Patent Publication Number: US-8978520-B2

Title: Tightening machine having socket unit

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Japanese Patent Application No. 2010-279170 filed on Dec. 15, 2010, entitled “TIGHTENING MACHINE HAVING SOCKET UNIT”, which is hereby incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a socket unit used for tightening a nut while interposing a reaction washer therebetween and to a tightening machine having the socket unit, and more particularly, the present invention relates to a tightening machine having a socket unit, even in a phase misalignment between a nut and a reaction washer, capable of easily fitting an outer socket to a reaction washer in a state where a nut is fitted to an inner socket. 
     2. Description of Related Art 
     As shown in  FIG. 15 , there is a known method in which when structures are to be tightened to each other, a reaction washer  95  is fitted around a bolt  91  projecting from the structure  90 , and a nut  93  is tightened from above the reaction washer  95 . The reaction washer  95  has a concavo-convex periphery  96  in which twelve V-shaped grooves are repeatedly formed in a peripheral surface of a disk-like washer at about 30° intervals, and the nut  93  is tightened using a tightening machine provided at its tip end with a socket unit while receiving a tightening reaction force by the reaction washer  95 . 
     The tightening machine is concentrically provided with an inner socket holder and an outer socket holder which rotate in opposite directions from each other. The socket unit includes an inner socket and an outer socket which are formed concentrically. 
     The inner socket is provided at its base end outer periphery with a mounting projection. The mounting projection detachably engages with the inner socket holder of the tightening machine. A mounting groove is formed in an inner periphery of the inner socket holder. The inner socket is fitted to a mounting groove formed in an inner periphery of the inner socket holder without clearance in its circumferential direction. The outer socket also detachably engages with the outer socket holder, and if the tightening machine is operated, the inner socket and the outer socket rotate in opposite directions from each other. 
     The nut  93  is tightened in such a manner that an inner peripheral surface of a tip end of the inner socket is fitted to the nut  93 , and then an inner peripheral surface of a tip end of the outer socket is fitted to the reaction washer  95  and in this state, the tightening machine is operated. 
     It is necessary to tighten the nut  93  under preset torque, but primary tightening (temporarily tightening) is carried out before a so-called final tightening operation for tightening the nut  93  under the set torque. In this primary tightening operation, positions in the rotation direction (“phase”, hereinafter) of the reaction washer  95  and the nut  93  align with each other in some cases as shown in  FIG. 16A , but the phases of the reaction washer  95  and the nut  93  are misaligned in most of cases as shown in  FIG. 16B . 
     When the nut  93  and the reaction washer  95  are compared with each other, the nut  93  is thicker than the reaction washer  95  as shown in  FIG. 15 . Therefore, when the socket unit is fitted to the nut  93  and the reaction washer  95 , the inner socket is firstly fitted to the nut  93 , and then, if the socket unit is further moved forward, the outer socket is fitted to the reaction washer  95 . 
     When the inner socket and the nut  93  are fitted to each other, even if the phases of the inner socket and the nut  93  are misaligned each other, if an operator rotates the tightening machine itself around its axis, the phase misalignment can be solved. Therefore, the inner socket can be fitted to the nut  93 . 
     However, when the reaction washer  95  is to be fitted to the outer socket after the nut  93  is fitted to the inner socket, the outer socket does not excellently mesh with the concavo-convex periphery  96  of the reaction washer  95  depending upon the phase misalignment between the reaction washer  95  and the nut  93 , as shown in the above and in  FIGS. 16A and 16B , and the outer socket cannot be fitted to the reaction washer  95 . 
     Hence, it is proposed that an inner surface shape of a nut-engaging hole  32  formed in the inner socket to which the nut  93  is fitted is not formed into the same hexagonal shape as that of the nut  93 , a width of a bottom surface  33   a  of a groove  33  is widened to about 30° and an inner socket  30  having a nut-engaging hole  32  having clearance is used. Concerning a shape of the groove  33  of the inner socket, see a front view of  FIG. 12  of the present invention. 
