Patent Publication Number: US-6666440-B2

Title: Rotary clamp

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a clamp of the type that rotates a clamp rod. 
     2. Explanation of Related Art 
     There is a conventional technique of the rotary clamp of this type which is constructed in the following manner, as disclosed in U.S. Pat. No. 5,820,118. 
     A clamp rod is inserted into a housing, an upper wall of which supports a halfway height portion of the clamp rod vertically movably. The clamp rod has a lower portion provided with a piston which is vertically movably supported by a barrel portion of the housing. The clamp rod is formed with a cam groove on an upper side of the piston. An engaging ball to be fitted into the cam groove is inserted into a recess provided in the barrel portion of the housing. 
     The conventional technique had a problem of being unable to smoothly rotate the clamp rod because a large frictional force acts from the engaging ball to the cam groove when rotating the clamp rod. 
     SUMMARY OF THE INVENTION 
     The present invention has an object to provide a clamp in which a clamp rod smoothly rotates. 
     In order to accomplish the above object, the present invention has constructed a rotary clamp in the following manner. 
     For example, as shown in FIGS. 1 to  4  or in FIGS. 7 to  10 , a housing  3  supports a clamp rod  5  axially movably and rotatably around the axis. The clamp rod  5  is made movable for clamping from a first end wall  3   a  of the housing  3  to a second end wall  3   b  thereof. Within the housing  3 , the clamp rod  5  has an outer periphery provided with guide grooves  26  peripherally in plural number. Each of the guide grooves  26  comprises a rotary groove  27  and a straight groove  28  which are provided in continuity with each other from the second end wall  3   b to the first end wall  3   a . Engaging balls  29  to be fitted into the respective guide grooves  26  are rotatably supported by through holes  31  provided in the housing  3 . A sleeve  35  is externally fitted over the engaging balls  29  rotatably around the axis. 
     The above-mentioned invention offers the following advantage. 
     When the clamp rod rotates, for instance, in a clockwise direction when seen in plan, the engaging balls fitted into the rotary grooves of the clamp rod roll in a counter-clockwise direction when seen in plan. Simultaneously, the sleeve externally fitted over the engaging balls freely rotates in the counter-clockwise direction. This allows almost only rolling friction to act between an inner peripheral surface of the sleeve and every engaging ball, but hardly allows sliding friction to act therebetween. Therefore, a resistance which acts from the sleeve to every engaging ball is decreased. As a result, a frictional force which acts from every engaging ball to the rotary groove is reduced, thereby smoothly rotating the clamp rod with a light force. 
     The present invention includes the following rotary clamp. 
     For instance, as shown in FIGS. 1 to  4  or in FIGS. 7 to  10 , the second end wall  3   b  of the housing  3  is constituted by a support cylinder  13 . The clamp rod  5  is inserted into an inner wall  13   a  of the support cylinder  13 . And the inner wall  13   a  is provided with the through holes  31 . This invention has inserted the clamp rod into the inner wall of the support cylinder, which results in being able to form the inserted portion into a structure of a reduced diameter and decrease an inclination angle of the rotary groove. In consequence, it is possible to reduce the stroke required for rotating the clamp rod, thereby making the rotary clamp compact. 
     Further, the present invention includes the following rotary clamp. 
     For example, as shown in FIGS. 1 to  4 , an annular piston  15  is inserted into the housing  3  axially movably. The clamp rod  5  is inserted into the piston  15 . A radial bearing  24  is arranged between these piston  15  and clamp rod  5 . This invention offers an advantage of being able to rotate the clamp rod more smoothly. 
     Moreover, the present invention includes the following rotary clamp. 
     For instance, as shown in FIGS. 1 to  4 , the piston  15  faces an input portion  14  of the clamp rod  5  from a side of the first end wall  3   a . There is provided between the piston  15  and the first end wall  3   a , a first chamber  21  in which a clamp spring  20  is attached. And there is provided between the piston  15  and the second end wall  3   b , a second chamber  22  to which pressurized fluid for unclamping is supplied. This invention offers the following advantage. 
     Upon unclamping, a force which has acted from the pressurized fluid in the second chamber to the piston is not applied to the clamp rod. This prevents an excessive force from acting on the rotary grooves and the engaging balls. Therefore, a rotary mechanism which comprises the rotary grooves and the engaging balls increases its service lifetime. 
     In addition, the present invention includes the following rotary clamp. 
