Abstract:
A load lifting apparatus suitable for use in a materials-handling operation, which enables loads of various shapes and structures, e.g. concrete products, to be clamped by a one-touch simple operation despite an extremely simple structure and allows the loads to be released by a one-touch simple operation at desired places after they have been lifted and moved thereto. The apparatus includes a link mechanism ( 14 ) in which a pair of clamping members ( 1  and  2 ) and a plurality of members ( 4, 5, 8, 8 - 1, 8 - 2 ) are supported by a plurality of pivot shafts. The upper part of the link mechanism ( 14 ) is arranged to be capable of being lifted and lowered by a lifting device. The clamping members ( 1  and  2 ) rotate about a pivot shaft ( 3  or  3′ ) in response to vertical extension and contraction of the link mechanism ( 14 ), thereby enabling an object ( 11 ) to be grasped by the distal end portions of the clamping members ( 1  and  2 ) directly or indirectly through grab members ( 10 ) by the extension of the link mechanism ( 14 ), and allowing the object ( 11 ) to be released therefrom by the contraction of the link mechanism ( 14 ). The apparatus has a lock-unlock mechanism ( 20, 30, 40, 50, 60,  or  70 ) actuated in response to the vertical extension and contraction of the link mechanism ( 14 ), caused by vertical movement of the lifting device, to lock the clamping members in respective predetermined positions of rotation about the pivot shaft and to unlock the clamping members in response to the vertical movement of the lifting device.

Description:
This application is a divisional of prior application Ser. No. 09/374,994 filed Aug. 16, 1999, now U.S. Pat. No. 6,123,376, which is a divisional of prior application Ser. No. 08/967,733 filed Nov. 12, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a load lifting apparatus suitable for use in a materials-handling operation, which enables loads (particularly, heavy objects) of various shapes and structures to be readily clamped and allows the loads to be readily released at a desired position after they have been lifted and moved thereto. 
     To lift and move to a desired position a heavy load, e.g. a concrete block or pipe or a steel pipe, the following method has heretofore been employed: A wire rope or chain is wound around the load, and the other end of the wire rope or chain is engaged with a hook or the like of materials-handling equipment. In this state, the load is lifted and moved to the desired position, and then the wire rope or chain is removed from the load. 
     Some loads are lifted and moved to a desired position as follows: Engagement members, e.g. bolts or hooks, for engaging a wire rope or chain have previously been attached to the loads, or the loads have been arranged such that these engagement members can be attached thereto. In a materials-handling operation, a wire rope or chain is engaged with engagement members previously attached to a load or engagement members attached thereto at the site of work, and the other end of the wire rope or chain is engaged with a hook of materials-handling equipment. After the load has been lifted and moved to a desired position, the wire rope or chain is disengaged from the engagement members attached to the load. 
     Materials-handling equipment, e.g. a crane, has a lifting apparatus that has clamping members for clamping a load as an object to be lifted. The clamping members of the lifting apparatus are opened by human power or using a jig, e.g. a hydraulic device, to pick up a load to be lifted. Then, the load is lifted and moved to a desired position by the materials-handling equipment. At the destination, the clamping members are removed from the load by human power or using a jig, e.g. a hydraulic device. 
     The above-described conventional method in which a wire rope or chain is wound around a load or engaged with engagement members by a manual operation suffers from the problem that the materials-handling operation is difficult because it is carried out by a manual operation and, at the same time, it is dangerous at a shaky work site. 
     The conventional method in which the clamping members of materials-handling equipment are operated by human power or using a jig, e.g. a hydraulic device, suffers from the problem that it is difficult to operate the clamping members at a job site and, at the same time, the use of a hydraulic device or other similar jig involves a mechanical failure and causes the overall size of the equipment to increase. 
     If such a jig as a hydraulic device is mounted on a vehicle or the like, it occupies an installation space, resulting in a reduction of the space for loading an object to be lifted, and also causing the cost of the equipment to increase. 
     SUMMARY OF THE INVENTION 
     In view of the above-described circumstances, an object of the present invention is to provide a load lifting apparatus suitable for use in a materials-handling operation, which enables loads of various shapes and structures, e.g. concrete products, to be clamped by a one-touch simple operation despite an extremely simple structure and allows the loads to be released by a one-touch simple operation at desired places after they have been lifted and moved thereto. 
     To solve the above-described problem, the present invention provides a load clamping and lifting apparatus including a link mechanism ( 14 ) in which a pair of clamping members ( 1  and  2 ) and a plurality of members ( 4 ,  5 ,  8 ,  8 - 1 ,  8 - 2 ) are supported by a plurality of pivot shafts. The upper part of the link mechanism ( 14 ) is arranged to be capable of being lifted and lowered by a lifting device. The clamping members ( 1  and  2 ) rotate about a pivot shaft ( 3  or  3 ′) in response to vertical extension and contraction of the link mechanism ( 14 ), thereby enabling an object ( 11 ) to be grasped by the distal end portions of the clamping members ( 1  and  2 ) directly or indirectly through grab members ( 10 ) by the extension of the link mechanism ( 14 ), and allowing the object ( 11 ) to be released therefrom by the contraction of the link mechanism ( 14 ). The load clamping and lifting apparatus has a lock-unlock mechanism ( 20 ,  30 ,  40 ,  50 ,  60 , or  70 ) that is actuated in response to the vertical extension and contraction of the link mechanism ( 14 ), which is caused by vertical movement of the lifting device, to lock the clamping members in respective predetermined positions of rotation about the pivot shaft and to unlock the clamping members in response to the vertical movement of the lifting device. 
     In the load clamping and lifting apparatus, the lock-unlock mechanism ( 20 ,  30 , or  40 ) has a frame ( 21 ,  31 , or  41 ) provided with a guide groove; a latch ( 24 ,  34 , or  44 ); and a shunting member ( 23 ,  33 , or  43 ). The frame ( 21 ,  31 , or  41 ) is attached to a clamping member ( 1  or  2 ) or other member ( 4  or  5 ) constituting the link mechanism ( 14 ) or to a member ( 8 ,  8 - 1 , or  8 - 2 ) that moves in response to extension and contraction of the link mechanism. The guide groove ( 22 ,  32 , or  42 ) is pierced with a projecting pin ( 15 ) provided on a clamping member ( 1  or  2 ) or other member ( 4  or  5 ) constituting the link mechanism ( 14 ) or on a member ( 8 ,  8 - 1 , or  8 - 2 ) that moves in response to extension and contraction of the link mechanism. The projecting pin ( 15 ) is movable in the guide groove ( 22 ,  32 , or  42 ). The latch ( 24 ,  34 , or  44 ) and the shunting member ( 23 ,  33 , or  43 ) are rotatably supported on the frame ( 21 ,  31 , or  41 ) by respective pivot shafts (pins  25 ,  35 , or  45 ;  27 ,  37 , or  47 ) such that the distal ends of the latch and the shunting member project into the guide groove ( 22 ,  32 , or  42 ) so that the latch and shunting member are each rotated by movement of the projecting pin ( 15 ) in one direction, and rotation of the latch and shunting member caused by movement of the projecting pin ( 15 ) in the opposite direction is limited by respective limiting devices (stopper pin  26 ,  36 , or  46 ; pin  27 ,  37 , or  47 ). The guide groove ( 22 ,  32 , or  42 ) is formed such that after the projecting pin ( 15 ) has moved in one direction past the latch ( 24 ,  34 , or  44 ), movement of the projecting pin ( 15 ) in the opposite direction is limited by the latch ( 24 ,  34 , or  44 ), and when the projecting pin ( 15 ) moves in the opposite direction after moving in the one direction by a predetermined distance past the shunting member ( 23 ,  33 , or  43 ), the shunting member ( 23 ,  33 , or  43 ) enables the projecting pin ( 15 ) to bypass the latch ( 24 ,  34 , or  44 ) and to move in the opposite direction (see FIGS. 2,  4  and  6 ). 