     By widening the width of the groove  33 , it is possible to provide clearance of about 30° in the case of the illustrated example. Therefore, if a phase misalignment between the reaction washer  95  and the nut  93  is as small as 5° or 10° and is much smaller than 30°, it is theoretically possible to engage the reaction washer  95  with the outer socket if an operator rotates the tightening machine itself around its axis after the nut  93  is made to mesh with the inner socket. 
     The maximum phase misalignment between the reaction washer and the nut is theoretically 30° but members vary and the nut and the reaction washer are not concentric. Therefore, the maximum phase misalignment between the reaction washer and the nut exceeds 30° in some cases. In such a case, even if the tightening machine itself is rotated, the reaction washer cannot be engaged with the outer socket. Hence, in such a case, a so-called inching operation is required. In the inching operation, the tightening machine is slightly operated to align the phases of the outer socket and the inner socket from each other. If the phases do not align by one inching operation, the inching operation must be repeated many times and this deteriorates operation efficiency. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a tightening machine having a socket unit, even if a phase misalignment occurs between a nut and a reaction washer, capable of easily fitting an outer socket to a reaction washer in a state where the nut is fitted to an inner socket. 
     To solve the above problem, the present invention provides a tightening machine having a socket unit for tightening a reaction washer fitted to a bolt which projects from a structure, the socket unit including an inner socket whose tip end can engage with the nut, and an outer socket which is formed concentrically with the inner socket and whose tip end can engage with the reaction washer, the tightening machine concentrically including an inner socket holder and an outer socket holder which can rotate in opposite directions from each other, the outer socket holder being formed on an outer periphery of the inner socket holder, the inner socket being detachably engaged with the inner socket holder, and the outer socket being detachably engaged with the outer socket holder, wherein the inner socket is engaged with the inner socket holder with clearance in its circumferential direction. 
     According to the tightening machine having the socket unit of the present invention, the inner socket has clearance in the rotation direction with respect to the inner socket holder. Hence, the tightening machine is not influenced by a phase misalignment between the reaction washer and the nut, and if an operator rotates the tightening machine itself around its axis in a state where the nut is engaged with the inner socket, the inner socket can be rotated with respect to the outer socket and the phase of the outer socket with respect to the inner socket can be shifted. Therefore, the phases of the reaction washer and the outer socket can align with each other, and the outer socket can be engaged with the reaction washer. 
     Hence, the inching operation for operating the tightening machine and aligning the phases of the reaction washer and the outer socket with each other becomes unnecessary. Therefore, it is possible to remarkably enhance the efficiency of the final tightening operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing a state where a socket unit is detached from a tightening machine; 
         FIG. 2  is a front view and a partial sectional view of a side surface of the socket unit; 
         FIG. 3  is a front view, a partial sectional view of a side surface and a rear view of an outer socket body from which a reaction washer-engaging portion is detached; 
         FIG. 4  is a sectional view and a front view of the reaction washer-engaging portion of a outer socket; 
         FIG. 5  is a front view, a partial sectional view of a side surface and a rear view of the inner socket; 
         FIG. 6  is a partial sectional view of a side surface of a tip end of the tightening machine; 
         FIG. 7  is a partial sectional perspective view of an inner socket holder; 
         FIG. 8  is a front view and a sectional view of the inner socket holder; 
         FIG. 9  is a sectional view showing an engaged state between the inner socket and the inner socket holder; 
         FIG. 10  is a sectional view showing an engaged state between the inner socket and the nut; 
         FIG. 11  shows another embodiment of the present invention, and is a front view and a partial sectional view of a side surface of a socket unit; 
         FIG. 12  is a front view and a sectional view of a side surface of a nut-side member; 
         FIG. 13  is a front view, a sectional view of a side surface and a rear view of a holder-side member; 
         FIG. 14  is a sectional view showing an engaged state between the holder-side member and the nut-side member; 
         FIG. 15  is a sectional view showing a state where a reaction washer and a nut are fitted to a bolt projecting from a structure; and 
         FIGS. 16A and 16B  are diagrams of  FIG. 15  as viewed from the nut, wherein  FIG. 16A  shows a state where phases of the nut and the reaction washer are the same and  FIG. 16B  shows a state where the phases of the nut and the reaction washer are different from each other. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a perspective view showing a state where a socket unit  20  is detached from a tightening machine  10 . The socket unit  20  is mounted on a tip end of the tightening machine  10  and as shown in  FIG. 15 , the socket unit  20  is for carrying out a final tightening operation of a nut  93  which is primary tightened to a bolt  91  projecting from a structure  90  through a reaction washer  95 . A configuration of the socket unit  20  and a configuration of the tightening machine  10  will be described hereinafter. 
