Patent Publication Number: US-11660713-B2

Title: Fastening device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional patent application of Ser. No. 17/048,807 filed on Oct. 19, 2020, which is a National Phase of International Application No. PCT/JP2019/019250 filed on May 15, 2019, and claims priority of Japanese Patent Application No. 2018-231252 filed on Dec. 11, 2018, the disclosures of which are hereby incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a fastening device that fastens a bolt to a fastening target object. 
     BACKGROUND ART 
     A bolt/nut fastener is used to assemble and fix a large number of mechanical structures including an automobile as a representative example. Patent Literature 1 discloses a bolt having a male screw section formed around the outer circumferential surface of the bolt head section. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent No. 6,381,840 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     An object of the present invention is to automate bolt fastening operation. 
     Solution to Problem 
     To achieve the object described above, the invention of the present application relates, as a viewpoint, to (1) a fastening device for fastening a bolt to a fastening target object, the bolt so configured that a bolt head section male screw section is formed around a side surface of a bolt head section and an engagement receiving section is formed at a top surface of the bolt head section, the fastening device including a tension rod including a rod female screw section that engages with the bolt head section male screw section, a bit that is so disposed inside the tension rod as to be movable in an upward/downward direction, includes an engagement section that engages with the engagement receiving section, and rotates the bolt around an axis extending in the upward/downward direction with the engagement receiving section engaging with the engagement section, a supporter that is disposed in a position where the supporter circumferentially surrounds the tension rod and has a lower end section protruding downward beyond a lower end section of the tension rod, a sensor for sensing compression force acting on the supporter in the upward/downward direction, a first motor for rotating the bit around the axis, and a second motor for rotating the tension rod around the axis. 
     (2) The fastening device described in item (1) above, in which a bolt shaft male screw section is formed around a shaft section of the bolt, and the rod female screw section and the bolt shaft male screw section have the same screw pitch. 
     (3) The fastening device described in item (1) or (2) above, in which a tubular holder is disposed inside the tension rod, and a magnet, the bit attracted by the magnet, and a spring that urges the magnet downward are disposed in the holder. 
     (4) The fastening device described in any one of (1) to (3) above, in which the engagement receiving section is an insertion hole, and the engagement section is a lower end section of the bit inserted into the insertion hole. 
     (5) The invention of the present application relates, as another viewpoint, to a fastening device for fastening a bolt to a fastening target object, the bolt so configured that a bolt head section female screw section is formed in an inner circumferential surface of a recess provided in an upper surface of a bolt head section, the fastening device including a tension rod including a rod male screw section that engages with the bolt head section female screw section, a drive socket that is disposed in a position where the drive socket circumferentially surrounds the tension rod and has a lower end section including a grasper for laterally grasping the bolt head section, a supporter that is disposed in a position where the supporter circumferentially surrounds the drive socket and has a lower end section protruding downward beyond a lower end section of the drive socket, a sensor for sensing compression force acting on the supporter in the upward/downward direction, a first motor for rotating the tension rod around an axis extending in the upward/downward direction, and a second motor for rotating the drive socket around the axis. 
     (6) The fastening device described in (5) above, in which a bolt shaft male screw section is formed around a shaft section of the bolt, and the rod male screw section and the bolt shaft male screw section have the same screw pitch. 
     (7) The fastening device described in (5) or (6) above, in which a projection is formed at a lower end section of the tension rod, and the rod male screw section is formed around an outer circumferential surface of the projection. 
     Advantageous Effects of Invention 
     The configuration of above-mentioned item (1) of the present invention allows automated tightening of a bolt so configured that a male screw section is formed around the outer circumferential surface of a bolt head section. The configuration of above-mentioned item (5) of the present invention allows automated tightening of a bolt so configured that a bolt head section female screw section is formed in the inner circumferential surface of a recess in the top surface of a bolt head section. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view of a fastening device (first embodiment). 
         FIG.  2    is a cross-sectional view of a fastening target object and a bolt (first embodiment). 
         FIG.  3    is a perspective view of the bolt (first embodiment). 
         FIG.  4 ( a )  is a perspective view of a bolt (variation). 
         FIG.  4 ( b )  is a cross-sectional view of a fastening target object and a bolt (variation). 
         FIG.  5    describes the action (first half) of the fastening device according to the first embodiment. 
         FIG.  6    describes the action (second half) of the fastening device according to the first embodiment. 
         FIG.  7    shows graphs illustrating the relationship among the angle of rotation of the bolt, axial force acting on the bolt, and tensile force acting on the bolt. 
