Patent Publication Number: US-2023151840-A1

Title: Part fastening structure and mounting tool

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2021-185075 filed on Nov. 12, 2021, incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a part fastening structure and a mounting tool. 
     2. Description of Related Art 
     Japanese Unexamined Patent Application Publication No. 2017-35220 (JP2017-35220 A) discloses a walking training device including a walking assisting device attached to a trainee’s leg. The walking assist device includes a thigh frame and a lower leg frame. The thigh frame is attached to the trainee’s thigh and the lower leg frame is attached to the trainee’s lower leg. 
     SUMMARY 
     In such a walking training device, various trainees wear walking assist devices (also referred to as leg braces or mounting tools) for training. Thus, it is necessary for an assistant to adjust the walking assist device according to the trainee. For example, the assistant adjusts a frame length according to a length of a trainee’s leg. In such a case, a part fastening structure for fastening two parts (for example, an upper frame and a lower frame) is used. That is, a frame is formed by fastening the two parts with the part fastening structure. 
     The frame length can be adjusted by the assistant loosening bolts and nuts and removing the two parts. That is, the assistant adjusts the frame length according to the leg length by changing the fastening position of the parts. Thus, it is desirable to easily and reliably attach and remove the parts. For example, it is desirable to attach and detach without a special tool. Further, a structure that does not loosen during use is desired. 
     The present disclosure has been made to solve such a problem, and provides a part fastening structure capable of simply and reliably fastening parts. 
     The part fastening structure in the present embodiment is a part fastening structure that fastens a part using a bolt and a nut. The bolt includes: a first spiral groove provided on a peripheral surface of the bolt; a second spiral groove that is provided on the peripheral surface of the bolt and that is provided on a tip end side of the bolt with respect to the first spiral groove; a slit that is provided on the peripheral surface of the bolt along an axial direction, and that extends from a tip end of the bolt to the first spiral groove through the second spiral groove; and a loosening suppression groove that extends from another end of the second spiral groove to the tip end side, the nut includes: a first nut member including a first pin that protrudes from an inner peripheral surface toward a central axis side and that is inserted into the first spiral groove; and a second nut member including a second pin that protrudes from the inner peripheral surface toward the central axis side and that is inserted into the second spiral groove; and an urging member that is disposed between the first nut member and the second nut member and that urges the first nut member toward a head portion of the bolt. 
     In the part fastening structure described above, the first spiral groove may be provided to be thicker than the second spiral groove, the first pin may be thicker than the second pin, and the first pin may be thicker than the second spiral groove. 
     In the part fastening structure described above, the first nut member may include: a cylindrical portion; and a dish portion protruding from the cylindrical portion to an outer peripheral side, the second nut member may be disposed on the outer peripheral side of the cylindrical portion, the cylindrical portion may be provided with a through hole extending through the cylindrical portion in a direction orthogonal to the axial direction, the through hole may be an elongated hole in which a longitudinal direction coincides with the axial direction, and the second pin may extend through the through hole. 
     In the part fastening structure described above, the urging member may be disposed between the dish portion and the second nut member. 
     In the part fastening structure described above, the first nut member may be provided with a plurality of the first pins spaced away from each other in a circumferential direction, the second nut member may be provided with a plurality of the second pins spaced away from each other in the circumferential direction, a plurality of the first spiral grooves may be provided corresponding to the first pins, a plurality of the second spiral grooves may be provided corresponding to the second pins, and a circumferential angle of the first spiral grooves and the second spiral grooves may be less than 180°. 
     In the part fastening structure described above, the nut may be provided with two first pins and two second pins, the two second pins may be disposed so as to face each other with a central axis in between, and the two first pins may be disposed so as to face each other with the central axis in between. 
     A mounting tool according to the present embodiment is a leg brace that is worn by a user, the mounting tool including: a first part including a plurality of first through holes; a second part including a second through hole; and the part fastening structure described above, in which the bolt is inserted through the first through hole and the second through hole. 
     According to the present disclosure, it is possible to provide a part fastening structure and a mounting tool capable of simply and reliably fastening parts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like signs denote like elements, and wherein: 
         FIG.  1    is a perspective view showing the two parts fastened by the part fastening structure; 
         FIG.  2    is a perspective view showing the two parts fastened by the part fastening structure; 
         FIG.  3    is a side view of a fastening portion of the part fastening structure; 
         FIG.  4    is a perspective view of the fastening portion of the part fastening structure; 
         FIG.  5    is a perspective view showing the configuration of a bolt; 
         FIG.  6    is an exploded perspective view of a nut; 
         FIG.  7    is a perspective view showing a state before the bolt and the nut are attached; 
         FIG.  8    is a perspective view showing a state in which the bolt and the nut are attached; 
         FIG.  9    is a cross-sectional view showing the part fastening structure that is cut; and 
         FIG.  10    is a schematic perspective view showing a mounting tool having the part fastening structure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, the present disclosure will be described through embodiments of the disclosure. However, the disclosure according to the claims is not limited to the following embodiments. Moreover, all of the configurations described in the embodiments are not necessarily indispensable as means for solving the issue. 