     &lt;Socket Unit&gt; 
     As shown in  FIGS. 1 and 2 , the socket unit  20  concentrically has an inner socket  30  disposed on its inner peripheral side and an outer socket  50  disposed on its outer peripheral side, a bush  22  is fitted between the inner socket  30  and the outer socket  50 , and the inner socket  30  can rotate with respect to the outer socket  50 . 
     As shown in  FIGS. 1 to 4 , the outer socket  50  includes a cylindrical outer socket body  51  which penetrates and opens in its axial direction, and a reaction washer-engaging portion  60  mounted on the outer socket body  51 . 
     The outer socket body  51  includes a plurality of mounting recesses  52  (see  FIG. 3 ) formed in its side (hereinafter referred to as “tip end” or “tip end side”) opposite from the tightening machine  10 , and the mounting recesses  52  mesh with a plurality of mounting projections  63  (see  FIG. 4 ) projecting from an outer periphery of the reaction washer-engaging portion  60  at equally-spaced intervals. A screw hole  53  is formed in a peripheral surface of the outer socket body  51  near its tip end for integrally fixing the reaction washer-engaging portion  60 . 
     A diameter of the outer socket body  51  on the side of the tightening machine  10  (hereinafter referred to as “base end” or “base end side”) is reduced, and the inner socket  30  rotatably and slidably abuts against a support surface  54  whose diameter is reduced and which is perpendicular to the axial direction. 
     A mounting flange  55  having a plurality of notches projects from a diameter-reduced side peripheral surface of the outer socket body  51 , the mounting flange  55  meshes with a flange receiving portion  81  of an outer socket holder  80 , and the mounting flange  55  functions as a detent member when the outer socket  50  is mounted on the outer socket holder  80 . A peripheral groove  56  into which a lock screw  82  is fitted is formed at a location closer to the base end side than the mounting flange  55 , and the lock screw  82  functions as a detent member when the mounting flange  55  is mounted on the outer socket holder  80 . 
     The reaction washer-engaging portion  60  is mounted on a tip end of the outer socket body  51 , and includes a reaction washer-engaging hole  61  having an inner periphery into which a concavo-convex periphery  96  of an outer periphery of the reaction washer  95  is fitted. When the reaction washer  95  has twelve V-shaped concavo-convex peripheries  96 , twelve V-shaped bottom grooves  62  are arranged in the reaction washer-engaging hole  61  at equally-spaced intervals in the circumferential direction. 
     A mounting projection  63  which is fitted into the mounting recess  52  of the outer socket body  51  projects from an outer periphery of the reaction washer-engaging portion  60  as described above. Notches are formed in the illustrated mounting projection  63  at predetermined intervals, projections formed between the notches mesh with the mounting recess  52  of the outer socket body  51 . 
     An inner diameter of the base end of the reaction washer-engaging portion  60  is greater than a diameter of the reaction washer-engaging hole  61 , and a step formed by this configuration functions as a later-described detent member  64  of the inner socket  30 . 
     As shown in  FIGS. 1 ,  2  and  5 , the inner socket  30  is configured based on a cylindrical inner socket body  31  which penetrates and opens in its axial direction, and the inner socket body  31  includes a nut-engaging hole  32 . A diameter of a tip end of the nut-engaging hole  32  is increased, and the nut  93  is integrally rotatably engaged with an inner surface of the nut-engaging hole  32 . The illustrated nut-engaging hole  32  has twelve V-shaped bottom grooves  33  at equally-spaced intervals in the circumferential direction so that the nut-engaging hole  32  can engage even if the grooves  33  are misaligned from the hexagonal nut  93  in phase 30°. 
