Patent Publication Number: US-7712546-B2

Title: Power tool having torque limiter

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a rotary fastening tool and typically to a rotary fastening tool with a torque limiter that interrupts torque transmission from the input side to the output side when torque acting on a tool bit reaches a set value. 
   2. Description of the Related Art 
   Japanese utility model publication No. 50-33759 discloses an electric screwdriver having a torque limiter that transits torque from the input side to the output side. In the known art, a pair of clutches for torque limiter is provided between the input side and the output side. The clutches have clutch teeth formed in the respective clutch surfaces facing with each other and engage with each other in the direction of rotation. One of the clutches is biased toward the other by a spring member. During screw-tightening operation, when the screw head is seated on the workpiece, torque acting upon the output side clutch increases. When the torque reaches a set value, power is interrupted. 
   In screw-tightening operation, a reaction force acts upon a housing that forms a driver body, in a direction opposite to the screw-tightening direction with respect to rotation on the axis of the tool bit. Therefore, the user holds the driver body (the handgrip) while applying a force in the screw-tightening direction in such a manner as to prevent the driver body from rotating by the reaction force. However, in this state, when the torque limiter is actuated and the reaction force acting upon the driver body is instantaneously eliminated, as its reaction, the user&#39;s hand holding the driver body is caused to move in the screw-tightening direction. Thus, in the known electric screwdriver having a torque limiter, the driver body unexpectedly rotates just after actuation of the torque limiter. Therefore, further improvement in ease of use is desired. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the invention to provide an effective technique for avoiding the influence of reaction during tightening operation in the rotary fastening tool. 
   Above-described problem can be solved by the features of claimed invention. 
   According to the invention, a representative rotary fastening tool is provided to have a tool body, a motor a driving-side rotating member, a driven-side rotating member, a tip-end side rotating member and a rotation control mechanism. The motor is housed within the tool body. The driving-side rotating member is rotationally driven by the motor. The driven-side rotating member is disposed coaxially with the driving-side rotating member. The tip-end side rotating member is disposed coaxially with the driven-side rotating member and rotationally driven via the driven-side rotating member. The tip-end side rotating member drives a tool bit to perform a tightening operation. 
   The rotation control mechanism allows the tip-end side rotating member to rotate in the tightening direction during the tightening operation of the tool bit. The rotation control mechanism may preferably be disposed between the tool body and the tip-end side rotating member. When the tip-end side rotating member is fixed to a workpiece with the tool bit during the tightening operation and when torque transmission from the driving-side rotating member to the driven-side rotating member is interrupted, the rotation control mechanism locks the tip-end side rotating member and the tool body together against rotation with respect to each other. As a result, the tool body is prevented from being rotated in the tightening direction, typically at the end of a tightening operation. 
   During the operation of tightening such as screws or bolts, a reaction force acts upon a tool body in a direction opposite to the tightening direction. Therefore, the user of the rotary fastening tool tends to hold the tool body in such a manner as to prevent the tool body from rotating by the reaction force. However, in this state, reaction force acting upon the tool body is instantaneously eliminated for example as a result of a torque limiter, the user&#39;s hands holding the tool body is caused to move in the tightening direction as a result of a reaction. According to the invention, the rotation control mechanism prevents the tool body from rotating in the tightening direction by locking the tool body to the tip-end side rotating member which is trapped and fixed to the workpiece with the tool bit at an end of the tightening operation. Thus, the force applied by the user in the tightening direction can be supported by the tip-end side rotating member fixed on the workpiece side. Therefore, the user&#39;s hand holding the tool body can be prevented from being caused to move in the tightening direction for example just after actuation of the torque limiter. 
   During the operation of loosening screws or bolts, by rotation of the driven-side rotating member in the loosening direction, the rotation control mechanism is disabled from performing the function of locking the tip-end side rotating member and the tool body together against rotation with respect to each other. Specifically, solely by driving the motor in the reverse direction, the rotation control mechanism can be disabled from performing the function of locking the tip-end side rotating member and the tool body. Therefore, there is no need to perform an additional operation for disabling the locking function of the rotation control mechanism, so that ease of operation in switching between tightening operation mode and loosening operation mode can be enhanced. 
   The representative rotary fastening tool may preferably include a torque limiter. The torque limiter may transmit torque of the driving-side rotating member to the driven-side rotating member when the torque acting upon the driven-side rotating member is lower than a predetermined set value. On the other hand, the torque limiter may interrupt the torque transmission when the torque acting upon the driven-side rotating member exceeds the set value. 
   Further, according to the representative rotary fastening tool, the motor may preferably be selectively driven both in a normal direction to perform a tightening operation and a reverse direction of rotation to perform a loosening operation by mode-selecting operation. During the loosening operation of the tool bit, the rotation control mechanism may be disabled from locking the tip-end side rotating member and the tool body by utilizing a rotation of the driven-side rotating member in the loosening direction. As a result, the tip-end side rotating member is allowed to rotate in the loosening direction to perform a loosening operation of the tool bit. 