     A part fastening structure  1  for fastening a first part  10  and a second part  20  will be described with reference to  FIGS.  1  to  4   . The first part  10  and the second part  20  configure, for example, a frame to be attached to a leg of a trainee who performs walking training. Here, the first part  10  and the second part  20  are fastened by the part fastening structure  1  to form a frame disposed along a lower leg of the trainee. An assistant (also called a user) who assists the trainee adjusts the length of the frame according to the trainee. 
       FIG.  1    is a perspective view showing a state before fastening.  FIG.  2    is a perspective view showing a state after fastening.  FIG.  3    is a side view showing a state after fastening.  FIG.  4    is a diagram showing the part fastening structure that is cut. 
     One of the first part  10  and the second part  20  is disposed on an upper side, and the other is disposed on a lower side. Here, the first part  10  is disposed on a knee side and the second part  20  is disposed on an ankle side. The part fastening structure  1  is disposed laterally to a shin. Of course, the first part  10  and the second part  20  are not limited to the lower leg frame and the leg brace. Further, the vertical arrangement of the first part  10  and the second part  20  is not particularly limited. 
     The part fastening structure  1  includes a bolt  30  and a nut  60 . In the part fastening structure  1 , the first part  10  and the second part  20  are fastened using the bolt  30  and the nut  60 . The bolt  30  is passed through through holes provided in the first part  10  and the second part  20 . In  FIG.  1    and the like, a straight line along an axis center of the bolt  30  is shown as a central axis AX. The bolt  30  and the nut  60  are attached by rotating the bolt  30  or the nut  60  around the central axis AX. By attaching the nut  60  to the bolt  30 , the first part  10  and the second part  20  are fastened. 
     The first part  10  and the second part  20  are members whose longitudinal direction is along the lower leg. In  FIG.  1   , the first part  10  and the second part  20  are both channel steels. For example, the first part  10  and the second part  20  are each made of a metal material such as aluminum. A direction along the central axis AX is a thickness direction of the first part  10  and the second part  20 . 
     As shown in  FIGS.  1  and  2   , the first part  10  is provided with a plurality of through holes  11 . The through hole  11  extends through in the thickness direction of the first part  10 . The through holes  11  are disposed in one row along the longitudinal direction of the first part  10 . Here, the through holes  11  are formed at equal intervals along the longitudinal direction of the first part  10 . Each of the through holes  11  has an elongated hole shape to suppress rotation on the bolt side. The through holes  11  have the same size and the same shape. 
     The second part  20  includes one through hole  21 . The through hole  21  extends through the second part  20  in a thickness direction of the second part  20 . The first part  10  and the second part  20  are disposed so as to partially overlap each other. The first part  10  and the second part  20  are disposed so that the through hole  21  of the second part  20   overlaps with one through hole  11  of the first part  10 . The bolt  30  is inserted into the through hole  21  of the second part  20  and the through hole  11  of the first part  10 . Then, the nut  60  is attached to the bolt  30  inserted into the through hole  11  and the through hole  21 . As a result, the first part  10  and the second part  20  are fixed. Here, the bolt  30  is in contact with the first part  10  and the nut  60  is in contact with the second part  20 . 
     Further, by changing the through hole  11  into which the bolt  30  is inserted, a length of an overlapping portion of the first part  10  and the second part  20  is changed. Thus, the total length of the frame can be adjusted. For example, by inserting the bolt  30  into the through hole  11  on the left side of  FIG.  1    among the through holes  11 , the overlapping portion of the first part  10  and the second part  20  is shortened. Thus, the frame length can be increased. By inserting the bolt  30  into the through hole  11  on the right side of  FIG.  1    among the through holes  11 , the overlapping portion of the first part  10  and the second part  20  becomes longer. Thus, the frame length can be shortened. In this way, the frame length can be made variable by changing the fastening position by the part fastening structure  1 . 
     Next, the part fastening structure  1  will be described with reference to  FIGS.  5  to  9   .  FIG.  5    is a perspective view showing the configuration of the bolt  30 .  FIG.  6    is an exploded perspective view of the nut  60 .  FIG.  7    is a perspective view showing the part fastening structure  1  in a state before the bolt  30  and the nut  60  are attached.  FIG.  8    is a perspective view showing the part fastening structure  1  in a state in which the bolt  30  and the nut  60  are attached.  FIG.  9    is a cross-sectional view showing the part fastening structure  1  in a state in which the bolt  30  and the nut  60  are attached. 