     A diameter of the inner socket  30  at a location closer to its base end than the nut-engaging hole  32  is reduced, and its step  34  slidably abuts against the support surface  54  of the outer socket body  51 . 
     A base end side of the inner socket body  31  projects more than a base end of the outer socket body  51  when the inner socket body  31  is mounted on the outer socket  50 , a plurality of projections  35  project from a projecting peripheral surface in a direction along its axis, and the projections  35  engage with recesses  71  of a later-described inner socket holder  70  with clearance. 
     Widths of the projections  35  and the recesses  71  can appropriately be set in accordance with an amount of clearance required by the inner socket  30  with respect to the inner socket holder  70 . It is preferable that the clearance has an angle exceeding 30° and in this case, it should be configured such that the projection  35  can rotate within the recess  71  beyond 30° in the circumferential direction. It is preferable that the clearance is set in a range of 35° or more and 45° or less, and more preferably 40° or more. By setting the clearance in this manner, the number of the phase alignment points of the reaction washer  95  matches with the phase of the outer socket  50  can be set to one or two. 
     In the illustrated embodiment, the number of the projections  35  is four, but if there is a margin of strength, the number of the projections  35  may be two or three. It is of course possible to provide five or more projections  35  depending upon a required amount of clearance. 
     In this embodiment, the projections  35  are formed on the inner socket  30  and the recesses  71  are formed in the inner socket holder  70 , the projections  35  and the recesses  71  are nominal names in terms of description and the recesses may be formed in the inner socket  30  and the projections may be formed on the inner socket holder  70 . The clearance is not limited to the engagement between the projection and the recess, and other configuration can also be employed. 
     The inner socket  30  having the above-described configuration is fitted into the outer socket body  51  with the bush  22  interposed therebetween, the mounting projections  63  of the reaction washer-engaging portion  60  is meshed with the mounting recess  52  of the outer socket body  51  and they are screwed, and the socket unit  20  shown in  FIGS. 1 and 2  can be formed. 
     &lt;Tightening Machine&gt; 
     As shown in  FIGS. 1 and 6 , the tightening machine  10  on which the socket unit  20  is detachably mounted has the inner socket holder  70  disposed on the inner side of its tip end and the outer socket holder  80  formed concentrically with the outer periphery of the inner socket holder  70  such that the inner socket holder  70  and the outer socket holder  80  can rotate in the opposite directions from each other. The inner socket holder  70  and the outer socket holder  80  are closely connected to a power mechanism (not shown) of the tightening machine  10  through a planet gear speed reducing mechanism (not shown) and they can rotate. 
     The present invention is characterized in that the inner socket  30  of the socket unit  20  is engaged with the inner socket holder  70  of the tightening machine  10  with clearance. Description of a mechanism which rotates the inner socket holder  70  and the outer socket holder  80  in the opposite directions will not be given. 
     The inner socket holder  70  is the cylindrical holder which rotatably engages with the inner socket  30  of the socket unit  20  with clearance, is disposed at a location behind the outer socket holder  80 , and as shown in  FIGS. 1 and 6  to  8 , the plurality of recesses  71 ,  71  are provided in the inner peripheral surface along the axial direction. When the inner socket  30  is fitted to the inner socket holder  70 , the projection  35  of the inner socket  30  can engage with the recess  71  as shown in  FIG. 9 . The number of recesses  71  is the same as that of the projections  35  of the inner socket  30  and the number of the recesses  71  is four in the illustrated embodiment. A width of the recess  71  is formed wider than the projection  35  of the inner socket  30 , and it is preferable that the width of the recess  71  is set such that the inner socket  30  can rotate by a desired amount of clearance with respect to the inner socket holder  70  in a state where the projection  35  is fitted to the recess  71 . It is preferable that the angle of the clearance exceeds 30°, and the angle is preferably set in a range of 35° or more and 45° or less, and more preferably 40° or more as described above. 