         FIG.  8    is a cross-sectional view of a fastening device (second embodiment). 
         FIG.  9    is a perspective view of part of a bolt (second embodiment). 
         FIG.  10    describes the action (first half) of the fastening device according to the second embodiment. 
         FIG.  11    describes the action (second half) of the fastening device according to the second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG.  1    is a cross-sectional view showing a fastening device  100 . The fastening device  100  includes a first motor  10 , a second motor  30 , and an enclosure  50 . The first motor  10  and the second motor  30  are independent of each other and driven and controlled by a controller  5 . That is, the controller  5  can control the first motor  10  and the second motor  30  independently of each other. The controller  5  can, for example, be a CPU (central processing unit). 
     The enclosure  50  includes a body  51  and a base  52 , which has a recessed upper end fixed to the lower surface of the body  51 . The body  51  accommodates a first decelerator  11  and a second decelerator  31 . The first decelerator  11  includes an output shaft  11   a . The output shaft  11   a  has a tubular shape, and a power transmitter  12  is inserted into and fixed to a tubular section of the output shaft  11   a . The power transmitter  12  rotates along with the output shaft  11   a . A lower end section of the power transmitter  12  protrudes downward beyond the output shaft  11   a , and the protruding section extends toward the interior of a tubular bit holder  13 , which extends in the upward/downward direction, and is fixed. The rotational output produced by the output shaft  11   a  is therefore transmitted to the bit holder  13  via the power transmitter  12  to allow rotation of the output shaft  11   a  and the bit holder  13  integrated with each other. 
     The output shaft  11   a  and the bit holder  13  are disposed in a roughly coaxial manner and so supported as to be rotatable relative to a bearing  14 . A magnet  13   a  is provided in the bit holder  13  and attracts a bit  13   b  for rotating a bolt. The bit  13   b  extends downward and extends beyond the lower end section of the bit holder  13 . In the configuration described above, rotating the bit holder  13  allows the bit  13   b  to rotate around an axis extending in the upward/downward direction. 
     The magnet  13   a  and the bit  13   b  are so accommodated in the tubular bit holder  13  as to be slidably movable in the upward/downward direction, and a spring  13   c  is interposed between the magnet  13   a  and the power transmitter  12 . The spring  13   c  is disposed along the inner wall of the bit holder  13 . When the magnet  13   a  slides in the direction in which the magnet  13   a  approaches the power transmitter  12 , the spring  13   c  is loaded in the compressed direction. The configuration described above allows the bit  13   b  to move in the upward/downward direction in such a way that the bit  13   b  follows a bolt  70 , which will be described later. 
     The bit  13   b  wears due to a load exerted thereon when the bolt rotates. The configuration in which the bit  13   b  is attracted and fixed to the magnet  13   a  as in the present embodiment allows the bit  13   b  to be readily exchanged when the bit  13   b  has worn. 
     The rotational force produced by the second motor  30  is transmitted to a tension rod  15  via the second decelerator  31 , a pinion gear  32 , and an idle gear  33 . That is, a drive gear  151 , which is formed around the outer circumferential surface of the tension rod  15 , engage with the idle gear  33 , and the second motor  30  can be operated to rotate the tension rod  15  around an axis extending in the upward/downward direction. 
     Bearings  34  are disposed to receive loads acting on the pinion gear  32  and the idle gear  33  when the shaft of the second motor  30  rotates. The idle gear  33  is rotationally driven around an idle shaft  35  extending in the upward/downward direction. 
     The tension rod  15  includes a tension accommodator  15   a  for accommodating the bit holder  13  and the bearing  14 . A gap that allows rotation of the tension rod  15  is formed between the bit holder  13  and the tension rod  15 . The tension rod  15  is rotatably supported by bearings  16  and a thrust bearing  17 . 
     A tension rod female screw section  15   a   1  is formed in the inner radial surface of a lower end section of the tension rod  15 . The tension rod female screw section  15   a   1  engages with a bolt male screw section formed around the bolt head section when the bolt is pulled. The engagement will be described later in detail. 
     A supporter  18  includes a supporter accommodator  18   a  for accommodating the tension rod  15 . The upper end section of the supporter  18  is in contact with the thrust bearing  17 , and the lower end section of the supporter  18  extends downward beyond the lower end section of the tension rod  15 . Therefore, when the entire fastening device  100  lowers toward a fastening target object H, the lower end section of the supporter  18  comes into contact with the fastening target object H. A gap that allows rotation of the tension rod  15  is formed between the tension rod  15  and the supporter  18 . 