     First, the configuration of the bolt  30  will be described with reference to  FIG.  5    and the like. The bolt  30  includes a shaft portion  31  and a head portion  32 . The shaft portion  31  is a substantially columnar portion, and is a portion inserted into the through hole  11  shown in  FIG.  1    or the like. In  FIG.  5   , the center of the shaft portion  31  is defined as the central axis AX. A direction parallel to the central axis AX of the shaft portion  31  is defined as an axial direction. The axial direction is the direction in which the bolt  30  is inserted into the through hole  11 . Further, as shown in  FIG.  5   , and the like, a direction around the central axis AX is defined as a circumferential direction. Thus, like a cylindrical coordinate system, a position in the circumferential direction is represented by an angle of 0 to 360° with a reference position (origin position) as 0°. As shown in  FIG.  5   , the head portion  32  side in the axial direction is a base end side, and the opposite side is a tip end si de. 
     The shaft portion  31  includes a tip end surface  34  and a peripheral surface  33 . The tip end surface  34  corresponds to a bottom surface of a cylinder, and the peripheral surface corresponds to a side surface of the cylinder. The tip end surface  34  is provided on the tip end side of the shaft portion  31 . The tip end surface  34  is the bottom surface located on the tip end side of the columnar shaft portion  31 . The tip end surface  34  is a plane orthogonal to the central axis AX. 
     The head portion  32  is provided on the base end side of the shaft portion  31 . The head portion  32  is a disc-shaped dish portion. An outer diameter of the head portion  32  is larger than an outer diameter of the shaft portion  31 . The outer diameter of the shaft portion  31  is smaller than the through holes  11  and  21  so as to be inserted into the through holes  11  and  21 . The outer diameter of the head portion  32  is larger than that of the through holes  11  and  21 . Therefore, the head portion  32  comes into contact with the first part  10 . A washer, a disc spring, or the like may be disposed between the head portion  32  and the first part  10 . 
     The peripheral surface  33  is a portion from the tip end surface  34  to the head portion  32 . That is, the peripheral surface  33  is a side surface (outer peripheral surface) of the shaft portion  31  having a substantially columnar shape. A first spiral groove  41 , a second spiral groove  42 , a slit  43 , and a loosening suppression groove  44  are formed on the peripheral surface  33 . The second spiral groove  42  is provided on the tip end side of the first spiral groove  41 . In the axial direction, the second spiral groove  42  is disposed between the tip end surface  34  and the first spiral groove  41 . That is, the first spiral groove  41  is disposed on the head portion  32  side, and the second spiral groove  42  is disposed on the tip end surface  34  side. 
     The first spiral groove  41  and the second spiral groove  42  are grooves (recess portions) formed in a spiral shape on the peripheral surface  33 . The distance of the first spiral groove  41  from the tip end surface  34  changes in depending on the position (angle) in the circumferential direction. One end of the first spiral groove  41  is disposed on the tip end surface  34  side, and the other end is disposed on the head portion  32  side. Similarly, the distance of the second spiral groove  42  from the tip end surface  34  changes depending on the position in the circumferential direction. One end of the second spiral groove  42  is disposed on the tip end surface side, and the other end is disposed on the head portion  32  side. The first spiral groove  41  and the second spiral groove  42  are formed in parallel with each other. That is, the distance between the first spiral groove  41  and the second spiral groove  42  in the axial direction is constant. For example, when rotated by 90° in the circumferential direction, the position of the first spiral groove  41  shifts by 1 mm in the axial direction. 
     Further, the peripheral surface  33  of the bolt  30  is provided with the slit  43  formed along the axial direction. The slit  43  is a groove parallel to the central axis AX direction. The slit  43  extends from the tip end surface  34  to the first spiral groove  41 . The slit  43  is connected to one end of the first spiral groove  41  on the tip end surface  34  side. The slit  43  is connected to one end of the second spiral groove  42  on the tip end surface  34  side. That is, the slit  43  reaches one end of the first spiral groove  41  from the tip end surface  34  via one end of the second spiral groove  42 . 
     The loosening suppression groove  44  is connected to the second spiral groove  42 . The loosening suppression groove  44  is a groove formed on the peripheral surface  33  along the axial direction. The loosening suppression groove  44  is formed from the other end of the second spiral groove  42  on the head portion  32  side toward the tip end surface  34  side. The loosening suppression groove  44  does not reach the tip end surface  34 . That is, the length of the loosening suppression groove  44  in the axial direction is shorter than the distance from the tip end surface  34  to the other end of the second spiral groove  42 . The slit  43  is connected to one end of the second spiral groove  42 , and the loosening suppression groove  44  is formed at the other end. 
     Further, in the present embodiment, two slits  43  are formed on the peripheral surface  33 . The two slits  43  are disposed so as to face each other with the central axis AX interposed therebetween. Similarly, the two loosening suppression grooves  44  are formed on the peripheral surface  33 . The two loosening suppression grooves  44  are disposed so as to face each other with the central axis AX interposed therebetween. 
     Assuming that the position of one slit  43  in the circumferential direction is 0°, the position of the other slit  43  in the circumferential direction is 180°. That is, the two slits  43  are disposed symmetrically with respect to the central axis AX. Assuming that the position of one loosening suppression groove  44  in the circumferential direction is θ1, the position of the other loosening suppression groove  44  in the circumferential direction is (θ1 + 180°). Note that θ1 is less than 180°. That is, the two loosening suppression grooves  44  are disposed symmetrically with respect to the central axis AX. 
