In a clamping device (1) for machine tools (2) that is equipped with a power-operated chuck (5) and an electric drive motor (11) with a changeover function for triggering clamping movements, a motion converter (31) as well as a force accumulator (51) for maintaining the clamping force, which comprises several spring packs (52) supported on an adjustment element (32) of the motion converter (31), the spring packs (52) are each only arranged on one side of the adjustment element (32). In addition, a pressure piece (53) interacting with the adjustment element (32) is firmly connected to several spacer elements (57), each of which carries a stop disc (61) and passes through the spring packs (52) as well as a spacer (66). The spacer elements (57) and the spacers (66) can be adjusted relative to one another and the spacers (66) interact with spacer pins (71) which are guided through the pressure piece (53) and are supported on the wall (24) of the housing (21) opposite to the spring packs (52).Due to this embodiment, the spring packs (52) are always preloaded and do not have any axial play, and the adjustment movements of the pressure piece can thus be registered and evaluated directly. Rather, the spring packs (52) have a defined preload at all times, with the effect that the operating method is improved compared to embodiments of prior art.