Patent Publication Number: US-9902051-B2

Title: Electric screwdriver

Description:
TECHNICAL FIELD 
     The present invention relates to motor-driven screwdrivers. More specifically, the present invention relates to a motor-driven screwdriver configured to transmit to a bit holder rotational driving forces different in magnitude from each other when the bit holder is to be rotated forward to tighten a screw and when the bit holder is to be rotated backward to loosen a screw. 
     BACKGROUND ART 
     A motor-driven screwdriver needs to be capable of tightening a screw with an appropriate rotational driving force because screw tightening with an excessive rotational driving force may damage the screw itself, or a member into which the screw is driven, or the motor-driven screwdriver itself. The motor-driven screwdriver is also used to loosen a tightened screw, and for this purpose, it is usually necessary to apply to the tightened screw a larger rotational driving force than that applied to tighten the screw. 
       FIG. 9  is a cross-sectional view of a rotational driving force transmission device  1  of a motor-driven screwdriver developed to meet the above-described technical demand, as seen toward the rear end of the motor-driven screwdriver opposite to the front end thereof provided with a screw bit. Accordingly, in the figure, counterclockwise rotation is forward rotation to tighten a screw, and clockwise rotation is backward rotation to loosen a screw. 
     The rotational driving force transmission device  1  has a rotational driving shaft  2  driven to rotate upon receiving rotational driving force from a driving motor, a circular cylindrical rotation output member  3  rotatable about the rotation center axis of the rotational driving shaft  2 , and balls  4  held in the rotation output member  3  movably in the radial direction of the rotation output member  3  and subjected to radially inward urging force shown by the arrows  5 . Rotational driving force from the rotational driving shaft  2  is transmitted to the rotation output member  3  through the balls  4 , but when the rotational driving force exceeds a predetermined value, the balls  4  are pushed radially outward against the urging force  5 , so that the rotational driving shaft  2  idles with respect to the rotation output member  3 , thereby preventing a rotational driving force exceeding the predetermined value from being transmitted to the rotation output member  3 . In addition, the rotational driving shaft  2  is shaped as shown in  FIG. 9 , thereby allowing the radial position for engagement of the rotational driving shaft  2  with each ball  4  to differ between backward rotation and forward rotation such that the rotational driving shaft  2  engages the ball  4  at a radially inner position during forward rotation than during backward rotation. Consequently, the proportion of the radially outward component of the force transmitted from the rotational driving shaft  2  to the balls  4  is smaller during backward rotation than during forward rotation, so that the rotational driving force required to move the balls  4  radially outward against the urging force  5  is larger during backward rotation. Accordingly, it is possible to transmit a larger rotational driving force when the rotation output member  3  is rotated backward to loosen a screw than when the rotation output member  3  is rotated forward to tighten a screw (Patent Literature 1). 
       FIGS. 10 and 11  show another rotational driving force transmission device  6 . The rotational driving force transmission device  6  has a rotation input member  7  driven to rotate upon receiving rotational driving force from a driving motor, driving rollers  9  disposed in roller retaining portions  8 , respectively, of the rotation input member  7 , a circular cylindrical rotation output member  10  rotatable about the rotation center axis of the rotation input member  7 , and driven balls  11  radially movably held by the rotation output member  10 . The rotation output member  10  has a screwdriver bit (not shown) attached thereto. When the rotation input member  7  rotates forward (counterclockwise as seen in the figures), as shown in  FIG. 10 , the driving rollers  9  engage first retaining portions  8 - 1  of the roller retaining portions  8 , respectively, and, in this state, engage the driven balls  11 , respectively, to transmit rotational driving force to the rotation output member  10 . When the rotation input member  7  rotates backward (clockwise as seen in the figures), as shown in  FIG. 11 , the driving rollers  9  engage second retaining portions  8 - 2  of the roller retaining portions  8 , respectively, and, in this state, engage the driven balls  11 , respectively, to transmit rotational driving force to the rotation output member  10 . The driven balls  11  are urged toward the inside of the rotation output member, and when a force exceeding a predetermined value is applied thereto through the driving rollers  9 , the driven balls  11  move outward, thereby allowing the rotation input member  7  to idle. The first retaining portion  8 - 1  and second retaining portion  8 - 2  of each roller retaining portion  8  are different in shape from each other as shown in  FIGS. 10 and 11 . The difference in shape allows the position for engagement of each driving roller  9  with the associated driven ball  11  to differ between forward and backward rotation such that the driving roller  9  engages the driven ball  11  at a position more away from the rotation center axis during backward rotation than during forward rotation, as in the case of the above-described example shown in  FIG. 9 . Accordingly, it is possible to transmit a larger rotational driving force during backward rotation than during forward rotation (Patent Literature 2). 
     PATENT LITERATURE 
     
