This application is a continuation of PCT/JP2005/009649, filed May 26, 2005, which claims priority to Japanese Application No. 2004-171883, filed Jun. 9, 2004. The entire contents of these applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a plug-socket assembly including a plug and a socket that receives and secures the plug. More particularly, the present invention relates to a plug-socket assembly suitable for use as a device for fastening an article. 
     BACKGROUND OF THE INVENTION 
     There is known a fastening device in which a socket is fixed to a wall, for example, and a plug is inserted into the socket and locked therein, thereby attaching to the wall a desired article such as a cover of lighting equipment previously attached to the plug. In order that the plug and the socket shall be surely connected together without play, the rear surface of the forward end head portion of the plug is formed into a slant surface that is sloped or extends radially inward toward the rear end thereof. A locking element fitted to the socket is resiliently pressed against the slant surface radially inward, thereby displacing the plug forward (i.e. drawing the plug into the socket; for example, see Japanese Patent Application Publication No. 2001-182726). In this type of fastening device, however, if an excessive pulling force is applied to the plug, the slant surface of the plug acts to displace the locking element radially outward, which may lead to a situation that the plug undesirably comes out of the socket. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a plug-socket assembly in which the plug cannot come out of the socket even if an excessive external pulling force is applied to the plug. 
     A plug-socket assembly according to the present invention comprises a socket body having a plug insertion hole, a locking element fitted to the socket body so as to be displaceable in the radial direction of the plug insertion hole, a locking element-actuating member fitted to the outside of the socket body so as to be displaceable in the axial direction of the plug insertion hole, and a resilient member that urges the locking element-actuating member in the axial direction. The plug has a forward end head portion that is slidably inserted into the plug insertion hole of the socket. The forward end head portion has a slant rear surface that is sloped or extends radially inward toward the rear end thereof as viewed in the plug insertion direction. 
     The locking element-actuating member of the socket has a locking element sliding surface provided on the inner side thereof. The locking element sliding surface is sloped or extends radially outward in an urging direction in which the locking element-actuating member is urged by the resilient member. The locking element sliding surface presses and engages the locking element by being urged in the axial direction by the resilient member. 
     The locking element sliding surface of the socket has a first portion that engages the locking element to allow it to project as far as a first position in the plug insertion hole when the plug is not inserted in the plug insertion hole, and a second portion that is spaced apart from the first portion by a predetermined distance in the urging direction of the resilient member. When engaging the second portion, the locking element is displaced to a second position that is radially outward of the first position to allow the forward end head portion of the plug to be inserted into the plug insertion hole beyond the locking element. The locking element sliding surface further has a third portion that is closer to the first portion than the second portion. The third portion presses and engages the locking element that engages the slant rear surface of the forward end head portion of the plug when the forward end head portion of the plug is inserted beyond the locking element. Further, the locking element sliding surface has a fourth portion that engages the locking element at an intermediate position between the second portion and the third portion and that is configured so that even if the locking element applies a radially outward force to the locking element sliding surface, the locking element-actuating member will not be forced to move in a direction opposite to the urging direction. 
     In the plug-socket assembly according to the present invention, if an excessive force is applied to the plug so as to pull it out of the socket, the locking element is moved radially outward by the slant rear surface of the forward end head portion of the plug, causing the locking element-actuating member to move against the urging force of the resilient member. Even if such occurs, when the locking element reaches a position where it engages the fourth portion of the locking element sliding surface, there is no longer any force from the locking element that causes the locking element-actuating member to move in the axial direction. Accordingly, the radially outward movement of the locking element is blocked, and thus the plug is prevented from being undesirably pulled out. 
     To pull the plug out of the socket, the locking element-actuating member is moved against the urging force of the resilient member. By doing so, the plug and the socket can be disengaged from each other. 
     Preferably, the slope between the first portion and the fourth portion is less steep than the slope between the fourth portion and the second portion. The reason for this is as follows. The slope between the first portion and the fourth portion is preferably minimized in order to maximize the radially inward force transmitted from the resilient member to the locking element through the locking element-actuating member to thereby increase the force for drawing the plug into the plug insertion hole. On the other hand, the slope between the fourth portion and the second portion is preferably made steep in order that a small axial displacement of the locking element-actuating member shall allow the locking element to be displaced radially outward to a considerable extent (i.e. in order to minimize the axial length of the locking element-actuating member). 