     In the load clamping and lifting apparatus, the lock-unlock mechanism ( 50 ) has a frame ( 51 ) provided with a guide groove ( 52 ); a latch projection ( 54 ) integrally formed with the frame ( 51 ) such that the latch projection ( 54 ) projects into the guide groove ( 52 ); and a shunting member ( 53 ). The frame ( 51 ) is attached to a clamping member ( 1  or  2 ) or other member ( 4  or  5 ) constituting the link mechanism ( 14 ) or to a member ( 8 ,  8 - 1 , or  8 - 2 ) that moves in response to extension and contraction of the link mechanism. The guide groove ( 52 ) is pierced with a projecting pin ( 15 ) provided on a clamping member ( 1  or  2 ) or other member ( 4  or  5 ) constituting the link mechanism ( 14 ) or on a member ( 8 ,  8 - 1 , or  8 - 2 ) that moves in response to extension and contraction of the link mechanism. The projecting pin ( 15 ) is movable in the guide groove ( 52 ). The shunting member ( 53 ) is rotatably supported on the frame ( 51 ) by a pivot shaft (pin  57 ) such that the distal end of the shunting member ( 53 ) projects into the guide groove ( 52 ) so that the shunting member ( 53 ) is rotated by movement of the projecting pin ( 15 ) in one direction, and rotation of the shunting member ( 53 ) caused by movement of the projecting pin ( 15 ) in the opposite direction is limited by a limiting device (stopper pin  56 ). The guide groove ( 52 ) is formed such that after the projecting pin ( 15 ) has moved in one direction past the latch projection ( 54 ), movement of the projecting pin ( 15 ) in the opposite direction is limited by the latch projection ( 54 ), and when the projecting pin ( 15 ) moves in the opposite direction after moving in the one direction by a predetermined distance past the shunting member ( 53 ), the shunting member ( 53 ) enables the projecting pin ( 15 ) to bypass the latch projection ( 54 ) and to move in the opposite direction (see FIGS.  7  and  8 ). 
     In the load clamping and lifting apparatus, the lock-unlock mechanism ( 60  or  70 ) has a frame ( 61  or  71 ) provided with a straight-line shaped guide groove ( 62  or  72 ); a latch member ( 63  or  73 ) having a latch projection ( 63   a  or  73   a ) and a projection ( 63   b  or  73   b ) for rotation; a limiting device (stopper  65  or  75 ) for limiting rotation in a predetermined direction of the latch member ( 63  or  73 ); and a holding device (magnet  66  or  76 ) for holding the latch member ( 63  or  73 ) in a predetermined rotational position. The frame ( 61  or  71 ) is attached to a clamping member ( 1  or  2 ) or other member ( 4  or  5 ) constituting the link mechanism ( 14 ) or to a member ( 8 ,  8 - 1 , or  8 - 2 ) that moves in response to extension and contraction of the link mechanism. The guide groove ( 62  or  72 ) is pierced with a projecting pin ( 15 ) provided on a clamping member ( 1  or  2 ) or other-member ( 4  or  5 ) constituting the link mechanism ( 14 ) or on a member ( 8 ,  8 - 1 , or  8 - 2 ) that moves in response to extension and contraction of the link mechanism. The projecting pin ( 15 ) is movable in the guide groove ( 62  or  72 ). The latch member ( 63  or  73 ) is rotatably supported on the frame ( 61  or  71 ) by a pivot shaft ( 64  or  74 ) such that the distal end of the latch projection ( 63   a  or  73   a ) projects into the guide groove ( 62  or  72 ) so that the latch member ( 63  or  73 ) is rotated by movement of the projecting pin ( 15 ) in one direction, and such that the projection ( 63   b  or  73   b ) for rotation extends across the guide groove ( 62  or  72 ), and rotation of the latch member ( 63  or  73 ) caused by movement of the projecting pin ( 15 ) in the opposite direction is limited by the limiting device (stopper  65  or  75 ). The latch member ( 63  or  73 ) is arranged such that after the projecting pin ( 15 ) has moved in the guide groove ( 62  or  72 ) in one direction past the latch projection ( 63   a  or  73   a ), movement of the projecting pin ( 15 ) in the opposite direction is limited by the latch projection ( 63   a  or  73   a ), and when the projecting pin ( 15 ) pushes the projection ( 63   b  or  73   b ) for rotation after moving further in the one direction, the latch member ( 63  or  73 ) rotates greatly to a predetermined position and is held at this position by the holding device (magnet  66  or  76 ), and thereafter, the latch member ( 63  or  73 ) is released from the holding device (magnet  66  or  76 ) by movement of the projecting pin ( 15 ) in the opposite direction and rotated to a position where rotation of the latch member ( 63  or  73 ) is limited by the limiting device (stopper  65  or  75 ) (see FIGS. 9 to  14 ). 
     In the load clamping and lifting apparatus, when the pair of clamping members ( 1  and  2 ) constituting the link mechanism ( 14 ) are not locked by the lock-unlock mechanism ( 20 ,  30 , or  40 ), a lifting operation of the lifting device causes the lower ends of the clamping members ( 1  and  2 ) to move toward each other to grasp an object lying therebetween directly or indirectly through grab members ( 10 ) (see FIGS. 1,  2  and  3 ). 
     In the load clamping and lifting apparatus, when the pair of clamping members ( 1  and  2 ) constituting the link mechanism ( 14 ) are not locked by the lock-unlock mechanism ( 20 ), a lifting operation of the lifting device causes the lower ends of the clamping members ( 1  and  2 ) to move away from each other to press the inner wall of an object ( 11 ) lying outside the clamping members ( 1  and  2 ) directly or indirectly through grab members ( 10 ), thereby grasping the object ( 11 ) (see FIG.  15 ). 