         
         
           
             Patent Literature 1: Japanese Examined Utility Model Application Publication No. Hei 2-12053 
             Patent Literature 2: Japanese Patent No. 3992676 
           
         
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the structure including a rotational driving shaft having a special shape as shown in  FIG. 9  suffers from the problem that machining of the parts becomes complicated. The structure in which driving rollers move therein as shown in  FIGS. 10 and 11  has the problem that the parts may wear at a high rate and may be broken particularly when forward rotation and backward rotation are repeated alternately. 
     Accordingly, it is an object of the present invention to provide a motor-driven screwdriver having a rotational driving force transmission device capable of solving the above-described problems. 
     Solution to Problem 
     The present invention provides a motor-driven screwdriver having a bit holder securely holding a screwdriver bit, and a rotational driving force transmission device for transmitting rotational driving force from a driving source to the bit holder to rotate the screwdriver bit forward and backward. The rotational driving force transmission device has a driving member driven to rotate about a rotation center axis upon receiving rotational driving force from the driving source, a driven member disposed around the driving member rotatably about the rotation center axis and drivably connected to the bit holder, the driven member having an outer peripheral surface and an inner peripheral surface in a radial direction with respect to the rotation center axis, the driven member further having a through-hole extending therethrough from the outer peripheral surface to the inner peripheral surface, a power transmission member movably held in the through-hole of the driven member, the power transmission member having a circular cross-section in a plane perpendicular to the rotation center axis, and an urging member urging the power transmission member inward in the radial direction so that a part of the power transmission member projects inward beyond the inner peripheral surface of the driven member. The driving member has a shaft portion extending along the rotation center axis, and a projecting portion projecting from the shaft portion outward in the radial direction toward the inner peripheral surface of the driven member. When the driving member is rotated forward and backward about the rotation center axis, the projecting portion engages the power transmission member to transmit rotational driving force from the driving member to the driven member through the power transmission member. The through-hole has a forward rotation guide surface against which the power transmission member is pressed when the driving member rotates forward and the projecting portion engages the power transmission member, and a backward rotation guide surface against which the power transmission member is pressed when the driving member rotates backward and the projecting portion engages the power transmission member. The through-hole is provided such that a through-hole center axis passing through a center between the forward rotation guide surface and the backward rotation guide surface in a plane perpendicular to the rotation center axis does not intersect the rotation center axis. When a rotational driving force exceeding a predetermined value is applied, the power transmission member is pushed by the projecting portion outward in the radial direction in the through-hole against urging force of the urging member. 
     According to the motor-driven screwdriver, the through-hole is provided such that the through-hole center axis does not intersect the rotation center axis, whereby the relationship between the direction in which the power transmission member is pushed by the projecting portion of the driving member and the direction in which the power transmission member moves while being guided by the guide surface can be made to differ between forward and backward rotation. Accordingly, the magnitude of pressing force applied to the urging member through the power transmission member relative to the magnitude of rotational driving force of the driving member is varied between forward and backward rotation, and thus the magnitude of rotational driving force required to push away the urging member against the urging force thereof can be made to differ between forward and backward rotation. As a result, it becomes possible to vary the magnitude of rotational driving force transmittable to the driven member between forward and backward rotation. It should be noted that the through-hole center axis corresponds to an axis of symmetry about which axial symmetry is established between hypothetical lines extending along the forward rotation guide surface and the backward rotation guide surface, respectively, in a plane perpendicular to the rotation center axis. 
     Preferably, the forward rotation guide surface and the backward rotation guide surface of the through-hole may be parallel to each other. 
     Thus, the shape of the through-hole is simplified, and it becomes possible to form the through-hole more easily. 
