Patent ID: 8403338

Claim:
A device for clamping a stem, the device comprising: a body forming a piston chamber, extending along an axis, and having an axially outwardly open hole adapted to receive the stem; a plurality of collet jaws in the hole having axially outer ends and shiftable axially between an outer releasing position with the ends radially spread and an inner holding position with the ends more closely radially juxtaposed, the stem being engageable axially in the hole between the jaws and lockable in the body in the inner holding position; a piston axially shiftable in the chamber and coupled to the jaws for shifting same between the outer and inner positions, the chamber being oriented relative to the piston such that pressurization of the chamber moves the piston axially outward; a main spring urging the piston axially inward and thereby urging the jaws into the inner holding position; a cover element having an outer head and 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, the head being dimensioned such that in the extended position it blocks radial inward movement of the outer ends of the jaws from the releasing position to the holding position; and a secondary spring braced between the cover element and the piston and urging the element into the extended position, such that in the extended position of the cover element, insertion of the stem axially inward into the hole pushes the head of the cover element axially inward to allow the outer ends of the jaws to move radially inward and grip the stem.