A device for clamping a stem has a body forming a piston chamber and having an axially outwardly open hole adapted to receive the stem and a plurality of collet jaws in the hole shiftable axially by a piston between an outer releasing position with its axial outer ends radially spread and an inner holding position with the ends more closely radially juxtaposed. The stem is engageable axially in the hole between the jaws and lockable in the body in the inner holding position. A main spring urges the piston and jaws axially inward into the inner position. A cover element has an outer head and is axially shiftable relative to the piston between an extended position with the head engaged radially between the outer ends of the jaws and a retracted position with the head spaced axially inward from the outer ends of the jaws.