Patent Publication Number: US-2013249156-A1

Title: Clamp apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a clamp apparatus which is arranged to cause a locking member to engage with a hole formed in an object to be fixed such as a work and a mold and to pull and fix the object via the locking member. 
     BACKGROUND ART 
     An example of such a clamp apparatus is recited in Patent Literature 1(Japanese Patent No. 4297511). This conventional art is arranged as follows. 
     Four grip members are fitted externally to an upper part of a clamp rod with predetermined gaps being formed in circumferential directions, and the grip members are radially biased inward by an O-ring made of rubber and pushed upward by a hydraulic advancing unit. When the clamp is driven, to begin with, the grip members maintained at an elevated position by a predetermined force of the advancing unit are pushed radially outward by tapered surfaces of the clamp rod, so that the peripheries of the grip members engage with a hole of a work. Thereafter, the grip members engaging with the work are brought down by the clamp rod against the advancing unit. As a result, the clamp rod pulls the work downward via the grip members and fixes the work to the upper surface of the housing. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent No. 4297511 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     The conventional art above involves the following problems. 
     When the clamp is driven, because the grip members are pushed radially outward, a wide gap is formed between neighboring grip members. If a foreign matter such as swarf or dust is caught in the gap, the grip members cannot move radially inward and are kept at radially outside positions. If one attempts to fit the hole of the work with the grip members in this state, the lower part of the circumferential wall of the hole collides the upper end of each grip member and the grip member is damaged. 
     Furthermore, according to the conventional art, parts where the tapered surfaces of the clamp rod engage with the tapered surfaces of the grip members are exposed upward. On this account, a foreign matter may be caught at the engaging parts and the tapered surfaces may be damaged. 
     For the reasons above, the above-described clamp apparatus of the conventional art breaks down often and is poor in maintainability. 
     An object of the present invention is to provide a clamp apparatus which is good in maintainability. 
     Solution to Problem 
     To achieve the object above, the clamp apparatus is arranged as below as shown in  FIG. 1A  to  FIG. 2B  or  FIG. 3  to  FIG. 6 . 
     A guide member  16  includes a top wall  17  and a circumferential wall  18  protruding to a leading end side as compared to a housing  1  and inserted into a hole  21  of an object  20  to be fixed. The guide member  16  is inserted into the housing  1  to be movable toward a base end side against an advancing unit  26  that pushes the guide member  16  toward the leading end side, within a locking stroke of moving the guide member  16  toward the base end side. A plurality of guide grooves  23  are formed on the circumferential wall  18  of the guide member  16  at predetermined intervals in a circumferential direction and radially penetrate the circumferential wall  18 . A plurality of locking members  24  are inserted into the respective guide grooves  23  to be radially movable, are biased radially inward by an elastic member  50  or  52 . Each of the locking members  24  includes a engaging portion  45  which is pressed onto an inner peripheral surface of the hole  21  of the object  20  to be fixed. A clamp rod  32  includes wedge surfaces  35  engaging with the respective locking members  24  from the leading end side, and the clamp rod  32  inserted into the guide member  16  is clamp-driven toward the base end side and unclamp-driven toward the leading end side by a driver D. 
     The present invention achieves the following functions and effects. 
     In the unclamped state, the guide member and the locking members are maintained at an elevated position by a predetermined force of the advancing unit. In the clamp driving, to begin with, the clamp rod pushes out the locking members, which has been maintained at the elevated position by the predetermined force, radially outward along the guide grooves formed on the circumferential wall of the guide member, and causes the engaging portion of each locking member to engage with the hole of the work. Subsequently, the clamp rod moves down the locking members engaging with the work and the guide member against the advancing unit. As a result, the clamp rod pulls the work downward via the locking members and the work is fixed to the upper surface of the housing. 
     Between the guide grooves formed on the circumferential wall of the guide member and the locking members, only a small gap allowing the locking members to be radially movable is required. For this reason, being different from the conventional art above, it is possible to prevent a foreign matter such as swarf and dust from being caught at the locking members, and the elastic member causes the locking members to smoothly return radially inward. The locking members are therefore smoothly attached and detached to and from the hole of the work, and hence the damage to the locking members is prevented. 
     Furthermore, because the parts where the wedge surfaces of the clamp rod engage with the locking members are housed in a space enclosed by the top wall and the circumferential wall of the guide member, it is possible to prevent a foreign matter from being caught at the engaging parts. 
