PRESSMASCHINE

Description

1. Technical field

[0001] The present invention relates to a press machine, in particular a hand-operated press machine, for the plastic deformation of preferably tubular workpieces. The invention further relates to a method for operating a hand-operated press machine. In particular, the press machine according to the invention is suitable for connecting pipes with press fittings in heating and plumbing installations by pressing. 2. State of the art

[0002] Several methods for joining tubular workpieces are known in the prior art. One of these methods places a larger pipe over a smaller pipe and presses them together. In other methods, pipes are joined by pressing press fittings. A possible device for simple pressing is a hand-held pressing machine, in particular a pipe pressing machine. Such hand-held pressing machines have a pressing tool with interchangeable jaws, for example, jaws for pressing, crimping, or cutting workpieces. In pipe pressing machines for pipe connections using a press fitting, the pressing jaws surround the press fitting positioned over the pipe. By closing the pressing jaws, the press fitting and the pipe are plastically deformed and thus permanently joined together.

[0003] A hand-operated press is typically powered by a DC electric motor. For power, the press can be connected to a mains supply or a battery. To generate the required high pressing forces, an electro-hydraulic or electromechanical conversion unit is usually positioned between the electric motor and the pressing tool. The electric motor generates a rotary motion, which is reduced by a gearbox connected to the electric motor. The gearbox output shaft, in turn, drives an eccentric on an eccentric shaft. In a hydraulic conversion unit, the driven eccentric moves a piston of a piston pump back and forth, thereby directing hydraulic fluid into a cylinder. The hydraulic fluid pumped by the piston pump pressurizes a working piston within the cylinder, moving it linearly towards the pressing tool.Rollers connected to the working piston then actuate the preferably interchangeable pressing tool, for example, pressing jaws, and cause the pressing jaws to close. As the pressing jaws close, they transmit a force to the workpieces to be pressed, thus pressing or joining them together.

[0004] A hand-operated pressing tool for crimping press fittings in plumbing and cable lugs is known from publication EP 2 501 523 B1. To generate the required high pressing forces, the pressing tool is connected to an electro-hydraulic conversion unit. A brushless electric motor is used as the drive motor. As soon as the required pressing force is reached, a purely mechanical pressure relief valve opens and the motor speed increases sharply. This is detected by the control unit of the pressing tool, and the electric motor is subsequently switched off.

[0005] Up to now, presses have always applied pressure to workpieces up to the machine's maximum pressing force. However, due to their material or shape properties, some workpieces are already fully pressed together at a lower pressure. In such cases, further pressing causes the hydraulic pressure in the cylinder to rise rapidly, as there is no plastic deformation of the workpiece, only elastic deformation of the tool. This elastic deformation does not result in any further pressing of the workpiece, but leads to significant wear on the press jaws and the press tool. Furthermore, it wastes electrical energy and unnecessarily prolongs the pressing process.

[0006] From the publication EP 2 014 418 A2, a press machine for the plastic deformation of a workpiece is known, in which a coupling device mechanically couples an electric motor of the hydraulic drive with a pressure relief valve, so that the electric motor opens the pressure relief valve when operating in a release direction opposite to the working direction of rotation.

[0007] The invention is therefore based on the objective of providing a hand-operated press machine that is optimized for different pressing operations and in which the wear of the press tools is significantly reduced. 3. Summary of the invention

[0008] The problems mentioned above are solved according to the invention by a press machine according to claim 1 and a method for operating a press machine according to claim 8.

[0009] In particular, the above-mentioned problems are solved by a press machine for plastically deforming a workpiece, comprising an electric motor, press jaws for applying a pressing force to a workpiece, a hydraulic power transmission unit for transmitting the force of the electric motor to the press jaws, comprising a piston pump driven by the electric motor rotating in the working direction and a pressure relief valve, and a coupling device that mechanically couples the electric motor to the pressure relief valve, so that the electric motor opens the pressure relief valve when operating in a release direction opposite to the working direction.