   As another aspect of the invention, the rotation control mechanism may include a rotation control member and a retainer. The rotation control member may be disposed between the tool body and the tip-end side rotating member. When the tip-end side rotating member is rotated in the tightening direction, the rotation control member allows said rotation. On the other hand, when the tip-end side rotating member is rotated in the loosening direction, the rotation control member engages with both the tool body and the tip-end side rotating member and is moved between an actuated position and a released position. In the actuated position, the tip-end side rotating member is locked to the tool body. In the released position, the engagement with the tool body and the tip-end side rotating member is released and the tip-end side rotating member can be freely rotate with respect to the tool body. 
   The retainer may be disposed between the tool body and the tip-end side rotating member such that the retainer is allowed to rotate with respect to the tool body and the tip-end side rotating member. The retainer moves the rotation control member between the actuated position and the released position and retains the rotation control member in that position. Typically, the rotation control member may wedge in the tool body and the tip-end side rotating member to lock both members. 
   When the motor is driven in the normal direction and the driven-side rotating member is rotated in the tightening direction, the driven-side rotating member may rotate the retainer in the tightening direction before rotationally driving the tip-end side rotating member to cause the retainer to move the rotation control member to the actuated position. As a result, when the tip-end side rotating member is rotated with respect to the tool body in the loosening direction, the retainer allows the rotation control member to lock the tip-end side rotating member and the tool body together. On the other hand, when the motor is driven in the reverse direction and the driven-side rotating member is rotated in the loosening direction, the driver-side rotating member rotates the retainer in the loosening direction before rotationally driving the tip-end side rotating member, which causes the retainer to move the rotation control member to the released position. As a result, the retainer disables the rotation control member from performing the function of locking the tip-end side rotating member and the tool body together and allows the loosening operation of the tool bit. 
   According to the preferred aspect of the invention, when the driven-side rotating member rotates in the tightening direction or the loosening direction, the retainer is rotated in the tightening direction or the loosening direction before the tip-end side rotating member is rotated. Therefore, during tightening operation, the rotation control member is moved to the actuated position by the retainer and can ensure the function of locking the tool body to the tip-end side rotating member when the torque limiter is actuated. During loosening operation, the rotation control member is moved to the released position by the retainer and can be disabled from performing the function of locking the tool body to the tip-end side rotating member. Thus, the operation of tightening or loosening screws or bolts can be smoothly performed. 
   As another aspect of the invention, the tip-end side rotating member may include a plane region in a predetermined extent in the circumferential direction. The rotation control member may include a member that has a circular section. During the tightening operation of the tool bit, when torque transmission is interrupted, the rotation control member moves toward one end of the plane region in the circumferential direction and engages with both the plane region and the inner wall surface of the tool body, thereby locking the tip-end side rotating member and the tool body together. Thus, rotation of the tool body in the tightening direction with respect to the tip-end side rotating member can be prevented. Further, the retainer may include an elastic element that biases the circular member toward the one end of the plane region in the circumferential direction. 
   When the torque transmission is interrupted during the tightening operation, the tip-end side rotating member and the tool body can be locked by the wedging effect of the circular member that engages in (a narrow-angle portion) between the plane region of the tip-end side rotating member and the inner wall surface of the tool body. Further, the circular member is biased in the direction of such engagement by the biasing member so that the circular member can instantaneously and reliably achieve such engagement. The member having a circular section may typically include a rod-like element having a circular section or a spherical element. When a rod-like element is used as the circular member, surface pressure exerted between the plane region of the tip-end side rotating member and the inner wall surface of the tool body during engagement can be reduced. As a result, the durability can be increased. When a spherical element is used as the circular member, ease of assembling can be enhance. 
   The torque limiter may include a plurality of first torque receiving parts and a plurality of first torque transmitting parts in the circumferential direction. The first torque receiving parts may be provided on the driven-side rotating member. The first torque transmitting parts may rotate together with the driving-side rotating member and transmit torque of the driving-side rotating member to the driven-side rotating member while being held in contact with the first torque receiving parts. 
   Further, the rotary fastening tool may include a second torque receiving part that protrudes radially outward from the tip-end side rotating member, and a second torque transmitting part having a predetermined phase difference in the circumferential direction with respect to the second torque receiving part. The second torque transmitting part may rotate together with the driven-side rotating member and transmits torque of the driven-side rotating member to the tip-end side rotating member while being held in contact with the second torque receiving part. The rotary fastening tool may further include a third torque receiving part that protrudes from the retainer in the direction of a rotation axis of the driven-side rotating member, and a third torque transmitting part having a predetermined phase difference in the circumferential direction with respect to the third torque receiving part. The third torque transmitting part rotates together with the driven-side rotating member and transmits torque of the driven-side rotating member to the retainer while being held in contact with the third torque receiving part. The terms of phase according to the invention may represent a phase of engagement in the direction of rotation or a phase with respect to the angle of engagement or a phase difference of the engagement angle in the direction of rotation between the torque transmitting part and the torque receiving part. In other words, it may represent a play region in which torque transmission is not effected in the direction of rotation. 