     In order to allow the plug to be inserted into the plug insertion hole without moving the locking element-actuating member in the axial direction, balls that are displaceable radially outward by the forward end head portion of the plug may be used to be pressed against the slant surface between the fourth portion and the second portion to move the locking element-actuating member in the axial direction, as will be described later. In this regard also, the slope between the fourth portion and the second portion should be made steep to minimize the force required to insert the plug. 
     Specifically, the fourth portion may be formed to extend parallel to the axial direction of the plug insertion hole of the socket. The fourth portion may have a surface configuration facing opposite to the urging direction of the resilient member. The fourth portion may be a recess curved radially outward. 
     As has been stated above, in order to allow the plug to be inserted into the plug insertion hole without moving the locking element-actuating member, the plug-socket assembly may be arranged such that the forward end head portion of the plug is provided with a slant front surface extending radially inward in the plug insertion direction, and radially displaceable balls are provided at a position of the socket body closer to the inlet of the plug insertion hole than the locking element. 
     The slant front surface of the plug engages the balls when the plug is inserted into the plug insertion hole, and displaces the balls radially outward so that the balls engage and press against the locking element sliding surface between the fourth portion and the second portion, thereby displacing the locking element-actuating member in the direction opposite to the urging direction to move the locking element to a position between the fourth portion and the second portion. The slant front surface of the forward end head portion of the plug as inserted presses and engages the locking element in the above-described position, thereby displacing the locking element-actuating member in the axial direction. 
     Specifically, the balls may include first balls provided closer to the inlet of the plug insertion hole than the locking element, and second balls provided closer to the inlet of the plug insertion hole than the first balls. 
     The slant front surface of the plug first engages the second balls when the plug is inserted into the plug insertion hole, and displaces the second balls radially outward so that the second balls engage and press against the locking element sliding surface between the fourth portion and the second portion, thereby displacing the locking element-actuating member in the direction opposite to the urging direction. Subsequently, the slant front surface engages the first balls and displaces them radially outward so that the first balls engage and press against the locking element sliding surface between the fourth portion and the second portion, thereby displacing the locking element-actuating member in the direction opposite to the urging direction. Finally, the slant front surface engages the locking element and displaces it radially outward so that the locking element engages and presses against the locking element sliding surface between the fourth portion and the second portion, thereby displacing the locking element-actuating member in the direction opposite to the urging direction, and thus allowing the locking element to engage the second portion. 
     The plug may have at the rear end thereof a flange extending in the radial direction. The flange can clamp and secure a desired member between itself and the rear end of the socket body in a state where the locking element is pressed and engaged with the slant rear surface of the forward end head portion of the plug as inserted into the plug insertion hole. The desired member may be connected and secured to the plug so as to be attached by inserting and connecting the plug into the socket. 
     Preferably, the socket body is provided with a flange extending radially outward so that the forward end of the locking element-actuating member, as viewed in the urging direction in which it is urged by the resilient member, engages the flange when the plug is not inserted. The reason for this arrangement is to prevent the locking element-actuating member from hitting the first balls or the second balls when the locking element-actuating member is released after it has been pulled back by a manual operation and then pushed back by the resilient member. 
     In the plug-socket assembly according to the present invention, when the plug is inserted into the socket, the locking element engages the slant rear surface of the forward end head portion of the plug, and at this time, the slanted locking element sliding surface of the locking element-actuating member is engaged with the locking element, and the pressing force of a resilient member, e.g. a spring, is applied to the locking element, thereby applying drawing force to the plug in the plug insertion direction. By so doing, the plug is locked to the socket without play. In addition, even if an excessive pulling force is applied to the plug, the locking element sliding surface will not be displaced to such an extent as to release the locking element, which might otherwise occur in the prior art, but allows the locking element to remain in the engaging position with the forward end head portion of the plug, whereby the plug can be prevented from being pulled out of the socket. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal sectional view of the plug-socket assembly according to the first embodiment of the present invention. 