[0010] The coupling device has the advantage that when the electric motor rotates in the working direction, the press machine performs a pressing operation as usual, while when the electric motor rotates in the releasing direction, the pressure relief valve is opened. Thus, by simply reversing the electric motor's rotation, the pressure relief valve can be opened programmatically at any desired time. The electric motor therefore also acts as the actuator for the pressure relief valve.

[0011] Opening the pressure relief valve terminates the pressing process, meaning that the end time can be controlled, particularly independently of reaching a maximum pressing force. Unlike the prior art, the press machine according to the invention avoids the unnecessary application of a maximum pressing force. This results in no, or only minimal, elastic deformation of the press jaws, thus preventing unnecessary wear of the press machine, especially of the press jaws.

[0012] Furthermore, the pressing time is reduced, enabling faster overall work. The maximum pressing pressure is usually set quite high to ensure a secure crimping, i.e., joining, of the workpieces. If only a lower pressing pressure is required, while still guaranteeing a secure crimp, the necessary pressing time is also reduced, making work with the crimping machine more efficient.

[0013] By reversing the electric motor's rotation from the working direction to the opposite release direction, and thereby opening the pressure relief valve, an additional valve, such as a high-pressure hydraulic solenoid valve, is not required. This simplifies the design of the press machine, reduces its cost, and also lowers its weight.

[0014] According to the invention, the press machine has a clutch that only acts in the disengagement direction of the electric motor. This allows the electric motor to operate in its usual direction of rotation to perform the pressing process. Therefore, no structural modifications to the press machine are necessary for the pressing process. This simplifies the design and manufacture of the blank press machine. In the disengagement direction, the clutch enables power transmission from the electric motor to the pressure relief valve, allowing it to be opened automatically and under program control.

[0015] Preferably, the coupling includes a freewheel and is preferably a freewheel ball bearing. This allows us to implement a coupling in a structurally simple and reliable manner that only operates in one direction of rotation, in this case the disengagement direction.

[0016] According to the invention, the press machine further comprises a cam driven by the electric motor via the coupling, which can open the pressure relief valve. By means of a cam, the rotary motion of the electric motor can be easily converted into a preferably linear motion for opening the pressure relief valve.

[0017] According to the invention, the press machine further comprises a lever of the pressure relief valve, upon which the cam can act. The movement of the cam can be transmitted to the pressure relief valve in a structurally simple manner by means of a lever. This provides a particularly space-saving control of the pressure relief valve. In addition, the lever can be used to increase the force exerted by the cam to open the pressure relief valve.

[0018] Preferably, the press machine further comprises an eccentric shaft driven by an electric motor, with an eccentric for actuating the piston pump. The piston pump can be operated in a particularly reliable manner by means of the eccentric shaft. Preferably, a ball bearing is arranged around the eccentric, so that the drive of the piston pump is also very low-friction.

[0019] Preferably, the cam is mounted on the eccentric shaft via the coupling and can be driven by it in the disengagement direction. This provides a structurally simple solution for driving the cam on the one hand and the piston pump on the other.

[0020] Preferably, the press machine has a control unit for driving the electric motor and at least one sensor for measuring at least one pressing parameter. Based on this at least one pressing parameter, the control unit is configured to recognize the end of the pressing process and to operate the electric motor in the opposite direction of rotation (the release direction) to the working direction, in order to open the pressure relief valve. The use of this control unit, which detects at least one pressing parameter via at least one sensor, has the advantage that the press machine does not need to apply maximum pressing force for the pressing operation. Instead, the control unit can terminate the pressing process at a precisely defined point, while still ensuring a reliable pressing operation. This prevents premature wear of the press jaws and other components within the press machine.The control system uses the evaluation of sensor values ​​to detect the end of the pressing process, whereupon the electric motor is controlled by the control system in the release direction opposite to the working direction, in order to reopen the press jaws and allow the hydraulic fluid to drain from the working piston into the oil reservoir, so that the press machine returns to its open starting position.