     A sensor  181  is provided on the side facing the upper end of the supporter  18 . The sensor  181  includes a straining element  181   a  and a strain gauge  181   b . The straining element  181   a  has a cylindrical shape having a reduced-diameter intermediate section. The strain gauge  181   b  is attached to the intermediate section of the straining element  181   a.    
     In the process of pulling the bolt, the straining element  181   a  receives upward force having the same magnitude as that of tensile force transmitted from the supporter  18  and is therefore distorted in the compression direction within an elastic deformation range of the straining element  181   a . The strain gauge  181   b  changes in terms of resistance in accordance with the strain of the straining element  181   a  and changes output voltage to be outputted to the controller  5 . The controller  5  calculates the tensile force acting on the screw section of the bolt based on the amount of change in the output voltage from the strain gauge  181   b . The sensor  181  is not limited to the combination of the straining element  181   a  and the strain gauge  181   b  and can be another sensor capable of detecting the tensile force acting on the bolt. 
     The bolt to be tightened by the fastening device  100  according to the present embodiment will next be described with reference to  FIGS.  2  and  3   .  FIG.  2    is a cross-sectional view of the bolt and the fastening target object, and  FIG.  3    is a perspective view of the bolt. The bolt  70  is a bolt with a hexagonal hole and is formed of a bolt shaft section  71  and a bolt head section  72 . The bolt shaft section  71  has a male screw formed therearound. The fastening target object H (in other words, workpiece) is formed of fastening target objects H 1  and H 2 , which are layered on each other in the upward/downward direction, and bolt holes H 1   a  and H 2   a  are formed in the fastening target objects H 1  and H 2 , respectively. 
     The bolt  70  is inserted into the bolt holes H 1   a  and H 2   a , and a nut  80  is caused to engage with the bolt shaft section  71  protruding downward beyond the end surface of the fastening target object H (H 2 ). The bolt  70  is thus fastened to the fastening target object H. It is, however, noted that the invention of the present application is applicable to a fastener formed only of the bolt  70  with no nut  80 . In this case, forming a female screw section that engages with the male screw section of the bolt shaft section  71  in the circumferential surface of the bolt hole H 2   a  allows the bolt  70  to be fastened to the fastening target object H. 
     An insertion hole  72   b  having a hexagonal shape (corresponding to engagement receiving section) is formed in the top surface of the bolt head section  72 . A lower end section (corresponding to engagement section) of the bit  13   b  can be inserted into the insertion hole  72   b  and rotated to fasten the bolt  70  to the fastening target object H. A bolt head section male screw section  72   a  is continuously formed around the side surface of the bolt head section  72  in the circumferential direction with no interruptions. Rotating the tension rod  15  allows the bolt  70  to be pulled with the tension rod female screw section  15   a   1  engaging with the bolt head section male screw section  72   a.    
     Now, let S 1  be the area over which the supporter  18  and the fastening target object H are in contact with each other and S 2  be the area over which the bolt head section  72  and the fastening target object H are in contact with each other, and it is desirable that the contact areas S 1  and S 2  are equal to each other. The seat surface of the bolt head section  72  and the supporter  18  exert surface pressure on the fastening target object H. Therefore, when the contact areas S 1  and S 2  differ from each other (that is, when difference in surface pressure is present), the amount of deformation of the fastening target object H undesirably differs from the amount of deformation in actual bolt fastening. Further, when the difference in surface pressure excessively increases, the supporter  18  is likely to deform and damage the fastening target object H, and the difference in the amount of deformation is likely to lower the fastening precision. 
     In the present embodiment, the insertion hole  72   b  has a hexagonal shape, but not necessarily in the present invention, and the insertion hole  72   b  may have an octagonal or any other polygonal shape. The invention of the present application is also applicable to a hexagonal bolt having a hexagonal bolt head section. In this case, an insertion hole  72   b ′ may be formed in the top surface of a bolt head section  72 ′, and bolt head section male screw sections  72   a ′ may be intermittently formed at curved sections of the side surface of the bolt head section  72 ′, as shown in  FIG.  4 ( a ) . Further, the bolt head section male screw section  72   a  ( 72   a ′) is not necessarily formed across the bolt head section  72  ( 72 ′) from the upper end to the lower end thereof and may instead be formed only part of the section from the upper end to the lower end as long as an engagement length necessary for the rotation of the tension rod  15  can be ensured. 
