Patent Publication Number: US-6338476-B1

Title: Clamp apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a clamp apparatus capable of clamping a workpiece by using an arm which is rotatable by a predetermined angle in accordance with a driving action of a driving mechanism. 
     2. Description of the Related Art 
     The clamp cylinder has been hitherto used, for example, in order to clamp a constitutive part when the constitutive part of an automobile or the like is welded. Such a clamp cylinder is disclosed, for example, in U.S. Pat. No. 4,458,889. 
     The clamp cylinder disclosed in U.S. Pat. No. 4,458,889 is shown in FIGS. 22 and 23. A piston rod  2  is arranged movably back and forth between a pair of dividable bodies  1   a ,  1   b . A coupling  3  is connected to one end of the piston rod  2 . A pair of links  5   a ,  5   b  and a pair of rollers  6   a ,  6   b  are attached rotatably by the aid of a first shaft  4  on both sides of the coupling  3  respectively. An arm  8 , which is rotatable by a predetermined angle, is connected between the pair of links  5   a ,  5   b  by the aid of a second shaft  7 . 
     In this arrangement, the pair of rollers  6   a ,  6   b  are provided rotatably by the aid of a plurality of needles  9   a  installed to a hole. The piston rod  2  is provided so that it is displaced integrally with the rollers  6   a ,  6   b  in accordance with the guiding action of the rollers  6   a ,  6   b  which make sliding movement along track grooves  9   b  formed on the bodies  1   a ,  1   b  respectively. 
     However, the clamp cylinder disclosed in U.S. Pat. No. 4,458,889 concerning the conventional technique described above involves the following inconvenience. That is, when an unillustrated workpiece is clamped by the arm  8 , the clamping force is decreased due to the change of the angle of rotation of the arm  8 . 
     In the case of the clamp cylinder described above, the reaction force, which is generated when the workpiece is clamped by the arm  8 , is applied to the first shaft  4 . Therefore, it is necessary to design the diameter of the first shaft  4  considering, for example, the surface pressure and the strength. As a result, an inconvenience arises in that the diameter of the first shaft  4  is increased. 
     Further, it is necessary to design the wall thickness and the diameter of the rollers  6   a ,  6   b  considering the surface pressure and the strength of the pair of rollers  6   a ,  6   b  which make sliding movement along the track grooves  9   b . As a result, the following inconvenience arises. That is, the shape of the pair of rollers  6   a ,  6   b  is increased, in accordance with which the size of the bodies  1   a ,  1   b  is increased. 
     SUMMARY OF THE INVENTION 
     A general object of the present invention is to provide a clamp apparatus which makes it possible to maintain a substantially constant clamping force even when the angle of rotation of an arm is changed when a workpiece is clamped. 
     A principal object of the present invention is to provide a clamp apparatus which makes it possible to decrease the diameter of a knuckle pin by receiving the reaction force generated upon the clamping by using a guide roller. 
     Another object of the present invention is to provide a clamp apparatus which makes it possible to realize a small size of a body. 