         FIG. 2A  is a view for explaining the operation of the plug-socket assembly according to the first embodiment, which shows the way in which the sleeve is retracted by a manual operation to insert the plug, thereby allowing the balls serving as the locking element to be displaced radially outward, so that the forward end head portion of the plug inserted pushes the balls aside and is inserted further inside the plug insertion hole. 
         FIG. 2B  is a view for explaining the operation of the plug-socket assembly according to the first embodiment, which shows the way in which the plug is further inserted from the position shown in  FIG. 2A , and when the forward end head portion has passed the balls serving as the locking element, the sleeve is released, so that the sleeve is returned rightward by the compression spring, and the slant inner surface (sliding surface) of the socket engages the balls and displaces them radially inward, causing the balls to engage the slant rear surface of the forward end head portion of the plug. 
         FIG. 2C  is a view for explaining the operation of the plug-socket assembly according to the first embodiment, which shows the way in which a rightward pulling force acts on the plug that is in the position shown in  FIG. 2B , and the force is transmitted to the sleeve through the balls, causing the sleeve to move leftward, so that the horizontal fourth portion at an intermediate part of the slant inner surface (sliding surface) engages the balls to prevent transmission of leftward force to the sleeve. 
         FIG. 3  is a schematic cross-sectional view as seen from the line III-III in  FIG. 1 , showing an example of arrangement of the balls. 
         FIG. 4A  is a fragmentary longitudinal sectional view of the sleeve in the first embodiment of the plug-socket assembly. 
         FIG. 4B  is a fragmentary longitudinal sectional view of a sleeve having a sliding surface similar to that shown in  FIG. 4A  but modified. 
         FIG. 4C  is a fragmentary longitudinal sectional view of a sleeve having another modified sliding surface. 
         FIG. 5  is a longitudinal sectional view of the plug-socket assembly according to the second embodiment of the present invention. 
         FIG. 6  is a schematic cross-sectional view as seen from the line VI-VI in  FIG. 5 , showing an example of ball arrangement. 
         FIG. 7  is a schematic cross-sectional view as seen from the line VII-VII in  FIG. 5 , showing an example of ball arrangement. 
         FIG. 8  is a schematic cross-sectional view as seen from the line VIII-VIII in  FIG. 5 , showing an example of ball arrangement. 
         FIG. 9A  is a view for explaining the operation of the plug-socket assembly according to the second embodiment, which shows a state before the plug is inserted. 
         FIG. 9B  shows a state where the insertion of the plug has been started, so that the forward end head portion of the plug has engaged the second balls and begun to move the sleeve leftward. 
         FIG. 9C  shows a state where the insertion of the plug has proceeded, so that the forward end head portion of the plug has begun to engage the first balls. 
         FIG. 9D  shows a state where the first balls have engaged the sliding surface of the sleeve by being pushed radially outward by the forward end head portion of the plug, and thus have begun to displace the sleeve leftward. 
         FIG. 9E  shows a state where the balls serving as the locking element have begun to engage the sliding surface of the sleeve by being pushed radially outward. 
         FIG. 9F  shows a state where the balls serving as the locking element have been engaged with the forward end head portion of the plug to engage the second portion of the sliding surface of the sleeve, thereby allowing the forward end head portion of the plug to be further inserted. 
         FIG. 9G  shows a state where the forward end head portion of the plug has been inserted beyond the balls serving as the locking element, and the compression spring has displaced the balls radially inward through the sleeve, causing the ball to engage the slant rear surface of the forward end head portion of the plug. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the plug-socket assembly according to the present invention will be explained with reference to  FIGS. 1 to 4A . In the following description, the term “axial direction” means a “direction along the longitudinal axis of the plug insertion hole” or a direction parallel thereto, and the term “radial direction” means a “direction extending radially” from the “axial direction”. 
       FIG. 1  is a longitudinal sectional view of a first embodiment of a plug-socket assembly  10 .  FIGS. 2A to 2C  are views for explaining the operation of the plug-socket assembly  10 .  FIG. 3  is a schematic cross-sectional view as seen from the line III-III in  FIG. 1 .  FIG. 4A  is a fragmentary longitudinal sectional view of a sleeve. 
