Abstract: A clamping device comprises a holder in which an actuation arm is movable in a longitudinal directions. A fixed clamping jaw is carried by the holder and a movable clamping jaw is arranged within the holder opposite to the fixed clamping jaw and it is supported by at least two longitudinally spaced lever members which have a first pivotal connection to the holder, a second pivotal connection to the movable clamping jaw and a third pivotal connection to the actuator arm. An actuator is connected to the arm to move it backwardly and forwardly for opening and closing the jaws. The pivotal connecting points of the levers are chosen so that the pivotal points are disposed at substantially right angles to each other. The pivot pins are cylindrical hinge pins and the line between the pivotal connection of the lever to the jaw and to the fixed holder makes an angle of 45.degree. to the horizontal so that a larger distance between the points of support is possible.

Abstract: The hydraulic activators are provided with a relief valve which serves to permit communication between the disengaging cylinder chamber and the work cylinder chamber. The open position of the relief valve occurs when the release apparatus for the rotatable friction brake member is closed. The closed position of the relief valve occurs after the rotating brake is released. The relief valve can be mounted in the release apparatus of the working piston.