[0021] Preferably, the control system detects the end of the pressing process when shutdown criteria are met, at which point the control system reverses the direction of rotation of the electric motor from the working direction to the release direction. Detecting shutdown criteria, either entered into the system before the pressing process or detected by sensors during or shortly before the pressing process, enables optimal pressing without having to apply the maximum possible pressing force. In particular, certain shutdown criteria can detect that continuing the pressing process would only result in elastic deformation of the press jaws. Terminating the pressing process before the maximum pressing force is unnecessarily applied counteracts premature wear of the press jaws and other components of the pressing machine.Another shutdown criterion could be, for example, reaching an adjustable hydraulic pressure, such as a hydraulic pressure of 400 bar.

[0022] Preferably, the at least one sensor comprises a Hall sensor that detects the revolutions of the electric motor and / or a current sensor that detects the supply current to the electric motor and / or a pressure sensor that detects the hydraulic oil pressure and / or a force sensor that detects a mechanical force characteristic of the pressing force and / or a torque sensor that detects a torque in the power transmission unit. The at least one sensor enables the control system of the press machine to detect various pressing parameters and / or their temporal profile. These pressing parameters are evaluated by a control system to determine when shutdown criteria have been reached. The shutdown criteria can be different pressing parameter values, such as a minimum value, a minimum duration, or similar. Based on the shutdown criteria, the control system can automatically decide when the current pressing process should be terminated.No additional user interaction is required to end the pressing process. Once the control system detects that one or more shutdown criteria have been met, the electric motor's direction of rotation is reversed from the working direction to the opposite release direction, thus ending the pressing process. The shutdown criteria can be flexibly defined using the pressing parameters. This allows for flexible adjustments to the shutdown process for different pressing machines, different materials being pressed, or changing user experience.

[0023] The problems mentioned above are also solved by a method for operating a press machine comprising the following steps: Operating a hydraulic power transmission unit by an electric motor rotating in a working direction; detecting the end of the pressing process by a control system of the press machine; after detecting the end of the pressing process, reversing the direction of rotation of the electric motor from the working direction to the release direction; and opening a pressure relief valve of the hydraulic power transmission unit by rotating the electric motor in the release direction.

[0024] By detecting the end of the pressing process, the control system ensures that the pressing machine only applies the pressing force required for the workpiece to achieve optimal crimping. A pressing operation that reaches the maximum pressing force of the machine is unnecessary. This also eliminates the associated wear and tear on the pipe pressing machine and the additional pressing time.

[0025] Once the control system detects the end of the pressing process, it reverses the motor's direction of rotation from the working direction to the release direction. By rotating the electric motor in the release direction, the pressure relief valve of the hydraulic power transmission unit is opened, and the pressing tool is returned to its open starting position. This method ensures simple, fast, and optimal pressing of the workpiece. Because the pressing process is detected early, the maximum pressing force of the press is not required in most cases, resulting in longer maintenance intervals, reduced wear on the press, shorter pressing times, and extended battery life.

[0026] The process is preferably used for the plastic deformation of a tubular workpiece, for example a press fitting of a heating or sanitary installation.

[0027] Preferably, the hydraulic power transmission unit and the press jaws of the press machine are returned to their end positions by opening the pressure relief valve.

[0028] Preferably, the method further includes the step of recognizing the end position of the power transmission unit and switching off the electric motor by the control system.

[0029] This further simplifies the control and operation of the press for the user. Manually switching off the electric motor is no longer necessary. The automatic shutdown saves energy and provides the user, especially in complex operating environments, with direct feedback on the completion of the press's return process to the open position.