     As shown in  FIG. 1 , the plug-socket assembly  10  includes a plug  20  and a socket  30  that receives and secures the plug  20 . The socket  30  has a socket body  31  having a plug insertion hole  311 , and a locking element (assuming the form of balls in the illustrated example; hereinafter referred to simply as “balls”)  32  fitted to the socket body  31  so as to be displaceable in the radial direction of the plug insertion hole  311 . The socket  30  further has a locking element-actuating member (assuming the form of a sleeve in the illustrated example; hereinafter referred to simply as “sleeve”)  33  fitted to the outside of the socket body  31  so as to be displaceable in the axial direction of the plug insertion hole  311 , and a resilient member (assuming the form of a compression coil spring in the illustrated example; hereinafter referred to simply as “compression spring”)  34  that urges the sleeve  33  toward an inlet  313  of the plug insertion hole  311 . 
     The socket  30  is fixed to a desired place, e.g. a wall W, with a bolt B or the like. 
     An annular space  341  is formed between the inner periphery of the sleeve  33  and the outer periphery of the socket body  31 , and the compression spring  34  is installed in the space  341  and compressed by a stop ring  35  secured to the socket body  31 . The compression spring  34  urges the sleeve  33  to abut against a stopper flange  314  provided at the end of the plug insertion hole  311  closer to the inlet  313 . 
     The socket body  31  is provided with ball receiving holes  315  radially extending through the socket body  31 . The balls  32  are received in the ball receiving holes  315  so as to be displaceable in the radial direction. As shown in  FIG. 3 , the ball receiving holes  315  are provided at a plurality of equally spaced positions (4 positions in this embodiment) along the circumferential direction. The ball receiving holes  315  are tapered radially inward of the socket body  31  to prevent the balls  32  from falling off. The balls  32  are substantially in contact with a sliding surface  332  of the sleeve  33  (described later) and project radially inward of the socket body  31  when the plug  20  is not inserted in the socket  30  (see  FIG. 1 ). 
     The plug  20  has a forward end head portion  21  that is slidably inserted into the plug insertion hole  311  of the socket  30 . The plug  20  further has a rear end flange  22  and a reduced-diameter portion  23  located between the forward end head portion  21  and the rear end flange  22 . The forward end head portion  21  has a slant front surface  211  and a slant rear surface  212 , which extend radially inward toward the front and rear ends, respectively, of the forward end head portion  21 , and a flat surface  213  located between the slant front surface  211  and the slant rear surface  212 . 
     As shown in  FIGS. 1 and 4A , the sleeve  33  of the socket  30  is provided on the inner side thereof with a locking element sliding surface (i.e. a sliding surface that is in sliding contact with the balls  32 ; hereinafter referred to simply as “sliding surface”)  332 . The sliding surface  332  is sloped or extends radially outward in the urging direction in which the sleeve  33  is urged by the compression spring  34 . 
     The sliding surface  332  of the sleeve  33  of the socket  30  is located at a position where it is substantially in contact with the balls  32  when the plug  20  is not inserted in the plug insertion hole  311  ( FIG. 1 ). The sliding surface  332  engages the balls  32  at a first portion P 1  ( FIGS. 4A and 1 ), thereby causing the balls  32  to project into the plug insertion hole  311  by a predetermined amount. 
     To insert the plug  20  into the plug insertion hole  311 , an operator displaces the sleeve  33  leftward ( FIG. 2A ), as viewed in  FIGS. 1 and 2 , so that the balls  32  are displaced radially outward by the slant front surface  211  of the forward end head portion  21  of the plug  20  inserted into the plug insertion hole  311 , and the flat surface  213  of the forward end head portion  21  is inserted beyond the balls  32 . That is, in the illustrated example, the sliding surface  332  has a second portion P 2  ( FIGS. 4A and 2A ) for allowing the balls  32  to be displaced radially outward as stated above. 
     The operator releases the sleeve  33  when the forward end head portion  21  of the plug  20  has been inserted beyond the balls  32 . Consequently, the sleeve  33  is displaced rightward by the compression spring  34 , and the sliding surface  332  presses and engages the balls  32  at a third portion P 3  located between the first portion P 1  and the second portion P 2  ( FIGS. 4A and 2B ), thus causing the balls  32  to press and engage the slant rear surface  212  of the forward end head portion  21  of the plug  20 . In this state, the plug  20  is drawn into the plug insertion hole  311  by the urging force of the compression spring  34  applied thereto through the balls  32  and the slant rear surface  212 . In the illustrated example, a plate-shaped article  40  is clamped between the flange  22  at the plug rear end and the stopper flange  314  of the socket  30  and firmly held without play by the drawing force acting on the plug  20 . 