     Preferably, the respective directions of the forward rotation guide surface and the backward rotation guide surface of the through-hole may be set so that the through-hole center axis of the through-hole extends between a straight line connecting between the rotation center axis of the driving member and the center of the power transmission member and a straight line connecting between the center of the power transmission member and a point of contact between the projecting portion and the power transmission member when the driving member rotates forward and the projecting portion engages the power transmission member. 
     With the above-described structure, rotational driving force transmittable to the driven member can be made greater during backward rotation than during forward rotation, and it is therefore possible to transmit appropriate rotational driving forces when tightening and loosening a screw. 
     Specifically, the projecting portion may have an arcuate surface centered on an axis parallel to the rotation center axis, so that the arcuate surface engages the power transmission member when the driving member rotates forward and backward. 
     With the above-described structure, the point and angle of contact between the projecting portion and the power transmission member are relatively identical as seen from the rotation center axis during forward and backward rotation. Therefore, the magnitudes of rotational driving forces transmittable to the driven member during forward and backward rotation can be easily set only by adjusting the inclination of the forward and backward rotation guide surfaces of the through-hole. 
     More specifically, the shaft portion may have a circular cylindrical outer peripheral surface centered on the rotation center axis, and the projecting portion may have an arcuate outer peripheral surface extending parallel to the rotation center axis. 
     Even more specifically, the projecting portion may be formed by a circular columnar member that is partially embedded in the circular cylindrical outer peripheral surface of the shaft portion so as to extend parallel to the rotation center axis, the circular columnar member having a portion projecting from the circular cylindrical outer peripheral surface toward the inner peripheral surface of the driven member to form the arcuate outer peripheral surface. 
     Preferably, the power transmission member may have a spherical shape. 
     With a spherical shape, the power transmission member can move smoothly when the rotational driving force exceeds a predetermined value. 
     Preferably, the urging member may comprise a taper ring having a tapered surface abutting against the power transmission member, and a spring pressing the taper ring in a direction parallel to the rotation center axis so that the tapered surface urges the power transmission member inward in the radial direction. 
     More preferably, the arrangement may be as follows. When the power transmission member is pushed outward in the radial direction, the taper ring moves against the spring, and a stop switch for stopping the driving source is activated in response to movement of the taper ring. 
     Because the driving source stops when a rotational driving force exceeding a predetermined value is applied, no excessive load will be applied to a screw or the like, and it is possible to prevent a failure of the motor-driven screwdriver itself, and to ensure safety for the user. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional side view of a motor-driven screwdriver according to the present invention. 
         FIG. 2  is a sectional view seen along the line A-A in  FIG. 1 , showing a rotational driving force transmission device during forward rotation (counterclockwise rotation as seen in the figure). 
         FIG. 3  is a sectional view seen along the line A-A in  FIG. 1 , showing the rotational driving force transmission device during backward rotation (clockwise rotation as seen in the figure). 
         FIG. 4  is a sectional view seen along the line A-A in  FIG. 1 , showing the rotational driving force transmission device when power transmission members are pushed outward. 
         FIG. 5  is a partially-sectioned perspective view showing an important part of the rotational driving force transmission device, which is partially cut away to clarify the relationship between a shaft portion of a driving member, circular columnar members (projecting portions) partially embedded in the shaft portion, spherical power transmission members, and a driven member holding the power transmission members. 
         FIG. 6  is a diagram obtained by rotating  FIG. 3  slightly clockwise so that the center axes of through-holes provided in the driven member are parallel to a line perpendicularly passing through the rotation center axis of the driving member. 
         FIG. 7  is a partially-sectioned perspective view similar to  FIG. 5 , showing an important part of a rotational driving force transmission device according to another embodiment. 
         FIG. 8  is a sectional view showing the rotational driving force transmission device according to the another embodiment. 