     The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a perspective view illustrating a clamp apparatus according to an embodiment of the present invention; 
     FIG. 2 shows a perspective view illustrating a state in which a cover member is detached from a body of the clamp apparatus; 
     FIG. 3 shows a longitudinal sectional view of the arrangement taken along a line III—III shown in FIG. 1; 
     FIG. 4 shows a cross-sectional view of the arrangement taken along a line IV—IV shown in FIG. 3; 
     FIG. 5 shows an exploded perspective view illustrating the body for constructing the clamp apparatus; 
     FIG. 6 shows, with partial cross section, an arrangement illustrating a modified example of a guide roller; 
     FIG. 7 shows a cross-sectional view of the arrangement taken along a line VII—VII shown in FIG. 6; 
     FIG. 8 shows the operation effected when an arm is at an initial position; 
     FIG. 9 shows the operation illustrating a state in which a rod member is moved upwardly from the initial position, and a curved surface of a link plate contacts with the guide roller; 
     FIG. 10 shows the operation effected when a workpiece is clamped; 
     FIG. 11 shows a partial magnified view for illustrating a route of transmission of the reaction force in the clamp cylinder concerning the conventional technique; 
     FIG. 12 shows a partial magnified view for illustrating a route of transmission of the reaction force in the clamp cylinder according to the embodiment of the present invention; 
     FIG. 13 illustrates an angle formed at the point of action of the force in the clamp cylinder concerning the conventional technique; 
     FIG. 14 illustrates a state in which the arm is displaced by an angle θ from the state shown in FIG. 13; 
     FIG. 15 illustrates an angle formed at the point of action of the force.in the clamp apparatus according to the embodiment of the present invention; 
     FIG. 16 illustrates a state in which the arm is displaced by an angle θ from the state shown in FIG. 15; 
     FIG. 17 illustrates the relationship between the angle of rotation θ of the arm and the clamping force; 
     FIG. 18 illustrates the point of contact between the curved surface of the link plate and the guide roller in a state in which the arm is rotated by an angle θ 3  with respect to the horizontal axis; 
     FIG. 19 illustrates the point of contact between the curved surface of the link plate and the guide roller in a state in which the arm is rotated by an angle θ 4  with respect to the horizontal axis; 
     FIG. 20 illustrates the point of contact between the curved surface of the link plate and the guide roller in a state in which the arm is substantially in the horizontal state; 
     FIG. 21 shows a partial longitudinal sectional view illustrating a clamp apparatus according to another embodiment of the present invention; 
     FIG. 22 shows an exploded perspective view illustrating major components of the clamp cylinder concerning the conventional technique; and 
     FIG. 23 shows, with partial cross section, a side view illustrating the clamp cylinder shown in FIG.  22 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, reference numeral  10  indicates a clamp apparatus according to an embodiment of the present invention. The clamp apparatus  10  comprises a body  12  which is formed to have a flat configuration, a cylinder unit (driving mechanism)  14  which is coupled in an air-tight manner to the lower end of the body  12 , and an arm  20  which is coupled to a rectangular bearing section  18  protruding to the outside through a pair of substantially circular openings  16   a ,  16   b  (see FIG. 5) formed through the body  12 . 
     The cylinder unit  14  includes an end block  24  which has an unillustrated elliptic recess formed on its upper surface, and a cylinder tube  26  which is composed of a cylinder having an elliptic cross section with its first end coupled in an air-tight manner to the recess of the end block  24  and with its second end coupled in an air-tight manner to the bottom surface of the body  12 . 
     As shown in FIG. 3, the cylinder unit  14  further comprises a piston  30  which is accommodated in the cylinder tube  26  and which makes reciprocating movement along a cylinder chamber  28 , a rod member  32  which is coupled to a central portion of the piston  30  and which is displaceable integrally with the piston  30 , and a ring-shaped spacer  33  which is provided at the connecting portion between the piston  30  and the rod member  32  and which is externally fitted to the rod member  32  via a hole. The spacer  33  is made of a metal material such as aluminum. The spacer  33  abuts against the wall surface of a projection  50  for forming the upper portion of the cylinder chamber  28  at the terminal end position of the displacement of the pinto  30 , and thus it functions as a stopper for regulating the displacement of the piston  30 . Alternatively, the piston  30  and the spacer  33  may be formed to be integrated into one unit. 
     As shown in FIG. 3, a wear ring  34  and a piston packing  36  are installed to the outer circumferential surface of the piston  30  respectively. Unillustrated attachment holes are bored through four corner portions of the end block  24 . The end block  24  and the cylinder tube  26  are assembled in an air-tight manner to the body  12  by the aid of four shafts  40  inserted through the attachment holes (see FIGS.  1  and  2 ). Pairs of pressure fluid inlet/outlet ports  42   a ,  42   b,  44  a ,  44   b , which are used to introduce and discharge the pressure fluid (for example, compressed air) with respect to the cylinder chamber  28  respectively, are formed mutually opposingly in the body  12  and the end block  24  respectively (see FIG.  3 ). 
     When the clamp apparatus  10  is practically used, unillustrated blank caps are screwed into any pair of the pressure fluid inlet/outlet ports  42   a ,  44   a  ( 42   b ,  44   b ). Thus, the clamp apparatus  10  is used in a state in which one of the pairs of pressure fluid inlet/outlet ports  42   a ,  44   a  ( 42   b ,  44   b ) are closed. 