     In the present embodiment, the bit  13   b  is inserted into the insertion hole  72   b  formed in the bolt head section  72  to rotate the bolt  70 , but not necessarily in the present invention. Instead, a protrusion  72   c  (corresponding to engagement receiving section) may be formed at the bolt head section  72  ( 72 ′), and the protrusion  72   c  may be inserted into a recess (corresponding to engagement section) that is not shown but formed in the lower end section of the bit  13   b  to rotate the bolt  70 , as shown in  FIG.  4 ( b ) . That is, the invention of the present application is widely applicable to bolts each having a bolt head section male screw section formed around the side surface of the bolt head section and an engagement receiving section for engagement with the engagement section of the bit formed at the bolt head section. The invention of the present application is further applicable to a flanged bolt having a flange formed around the bolt head section. 
     The action of the fastening device  100  will next be described with reference to  FIGS.  5  and  6   , which are descriptive diagrams of the action. It is now assumed that the bolt  70  is temporarily attached to the fastening target object H, and that the bolt head section  72  is located in a temporal attachment position separate from the upper surface of the fastening target object H in an initial state (see  FIG.  5 ( a ) ). It is further assumed that the bit  13   b  is inserted into the insertion hole  72   b  of the bolt head section  72 . It is assumed that the control described below is performed by the controller  5  unless otherwise stated. 
     When the first motor  10  is operated, the first decelerator  11 , the output shaft  11   a , the power transmitter  12 , and the bit holder  13  rotate, and the bit  13   b  held by the bit holder  13  rotates in the direction indicated with the arrow K 1 . When the bit  13   b  rotates, the bolt  70  screws downward. The fastening device  100  is so supported as to be allowed to lower by its own weight relative to a support section (not shown), and the entire fastening device  100  therefore lowers with the bit  13   b  rotating. As a result, the tension rod female screw section  15   a   1  and the bolt head section male screw section  72   a  become engageable with each other (see  FIG.  5 ( b ) ). 
     When the bolt  70  further rotates and the bolt head section  72  is seated on the upper surface of the fastening target object H, the force that is received by the first motor  10  and attempts to rotate the shaft of the first motor  10  abruptly increases, resulting in an abrupt increase in feedback torque (see  FIG.  5 ( c ) ). The controller  5  senses based on the feedback torque produced by the first motor  10  that the bolt  70  has been seated and stops the operation of the first motor  10 . The shaft of the first motor  10  is then reversely rotated in the direction indicated by the arrow K 2  to move the bolt head section  72  back to the position slightly separate from the upper surface of the fastening target object H (see  FIG.  5 ( d ) ). 
     Thereafter, the operation of the first motor  10  is stopped, and the second motor  30  is operated. When the second motor  30  is operated, the entire fastening device  100  moves downward with the tension rod  15  rotating in the direction indicated by the arrow K 1 , so that the tension rod female screw section  15   a   1  and the bolt head section male screw section  72   a  engage with each other (see  FIG.  6 ( e ) ). In this process, the bit  13   b  slides upward in the interior of the bit holder  13  while resisting the elastic force provided by the spring  13   c.    
     When the tension rod  15  is further caused to screw downward while rotating, the lower end section of the supporter  18  comes into contact with the upper surface of the fastening target object H, so that the fastening device  100  stops lowering. Since the supporter  18  is in contact with the fastening target object H, an attempt to further rotate the tension rod  15  in the direction indicated with the arrow K 1  cannot cause the tension rod  15  to screw downward. In this process, since the bit  13   b  inserted into the insertion hole  72   b  of the bolt head section  72  prevents the bolt  70  from rotating, the tension rod  15  exerts a load in the pulling direction (that is, upward) on the bolt  70 . However, since the supporter  18  is in contact with the fastening target object H, rotation of the tension rod  15  cannot move the bolt  70  in the pulling direction. As a result, tensile force P acts on the nut  80  and the supporter  18  (see  FIG.  6 ( f ) ). Since the upper end section of the supporter  18  is in contact with the thrust bearing  17 , the thrust bearing  17  and the fastening target object H compress the supporter  18 , and the compression force is detected as tensile force P with the sensor  181 . 
     A target value of the tensile force P is now defined as target tensile force (10 kN, for example). To prevent overshooting operation of the second motor  30 , it is desirable to decelerate the rotation of the shaft of the second motor  30  as the tensile force P approaches the target tensile force and stop the operation of the second motor  30  when the tensile force P reaches the target tensile force (corresponding to second step). The target tensile force is also target axial force acting on the bolt  70 . 