     Similarly, the two first spiral grooves  41  and the two second spiral grooves  42  are formed on the peripheral surface  33 . In the circumferential direction, one first spiral groove  41  is formed so as not to overlap with the other first spiral groove  41 . That is, the size (angle range) of each first spiral groove 4  1  in the circumferential direction is formed so as to be less than 180°. 
     Specifically, in the circumferential direction, one first spiral groove  41  is formed in the range of 0° to θ1, and the other first spiral groove  41  is formed in the range of 180° and above (θ1 + 180°). The two first spiral grooves  41  are disposed symmetrically with respect to the central axis AX. The circumferential angle of each first spiral groove  41  is less than 180°. 
     Similarly, in the circumferential direction, one second spiral groove  42  is formed so as not to overlap with the other second spiral groove  42 . That is, the size (angle range) of each second spiral groove  42  in the circumferential direction is formed so as to be less than 180°. 
     Specifically, in the circumferential direction, one second spiral groove  42  is formed in the range of 0° to θ1, and the other second spiral groove  42  is formed in the range of 180° and above (θ1 + 180°). The two second spiral grooves  42  are disposed symmetrically with respect to the central axis AX. The circumferential angle of each second spiral groove  42  is less than 180°. Here, the first spiral groove  41  and the second spiral groove  42  have substantially the same length in the circumferential direction. 
     As described above, the circumferential angle of the first spiral groove  41  and the second spiral groove  42  is less than 180°. Thus, two first spiral grooves  41  and two second spiral grooves  42  can be formed on the peripheral surface  33 . The peripheral surface  33  may be flat at the position in the circumferential direction in which the slit  43  is formed. The bolt  30  is made of a metal material such as iron or stainless steel. The diameter of the shaft portion  31  is about 7 mm to 8 mm. 
     Next, the configuration of the nut  60  will be described with reference to  FIGS.  6 ,  7   , and the like. The nut  60  includes a first nut member  61 , a second nut member  62 , and a spring  63 . The second nut member  62  is a cylindrical member or a ring-shaped member. That is, the second nut member  62  is a member having a hollow portion  62   a  provided along the central axis AX. The hollow portion  62   a  is a columnar space along the axial direction. The surface of the second nut member  62  on the hollow portion  62   a  side is defined as an inner peripheral surface  62   b . The outer peripheral surface of the second nut member  62  is referred to as an outer peripheral surface  62   c . 
     The second nut member  62  has a second pin  72 . For example, the second nut member  62  is formed with a pin hole  62   h  for fixing the second pin  72 . The pin hole  62   h  is a through hole that reaches the inner peripheral surface  62   b  from the outer peripheral surface  62   c . 
     The second pin  72  is press-fitted into the pin hole  62   h  from the outer peripheral surface  62   c  side. The second pin  72  protrudes from the inner peripheral surface  62   b  of the second nut member  62  toward the central axis AX side. The second pin  72  is disposed along a direction that passes through the central axis AX and that is extended along a direction orthogonal to the central axis AX. When the second pin  72  is inserted into the pin hole  62   h , the second nut member  62  holds the second pin  72 . 
     As shown in  FIG.  9   , the second pin  72  is inserted into the second spiral groove  42  of the bolt  30 . Here, the second nut member  62  is provided with two second pins  72  and two pin holes  62   h . A plurality of the second pins  72  is provided away from each other in the circumferential direction. The two second pins  72  are disposed symmetrically with respect to the central axis AX. That is, the two second pins  72  are disposed on the same straight line orthogonal to the central axis AX. The two second pins  72  correspond to the two second spiral grooves  42 . That is, at the time of fastening the parts, one second pin  72  is inserted into one second spiral groove  42 , and the other second pin  72  is inserted into the other second spiral groove  42 . 
     As shown in  FIG.  6    and the like, the first nut member  61  includes a cylindrical portion  65  and a dish portion  66 . The cylindrical portion  65  is a cylindrical member having a hollow portion  65   a . The hollow portion  65   a  is a columnar space along the axial direction. An outer diameter of the cylindrical portion  65  is smaller than a diameter of the hollow portion  62   a  of the second nut member  62 . The cylindrical portion  65  is inserted into the hollow portion  62   a  of the second nut member  62 . The surface of the cylindrical portion  65  on the hollow portion  65   a  side is defined as an inner peripheral surface  65   b . An outer peripheral surface of the cylindrical portion  65  is defined as the outer peripheral surface  65   c . The outer peripheral surface  65   c  is a surface facing the inner peripheral surface  62   b  of the second nut member  62 . 