[0030] Preferably, the termination of the pressing process is detected by reaching one or more of the following shutdown criteria: Increase in supply current or reaching a predetermined supply current of the electric motor; increase in pressing force or reaching a predetermined maximum pressing force; increase in torque or reaching a predetermined torque; reaching a maximum number of revolutions of the electric motor; increase in hydraulic pressure or reaching a predetermined maximum hydraulic pressure;

[0031] The sensors and control electronics can detect and calculate increases in supply current, pressing force, torque, and hydraulic pressure, which are then used as shutdown criteria. This allows the press to determine a material- and tool-specific shutdown point during the pressing process, thus shutting down the press without applying its maximum pressing force. Optimal workpiece compression is still achieved. This eliminates the need for the user to manually define and / or input maximum shutdown values ​​based on material and / or tool characteristics. However, it is also possible to predefine maximum parameters for supply current, pressing force, torque, hydraulic pressure, and / or a maximum number of revolutions of the press's electric motor.

[0032] Preferably, the method further includes the step that the pressure relief valve of the hydraulic power transmission unit closes again after reaching its end position. This allows another pressing operation to be started immediately after the completion of the previous one. 4. Brief description of the characters

[0033] Preferred embodiments of the present invention are illustrated below with reference to the accompanying figures. These figures show: Fig. 1 a schematic sectional view of the drive unit of an embodiment of a press machine; Fig. 2 a schematic side view of parts of the drive unit according to Fig. 1 with a pressure relief valve and its control in the closed state; and Fig. 2b side view according to Fig. 2a with a pressure relief valve and its control in the open state. 5. Detailed description of preferred embodiments

[0034] Preferred embodiments of the present invention are described in detail below with reference to the accompanying figures.

[0035] Fig. 1Figure 1 shows the drive and control unit of a preferably hand-operated pressing machine 1, such as those used for connecting pipes using press fittings. The hand-operated pressing machine 1 has an electric motor 2, which in one embodiment is a DC electric motor. The electric motor 2 is powered by current from a battery or accumulator, or by a wired power supply, or a combination thereof (not shown). The electric motor 2 is connected to a gearbox 4 on the output side to reduce its speed to a speed suitable for the load, here a piston pump 20. The electric motor 2 and the gearbox 4 can also be designed as a single, integrated electric motor-gearbox unit. Furthermore, instead of an electric motor with a gearbox, an electric motor with a suitable speed and / or torque can be used.

[0036] On the output side of the gearbox 4, an eccentric shaft 6 connects to the output shaft of the gearbox 4. The eccentric shaft 6 is rotatably mounted on ball bearings 7. The eccentric shaft 6 includes an eccentric 6a which moves a piston 22 of the piston pump 20 linearly up and down. To reduce friction, the eccentric 6a is surrounded by a ball bearing 6b, which transmits the eccentric force to the piston 22. The ball bearing 6b prevents the eccentric surface from sliding on the piston surface and thus minimizes friction.

[0037] The piston 22 of the piston pump 20 is biased towards the eccentric 6a by means of a spring 21. If the eccentric 6a is rotated by turning the eccentric shaft 6, the piston 22 is pushed against the spring 21. This opens a check valve 24 on the piston pump 20, and hydraulic fluid 44 is pumped by the piston pump 20 from a fluid reservoir 40 via a line 23 and a line 26 into a cylinder chamber 14a above the working piston 12 of the hydraulic power transmission unit 10.

[0038] As the eccentric shaft 6 rotates further, the eccentric 6a is rotated away from the piston 22 of the piston pump 20, so that, due to the restoring force of the spring 22, the piston 22 moves towards the eccentric shaft 6. This closes the check valve 24 of the piston pump 20 and prevents backflow of hydraulic fluid 44 from the cylinder chamber 14a.

[0039] During continuous rotation of the eccentric shaft 6 in the working direction RA (see Fig. 2a The piston pump 20 pumps hydraulic fluid 44 into the cylinder chamber 14a, thereby increasing the pressure there and moving the working piston 12a towards the tool side 16 of the press machine 1, or into Fig. 1 to the left. The head 12a of the axially movable working piston 12 in the cylinder 14 can have a seal 13, preferably a sealing ring.

