The present invention relates to an arrangement for tightening screw connections.
Arrangements of the above mentioned general type are known in the art. Known arrangements have multi-stage rotary drive which allows a screwing-in with a high rotary speed in a first stage and tightening with a high torque in a further stage. One of such arrangements is disclosed for example in the German document DE 38 01 972 A1, in which a mechanical switching transmission is arranged after the drive motor and at a certain loading movement switches from a low torque to a high torque. A switching off device which switches off the rotary drive at reaching a predetermined screwing-in condition is however not provided. Such switching off devices are generally known and can be formed for example as mechanical overload couplings. However, there is a disadvantage here that the screwing arrangements provided with such switching off devices operate inaccurately for example due to the influence of inertia, so that the tightening moment of the screw connections tightened in this manner substantially fluctuates.
German reference DE-41 28 427 A1 discloses a power operated screwing tool which has a direct current motor operating in different working stages. In a first stage a screw connection is pre-tightened at a higher rotary speed to a predetermined screwing condition. The further tightening of the screw connection is performed then at a lower rotary speed, and the drive motor after reaching a desired screwing condition is turned off. The determination of the tightening moment is performed from the filled current taken by the motor. Screw connections can be tightened with this arrangement relatively accurately. However, the drive and the associated drive control are expensive so that such an arrangement is questionable for many applications from cost reasons.