EP1319475B1 - Method of operating a hydraulic pressing tool and hydraulic pressing tool - Google Patents

Abstract

Hydraulic pressing device comprises a hydraulic pump, a moving part (4), a fixed part (9), and a non-return valve (6) acting as an over pressure valve. The moving part runs from a starting position into a pressing position, the non-return valve moves automatically into an open position depending on a prescribed pressing pressure and the moving part moves back under the action of a restoring spring (5). Preferred Features: The non-return valve has a locking arrangement (10) in which a protrusion (12) interacts. The protrusion stays in its locking position under the spring tension.

Description

The invention relates firstly to a method for operating a hydraulic pressing device, in particular hand-pressing device, wherein the pressing device comprises a hydraulic pump, a moving part, a fixed part and a return valve, wherein further the moving part is moved by the construction of a hydraulic pressure in a Verpressstellung and the return valve automatically moves into an open position only at a predetermined, corresponding to an injection pressure hydraulic pressure, wherein further the moving part is adapted to automatically move under the action of a return spring of the injection position in an end position and the return valve is designed to close only after the Movement part has reached the end position.

The state of the art is on the WO 99/19947 the applicant. In addition to that EP 1121997 A1 and the GB 2057945 A ,

In the device known from the first-mentioned document, such a method has already been realized with great benefit and has found a broad application. As a rule, the embodiment is also advantageous and satisfactory. However, there are cases in which premature stopping of the moving part is desired without the movement of the moving part being prevented from reaching the end position in other cases. The object is therefore to provide a hydraulic hand-pressing device that allows an optional stopping of the moving part in one position.

This object is achieved in a method according to claim 1. The invention proposes that is acted on the return valve before reaching the end position of the moving part with a corresponding closure force.

This can optionally be done in any position of the moving part by direct action on the return valve. If necessary, also by hand. For this purpose, the fact can advantageously be exploited that the return valve has an operating section continuing to the outside of the device. On the user can act in the simplest case, for example, by hand with the required force. Thus, an interruption of the return in any position of the moving part readily possible. Optionally, via a lever translation, this action possibility for the user can also be conveniently moved in the area of the device handle. Approximately there, optionally via another button, where the trigger button of the device is arranged.

In further detail, it can also be provided that the return valve is locked until reaching a preselected or predetermined return position of the moving part. This makes it possible to provide said closure force approximately by means of a force acting on the return valve return spring high power. In the course of reaching the injection pressure, this return spring high force is automatically driven by acting on the moving part hydraulic pressure in a biasing position and locked there immediately. The return valve is thus basically exposed to the return of the moving part of this force, however, the catch captures this force until it is released by the user by an operation, or by the device itself, upon reaching a predetermined return position of the moving part. The action on the return valve takes place accordingly by releasing a previously stored locking force.

In an embodiment of the aforementioned known device, it is also possible to magnetically form the return valve and to generate said force characterized in that an electromagnet is switched on at a given time. The solenoid can then either pull the return valve into its closed position or, in the case of a opposite pole arrangement, push the return valve into its closed position. Further, it is also possible to make an electromechanical lock. In this case, by an electric actuator, for example. Again, an electromagnet, a mechanical locking part in the travel of the return valve, the open position, pushed. In the same way, this lock can then be pulled out again by electrical actuation.

However, the magnetic or electromagnetic action is obviously not or at least not necessary by the release of a previously stored locking force. Rather, it is done by voltage application of a corresponding electromagnetic or electromechanical arrangement.

It is advisable to integrate the operation for locking or canceling the locking of the return valve or the operation for the action of the required closure force, regardless of the position of the return part, in the already given operating handle of the device. In a simple way about, as already mentioned in principle, via a mechanical lever device which acts on the end of the return valve.

In an advanced embodiment it can be provided that the actuating button for starting the electric motor or the associated hydraulic pump is also provided with an action button on the locking part of the return valve at the same time.

In further concretization, the release button can be suspended like a lever and be provided with a recoverable against spring locking mandrel, which in turn intervenes against a spring force adjustable locking member which acts on the return valve, and this can overflow again.