     The shaft of the first motor  10  is then caused to rotate again in the direction indicated with the arrow K 1  with the tensile force P applied to cause the bolt head section  72  to be seated on the fastening target object H (corresponding to third step). It is desirable that the tension rod female screw section  15   a   1  and the male screw formed around the bolt shaft section  71  have the same screw pitch. If the screw pitches differ from each other, the rotation control performed on the bit  13   b  (that is, drive control performed on first motor  10 ) and the rotation control performed on the tension rod  15  (that is, drive control performed on second motor  30 ) need to be performed simultaneously, resulting in cumbersome control. On the other hand, when the screw pitches are equal to each other, the bolt is tightened only by rotation of the bit  13   b , resulting in no cumbersome synchronous control. 
     When the bolt head section  72  comes into contact with the fastening target object H, the tensile force P starts decreasing, whereas axial force F acting on the bolt  70  increases (see  FIGS.  6 ( g )  and  7 ). The derivative of the tensile force P, dP/dθ, is regularly monitored, and the rotation of the bit  13   b  is stopped when the behavior of dP/dθ changes from an unstable curve (nonlinear region) to a straight line (corresponding to fourth step). The term “dθ” corresponds to the angle of rotation of the bolt  70 . 
     Finally, the shaft of the second motor  30  is rotated in the direction opposite the direction at the time of bolt tightening to retract the fastening device  100  from the fastening target object H and cause the bolt head section  72  and the tension rod  15  to disengage from each other (see  FIG.  6 ( h ) ). At this point, the axial force F acting on the bolt  70  shows a value close to the target tensile force, whereby precise axial force F can be provided. 
     In the present embodiment, the operation of tightening the bolt  70  is stopped when the axial force becomes greater than the target axial force F to cause the bolt head section  72  and the tension rod  15  to disengage from each other, as shown in  FIG.  7   . When the operation of tightening the bolt  70  is stopped immediately after the bolt  70  is seated on the fastening target object H to cause the bolt head section  72  and the tension rod  15  to disengage from each other, the axial force decreases by the amount corresponding to the elastic deformation of the bolt  70  and the fastening target object H. A decrease σ representing the decrease described above corresponds to the difference between the axial force produced when the behavior of dP/dθ changes from a curve to a straight line and the target axial force F. It is, however, noted that the tensile force P may be set in advance at a value greater by the decrease σ in expectation of the decrease in the axial force after the bolt  70  is seated, and the operation of tightening the bolt  70  may be stopped immediately after the bolt  70  is seated. In this case, when the bolt head section  72  and the tension rod  15  are caused to disengage from each other, the axial force acting on the bolt  70  decreases toward the target axial force F. 
     Second Embodiment 
       FIG.  8    is a cross-sectional view of a fastening device  200 .  FIG.  9    is a perspective view of part of the bolt. A bolt  90  includes a bolt head section  91  and a bolt shaft section  92 . A bottomed, tubular head section opening  91   a  (corresponding to recess) is formed in the top surface of the bolt head section  91 , and a head section female screw section  91   b  is formed in the inner circumferential surface of the head section opening  91   a . A male screw  92   a  is formed around the bolt shaft section  92 . The bolt  90  is fastened along with a nut  93  (see  FIGS.  10  and  11   ), which will be described later, to the fastening target object H. The fastening target object H is the same as that in the first embodiment and will therefore not be described. 
     The fastening device  200  includes a first motor  210 , a second motor  220 , and an enclosure  223 . The first motor  210  and the second motor  220  are independent of each other and driven and controlled by a controller  9 . That is, the controller  9  can control the first motor  210  and the second motor  220  independently of each other. The controller  9  can, for example, be a CPU (central processing unit). 
     The enclosure  223  includes a body  223   a  and a base  223   b , which has a recessed upper end fixed to the lower surface of the body  223   a . The body  223   a  accommodates a first decelerator  211  and a second decelerator  221 . A tension rod  201  for rotating the bolt  90  protrudes beyond the lower surface of the enclosure  223 . A columnar protrusion  201   a  having a columnar shape (corresponding to projection) is provided at the lower end section of the tension rod  201 , and a rod male screw section  201   b  are formed around the outer circumferential surface of the columnar protrusion  201   a.    
     The rotational force produced by the first motor  210  is transmitted to the tension rod  201  via the first decelerator  211 , and the tension rod  201  rotates around an axis extending in the upward/downward direction. Rotating the tension rod  201  with the rod male screw section  201   b  of the tension rod  201  and the head section female screw section  91   b  engaging with each other allows the bolt  90  to be tightened. The fastening device  200  is so supported as to be allowed to lower by its own weight relative to a support section (not shown). Therefore, when the bolt  90  screws toward the fastening target object H, the entire fastening device  200  lowers along with the bolt  90 . 