     In the load clamping and lifting apparatus, a pair of link mechanisms ( 14  and  14 ) are provided to extend between a pair of upper and lower support members ( 8 - 1  and  8 - 2 ) such that the link mechanisms lie in a side-by-side relation to each other at a predetermined distance therebetween. When the pair of clamping members ( 1  and  2 ) constituting each of the link mechanisms ( 14  and  14 ) are not locked by the lock-unlock mechanism ( 20 ), a lifting operation of the lifting device causes the lower ends of the clamping members of each link mechanism to move toward each other so that side walls of an object ( 11 ) which are at the predetermined distance from each other are grasped by the lower ends of the clamping members ( 1  and  2 ) of the pair of link mechanisms, respectively, directly or indirectly through grab members ( 10 ) (see FIG.  16 ). 
     In the load clamping and lifting apparatus, a pair of link mechanisms ( 14  and  14 ) are provided to extend between a pair of upper and lower support members ( 8 - 1  and  8 - 2 ) at both ends, respectively, of the pair of support members. When the pair of clamping members ( 1  and  2 ) constituting each of the link mechanisms ( 14  ad  14 ) are not locked by the lock-unlock mechanism ( 30 ), a lifting operation of the lifting device causes the lower ends of the clamping members ( 1  and  2 ) of each link mechanism to move toward each other to grasp an object ( 11 ) lying therebetween directly or indirectly through grab members ( 10 ) (see FIGS.  17  and  18 ). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing an example of a structure of a load lifting apparatus according to the present invention. 
     FIG. 2 is a diagram showing an example of a structure of a lock-unlock mechanism used in a load lifting apparatus according to the present invention. 
     FIG. 3 is a diagram showing an example of a structure of a load lifting apparatus according to the present invention. 
     FIG. 4 is a diagram showing an example of a structure of a lock-unlock mechanism used in a load lifting apparatus according to the present invention. 
     FIG. 5 is a diagram showing an example of a structure of a load lifting apparatus according to the present invention. 
     FIG. 6 is a diagram showing an example of a structure of a lock-unlock mechanism used in a load lifting apparatus according to the present invention. 
     FIG. 7 is a diagram showing an example of a structure of a lock-unlock mechanism used in a load lifting apparatus according to the present invention. 
     FIG. 8 is a diagram showing an example of a structure of a lock-unlock mechanism used in a load lifting apparatus according to the present invention. 
     FIG. 9 is a diagram showing an example of a structure of a lock-unlock mechanism used in a load lifting apparatus according to the present invention. 
     FIG. 10 is a diagram showing an example of a structure of a lock-unlock mechanism used in a load lifting apparatus according to the present invention. 
     FIG. 11 is a diagram showing an example of a structure of a lock-unlock mechanism used in a load lifting apparatus according to the present invention. 
     FIG. 12 is a diagram showing an example of a structure of a lock-unlock mechanism used in a load lifting apparatus according to the present invention. 
     FIG. 13 is a diagram showing an example of a structure of a lock-unlock mechanism used in a load lifting apparatus according to the present invention. 
     FIG. 14 is a diagram showing an example of a structure of a lock-unlock mechanism used in a load lifting apparatus according to the present invention. 
     FIG. 15 is a diagram showing an example of a structure of a load lifting apparatus according to the present invention. 
     FIG. 16 is a diagram showing an example of a structure of a load lifting apparatus according to the present invention. 
     FIGS.  17 ( a ) and  17 ( b ) are diagrams showing an example of a structure of a load lifting apparatus according to the present invention. 
     FIGS.  18 ( a ) and  18 ( b ) are diagrams showing an example of a structure of a load lifting apparatus according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the accompanying drawings. 
     FIG. 1 is a diagram showing a structure of a load lifting apparatus according to the present invention. The load lifting apparatus has a pair of clamping members  1  and  2 . The clamping member  1  and the clamping member  2  are connected by a pivot shaft  3  such that the clamping members  1  and  2  intersect each other in an X-shape at the pivot shaft  3  and are rotatable about it. 
     The lower end of a link member  4  and the lower end of a link member  5  are connected to the upper ends of the clamping members  2  and  1  by pivot shafts  6  and  7 , respectively, such that the link members  4  and  5  are rotatable about the respective pivot shafts  6  and  7 . The upper ends of the link members  4  and  5  are connected by a pivot shaft  9  such that these upper ends intersect each other at the pivot shaft  9  and are rotatable about it. 
     The clamping member  1 , the clamping member  2 , the link member  4  and the link member  5  constitute a link mechanism  14  supported at four points (pivot shafts  3 ,  6 ,  7  and  9 ). 
     Grab members  10  are secured to the mutually opposing lower end portions of the clamping members  1  and  2 . A suspended ring  12  is secured to the pivot shaft  9 . 
     A wire rope  13 , for example, is engaged with the suspended ring  12 . The suspended ring  12  is lifted and lowered through-the wire rope  13 , thereby enabling an object (load)  11  to be clamped between the grab members  10  and released at a desired position after the object  11  has been lifted and moved thereto, as described later in detail. More specifically, when the suspended ring  12  is lifted, the link mechanism  14  extends vertically, causing the lower ends of the clamping members  1  and  2  to move toward each other. Consequently, the object  11  is clamped between the grab members  10 . Conversely, when the suspended ring  12  is lowered after the object  11  has been placed, for example, on the ground, the link mechanism  14  contracts vertically, causing the lower ends of the clamping members  1  and  2  to move away from each other, thereby releasing the object  11 . 
     A lock-unlock mechanism  20  locks the clamping members  1  and  2  in respective predetermined positions of rotation about the pivot shaft  3  and unlocks them in response to the vertical extension and contraction of the link mechanism  14 . The lock-unlock mechanism  20  is provided to extend between the clamping members  1  and  2  at a position closer to the left end of the link mechanism  14  than the pivot shaft  3 . 
     FIG. 2 is a diagram showing the structure of the lock-unlock mechanism  20  in detail. The lock-unlock mechanism  20  has a plate-shaped frame  21  provided with a guide groove  22 ; a shunting member  23 ; and a latch  24 . 
     The upper-right end portion of the frame  21  is rotatably attached to the clamping member  2  by a pin  16 . The guide groove  22  is pierced with a projecting pin  15  provided on the clamping member  1 . The projecting pin  15  is movable along the guide groove  22 . 
     The shunting member  23  is rotatably attached to the frame  21  by a pin  25 . The distal end of the shunting member  23  projects into the guide groove  22 . Clockwise rotation of the shunting member  23  is limited by a stopper pin  26 . 
     The latch  24  is rotatably attached to the frame  21  by a pin  27 . The distal end of the latch  24  projects into the guide groove  22 . Clockwise rotation of the latch  24  is limited by a stopper pin  28 . 
     A recessed portion  22   a  is formed in a side wall of the guide groove  22  at a position facing opposite to the distal ends of the shunting member  23  and the latch  24 . The distance between the recessed portion  22   a  and the distal ends of the shunting member  23  and the latch  24  is set equal to the width of a space through which the projecting pin  15  can pass. 
     The frame  21  is urged to rotate in the direction of the arrow A about the pin  16  by its own weight. If the suspended ring  12  is lowered in the state shown in FIG. 1, the link mechanism  14  contracts vertically, causing the projecting pin  15  to move upward in the guide groove  22 . When the projecting pin  15  comes in contact with the latch  24 , the latch  24  rotates counterclockwise about the pin  27 . After the projecting pin  15  has passed the latch  24 , the latch  24  rotates about the pin  27  by its own weight to return to the position where it abuts on the stopper pin  28 . 