   Further, a phase angle between the third torque receiving part and the third torque transmitting part in the circumferential direction may be larger than a phase angle between the first torque transmitting parts in the circumferential direction. With such configuration, when the torque transmission is interrupted during the tightening operation of the tool bit, the retainer is prevented from being rotated by rotation of the driven-side rotating member in the loosening direction. Thus, the rotation control member is held in the actuated position. 
   Further, a phase angle between the third torque receiving part and the third torque transmitting part in the circumferential direction may be smaller than a phase angle between the second torque receiving part and the second torque transmitting part in the circumferential direction. With such configuration, during the tightening operation of the tool bit, the driven-side rotating member rotates the retainer in the tightening direction before rotationally driving the tip-end side rotating member. Thus, the rotation control member is moved to the actuated position. During the loosening operation of the tool bit, the driven-side rotating member rotates the retainer in the loosening direction before rotationally driving the tip-end side rotating member. Thus, the rotation control member is moved to the released position. 
   As described above, the driven-side rotating member and the tip-end side rotating member are connected to each other via a play region in which torque transmission is not effected due to a phase difference provided between the second torque receiving part and the second torque transmitting part in the circumferential direction (the direction of rotation). Further, the driven-side rotating member and the retainer are connected to each other via a play region in which torque mission is not effected due to a phase difference provided between the third torque receiving part and the third torque transmitting part in the circumferential direction. 
   During the tightening operation, when the torque transmission is interrupted or when the first torque receiving part of the driven-side rotating member is disengaged from the first torque transmitting part of the driving-side rotating member, a force acts upon the driven-side rotating member in a direction that causes the driven-side rotating member to rotate in the loosening direction. According to the invention, with the construction in which the play region between the driven-side rotating member and the retainer is larger than the intervals (the play region) between the plurality of the first torque transmitting parts, even if the driven-side routing member is caused to rotate in the loosening direction when the torque limiter interrupts the torque transmission, this rotation of the driven-side rotating member can be limited within the play region between the driven-side rotating member and the retainer. Therefore, rotation of the driven-side rotating member does not affect the retainer. Specifically, during the tightening operation, the rotation control member can be held in the actuated position, so that the locking function of the rotation control member can be maintained. 
   Further, according to the invention, the play region between the driven-side rotating member and the retainer is smaller than the play region between the driven-side rotating member and the tip-end side rotating member. With this configuration, when the driven-side rotating member starts to rotate in the tightening direction or the loosening direction, the retainer can be rotated in the tightening direction or the loosening direction before the tip-end side rotating member is rotated. Thus, it can be ensured that rotation of the retainer positively precedes rotation of the tip-end side rotating member at the time of mode change between tightening operation and loosening operation. 
   The retainer may preferably be held by friction by the elastic member in such a manner as to be prevented from rotating with respect to the tip-end side rotating member unless rotated by the driven-side rotating member rotating in the loosening direction. The retainer can be prevented from freely moving so that its proper functioning can be ensured. The elastic member may typically include an O-ring or a torsion spring. 
   Other objects, features and advantages of the present invention will be readily understood after reading the following detailed description together with the accompanying drawings and the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional side view schematically showing an entire electric screwdriver according to an embodiment of the invention. 
       FIG. 2  is a sectional view of an essential part of the screwdriver, showing the construction of a torque limiter, a first spindle, a second spindle and a one-way clutch. 
       FIG. 3  is a schematic view of the torque limit in development. 
       FIG. 4  is a side view of a driven-side clutch member of the torque limiter. 
       FIG. 5  is a side view of a driving-side clutch member of the torque limiter. 
       FIG. 6  is a sectional view taken along line A-A in  FIG. 2 . 
       FIG. 7  is a sectional view taken along line B-B in  FIG. 2 . 
       FIG. 8  is a sectional view taken along line C-C in  FIG. 2 . 
       FIG. 9  is a graph showing the relationship between the torque and time during tightening operation (normal rotation). 
       FIG. 10  is a view illustrating the operations of the torque limiter, the first and second spindles and the one-way clutch during tightening operation (normal rotation). 
       FIG. 11  is a view illustrating the operations of the torque limiter, the first and second spindles and the one-way clutch during tightening operation (normal rotation). 
       FIG. 12  is a view illustrating the operations of the torque limiter, the first and second spindles and the one-way clutch during tightening operation (normal rotation). 
       FIG. 13  is a view illustrating the operations of the torque limiter, the first and second spindles and the one-way clutch during loosening operation (reverse rotation). 