         FIG. 9  is a sectional view of a rotational driving force transmission device of a conventional motor-driven screwdriver, in which the one-dot chain lines show a rotational driving shaft engaging balls during backward rotation (clockwise rotation as seen in the figure), and the solid lines show the rotational driving shaft engaging the balls during forward rotation (counterclockwise rotation as seen in the figure). 
         FIG. 10  is a sectional view showing a rotational driving force transmission device of another conventional motor-driven screwdriver during forward rotation. 
         FIG. 11  is a sectional view showing the rotational driving force transmission device of the other conventional motor-driven screwdriver during backward rotation. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As shown in  FIG. 1 , a motor-driven screwdriver  20  according to the present invention has a bit holder  22  securely holding a screwdriver bit (not shown; inserted from the right end as seen in the figure), and a rotational driving force transmission device  24  for transmitting rotational driving force from a driving motor (not shown) as a driving source (installed at the left end as seen in the figure) to the bit holder  22 . Rotational driving force from the driving motor is transmitted to the rotational driving force transmission device  24  through a speed reducer  26 . 
     As shown in  FIG. 2 , the rotational driving force transmission device  24  has a driving member  30  driven to rotate about a rotation center axis  32  upon receiving rotational driving force from the driving motor, a driven member  40  disposed around the driving member  30  rotatably about the rotation center axis  32  and drivably connected to the bit holder  22 , power transmission members  50  movably held in respective radially extending circular cylindrical through-holes  42  provided in the driven member  40 , and an urging member  60  positioning the power transmission members  50  so that a part of each power transmission member  50  projects inward beyond an inner peripheral surface  44  of the driven member  40 . The urging member  60  urges the power transmission members  50  inward when the power transmission members  50  are pushed radially outward. As will be understood from  FIG. 5 , the power transmission members  50  are spherical in shape. 
     The driving member  30  comprises a shaft portion  34  extending along the rotation center axis  32  and circular columnar members  36  partly embedded in a circular cylindrical outer peripheral surface  35  near the front end of the shaft portion  34 . Each columnar member  36  extends with its longitudinal direction parallel to the rotation center axis  32  and is secured with a part thereof embedded in the shaft portion  34 . The rest of each columnar member  36  projects from the cylindrical outer peripheral surface  35  of the shaft portion  34  to form a projecting portion  38 . When the driving member  30  rotates about the rotation center axis  32  forward (counterclockwise as seen in the figures) to tighten a screw and backward (clockwise as seen in the figures) to loosen a screw, the projecting portions  38  engage the power transmission members  50 , respectively, held in the through-holes  42  of the driven member  40 , thereby transmitting rotational driving force from the driving member  30  to the driven member  40  through the power transmission members  50 . 
     The through-holes  42  each have a forward rotation guide surface  42 - 1  against which the associated power transmission member  50  is pressed when the driving member  30  rotates forward and the projecting portions  38  engage the power transmission members  50 , respectively, as shown in  FIG. 2 , and a backward rotation guide surface  42 - 2  against which the associated power transmission member  50  is pressed when the driving member  30  rotates backward and the projecting portions  38  engage the power transmission members  50 , respectively, as shown in  FIG. 3 . The forward rotation guide surface  42 - 1  and the backward rotation guide surface  42 - 2  are provided parallel to each other. The direction of each through-hole  42  is set so that a through-hole center axis  46  defined as a center line passing through a center between the forward rotation guide surface  42 - 1  and the backward rotation guide surface  42 - 2  in a plane perpendicular to the rotation center axis  32  does not intersect the rotation center axis  32  (alternatively, the through-hole center axis  46  may be defined as an axis of symmetry about which axial symmetry is established between hypothetical lines extending along the forward rotation guide surface  42 - 1  and the backward rotation guide surface  42 - 2 , respectively, in a transverse plane perpendicular to the rotation center axis  32 ). The reason for this is as follows. By configuring each through-hole  42  such that the way in which the through-hole surface receives the associated power transmission member  50  when subjected to rotational driving force from the driving member  30  differs between forward and backward rotation, the force that the urging member  60  receives through the power transmission member  50  is made to differ between forward and backward rotation for the same magnitude of rotational driving force, thereby allowing the magnitude of transmittable rotational driving force to differ between forward and backward rotation. This will be explained below in detail. 