     The dish portion  66  is formed at one end of the cylindrical portion  65 . The dish portion  66  is a flange-shaped (eaves-shaped) portion extending outward from the outer peripheral surface  65   c  of the cylindrical portion  65 . That is, an outer diameter of the dish portion  66  is larger than the outer diameter of the cylindrical portion  65 . The outer diameter of the dish portion  66  is substantially the same as an outer diameter of the second nut member  62 . The dish portion  66  is formed at the end portion of the first nut member  61  on the head portion  32  side (see  FIG.  8   ). 
     In a plan view orthogonal to the central axis AX, the centers of the cylindrical portion  65 , the dish portion  66 , the hollow portion  62   a , and the hollow portion  65   a  coincide with the central axis AX. The cylindrical portion  65 , the dish portion  66 , the hollow portion  62   a , and the hollow portion  65   a  are circular in a plan view orthogonal to the central axis AX. The cylindrical portion  65 , the dish portion  66 , the hollow portion  62   a , and the hollow portion  65   a  are concentric circles in a plan view orthogonal to the central axis AX. 
     As shown in  FIG.  8   , in the axial direction, the dish portion  66  is disposed between the second nut member  62  and the head portion  32 . One surface of the dish portion  66  faces the head portion  32 , and the other surface faces the second nut member  62 . Then, as shown in  FIGS.  8  and  9   , a space S in which the first part  10  and the second part  20  are disposed is formed between the dish portion  66  and the head portion  32 . 
     As shown in  FIG.  6   , the first nut member  61  includes a first pin  71 . For example, the cylindrical portion  65  is formed with a pin hole  65   h  for fixing the first pin  71 . The pin hole  65   h  is a through hole that reaches the inner peripheral surface  65   b  from the outer peripheral surface  65   c  of the cylindrical portion  65 . 
     The first pin  71  is press-fitted into the pin hole  65   h  from the outer peripheral surface  65   c  side. The first pin  71  protrudes from the inner peripheral surface  65   b  toward the central axis AX side. The first pin  71  is disposed in the direction that passes through the central axis AX and that is extended along the direction orthogonal to the central axis AX. When the first pin  71  is inserted into the pin hole  65   h , the first nut member  61  holds the first pin  71 . 
     As shown in  FIG.  9   , the first pin  71  is inserted into the first spiral groove  41 . Here, the first nut member  61  is provided with two first pins  71  and two pin holes  65   h . The multiple first pins  71  are provided apart from each other in the circumferential direction. The two first pins  71  are disposed symmetrically with respect to the central axis AX. That is, the two first pins  71  are disposed on the same straight line orthogonal to the central axis AX. The two first pins  71  correspond to the two first spiral grooves  41 . That is, one first pin  71  is inserted into one first spiral groove  41 , and the other first pin  71  is inserted into the other first spiral groove  41 . 
     In the axial direction, the first pin  71  is disposed between the dish portion  66  and the second pin  72 . In the circumferential direction, the positions of the first pin  71  and the second pin  72  coincide with each other. The distance between the two first pins  71   in the circumferential direction is 180°. The distance between the two second pins  72  in the circumferential direction is 180°. 
     As shown in  FIGS.  6  and  9   , the cylindrical portion  65  is formed with an elongated hole  65   d  for inserting the second pin  72 . The elongated hole  65   d  is a through hole that reaches the inner peripheral surface  65   b  from the outer peripheral surface  65   c  of the cylindrical portion  65 . The elongated hole  65   d  extends through the cylindrical portion  65  along a direction orthogonal to the central axis AX. The cross-sectional shape of the elongated hole  65   d  is an elongated hole shape with the axial direction serving as the longitudinal direction. 
     Two elongated holes  65   d  are formed in the cylindrical portion  65 . The two elongated holes  65   d  correspond to the two second pins  72 . That is, one second pin  72  is inserted into one elongated hole  65   d , and the other second pin  72  is inserted into the other elongated hole  65   d . The size of the elongated hole  65   d  is larger than the diameter of the second pin  72 . Thus, the second pin  72  is displaced along the longitudinal direction of the elongated hole  65   d . That is, the second pin  72  moves in the axial direction in the elongated hole  65   d . In the axial direction, the size of the elongated hole  65   d  in the longitudinal direction defines the amount of displacement (stroke) of the second pin  72  with respect to the first pin  71 . That is, the axial distance between the first pin  71  and the second pin  72  changes by the size of the elongated hole  65   d . 
     A spring  63  is provided between the second nut member  62  and the first nut member  61 . For example, the second nut member  62  is provided with a recess portion or the like for disposing the spring  63 . The spring  63  is, for example, a coil spring and is disposed along the axial direction. The spring  63  expands and contracts in the axial direction. 
     The spring  63  is disposed between the second nut member  62  and the dish portion  66 . One end of the spring  63  is in contact with the second nut member  62  and the other end is in contact with the dish portion  66 . The spring  63  urges the dish portion  66  in an urging direction shown by an arrow B in  FIG.  8   . That is, the spring  63  generates an urging force along the axial direction so as to separate the dish portion  66  from the second nut member  62 . 