     As shown in FIG. 5, the body  12  comprises a first casing  46  and a second casing  48  which are asymmetric and which are assembled in an integrated manner. A projection  50 , which protrudes by a predetermined length in a substantially horizontal direction and which functions as a rod cover, is formed in an integrated manner at the lower end of the first casing  46 . The second casing  48  is formed to have a size in the longitudinal direction which is shortened by a thickness of the projection  50  as compared with the first casing  46 . In this arrangement, as shown in FIG. 5, the body  12  can be conveniently disassembled by detaching the second casing  48  from the first casing  46  without disassembling the cylinder unit  14 . 
     As shown in FIG. 5, a chamber  54  is formed in the body  12  by recesses  52   a ,  52   b  formed for the first casing  46  and the second casing  48  respectively (provided that the recess  52   b  is omitted from the illustration because it has the same structure as that of the recess  52   a ). The free end of the rod member  32  is provided to face in the chamber  54 . In this arrangement, the rod member  32  is guided linearly reciprocatively by the aid of guide grooves  58  which are formed on the respective inner wall surfaces of the first casing  46  and the second casing  48  respectively and on which a knuckle block  56  is slidable as described later on. A rod packing  60  (see FIG. 3) for surrounding the outer circumferential surface of the rod member  32  is provided at a through-hole formed in the projection  50 . 
     As shown in FIG. 5, a toggle link mechanism  64 , which is used to convert the rectilinear motion of the rod member  32  into the rotary motion of the arm  20  by the aid of a knuckle Joint  62 , is provided at a first end of the rod member  32 . The knuckle joint  62  comprises a knuckle block  56  having a forked section with branches separated by a predetermined spacing distance and branched substantially in parallel to one another, and a knuckle pin  70  for being rotatably attached to a hole formed in the forked section. 
     As shown in FIG. 5, a groove  68  having a T-shaped cross section, with which a disk-shaped projection  66  of the rod member  32  is engaged, is formed at a bottom surface portion of the knuckle block  56  to extend in a substantially horizontal direction. In this arrangement, predetermined clearances are formed between the groove  68  and the projection  66  formed integrally with the rod member  32  and between the knuckle block  56  and the guide groove  58 . The knuckle block  56  is provided displaceably substantially horizontally along the groove  68 . Thus, the rod member  32  is prevented from transmission of any load in the lateral direction. In other words, by providing the degree of freedom for the knuckle block  56 , for example, when a workpiece is clamped, then no lateral load is applied, for example, to the rod member  32  and the rod packing  60 , and the stroke of the rod member  32  can be efficiently transmitted to the toggle link mechanism  64 . 
     As shown in FIG. 5, the toggle link mechanism  64  includes a link plate (link member)  72  which is coupled to the forked section of the knuckle Joint  62  by the aid of a knuckle pin  70 , and a support lever  74  which is rotatably supported by the pair of substantially circular openings  16   a ,  16   b  formed through the first casing  46  and the second casing  48  respectively. 
     The link plate  72  is allowed to intervene between the knuckle Joint  62  and the support lever  74 , and it functions to link the knuckle joint  62  and the support lever  74 . Specifically, the link plate  72  is formed with a pair of holes  76   a ,  76   b  which are separated from each other by a predetermined spacing distance. The link plate  72  is coupled to the free end of the rod member  32  via the knuckle Joint  62  and the knuckle pin  70  rotatably attached to the first hole  76   a . The link plate  72  is coupled to the forked section of the support lever  74  via a link pin  78  rotatably attached to the second hole  76   b . A curved surface  81  for making contact with a guide roller (rotary means)  79  described later on is formed at a first end of the link plate  72  disposed in the vicinity of the first hole  76   a.    