     The second motor  220  is located next to the first motor  210  in a side-by-side fashion and connected to a second decelerator  221  located immediately below the second motor  220 . The second decelerator  221  decelerates the rotation inputted from the second motor  220  and outputs the decelerated rotation via an output shaft  229 . The output shaft  229  is so supported as to be rotatable relative to a bearing  222 . 
     The rotational force produced by the second motor  220  is transmitted to a drive socket  202  via the second decelerator  221 , the output shaft  229 , a pinion gear  227 , and an idle gear  228 . That is, a drive gear  316 , which is formed around the outer circumferential surface of the drive socket  202 , engage with the idle gear  228 , and the second motor  220  can be operated to rotate the drive socket  202  around an axis extending in the upward/downward direction. 
     Bearings  225  are disposed to receive loads acting on the pinion gear  227  and the idle gear  228  when the shaft of the second motor  220  rotates. The idle gear  228  is rotationally driven via an idle shaft  226  extending in the upward/downward direction. 
     The drive socket  202  protrudes beyond the lower surface of the enclosure  223 . The drive socket  202  is so supported as to be rotatable relative to a pair of upper and lower bearings  217   a ,  217   b  and a bearing  312 . A retaining ring  216  is provided to prevent the upper bearing  217   a  from falling off. 
     The drive socket  202  has a tubular shape and accommodates the tension rod  201 . A bolt head section grasper  202   a , which protrudes inward in the radial direction, is formed at a lower end section of the drive socket  202 . The inner radial surface of the bolt head section grasper  202   a  has a shape corresponding to the side surface of the bolt head section  91  (that is, hexagonal shape in the present embodiment). Rotating the drive socket  202  with the bolt head section grasper  202   a  fit to the bolt head section  91  allows the bolt  90  to screw relative to the fastening target object H. An attempt to rotate the bolt  90  via the tension rod  201  with the drive socket  202  being stationary cannot rotate the bolt  90  because the bolt head section grasper  202   a  grasps the bolt head section  91 . 
     A supporter  203  includes a supporter accommodator  203   a  for accommodating the drive socket  202 . The upper end section of the supporter  203  is in contact with the bearing  217   b , and the lower end section of the supporter  203  extends downward beyond the lower end section of the drive socket  202 . Therefore, when the entire fastening device  200  lowers toward the fastening target object H, the lower end section of the supporter  203  comes into contact with the fastening target object H. A gap that allows the rotation of the drive socket  202  is formed between the drive socket  202  and the supporter  203 . 
     Now, let S 1  be the area over which the supporter  203  and the fastening target object H are in contact with each other and S 2  be the area over which the bolt head section  91  and the fastening target object H are in contact with each other, and it is desirable that the contact areas S 1  and S 2  are equal to each other. The seat surface of the bolt head section  91  and the supporter  203  exert surface pressure on the fastening target object H. Therefore, when the contact areas S 1  and S 2  differ from each other (that is, when difference in surface pressure is present), the amount of deformation of the fastening target object H undesirably differs from the amount of deformation in actual bolt fastening. Further, when the difference in surface pressure excessively increases, the supporter  203  is likely to deform and damage the fastening target object H, and the difference in the amount of deformation is likely to lower the fastening precision. 
     A sensor  21  includes a straining element  213  and a strain gauge  214 . The straining element  213  has a cylindrical shape having a reduced-diameter intermediate section. The straining element  213  can be made of metal. The straining element  213  is disposed radially outside the tension rod  201  and between the decelerator  211  and the drive socket  202 . The upper portion of the straining element  213  is pressed by the decelerator  211  via a thrust bearing  212 . A lower portion of the straining element  213  is supported by the drive socket  202  via a thrust bearing  215 . The strain gauge  214  is attached to the intermediate section of the straining element  213 . The sensor  21  may instead be provided in the supporter  203 . 
     In the process of pulling the bolt  90 , the straining element  213  receives upward force having the same magnitude as that of tensile force transmitted from the supporter  203  and is therefore distorted in the compression direction within an elastic deformation range of the straining element  213 . The strain gauge  214  changes in terms of resistance in accordance with the strain of the straining element  213  and changes output voltage to be outputted to the controller  9 . The controller  9  calculates the tensile force acting on the screw section of the bolt  90  based on the amount of change in the output voltage from the strain gauge  214 . Thrust bearings  212  and  215 , which are provided on the upper and lower sides of the sensor  21 , have the function of suppressing transmission of turbulent force received from an object other than the supporter  203  to the sensor  21 . The sensor  21  is not limited to the combination of the straining element  213  and the strain gauge  214  and can be another sensor capable of detecting the tensile force acting on the bolt  90 . 