     The sliding surface  332  has a fourth portion P 4  extending horizontally between the second portion P 2  and the third portion P 3  ( FIG. 4A ). The fourth portion P 4  is intended to act as follows. When an external pulling force is applied to the plug  20 , the sleeve  33  is displaced leftward by the balls  32  pressed radially outward by the slant rear surface  212  of the plug forward end head portion  21 . At this time, when the balls  32  come in engagement with the fourth portion P 4  ( FIG. 2C ), there is no longer leftward force applied to the sleeve  33  from the balls  32  through the sliding surface  332 . Thus, the sleeve  33  is prevented from being further displaced leftward. That is, the fourth portion P 4  prevents the pulling out of the plug  20 . 
       FIGS. 4B and 4C  show modifications of the fourth portion P 4  of the sliding surface  332 . That is, the fourth portion P 4  in  FIG. 4B  is a recess that is curved radially outward. The fourth portion P 4  in  FIG. 4C  is a slant surface facing opposite to the urging direction of the compression spring  34 . These fourth portions P 4  are adapted so that when the balls  32  engage either of the fourth portions P 4 , force acting in the direction opposite to the urging direction of the compression spring  34  will not be transmitted to the sleeve  33  from the balls  32 . Thus, the fourth portions P 4  have an action similar to the above-described action of preventing the plug  20  from being pulled out undesirably, which is performed by the fourth portion P 4  shown in  FIG. 4A . 
     To unlock the plug  20  from the socket  30 , the sleeve  33  is forced to move leftward against the urging force of the compression spring  34  by a manual operation, for example. By doing so, the plug  20  can be readily disengaged from the socket  30 . 
     A second embodiment of the plug-socket according to the present invention will be explained with reference to  FIGS. 5 to 9 .  FIG. 5  is a longitudinal sectional view of the second embodiment of the plug-socket assembly  10 .  FIG. 6  is a schematic cross-sectional view as seen from the line VI-VI in  FIG. 5 .  FIG. 7  is a schematic cross-sectional view as seen from the line VII-VII in  FIG. 5 .  FIG. 8  is a schematic cross-sectional view as seen from the line VIII-VIII in  FIG. 5 .  FIG. 9  is a view for explaining the operation of the plug-socket assembly  10  according to the second embodiment. 
     The basic arrangement of the plug-socket assembly  10  is substantially the same as that of the foregoing first embodiment. Therefore, only the points in which the second embodiment differs from the first embodiment will be explained below. 
     In the plug-socket assembly  10  according to the second embodiment, the socket body  31  has, as shown in  FIGS. 5 to 7  and  9 , first and second sleeve-actuating balls  32   a  and  32   b  (see  FIGS. 5 and 7 ) for displacing the sleeve  33  in addition to the above-described balls  32  serving as the locking element. The first and second sleeve-actuating balls  32   a  and  32   b  are provided closer to the inlet  313  of the plug insertion hole  311  than the balls  32 . More specifically, the first sleeve-actuating balls  32   a  are located closer to the plug insertion hole inlet than the balls  32 . The second sleeve-actuating balls  32   b  are located even more closer to the plug insertion hole inlet than the balls  32 . The first sleeve-actuating balls  32   a  are circumferentially spaced from the balls  32  by a required angle, as shown in  FIGS. 6 and 7 , so as not to interfere with the balls  32 . 