       FIG. 14  is a view illustrating the operations of the torque limiter, the first and second spindles and the one-way clutch during loosening operation (reverse rotation). 
       FIG. 15  is a view illustrating the operations of the torque limiter, the first and second spindles and the one-way clutch during loosening operation (reverse rotation). 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Each of the additional features and method steps disclosed above and below may be utilized separately or in conjunction with other features and method steps to provide and manufacture improved rotary fastening tools and method for using such rotary fastening tools and devices utilized therein. Representative examples of the present invention, which examples utilized many of these additional features and method steps in conjunction, will now be described in detail with reference to the drawings. This detailed description is merely intended to teach a person skilled in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Only the claims define the scope of the claimed invention. Therefore, combinations of features and steps disclosed within the following detailed description may not be necessary to pace the invention in the broadest sense, and are instead taught merely to particularly describe some representative examples of the invention, which detailed description will now be given with reference to the accompanying drawings. 
   A representative embodiment of the invention will now be described with reference to the drawing.  FIG. 1  shows an entire electric screwdriver  100  as a representative embodiment of the rotary fastening tool according to the present invention. The screwdriver  100  includes a body  101 , a driver bit  119  detachably coupled to the tip end region (on the left side as viewed in  FIG. 1 ) of the body  101  via a tool holder  141 , and a handgrip (handle  107  connected to the body  101 . In the present embodiment, for the sake of convenience of explanation, the side of the driver bit  119  is taken as the front side and the opposite side as the rear side. 
   The body  101  includes a motor housing  103  that houses a driving motor  111 , and a gear housing  105  that houses a speed reducing mechanism  113 , a torque limiter  120 , a first spindle  130 , a second spindle  140  and a one-way clutch  150 . The driving motor  111  is driven when a trigger  107   a  on the handgrip  107  is depressed. The direction of rotation of the motor shaft of the driving motor  111  can be selected between normal rotation (clockwise forward or “screw-tightening direction”) and reverse rotation (counterclockwise forward or “screw-loosening direction”) by operating a rotation selection switch (not shown). 
   The rotating output of the driving motor  111  is transmitted from a power transmitting mechanism in the form of the speed reducing mechanism  113  to the second spindle  140  as a rotating force via a rotation drive disc  115 , the torque limiter  120  and the first spindle  130 . The tool holder  141  is disposed in the tip end region of the second spindle  140  and rotates together with the second spindle  140 . The driver bit  119  held by the tool holder  141  is rotationally driven together with the tool holder  141 . The speed reducing mechanism  113  comprises a planetary gear mechanism, but the construction is a known art and therefore will not be described in detail. Further, the rotation drive disc  115  corresponds to a carrier that supports planetary gears of the planetary gear mechanism for free rotation, and forms an output shaft of the speed reducing mechanism  113 . The rotation drive disc  115 , the torque limiter  120 , the first spindle  130 , the second spindle  140  and the one-way clutch  150  are all disposed on the same axis. 
     FIG. 2  shows the construction of the torque limiter  120 , the first spindle  130 , the second spindle  140  and the one-way clutch  150 . The torque limiter  120  includes a driving-side clutch member  121  and a driven-side clutch member  123  which face each other, a plurality of first steel balls  125 , and a compression coil spring  127 . The first steel balls  125  are disposed between the clutch members  121  and  123  and serve to transmit torque of the driving-side clutch member  121  to the driven-side clutch member  123 . The compression coil spring  127  serves as a biasing member for biasing the driven-side clutch member  123  toward the driving-side clutch member  121 . 
   The driving-side clutch member  121  is mounted on the rotation drive disc  115  such that it is prevented from moving in the axial direction and from rotating on the axis (in the direction of rotation) with respect to the rotation drive disc  115 . The driven-side clutch member  123  is fitted on the rear end portion (on the right side as viewed in  FIG. 1 ) of the first spindle  130  in the axial direction. Elongated grooves  130   a  and  123   a  are formed in the outside surface of the first spindle  130  and the inside surface of the driven-side clutch member  123 , respectively, and extend to a predetermined length in the axial direction. A second steel ball  128  is disposed in the elongated grooves  130   a ,  123   a . Thus, the driven-side clutch member  123  is allowed to move in the axial direction while being prevented from rotating on the axis with respect to the first spindle  130 . 
