Abstract: A power drive and transmission unit for self centering clamps and chucks has a housing with a cylindrical bore and a piston which is rotatably mounted therein and is acted upon by two linear operating, piston cylinder drives to obtain rotation of the rotating piston in one or the opposite direction. Two or three linearly moving central slide elements which are mounted on said housing for obtaining the centering chuck action elements are connected to the rotating piston by means of pins running in oblong slots in order to convert the rotation of the rotating piston into oppositely linear motion of the slide elements, so that they move towards or away from a common center depending on the direction of rotation of the rotating piston.