     The outer socket holder  80  is a cylindrical holder disposed on the outer periphery of the inner socket holder  70  such that the outer socket holder  80  can rotate in the direction opposite from that of the inner socket holder  70 , and the outer socket holder  80  is provided at its tip end with the flange receiving portion  81  including the plurality of notches with which the mounting flange  55  of the outer socket holder  80  meshes. Screw holes  83 ,  83  are formed in the outer socket holder  80  at locations close to its tip end. Lock screws  82  which enter peripheral grooves  56  of the base end of the outer socket  50  are fitted into the screw holes  83 ,  83  when the outer socket  50  is mounted. 
     The socket unit  20  is detachably mounted on the inner socket holder  70  and the outer socket holder  80  of the tightening machine  10 . The socket unit  20  is fitted in a state where it is aligned such that the projections  35  of the tip end of the inner socket  30  are fitted to the recesses  71  of the inner periphery of the inner socket holder  70  (see  FIG. 9 ) and then, the mounting flange  55  of the outer socket  50  is fitted to the flange receiving portion  81  of the outer socket holder  80 , the lock screws  82  are tightened such that the socket unit  20  does not come out, and the socket unit  20  is mounted on the tightening machine  10 . 
     Since the outer socket holder  80  and the inner socket holder  70  are closely connected to the power mechanism of the tightening machine  10 , free rotation is prevented. Therefore, if the socket unit  20  is mounted on the tightening machine  10 , the outer socket  50  and the outer socket holder  80  become one piece and they cannot be rotated. On the other hand, the inner socket  30  can rotate with respect to the outer socket  50  and the inner socket  30  has clearance in the rotation direction with respect to the inner socket holder  70 . Therefore, the inner socket  30  can rotate within a range of clearance. 
     In the tightening machine  10  having the socket unit  20 , the primary tightened nut  93  is finally tightened under predetermined torque. The final tightening operation is carried out in such a manner that the axis of the bolt  91  and the axis of the socket unit  20  are aligned with each other, the tightening machine  10  is moved forward such that the socket unit  20  approaches the nut  93  and the reaction washer  95  and in this state, the nut  93  is engaged with the nut-engaging hole  32  of the inner socket  30 . 
     If the phases of the nut  93  and the groove  33  of the nut-engaging hole  32  are misaligned with each other, an operator rotates the tightening machine itself around its axis. In this embodiment, since the nut-engaging hole  32  has the twelve grooves  33  as shown in  FIG. 5 , if the operator rotates the tightening machine itself 30° at a maximum around its axis, the nut-engaging hole  32  and the nut  93  are engaged with each other. 
     The tightening machine  10  is further moved forward in the state where the inner socket  30  and the nut  93  are engaged with each other, and the outer socket  50  and the reaction washer  95  are engaged with each other. At that time, if the concavo-convex periphery  96  of the reaction washer  95  and the grooves  62  of the outer socket  50  are not misaligned in phase from each other, the outer socket  50  can be engaged with the reaction washer  95  by moving the tightening machine  10  forward as it is. However, if the nut  93  and the reaction washer  95  have phase misalignment generated at the time of primary tightening as shown in  FIG. 16B  or if the inner socket  30  and the outer socket  50  are misaligned each other in phase, the outer socket  50  cannot excellently be engaged with the reaction washer  95 . 
     In the present invention, since the inner socket  30  has clearance exceeding 30° in the rotation direction with respect to the inner socket holder  70 , if an operator rotates the tightening machine itself around its axis in a state where the nut  93  is engaged with the inner socket  30 , the inner socket  30  rotates with respect to the outer socket  50  and the phase of the outer socket  50  with respect to the inner socket  30  is shifted. Therefore, the phase of the concavo-convex periphery  96  of the reaction washer  95  and the phase of the groove  62  of the reaction washer-engaging hole  61  of the outer socket  50  can align with each other and the outer socket  50  can be engaged with the reaction washer  95 . Further, if the clearance is 30°, the number of the phase alignment point becomes one or less, but if the clearance is set to the angle exceeding 30°, the number of the phase alignment points become one or two, and it becomes easy to engage the reaction washer  95  with the outer socket  50 . 
