1.  Field of the Invention
The present invention relates to a clamping device for machine tools, the device comprising a power-operated chuck for holding a workpiece, and clamping jaws that can be actuated using the clamping device by means of a movement converter and an axially moveable draw rod, wherein the clamping device includes an electric drive motor with a changeover function connectable to the movement converter for triggering clamping movements. 
2.  Description of the Prior Art
A device for the releasable connection of a workpiece carrier configured as a power-operated chuck with an actuating element is disclosed in EP 0344451 A1, in which is two pneumatically operated servo devices arranged coaxially, one after another, are provided and comprise pistons disposed in cylinders and adapted to change the power-operated chuck within a short time.  In this case, the piston rod of the actuating element of one servo device can act on the clamping jaws of the power-operated chuck by means of wedge hooks.  The piston rod of the other servo device, on the other hand, is connected to a coupling element by means of another axially movable draw rod.  Releasing and restoring the driving connection between the actuating element of one servo device, and a clamping piston arranged in the power-operated chuck, is undertaken by means of an actuating element provided with a special configuration allocated to the second servo device. 
The connection between the actuating element and the clamping piston of the power-operated chuck can be reliably established, or released, by means of the further adjustment element, although this involves considerable structural complexity.  In addition, the servo devices arranged axially, one after another, require a relatively large space, which means the application range of this fixture of the prior art is restricted to the specified purpose.