     Because of the above, the clamp apparatus of the present invention prevents the occurrence of a failure and is good in maintainability. 
     The present invention is preferably arranged so that the top wall  17  and the circumferential wall  18  of the guide member  16  are seamlessly formed in an integrated manner, and the clamp rod  32  is arranged to be inserted into the guide member  16  from the base end side of the circumferential wall  18 . 
     With this arrangement, the guide member is easily manufactured and attains high durability with a simple structure. Furthermore, because the clamp rod is easily attached and detached, and this contributes to the improvement in the maintainability. 
     In addition to the above, the following arrangements are preferably added to the present invention. 
     The circumferential wall  18  of the guide member  16  includes a guide portion  40  inserted into the hole  21  of the object  20  to be fixed and a guide base portion  41  which is formed to be larger in diameter than the guide portion  40  and inserted into the housing  1 . A base end face  24   a  of each of the locking members  24  is supported to be radially movable by either the guide base portion  41  of the guide member  16  or an interlocked member  27  that axially moves together with the guide member  16 . 
     In this case, because the base end face of the locking member is supported by the large-diameter guide base portion or the interlocked member, the inclination of the locking member at the time of the clamp driving is prevented and the locking member smoothly moves. 
     In addition to the above, the present invention is preferably arranged so that the guide grooves  23  are formed to be lined up on the guide portion  40  and the guide base portion  41  of the guide member  16 , and each of the locking members  24  is constituted by the engaging portion  45  formed on the leading end side and a thick lock base portion  46  that is on the base end side as compared to the engaging portion  45  and protrudes radially outward as compared to the engaging portion  45 . 
     According to this arrangement, the height of the locking member inserted into the guide groove can be increased. Furthermore, because the large-diameter lock base portion of the locking member is guided by the large-diameter guide base portion or the interlocked member, the prevention of the inclination of the locking members at the time of the clamp driving is ensured, and the locking members further smoothly move. 
     In addition to the above, preferably, in an unclamped state in which the clamp rod  32  is unclamp-driven, an outer peripheral surface of the engaging portion  45  of each of the locking members is provided radially inside as compared to an outer peripheral surface of the guide portion  40  of the circumferential wall  18 . 
     According to this arrangement, when the guide member is inserted into the hole of the work at the time of bringing the work, the guide member prevents the circumferential wall of the hole from colliding the locking members. It is therefore possible to properly use the locking members for a long time. 
     Preferably, the elastic member  50  is formed to have an annular shape and attached to the outer peripheral surface of the guide base portion  41  and the outer peripheral surface of the lock base portion  46 . 
     Furthermore, preferably, attaching grooves  48  and  49  are formed on the outer peripheral surface of the guide base portion  41  and the outer peripheral surface of the lock base portion  46 , respectively, to circumferentially extend, and the elastic member  50  which is formed to be a thin strip is attached to the attaching grooves  48  and  49 . 
     According to this arrangement, on the circumferential wall of the guide member, the attaching grooves are formed on the guide base portion which is formed to be thicker than the guide portion. The strength of the guide member is therefore improved as compared to a case where attaching grooves are formed on the guide portion. 
     The elastic member  52  may be provided in an annular space between the housing  1  and the guide base portion  41  to bias at least one of the guide member  16  and the locking members  24  radially inward. 
     Furthermore, preferably, the elastic member  52  provided between the housing  1  and the guide base portion  41  is constituted by dust seal, and an inner circumferential part of the dust seal contacts an outer peripheral surface of the elastic member  50  attached to an outer peripheral surface of the guide base portion  41  and an outer peripheral surface of the lock base portion  46 . 
     This ensures the prevention of the intrusion of a foreign matter such as swarf and dust into the housing. 
     For example, as shown in  FIG. 1A  to  FIG. 2B , further preferably, a receiving portion  54 ,  55  that prevents a base end portion of a radially inside surface of the lock base portion  46  from moving inward is provided on an outer circumference of the clamp rod  32 . 
     With this, the prevention of the inclination of the locking members at the time of the clamp driving is ensured, and the locking members further smoothly move. 
     In addition to the above, to ensure the prevention of the inclination of the locking members  24  at the time of the clamp driving, as shown in  FIG. 5 , for example, a plurality of receiving surfaces  34  are formed to be axially lined up on a radially inside surface of each of the locking members  24 , and wedge surfaces  35  engaging with the respective receiving surfaces  34  from the leading end side are formed on the clamp rod  32  to be axially lined up. 