     Therefore, the inching operation for operating the tightening machine  10  and making the phases of the reaction washer  95  and the outer socket  50  align with each other becomes unnecessary. Hence, it is possible to remarkably enhance the efficiency of the final tightening operation. 
     The nut-engaging hole  32  of the inner socket  30  has the twelve grooves  33 , but the number of the grooves  33  can be six or eighteen. 
     Concerning the nut-engaging hole  32  of the inner socket  30 , if the bottom surfaces  33   a  of the grooves  33  with which the nut  93  engages are made as six groove-shapes each having a width in the circumferential direction as shown in  FIG. 10 , a total of clearance of the nut  93  with respect to the nut-engaging hole  32  and clearance between the inner socket  30  and the inner socket holder  70  in the circumferential direction can be set to an angle exceeding 30°. It is preferable that the total of clearance is 35° or more and 45° or less and more preferably 40° or more. When the total of clearance is set to 40°, if the clearance between the nut-engaging hole  32  and the nut  93  is 30°, the clearance between the inner socket  30  and the inner socket holder  70  should be set to 10°. 
     Another Embodiment 
       FIGS. 11 to 14  show different embodiments of the present invention. In the above embodiment, clearance is provided between the inner socket  30  and the inner socket holder  70  so that the inner socket  30  can turn with respect to the inner socket holder  70 , but in the present embodiment, the inner socket  30  is divided into two members, i.e., a nut-side member  40  and a holder-side member  45 , clearance is provided between the nut-side member  40  and the holder-side member  45 , and the nut-side member  40  can turn with respect to the holder-side member  45 . Since the outer socket  50  is the same as that of the above embodiment, explanation thereof will not be given. 
     As shown in  FIG. 11 , the inner socket  30  includes the tip end nut-side member  40  and the base end holder-side member  45 , and the inner socket  30  is accommodated in the outer socket  50 . 
     An engaging hole  42  is formed in the nut-side member  40  over substantially the entire length in its axial direction to penetrate the nut-side member  40  where a nut  93  and the holder-side member  45  can engage with the engaging hole  42 . The engaging hole  42  is provided at its inner periphery with grooves  33  with which the nut  93  and the holder-side member  45  can engage, each of the grooves  33  has a width in the circumferential direction of the bottom surface  33   a  as shown in  FIG. 12 , and the nut  93  and the holder-side member  45  can be fitted to the grooves  33  with clearance in the circumferential direction (see  FIG. 10  of the above embodiment). 
     In the illustrated embodiment, each of the groove bottom surfaces  33   a  has the width of about 30°, and the nut  93  and the holder-side member  45  can be fitted into the engaging hole  42  with clearance of about 30° each. 
     A diameter of an inner periphery of the base end of the nut-side member  40  is increased, and forms a receiving portion  43  for the holder-side member  45  which will be described next. 
     As shown in  FIGS. 11 and 13 , the holder-side member  45  is provided at its tip end with a hexagonal fitting member  46  which is substantially the same shape as that of the nut  93  to be tightened. As shown in  FIGS. 11 and 14 , the fitting member  46  is fitted into the engaging hole  42  of the nut-side member  40 . 
     A diameter of a base end of the fitting member  46  is increased, and this configures a projection  47  which is turnably fitted into the receiving portion  43  of the nut-side member  40 . A plurality of mounting projections  48  project in a direction along the axial direction from a side closer to the base end than the projection  47 . The mounting projections  48  engaged with a mounting recess (not shown) formed in the inner socket holder  70  without clearance, and the mounting projections  48  rotate to follow the inner socket holder  70 . 
     The fitting member  46  is fitted into the engaging hole  42  of the nut-side member  40 , the holder-side member  45  is accommodated in the outer socket body  51 , and the holder-side member  45  cannot come out due to the reaction washer-engaging portion  60  of the outer socket  50 . 