In concrete terms, the invention proposes a hydraulic pressing device with a hydraulic piston running in a cylinder, wherein the hydraulic piston can be moved back against the force of a return spring.

To solve the problem of providing a more advantageous pressing device, in particular hand-pressing device, the invention proposes that the return of the hydraulic piston before reaching the starting position by a triggering device, acting on the return valve, is sustainable.

In a first embodiment, this can be achieved in that the triggering device consists of a train or pressure part connected to the return valve. The tensile or compression part penetrates not only the valve cylinder but also a usually outside of the valve, the device still surrounding housing. This can be acted about by hand. When the return valve in one embodiment according to the WO 99/19947 is pulled or pushed into the closed position during the return of the moving part, the hydraulic volume is prevented from further outflow. The movement part comes to a standstill. On a renewed Tripping operation of the electric motor towards the moving part can then be moved from the point where it was stopped again in the closed position.

With regard to a tensile part, for example, the tip of the closure valve may be formed as a rod passing through the housing. Of course, with appropriate sealing. In the known device, an extension of the return valve, which points in the opening direction and projects through the housing wall, is designed as a tensile part. About an associated here rocker part, which ultimately only passes through the outer housing cover, it can be used to open the return valve. This extension is now designed as a pressure part. It has a freely accessible end area on which the closure of the return valve can be acted.

In a further embodiment, it can also be provided that an electromagnet acts on the return valve. Such a hand-pressing device is operated in some embodiments with an accumulator. Or also with a direct connection to an electrical network, for example in an assembly hall. This can be used to provide an electromagnet associated with the return valve. Depending on the polarity with the return valve, the electromagnet can either push or pull it into the closed position.

In a further embodiment, it may be provided that the return valve is acted upon by a biasing force sufficient for moving into the closed position at any time during the return movement of the moving part. This can first, regardless of a possible method in the closed position at any time of the return of the moving part, from the point of importance and advantageous that the automatic return of the moving part in the least possible dependence on the pressure generated by acting on the return valve compression spring. This also with independence of the return valve from the pressure generated by the return spring.

In this aspect, the invention initially proposes that the open position of the return valve is locked until reaching a predetermined return position of the moving part. According to the invention it is provided that the open position of the return valve is mechanically locked. And until the moving part has moved to its original position, or a desired return position of the moving part.

In an embodiment, it can be provided that the return valve has a latching receptacle into which a locking projection engages when being locked. The latching receptacle is suitably provided rearwardly of the effective piston area of the return valve.

The retracting in the locking receptacle locking projection is suitably under spring tension.

With regard to the locking projection, a separate release part is provided, which moves out the locking projection from the locking receptacle. This depending on given mechanical conditions or pressure conditions, as explained in more detail below.

The release part can cooperate with a ramp of the locking projection. Suitably, a conical surface or another ramp-like surface is formed on the locking projection for this purpose. The Release part has a corresponding surface. It thus results in a Keilschieberwirkung.

The release part is mechanically acted upon by the moving part according to a first embodiment. The moving part is usually a piston of the pressing device. The release part is for mechanical loading in the direction of travel of the piston, so that the traversing piston or the moving movement part of this in a certain path portion of the travel, preferably at the end of the travel, mechanically actuated.

According to a second embodiment, the release part can be under spring tension. The spring bias drives the release part in the release position. The spring preload is exceeded by the pressure prevailing during the pressing and return of the moving part hydraulic pressure. Only after standstill of the moving part of the hydraulic pressure drops so far that the release part moves out through the imprinted spring bias from a trajectory of Arretierungsvorsprungs, which then because he is under spring tension, moves out of the locking receptacle of the return valve and this releases it to the closure ,

The release member may also be used to allow hand-operated removal of the locking projection from the locking position. For this purpose, in a simple version, the locking projection can be brought out of the housing at one end and pulled there by hand approximately against its biasing force into the locking position. The release part is then the rearwardly protruding portion of the locking projection.