     FIG. 3  shows the torque limiter  120  in development. As shown in  FIG. 3 , three spherical recesses  123   b  are formed in the rear side surface (the lower sure as viewed in  FIG. 3 ) of the driven-side clutch member  123  and arranged equidistantly in the circumferential direction (at intervals of 120 degrees). The recesses  123   b  receive the first steel balls  125 . An annular groove  121   a  is formed corresponding to the travel path of the first steel balls  125  in the front side surface of the driving-side clutch member  121 . Six mountain-like cams  121   b  are formed in the annular groove  121   a  and arranged equidistantly in the circumferential direction (at intervals of 60 degrees).  FIG. 4  shows the driven-side clutch member  123  in side view, and  FIG. 5  shows the driving-side clutch member  121  in side view. Each of the first steel balls  125  held by the driven-side clutch member  123  is movably fitted in the annular groove  121   a  of the driving-side clutch member  121 . The first steel ball  125  transmits the torque of the driving-side clutch member  121  to the driven-side clutch member  123  when the first steel ball  125  engages with the associated cam  121   b  from the circumferential direction. When the torque (rotational load) acting upon the first steel ball  125  exceeds a set value, the first steel ball  125  climbs over the cam  121   b  while moving the driven-side clutch member  123  away from the driving-side clutch member  121  against the spring force of the compression coil spring  127 . As a result, the first steel ball  125  is disengaged from the cam  121   b , so that the torque transmission from the driving-side clutch member  121  to the driven-side clutch member  123  is interrupted. The first steel ball  125  and the cam  121   b  are features that correspond to the “first torque receiving part” and the “first torque transmitting part”, respectively, according to this invention. 
   As shown in  FIG. 2 , the compression coil spring  127  is disposed between the front surface of the driven-side clutch member  123  and a spring receiving member  129  threadably mounted on the first spindle  130 . The compression coil spring  127  can change its position with respect to the first spindle  130  in the axial direction by rotating a nut  129   a  disposed on the front side of the spring receiving member  129 . In this manner, the compression coil spring  127  can change its biasing force in order to adjust the torque setting for interruption of torque transmission. 
   A carrier  131  is disposed on the side of the front end portion of the first spindle  130  in the axial direction and rotates together with the first spindle  130 . The first spindle  130  is connected to the second spindle  140  via the carrier  131 . The carrier  131  includes a square shank  133  and a cylindrical portion  135 . The square shank  133  is inserted into a square hole  130   b  of the first spindle  130 , so that the carrier  131  rotates together with the first spindle  130 . As shown in  FIGS. 2 and 8 , the cylindrical portion  135  of the carrier  131  is disposed in the outside region of the axial rear end portion of the second spindle  140 . Two radially outwardly protruding driven-side claws  143  are formed in the rear end portion of the second spindle  140  with a phase difference of 180 degrees in the circumferential direction. 
   In a corresponding manner, two radially inwardly protruding driving-side claws  135   a  are formed in the inside surface of the cylindrical portion  135  of the carrier  131  with a phase difference of 180 degrees in the circumferential direction. When the carrier  131  is caused to rotate together with the fit spindle  130  in the normal direction (tightening direction) or the reverse direction (loosening direction), the driving-side claws  135   a  contact the driven-side claws  143  and transmit the torque of the first spindle  130  to the second spindle  140 . The driven-side claws  143  and the driving-side claws  135   a  are features that correspond to the “second torque receiving part” and the “second torque transmitting part”, respectively, according to this invention. One driven-side claws  143  in contact with one driving-side claw  135   a  is positioned at a predetermined phase angle α 1  in the circumferential direction from the other driving-side claw  135   a  in contact with the other driven-side claw  143 . Thus, the carrier  131  and the second spindle  140  are connected to each other via a play region in which torque transmission is not effected in the circumferential direction (see  FIG. 8 ). 
   As mainly shown in  FIGS. 2 ,  6  and  7 , one-way clutch  150  includes a fixed ring  151  fitted in the gear housing  105 , a plurality of (four in this embodiment) needle pins  153  and a retainer  155  for holding the needle pins  153 . The needle pins  153  are disposed between the fixed ring  151  and the second spindle  140  and serve to allow the second spindle  140  to rotate in the normal direction and prevent it from rotating in the reverse direction. The needle pins  153  correspond to the “rotation control member” and the “member having a circular section” according to this invention. 
   The fixed ring  151  has an annular inner peripheral surface  151   a  having an inside diameter slightly larger than the outside diameter of the retainer  155 . Four planar regions  140   a  having a predetermined width are formed in the outer peripheral surface of the second spindle  140  and arranged equidistantly (at intervals of 90 degrees) in the circumferential direction. The needle pins  153  are disposed between the planar regions  140   a  and the inner peripheral surface  151   a  of the fixed ring  151 . The planar regions  140   a  correspond to the “plane region” according to this invention. The needle pins  153  are disposed such that its axial direction coincides with the axial direction of the second spindle  140 . 