     As shown in  FIG. 2 , when the driving member  30  rotates forward, a forward rotation engagement surface  38 - 1  on the outer surface of each columnar member  36  engages the associated power transmission member  50  projecting from the inner peripheral surface  44  of the driven member  40 , thereby applying a force shown by the arrow R f  to the power transmission member  50 . Simultaneously, the power transmission member  50  is urged radially inward by the urging member  60  with a force shown by the arrow F f . The power transmission member  50  engaged with the driving member  30  is also pressed against the forward rotation guide surface  42 - 1  of the through-hole  42 ; accordingly, a force shown by the arrow W f  is applied to the power transmission member  50  as a reaction force from the forward rotation guide surface  42 - 1 . 
     As shown in  FIG. 3 , when the driving member  30  rotates backward, a backward rotation engagement surface  38 - 2  on the outer surface of each columnar member  36  engages the associated power transmission member  50  to apply a force shown by the arrow R b  to the power transmission member  50 . To the power transmission member  50  are also applied a force shown by the arrow F b  from the urging member  60  and a force shown by the arrow W b  from the backward rotation guide surface  42 - 2  of the through-hole  42 , as in the case of the forward rotation of the driving member  30 . 
     When the driving member  30  is rotating forward or backward, while the rotational driving force from the driving member  30  is not greater than a predetermined magnitude, the force with which each projecting portion  38  of the driving member  30  pushes the associated power transmission member  50  outward is smaller than the force with which the urging member  60  can push the power transmission member  50  inward; therefore, the power transmission member  50  does not move radially. Accordingly, the engagement between the driving member  30  and the power transmission member  50  is maintained, and hence the rotational driving force of the driving member  30  is transmitted to the driven member  40  by W f  or W b  to rotate the bit holder  22  with the rotational driving force. However, when the rotational driving force from the driving member  30  exceeds the predetermined magnitude, the force with which the driving member  30  pushes the power transmission member  50  outward becomes greater than the force with which the urging member  60  pushes the power transmission member  50  inward, and the power transmission member  50  is pushed outward against the urging force of the urging member  60 . Consequently, as shown in  FIG. 4 , the projecting portion  38  of the driving member  30  passes across the position where the power transmission member  50  is provided, and idles with respect to the driven member  40 ; therefore, any rotational driving force greater than the predetermined magnitude cannot be transmitted to the driven member  40 . Thus, the motor-driven screwdriver  20  limits, to a predetermined magnitude, the rotational driving force transmittable to the bit holder  22  drivably connected to the driven member  40 . 
     Each through-hole  42 , which is provided in the driven member  40  to extend therethrough from the outer peripheral surface  48  to the inner peripheral surface  44 , is configured such that the through-hole center axis  46  does not intersect the rotation center axis  32 , as has been stated above. Assuming that an angle φ is an angle between a straight line L connecting between the rotation center axis  32  of the driving member  30  and the center of the power transmission member  50  and a straight line M connecting between the center of the power transmission member  50  and a point of contact between the projecting portion  38  and the power transmission member  50  when the projecting portion  38  is engaged with the power transmission member  50  ( FIG. 2 ) during forward rotation of the driving member  30 , an angle θ between the straight line L and the through-hole center axis  46  is preferably set smaller than the angle φ (θ&lt;φ). 
     Now, let us make a comparison in magnitude between rotational driving force T f  during forward rotation when the power transmission member  50  is pushed outward of the driven member  40  along the forward rotation guide surface  42 - 1  and rotational driving force T b  during backward rotation when the power transmission member  50  is pushed outward of the driven member  40  along the backward rotation guide surface  42 - 2 . Assuming that R f  represents the force applied to the power transmission member  50  when the rotational driving force T f  is generated in the driving member  30  during forward rotation, and that F f  represents the urging force of the urging member  60  during the forward rotation, the equation of equilibrium of force in the direction of the forward rotation guide surface  42 - 1  (i.e. in the direction of the through-hole center axis  46 ) is as follows:
 