     For example, as shown in FIGS. 7 to  10 , the piston  15  faces an input portion  14  of the clamp rod  5  from a side of the first end wall  3   a . There is provided between the piston  15  and the first end wall  3   a , a first chamber  21  to which pressurized fluid for clamping is supplied. And there is also provided between the piston  15  and the second end wall  3   b , a second chamber  22  to which pressurized fluid for unclamping is supplied. This invention offers the following advantage as well as the above-mentioned invention. 
     Upon unclamping, a force which has acted from the pressurized fluid in the second chamber to the piston is not applied to the clamp rod. This prevents an excessive force from acting on the rotary grooves and the engaging balls. Therefore, a rotary mechanism which comprises the rotary grooves and the engaging balls increases its service lifetime. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to  4  show a first embodiment of the present invention; 
     FIG. 1 is a partial sectional view of a rotary clamp when seen in elevation; 
     FIG. 2 is a sectional view of a rotary mechanism provided in the clamp when seen in plan; 
     FIG. 3 is an enlarged view of an essential portion in FIG.  1  and corresponds to a sectional view when seen along a line III—III in FIG. 2 in a direction indicated by arrows; 
     FIG. 4 is an enlarged and developed view of a lower slide portion provided in a clamp rod of the clamp; 
     FIG. 5 shows a first modification of the first embodiment and is similar to FIG. 4; 
     FIG. 6 shows a second modification of the first embodiment and is similar to FIG. 4; 
     FIGS. 7 to  10  show a second embodiment of the present invention; 
     FIG. 7 is a partial sectional view of the clamp when seen in elevation and is similar to FIG. 1; 
     FIG. 8 is a sectional view of a rotary mechanism provided in the clamp when seen in plan and is similar to FIG. 2; 
     FIG. 9 is an enlarged view of an essential portion in FIG.  7  and corresponds to a sectional view when seen along a line  1 X— 1 X in FIG. 8 in a direction indicated by arrows; and 
     FIG. 10 is an enlarged and developed view of a lower slide portion provided in a clamp rod of the clamp and is similar to FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the present invention is explained with reference to FIGS. 1 to  4 . First, an explanation is given for a whole structure of a rotary clamp by resorting to FIG.  1 . FIG. 1 is a partial sectional view of the clamp when seen in elevation. 
     A housing  3  of a clamp  2  is fixed to a work pallet  1  through a plurality of bolts (not shown). The housing  3  has a cylindrical hole  4  into which a clamp rod  5  is inserted. The clamp rod  5  has an upper end portion to which an arm  6  is secured at a desired rotation position by a nut  7 . The arm  6  has a leading end portion to which a push bolt  8  is fixed. 
     The housing  3  has an upper end wall (first end wall)  3   a  which supports an upper slide portion (first slide portion)  11  provided in a rod main body  5   a  of the clamp rod  5  slidably and hermetically. Further, a support cylinder  13  forms part of a lower end wall (second end wall)  3   b  of the housing  3  and slidably supports a lower slide portion (second slide portion)  12  which projects downwards of the rod main body  5   a . The upper slide portion  11  and the lower slide portion  12  are tightly fitted into the upper end wall  3   a  and the lower end wall  3   b , respectively. 
     The lower slide portion  12  has an outer diameter set to a value smaller than that of an outer diameter of the upper slide portion  11 . 
     A means for driving the clamp rod  5  is constructed as follows. 
     The clamp rod  5  is provided with an input portion  14  in the shape of a flange between the upper slide portion  11  and the lower slide portion  12 . Further, an annular piston  15  is externally fitted onto the clamp rod  5  vertically movably and hermetically through a sealing member  16 . The piston  15  faces the input portion  14  from above. And the piston  15  is inserted into the cylindrical hole  4  hermetically through another sealing member  15   a.    
     In addition, a radial bearing  24  is arranged between the input portion  14  and the piston  15 . A snap ring  25  prevents the removal of the piston  15 . Here the radial bearing  24  is composed of many metal balls and can receive not only a radial force but also a vertical thrust. 
     A first chamber  21  for clamping is provided between the piston  15  and the upper end wall  3   a . A clamp spring  20  made of a compressed coil spring is attached in the first chamber  21 . A second chamber  22  for unclamping is provided between the piston  15  and the lower end wall  3   b . Pressurized oil is supplied to and discharged from the second chamber  22  through a pressurized oil supply and discharge port  19  for unclamping and a restricting oil passage  18 . 