     Furthermore, as shown in  FIG. 1A , for example, a seated surface  58  for receiving the object  20  to be fixed is provided on the housing  1 , and an ejection hole  59  for pressurized air is opened on the seated surface  58  to be used by a seating sensor, and a mounting surface  61 , which supports the object  20  to be fixed at a position on the leading end side as compared to the seated surface  58  when the guide member  16  is advanced to the leading end side by the advancing unit  26 , is provided on the guide member  16 . 
     In addition to the above, the guide member  16  and the clamp rod  32  are provided to be movable radially with respect to the housing  1 . 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  relates to First embodiment of the present invention and is a cross section of a clamp apparatus in an unclamped state, when viewed in elevation. 
         FIG. 1B  is a cross section taken at the  1 B- 1 B line in  FIG. 1A . 
         FIG. 1C  is a cross section taken at the  1 C- 1 C line in  FIG. 1A . 
         FIG. 2A  is a cross section of the clamp apparatus in a clamped state, when viewed in elevation. 
         FIG. 2B  is a cross section taken at the  2 B- 2 B line in  FIG. 2A . 
         FIG. 3  relates to Second Embodiment of the present invention and is a partial view similar to  FIG. 1A  above. 
         FIG. 4  relates to Third Embodiment of the present invention and is a partial view similar to  FIG. 1A  above. 
         FIG. 5  relates to Fourth Embodiment of the present invention and is a partial view similar to  FIG. 1A  above. 
         FIG. 6  relates to Fifth Embodiment of the present invention and is a partial view similar to the clamped state shown in  FIG. 2A  above. 
     
    
    
     REFERENCE SIGNS LIST 
       1 : HOUSING,  16 : GUIDE MEMBER,  17 : TOP WALL,  18 : PERIPHERAL WALL,  20 : OBJECT TO BE FIXED (WORK),  21 : HOLE,  23 : GUIDE GROOVE,  24 : LOCKING MEMBER,  24   a : BASE END FACE,  26 : ADVANCING UNIT,  27 : INTERLOCKED MEMBER,  32 : CLAMP ROD,  34 : RECEIVING SURFACE,  35 : WEDGE SURFACE,  40 : GUIDE PORTION,  41 : GUIDE BASE PORTION,  45 : ENGAGING PORTION,  46 : LOCK BASE PORTION,  48 ,  49 : ATTACHING GROOVES,  50 : ELASTIC MEMBER,  52 : ELASTIC MEMBER (DUST SEAL),  54 : RECEIVING PORTION (FIRST RECEIVING PORTION),  55 : RECEIVING PORTION (SECOND RECEIVING PORTION),  58 : SEATED SURFACE,  59 : EJECTION HOLE,  61 : MOUNTING SURFACE, D: DRIVER 
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1A  to  FIG. 2B  relate to First embodiment of the present invention. A clamp apparatus of this First embodiment is used for fixing a work which is an object to be fixed. To begin with, the structure of the clamp apparatus will be described with reference to mainly  FIG. 1A  to  FIG. 1C . 
     To a fixing board T such as a table, a housing  1  is attached. The housing  1  includes an upper housing  2  and a lower housing  3 . The upper and lower housings  2  and  3  are fastened with each other by a plurality of bolts (not illustrated). In an upward direction, a large-diameter hole  2   a , a middle-diameter hole  2   b , and a small-diameter hole  2   c  are formed in the upper housing  2  in this order. Furthermore, in the upward direction, a large-diameter cylinder hole  3   a , a small-diameter cylinder hole  3   b , and a middle-diameter upper hole  3   c  are formed in the lower housing  3  in this order. 
     In the large-diameter cylinder hole  3   a  a piston main body  6  of a piston  5  is hermetically inserted, whereas in the small-diameter cylinder hole  3   b  a piston rod  7  is hermetically inserted. From and to a clamp chamber  10  formed above the piston main body  6 , pressurized oil is supplied and discharged via a supply and discharge port  11 . Furthermore, from and to an unclamp chamber  12  formed below the piston main body  6 , pressurized oil is supplied and discharged via another supply and discharge port  13 . 
     The piston  5 , the clamp chamber  10 , and the unclamp chamber  12  constitute a driver D of a later-described clamp rod  32 . 
     Into the upper housing  2 , a guide member  16  is inserted to be movable in a vertical direction (axial direction). The guide member  16  includes a top wall  17  having a guide surface  17   a  and a circumferential wall  18 . The top wall  17  and the circumferential wall  18  are seamlessly formed in an integrated manner. These walls protrude upward (i.e., on the leading end side) as compared to the upper end (leading end) of the upper housing  2  and can be inserted into a hole  21  of a work  20 . 