But it can also be provided insofar as a separate release part, which, at least over a certain range of motion, form-fitting with the Locking projection cooperates. For example in the form of a seesaw. Upon movement of the release part of the locking projection is then necessarily pulled out of its locking position. In further detail, the release part for this purpose can also be coupled to an actuation switch of the device. For example, with the trigger or start switch of the device. Approximately in such a way that a repeated actuation of the start switch leads to a withdrawal of the locking projection from the locking position and thus to a stop of the moving part. A subsequent actuation of the start switch then leads again to a start of the electric motor or a work of the pump, so that a new pressing operation begins.

Fig. 1

eine Querschnittsansicht eines hydraulischen Verpressgerätes mit einer Einrichtung zum wahlweisen Schließen des Rücklaufventils;

Fig. 2

eine ausschnittsweise Querschnittsdarstellung des Verpressgerätes mit einer tasterbetätigten Auslösung zum Verfahren des Rücklaufventils in die Verschlussstellung, in einer ersten Stellung;

Fig. 3

eine Darstellung gemäß Fig. 2, beim Start des Gerätes;

Fig. 4

eine Darstellung folgend der Fig. 3, nach weiterem Durchdrücken der Starttaste und angelaufener Pumpe:

Fig. 5

folgend der Fig. 4 nach Erreichen des Auslösedrucks des Rücklaufventils und Abschalten der Pumpe; Freigabe der Rückbewegung der Starttaste und Verriegelung des Rücklaufventils in der Offenstellung;

Fig. 6

folgend der Fig. 5 Wieder-Unterfahren des Löseteils bei freigegebener Starttaste und in der Öffnungsstellung verriegeltem Rücklaufventil;

Fig. 7

eine Grundstellung eines zweiten Ausführungsbeispiels, das Bewegungsteil ist in seiner Anfangsstellung und das Rücklaufventil geschlossen;

Fig. 8

das zweite Ausführungsbeispiel, nachdem Verpressdruck aufgebracht worden ist und das Bewegungsteil in seine Verpressstellung verfährt; das Rücklaufventil ist geschlossen;

Fig. 9

einen Zustand des zweiten Ausführungsbeispiels in welchem das Rücklaufventil geöffnet hat und das Bewegungsteil zurückverfährt;

Fig. 10

die Ausgangsstellung bei einem dritten Ausführungsbeispiel, das Rücklaufventil ist geschlossen;

Fig. 11

einen Zustand des dritten Ausführungsbeispiels, bei welchem Verpressdruck aufgebaut wird, das Rücklaufventil ist geschlossen;

Fig. 12

einen weiteren Zustand des dritten Ausführungsbeispiels, das Rücklaufventil ist geöffnet und das Bewegungsteil fährt in Richtung seiner Ausgangsstellung zurück.

In the following, the invention will be explained further with reference to the attached drawing, which, however, merely shows exemplary embodiments. Hereby shows:

Fig. 1

a cross-sectional view of a hydraulic Verpressgerätes with means for selectively closing the return valve;

Fig. 2

a partial cross-sectional view of the pressing device with a button-actuated release for moving the return valve in the closed position, in a first position;

Fig. 3

a representation according to Fig. 2 when starting the device;

Fig. 4

a representation following the Fig. 3 , after further pressing the start button and started pump:

Fig. 5

following the Fig. 4 after reaching the release pressure of the return valve and switching off the pump; Releasing the return movement of the start button and locking the return valve in the open position;

Fig. 6

following the Fig. 5 Re-lowering the release part with released start button and in the open position locked return valve;

Fig. 7

a basic position of a second embodiment, the moving part is closed in its initial position and the return valve;

Fig. 8

the second embodiment, after Verpressdruck has been applied and moves the moving part in its Verpressstellung; the return valve is closed;

Fig. 9

a state of the second embodiment in which the return valve has opened and the moving part zurückverfährt;

Fig. 10

the starting position in a third embodiment, the return valve is closed;

Fig. 11

a state of the third embodiment in which pressure is built up, the return valve is closed;

Fig. 12

another state of the third embodiment, the return valve is open and the moving part moves back in the direction of its initial position.