     A fitting gap (G) between a peripheral wall of the second chamber  22  and an outer peripheral surface of the piston  15  limits supply amount of pressurized oil from the oil passage  18  to the second chamber  22  as well as discharge amount of the pressurized oil from the second chamber  22  to the oil passage  18 . 
     A rotary mechanism is provided over the lower slide portion  12  of the clamp rod  5  and an upper portion of an inner wall  13   a  of the support cylinder  13 . The rotary mechanism is constructed in the following manner as shown in FIG.  1  and FIG. 2 to FIG.  4 . 
     FIG. 2 is a sectional view of the rotary mechanism when seen in plan. FIG. 3 is an enlarged view of an essential portion in FIG.  1  and corresponds to a sectional view when seen along a line III—III in FIG. 2 in a direction indicated by arrows. FIG. 4 is an enlarged and developed view of an outer peripheral surface of the lower slide portion  12 . 
     The lower slide portion  12  has the outer peripheral surface provided with three guide grooves  26  peripherally at substantially the same spacing. Each of the guide grooves  26  is formed from a groove in the shape of an arc or a segment when seen in section. And it comprises a helical rotary groove  27  and a straight groove  28  which is in upward continuity with the helical rotary groove  27 . The rotary grooves  27  as well as the straight grooves  28  are arranged in parallel with one another. As for the adjacent guide grooves  26 ,  26 , a partition wall is minimum in thickness between a lower portion of a right rotary groove  27  and an upper portion of a left rotary groove  27  in FIG.  4 . The minimum thickness (M) of the partition wall is set to a value smaller than a groove width (W) of the guide groove  26 . Further, the rotary groove  27  is inclined at an angle (A) which is set to a small value within a range of about 11 degrees to about 25 degrees. In the exemplified clamp which relies on a spring force, the inclination angle (A) is preferably set to a value within a range of about 11 degrees to about 20 degrees for reducing the rotation stroke. 
     As such the inclination angle (A) of the helical rotary groove  27  has been made small to result in largely shortening a lead of the rotary groove  27 . This decreases the stroke for rotating the clamp rod  5 . 
     An engaging ball  29  is fitted into each of the guide grooves  26 . Numeral  29   a  in FIGS. 3 and 4 designates a fitting portion of the engaging ball  29 . The engaging ball  29  has a diameter (D) (see FIG. 3) set to a value larger than the minimum thickness (M) of the partition wall between the adjacent rotary grooves  27 ,  27 . The respective engaging balls  29  are rotatably supported by three through holes  31  provided in the upper portion of the inner wall  13   a  of the support cylinder  13 . A sleeve  35  is externally fitted over these three engaging balls  29  rotatably around the axis. Speaking it in more detail, the sleeve  35  has an inner peripheral surface formed with a groove  36  in the shape of a letter ‘V’. The V-shaped groove  36  has two vertical points at which the engaging ball  29  can roll. 
     The engaging ball  29  is inserted into the through hole  31  via an internally threaded hole  49  which is provided in the sleeve  35 . A closure bolt  50  is attached to the internally threaded hole  49 . A projection  50   a  at a leading end of the closure bolt  50  can receive the engaging ball  29 . 
     The rotary groove  27  has a lower end portion provided with a stopper wall  45  which receives the fitting portion  29   a  of the engaging ball  29 . The stopper wall  45  has a receiving surface  45   a  which can fit with the engaging ball  29 . 
     Besides, the guide groove  26  has an opening which is provided at its edge portion with a cutting surface  34  for preventing interference. Owing to this arrangement, even if the opening edge portion of the guide groove  26  undergoes a plastic deformation by a surface pressure of the engaging ball  29  and heaps up, it is possible to prevent the interference between the heaped-up portion and the inner wall  13   a  of the support cylinder  13 . As a result, the clamp rod  5  smoothly rotates for a long period of time. 
     Further, as shown in FIG. 1, an outer wall  13   b  of the support cylinder  13  is attached to a barrel portion  3   c  of the housing  3  through a positioning pin  38  which extends vertically, so as to be prevented from rotating. This makes it possible to accurately determine a rotation phase of the clamp rod  5  with respect to the housing  3 . The support cylinder  13  is secured to the housing barrel portion  3   c  by a lock member  39  made of a snap ring. 
     The rotary clamp  2  operates as follows. 