     As shown in FIG. 5, the support lever  74  includes a forked section which is formed with holes for rotatably attaching the link pin  78 , the bearing section  18  which is formed to protrude in a direction substantially perpendicular to the axis of the rod member  32  and which has a rectangular cross section exposed to the outside from the body  12  through the opening  16   b , a pair of circumferential sections  80   a , 80 b  which are formed adjacently with the forked section interposed therebetween and which are fitted to the substantially circular openings  16   a ,  16   b  of the body  12  respectively, and a pair of circular arc-shaped projections  82   a ,  82   b  which are formed to slightly protrude in the lateral direction from the circumferential sections  80   a ,  80   b  and which are exposed to the outside from the body  12  through the openings  16   a ,  16   b  respectively. The arm  20  for claiming the unillustrated workpiece is detachably installed to the bearing section  18 . 
     The support lever  74  is provided to make the rotary action integrally with the arm  20 . The circular arc-shaped projections  82   a ,  82   b , which are formed on the support lever  74 , abut against plates  84   a    84   b  fixed to the body  12 . Accordingly, the circular arc-shaped projections  82   a ,  82   b  function as the stopper for stopping the rotary action of the arm  20 . 
     The rectilinear motion of the rod member  32  is transmitted to the support lever  74  via the knuckle joint  62  and the link plate  72 . The support lever  74  is provided rotatably by a predetermined angle about the center of rotation of the circumferential sections  80   a ,  80   b  which are supported by the pair of openings  16   a ,  16   b  formed through the body  12 . 
     As shown in FIG. 5, oblong recesses  86  are formed on the side surfaces of the first casing  46  and the second casing  48  for constructing the body  12  respectively. The recesses  86  are closed by a pair of cover members  88   a ,  88   b . The cover members  88   a ,  88   b  are installed detachably by the aid of screw members  89 . In this arrangement, the bearing section  18  of the support lever  74  is provided to be exposed to the outside through a substantially circular opening  90  which is formed at a substantially central portion of the cover member  88 b. 
     The plates  84   a ,  84   b , which make abutment against the circular arc-shaped projections  82   a ,  82   b  of the support lever  74  to stop the rotary action of the arm  20 , are fixed detachably on the wall surfaces of the recesses  86  by the aid of screw members  92 . 
     As shown in FIG. 5, the plate  84   b  ( 84   a ) has a first abutment surface  96  for making abutment against a first end surface  94  of the circular arc-shaped projection  82   b  ( 82   a ), and a second abutment surface  100  for making abutment against a second end surface  98  of the circular arc-shaped projection  82   b  ( 82   a ). A curved surface  102  for surrounding the support lever  74  is formed between the first abutment surface  96  and the second abutment surface  100 . The first end surface  94  and the second end surface  98  of the support lever  74  are formed so that they are separated from each other by an angle of about 90 degrees. It is a matter of course that the angle of separation between the first end surface  94  and the second end surface  98  of the support lever  74  is not limited to 90 degrees. 
     In this arrangement, the pair of plates  84   a ,  84   b  can be conveniently exchanged with other plates (as described later on) with ease by detaching the pair of cover members  88   a ,  88   b  from the body  12  respectively, and loosening the screw members  92 . When the pair of cover members  88   a ,  88   b  are detached from the body  12  respectively, the first end surface  94  and the second end surface  98  of the circular arc-shaped projection  82   b  ( 82   a ) formed on the support lever  74  are exposed to the outside as shown in FIG. 2 (however, the first end surface  94  is not shown). 
     As shown in FIG. 5, recesses  106  having a circular arcshaped cross section are formed on upper side portions of the inner wall surfaces of the first casing  46  and the second casing  48  for constructing the body  12  respectively. The guide roller  79 , which makes the rotary action by a predetermined angle by making contact with the curved surface  81  of the link plate  72 , is provided in the recesses  106 . A pin member  110 , which rotatably supports the guide roller  79 , is secured to holes  108  formed on the first casing  46  and the second casing  48 . A plurality of needle bearings  112  are installed in the circumferential direction in a through-hole of the guide roller  79 . The guide roller  79  is provided to smoothly make the rotary action in accordance with the rolling action of the needle bearings  112 . 
     Alternatively, another arrangement is available as shown in FIGS. 6 and 7. That is, a pin member  114  may be provided to make direct contact with the curved surface  81  of the link plate  72 . Both ends of the pin member  114  may be rotatably supported by a plurality of short-length needle bearings  118  installed to cap members  116   a ,  116   b  respectively. 