   Space  156  is formed between the planar region  140   a  of the second spindle  140  and the inner peripheral surface  151   a  of the fixed ring  151 . The radial width of the space  156  is at the maximum in the middle of the planar region  140   a  in the circumferential direction and at the minimum at the ends of the planar region  140   a . Each of the needle pins  153  has the outside diameter smaller than the maximum width of the space  156  and larger than the minimum width of the space  156 . The needle pin  153  is thus allowed to move between the minimum width position and the maximum width position in the space  156 . In the state in which the needle pin  153  is in the minimum width position, when the second spindle  140  rotates in the normal direction (tightening direction), the needle pin  153  is pushed backed toward the maximum width position and allows the second spindle  140  to rotate in the tightening direction. 
   On the other hand, when the second spindle  140  rotates in the reverse direction (loosening direction), the needle pin  153  engages in the planar region  140   a  and the inner peripheral surface  151   a , so that the second spindle  140  and the fixed ring  151  are locked together. Thus, the second spindle  140  is prevented from rotating. In the state in which the needle pin  153  is in the maximum width position, the needle pin  153  is disengaged from the planar region  140   a  and the inner peripheral surface  151   a , so that the second spindle  140  is allowed to rotate both in the tightening direction and the loosening direction. The minimum width position and the maximum width position respectively correspond to the “actuated position” and the “released position” according to the invention. 
   The retainer  155  is generally cylindrically shaped and disposed between the fixed ring  151  and the second spindle  140  and can rotate with respect to both the fixed ring  151  and the second spindle  140 . An O-ring  158  is disposed between the inner peripheral surface of the retainer  155  and the outer peripheral surface of the second spindle  140 . Thus, the retainer  155  is provided with frictional resistance to rotation with respect to the second spindle  140 . Therefore, unless forcibly torqued, the retainer  155  is held on the second spindle  140 . The O-ring  158  is a feature that corresponds to the “elastic member” according to this invention. Four recesses  155   a  for retaining the needle pins  153  are formed in the retainer  155  and arranged at intervals of 90 degrees in the circumferential direction. Each of the recesses  155   a  has a notch-like shape having a predetermined depth extending forward from the axial rear end of the retainer  155 . Each of the needle pins  153  is allowed to move between the minimum width position and the maximum width position within the associated recess  155   a . Further, the needle pin  153  is normally biased toward one end of the planar region  140   a  in the circumferential direction or toward the minimum width position by a flat spring  157  mounted on the retainer  155 . When the driving motor  111  is not driven, the needle pin  153  is held in the minimum width position. 
   Further, the retainer  155  has two rotation following pins  159  protruding to the carrier  131  side in order to be caused to rotate following the second spindle  140  when the second spindle  140  rotates. The rotation following pins  159  are disposed in the retainer  155  at intervals of 180 degrees in the circumferential direction. Correspondingly, two notch-like recesses  135   b  (see  FIG. 8 ) are formed in the cylindrical portion  135  of the carrier  131  at intervals of 180 degrees in the circumferential direction. Each of the recesses  135   b  has a predetermined length in the circumferential direction. The protruding portion of each of the rotation following pins  159  is disposed within the associated recess  135   b . When the carrier  131  rotates, the rotation following pin  159  is pushed by the carrier  131  in the circumferential direction in contact with an engagement surface  135   c  for normal rotation or an engagement surface  135   d  for reverse rotation which extends in a direction crossing the circumferential direction of the recess  135   b . Thus, the retainer  155  is caused to rotate following the carrier  131 . The rotation following pin  159  and the normal and reverse rotation engagement surfaces  135   c ,  135   d  we features that respectively correspond to the “third torque receiving part” and the “third torque transmitting part” according to the invention. 
   A predetermined phase angle α 2  is provided in the circumferential direction between the rotation following pin  159  in contact with one of the engagement surfaces  135   c ,  135   d  and the other of the engagement surfaces  135   c ,  135   d  in the recess  135   b  (see  FIG. 8 ). Thus, the carrier  131  and the retainer  155  are connected to each other via a play region in which torque transmission is not effected in the circumferential direction. The phase angle α 2  between the rotation following pin  159  and the engagement surface  135   c  or  135   d  is larger man the intervals (the phase angle of 60 degrees) between the cams  121   b  of the torque limiter  120  and smaller than the phase angle α 1  between the driven-side claw  143  and the driving-side claw  135   a.    
   Operation and usage of the electric screwdriver  100  according to this embodiment will now be explained. First an operation of tightening screws (not shown) will be explained with reference mainly to  FIGS. 9 to 12 . The driving motor  11  is driven in the normal direction of rotation (clockwise) with a screw pressed against the workpiece via the driver bit  119 . Then, the second spindle  140  is rotationally driven in the normal direction via the speed reducing mechanism  113 , the torque limiter  120 , the first spindle  130  and the carrier  131 . Thus, the screw-tightening operation is performed via the tool holder  141  that rotates together with the second spindle  140 , and the driver bit  119 . 