[0040] A pressing tool (not shown) can be attached to the tool side 16 of the press 1 and is actuated by the working piston 12. The force of the working piston 12 as it moves towards the tool side 16 closes the pressing jaws of the tool (not shown) to press a workpiece. The pressing jaws then generate a pressing force F that is directly proportional to the hydraulic pressure P. F = P x A ( A(Area of ​​the working piston)). When the press jaws close around a workpiece or press fitting, a counterforce is generated and the hydraulic pressure in cylinder chamber 14a increases. Due to the increasing hydraulic pressure and the resulting force between the press jaws, the workpiece or press fitting is plastically deformed.

[0041] The pressing process is controlled by an electronic control unit 60, which evaluates the signals from at least one sensor 52-60. The sensors 52-60 measure process parameters, and the control unit 60 is configured to detect the end of the pressing process based on at least one of these process parameters.

[0042] In one embodiment, a pressure sensor 52 on the hydraulic power transmission unit 10 can determine the hydraulic pressure in the cylinder chamber 14a of the cylinder 14 (pressure side) during a pressing operation and transmit its measurement data to the control unit 50. The pressure sensor 52 can be designed as an analog or digital pressure sensor 52. In the case of an analog pressure sensor 52, an analog-to-digital converter (ADC) can additionally be provided, which converts the measured analog signals into digital signals.

[0043] Furthermore, a torque sensor 54 can be provided which measures a torque in the power transmission device 10. For this purpose, for example, a strain gauge could be arranged on the eccentric shaft 6.

[0044] Furthermore, a rotation sensor 56 can be provided to detect the rotations of the electric motor 2. The rotation sensor 56 can preferably be a Hall sensor or an optical sensor for detecting the rotations of the electric motor 2.

[0045] A current sensor 60 can also be provided to detect the supply current of the electric motor 2. The current sensor 60 can preferably be integrated into the control unit 50.

[0046] Furthermore, a force sensor 58 can be provided which detects a mechanical force characteristic of the pressing force. For this purpose, a load cell could preferably be integrated into the piston 12.

[0047] The sensors 52-60 can be used individually or in various combinations and transmit all their measured values ​​to the controller 50, which evaluates the measured values ​​to determine, in particular, whether the pressing process has ended. For this purpose, the controller can use one or more of the following shutdown criteria: the increase in the supply current or reaching a predetermined supply current of the electric motor 2, in particular measured by the current sensor 60; the increase in the pressing force or reaching a predetermined maximum pressing force, in particular measured by the force sensor 58; the increase in the torque or reaching a predetermined torque, in particular measured by the torque sensor 54; reaching a maximum number of revolutions of the electric motor, in particular measured by the revolution sensor 56; the increase in the hydraulic pressure or reaching a predetermined maximum hydraulic pressure, in particular measured by the pressure sensor 52;

[0048] Depending on the evaluation results, the controller 50 then activates the electric motor 2. This motor can be stationary, rotate in the working direction RA to operate the piston pump 20, or rotate in the release direction RL to open a pressure relief valve 30. When the pressure relief valve 30 opens, the hydraulic pressure in the cylinder chamber 14a is released, and the hydraulic fluid can flow back from the cylinder chamber through line 42 into the reservoir 40. During this process, the piston 12 returns to its initial position, in which the jaws of the pressing tool (not shown) are open.

[0049] In the Figure 1 , 2a and 2bFigure 30 shows a preferred embodiment of a pressure relief valve 30. The pressure relief valve 30 is actuated by a cam 34, which is arranged on the eccentric shaft 6 via a coupling 33. The coupling 33 is preferably a freewheel coupling that transmits the rotary motion of the eccentric shaft 6 to the cam 34 when the electric motor 2 is in the release direction RL, but rotates freely and does not drive the cam 34 when the electric motor 2 and the eccentric shaft 6 rotate in the opposite direction RA. Preferably, the coupling 33 is designed as a ball bearing with a freewheel. However, the freewheel and the cam bearing can also be implemented as separate components. The coupling 33 can also be designed as a slip clutch or similar, which then acts as a freewheel together with the cam 34 and the subsequent mechanism.