     On the circumferential wall  18 , three guide grooves  23  are formed at predetermined intervals in circumferential directions. The guide grooves  23  radially penetrate the circumferential wall  18 . Into each guide groove  23 , a locking member  24  is inserted to be radially movable. 
     An advancing unit  26  is provided to push the guide member  16  upward with a predetermined force. This advancing unit  26  includes a cylindrical interlocked member  27  that is inserted into the large-diameter hole  2   a  of the upper housing  2  and is movable in the vertical direction together with the guide member  16  and a plurality of coned disc springs  28  that bias the interlocked member  27  upward. The upward movement of the interlocked member  27  is prevented by a stepped portion  30  formed between the large-diameter hole  2   a  and the middle-diameter hole  2   b . The coned disc springs  28  are attached to the above-described upper hole  3   c  of the lower housing  3 . 
     Into the guide member  16 , the clamp rod  32  can be inserted from the lower side (base end side) of the circumferential wall  18 . A lower part of this clamp rod  32  is connected to the piston rod  7  to be radially movable. At an upper part of the clamp rod  32 , an wedge surface  35  is formed to engage from above with a receiving surface  34  formed on an inner side of each locking member  24 . The receiving surface  34  and the wedge surface  35  are sloped downward toward the axis of the clamp rod  32 . These surfaces are flat surfaces in the present case. 
     Furthermore, to an annular gap formed between the outer peripheral surface of the clamp rod  32  and the above-described interlocked member  27 , a U-shaped packing  37  is attached. This packing  37  prevents a foreign matter such as cutting oil from entering the driver D and biases the clamp rod  32  radially inward. 
     Now, the guide member  16 , the locking member  24 , and the clamp rod  32 , which have been briefly described above, will be detailed. 
     The circumferential wall  18  of the guide member  16  includes a guide portion  40  inserted into the hole  21  of the work  20  and a guide base portion  41  which is larger in diameter than the guide portion  40  and inserted into the upper housing  2 . From a lower part of this guide base portion  41 , a flange  42  protrudes radially outward, and this flange  42  is inserted into the middle-diameter hole  2   b  to be radially movable. 
     As a result, the guide member  16  and the clamp rod  32  are arranged to be radially movable with respect to the housing  1 . 
     The above-described guide grooves  23  are formed on the guide portion  40  of the guide member  16  and the guide base portion  41  to be lined up in the vertical direction. 
     Furthermore, each of the locking members  24  inserted into the guide groove  23  is constituted by a engaging portion  45  pressed onto the inner peripheral surface of the hole  21  of the work  20  and a lock base portion  46  formed below the engaging portion  45 . The lock base portion  46  is a thick portion and protrudes radially outward as compared to the engaging portion  45 . The lower surface of the lock base portion  46 , which surface is a lower end surface (base end surface)  24   a  of the locking member  24 , is supported by the upper surface of the flange  42  of the guide base portion  41  to be radially movable. 
     On the outer peripheral surface of the guide base portion  41 , attaching grooves  48  are formed to extend in circumferential directions. An attaching groove  49  is also formed on the outer peripheral surface of each lock base portion  46  to extend in circumferential directions. To these attaching grooves  48  and  49 , an elastic member  50  which is a thin strip made of rubber or synthetic resin is attached. With this arrangement, the elastic member  50  biases the locking members  24  radially inward. 
     In addition to the above, in an annular space formed between the above-described small-diameter hole  2   c  of the upper housing  2  and the guide base portion  41 , a dust seal (elastic member)  52  is provided to bias the guide member  16  radially inward. This dust seal  52  is made of synthetic resin or the like, and contacts, at its inner circumferential part, the outer peripheral surface of the elastic member  50 . With this arrangement, the dust seal  52  biases the locking member  24  and the guide member  16  radially inward. 
     Furthermore, to prevent the lower end portion of the radially inside surface of the lock base portion  46  of the locking member  24  from moving radially inward, two receiving portions, i.e., first and second receiving portions  54  and  55  are provided on the outer circumference of the clamp rod  32 . That is to say, in the initial stage of clamp driving of switching the unclamped state shown in  FIG. 1A  to the clamped state shown in  FIG. 2A , as shown in  FIG. 1A  and  FIG. 1B , the first receiving portion  54  contacts the lower end portion of the inside surface of the lock base portion  46 . In the middle and late stages of the clamp driving, as shown in  FIG. 2B , the second receiving portion  55  contacts the lower end portion of the inside surface of the lock base portion  46 . These two receiving portions  54  and  55  are preferably formed to be adjacent in the vertical direction. 
