Patent ID: 6988720

Claim:
A clamping device that is engageable detachably with a vertical bore of a workpiece for locking it, comprising: a clamp body, a pull rod mounted in the clamp body in a manner in which it is vertically movable and protrudes upwardly, a tapered rod portion provided at the top end portion of the pull rod and having a diameter increasing toward the top, a collet member provided with a collet fitted on the tapered rod portion, which is elastically deformable in a diametrically expanding manner and which is slidably fitted on an upper half of the pull rod, an annular collet supporting member that abuts against a bottom of the collet member for supporting the collet member from below, and a hydraulic cylinder for biasing the collet supporting member upward and driving the pull rod downward; wherein the hydraulic cylinder comprises: a cylinder bore formed in the clamp body, a piston member slidably fitted in the cylinder bore for driving the pull rod downward, an annular hydraulic chamber formed by the cylinder bore, piston member, and collet supporting member, wherein the piston member has a greater pressure receiving area than a pressure receiving area of the collet supporting member in the annular hydraulic chamber; and wherein the pull rod is horizontally movable by a specific small distance relative to the piston member, and the collet member is horizontally movable relative to the clamp body together with the pull rod.