Patent ID: 8777197

Claim:
A clamping apparatus comprising a clamp rod ( 7 ) inserted into a cylinder hole ( 6 ) of a housing ( 2 ) is rotated in a released raised position and then is driven straight down to a locked lowered position, the clamping apparatus further comprising: an annular piston ( 13 ) which is hermetically inserted between the cylinder hole ( 6 ) and the clamp rod ( 7 ) so as to be movable in an axial direction and to be rotatable about an axis; a guide groove ( 18 ) which is provided in either one of (i) an outer peripheral portion of the annular piston ( 13 ) and (ii) an inner peripheral wall of the cylinder hole ( 6 ), the guide groove ( 18 ) having a rotational groove ( 23 ) and a straight advance groove ( 24 ) which are provided so as to be contiguous with each other upwardly; an engaging member ( 19 ) which is provided on the other of (i) the outer peripheral portion of the annular piston ( 13 ) and (ii) the inner peripheral wall of the cylinder hole ( 6 ), and is fitted into the guide groove ( 18 ); a transmission mechanism ( 26 ) which is provided between the clamp rod ( 7 ) and the annular piston ( 13 ), and prevents the clamp rod ( 7 ) and the annular piston ( 13 ) from rotating relatively to each other about the axis and permits the clamp rod ( 7 ) and the annular piston ( 13 ) to move relatively to each other in the axial direction; and a lock actuation chamber ( 40 ) which is formed above the annular piston ( 13 ) and pressurized fluid is supplied to and discharged from; wherein a diameter (D 1 ) of a portion of the clamp rod ( 7 ) that is sealed in an upper end wall ( 3 a ) of the housing ( 2 ) is designed to be larger than a diameter (D 2 ) of a portion of the clamp rod ( 7 ) that is sealed in the annular piston ( 13 ); wherein, the apparatus is configured so that when the clamp rod ( 7 ) is moved from the released raised position to the locked lowered position, pressurized fluid is supplied to the lock actuation chamber ( 40 ), and thereby the annular piston ( 13 ) descends while rotating via the rotational groove ( 23 ) and the engaging member ( 19 ) with the clamp rod ( 7 ) being kept lifted in the released raised position by a force differential between upward and downward forces exerted on the clamp rod ( 7 ) by the pressurized fluid in the lock actuation chamber ( 40 ), and thereby the annular piston ( 13 ) causes, via the transmission mechanism ( 26 ), the clamp rod ( 7 ) to rotate in the released raised position, and thereafter, the annular piston ( 13 ) causes the clamp rod ( 7 ) to descend straight down via a driven portion fittingly attached to the clamp rod; and wherein an erroneous operation prevention mechanism (M) which prevents the clamp rod ( 7 ) from descending during its rotation and permits the clamp rod ( 7 ) to descend during its straight descent is provided to a lower portion of the clamp rod ( 7 ) and in a lower end wall ( 5 ) of the housing ( 2 ), and wherein said erroneous operation prevention mechanism has a dissimilar shape from said lower end wall of the housing.