Shown and described is first in Fig. 1 a section of a hydraulic pressing device, as with the exception of the features described here and in more detail, for example. In the WO 99/19947 is shown. To explain the construction and operation of the return valve and the pressing device in general, and for the other embodiments described herein, is also incorporated in full to the cited document, also for the purpose of this document in claims of this application, incorporated herein by reference.

There are two also separately from each other having having embodiments in the Fig. 1 added.

First of all, it is important that the return valve 1 has an outward pressure on the device 2 pressure paragraph. With this, the return valve 1 can be brought from its open position optionally, in each position of the moved back by the spring 3 moving part 4, here a hydraulic piston, in its closed position. The movement part 4 or the hydraulic piston is then stopped at the point to which it is at this time.

Additionally or alternatively, an electromagnet 5 may be provided, which also acts on the return valve 1. In the exemplary embodiment, coil windings 6 are provided which, when electrically acted upon, magnetically have an associated section, for example the section 7 of FIG Return valve cooperate. In that case a pulling effect is given. It can just as well be formed a pressure effect. Also, the penetrating portion of the return valve may be used as an armature moved by the magnet.

The embodiment of the Fig. 2 to 6 basically refers to a hydraulic pressing device as it is called by the WO 99/19947 is known. Here, however, the return valve 1 is constantly biased in the case of the embodiment by a compression spring 20 with such a force in the closed position that regardless of the pressure exerted by the moving part 4 when returning pressure, the return valve would move at any time in the closed position. This is prevented by a Arretierungsvorsprung 8, which, as further below in further detail with respect to the embodiments of Fig. 9 to 12 explained, enters a corresponding recess of the return valve.

It is important now that the locking projection 8 is led out of the back of the device body 9 and forms a Beaufschlagungsabsatz 10. This is under attack by a release part 11, which in turn is mounted like a rocker about a pivot point 12 on the device body 9. At the same time, the release part 11 or the holder 13 of the release part 11 is rigidly connected to a release button 14 of the device. The trigger button 14 acts on actuation of an electric switch 15th

Specifically, this is described below with reference to Fig. 3 to 6 further explained.

The Fig. 2 shows the state of the switched-off electric motor. The hydraulic groover is at rest. The return valve 1 is located in a Opening position and is locked by the locking projection 8 herein. The moving part 4 is in a fully retracted position.

In Fig. 3 the state is shown that the user wants to perform a compression. He begins to act on the trigger button 14. This has, against the action of a compression spring 16, which abuts the other end of a fixed housing part 17, moved in the direction of the electric switch 15. The release projection 11 in this case moves on a circular path around the joint 12. It has already partially pulled out the locking projection 8 from its locking position.

Furthermore, cf. Fig. 4 , the user has by pressing the trigger button 14 on the one hand, the electric switch 15 is actuated, so that the electric pump starts and the hydraulic chamber 18 is filled with hydraulic fluid. At the same time, however, the release projection 8 has been excavated so far that the return valve 1 is moved under the action of the compression spring 7 in the closed position. The hydraulic piston or the movement part 4 has already moved away from its end position, it is in Fig. 4 no longer visible. Since in further detail the release member 11 can be moved back against the action of a compression spring 19 from its foremost position, when releasing the trigger button 14, as in the Fig. 5 and 6 shown, the release member 11 again snap under the projection 10, without thereby the locking position of the locking part 8 is impaired.

Subsequent to the position according to Fig. 6 then takes the release part again the position of Fig. 2 a, but with the moving part is moved further in the direction of Verpressstellung, by the constantly running pump. As soon as the injection position has been reached, the pump switches itself off automatically.

This even if, as a rule, the release button 14 is still pushed. At the same time the return valve 1 moves automatically in its open position, against the action of the compression spring. 7

The Fig. 7 to 9 refer to a second embodiment, which relates only to the locking of the acted upon by an overpressure in the open position return valve. Overpressure here means that the pressure is higher than it is needed, for example, only at the end of the return path of the moving part to close the return valve. Rather, it is so high that it is suitable in any position of the return of the moving part to close the return valve.