     As shown in FIG. 5, the pair of guide grooves  58 , which are composed of rectangular grooves and which extend in the vertical direction, are provided mutually opposingly on the inner wall surfaces of the first casing  46  and the second casing  48 . The knuckle block  58  is interposed between the pair of guide grooves  58 . The knuckle block  56  is provided slidably in the vertical direction in accordance with the guiding action of the guide grooves  58 . 
     The clamp apparatus  10  according to the embodiment of the present invention is basically constructed as described above. Next, its operation, function, and effect will be explained. 
     At first, the clamp apparatus  10  is fixed to a predetermined position by the aid of an unillustrated fixing means. First ends of pipes such as unillustrated tubes are connected to the pair of pressure fluid inlet/outlet ports  42   a ,  44   a  ( 42   b ,  44   b ) respectively. Second ends of the pipes are connected to an unillustrated pressure fluid supply source. FIG. 8 shows the clamp apparatus  10  in the unclamping state, and FIG. 10 shows the clamp apparatus  10  in the clamping state. The following description will be made assuming that the unclamping state shown in FIG. 8 represents the initial position. 
     After performing the preparatory operation as described above, the unillustrated pressure fluid supply source is energized at the initial position shown in FIG. 8 to introduce the pressure fluid (for example, compressed air) from the first pressure fluid inlet/outlet port  44   a  into the cylinder chamber  28  disposed at the lower side of the piston  30 . The piston  30  is pressed in accordance with the action of the pressure fluid introduced into the cylinder chamber  28 . The piston  30  is moved upwardly along the cylinder chamber  28 . 
     The rectilinear motion of the piston  30  is transmitted to the toggle link mechanism  64  via the rod member  32  and the knuckle joint  62 , and it is converted into the rotary motion of the arm  20  in accordance with the rotary action of the support lever  74  which constitutes the toggle link mechanism  64 . 
     That is, the rectilinear motion (upward movement) of the piston  30  allows the force to act so that the link plate  72  and the knuckle joint  62  engaged with the free end of the piston rod  32  are pressed in the upward direction. Owing to the pressing force exerted on the link plate  72 , the link plate  72  is rotated by a predetermined angle about the support point of the knuckle pin  70 , and the support lever  74  is rotated in the direction of the arrow A in accordance with the linking action of the link plate  72 . 
     Therefore, the arm  20  is rotated by a predetermined angle in the direction of the arrow B about the support point of the bearing section  18  of the support lever  74 . Accordingly, the circular arc-shaped projection  82   b  ( 82   a ) is rotated by the predetermined angle integrally with the support lever  74 . 
     During the process in which the arm  20  is rotated in the direction of the arrow B as described above, the curved surface  81  of the link plate  72  contacts with the guide roller  79  as shown in FIG.  9 . The guide roller  79  is rotated about the center of the pin member  110  while maintaining the state of contact with the curved surface  81 . 
     The arm  20  is further rotated, and the first end surface  94  of the circular arc-shaped projection  82   b  ( 82   a ) abuts against the first abutment surface  96  of the plate  84   b  ( 84   a ) gwhich is fixed to the body  12  as shown in FIG.  10 . Accordingly, the arm  20  stops the rotary action. As a result, the clamping state is given, in which the workpiece is clamped by the arm  20 . 
     After the arm  20  stops the rotary action to give the clamping state, the piston  30  and the rod member  32  are further moved slightly upwardly. The spacer  33  abuts against the wall surface of the projection  50 . Accordingly, the piston  30  and the rod member  32  are stopped to give the terminal end position of the displacement (see FIG.  3 ). On the other hand, when the pressure fluid is supplied to the pressure fluid Inlet/outlet port  42   a  in accordance with the switching action of an unillustrated changeover valve in the state shown in FIG. 3, the piston  30  is moved downwardly. Further, the support lever  74  is rotated in a direction opposite to the direction described above by the aid of the link plate  72  in accordance with the. downward movement action of the rod member  32 . Accordingly, the arm  20  is rotated in a direction to make separation from the workpiece. 