     FIG. 9  is a graph showing the relationship between the torque and time during tightening operation (normal rotation). In the graph, step  1  represents a step just before actuation of the torque limiter  120  (just before interruption of torque transmission), step  2  is a step just after actuation of the torque limiter  120  (just after interruption of torque transmission), and step  3  is a step following step  2  after a lapse of a short period of time. Further,  FIGS. 10 to 12  show the states in steps  1  to  3 , respectively, with the torque limiter  120  shown at right, the carrier  131  and the second spindle  140  in the middle, and the one-way clutch  150  at left. 
   In step  1  as shown in  FIG. 10 , the driven-side clutch member  123  is placed in engagement with the cams  121   b  of the driving-side clutch member  121  via the first steel balls  125 , so that the torque transmission of the torque limiter  120  is maintained. In this torque transmission state, the normal rotation engagement surfaces  135   c  of the carrier  131  that rotates together with the first spindle  130  are in contact with the rotation following pins  159  of the retainer  155 . Therefore, the retainer  155  rotates in the tightening direction (clockwise). Further, the driving-side claws  135   a  of the carrier  131  are in contact with the driven-side claws  143  of the second spindle  140 . Therefore, the second spindle  140  rotates in the tightening direction (clockwise) together with the carrier  131 . In this state, in the one-way clutch  150 , the needle pins  153  held in the minimum width position are pushed back to the maximum width position, so that the second spindle  140  is allowed to rotate. This state remains unchanged from the start to the final stage of the screw-tightening operation in which the seating surface of the screw head is seated on the workpiece. 
   When a screw is fastened to the workpiece with the seating surface of the screw head seated on the workpiece, the torque (rotational load) acting upon the first spindle  130  via the second spindle  140  and the carrier  131  exceeds a set value. Then, the torque limiter  120  is actuated, which brings about the state of step  2  and then the state of step  3 . Specifically, as shown in  FIG. 11 , the first steel balls  125  climb over the cams  121   b  while moving the driven-side clutch member  123  away firm the driving-side clutch member  121  against the spring force of the compression coil spring  127 . As a result, the torque transmission is interrupted. In the torque limiter  120 , immediately after the torque transmission, a force acts upon the driven-side clutch member  123  in a direction that causes the driven-side clutch member  123  to rotate in the loosening direction (counterclockwise) (see the torque limiter  120  shown at right in  FIGS. 11 and 12 ). As a result, the carrier  131  is caused to rotate in the loosening direction. 
   Therefore, in this embodiment, the phase angle α 2  between the rotation following pin  159  of the retainer  155  and the engagement surface  135   c  that transmits the torque of the carrier  131  in contact with the rotation following pin  159  is larger than the intervals (of 60 degrees) between the cams  121   b . With this construction, even if the carrier  131  is caused to rotate in the loosening direction when the torque limiter  120  is actuated to interrupt the torque transmission, the rotation of the carrier  131  is avoided from causing the retainer  155  to rotate (see the drawings shown in the middle of  FIGS. 11 and 12 ). Specifically, the retainer  155  stops together with the second spindle  140  and holds the needle pins  153  in the minimum width position. In this state, when a force acts upon the fixed ring  151  in a direction that causes the fixed ring  151  to rotate in the tightening direction (clockwise), each of the needle pins  153  engages in the planar region  140   a  of the second spindle  140  and the inner peripheral surface  151   a  of the fixed ring  151 , so that the second spindle  140  and the fixed ring  151  are locked together. Thus, the fixed ring  151  is prevented from rotating in the tightening direction by actuation of the one-way clutch  150 . 
   When a tightening operation is performed by using the screwdriver  100 , a reaction force acts upon the body  101  in a direction opposite to the tightening direction with respect to rotation on the axis of the driver bit  119 . Therefore, the user holds the handgrip  107  in such a manner as to prevent the body  101  from being caused to rotate by the reaction force (i.e. the user applies a force in the tightening direction). In this state, when the torque limiter  120  is actuated and the reaction force on the body  101  is instantaneously eliminated, as its reaction, the user&#39;s hand holding the handgrip  107  may be caused to move in the tightening direction. At this time, according to this embodiment, as described above, the one-way clutch  150  prevents the body  101  (the fixed ring  151 ) from rotating in the tightening direction. Thus, the force applied by the user in the tightening direction can be supported by the second spindle  140  fixed on the workpiece side. Therefore, the user&#39;s hand holding the handgrip  107  can be prevented from moving in the tightening direction just after actuation of the torque limiter  120 . 