     In a state of FIG. 1, pressurized oil is supplied to the second chamber  22  for unclamping, thereby raising the clamp rod  5  to an illustrated rotation and retreat position. 
     When switching over the clamp  2  to a clamping condition, the pressurized oil in the second chamber  22  is discharged to push down the input portion  14  of the clamp rod  5  by the clamp spring  20 . Then the clamp rod  5  goes down along the rotary grooves  27  while rotating in a clockwise direction when seen in plan. Subsequently, it descends straightly along the straight grooves  28 . This enables the clamp rod  5  to switch over to a clamping position (not shown). 
     As shown by an arrow in FIG. 2, when the clamp rod  5  rotates in the clockwise direction when seen in plan, every engaging ball  29  fitted into the rotary groove  27  rolls in a counter-clockwise direction when seen in plan and at the same time the sleeve  35  externally fitted over the respective engaging balls  29  freely rotates in the counter-clockwise direction. This allows almost only rolling friction to act between an inner peripheral surface of the sleeve  35  and every engaging ball  29 , but hardly allows sliding friction to act therebetween. This reduces a resistance which acts from the sleeve  35  to every engaging ball  29 , which results in decreasing a frictional force which acts from every engaging ball  29  to the rotary groove  27  and therefore smoothly rotating the clamp rod  5  with a light force. 
     Here, the sleeve  35  has an inner diameter set to a value which is about one and half times a value of an outer diameter of the lower slide portion  12  of the clamp rod  5 . Thus in the case of rotating the clamp rod  5  by 90 degrees, the sleeve  35  rotates by about 60 degrees. 
     When switching over the clamp  2  from the clamping condition to a rotated and retreated condition in FIG. 1, the pressurized oil is supplied to the second chamber  22  for unclamping. Then, first, the piston  15  goes up by an upward oil pressure force which acts on an annular sectional area of the piston  15 . Simultaneously, the clamp rod  5  straightly ascends along the straight grooves  28  by an upward oil pressure force which acts on an inner sectional area of the sealing member  16 . Subsequently, the clamp rod  5  ascends along the rotary groove  27  while rotating in the counter-clockwise direction when seen in plan, whereby the clamp rod  5  and the arm  6  switch over to the rotation and retreat position in FIG.  1 . 
     In this case, as mentioned above, the upward force which acts from the pressurized oil in the second chamber  22  to the piston  15  does not apply to the clamp rod  5 . This prevents an excessive force from acting on the rotary grooves  27  and the engaging balls  29 . 
     At the above time of rotating and retreating, if the clamp rod  5  rotates in the counter-clockwise direction, every engaging ball  29  and the sleeve  35  rotates in a direction opposite to the direction indicated by the arrow in FIG.  2 . 
     Further, at the above time of rotating and retreating, as shown in FIGS. 1 and 4, the stopper wall  45  has the receiving surface  45   a  fitted with the fitting portion  29   a  of the engaging ball  29 , thereby inhibiting the rotation of the clamp rod  5 . This results in stopping the rotation of the clamp rod  5  with a high accuracy. Moreover, the clamp rod  5  is provided with the stopper wall  45  and therefore offers the following advantage, when compared with a case where the barrel portion  3   c  of the housing  3  is provided with the stopper wall  45 . 
     The cylindrical hole  4  of the housing  3  need not be provided with a stepped portion for the stopper wall and therefore can be formed straight. This can facilitate the machining of the cylindrical hole  4  and besides can make the clamp spring  20  large and strong. 
     The first embodiment further offers the following advantages. 
     The clamp rod  5  is provided with the guide grooves  26 , into which the engaging balls  29  are fitted, respectively. This enables the support cylinder  13  to support the clamp rod  5  peripherally and substantially evenly through the engaging balls  29 . Accordingly, when driven for clamping and for unclamping, the clamp rod  5  can be prevented from inclining. This results in improving the accuracy of placing the push bolt  8  provided in the arm  6  at a clamping position and at an unclamping position. 
     The partition wall between the adjacent guide grooves  26 ,  26  has the minimum thickness (T) set to the value smaller than the groove width (W) of the guide groove  26 . Consequently, many guide grooves can be provided in the clamp rod  5  to result in the possibility of peripherally and substantially evenly supporting the clamp rod  5  and at the same time decreasing the inclination angle (A) of the rotary groove  27 . This can reduce the stroke required for rotating the clamp rod  5  to thereby make the rotary clamp  2  compact. 