     In addition to the above, the upper housing  2  is provided with a seated surface  58  for receiving the work  20 , and an ejection hole  59  for ejecting pressurized air is opened on the seated surface  58  to be used by a seating sensor (not illustrated). Furthermore, a mounting surface  61  supporting the work  20  is provided on the upper surface of the guide base portion  41  of the guide member  16 . The mounting surface  61  supports, as shown in  FIG. 1A , the work  20  at a position above the seated surface  58  when the advancing unit  26  advances the guide member  16  upward. 
     The clamp apparatus above operates as described below. 
     In the unclamped state shown in  FIG. 1A  to  FIG. 1C , the pressurized oil in the clamp chamber  10  is discharged and the pressurized oil is supplied to the unclamp chamber  12 . With this, the piston  5  moves the clamp rod  32  upward to an unclamped position, the coned disc springs  28  move the guide member  16  and the locking member  24  upward to a released position via the interlocked member  27 , and each locking member  24  is biased by the elastic member  50  radially inward. Because of the above, the outer peripheral surface of the engaging portion  45  of the locking member  24  is positioned inside as compared to the outer peripheral surface of the guide portion  40  of the circumferential wall  18  of the guide member  16 . It is therefore possible to prevent, by the guide member  16 , the circumferential wall of the hole  21  of the work  20  from colliding each locking member  24  when the work is brought as described below. 
     As the work  20  is moved down in the unclamped state, the hole  21  of the work  20  is externally fitted with the guide portion  40  of the circumferential wall  18  of the guide member  16  and the engaging portion  45  of the locking member  24 , with a predetermined gap being formed therebetween, and the lower surface of the work  20  is received by the mounting surface  61  of the guide member  16 . 
     At this stage, because a contact gap G is formed between the seated surface  58  of the upper housing  2  and the lower surface of the work  20 , pressurized air is exhausted through the ejection hole  59 , and hence the pressure in an air supply port (not illustrated) connected to the ejection hole  59  is lowered. When the pressure switch (seating sensor) detects this pressure drop, the state of the clamp apparatus is confirmed to be the unclamped state. 
     When the unclamped state shown in  FIG. 1A  is switched to the clamped state shown in  FIG. 2A , the pressurized oil in the unclamp chamber  12  is discharged and the pressurized oil is supplied to the clamp chamber  10 , so that the piston  5  is lowered. 
     In response to this, first of all, the clamp rod  32  is lowered with respect to the guide member  16  and the locking members  24  maintained at the elevated position by the urging force of the coned disc spring  28 , the wedge surface  35  of the clamp rod  32  pushes the locking member  24  radially outward, and the engaging portion  45  of each locking member  24  engages with the hole  21  of the work  20  as the engaging portion  45  plastically deforms the inner peripheral surface of the hole  21  or frictionally contacts the inner peripheral surface. Subsequently, in this engaged state, the clamp rod  32  locking-drives the locking members  24  and the guide member  16  downward against the urging force of the coned disc springs  28 . As a result, the locking member  24  pulls the work  20  downward and the lower surface of the work  20  is fixed to the seated surface  58  of the upper housing  2 . 
     That is to say, the guide member  16  is, within its locking stroke, inserted into the upper housing  2  to be movable downward against the urging force of the coned disc springs  28  that push the guide member  16  upward. 
     In the initial stage of the locking stroke of the guide member  16  (i.e., in the initial stage of the clamp driving), as shown in  FIG. 1A  and  FIG. 1B , the first receiving portion  54  prevents the lower end portion of the inside surface of the lock base portion  46  of each locking member  24  from moving radially inward. In the middle and late stages of the locking stroke (i.e., in the middle and late stages of the clamp driving), as shown in  FIG. 2B , the second receiving portion  55  prevents the lower end portion of the inside surface of each lock base portion  46  from moving radially inward. This arrangement prevents each locking member  24  pushed radially outward by the wedge surface  35  of the clamp rod  32  from being inclined, and hence each locking member  24  smoothly moves radially outward. 