     When the suspended ring  12  is raised in this state, the link mechanism  14  is caused to extend vertically, and the projecting pin  15  is also caused to move downward in the guide groove  22 . However, the downward movement of the projecting pin  15  is blocked by the latch  24  (see B in FIG.  2 ). Consequently, the clamping members  1  and  2  are stopped from rotating about the pivot shaft  3 . Thus, the distance between the mutually opposing lower ends of the clamping members  1  and  2  is maintained (locked) in this state. 
     At this time, the projecting pin  15  surely lies on the latch  24  because the frame  21  is urged to rotate in the direction of the arrow A about the pin  16  by its own weight. 
     With the above state being maintained, the link mechanism  14  is moved to the position of the object  11 . Then, the suspended ring  12  is lowered such that the object  11  lies between the clamping members  1  and  2  of the link mechanism  14 . 
     The lowering of the suspended ring  12  causes the projecting pin  15  to push up the shunting member  23 . Consequently, the shunting member  23  rotates counterclockwise about the pin  25 . After the projecting pin  15  has passed the shunting member  23 , the shunting member  23  returns by its own weight until it comes in contact with the stopper pin  26 . 
     When the suspended ring  12  is lifted in this state, the link mechanism  14  extends vertically, causing the projecting pin  15  to move downward through the space between the recessed portion  22   a  of the guide groove  22  and the distal ends of the shunting member  23  and the latch  24  (see C and D in FIG.  2 ). Consequently, the grab members  10  move toward each other to clamp the object  11 . 
     After being clamped as described above, the object  11  is lifted and moved to a desired place. Then, the suspended ring  12  is lowered. Consequently, the projecting pin  15  moves upward in the guide groove  22  as in the case of the above, and at the same time, the distance between the grab members  10  enlarges. Thus, the object  11  is released. 
     When the suspended ring  12  is lifted after the projecting pin  15  has further moved upward past the latch  24 , the projecting pin  15  is prevented from moving downward by the latch  24 . Accordingly, the distance between the mutually opposing lower ends of the clamping members  1  and  2 , that is, the distance between the grab members  10 , is maintained (locked) at a value determined by the position of the projecting pin  15  (see B in FIG.  2 ). 
     At this time, the projecting pin  15  surely lies on the latch  24  because the frame  21  is urged to rotate in the direction of the arrow A about the pin  16  by its own weight as in the case of the above. 
     FIG. 3 is a diagram showing another structure of a load lifting apparatus according to the present invention. The load lifting apparatus differs from the load lifting apparatus shown in FIG. 1 in that the lock-unlock mechanism  30  is provided to extend between the clamping members  1  and  2  at a position closer to the right end of the link mechanism  14  than the pivot shaft  3 , and the lower-left end portion of the frame  31  is rotatably attached to the clamping member  2  by the pin  16 , and further that the projecting pin  15  provided on the clamping member  1  extends through the guide groove  32 . In addition, the structure of the lock-unlock mechanism  30  slightly differs from that of the lock-unlock mechanism  20 . 
     FIG. 4 is a diagram showing the structure of the lock-unlock mechanism  30  in detail. The lock-unlock mechanism  30  has a plate-shaped frame  31  provided with a guide groove  32 ; a shunting member  33 ; and a latch  34 . The lower-left end portion of the frame  31  is rotatably attached to the clamping member  2  by the pin  16 . The guide groove  32  is pierced with the projecting pin  15  provided on the clamping member  1 . 
     The shunting member  33  is rotatably attached to the frame  31  by a pin  35 . The distal end of the shunting member  33  projects into the guide groove  32 . Counterclockwise rotation of the shunting member  33  is limited by a stopper pin  36 . 
     The latch  34  is rotatably attached to the frame  31  by a pin  37 . The distal end of the latch  34  projects into the guide groove  32 . Counterclockwise rotation of the latch  34  is limited at a predetermined position by a stopper pin  38 . 
     A recessed portion  32   a  is formed in a side wall of the guide groove  32  at a position facing opposite to the distal ends of the latch  34  and the shunting member  33  as in the case of the lock-unlock mechanism  20 . 
     The frame  31  is urged to rotate in the direction of the arrow E about the pin  16  by its own weight. 
     If the suspended ring  12  is lowered in the state shown in FIG. 3, the link mechanism  14  contracts vertically, causing the projecting pin  15  to move downward in the guide groove  32 . When the projecting pin  15  comes in contact with the latch  34 , the latch  34  rotates clockwise about the pin  37 . After the projecting pin  15 , passes the latch  34 , the latch  34  rotates about the pin  37  by its own weight to return to a position where the latch  34  abuts on the stopper pin  38 . 
     When the suspended ring  12  is raised in this state, the link mechanism  14  is caused to extend vertically, and the projecting pin  15  is also caused to move upward. However, the upward movement of the projecting pin  15  is blocked by the latch  34  (see F in FIG.  4 ). 
     Consequently, the clamping members  1  and  2  are stopped from rotating about the pivot shaft  3 . Thus, the distance between the mutually opposing lower ends of the clamping members  1  and  2  is maintained (locked) in this state. At this time, the projecting pin  15  surely slides into a position where it abuts on the lower end of the latch  34  because the frame  31  is urged to rotate in the direction of the arrow E about the pin  16  by its own weight. 
     With the above state being maintained, the link mechanism  14  is moved to the position of the object  11 , and the suspended ring  12  is lowered. Consequently, the projecting pin  15  moves downward in the guide groove  32 . The projecting pin  15  pushes the shunting member  33  to rotate clockwise about the pin  35 . After the projecting pin  15  has passed the shunting member  33 , the shunting member  33  returns by its own weight until it comes in contact with the stopper pin  36 . 
     When the suspended ring  12  is lifted in this state, the link mechanism  14  extends vertically, causing the projecting pin  15  to move upward through the space between the recessed portion  32   a  of the guide groove  32  and the distal ends of the shunting member  33  and the latch  34  (see G and H in FIG.  4 ). Consequently, the grab members  10  move toward each other to clamp the object  11 . 
     After being clamped as described above, the object  11  is lifted and moved to a desired place. Then, the suspended ring  12  is lowered. Consequently, the projecting pin  15  moves downward in the guide groove  32  as in the case of the above, and at the same time, the distance between the grab members  10  enlarges. Thus, the object  11  is released. 
     When the suspended ring  12  is lifted after the projecting pin  15  has further moved downward past the latch  34 , the projecting pin  15  is urged to move upward. However, the upward movement of the projecting pin  15  is prevented by the latch  34 . Accordingly, the distance between the mutually opposing lower ends of the clamping members  1  and  2 , that is, the distance between the grab members  10 , is maintained (locked) at a value determined by the position of the projecting pin  15  (see F in FIG.  4 ). At this time, the projecting pin  15  surely slides into a position where it abuts on the lower end of the latch  34  because the frame  31  is urged to rotate in the direction of the arrow E about the pin  16  by its own weight as in the case of the above. 