     During the process in which the arm  20  is rotated in the direction to make separation from the workpiece, the second end surface  98  of the circular arc-shaped projection  82   b  ( 82   a ) abuts against the second abutment surface  100  of the plate  84   b  ( 84   a ) which is fixed to the body  12 . Accordingly, the arm  20  stops the rotary action. As a result, the clamp apparatus  10  is restored to the initial position shown in FIG.  8 . 
     Next, explanation will be made for the case in which the reaction force, which is generated corresponding to the clamping force when the workpiece is clamped, is balanced with the force to be balanced with the reaction force. 
     In the case of the clamp cylinder concerning the conventional technique, as shown in FIG. 11, when the workplace is clamped, the reaction force is generated in the direction opposite to the clamping force. The reaction force is transmitted to the second shaft  7  via the arm  8 , and it is further transmitted via the links  5   a ,  5   b  to the pair of rollers  6   a ,  6   b  rotatably supported by the first shaft  4  to balance the reaction force with the force to be balanced with the reaction force. Therefore, in the case of the clamp cylinder concerning the conventional technique, the force corresponding to the reaction force is applied to the first shaft  4  which rotatable supports the pair of rollers  6   a ,  6   b  . For this reason, it is necessary to design the first shaft to have the large diameter. 
     On the contrary, in the case of the embodiment of the present invention, as shown in FIG. 12, the reaction force is transmitted via the arm  20  to the link pin  78 , and it is further transmitted to the guide roller  79  which contacts with the curved surface  81  of the link plate  72 . In this arrangement, the guide roller  79  is rotatably supported by the pin member  110  fixed to the holes  108  (see FIG. 5) of the first casing  46  and the second casing  48 . The reaction force, which is transmitted to the guide roller  79 , is applied to the pin member  110  fixed to the first and second casings  46 ,  48 . 
     Therefore, the embodiment of the present invention is designed such that the force corresponding to the reaction force is not applied at all to the knuckle pin  70 . Accordingly, it is possible to decrease the diameter of the knuckle pin  70 . Further, it is possible to improve the durability of the connecting portion between the knuckle block  56  and the link plate  72 . 
     In the conventional technique, it is necessary to design the width and the diameter of the rollers  6   a ,  6   b , considering the surface pressure and the strength of the pair of rollers  6   a ,  6   b  which slidably move on the track grooves  9   b . On the contrary, in the embodiment of the present invention, it is unnecessary to make such a design, and hence it is possible to realize a small size of the body  12 . 
     Next, investigation will be made for the angle formed at the point of action of the force when the workpiece is clamped by the arm. 
     In the clamp cylinder concerning the conventional technique, it is assumed that θ 1  represents the angle formed at the point of action of the force in the state in which the workpiece is clamped by the arm  8  substantially in the horizontal state (see FIG.  13 ), and θ 2  represents the angle formed at the point of action of the force when the angle of the arm  8  during the clamping is changed by an angle θ in the clockwise direction (see FIG.  14 ). As clearly understood from comparison between FIG.  13  and FIG. 14, in the case of the conventional technique, when the angle of rotation of the arm  8  during the clamping of the workplace is changed, the angle (θ 1 , θ 2 ) formed at the point of action of the force is greatly changed. 
     On the contrary, in the embodiment of the present invention, the angle formed at the point of action of the force is substantially constant even when the angle of rotation of the arm  20  during the clamping of the workpiece is changed by the angle θ from the angle θ 1  formed at the point of action of the force in the state in which the workpiece is clamped by the arm  20  substantially in the horizontal state (see FIG. 15) to the angle θ 2  (see FIG.  16 ). 
     According to this fact, as clearly understood from FIG. 17 as well, the clamping force is substantially constant in the embodiment of the present invention (solid line) even when the angle of rotation of the arm  20  is increased. On the contrary, the conventional technique (broken line) is inconvenient in that the clamping force is quickly decreased when the angle of rotation of the arm  8  is increased. 
     Therefore, the embodiment of the present invention has the following effect. That is, even when the apparatus is set such that the workpiece is clamped by the arm  20  at a desired angle of rotation depending on, for example, the condition of the use by a user, it is possible to obtain a substantially constant clamping force. 