 R   f  cos(φ−θ)= F   f  cos θ  (1)
 
     Assuming that R b  represents the force applied to the power transmission member  50  when the rotational driving force T b  is generated in the driving member  30  during backward rotation, and that F b  represents the urging force of the urging member  60  during the backward rotation, the equation of equilibrium of force in the direction of the backward rotation guide surface  42 - 2  (i.e. in the direction of the through-hole center axis  46 ) is as follows:
 
 R   b  cos(φ+θ)= F   b  cos θ  (2)
 
     Here, the urging force of the urging member  60  is the same during forward and backward rotation as follows:
 
 F   f   =F   b   =F   (3)
 
     Therefore, from the above expressions (1) to (3), we obtain the following:
 
 R   f  cos(φ−θ)= R   b  cos(φ+θ)  (4)
 
 R   f   /R   b =cos(φ+θ)/cos(φ−θ)&lt;1) (0&lt;φ−θ&lt;φ+θ&lt;90°)  (5)
 
 R   f   &lt;R   b   (6)
 
     The relation between the rotational driving force T f  and the force R f  during the forward rotation may be expressed as follows by using a constant C:
 
 T   f   =CR   f (7)
 
     Here, the magnitude of force applied to the power transmission member  50  by the rotational driving force varies according to both the distance between the rotation center axis  32  and the point of contact between the projecting portion  38  and the power transmission member  50  and the direction of the rotational driving force. In this regard, however, the point and angle of contact between the projecting portion  38  and the power transmission member  50  are symmetric about a radial axis  28  between forward and backward rotation. Therefore, the distance between the rotation center axis  32  and the point of contact is the same during forward and backward rotation. Moreover, the direction of the force is symmetric about the radial axis  28  between forward and backward rotation. Therefore, the relation between the rotational driving force T b  and the force R b  during backward rotation can be expressed as follows in the same way as during forward rotation:
 
 T   b   =CR   b   (8)
 
     Therefore, from the expressions (6) to (8), we can derive the following relation:
 
 T   f   &lt;T   b   (9)
 