[0050] In the illustrated embodiment, when rotated in the release direction RL, the cam 34 actuates a lever 32 by moving its head 32a away and thus pivoting the lever 32, as shown in Fig. 2b depicted. Fig. 2b The pressure relief valve 30 is shown in its open position, actuated by the cam 34 and the lever 32, in the release direction RL of the eccentric shaft 6. Fig. 2a The pressure relief valve 30 is shown in its closed position in the working direction RA of the eccentric shaft 6.

[0051] The lever 32 is movably connected at a connection point 32b to a valve closure 38 of the pressure relief valve 30. The valve closure 38 regulates the flow of hydraulic fluid 44 from the cylinder chamber 14a to the reservoir 40 via lines 42 and 26. The valve closure 38 is biased into its closed position by a spring 39. The lever 32 is thereby pressed against a stop 36 by the force of the spring 39. The lever 32 can pivot about a pivot point 32c at its end opposite the lever head 32a. At the pivot point 32c, the lever is rotatably mounted to the hydraulic power transmission unit 10.

[0052] If the electric motor 2 is controlled by the control unit 50 in the release direction RL at the end of the pressing process, as shown in Fig. 2bAs shown, the cam 34, during its rotational movement, abuts the head 32a of the lever 32. Depending on the initial position of the cam 34, this occurs after a maximum of one revolution of the eccentric shaft 6. With further rotation of the eccentric shaft 6, and thus of the cam 34, in the release direction RL, the cam 34 moves the lever 32 against its preload. Fig. 2b to the right, which causes the lever 32 to move the valve closure 38 into its open position, thereby opening the pressure relief valve 30 by motor. This allows the pressurized hydraulic fluid 44 to flow from the cylinder chamber 14a back into the reservoir 40.

[0053] If a single opening of the pressure relief valve 30 is insufficient to return the working piston 12 to its initial position, further rotations of the electric motor 2 and the eccentric shaft 6 in the release direction RL allow additional hydraulic fluid 44 to flow from the cylinder chamber 14a into the reservoir 40. By appropriately controlling the electric motor 2 via the control unit 50, the eccentric shaft 6 can also be stopped in a position where the pressure relief valve 30 is open. This results in a faster retraction of the working piston 12 and further savings in time and energy.

[0054] The pressure sensor 52 can measure when the working piston 12 reaches its starting position, or when the hydraulic pressure in the first area 14a of the cylinder chamber 14 is released, and this information can be transmitted to the control unit 50. The control unit 50 can then cause the electric motor 2 to stop completely. The press 1 is then back in its open starting position and can be used immediately for another pressing operation.

[0055] Particularly when crimping soft workpieces, such as copper or plastic fittings, it is advantageous to terminate the crimping process before the crimping machine 1 reaches its maximum crimping force. Nevertheless, a pressure relief valve can be provided to protect the crimping machine 1 from unwanted overpressure, e.g., at a maximum pressure of around 170 bar. The pressure relief valve 30 can also be designed to function as a conventional pressure relief valve in addition to its motorized opening function. For this purpose, the force of the spring 39 must be selected according to the desired opening pressure. Reference symbol list

[0056] 1 Press machine 2 Electric motor 4 Gearbox 6 Eccentric shaft 6a Eccentric 6b Ball bearing 7 Bearing 10 Hydraulic power transmission unit 12 Working piston 12a Working piston head 13 Seal 14 Cylinder 14a Cylinder chamber 16 Tool side 20 Piston pump 21 Spring 22 Piston 23 Line 24 Valve cover 26 Line 30 Pressure relief valve 32 Lever 32a Lever head 32b Connection point 32c Pivot point 33 Clutch, freewheel ball bearing 34 Cam 36 Stopper 38 Valve cover 39 Spring 40 Reservoir 42 Line 44 Hydraulic fluid 50 Control 52 Pressure sensor RA Working direction RLL Release direction

