Device for clamping workpiece to a surface

A self-locking device for clamping a workpiece to a supporting surface, said clamping device comprising a housing (1), a first bore (3) in said housing, a clamping bolt (2) slidable in said first bore and projecting out of said first bore for contacting said workpiece, a second bore (4) crossing said first bore (3) in said housing, a locking piston (6) slidably movable in said second bore (4) and dividing said second bore (4) into a first chamber (12) and a second chamber (13), said locking piston (6) including a piston rod (5) reaching into said second chamber (13), a first wedging incline (8) on said piston rod (5), a second wedging incline (2′) on an inner end of said clamping bolt (2) cooperating with said first wedging incline (8) of said piston rod (5) for locking said clamping bolt (2) in a fixed position in response to moving said locking piston (6) and piston rod (5) into a bolt locking position, said clamping device further comprising a tongue and groove combination between said piston rod (5) and said inner end of said clamping bolt (2), said tongue and groove combination comprising a tongue (14,15) and a groove (9,10) wherein said tongue is guided in said groove in a force transmitting manner, said groove comprising a first groove portion (9) and a second groove portion (10) interconnected by an open neck (11), said tongue comprising a first tongue section (14) and a second tongue section (15) slidingly received in said first and second groove portions (9,10) for slidingly guiding said tongue in said groove when said locking piston (6) and piston rod (5) are moved relative to said clamping bolt (2), said housing comprising a first inlet (12′) for admitting fluid under pressure into said first chamber (12) for moving said piston (6) and piston rod (5) into a bolt locking position, and wherein said housing comprises a second inlet (20,21) for admitting fluid under pressure into said second chamber (13) for driving said piston (6) and piston rod (5) into a bolt release position, and wherein said second groove portion (10) has a flow cross-sectional area that is larger than a cross-sectional area of said second tongue section (15) thereby forming a fluid flow channel (22) communicating with said second chamber (13) in said second bore (4), said fluid flow channel (22) extending between said tongue (14,15) and said groove (9,10) for admitting fluid under pressure from said second chamber (13) into said fluid flow channel (22) for separating said locking piston (6) and piston rod (5) from said clamping bolt (2) to thereby facilitate movement of said locking piston (6) and piston rod (5) into said bolt release position.