     The clamp rod  5  is provided with the upper slide portion (first slide portion)  11  and the lower slide portion (second slide portion)  12  outside the opposite ends of the piston  15 . Therefore, notwithstanding the existence of a fitting gap of the piston  15 , the two slide portions  11 ,  12  axially spaced apart from each other can prevent the inclination of the clamp rod  5 . In consequence, the housing  3  can surely guide the clamp rod  5  with a high accuracy. 
     The rotary mechanism which comprises the rotary grooves  27  and the engaging balls  29  is provided between the support cylinder  13  which has the above-mentioned guiding strength, and the lower slide portion  12 . Therefore, it can fully endure a rotary torque and increase its service lifetime. In addition, the engaging balls  29  are provided in the support cylinder  13 , thereby enabling portions for installing the engaging balls  29  to serve as a portion for supporting the lower slide portion  12 . Thus it is possible to reduce a height of the housing  3  and make the rotary clamp  2  compact. 
     Moreover, the lower slide portion  12  has the outer diameter set to the value smaller than that of the outer diameter of the upper slide portion  11  to result in shortening the lead of the rotary groove  27  formed in the lower slide portion  12 . This further reduces the stroke for rotating the clamp rod  5  and as a result can make the rotary clamp  2  more compact. Additionally, the pressurized oil for driving the piston  15  is decreased in supply and discharge amount. 
     FIG. 5 shows a first modification of the first embodiment and is similar to FIG.  4 . In FIG. 5, the partition wall between the adjacent rotary grooves  27 ,  27  has the minimum thickness (M) set to a value smaller than that shown in FIG.  4 . The adjacent cutting surfaces  34 ,  34  overlap one another at a portion of the minimum thickness (M). Further, in FIG. 5, the inclination angle (A) of the rotary groove  27  is set to a value within a smaller range (about 11 degrees to about 15 degrees) than that of FIG.  4 . 
     FIG. 6 shows a second modification of the first embodiment and is similar to FIG.  4 . In this case, the clamp rod  5  has the lower slide portion  12  provided with four guide grooves  26 . A pair of the adjacent guide grooves  26 ,  26  and the corresponding engaging balls  29  are displaced not only peripherally of the clamp rod  5  but also axially thereof. And the partition wall between a pair of the adjacent rotary grooves  27 ,  27  has the minimum thickness (M) set to a value smaller than the groove width (W). The partition wall between a pair of the adjacent straight grooves  28 ,  28  has a minimum thickness (N) set to a value smaller than the groove width (W). Additionally, the latter minimum thickness (N) is set to a value smaller than that of the former minimum thickness (M). Thus the partition wall between the adjacent guide grooves  26 ,  26  has a minimum thickness (T) set to a value smaller than the groove width (W) and the diameter of the engaging ball  29 . 
     The first embodiment and its modifications can be modified as follows. 
     It is possible to provide the through holes  31  which rotatably support the engaging balls  29 , in the barrel portion  3   c  of the housing  3  and the like instead of providing them in the support cylinder  13  (lower end wall  3   b ) as exemplified. 
     The inner peripheral surface of the sleeve  35  may be provided with a U-shaped groove or an arcuate groove instead of the exemplified V-shaped groove  36 . Further, it may be a straight inner peripheral surface. With the straight inner peripheral surface, in order to inhibit the vertical movement of the sleeve  35  with respect to the engaging balls  29 , it is considered to provide a snap ring or the like stopper between the inner wall  13   a  of the support cylinder  13  and the sleeve  35 . 
     The helically formed rotary groove  27  is inclined at the angle (A) preferably within a range of 10 degrees to 30 degrees and more preferably within a range of 11 degrees to 20 degrees. 
     FIGS. 7 to  10  show a second embodiment. In the second embodiment, the members similar to the constituent members in the first embodiment are, in principle, designated by the same characters. 
     In the second embodiment shown in FIGS. 7 to  10 , FIG. 7 is a partial sectional view of the rotary clamp  2  when seen in elevation and is similar to FIG.  1 . FIG. 8 is a sectional view of the rotary mechanism provided in the clamp  2  when seen in plan and is similar to FIG.  2 . FIG. 9 is an enlarged view of an essential portion in FIG.  7  and corresponds to a sectional view when seen along a line IX—IX in FIG. 8 in a direction indicated by arrows. FIG. 10 is an enlarged and developed view of the lower slide portion  12  provided in the clamp rod  5  of the clamp  2 . 