     In addition to the above, because the lower surface of the thick lock base portion  46  of the locking member  24  is supported on the upper surface of the large-diameter flange  42  of the guide base portion  41  of the guide member  16 , each locking member  24  is moved smoothly and securely in a horizontal direction. 
     When the lower surface of the work  20  is fixed to the seated surface  58  of the upper housing  2  as described above, the ejection hole  59  is closed and hence the pressure in the air supply port (not illustrated) connected to the ejection hole is increased to a predetermined pressure. When this pressure increase is detected by a pressure switch (seating sensor), the state of the work  20  is confirmed to be the seated state. 
       FIG. 3  to  FIG. 6  show Second Embodiment to Fifth Embodiment of the present invention, respectively. In these embodiments, members identical with (or similar to) those of First embodiment will be basically denoted by the same reference numerals. 
     Second Embodiment show in  FIG. 3  is arranged so that the guide member  16  is integrated with the interlocked member  27 . 
     In this Second Embodiment, an annular gap may be formed between the outer peripheral surface of the interlocked member  27  and the large-diameter hole  2   a  of the upper housing  2 . This allows the guide member  16  and the clamp rod  32  to radially move with respect to the housing  1 . 
     In Third Embodiment shown in  FIG. 4 , the guide grooves  23  are opened on the lower surface of the circumferential wall  18  of the guide member  16 . The locking members  24  are inserted into these guide grooves  23 , and a flange  65  protrudes radially outward from a lower part of the lock base portion  46  of each locking member  24 . And, on the upper surface of the interlocked member  27  that moves in a vertical direction together with the guide member  16 , the lower surface of the flange  65  is supported to be radially movable. This ensures the prevention of the inclination of the locking members  24 . 
     Fourth Embodiment shown in  FIG. 5  is arranged so that a plurality of (two in this example) receiving surfaces  34  are lined up in the vertical direction on the radially inside surface of the locking member  24 , and wedge surfaces  35  engaging with the respective receiving surfaces  34  from above are lined up in the vertical direction on the clamp rod  32 . 
     With this structure, because the lower wedge surface  35  contacts the lower end portion of the radially inside surface of the locking member  24  in the whole locking stroke of the guide member  16 , the prevention of the inclination of this locking member  24  is ensured. 
       FIG. 6  relating to Fifth Embodiment shows a clamped state. In this Fifth Embodiment, the locking member  24  is provided only with the engaging portion  45 . The lower end surface  24   a  of such a locking member  24  is supported by the upper end surface of the guide base portion  41  of the guide member  16 . The attaching groove  49  is formed at a vertically central part of each locking member  24  and the attaching grooves  48  are formed at a vertically central part of the guide portion  40  of the guide member  16 . The strip-shaped elastic member  50  is attached to these grooves  48  and  49 . 
     When the state of the clamp apparatus is switched to the unclamped state (not illustrated), the outer circumference of the locking member  24  and the outer circumference of the elastic member  50  are positioned inside the outer circumference of the guide portion  40 . 
     The embodiments may be modified as below. 
     The top wall  17  and the circumferential wall  18  of the guide member  16  may not be seamlessly formed in an integrated manner. The top wall  17  may be fastened to the circumferential wall  18  by a screw. 
     The advancing unit  26  may include an elastic member such as a compression coil spring and rubber instead of the coned disc springs  28  above. Furthermore, instead of such an elastic member, a structure utilizing pressurized oil (see e.g., Japanese Patent No. 4297511 which is the conventional art above) may be used. Furthermore, a structure utilizing compressed air or the like instead of pressurized oil may be used. 
     The elastic member  50  that biases the locking members  24  radially inward may be constituted by a flat spring made of belt metal, instead of rubber or synthetic resin. 
     Alternatively, the elastic member  50  is omitted and the locking member  24  is biased radially inward solely by the dust seal (elastic member)  52 . Alternatively, the dust seal  52  solely has its primal function of preventing the intrusion of foreign matters, and the locking members  24  are biased radially inward solely by the elastic member  50 . 
     While the number of the guide grooves  23  and the number of the locking members  24  are preferably 3 as in the example above, the number may be 2, 4, 5, or more. 
     Each guide groove  23  and each locking member  24  may be rectangular or oval in profile. 
     The outer peripheral surface of the engaging portion  45  of the locking member  24  may be flat or spherical in shape, instead of the serrated shape in the example above. 
     The wedge surface  35  of the clamp rod  32  may be an arc surface instead of the flat surface as in the example above. 
     Various modifications within the scope of the disclosure will be apparent to those skilled in the art.