Here too, the hydraulic pressing device 21 has a hydraulic pump (not shown), by means of which hydraulic fluid, usually oil, into the hydraulic chamber 18, cf. also Fig. 8 , is pumped. By means of the hydraulic pressure is a moving part 4, here a piston, against the action of a return spring 3 movable. The hydraulic pressure is built up so long until a predetermined compression pressure is reached at which the same time acting as a pressure relief valve return valve 1 opens.

The return valve 1 has for this purpose an initially effective comparatively small valve surface, which corresponds in the embodiment given by the bore 23 cross section.

This cross-section requires to release the return valve 1 as high a pressure as it corresponds to the pressure. After lifting the return valve 1, the given by the diameter of the piston portion 24 larger area comes into effect. Therefore, the return valve becomes 1 after its release by a much lower pressure - still - held in the open position.

The movement part 4 is moved relative to a fixed part 25. The terms moving part and fixed part can also be related to not shown in detail elements of the working range of the pressing device. The moving part or the moving parts are for this purpose, for example, together jaws or the moving part is a knife or a die, which moves against a stop or a fixed counter-mold.

In the course of building up a pressure, which state in Fig. 8 is shown, the hydraulic chamber 18 is acted upon by increasing increasing pressure, by the aforementioned pump, not shown, and the piston of the moving part 4 is acted upon by increasing pressure until reaching a compression pressure.

The return valve 1 has a latching receptacle 26, which is formed in the embodiment by a circumferential groove in a piston portion of the return valve 1. It is further provided a locking part 27 which is biased under the action of a spring 28 to the return valve 1 point. The locking member 27 has a locking projection 8, which moves into the locking receptacle 26 in its open position in the process of the return valve 1.

The return valve 1 is in turn under bias in its closed position by the action of the spring 20th

The back of the return valve 1, in extension of a shaft of the return valve 1, a handle 29 is further formed, with which the Return valve can be moved by hand in its open position. However, it can not be closed by hand when engaging the Arretierungsvorsprungs 8.

The locking member 27 in turn cooperates with a release member 11 which is formed as a valve piston. The release member 11 has an urging projection 30 which projects into the travel of the moving member 4, here the piston.

The release part 11 further has an actuating tip 31 which cooperates with an actuating formation 32 on the locking part 27. In the embodiment, the operating tip 31 and the Betätigungsausformung 32 are each cone-shaped.

When starting from the starting position in Fig. 7 Oil is pumped into the hydraulic chamber 18 to move the moving part 4, the moving part 4 moves against the action of the spring 3, as in Fig. 8 is shown. Uniformly with a release of the actuating projection 30 moves the locking member 27 by the action of the spring 28 to the locking projection 8 abuts the peripheral surface of the piston portion 33 which is formed in the closing direction of the return valve 1 in front of the locking receptacle 26. The piston portion 33 corresponds in diameter to the free diameter of the cylinder 34, in which the return valve 1 moves.

In the starting position according to Fig. 7 is the locking projection 8 by the action of the spring 3, which presses the movement part 4 on the Beaufschlagungsvorsprung 30 and so the Löseteil 11 zurückverfährt, retracted under the formation of a free edge 35 of the piston portion 33.

If now according to Fig. 8 the hydraulic pressure is built up to the compression pressure is, with overcoming bias, as off Fig. 8 can be seen, the Arretierungsvorsprung 8 is moved up to the peripheral piston surface of the piston portion 33. Taking into account the angular relationship between the release part and the locking part, given by the conical surfaces 31 and 32, the bias of the Arretierungsvorsprungs 8 on the piston portion 33 is so high that the pressure in the cylinder chamber 18 up to the pressure of the action of the spring 28 is exceeded.

The latter is not necessary. It could also be that the effect of the spring 28 is much lower. Then, the lock would not move in the course of the back process of the moving part 4, under a much lower pressure in the cylinder chamber 18, the locking projection into the locking recess 26.

