transmission member  18 , which face each other in the axial direction. Also, the preload member  20  is elastically compressed and clamped between the leading end face of the output shaft  12   a  of the electric motor  7  and the base end face of the worm shaft  6   a , which face each other in the axial direction. For this reason, it is possible to restrain the axial position of the coupling  19  with respect to the drive-side transmission member  17  and the driven-side transmission member  18  by the partition wall part  26  and the preload member  20 . Therefore, in the first example, it is not necessary to separately provide the outer peripheral surfaces of the drive-side transmission member  17  and the driven-side transmission member  18  with collar parts facing both axial end faces of the coupling  19  so as to restrain the axial position of the coupling  19 . Accordingly, it is possible to shorten the axial dimensions of the drive-side transmission member  17  and the driven-side transmission member  18 , thereby implementing axial miniaturization. In the meantime, when implementing the present invention, the preload member  20  may be arranged with an axial gap being interposed between the leading end face of the output shaft  12   a  of the electric motor  7  and the base end face of the worm shaft  6   a.    
     Also, at least in the initial mounted state, the partition wall part  26  is not in contact with the drive-side transmission member  17 , the driven-side transmission member  18 , the output shaft  12   a  of the electric motor  7  and the worm shaft  6   a . Therefore, even though the partition wall part is contacted to the members  17 ,  18 ,  12   a ,  6   a  upon occurrence of the misalignment, the force of the contact is lowered as the preload member  20  is elastically deformed. Accordingly, it is possible to suppress the abnormal sound accompanied by the contact. 
     Also, based on the elastic force of the preload member  20 , the worm shaft  6   a  is urged toward the other axial side, so that the axial preload is applied to the rolling bearing  9   b  configured to rotatably support the worm shaft  6   a  to the housing  3 . Accordingly, it is possible to suppress the abnormal sound associated with the rattling of the rolling bearing  9   b.    
     Also, the guide concave parts  36   a ,  36   a  are provided at the portions corresponding to one axial end opening peripheral edge portions of the concave parts configuring the first coupling-side concave-convex part  27  at the plurality of circumferential places on one axial side face of the coupling  19 , and the guide concave parts  36   b ,  36   b  are provided at the portions corresponding to the other axial end opening peripheral edge portions of the concave parts configuring the second coupling-side concave-convex part  28  at the plurality of circumferential places on the other axial side surface of the coupling  19 . Therefore, when performing the assembling, it is possible to easily perform an operation of inserting the convex parts configuring the drive-side concave-convex part  21  into the inner sides of the concave parts configuring the first coupling-side concave-convex part  27  from one axial end openings of the respective concave parts by using the respective guide concave parts  36   a ,  36   a  as guide parts. Also, likewise, it is possible to easily perform an operation of inserting the convex parts configuring the driven-side concave-convex part  22  into the inner sides of the concave parts configuring the second coupling-side concave-convex part  28  from the other axial end openings of the respective concave parts by using the respective guide concave parts  36   b ,  36   b  as guide parts. 
     Also, both axial end faces of the drive-side low-stiffness body  24  are provided with the respective guide concave parts  36   a ,  36   a  (axial depth dimensions of the respective guide concave parts  36   a ,  36   a ), so that a range of the engagement part (contact part) between the drive-side concave-convex part  21  and the first coupling-side concave-convex part  27  is narrowed as much as the dimensions. Specifically, a contact area between each of the concave parts  45  configuring the first coupling-side concave-convex part  27  and each of the convex parts  42  configuring the drive-side concave-convex part  21  is reduced. For this reason, when the misalignment occurs (when the torque transmission (rotation) is performed with the central axes of the drive-side transmission member  17  and the coupling  19  being inclined relative to each other), it is possible to suppress a resistance force (frictional force to be applied between the concave part  45  and the convex part  42 ) against the rotation occurring at the engagement part. Also, since the respective guide concave parts  36   a ,  36   a  are provided, the inclination of the central axes of the drive-side transmission member  17  and the coupling  19  is likely to be absorbed. In the meantime, a circumferential dimension W 45  (refer to  FIG. 12 ) of the concave part  45  of the first coupling-side concave-convex part  27  is preferably equal to or larger than the circumferential width dimension W A  (refer to  FIG. 7B ) of the convex part  42  of the drive-side concave-convex part  21  (W 45 ≥W A ). 
     Also, both axial end faces of the driven-side low-stiffness body  25  are provided with the respective guide concave parts  36   b ,  36   b  (axial depth dimensions of the respective guide concave parts  36   b ,  36   b ), so that a range of the engagement part (contact part) between the driven-side concave-convex part  22  and the second coupling-side concave-convex part  28  is narrowed as much as the dimensions. Specifically, a contact area between each of the concave parts  47  configuring the second coupling-side concave-convex part  28  and each of the convex parts  44  configuring the driven-side concave-convex part  22  is reduced. For this reason, when the misalignment occurs (when the torque transmission (rotation) is performed with the central axes of the driven-side transmission member  18  and the coupling  19  being inclined relative to each other), it is possible to suppress a resistance force (frictional force to be applied between the concave part  47  and the convex part  44 ) against the rotation occurring at the engagement part. Also, since the respective guide concave parts  36   b ,  36   b  are provided, the inclination of the central axes of the driven-side transmission member  18  and the coupling  19  is likely to be absorbed. In the meantime, a circumferential dimension W 47  (refer to  FIG. 12 ) of the concave part  47  of the second coupling-side concave-convex part  28  is preferably equal to or larger than the circumferential width dimension W B  (refer to  FIG. 8A ) of the convex part  44  of the driven-side concave-convex part  22  (W 47 ≥W B ). 
