Pressing tool

1. A pressing tool for plastically deforming a workpiece, comprising:

an electric motor;

pressing jaws for applying a pressing force to a workpiece;

a hydraulic force transmitting unit for transmitting a force of the electric motor to the pressing jaws, comprising a piston pump driven by the electric motor rotating in operating rotation direction RA, and a pressure relief valve;

a coupling arrangement which mechanically couples the electric motor to the pressure relief valve, so that the electric motor opens the pressure relief valve when operated in a loosening rotation direction RL opposite to the operating rotation direction RA; and

wherein the pressure relief valve includes a spring biasing the pressure relief valve in a closed position.

2. The pressing tool according to claim 1 , further comprising the coupling which only acts in loosening rotation direction RL of the electric motor.

3. The pressing tool according to claim 2 , wherein the coupling comprises a freewheel.

4. The pressing tool according to claim 3 wherein the freewheel comprises a freewheel ball bearing.

5. The pressing tool according to claim 2 , further comprising a cam driven by the electric motor via the coupling, wherein the cam may open the pressure relief valve.

6. The pressing tool according to claim 5 , further comprising a lever of the pressure relief valve upon which the cam may act.

7. The pressing tool according to claim 1 , further comprising an eccentric shaft driven by the electric motor with an eccentric for actuating the piston pump.

8. The pressing tool according to claim 7 , further comprising a cam driven by the electric motor via the coupling, wherein the cam may open the pressure relief valve, wherein the cam is mounted on the eccentric shaft via the coupling and is driven by the eccentric shaft in loosening rotation direction RL.

9. The pressing tool according to claim 1 , further comprising a control for controlling the electric motor and at least one sensor for measuring at least one pressing parameter, wherein the control is adapted to detect an end of a pressing process based on the at least one pressing parameter and to operate the electric motor in the loosening rotation direction RL opposite to the operating rotation direction RA in order to open the pressure relief valve.

10. The pressing tool according to claim 9 , wherein the control detects the end of the pressing process when switch-off criteria are met, wherein the control then reverses the rotation direction of the electric motor from the operating rotation direction RA to the loosening rotation direction RL.

11. The pressing tool according to claim 9 , wherein the at least one sensor comprises at least one sensor selected from a group consisting of a sensor which captures the revolutions of the electric motor, a current sensor which captures the supply current of the electric motor, a pressure sensor which captures the hydraulic oil pressure, a force sensor which captures a mechanical force characteristic of the pressing force, a torque sensor which captures a torque in the force transmitting unit, and combinations thereof.

12. The pressing tool according to claim 1 , wherein the force of the spring is selected upon a desired opening pressure of the pressure relief valve.

13. A method of operating a pressing tool, wherein the method comprises the following steps:

providing a pressure relief valve and a spring biasing the pressure relief valve in a closed position;

operating a hydraulic force transmitting unit by an electric motor rotating in an operating rotation direction RA;

detecting an end of a pressing process by a control of the pressing tool; reversing a rotation direction of the electric motor from the operating rotation direction RA to a loosening rotation direction RL after the detection of an end of a pressing process; and

opening a pressure relief valve of the hydraulic force transmitting unit by the electric motor rotating in the loosening rotation direction RL.

14. The method according to claim 13 , wherein a servo piston of the hydraulic force transmitting unit and pressing jaws of pressing tool return to their final positions due to the opening of the pressure relief valve.

15. The method according to claim 13 , further comprising a step of detecting a final position of pressing tool and of switching off the electric motor by the control.

16. The method according to claim 13 , wherein the end of the pressing process is detected by the reaching of one or more of the following switch-off criteria:

increase of a supply current or reaching a predefined supply current of the electric motor;

increase of a pressing force or reaching a predefined maximum pressing force;

increase of a torque or reaching a predefined torque;

reaching a maximum number of revolutions of the electric motor;

increase of a hydraulic pressure or reaching a predefined maximum hydraulic pressure.

17. The method according to claim 13 , wherein the pressure relief valve of the hydraulic force transmitting unit closes again after the force transmitting unit has reached its final position.