   Screw-loosening operation is explained with reference mainly to  FIGS. 13 to 15 . The driving motor  11  is driven in the reverse direction of rotation (counterclockwise) with the driver bit  119  pressed against a screw to be loosened.  FIG. 13  shows the state just after the start of rotation in the reverse direction. In the torque limiter  120 , the first steel balls  125  held by the driven-side clutch member  123  engage with the cams  121   b  of the driving-side clutch member  121 , so that the carrier  131  is rotated together with the first spindle  130  in the reverse direction  FIG. 14  shows an advanced state of the revere rotation. When the carrier  131  is rotated, the reverse rotation engagement surface  135   d  of the carrier  131  contacts the rotation following pin  159  of the retainer  155 . Thereafter, the driving-side claw  135   a  of the carrier  131  contacts the driven-side claw  143  of the second spindle  140 . Specifically, with the construction in which the phase angle (engagement angle) α 2  for contact (engagement) between the engagement surface  135   d  and the rotation following pin  159  in the direction of rotation is smaller than the phase angle (engagement angle) α 1  for contact (engagement) between the driving-side claw  135   a  and the driven-side claw  143  in the direction of rotation, contact between the engagement surface  135   d  and the rotation following pin  159  precedes contact between the driving-side claw  135   a  and the driven-side claw  143 . 
   When the retainer  155  is caused to rotate in the reverse direction by contact between the engagement surface  135   d  and the rotation following pin  159 , the needle pin  153  in the recess  155   a  of the retainer  155  is pushed by the wall surface  155   b  of the recess  155   a  and moved from the minimum width position to the maximum width position of the space  156  formed between the planar region  140   a  of the second spindle  140  and the inner peripheral surface  151   a  of the fixed ring  151 . As shown at left in  FIG. 15 , when the needle pin  153  is moved to the maximum width position of the space  156 , the needle pin  153  is disengaged from the fixed ring  151  and the second spindle  140 . As a result, the function of the one-way clutch  150  is disabled and the second spindle  140  is allowed to rotate. Thereafter, the driving-side claw  135   a  of the carrier  131  contacts the driven-side claw  143  of the second spindle  140 , so that the torque of the carrier  131  is transmitted to the second spindle  140 . Thus, the screw-loosening operation is smoothly performed. 
   As described above, during the screw-tightening operation, the one-way clutch  150  disposed between the second spindle  140  and the gear housing  105  can eliminate the problem of reaction which may be caused when the torque limiter  120  is actuated. Further, during the loosening operation, the engaging function of the one-way clutch  150  can be automatically disabled, so that the loosening operation is smoothly performed. Particularly, in this embodiment, when the driving motor  111  is driven in the reverse direction of rotation, the engaging function of the one-way clutch  150  can be automatically disabled by utilizing rotation of the carrier  131  driven in the reverse direction. Therefore, the need to perform an additional operation for disabling the engaging function of the one-way clutch  150  can be eliminated, so that case of operation in switching between tightening operation mode and loosening operation mode can be enhanced. 
   Further, according to this embodiment, in the tightening operation in which the second spindle  140  is rotationally driven in the tightening direction, the second spindle  140  and the fixed ring  151  can be reliably locked by the wedging effect of the needle pin  153  that engages in (a narrow-angle portion) between the planar region  140   a  of the second spindle  140  and the inner peripheral surface  151   a  of the fixed ring  151  when the torque limiter  120  is actuated. Further, the needle pin  153  is biased toward the minimum width position by the flat spring  157 , so that the needle pin  153  can be instantaneously and reliably engaged between the planar region  140   a  and the inner peripheral surface  151   a.    
   Further, in this embodiment, with the construction in which the retainer  155  is held on the second spindle  140  via the O-ring  158  by friction, the retainer  155  can be prevented from finely moving, unless forcibly rotated by the carrier  131  rotating in the loosening direction. Thus, the proper functioning of the one-way clutch  150  can be ensured. 
   Further, in this embodiment, the electric screwdriver  100  having the torque limiter  120  is described as an example of the rotary fastening tool of the present invention. However, this invention can also be applied to any other rotary fastening tool having the torque limiter  120 . 
   DESCRIPTION OF NUMERALS 
   
       
         100  electric screwdriver 
         101  body 
         103  motor housing 
         105  gear housing 
         107  handgrip 
         107   a  trigger 
         111  driving motor 
         113  speed reducing mechanism 
         115  rotation drive disc 
         119  driver bit 
         120  torque limiter 
         121  driving-side clutch member 
         121   a  annular groove 
         121   b  cam 
         123  driven-side clutch member 
         123   a  elongated groove 
         123   b  recess 
         125  firm steel ball 
         127  compression coil spring 
         128  second steel ball 
         129  spring receiving member 
         129   a  nut 
         130  first spindle 
         130   a  elongated groove 
         130   b  square hole 
         131  carrier 
         133  square shank 
         135  cylindrical portion 
         135   a  driving-side claw 
         135   b  recess 
         135   c ,  135   d  engagement surface 
         140  second spindle 
         140   a  planar region 
         141  tool holder 
         143  driven-side claw 
         150  one-way clutch 
         151  fixed ring 
         151   a  inner peripheral surface 
         153  needle pin 
         155  retainer 
         155   a  recess 
         155   b  wall surface 
         156  space 
         157  flat spring 
         158  O-ring 
         159  rotation following pin