     When the plug  20  is inserted into the plug insertion hole  311  ( FIG. 9A ), the slant front surface  211  of the plug  20  first engages the second sleeve-actuating balls  32   b  and displaces them radially outward ( FIG. 9B ) so that the second sleeve-actuating balls  32   b  engage and press against the sliding surface  332  at a position between the fourth portion P 4  and the second portion P 2 , thereby displacing the sleeve  33  in the direction opposite to the urging direction of the compression spring  34  ( FIG. 9C ). Subsequently, the slant front surface  211  of the plug  20  engages the first sleeve-actuating balls  32   a  ( FIG. 9D ) and displaces them radially outward ( FIG. 9E ) so that the first sleeve-actuating balls  32   a  engage and press against the sliding surface  332  at a position between the fourth portion P 4  and the second portion P 2 , thereby displacing the sleeve  33  in the direction opposite to the urging direction of the compression spring  34 . Finally, the slant front surface  211  of the plug  20  engages the balls  32  serving as the locking element and displaces them radially outward ( FIG. 9F ) so that the balls  32  engage and press against the sliding surface  332  at a position between the fourth portion P 4  and the second portion P 2 , thereby displacing the sleeve  33  in the direction opposite to the urging direction. Consequently, the balls  32  engage the second portion P 2  to allow the flat surface  213  of the forward end head portion  21  of the plug  20  to be inserted into the plug insertion hole  311  beyond the balls  32 . When the flat surface  213  of the forward end head portion  21  of the plug  20  has been inserted beyond the balls  32 , the sleeve  33  is pressed and displaced rightward by the urging force of the compression spring  34 , causing the balls  32  to press and engage the slant rear surface  212  of the forward end head portion  21  of the plug  20 . Thus, the plug  20  is forced to be drawn into the plug insertion hole  311 , thereby being connected and locked. 
     In the plug-socket assembly according to this embodiment, the plug can be inserted without moving the sleeve leftward in advance. That is, by simply inserting the plug, the sleeve is moved to allow the plug to be inserted so as to be connected ( FIG. 9G ). 
     Although some embodiments of the plug-socket assembly according to the present invention have been described above, the present invention is not necessarily limited to these embodiments but can be modified in a variety of ways without departing from the scope of the invention set forth in the appended claims. For example, the plug-socket assembly according to the second embodiment has the first and second sleeve-actuating balls for moving the sleeve  33  leftward. In other words, these sleeve-actuating balls are provided to move the sleeve as far as a position where the portion of the sliding surface between the second portion and the fourth portion radially is aligned with the balls  32  serving as the locking element. Therefore, only either the first or second sleeve-actuating balls may be used to move the sleeve so that the sliding surface and the balls  32  are radially aligned with each other as stated above. Further, in the foregoing embodiments, a plate-shaped article is arranged to be clamped between the flange provided at the rear end of the socket and the flange provided at the rear end of the plug. The arrangement may, however, be such that the article is attached directly to the plug by some means instead of being clamped as stated above. In such a case, the flange at the rear end of the plug is arranged to abut against the rear end of the socket, whereby the flanges at the plug rear end and the socket rear end can be firmly secured to each other without play by the force applied by the compression spring to the plug to draw it into the plug insertion hole. Accordingly, the article can be attached even more surely. Although in the foregoing embodiments, the socket is fixed to a wall, for example, the arrangement may be such that the plug is fixed to the wall and an article to be attached is secured to the socket, thereby attaching the article to the wall. 
     The plug-socket assembly according to the present invention is usable not only to attach a desired article to a wall or the like but also to serve in a plug-socket assembly type connector to prevent disconnection of the plug and the socket that might otherwise be caused by an excessive external force unexpectedly applied to the connected plug and socket.
An object of the present invention is to provide a plug/socket assembly in which the plug will never pull out of the socket even if an excessive pull-out force is applied to the plug. A plug/socket assembly ( 10) comprises a plug ( 20) and a socket ( 30) for receiving and fixing the plug ( 20) therein. The socket ( 30) includes a socket main body ( 31) having a plug insertion hole ( 311), balls ( 32) which are provided with the socket main body ( 31) in a radially displaceable manner with respect to the plug insertion hole ( 311), a sleeve ( 33) which is mounted over the outer surface of the socket main body ( 31) and is displaceable along the axial direction of the plug insertion hole ( 311), and a compression spring ( 34) for urging the sleeve ( 33) along the axial direction toward the inlet port ( 313) of the plug insertion hole. The inner surface of the sleeve is provided with a slide surface ( 332). The slide surface has first to fourth regions (P 1 -P 4). The fourth region (P 4) is shaped such that with an external pull-out force being applied to the plug, even if the balls apply a radially outward force to the slide surface, no leftward force to push back the sleeve will be produced.