     The second embodiment is different from the first embodiment on the following points. 
     The driving means for the clamp rod  5  is formed into a double-acting system. More specifically, pressurized oil for clamping is supplied to and discharged from the first chamber  21  provided upwards of the piston  15 , through a pressurized oil supply and discharge port  17  for clamping. Further, pressurized oil for unclamping is supplied to and discharged from the second chamber  22  provided downwards of the piston  15 , through a pressurized oil supply and discharge port for unclamping (not shown) and the oil passage  18 . 
     Outside upper and lower opposite sides of another sealing member  15   a  attached to an outer periphery of the piston  15  in fitting relationship, there are formed relatively large fitting gaps between the outer peripheral surface of the piston  15  and the cylindrical hole  4 . This enables the housing  3  to smoothly support the clamp rod  5  with a good accuracy at vertical two portions of the upper slide portion  11  and the lower slide portion  12 . 
     The lower slide portion  12  has the outer peripheral surface provided with four guide grooves  26  peripherally at substantially the same spacing. Likewise the first embodiment, each of the guide grooves  26  comprises the helical rotary groove  27  and the straight groove  28  which is in upward continuity with the rotary groove  27 . However, the rotary groove  27  has a lower portion opened to an under surface of the clamp rod  5  through a vertically extending groove (designated by no numeral). The engaging ball  29  can be inserted into the guide groove  26  through the opening. 
     Likewise the first embodiment, as for the adjacent guide grooves  26 ,  26 , the partition wall is minimum in thickness between a lower portion of a right rotary groove  27  and an upper portion of a left rotary groove  27  in FIG.  10 . The partition wall has the minimum thickness (M) set to a value smaller than the groove width (W) of the guide groove  26  and the diameter of the engaging ball  29 . 
     The engaging balls  29  fitted into the respective guide grooves  26  are rotatably supported by the four through holes  31  provided in the upper portion of the inner wall  13   a  of the support cylinder  13 . The sleeve  35  is externally fitted over these four engaging balls  29  rotatably around the axis. The rotary groove  27  is concaved to provide an arcuate recess  37 . Every engaging ball  29  is rollable in the rotary groove  27  at two vertical outside positions of the recess  37 . 
     A cylindrical spacer  32  is attached between a lower portion of a peripheral wall of the second chamber  22  for unclamping and an upper surface of the support cylinder  13 . The spacer  32  has an upper surface formed with a restricting groove  33 . The restricting groove  33  controls supply amount of the pressurized oil from the oil passage  18  to the second chamber  22 . A though hole or the like is employable instead of the groove  33 . 
     The support cylinder  13  is pushed and fixed to the housing barrel portion  3   c  by the lock member  39  made of an externally threaded cylinder. 
     Likewise the first embodiment, the lower slide portion  12  has the outer diameter set to a value smaller than that of the outer diameter of the upper slide portion  11 . This shortens the lead of the helical rotary groove  27  to result in reducing the rotation stroke of the clamp rod  5 . 
     The respective embodiments and modifications can be further modified as follows. 
     The clamp rod  5  is preferably provided with three or four guide grooves  26 , but it may be provided with two guide grooves. Further, at least five guide grooves may be provided. And the guide groove  26  may have a groove in the shape of a cam instead of the exemplified helical rotary groove  27 . 
     It is sufficient if the minimum thickness (T) of the partition wall between the adjacent guide grooves  26 ,  26  has a value smaller than the diameter of the engaging ball  29 . In consequence, the minimum thickness (T) can be made to have a value larger than the groove width (W) of the guide groove  26 . 
     The pressurized fluid which is supplied to and discharged from the first chamber  21  or the second chamber  22  may be other kinds of liquid, and air or the like gas, instead of the exemplified pressurized oil. 
     Besides, the rotary clamp of the present invention, which comprises the engaging balls  29  and the rotatable sleeve  35 , may be of a single-acting and spring-return type instead of the type that clamps by a spring force or the double-acting type as exemplified. 
     On performing clamping operation, the clamp rod  5  rotates in the clockwise direction when seen in plan. Instead, on performing the clamping operation, it may rotate in the counter-clockwise direction when seen in plan. Further, it is a matter of course that the rotation angle of the clamp rod  5  may be set to a desired angle, for example, such as 90 degrees, 60 degrees and 45 degrees.