     Thus, as shown in  FIG.  8   , the spring  63  creates a gap G between the dish portion  66  and the second nut member  62 . In this way, the spring  63  is disposed between the second nut member  62  and the first nut member  61 . The spring  63  is an urging member that urges the first nut member  61  toward the head portion  32  of the bolt  30 . In other words, the spring  63  urges the second nut member  62  in a direction away from the head portion  32 . Of course, the urging member is not limited to the spring  63 , and an elastic body such as resin may be used. 
     The spring  63  has, for example, an outer diameter of 3 mm and a natural length of 10 mm. The spring constant k = 0.3 N/mm. The length of the spring at the time of mounting is 7 mm, and the mounting load is 0.9 N. The maximum expansion/contraction amount of the spring  63  is 5 mm, and the maximum load is 1.5 N. Since the spring  63  expands and contracts in the range of 5 mm to 7 mm, the size of the elongated hole  65   d  in the longitudinal direction is 2 mm. 
     A method of attaching the second nut member  62  and the first nut member  61  will be described. First, the first nut member  61  to which the first pin  71  is attached is prepared. Specifically, the first pin  71  is inserted into the pin hole  65   h  from the outer peripheral surface  65   c  side. By press-fitting the first pin  71  into the pin hole  65   h , the first pin  71  is fixed to the first nut member  61 . The first pin  71  protrudes from the inner peripheral surface  65   b  toward the central axis AX. Further, the second nut member  62  to which the second pin  72  is not attached is prepared. 
     The spring  63  is disposed between the first nut member  61  and the second nut member  62 . Then, the cylindrical portion  65  is inserted into the hollow portion  62   a  of the second nut member  62 . Here, the relative positions of the first nut member  61  and the second nut member  62  are adjusted so that the positions in the circumferential direction of the elongated hole  65   d  and the pin hole  62   h  match. As a result, the elongated hole  65   d  and the pin hole  62   h  are connected. The second pin  72  is inserted into the pin hole  65   h  and the elongated hole  65   d  from the outer peripheral surface  62   c  side. By press-fitting the second pin  72  into the pin hole  65   h , the second pin  72  is fixed to the second nut member  62 . 
     As a result, as shown in  FIGS.  7  to  9   , and the like, the second nut member  62  and the first nut member  61  are fixed. Since the second pin  72  protrudes from the inner peripheral surface  65   b  of the cylindrical portion  65  toward the central axis AX, the second nut member  62  and the first nut member  61  become unremoveable. 
     As described above, the cylindrical portion  65  is provided with the elongated hole  65   d  extending in the axial direction. The second pin  72  is movable along the axial direction in the elongated hole  65   d . The expansion and contraction of the spring  63  changes the distance between the first pin  71  and the second pin  72  in the axial direction. In other words, the elongated hole  65   d  defines the expansion and contraction length of the spring  63 . The distance between the first pin  71  and the second pin  72  changes by the distance corresponding to the length of the elongated hole  65   d  in the axial direction. 
     Here, two springs  63  are disposed between the second nut member  62  and the first nut member  61 . In the circumferential direction, the two springs  63  are disposed symmetrically with respect to the central axis AX. Specifically, the first pin  71 , the spring  63 , the first pin  71 , and the spring  63  are disposed at intervals of 90° in the circumferential direction. Further, in the circumferential direction, the positions of the first pin  71  and the second pin  72  coincide with each other. 
     Next, a method of attaching the bolt  30  and the nut  60  will be described with reference to  FIGS.  7  to  9   , and the like. The nut  60  is inserted into the shaft portion  31  of the bolt  30 . That is, the shaft portion  31  is inserted into the hollow portion  65   a . Here, the positions of the first pin  71  and the second pin  72  in the circumferential direction coincide with the slit  43 . That is, the nut  60  is inserted into the bolt  30  so that the first pin  71  and the second pin  72  move along the slit  43 . As it is inserted into the bolt  30 , the first pin  71  and the second pin  72  move axially in the slit  43 . Then, when the bolt  30  is inserted all the way, the first pin  71  reaches the first spiral groove  41 . Here, when a force for contracting the spring  63  is applied to further push the member  62 , the second pin  72  reaches the second spiral groove  42 . 
     Next, the bolt  30  or nut  60  is rotated around the axis. That is, the bolt  30  rotates relative to the nut  60  about the axis. As a result, the first pin  71  moves along the first spiral groove  41 , and the second pin  72  moves along the second spiral groove  42 . That is, the first pin  71  spirally moves in the first spiral groove  41 , and the second pin  72  spirally moves in the second spiral groove  42 . That is, as the bolt  30  rotates, the first pin  71  moves from one end to the other end of the first spiral groove  41  in the axial direction and the circumferential direction. Similarly, the second pin  72  moves axially and circumferentially from one end to the other end of the second spiral groove  42 . By rotating the bolt  30  or the nut  60  in the circumferential direction in this way, the bolt  30  and the nut  60  are screwed together. 