     Also, the parts, which are located at both axial sides with the partition wall part  26  being interposed therebetween, of the respective concave parts  49 ,  49  configuring the third coupling-side concave-convex part  29  are formed to be connected with each other through the respective through-holes  33 ,  33  formed at the plurality of circumferential places on the radially outer half part of the partition wall part  26 . Also, the diameter d A  of the inscribed circle of the respective through-holes  33 ,  33  is smaller than the diameter d B  of the inscribed circle of the convex parts configuring the third coupling-side concave-convex part  29  (d A &lt;d B ). By adopting this configuration, when the high torque is transmitted, the excessive stress is suppressed from being applied to the inner peripheral surface of the main part  30  of the high-stiffness body  23 , so that the durability of the high-stiffness body  23  is improved. 
     Also, a distance ϕF between the bottom surfaces of the concave parts  45  radially facing each other (a distance ϕF between the bottom surfaces of the concave parts  47  radially facing each other) (refer to  FIG. 12 ) is smaller than the distance ϕS between the bottom surfaces of the concave parts  49  radially facing each other (refer to  FIG. 6 ) (ϕF&lt;ϕS). Thereby, even when the misalignment occurs between the output shaft  12   a  of the electric motor  7 , which is a drive shaft, and the worm shaft  6   a , which is a driven shaft, the drive-side low-stiffness body  24  and the driven-side low-stiffness body  25  are contacted to the convex parts  42  configuring the drive-side concave-convex part  21  and the convex parts  44  configuring the driven-side concave-convex part  22 . 
     Also, a distance d x  (refer to  FIG. 12 ) between the respective clamped pieces  35   a ,  35   b  radially facing each other is smaller than the diameter d B  (refer to  FIG. 10 ) of the inscribed circle of the convex parts configuring the third coupling-side concave-convex part  29  (d X &lt;d B ). Thereby, even when the misalignment occurs between the output shaft  12   a  of the electric motor  7 , which is a drive shaft, and the worm shaft  6   a , which is a driven shaft, the drive-side low-stiffness body  24  and the driven-side low-stiffness body  25  are contacted to the concave parts  41  configuring the drive-side concave-convex part  21 . 
     In the meantime, when implementing the present invention, the circumferential width dimension W A  (W B ) of the convex part  42  ( 44 ) configuring the drive-side concave-convex part  21  (the driven-side concave-convex part  22 ) may be changed in the axial direction, as shown in  FIGS. 9B and 9C , for example. Specifically, as shown in  FIG. 9B , the width dimension W A  (W B ) of the convex part  42  ( 44 ) may be decreased toward the partition wall part  26  in the axial direction (the left in  FIG. 9B ) or, as shown in  FIG. 9C , the width dimension W A  (W B ) of the convex part  42  ( 44 ) may be decreased from an axially intermediate part toward both end edge portions. When the configuration as shown in  FIGS. 9B and 9C  are adopted, the range of the engagement part (contact part) between the drive-side concave-convex part  21  and one axial side half parts of the first coupling-side concave-convex part  27  and third coupling-side concave-convex part  29  (the range of the engagement part (contact part) between the driven-side concave-convex part  22  and the other axial side half parts of the second coupling-side concave-convex part  28  and third coupling-side concave-convex part  29 ) is narrowed. Accordingly, when the misalignment occurs, it is possible to suppress the resistance force against the rotation occurring at the engagement part. Also, when the misalignment occurs, it is possible to efficiently prevent the drive-side concave-convex part  21  (the driven-side concave-convex part  22 ) and one axial side half parts of the first coupling-side concave-convex part  27  and third coupling-side concave-convex part  29  (the other axial side half parts of the second coupling-side concave-convex part  28  and third coupling-side concave-convex part  29 ) from partially colliding with each other. 
     Second Example of Embodiment 
     A second example of the embodiment of the present invention is described with reference to  FIGS. 15 to 20 . 
     In a torque-transmission joint  16   a  of the second example, an outer peripheral surface of one axial end portion of a drive-side transmission member  17   a  is provided over an entire circumference with a drive-side collar part  39 , so that one axial side openings of the concave parts  41  configuring the drive-side concave-convex part  21  are blocked. Also, an outer peripheral surface of the other axial end portion of a driven-side transmission member  18   a  is provided over an entire circumference with a driven-side collar part  40 , so that the other axial side openings of the concave parts  43  configuring the driven-side concave-convex part  22  are blocked. The other axial side surface of the drive-side collar part  39  and one axial side surface of the driven-side collar part  40  are respectively brought close to one axial side surface of the coupling  19  and the other axial side surface of the coupling  19  with gaps of axial dimensions δ X , δ Y  being interposed. Thereby, it is possible to restrain the axial position of the coupling  19  relative to the drive-side transmission member  17   a  and the driven-side transmission member  18   a  by the drive-side collar part  39  and the driven-side collar part  40 , too. 