     FIG. 5 is a diagram showing another structure of a load lifting apparatus according to the present invention. The load lifting apparatus differs from the load lifting apparatus shown in FIG. 3 in that the lock-unlock mechanism  40  is provided to extend horizontally above the pivot shaft  3  of the link mechanism  14 , and the left end portion of the frame  41  is rotatably attached to the clamping member  2  by the pin  16 , and further that the projecting pin  15  provided on the clamping member  1  extends through the guide groove  42 . 
     FIG. 6 is a diagram showing the structure of the lock-unlock mechanism  40  in detail. The lock-unlock mechanism  40  has a plate-shaped frame  41  provided with a guide groove  42 ; a shunting member  43 ; and a latch  44 . 
     The left end portion of the frame  41  is rotatably attached to the clamping member  2  by the pin  16 . The guide groove  42  is pierced with the projecting pin  15  provided on the clamping member  1 . The shunting member  43  is rotatably attached to the frame  41  by a pin  45 . The distal end of the shunting member  43  projects into the guide groove  42 . Clockwise rotation of the shunting member  43  is limited by a stopper pin  46 . 
     The latch  44  is rotatably attached to the frame  41  by a pin  47 . The distal end of the latch  44  projects into the guide groove  42 . Clockwise rotation of the latch  44  is limited by a stopper pin  48 . 
     A recessed portion  42   a  is formed in a side wall of the guide groove  42  at a position facing opposite to the distal ends of the shunting member  43  and the latch  44  as in the case of the lock-unlock mechanisms  20  and  30 . 
     The frame  41  is urged to rotate in the direction of the arrow I about the pin  16  by its own weight. 
     If the suspended ring  12  is lowered in the state shown in FIG. 5, the link mechanism  14  contracts vertically, causing the projecting pin  15  to move rightward in the guide groove  42 . 
     When the projecting pin  15  comes in contact with the latch  44 , the latch  44  rotates counterclockwise about the pin  47 . After the projecting pin  15  passes the latch  44 , the latch  44  rotates about the pin  47  by its own weight to return to a position where the latch  44  abuts on the stopper pin  48 . 
     When the suspended ring  12  is raised in this state, the link mechanism  14  is caused to extend vertically, and the projecting pin  15  is caused to move leftward. However, the leftward movement of the projecting pin  15  is blocked by the latch  44  (see J in FIG.  6 ). 
     Consequently, the clamping members  1  and  2  are stopped from rotating about the pivot shaft  3 . Thus, the distance between the mutually opposing lower ends of the clamping members  1  and  2  is maintained (locked) in this state. 
     At this time, the projecting pin  15  surely slides into a position where it abuts on the right edge of the latch  44  because the frame  41  is urged to rotate in the direction of the arrow I about the pin  16  by its own weight. 
     With the above state being maintained, the link mechanism  14  is moved to the position of the object  11 , and the suspended ring  12  is lowered such that the object  11  lies between the clamping members  1  and  2  of the link mechanism  14 . Consequently, The projecting pin  15  pushes the shunting member  43  to rotate counterclockwise about the pin  45 . 
     When the suspended ring  12  is lifted in this state, the link mechanism  14  extends vertically, causing the projecting pin  15  to move leftward through the space between the recessed portion  42   a  of the guide groove  42  and the distal ends of the shunting member  43  and the latch  44  (see K and L in FIG.  6 ). Consequently, the grab members  10  move toward each other to clamp the object  11 . 
     After being clamped as described above, the object  11  is lifted and moved to a desired place. Then, the suspended ring  12  is lowered. Consequently, the projecting pin  15  moves rightward in the guide groove  42  as in the case of the above, and at the same time, the distance between the grab members  10  enlarges. Thus, the object  11  is released. 
     When the suspended ring  12  is lifted after the projecting pin  15  has further moved rightward past the latch  44 , leftward movement of the projecting pin  15  is blocked by the latch  44 . Accordingly, the distance between the mutually opposing lower ends of the clamping members  1  and  2 , that is, the distance between the grab members  10 , is maintained at a value determined by the position of the projecting pin  15  (see J in FIG.  6 ). 
     At this time, the projecting pin  15  surely slides into a position where it abuts on the right edge of the latch  44  because the frame  41  is urged to rotate in the direction of the arrow I about the pin  16  by its own weight as in the case of the above. 
     As stated above, the load clamping and lifting apparatus according to the present invention has a four-point link mechanism  14  comprising the clamping member  1 , the clamping member  2 , the link member  4  and the link member  5 , and the link mechanism  14  is provided with the lock-unlock mechanism  20  ( 30  or  40 ), thereby enabling the object  11  to be clamped, lifted and moved to a desired place and released simply by raising and lowering the suspended ring  12  by a crane or the like through the wire rope  13 . Thus, it is possible to dispense with an operation of winding a wire rope around the object  11  for each materials-handling operation, which is difficult and may be dangerous according to the conditions of the place where the materials-handling operation is carried out. 
     FIG. 7 is a diagram showing another example of the structure of a lock-unlock mechanism used in the load clamping and lifting apparatus according to the present invention. The lock-unlock mechanism  50  has a structure in which a latch projection  54  is provided to project into the guide groove  52  in place of the latch  34  having the structure shown in FIG.  4 . 
     In addition, the frame  51  has a projection  51   a  extending leftwardly from the lower end thereof. The lock-unlock mechanism  50  having such a structure is mounted on the link mechanism  14  in place of the lock-unlock mechanism  30  shown in FIG.  3 . More specifically, the projection  51   a  is rotatably attached to the clamping member  2  by the pin  16 , and the guide groove  52  is pierced with the projecting pin  15  provided on the clamping member  1 . 
     In this state, the frame  51  is urged to rotate in the direction of the arrow M about the pin  16  by its own weight. 
     With the lock-unlock mechanism  50  having the above-described structure, when the suspended ring  12  is lowered, the link mechanism  14  contracts vertically, causing the projecting pin  15  to move downward in the guide groove  52 . 
     During the downward movement, the projecting pin  15  first comes in contact with the latch projection  54  and then passes through the space between the distal end of the latch projection  54  and the recessed portion  52   a  of the guide groove  52  (see N in FIG.  7 ). Because the frame  51  is urged to rotate in the direction of the arrow M about the pin  16  as stated above, the projecting pin  15  enters the space between the latch projection  54  and the shunting member  53  (see O in FIG. 7) after passing through the space between the distal end of the latch projection  54  and the recessed portion  52   a  of the guide groove  52 . 
     When the suspended ring  12  is raised in this state, the link mechanism  14  is caused to extend vertically, and the projecting pin  15  is also caused to move upward. However, the upward movement of the projecting pin  15  is blocked by the latch projection  54 . Consequently, the clamping members  1  and  2  are stopped from rotating about the pivot shaft  3 . Thus, the distance between the mutually opposing lower ends of the clamping members  1  and  2  is maintained (locked) in this state. 