As soon as the pressure has been reached and the return valve 1 has opened, this state is in Fig. 9 shown, the locking receptacle 26 is in register with the locking projection 8 so that it retracts by the action of the spring 28 (or, as indicated above, retires later in time, when the pressure in the cylinder chamber 8 has fallen so far that the effect of Spring 28 is sufficient, but the moving part 14 is not yet back to the stop process).

In this way, further, the release part 11, which is not under any spring bias, but only by the spring 28 and in the cylinder chamber 18th prevailing oil pressure, further back process. It is thus still further in the trajectory of the moving part 4.

The oil from the cylinder chamber 18 can flow through the indicated line 36, which is released by the piston section 24 when the return valve is opened, into the reservoir 22.

Now, when the moving member 4 abuts on the end face of the urging projection 30 of the release member 11, this is pushed by the action of the spring 3 in the direction of the locking member 27 and thus moves over the gear surfaces 31, 32, the locking member 27 against the action of the spring 28th back.

The return valve 1 can then by operation of the spring 20 back into its closed position according to Fig. 7 respectively. Fig. 8 method.

The third embodiment is in the 10 to 12 shown.

Here, in principle, the locking part described above is functionally combined with the release part.

The return valve 1 corresponds to the return valve 1 described above. In this respect, reference is made to this.

The same thing is the fixed part 25 with the moving part 4 with acting thereon return spring 3 the same.

In the presentation of Fig. 10 the basic position is shown, with the return valve 1 closed and with the locking projection 8 in a distance to the associated piston portion 33 of the return valve. 1

The locking projection 8 is in the embodiment of the 10 to 12 initially under bias by the spring 36. The locking projection 8 is connected to a directly acted upon by the force acting in the cylinder chamber 18 hydraulic pressure piston part 37. Next, however, the Arretierungsvorsprung within the piston member 37 is movable. For this he stands within the piston member 37 under the action of a spring 38 which biases the locking projection 8, in response to a force acting on the piston member 37 pressure against the return valve 1.

If, according to the Fig. 8 described state, in the second embodiment, a compression pressure is built up, see. Fig. 12 , the piston part 25 is exposed to this - increasing - pressure. This compression pressure overcomes the action of the spring 36, so that the piston member 37 is moved together with its shaft 39 to the return valve 1 back.

The piston part 37 can be moved until the shaft 39 comes to the stop 40 at the end 40 of the associated cylinder bore 41. In this state, the end face of the Arretierungsvorsprunges 8, which is associated with the return valve 1, already encountered on the circumference of the piston portion 33 of the return valve 1. In the course of the increasing process of the piston member 37 of the locking projection 8 is moved back, the spring 38 comes into effect.

For this purpose, the locking projection 8 passes through a bore 42 in the piston section 39 of the piston part 37.

In the hollow piston portion 39 of the locking projection 8 is movably guided by means of a piston portion 43, on whose side facing away from the return valve 1, the spring 38 acts.

As soon as the predetermined compression pressure is reached and as a result the return valve 1 is moved into its open position, the locking projection 8 enters the latching recess 26, in principle as already described in the second exemplary embodiment. This condition is in Fig. 12 shown. The further prevailing pressure in the cylinder chamber 18, caused by the spring 3, holds the locking projection 8 in the locked position according to Fig. 12 , Only when the moving part 4, in this case the piston, comes to rest on the end wall 44 of the cylinder chamber 18 does the pressure drop in the cylinder chamber 18 so that the piston part 37 moves into the position according to the action of the spring 36 Fig. 10 moves. As a result, the locking projection 26 is moved out, so that due to the action of the spring 20, the return valve in its closed position, according to Fig. 10 can be moved again.