     That is, the rotational driving force required to push the power transmission member  50  outward of the driven member  40  is greater during backward rotation than during forward rotation. In the motor-driven screwdriver  20 , the rotational driving force transmittable to loosen a screw is greater than that to tighten a screw. It should be noted that the difference in rotational driving force between forward and backward rotation can be set as desired by adjusting the degree of inclination of the forward rotation guide surface  42 - 1  and the backward rotation guide surface  42 - 2 , i.e. the degree (angle θ) of inclination of the through-hole center axis  46 , as will also be understood from the expression (5). 
     Thus, the motor-driven screwdriver  20  according to the present invention is configured to allow the transmittable rotational driving force to differ in magnitude between forward and backward rotation by obliquely forming the forward rotation guide surface  42 - 1  and the backward rotation guide surface  42 - 2 , which guide the associated power transmission member  50 , and does not require the projecting portions  38  of the driving member  30  to be formed into a complicated shape. Therefore, the parts can be made relatively simple in shape. 
     Although in the foregoing description the through-hole  42  is formed such that the through-hole center axis  46  is inclined by an angle θ, it should be noted that the through-hole  42  may be formed, as shown in  FIG. 6 , which is a drawing obtained by slightly rotating  FIG. 2  clockwise. That is, the through-hole  42  may be formed such that the through-hole center axis  46  is displaced parallel by a distance D from a line extending perpendicularly through the rotation center axis  32  of the driving member  30 , which results in a configuration similar to the above. In actual manufacturing process, it may be often easier to set a distance D from the center and to form the through-hole  42  perpendicularly at a position displaced by the distance D from the center than to set an amount of inclination and to obliquely form the through-hole  42  by cutting or the like. 
     The power transmission members  50  may have, besides a spherical shape, any other shape having a circular cross-section in a plane perpendicular to the rotation center axis  32 . For example, the power transmission members  50  may be circular cylindrical members disposed such that the longitudinal axes thereof are parallel to the rotation center axis  32 , as shown in  FIG. 7 . 
     Further, the through-holes  42  may be tapered holes, as shown in  FIG. 8 , in which the forward rotation guide surface  42 - 1  and the backward rotation guide surface  42 - 2  are not parallel to each other. In this case, the through-hole center axis  46  (i.e. an axis extending in the longitudinal direction of the through-hole through the center of the transverse section of the through-hole) is defined as a bisector bisecting an angle α formed by extended lines  42 - 3  and  42 - 4  of the forward rotation guide surface  42 - 1  and the backward rotation guide surface  42 - 2  in a plane perpendicular to the rotation center axis  32 , and thus the through-hole center axis  46  does not intersect the rotation center axis  32 . With this configuration, the respective directions of the forward and backward rotation guide surfaces  42 - 1  and  42 - 2  of the through-hole  42  can be set independently of each other; therefore, rotational driving forces transmittable during forward and backward rotation can be set with a higher degree of freedom. 
     The urging member  60  comprises, as shown in  FIG. 1 , a taper ring  62  and a spring  64 . The taper ring  62  has a tapered surface which engages the power transmission members  50 , so that, when the power transmission members  50  are pushed outward, a rightward (as seen in the figure) force is applied to the taper ring  62  from the power transmission members  50 . The spring  64  urges the taper ring  62  leftward (as seen in the figure) in the direction of the rotation center axis  32  to maintain, in the radial axis direction, the positions of the power transmission members  50  pushed by the projecting portions  38  of the driving member  30 . When a rotational driving force exceeding a predetermined value is applied, as has been stated above, the power transmission members  50  move outward along the through-hole center axis  46  while pushing the taper ring  62  rightward in the direction of the rotation center axis  32  against the urging force of the spring  64 . Consequently, an inclined surface  72  of a circular cylindrical member  70  presses balls  74  radially inward, and this causes an inclined surface  78  of a pilot pin holding member  76  to be pushed, which in turn pushes a pilot pin  80  leftward. The movement of the pilot pin  80  actuates a start switch of the driving motor to stop the driving motor. Stopping the driving motor in this way prevents the driving member from continuing to idle when a rotational driving force exceeding a predetermined value is generated, and it is therefore possible to reduce excess load on a screw or the like and to ensure safety for the user. 
     In this embodiment, the projecting portions  38  of the driving member  30  are formed by embedding the columnar members  36 , which are separate members, in the circular cylindrical outer peripheral surface  35  of the shaft portion  34 , but the projecting portions  38  may be formed integrally with the shaft portion  34 . Further, although the outer surface of each projecting portion  38  is arcuate, the projecting portion  38  may have an outer surface with a shape other than a circular arc, provided that the forward rotation engagement surface  38 - 1  and the backward rotation engagement surface  38 - 2  are symmetric about a radial axis  29  extending through the projecting portion  38 . Alternatively, the outer surface of the projecting portion  38  may have any asymmetric shape about the radial axis  29 . 
     LIST OF REFERENCE SIGNS 
     
         
         
           
               20 : motor-driven screwdriver 
               22 : bit holder 
               24 : rotational driving force transmission device 
               26 : speed reducer 
               28 : radial axis 
               29 : radial axis 
               30 : driving member 
               32 : rotation center axis 
               34 : shaft portion 
               35 : circular cylindrical outer peripheral surface 
               36 : circular columnar member 
               38 : projecting portion 
               38 - 1 : forward rotation engagement surface 
               38 - 2 : backward rotation engagement surface 
               40 : driven member 
               42 : through-hole 
               42 - 1 : forward rotation guide surface 
               42 - 2 : backward rotation guide surface 
               42 - 3 : extended line 
               42 - 4 : extended line 
               44 : inner peripheral surface 
               46 : through-hole center axis 
               48 : outer peripheral surface 
               50 : power transmission member 
               60 : urging member 
               62 : taper ring 
               64 : spring 
               70 : circular cylindrical member 
               72 : inclined surface 
               74 : ball 
               76 : pilot pin holding member 
               78 : inclined surface 
               80 : pilot pin