     The action of the fastening device  200  will next be described with reference to  FIGS.  10  and  11   , which are descriptive diagrams of the action. It is now assumed that the bolt  90  is temporarily attached to the fastening target object H, and that the bolt head section  91  is located in a temporal attachment position separate from the upper surface of the fastening target object H in an initial state (see  FIG.  10 ( a ) ). It is further assumed that the bolt head section grasper  202   a  of the drive socket  202  grasps an upper end section of the bolt head section  91 . It is still further assumed that the columnar protrusion  201   a  of the tension rod  201  is in contact with the upper end of the head section female screw section  91   b  of the bolt head section  91 . It is still further assumed that the control described below is performed by the controller  9  unless otherwise stated. 
     When the second motor  220  is operated, the second decelerator  221 , the output shaft  229 , the pinion gear  227 , the idle gear  228 , and the drive gear  316  rotate, and the bolt  90  grasped by the grasper  202   a  of the drive socket  202  rotates in the direction indicated by the arrow K 1  (in other words, clockwise direction in plan view). When the drive socket  202  rotates, the bolt  90  screws downward. The fastening device  200  is so supported as to be allowed to lower by its own weight relative to a support section (not shown), and the entire fastening device  200  therefore lowers with the drive socket  202  rotating. 
     When the drive socket  202  further rotates the bolt  90  in the direction indicated by the arrow K 1  and the bolt head section  91  is seated on the upper surface of the fastening target object H, the force that is received by the second motor  220  and attempts to rotate the shaft of the second motor  220  abruptly increases, resulting in an abrupt increase in the feedback torque. The controller  9  senses based on the feedback torque produced by the second motor  220  that the bolt  90  has been seated and stops the operation of the second motor  220 .  FIG.  10 ( b )  shows the state immediately after the operation of the second motor  220  is stopped. 
     After the operation of the second motor  220  is stopped, the second motor  220  is so operated that the shaft thereof rotates in the opposite direction to rotate the drive socket  202  in the direction indicated with the arrow K 2  (in other words, counterclockwise direction in plan view). When the drive socket  202  is rotated in the direction indicated with the arrow K 2 , the bolt  90  screws upward, and the tension rod  201 , the drive socket  202 , and the supporter  203  integrally move upward. The bolt head section  91  thus moves to a position slightly separate from the upper surface of the fastening target object H (in other words, tension rod drive start position).  FIG.  10 ( c )  shows the state immediately after the bolt  90  reaches the tension rod drive start position. 
     Thereafter, the operation of the second motor  220  is stopped, and the first motor  210  is operated. When the first motor  210  is operated, the tension rod  201  screws toward the bottom surface of the head section opening  91   a  of the bolt head section  91  while rotating in the direction indicated by the arrow K 1 , and the drive socket  202  and the supporter  203  lower along with the tension rod  201  toward the fastening target object H.  FIG.  11 ( d )  shows the state in a halfway position where the tension rod  201 , the drive socket  202 , and the supporter  203  lower toward the fastening target object H. The position of the bolt  90  does not change in the course of transition from the step shown in  FIG.  10 ( c )  to the step shown in  FIG.  11 ( d ) . 
     When the tension rod  201  further screws, the lower end section of the supporter  203  comes into contact with the upper surface of the fastening target object H, so that the fastening device  200  stops lowering.  FIG.  11 ( e )  shows the state immediately after the supporter  203  comes into contact with the upper surface of the fastening target object H. Since the supporter  203  is in contact with the fastening target object H, an attempt to further rotate the tension rod  201  in the direction indicated with the arrow K 1  cannot cause the tension rod  201  to screw downward. 
     In this process, since the grasper  202   a  of the drive socket  202  prevents the bolt  90  from rotating, the tension rod  201  exerts a load in the pulling direction (that is, upward) on the bolt  90 . However, since the supporter  203  is in contact with the fastening target object H, rotation of the tension rod  201  cannot move the bolt  90  in the pulling direction. As a result, tensile force P acts on the nut  93  and the supporter  203  (see  FIG.  11 ( e ) ). In this process, the members (such as straining element  213 ) sandwiched between the thrust bearing  212  and the supporter  203  are compressed, and the compression force is detected as tensile force P with the strain gauge  214 . 