     When the second pin  72  moves to the other end of the second spiral groove  42 , it reaches the loosening suppression groove  44 . The spring  63  urges the second nut member  62  in a direction away from the dish portion  66 . Thus, when the second pin  72  reaches the loosening suppression groove  44 , it moves toward the tip end surface  34  side along the loosening suppression groove  44 . That is, the second pin  72  is pushed back toward the tip end surface  34  side of the bolt  30 . As a result, the second pin  72  moves to the end portion of the loosening suppression groove  44  on the tip end surface  34  side. Thus, the gap G between the dish portion  66  and the second nut member  62  becomes wider in the axial direction. 
     By doing so, the bolt  30  and the nut  60  are fixed. That is, the first part  10  and the second part  20  shown in  FIG.  1    and the like are fastened in the space S between the head portion  32  of the bolt  30  and the dish portion  66  of the nut  60 . 
     Since the spring  63  urges the second nut member  62  in the direction away from the head portion  32 , the second pin  72  moves toward the tip end side in the loosening suppression groove  44 . As a result, since the second pin  72  is restricted from moving along the second spiral groove  42 , the bolt  30  and the nut  60  are suppressed from loosening. That is, it is possible to suppress the second pin  72  from returning from the other end of the second spiral groove  42  on the loosening suppression groove  44  side to one end on the slit  43  side. Thus, at the time of fastening, the relative rotation between the bolt  30  and the nut  60  is restricted, and the second pin  72  is not moved to the slit  43 . Thus, it is possible to suppress the bolt  30  from coming off the nut  60  along the axial direction. 
     Next, a method of removing the bolt  30  and the nut  60  will be described. In the state where the bolt  30  and the nut  60  are fixed, the second pin  72  is inserted into the loosening suppression groove  44  as described above. That is, the second pin  72  is located on the tip end surface  34  side of the loosening suppression groove  44 . The user pushes the second nut member  62  toward the head portion  32 . That is, the user applies an axial force to the second nut member  62  so as to bring the second nut member  62  closer to the dish portion  66 . Since the spring  63  contracts due to the force of the user, the second pin  72  moves toward the head portion  32  side along the loosening suppression groove  44 . The user pushes in the second nut member  62  until the second pin  72  reaches the end portion of the loosening suppression groove  44  on the head portion  32  side. As a result, since the second pin  72  reaches the second spiral groove  42 , the nut  60  can rotate around the axis. 
     Then, with the user pushing in the second nut member  62 , the nut  60  is rotated around the axis. By doing so, the second pin  72  moves along the second spiral groove  42 . When the second pin  72  moves from the other end to one end of the second spiral groove  42 , the second pin  72  reaches the slit  43 . Similarly, the first pin  71  moves along the first spiral groove  41 . When the first pin  71  moves from the other end to one end of the first spiral groove  41 , the first pin  71  reaches the slit  43 . 
     This makes it possible to pull out the nut  60  from the bolt  30 . That is, the nut  60  is moved in the axial direction so that the dish portion  66  of the nut  60  is separated from the head portion  32  of the bolt  30 . The first pin  71  and the second pin  72  move toward the tip end surface  34  side along the slit  43 . Thus, the user can remove the nut  60  from the bolt  30 . 
     As described above, the peripheral surface  33  of the shaft portion  31  of the bolt  30  is provided with the first spiral groove  41 , the second spiral groove  42 , the slit  43 , and the loosening suppression groove  44 . Further, the nut  60  includes the first pin  71  that moves along the first spiral groove  41  and the second pin  72  that moves along the second spiral groove  42 . 
     When the first pin  71  and the second pin  72  are located in the circumferential position of the slit  43 , the user can move the nut  60  relative to the bolt  30  along the axial direction. The bolt  30  and the nut  60  can be fixed by just inserting the first pin  71  and the second pin  72  into the slit  43  and rotating the nut  60 . Further, when the first pin  71  and the second pin  72  are located in the circumferential direction of the slit  43 , the nut  60  can be removed from the bolt  30 . When the first pin  71  and the second pin  72  deviate from the circumferential position of the slit  43 , the nut  60  cannot be attached to or detached from the bolt  30 . 
     Then, when the second pin  72  moves from one end to the other end of the second spiral groove  42 , the spring  63  urges the second nut member  62  in the direction away from the head portion  32 . Thus, the second pin  72  moves to the end portion on the tip end side of the loosening suppression groove  44 . The movement and rotation of the nut  60  with respect to the bolt  30  is restricted. That is, the nut  60  cannot rotate about the axis and cannot move along the axial direction. 
     In other words, unless the user pushes the second nut member  62  toward the head portion  32  side, the nut  60  cannot be removed from the bolt  30 . As a result, the bolt  30  and the nut  60  can be securely fixed. Loosening of the nut  60  can be suppressed, and the two parts can be securely fastened. That is, it is possible to suppress the bolt  30  from coming off from the nut  60  when the parts are fastened. 
