The invention is based on a method for operating an electric power tool and an electric power tool according to the preambles to the independent claims.
In known electric power tools, a torque limitation is implemented in a known fashion in that either a mechanical overload clutch on the driven shaft of the transmission periodically interrupts the frictional engagement when a predetermined load moment is reached or the power consumption of the electric motor, which is proportional to the torque, is regulated to a constant value.
As a rule, the value of the desired load moment is preset and is not changed during operation. Usually, the user of the electric power tool makes changes when the motor is switched off. Through the actuation of a pushbutton of the electric power tool by the pressure of the user's finger, the speed of the motor is either changed by means of a pulse width modulation or is preset proportional to the pushbutton position by means of a regulator. When the load moment preset by means of the torque limitation is reached, the moment output by the motor is kept constant either by the mechanical clutch periodically interrupting the frictional engagement or by the motor current being regulated to a constant value.
In the case of the mechanical overload clutch, the motor continues to rotate at an undiminished speed; with the periodic engagement and renewed releasing of the overload clutch, a high torque peak is temporarily exerted on the work piece. In screwdrivers, this leads to the screw being turned a certain amount furthers e.g. with an impulse screwdriver, and can potentially lead to a stripping of the screw. The reaction of the overload clutch is also connected with a significant generation of noise that is often found to be unpleasant.
If the torque is electronically limited through regulation of the motor current, then after the preset load moment has been reached, the motor is supplied with current until the user releases the pushbutton. This also applies to the mechanical torque limitation.