Chuck, in particular machine vise

In a chuck, in particular a machine vise comprising a hollow spindle which is threadedly engaged with a stationary vise part, a pressure rod which is axially movably supported in said hollow spindle and which exerts the clamping pressure, a hydraulic or mechanical power amplifier which is arranged in a cylindrical housing which is connected to the hollow spindle, a secondary member of said power amplifier acting onto the pressure rod and the primary member of said power amplifier being under the power of an air-operated piston, which itself is arranged in an air cylinder which is connected to the power amplifier housing and is fixedly connected to same and the air cylinder has at its rear end not facing the hollow spindle a plate which seals off the cylinder chamber, at the rear face of which plate there sealingly abuts a rotary slide valve which can be rotated with respect to the plate between two end positions and which is coaxial to said plate, in the first end position of which rotary slide valve an axial air-supply opening, which is constantly in connection with a compressed-air source, is in alignment with a first axial opening of the plate, which leads into the cylinder chamber, and in the second end position of which rotary slide valve an axial vent is connected to a corresponding axial opening of the plate and including engaging means for a handcrank are provided on the rotary slide valve and between the rotary slide valve and the plate there is provided a spring-loaded torque coupling which holds the rotary slide valve in the ventilating position, the improvement comprising wherein the rotary slide valve is arranged in a sleeve which concentrically surrounds said rotary slide valve, is connected to the air cylinder at its one end through a fine thread connection and is secured against rotation after axial adjustment, which sleeve has at its other end a radially inwardly directed flange or the like, wherein between said flange and the rotary slide valve there is provided an initially tensioned axial ball bearing.