     Concerning the socket unit  20  of the above configuration, since the fitting member  46  of the holder-side member  45  and the engaging hole  42  of the nut-side member  40  are engaged with each other with clearance as shown in  FIG. 14 , the nut-side member  40  can turn by the amount of clearance with respect to the holder-side member  45 . In this embodiment, the clearance between the holder-side member  45  and the nut-side member  40  is about 30°. The engaging hole  42  of the nut-side member  40  can also engage with the nut  93  with clearance. Therefore, the nut  93  can be fitted into the engaging hole  42  with clearance in the circumferential direction. In this embodiment, the clearance between the nut-side member  40  and the nut  93  is about 30°. 
     Therefore, the nut  93  has clearance of two times of the above clearance, i.e., clearance of about 60° in this embodiment with respect to the holder-side member  45  which is non-turnably mounted on the inner socket holder  70 . 
     In the tightening machine  10  having the socket unit  20 , the primary tightened nut  93  is finally tightened under predetermined torque. The final tightening operation is carried out in such a manner that an axis of the bolt  91  and an axis of the socket unit  20  are made to align with each other, the tightening machine  10  is moved forward such that the socket unit  20  approaches the nut  93  and the reaction washer  95  and in this state, the nut  93  is engaged with the engaging hole  42  of the inner socket  30 . 
     If the phases of the nut  93  and the groove  33  of the nut-engaging hole  32  are misaligned with each other, an operator rotates the tightening machine itself around its axis. In this embodiment, since the groove  33  of the engaging hole  42  has the bottom surface  33   a  of about 30° as shown in  FIG. 10 , if the operator rotates the tightening machine itself around its axis 30° at the maximum, the engaging hole  42  and the nut  93  engage with each other. 
     The tightening machine  10  is further moved forward in the state where the inner socket  30  and the nut  93  are engaged with each other, and the outer socket  50  and the reaction washer  95  are engaged with each other. At that time, if the concavo-convex periphery  96  of the reaction washer  95  and the grooves  62  of the outer socket  50  are not misaligned in phase from each other, the outer socket  50  can be engaged with the reaction washer  95  by moving the tightening machine  10  forward as it is. However, if the nut  93  and the reaction washer  95  have phase misalignment generated at the time of primary tightening as shown in  FIG. 16B  or if the inner socket  30  and the outer socket  50  are misaligned in phase, the outer socket  50  cannot excellently be engaged with the reaction washer  95 . 
     According to the inner socket  30  of the present invention, the nut-side member  40  has the clearance in the circumferential direction with respect to the holder-side member  45 , and the nut  93  has the clearance in the circumferential direction with respect to the nut-side member  40 . Therefore, if an operator rotates the tightening machine itself around its axis in the state where the nut  93  is engaged with the inner socket  30 , the nut-side member  40  rotates with respect to the holder-side member  45  and the nut  93  rotates with respect to the nut-side member  40 . Hence, the phase of the outer socket  50  with respect to the nut  93  can be shifted, the phase of the concavo-convex periphery  96  of the reaction washer  95  and the phase of the groove  62  of the reaction washer-engaging hole  61  of the outer socket  50  can be made to align with each other, and the outer socket  50  can be engaged with the reaction washer  95 . 
     Therefore, the inching operation for operating the tightening machine  10  and making the phases of the reaction washer  95  and the outer socket  50  align with each other becomes unnecessary. Hence, it is possible to remarkably enhance the efficiency of the final tightening operation. 
     In the above embodiment, the clearance between the nut-side member  40 , the holder-side member  45  and the nut  93  is set to about 30° (total about 60°). However, the total clearance is not limited to this, and it is preferable that the clearance is set to an angle exceeding 30°, more preferably 35° or more and 45° or less, and more preferably 40° or more. 
     The present invention is effective as a tightening machine having a socket unit capable of easily fitting an outer socket to a reaction washer in a state where a nut is fitted to an inner socket even if phases of the nut and the reaction washer are misaligned each other.