Claims

1. A press machine (1) for plastically deforming a workpiece, comprising: a. an electric motor (2); b. press jaws for applying a pressing force to a workpiece; c. a hydraulic force transmission unit (10) for transmitting the force of the electric motor (2) to the press jaws, comprising a piston pump (20) driven by the electric motor (2) rotating in the working rotational direction (RA) and a pressure relief valve (30); d. a coupling device (32, 33, 34) which mechanically couples the electric motor (2) to the pressure relief valve (30), so that the electric motor (2) opens the pressure relief valve (30) during operation in a release rotational direction (RL) opposite to the working rotational direction (RA); characterized by e. a cam (34) driven by the electric motor (2) via a clutch (33), wherein the clutch (33) acts only in the release rotational direction (RL) of the electric motor (2); and f. a lever (32) of the pressure relief valve (30) on which the cam (34) acts.

2. The press machine according to claim 1, wherein the clutch (33) comprises a freewheel and is preferably a freewheel ball bearing.

3. The press machine according to one of claims 1 or 2, further comprising an eccentric shaft (6) driven by the electric motor (2) with an eccentric (6a) for actuating the piston pump (20).

4. The press machine according to claim 3, wherein the cam (34) is mounted on the eccentric shaft (6) via the clutch (33) and is driven by the latter in the release rotational direction (RL).

5. The press machine according to one of claims 1-4, further comprising a controller (50) for controlling the electric motor (2) and at least one sensor (52-60) for measuring at least one pressing parameter, wherein the controller (50) is configured to detect the end of the pressing process based on the at least one pressing parameter and to operate the electric motor (2) in the release rotational direction (RL) opposite to the working rotational direction (RA) in order to open the pressure relief valve (30).

6. The press machine according to claim 5, wherein the controller (50) detects the end of the pressing process if switch-off criteria are met, wherein the controller (50) then reverses the rotational direction of the electric motor (2) from the working rotational direction (RA) to the release rotational direction (RL).

7. The press machine according to one of claims 5 or 6, wherein the at least one sensor (52-60) comprises: a. a sensor (56) which detects the revolutions of the electric motor (2); and / or b. a current sensor (60) which detects the supply current of the electric motor (2); and / or c. a pressure sensor (52) which detects the hydraulic oil pressure; and / or d. a force sensor (58) which detects a mechanical force characteristic of the pressing force; and / or e. a torque sensor (54) which detects a torque in the force transmission unit (10).

8. A method for operating a press machine (1), wherein the method comprises the following steps: a. operating a hydraulic force transmission unit (10) by an electric motor (2) rotating in a working rotational direction (RA); b. detecting the end of the pressing process by a controller (50) of the press machine (1); characterized by the following steps: c. after detecting the end of the pressing process, reversing the rotational direction of the electric motor (2) from the working rotational direction (RA) to the release rotational direction (RL); d. rotating a cam (34) driven by the electric motor (2) via a clutch (33), wherein the clutch (33) acts only in the release rotational direction (RL) of the electric motor (2); e. operating a lever (32) of the pressure relief valve (30) by the cam (34); and thereby f. opening a pressure relief valve (30) of the hydraulic force transmission unit (10) by rotating the electric motor (2) in the release rotational direction (RL).

9. The method according to claim 8, wherein the hydraulic force transmission unit (10) and press jaws of the press machine (1) move back into their end positions by opening the pressure relief valve (30).

10. The method according to one of claims 8 or 9, further comprising the step of detecting the end position of the force transmission unit (10) and switching off the electric motor (2) by the controller (10).

11. The method according to one of claims 8-10, wherein the end of the pressing process is detected by reaching one or more of the following switch-off criteria: a. increase in the supply current or reaching a predetermined supply current of the electric motor (2); b. increase in the pressing force or reaching a predetermined maximum pressing force; c. increase in the torque or reaching a predetermined torque; d. reaching a maximum number of revolutions of the electric motor (2); e. increase in the hydraulic pressure or reaching a predetermined maximum hydraulic pressure.

12. The method according to one of claims 8-11, wherein the pressure relief valve (30) of the hydraulic force transmission unit (10) closes again after reaching its end position.