Claims (19)

1. A method of operating a hydraulic pressing unit, in particular a manual pressing unit, the pressing unit having a hydraulic pump, a moving part (4), a stationary part and a non-return valve (1), furthermore, the moving part (4) being displaced into a pressing position by the build-up of a hydraulic pressure and the non-return valve (1) moving automatically into an open position only in the presence of a predetermined hydraulic pressure corresponding to a pressing pressure, and, furthermore, the moving part (4) being configure for moving back automatically from the pressing position into an end position under the action of a restoring spring, and the non-return valve (1) being configured only to close once the moving part (4) has reached the end position, characterized in that during the return movement of the moving part (4) the return valve is subjected to the action of a necessary closure force, when a pre-selected return position is reached or a predetermined return position of the moving part (4) is reached.
2. A method according to Claim 1, characterized in that there is effected a mechanical latching of the non-return valve (1) in the open position.
3. A method according to Claim 1, characterized in that there is effected a magnetic latching of the non-return valve (1) in the open position.
4. A method according to Claim 1, characterized in that there is effected an electromechanical latching of the non-return valve (1) in the open position.
5. A hydraulic pressing unit, in particular a manual pressing unit, the pressing unit having a hydraulic plump, a moving part (4), a stationary part and a non-return valve (1), the moving part (4) being displaceable into a pressing position by the build-up of a hydraulic pressure and the non-return valve (1) moving automatically into its open position only in the presence of a predetermined hydraulic pressure corresponding to a pressing pressure, and the moving part (4) being configured for moving back automatically from the pressing position into an end position under the action of a restoring spring, and the non-return valve (1) being configured only to close once the moving part (4) has reached the end position, characterized in that it is possible to stop the return of the moving part (4), before it reaches the end position, by a triggering device acting on the non-return valve (1).
6. A pressing unit according to Claim 5, characterized in that the triggering device comprises a pulling or pushing part connected to the non-return valve (1).
7. A pressing unit according to either of Claims 5 and 6, characterized in that an electromagnet acts on the non-return valve (1).
8. A pressing unit according to any of Claims 5 to 7, characterized in that the non-return valve (1) is subjected to the action of a prestressing force which is sufficient for the displacement into the closed position at any point in time during the return movement of the moving part (4).
9. A pressing unit according to any of the preceding Claims 5 to 8, characterized in that the open position of the non-return valve (1) is latched until a predetermined return position of the moving part (4) has been reached.
10. A pressing unit according to Claim 9, characterized in that the open position of the non-return valve (1) is latched mechanically.
11. A pressing unit according to any of the preceding Claims 5 to 10, characterized in that a separate releasing part provided for the purpose of releasing a latching protrusion, the releasing part moving the latching protrusion out of the latching seat.
12. A hydraulic pressing unit having a hydraulic pump, a moving part (4) and a stationary part (9) and a non-return valve (6), the moving part (4) moving from a starting position into a pressing position, the non-return valve (6) moving automatically into an open position in dependence on a hydraulic pressure corresponding to the pressing pressure, and the moving part (4) moving back under the action of a restoring spring (5), characterized in that the open position of the non-return valve (6) is latched until a predetermine return position of the moving part (4) has been reached.
13. A pressing unit according to Claim 12, characterized in that the non-return valve (6) has a latching seat (10) into which a latching protrusion (12) engages for latching action.
14. A pressing unit according to Claim 13, characterized in that the latching protrusion (12) is subject to spring prestressing into the latching position.
15. A pressing unit according to either of Claims 13 and 14, characterized in that a separate releasing part (16) is provided, which moves the latching protrusion (12) out of the latching seat (10).
16. A pressing unit according to Claim 15, characterized in that the releasing part (16) interacts with a ramp of the latching protrusion (12).
17. A pressing unit according to either of Claims 15 and 16, characterized in that the moving part (4) can act on the releasing part (16).
18. A pressing unit according to any of Claims 15 to 17, characterized in that the moving part (4) is a piston, and that the releasing part (16) projects into the displacement path of the piston.
19. A pressing unit according to any of Claims 15 to 18, characterized in that the releasing part (16) is subject to spring prestressing into its release position, and that the spring prestressing is exceeded by the hydraulic pressure which prevails during pressing and return of the moving part (4).

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