     With the above state being maintained, the link mechanism  14  is moved to the position of the object  11 , and then the suspended ring  12  is lowered. Consequently, the projecting pin  15  also moves downward and pushes the shunting member  53  to rotate clockwise about a pin  57 . After the projecting pin  15  has passed the shunting member  53 , the shunting member  53  returns by its own weight until it comes in contact with the stopper pin  56 . 
     When the suspended ring  12  is lifted in this state, the link mechanism  14  extends vertically, causing the projecting pin  15  to move upward through the space between the distal end of the shunting member  53  and the recessed portion  52   a  of the guide groove  52  and through the space between the distal end of the latch projection  54  and the recessed portion  52   a  (see P and N in FIG.  7 ). 
     Consequently, the grab members  10  move toward each other to clamp the object  11 . 
     After being clamped as described above, the object  11  is lifted and moved to a desired place. Then, the suspended ring  12  is lowered. Consequently, the projecting pin  15  moves downward in the guide groove  32  as in the case of the above, and at the same time, the distance between the grab members  10  enlarges. Thus, the object  11  is released. 
     When the suspended ring  12  is lifted after the projecting pin  15  has further moved downward past the latch projection  54 , the projecting pin  15  is prevented from moving upward by the latch projection  54 . Accordingly, the distance between the mutually opposing lower ends of the clamping members  1  and  2 , that is, the distance between the grab members  10 , is maintained at a value determined by the position of the projecting pin  15  (see O in FIG.  7 ). 
     At this time, the projecting pin  15  surely slides into a position where it abuts on the lower end of the latch projection  54  because the frame  51  is urged to rotate in the direction of the arrow M about the pin  16  as in the case of the above. 
     FIG. 8 is a diagram showing another example of the structure of a lock-unlock mechanism used in the load clamping and lifting apparatus according to the present invention. The lock-unlock mechanism  50  is approximately the same as the lock-unlock mechanism  50  shown in FIG.  7 . The lock-unlock mechanism shown in FIG.  7  and that shown in FIG. 8 are in an upside-down relation to each other. 
     The lock-unlock mechanism  50  is used in place of the lock-unlock mechanism of the load lifting apparatus shown in FIG.  1 . The operation of the lock-unlock mechanism  50  is approximately the same as that of the lock-unlock mechanism  50  shown in FIG. 7; therefore, a description thereof is omitted. 
     FIG. 9 is a diagram showing another example of the structure of a lock-unlock mechanism used in the load clamping and lifting apparatus according to the present invention. The lock-unlock mechanism  60  has a structure in which a frame  61  having a straight-line shaped guide groove  62  is provided with a latch member  63  that rotates about a pin  64 . 
     The latch member  63  is provided with a latch projection  63   a  such that the latch projection  63   a  projects into the guide groove  62 . The latch member  63  is further provided with a projection  63   b  for rotation such that the projection  63   b  extends across the guide groove  62 . Further, the latch member  63  is provided with a projection  63   c  for release above the latch projection  63   a.    
     Counterclockwise rotation of the latch member  63  is limited by a stopper  65 . 
     It should be noted that reference numeral  66  denotes a magnet for holding the latch member  63  in a predetermined rotational position. 
     The lock-unlock mechanism  60  having the above-described structure is mounted on the link mechanism  14  in place of the lock-unlock mechanism  30  shown in FIG.  3 . More specifically, the frame  61  is secured to the clamping member  2  by the pin  16  (the frame  61  is not completely fixed but slightly loosely secured), and the guide groove  62  is pierced with the projecting pin  15  provided on the clamping member  1 . 
     If the suspended ring  12  is lowered in the state shown in FIG. 3, the projecting pin  15  also moves downward. When the projecting pin  15  comes in contact with the latch projection  63   a,  the latch member  63  rotates clockwise about the pin  64 . 
     After the projecting pin  15  has passed the distal end of the latch projection  63   a,  the latch member  63  rotates counterclockwise about the pin  64  by its own weight to return to a position where the latch member  63  abuts on the stopper  65 . 
     When the suspended ring  12  is raised in this state, the link mechanism  14  is caused to extend vertically, and the projecting pin  15  is also caused to move upward. However, the upward movement of the projecting pin  15  is blocked by the latch projection  63   a  (see Q in FIG.  9 ). 
     Consequently, the clamping members  1  and  2  are stopped from rotating about the pivot shaft  3 . Thus, the distance between the mutually opposing lower ends of the clamping members  1  and  2  is maintained (locked) in this state. 
     With the above state being maintained, the link mechanism  14  is moved to the position of the object  11 , and the suspended ring  12  is lowered. Consequently, the projecting pin  15  moves downward and pushes the projection  63   b  for rotation, causing the latch member  63  to rotate clockwise about the pin  64  as shown in FIG.  10 . 
     When the projecting pin  15  further moves downward to reach the distal end of the projection  63   b  as shown in FIG. 11, the latch member  63  is held by the magnet  66 . Thus, the latch member  63  is maintained in this position, and the distance between the mutually opposing lower ends of the clamping members  1  and  2  of the link mechanism  14  reaches a maximum. 
     When the suspended ring  12  is raised in this state, the projecting pin  15  moves upward in the guide groove  62  past the latch projection  63   a  and comes in contact with the projection  63   c  for release. Consequently, the latch member  63  rotates counterclockwise about the pin  64 . 
     Thus, the latch member  63  is released from the magnet  66 , and the latch member  63  rotates about the pin  64  to return to the position shown in FIG. 9 by its own weight. 
     As the projecting pin  15  further moves upward, the grab members  10  move toward each other to clamp the object  11 . 
     After being clamped as described above, the object  11  is lifted and moved to a desired place. Then, the suspended ring  12  is lowered. Consequently, the projecting pin  15  moves downward in the guide groove  32  as in the case of the above, and at the same time, the distance between the grab members  10  enlarges. Thus, the object  11  is released. 
     When the projecting pin  15  further moves downward past the distal end of the latch projection  63   a,  the latch member  63  rotates about the pin  64  to return to the position shown in FIG. 9 by its own weight. Thus, if the suspended ring  12  is raised in this state, the clamping members  1  and  2  are locked in the respective positions. 
     FIG. 12 is a diagram showing another example of the structure of a lock-unlock mechanism used in the load clamping and lifting apparatus according to the present invention. The lock-unlock mechanism  70  has a structure in which a frame  71  having a straight-line shaped guide groove  72  is provided with a latch member  73  that rotates about a pin  74 . 
     As shown in FIG. 13, the latch member  73  is provided with a latch projection  73   a  such that the latch projection  73   a  projects into the guide groove  72 . The latch member  73  is further provided with a projection  73   b  for rotation such that the projection  73   b  extends across the guide groove  72 . 
     Counterclockwise rotation of the latch member  73  is limited by a stopper  75 . 
     It should be noted that reference numeral  76  denotes a magnet for holding the latch member  73  in a predetermined rotational position. 