     Further, in the embodiment of the present invention, the spacer  33 , which regulates the terminal end position of the displacement of the piston  30 , is provided at the connecting portion between the rod member  32  and the piston  30  at the inside of the cylinder unit  14 . Accordingly, it is possible to reliably avoid the invasion into the top dead center (dead point) by means of the simple structure. 
     Next, explanation will be made with reference to FIGS. 18 to  20  for the arrangement in which the contact portion between the curved surface  81  of the link plate  72  and the guide roller  79  is maintained at a substantially constant position irrelevant to the angle of rotation of the arm  20 . 
     FIG. 18 shows a state in which the arm  20  makes the rotary action from the initial position of the unclamping state to clamp the workpiece at an angle θ 3  with respect to the horizontal axis. FIG. 19 shows a state in which the arm  20  further makes the rotary action from the state shown in FIG. 18 to clamp the workpiece at an angle θ 4  with respect to the horizontal axis. FIG. 20 shows a state in which the arm  20  clamps the workpiece substantially in the horizontal state. 
     In the arrangement shown in FIGS. 18 to  20 , a long hole  119 , which functions as a contact position-holding mechanism, is formed in the link plate  72  so that the knuckle pin  70  is engaged with the long hole  119 . The long hole  119  is formed in the link plate  72  to provide the looseness for the knuckle pin  70 . Accordingly, the contact portion between the guide roller  79  and the curved surface  81  formed on the link plate  72  can be maintained at a substantially constant position regardless of the angle of rotation of the arm  20 . 
     The degree of freedom of the knuckle pin  70  is ensured, and thus the central point of the knuckle pin  70  can be set on the extension line of the axis T of the rod member  32 . As a result, the linear accuracy of the rod member  32  is effectively maintained, and thus it is possible to improve the durability of the cylinder unit  14 . 
     The contact portion between the curved surface  81  and the guide roller  79  is actually based on the line-to-line contact. However, in FIGS. 18 to  20 , the contact portion is depicted as the contact point P for convenience of the explanation. The position of the contact point P is represented by the X coordinate and the Y coordinate (X, Y) assuming that the center of rotation of the arm  20  is the origin O. 
     As clearly understood from FIGS. 18 to  20 , even when the angle of rotation of the arm  20  is changed, the contact point P between the guide roller  79  and the curved surface  81  of the link plate  72  is always identical, and it is at the constant position (X, Y). Therefore, it is possible to obtain the further linear clamping characteristic so that the clamping force is substantially constant irrelevant to the change of the angle of rotation of the arm  20  by maintaining, at the constant position, the contact point P between the guide roller  79  and the curved surface  81  of the link plate  72  to avoid the occurrence of any scuffing at the contact portion between the link plate  72  and the guide roller  79 . 
     In FIGS. 18 to  20 , the origin O of the rectangular coordinates system is set at the center of rotation of the arm  20 . However, it is a matter of course that the center of rotation of the guide roller  79  is alternatively used as the origin O. 
     Next, a clamp apparatus  120  according to another embodiment is shown in FIG.  21 . The same constitutive components as those of the clamp apparatus  10  shown in FIG. 3 are designated by the same reference numerals, detailed explanation of which will be omitted. 
     The clamp apparatus  120  has the following feature. That is, a projection  126  is coaxially connected on a side opposite to a piston  124  provided with a rod member  122 . A lock mechanism  132  is provided for locking the piston  124  by using a pair of balls  130   a ,  130   b  which are engaged with an annular recess  128  of the projection  126 . 
     The lock mechanism  132  has a pair of pressing members  136   a ,  136   b  for pressing the balls  130   a ,  130   b  toward the annular recess  128  in accordance with the action of the resilient force of spring members  134   a ,  134   b , and it functions to maintain the arm  20  at the initial position. The provision of the lock mechanism  132  has the following advantage. That is, even when the piston  124  is allowed to be in a free state by discharging the pressure fluid from the cylinder chamber  28 , then the piston  124  is in the locked state, and it is prevented from displacement. Therefore, the rotary action of the arm  20  is avoided, and the arm  20  can be locked at the initial position. 
     In the embodiment of the present invention, the cylinder is used as the driving mechanism. However, there is no limitation thereto. It is also preferable that the rod member  32  is displaced by using, for example, an unillustrated linear actuator or an electric motor.