A plurality of finger-like grippers (12, 13, 14) pivotably supported in a housing (4, 5), and a plunger or piston (7) with a head part (8a) acting on the grippers (12, 13, 4), the piston being supported in such a way that it is free to slide in the housing (4). The grippers are held and guided in openings (17, 18) in the top part (5) of the housing (4) and can pivot about axes transverse to the longitudinal axis (L) of the housing (4, 5), where the pivot axes (a, b, c) of all the grippers are in the same plane (e) and together form a regular polygon surrounding the longitudinal axis (L). The ends of the grippers (12, 13, 14) facing the piston (7) are surrounded as a group by a ring of elastic material or by a garter spring (15), which pulls them toward stop surfaces (4b), which limit the pivoting movement.