     In the second example, the gaps of the axial dimensions δ X , δ Y  are provided. Accordingly, it is possible to prevent a problem that when the central axes of the drive-side transmission member  17   a  and driven-side transmission member  18   a  and the coupling  19  are inclined relative to each other, both axial end faces of the coupling  19  interfere with the drive-side collar part  39  and the driven-side collar part  40  and the inclination is thus obstructed. 
     The other configurations and operations are similar to the first example of the embodiment. 
     Third Example of Embodiment 
     A third example of the embodiment of the present invention is described with reference to  FIGS. 21 and 22 . 
     In the third example, a structure of a high-stiffness body  23   a  configuring the coupling is different from the first and second examples of the embodiment. Also, in the third example, the preload member is omitted. 
     That is, in the third example, partition wall parts  26   a ,  26   a  are provided (with being circumferentially spaced) only at the inner sides of the concave parts  49 ,  49 , which configure the third coupling-side concave-convex part  29 , of the axially central portion of the third coupling-side concave-convex part  29  provided on an inner peripheral surface of a main part  30   a  configuring the high-stiffness body  23   a . A diameter d z  of an inscribed circle of the respective partition wall parts  26   a ,  26   a  is smaller than a diameter of a circumscribed circle of the convex parts  42 ,  42  ( 44 ,  44 ) (for example, refer to  FIGS. 5 to 8  and  FIGS. 18 and 19 ) configuring the drive-side concave-convex part  21  (the driven-side concave-convex part  22 ). In an assembled state of the torque-transmission joint of the third example, at least a radially inner end portion of each of the partition wall parts  26   a ,  26   a  is arranged without an axial gap (are axially clamped) or with an axial gap between the other axial end face of the drive-side transmission member  17  (each of the convex parts  42 ,  42 ) and one axial end face of the driven-side transmission member  18  (each of the convex parts  44 ,  44 ), which face each other in the axial direction (for example, refer to  FIGS. 4, 7 and 8 , and  FIGS. 17, 19 and 20 ). Thereby, the respective partition wall parts  26   a ,  26   a  are provided, so that it is possible to restrain the axial position of the coupling relative to the drive-side transmission member  17  and the driven-side transmission member  18 . 
     The other configurations and operations are similar to the first and second examples of the embodiment. 
     The subject application is based on Japanese Patent Application No. 2016-044360 filed on Mar. 8, 2016, the contents of which are incorporated herein by reference. 
     INDUSTRIAL APPLICABILITY 
     When implementing the present invention, if the preload member is omitted, the support hole of the radially inner side of the partition wall part configuring the coupling may be omitted to configure the partition wall part into a simple flat plate shape (circular plate shape). Also, in this case, the partition wall part may be arranged without an axial gap (are axially clamped) or with an axial gap between the axial end faces of the drive-side transmission member and the driven-side transmission member, which face each other in the axial direction. 
     DESCRIPTION OF REFERENCE NUMERALS 
       1 : steering wheel 
       2 : steering shaft 
       3 : housing 
       4 : worm wheel 
       5 : worm teeth 
       6 ,  6   a : worm shaft 
       7 : electric motor 
       8 : worm 
       9   a ,  9   b : rolling bearing 
       10 : pressing piece 
       11 : coil spring 
       12 ,  12   a : output shaft 
       13 : spline hole 
       14 : spline shaft part 
       15 : preload applying mechanism 
       16 ,  16   a : torque-transmission joint 
       17 ,  17   a : drive-side transmission member 
       18 ,  18   a : driven-side transmission member 
       19 : coupling 
       20 : preload member 
       21 : drive-side concave-convex part 
       22 : driven-side concave-convex part 
       23 ,  23   a : high-stiffness body 
       24 : drive-side low-stiffness body 
       25 : driven-side low-stiffness body 
       26 : partition wall part 
       27 : first coupling-side concave-convex part 
       28 : second coupling-side concave-convex part 
       29 : third coupling-side concave-convex part 
       30 ,  30   a : main part 
       31 : core part 
       32 : support hole 
       33 : through-hole 
       34   a ,  34   b : coupling cylinder part 
       35   a ,  35   b : clamped piece 
       36   a ,  36   b : guide concave part 
       37 : engaging groove 
       38 : chamfered portion 
       39 : drive-side collar part 
       40 : driven-side collar part 
       41 : concave part 
       42 : convex part 
       43 : concave part 
       44 : convex part 
       45 : concave part 
       46 : convex part 
       47 : concave part 
       48 : convex part 
       49 : concave part 
       50 : convex part