     The lock-unlock mechanism  70  having the above-described structure is mounted on the link mechanism  14  in place of the lock-unlock mechanism  20  shown in FIG.  1 . More specifically, the frame  71  is secured to the clamping member  2  by the pin  16  (the frame  71  is not completely fixed but slightly loosely secured), and the guide groove  72  is pierced with the projecting pin  15  provided on the clamping member  1 . 
     If the suspended ring  12  is lowered in the state shown in FIG. 12, the projecting pin  15  moves upward. When the projecting pin  15  comes in contact with the latch projection  73   a,  the latch member  73  rotates counterclockwise about the pin  74 . 
     After the projecting pin  15  has passed the distal end of the latch projection  73   a,  the latch member  73  rotates clockwise about the pin  74  by its own weight to return to a position where the latch member  73  abuts on the stopper  75  as shown in FIG.  13 . 
     When the suspended ring  12  is raised in this state, the link mechanism  14  is caused to extend vertically, and the projecting pin  15  is caused to move downward. However, the downward movement of the projecting pin  15  is blocked by the latch projection  73   a  (see R in FIG.  13 ). 
     Consequently, the clamping members  1  and  2  are stopped from rotating about the pivot shaft  3 . Thus, the distance between the mutually opposing lower ends of the clamping members  1  and  2  is maintained (locked) in this state. 
     With the above state being maintained, the link mechanism  14  is moved to the position of the object  11 , and the suspended ring  12  is lowered. Consequently, the projecting pin  15  moves upward and pushes the projection  73   b  for rotation, causing the latch member  73  to rotate counterclockwise about the pin  74 . 
     When the projecting pin  15  further moves upward to reach the distal end of the projection  73   b  as shown in FIG. 14, the latch member  73  is held by the magnet  76 . Thus, the latch member  73  is maintained in the rotational position, and the distance between the mutually opposing lower ends of the clamping members  1  and  2  of the link mechanism  14  reaches a maximum. 
     When the suspended ring  12  is raised in this state, the projecting pin  15  moves downward in the guide groove  72  past the latch projection  73   a  and comes in contact with a side portion  73   c  of the latch member  73 . Consequently, the latch member  73  rotates clockwise about the pin  74 . 
     Thus, the latch member  73  is released from the magnet  76 , and the latch member  73  rotates about the pin  74  by its own weight. Consequently, the grab members  10  move toward each other to clamp the object  11 . 
     After being clamped as described above, the object  11  is lifted and moved to a desired place. Then, the suspended ring  12  is lowered. Consequently, the projecting pin  15  moves upward in the guide groove  32  as in the case of the above, and at the same time, the distance between the grab members  10  enlarges. Thus, the object  11  is released. 
     When the projecting pin  15  further moves upward past the distal end of the latch projection  73   a,  the latch member  73  rotates about the pin  74  to return to the position shown in FIG. 13 by its own weight as in the case of the above. Thus, if the suspended ring  12  is raised in this state, the clamping members  1  and  2  are locked in the respective positions. 
     FIG. 15 is a diagram showing another structure of a load lifting apparatus according to the present invention. This load lifting apparatus is used to lift a frusto-conical object  11 , e.g. a frusto-conical concrete pipe. 
     As illustrated in the figure, a clamping member  1  and a clamping member  2  are rotatably attached to a support member  8  by respective pivot shafts  3 ′. The upper end of the clamping member  1  is rotatably connected to the lower end of a link member  4  by a pivot shaft  6 . Similarly, the upper end of the clamping member  2  is rotatably connected to the lower end of a link member  5  by a pivot shaft  7 . The upper ends of the link members  4  and  5  are rotatably connected to each other by a pivot shaft  9 . 
     A suspended ring  12  is secured to the pivot shaft  9 . A wire rope  13 , for example, is engaged with the suspended ring  12  so that the suspended ring  12  can be lifted and lowered through the wire rope  13 . 
     The clamping member  1 , the clamping member  2 , the link member  4 , the link member  5  and the support member  8  constitute a link mechanism  14  supported at five points. 
     A lock-unlock mechanism  20  having the structure shown in FIG. 2 is provided to extend between the link member  4  and the support member  8 . More specifically, the frame  21  is rotatably attached to the link member  4  by the pin  16 , and the guide groove  22  of the frame  21  is pierced with the projecting pin  15  provided on the support member  8 . 
     In the load lifting apparatus arranged as stated above, when the projecting pin  15  lies on the latch  24  shown in FIG. 2 (i.e., when the link mechanism  14  is in a locked state), the clamping members  1  and  2  are in the respective positions R. That is, the distance between the outer sides of the clamping members  1  and  2  is smaller than the inner diameter of the object  11 . 
     In this state, the link mechanism  14  is moved to a position directly above the object  11 , and the suspended ring  12  is lowered. Consequently, the support member,  8  is placed on the upper end of the object  11 . As the suspended ring  12  is further lowered, the frame  21  of the lock-unlock mechanism  20  moves downward. In other words, the projecting pin  15  moves upward in the guide groove  22 . 
     The projecting pin  15  pushes up the shunting member  23 . After the projecting pin  15  has passed the shunting member  23 , the shunting member  23  returns to come in contact with the stopper pin  26 . When the suspended ring  12  is raised after the shunting member  23  has abutted on the stopper pin  26 , the link mechanism  14  extends vertically. 
     Consequently, the distance between the lower ends of the clamping members  1  and  2  enlarges, and the grab members  10  come in contact with the inner wall of the object  11 . 
     When the suspended ring  12  is raised in this state, the object  11  is lifted. After the object  11  has been moved to a desired place, the suspended ring  12  is lowered to release the object  11 . The release of the object  11  and the locking of the clamping members  1  and  2  can be effected simply by lowering and raising the suspended ring  12  as stated above. Therefore, a description thereof is omitted. 
     It should be noted that the grab members  10  are swingably mounted by respective hinge pins  10   a  so that the grab members  10  can come in close contact with the slanted inner surface of the object. 
     The lock-unlock mechanism  20  may be replaced by either of the lock-unlock mechanism  50  and  70  having the structures shown in FIGS. 8,  12  to  14 . More specifically, the frame  51  or  71  is secured to the link member  4  by the pin  16  (the frame  51  or  71  is not completely fixed but secured such that the frame  51  or  71  is slightly rotatable), and the guide groove  52  or  72  of the frame  51  or  71  is pierced with the projecting pin  15  provided on the support member  8 . In this case also, the operation is substantially the same as the above; therefore, a description thereof is omitted. 
     FIG. 16 is a diagram showing another structure of a load lifting apparatus according to the present invention. This load lifting apparatus is used to lift a tubular object  11 , e.g. a manhole tube, in a vertical position. 
     As illustrated in the figure, the load lifting apparatus has two link mechanisms  14  each comprising a clamping member  1 , a clamping member  2 , a link member  4  and a link member  5 . The two link mechanisms  14  are provided in a side-by-side relation to each other between a pair of upper and lower support members  8 - 2  and  8 - 1 . A lock-unlock mechanism  20  having the structure shown in FIG. 2 is provided to extend between the support members  8 - 2  and  8 - 1 . 