     Further, the bolt  30  and the nut  60  can be removed by just rotating the nut  60  around the shaft with the nut  60  pushed in. Thus, simple attachment and detachment is possible. No special tools are required for installation and removal. That is, attachment and detachment can be performed with the user’s own hand or finger. Further, durability and workability can be improved. 
     Further, the nut  60  is provided with two first pins  71  and two second pins  72 . Two first spiral grooves  41 , two second spiral grooves  42 , two slits  43 , and two loosening suppression grooves  44  are formed on the peripheral surface  33  of the bolt  30 . This makes it possible to more reliably suppress the nut  60  from loosening. 
     In this case, the formation range of one first spiral groove  41  may be set to less than 180° in the circumferential direction. As a result, the pair of first spiral grooves  41  can be formed symmetrically with respect to the central axis AX. Similarly, the formation range of one second spiral groove  42  may be set to less than 180°. As a result, the pair of second spiral grooves  42  can be formed symmetrically with respect to the central axis AX. Thus, it can be installed more reliably and easily. 
     The first spiral groove  41  may be formed thicker than the second spiral groove  42 . For example, the groove width of the first spiral groove  41  can be about 1.5 mm, and the groove width of the second spiral groove  42  can be about 1.2 mm. The first pin  71  may be formed thicker than the second pin  72 . The first pin  71  may be thicker than the second spiral groove  42  and thinner than the first spiral groove  41 . By doing so, it is possible to suppress the first pin  71  from accidentally entering the second spiral groove  42 . This makes it possible to suppress the first pin  71  from accidentally entering the second spiral groove  42  on the tip end side even when the user rotates the nut  60  in the middle of the stroke of the slit  43  in the axial direction. Thus, it is possible to suppress erroneous mounting. 
     Further, the parts may be fastened with a wave washer, an E ring, or the like sandwiched in the space S between the head portion  32  of the bolt  30  and the dish portion  66 . 
       FIG.  10    is a diagram showing an example of a mounting tool using the part fastening structure  1 .  FIG.  10    is a perspective view schematically showing a mounting tool  120  worn by the user. The mounting tool  120  mainly includes a control unit  121 , a plurality of frames that support each part of the affected leg, and a load sensor  222  for detecting a load applied to the sole of the foot. The mounting tool  120  functions as a walking assist device having a drive unit and a control unit. 
     The control unit  121  includes an auxiliary control unit  220  that controls the mounting tool  120 , and also includes a motor (not shown) that generates a driving force for assisting the extension movement and the flexion movement of the knee joint. The frame supporting each part of the affected leg includes a thigh frame  122  and a lower leg frame  123  rotatably connected to the thigh frame  122 . Further, this frame includes a foot flat frame  124  rotatably connected to the lower leg frame  123 , a front side connecting frame  127 , and a rear side connecting frame  128 . 
     The thigh frame  122  and the lower leg frame  123  rotate relative to each other around a hinge axis H a  shown in the figure. The motor of the control unit  121  rotates in accordance with an instruction of the auxiliary control unit  220  to assist the thigh frame  122  and the lower leg frame  123  to open or close relatively around the hinge axis H a . An angle sensor  223  housed in the control unit  121  is, for example, a rotary encoder, and detects an angle formed by the thigh frame  122  and the lower leg frame  123  around the hinge axis H a . The lower leg frame  123  and the foot flat frame  124  rotate relative to each other around the hinge axis H b  shown in the figure. The relative rotating angle range is pre-adjusted by the adjusting mechanism  126 . 
     The thigh frame  122  includes a thigh belt  129 . The thigh belt  129  is a belt integrally provided on the thigh frame, and is wrapped around the thigh portion of the affected leg to fix the thigh frame  122  to the thigh portion. This suppresses the entire mounting tool  120  from shifting with respect to the trainee’s legs. 
     The load sensor  222  is a load sensor embedded in the foot flat frame  124 . The load sensor  222  can also be configured to detect the magnitude and distribution of the vertical load received by the trainee’s sole, for example, to detect a center of pressure (COP). The load sensor  222  is, for example, a resistance change detection type load detection sheet in which electrodes are disposed in a matrix. 
     The lower leg frame  123  includes the first part  10  and the second part  20  shown in  FIG.  1   . Then, the first part  10  and the second part  20  are connected via the part fastening structure  1 . That is, the user attaches the first part  10  and the second part  20  by the part fastening structure  1 . Thus, the length of the lower leg frame  123  can be adjusted in accordance with the leg length of the user who wears the mounting tool  120 . The user can easily and surely fasten the first part  10  and the second part  20  of the lower leg frame  123 . Therefore, the user can easily adjust the frame length. The number of parts to be fastened by the part fastening structure  1  may be 3 or more. 
     Although the disclosure made by the present inventor has been specifically described above based on the embodiment, the present disclosure is not limited to the above embodiment and can be variously modified within a range that does not depart from the gist thereof.