     A target value of the tensile force P is now defined as target tensile force (10 kN, for example). To prevent overshooting operation of the first motor  210 , it is desirable to decelerate the rotation of the shaft of the first motor  210  as the tensile force P approaches the target tensile force and stop the operation of the first motor  210  when the tensile force P reaches the target tensile force. The target tensile force is also target axial force acting on the bolt  90 . 
     The shaft of the second motor  220  is then caused to rotate again in the direction indicated with the arrow K 1  with the tensile force P applied to cause only the bolt  90  out of the tension rod  201 , the drive socket  202 , the supporter  203 , and the bolt  90  to screw downward. In other words, the bolt  90  moves in the direction in which the bolt  90  is pulled out of the tension rod  201 . When the bolt  90  further screws downward, the bolt head section  91  is seated on the fastening target object H. It is desirable that the head section female screw section  91   b  and the male screw  92   a  of the bolt shaft section  92  have the same screw pitch. If the screw pitches differ from each other when performing from the step shown in  FIG.  11 ( e )  to the step shown in  FIG.  11 ( f ) , the rotation control performed on the tension rod  201  (that is, drive control performed on first motor  210 ) and the rotation control performed on the drive socket  202  (that is, drive control performed on second motor  220 ) need to be performed simultaneously, resulting in cumbersome control. On the other hand, when the screw pitches are equal to each other, the bolt  90  is tightened only by rotation of the drive socket  202 , resulting in no cumbersome control. 
     When the bolt head section  91  comes into contact with the fastening target object H, the tensile force P starts decreasing, whereas axial force F acting on the bolt  90  increases (see  FIGS.  11 ( f )  and  7 ). The derivative of the tensile force P, dP/dθ, is regularly monitored, and the rotation of the drive socket  202  is stopped when the behavior of dP/dθ changes from an unstable curve (nonlinear region) to a straight line. The term “dθ” corresponds to the angle of rotation of the bolt  90 . 
     Finally, the shaft of the first motor  210  is rotated in the direction opposite the direction at the time of bolt tightening to retract the tension rod  201  to a position above the bolt head section  91  and cause the drive socket  202  and the bolt head section  91  to disengage from each other. The bolt  90  is thus removed from the fastening device  200 . At this point, the axial force F acting on the bolt  90  shows a value close to the target tensile force, whereby precise axial force F can be provided. 
     (Example of Other Usage of Fastening Device  100 ) 
     In the embodiments described above, the fastening device  100  ( 200 ) is used to newly tighten the bolt  70  ( 90 ), but not necessarily in the present invention, and the fastening device  100  ( 200 ) may be used to retighten the existing bolt  70  ( 90 ) determined to have insufficient axial force in an axial force detection step. For example, when bolts are successively fastened to a flange-shaped multi-axis workpiece (fastening target object), the axial force acting on a bolt fastened in an early stage of the fastening process decreases in some cases. In such cases, after the fastening device  100  ( 200 ) fastens the bolt, the axial force acting on the bolt can be detected by pulling the bolt. An axial force detection theory is described in Japanese Patent No. 4,028,254 and will therefore not be described in detail. When it is determined that the axial force is insufficient as a result of the axial force detection, the fastening device  100  ( 200 ) can perform retightening to manage the axial force with higher reliability. Moreover, the fastening devices  100  ( 200 ) according to the embodiments of the present invention can be arranged (in the form of a matrix, for example) and simultaneously tighten a plurality of bolts  70  ( 90 ). For example, even in the case of a multi-axis workpiece, such as bolts for a cylinder head of an engine, the fastening devices  100  ( 200 ) can perform simultaneous tightening, whereby stable axial force is provided across the entire product, resulting in improvement in the quality of the product. 
     REFERENCE SIGNS LIST 
     
         
           5 ,  9 : Controller 
           10 ,  210 : First motor 
           11 ,  211 : First decelerator 
           11   a : Output shaft 
           12 : Power transmitter 
           13 : Bit holder 
           13   a : Magnet 
           13   b : Bit 
           13   c : Spring 
           14 : Bearing 
           15 ,  201 : Tension rod 
           15   a   1 : Tension rod female screw section 
           18 ,  203 : Supporter 
           30 ,  220 : Second motor 
           70 : Bolt 
           71 : Bolt shaft section 
           72 : Bolt head section 
           72   a : Bolt head section male screw section 
           72   b : Insertion hole 
           100 : Fastening device 
           200 : Fastening device