     FIG. 16 is a diagram showing a state where each wall portion of an object  11  is clamped between grab members  10  attached to the lower ends of the clamping members  1  and  2  of each link mechanism  14 , that is, a state where the object  11  is in a lifted state. 
     It should be noted that reference numeral  19  denotes a guide for guiding the load lifting apparatus when it is lowered onto the object  11  such that the center of the load lifting apparatus, that is, the center line between the two link mechanisms  14 , and the center line of the object  11  coincide with each other. 
     In the above-described state, the object  11  is moved to a desired position, and then the suspended ring  12  is lowered to place the object  11  in the destination. When the suspended ring  12  is further lowered to lower the support member  8 - 2 , the two link mechanisms  14  contract vertically, and each pair of grab members  10  move away from each other to release the object  11 . 
     The projecting pin  15  provided on the lower support member  8 - 1  moves upward in the guide groove  22 . If the suspended ring  12  is raised after the projecting pin  15  has passed the latch  24 , each pair of clamping members  1  and  2  are locked in respective positions of rotation about the pivot shafts  3 ′ reached at that time. 
     The lock-unlock mechanism  20  may be replaced by either of the lock-unlock mechanism  50  and  70  having the structures shown in FIGS. 8,  12  to  14 . More specifically, the frame  51  or  71  is secured to the upper support member  8 - 2  by the pin  16  (the frame  51  or  71  is not completely fixed but secured such that the frame  51  or  71  is slightly rotatable), and the guide groove  52  or  72  of the frame  51  or  71  is pierced with the projecting pin  15  provided on the lower support member  8 - 1 . In this case also, the operation is substantially the same as the above; therefore, a description thereof is omitted. 
     FIGS.  17 ( a ) and  17 ( b ) are diagrams showing another structure of a load lifting apparatus according to the present invention. This load lifting apparatus is used to lift a plate-shaped object  11 , e.g. an L-shaped concrete wall, in a vertical position. 
     As illustrated in the figures, the load lifting apparatus has two link mechanisms  14  each comprising a clamping member  1 , a clamping member  2 , a link member  4  and a link member  5 . The two link mechanisms  14  are provided to extend between a pair of upper and lower support members  8 - 2  and  8 - 1  at both ends of the pair of support members  8 - 2  and  8 - 1 , respectively. A lock-unlock mechanism  30  having the structure shown in FIG. 4 is provided to extend between the support members  8 - 2  and  8 - 1 . 
     FIGS.  17 ( a ) and  17 ( b ) show a state where each side wall of the object  11  is clamped between grab members  10  attached to the lower ends of the clamping members  1  and  2  of each link mechanism  14 , that is, a state where the object  11  is in a lifted state. 
     In the above-described state, the object  11  is moved to a desired position, and then the suspended rings  12  are lowered to place the object  11  in the destination. When the suspended rings  12  are further lowered to lower the support member  8 - 2 , the two link mechanisms  14  contract vertically, and each pair of grab members  10  move away from each other as indicated by reference character S to release the object  11 . 
     The projecting pin  15  provided on the upper support member  8 - 2  moves downward in the guide groove  32 . If the suspended rings  12  are raised after the projecting pin  15  has passed the latch  34 , each pair of clamping members  1  and  2  are locked in respective positions of rotation about the pivot shafts  3 ′ reached at that time. 
     The lock-unlock mechanism  30  may be replaced by either of the lock-unlock mechanisms  50  and  60  having the structures shown in FIGS. 7,  9  to  11 . More specifically, the frame  51  or  61  is secured to the lower support member  8 - 1  by the pin  16  (the frame  51  or  61  is not completely fixed but secured such that the frame  51  or  61  is slightly rotatable and the guide groove  52  or  62  of the frame  51  or  61  is pierced with the projecting pin  15  provided on the upper support member  8 - 2 . In this case also, the operation is substantially the same as the above; therefore, a description thereof is omitted. 
     FIGS.  18 ( a ) and  18 ( b ) are diagrams showing another structure of a load lifting apparatus according to the present invention. This load lifting apparatus is used to lift a tubular object  11  of continuous length, e.g. a concrete pipe or steel pipe, in a horizontal position. 
     As illustrated in the figures, the load lifting apparatus has two link mechanisms  14  each comprising a clamping member  1  with an arc-shaped distal end portion, a clamping member  2  with an arc-shaped distal end portion, a link member  4  and a link member  5 . The two link mechanisms  14  are provided to extend between a pair of upper and lower support members  8 - 2  and  8 - 1  at both ends of the pair of support members  8 - 2  and  8 - 1 , respectively. A lock-unlock mechanism  30  having the structure shown in FIG. 4 is provided between the support members  8 - 2  and  8 - 1 . 
     FIGS.  18 ( a ) and  18 ( b ) show a state where the tubular object  11  is grasped by the clamping members  1  and  2  of the two link mechanisms  14 , that is, a state where the object  11  is in a lifted state. 
     In the above-described state, the object  11  is moved to a desired position, and then the suspended rings  12  are lowered to place the object  11  in the destination. When the suspended rings  12  are further lowered to lower the support member  8 - 2 , the two link mechanisms  14  contract vertically, and the arc-shaped portions of the clamping members  1  and  2  move away from each other to release the object  11 . 
     The projecting pin  15  provided on the upper support member  8 - 2  moves downward in the guide groove  32 . If the suspended rings  12  are raised after the projecting pin  15  has passed the latch  34 , each pair of clamping members  1  and  2  are locked in respective positions of rotation about the pivot shaft  3  reached at that time. 
     The lock-unlock mechanism  30  may be replaced by either of the lock-unlock mechanisms  50  and  60  having the structures shown in FIGS. 7,  9  to  11 . More specifically, the frame  51  or  61  is secured to the lower support member  8 - 1  by the pin  16  (the frame  51  or  61  is not completely fixed but secured such that the frame  51  or  61  is slightly rotatable), and the guide groove  52  or  62  of the frame  51  or  61  is pierced with the projecting pin  15  provided on the upper support member  8 - 2 . In this case also, the operation is substantially the same as the above; therefore, a description thereof is omitted. 
     It should be noted that the foregoing structures of load clamping and lifting apparatuses and lock-unlock mechanisms are embodiments of the present invention, and that the present invention is not necessarily limited thereto. The embodiments can be modified in various ways according to the structure, shape, weight, etc. of an object to be handled. 
     As has been described above, the load clamping and lifting apparatus according to the present invention has a lock-unlock mechanism that is actuated in response to the vertical extension and contraction of a link mechanism, which is caused by vertical movement of a lifting device, to lock the clamping members in respective predetermined positions of rotation about the pivot shaft and to unlock the clamping members in response to the vertical movement of the lifting device. Accordingly, a series of operations, i.e. grasping of a load, lifting, movement, release of the load, and locking of the clamping members in respective predetermined positions of rotation about the pivot shaft, can be executed by only the vertical movement of the lifting device without requiring human power. Thus, it is advantageously possible to achieve an efficient and labor-saving materials-handling operation.