Abstract:
A manually operated press comprises an actuation lever coupled to a shaft assembly. An actuation of the actuation lever is transformed into a stroke movement of a press ram coupled to the shaft assembly and, accordingly, into a change of a relative displacement position of the press ram. A clutch assembly is provided for interrupting a flow of force between the actuation lever and the press ram. The shaft is at least two-piece, namely configured with an input shaft and an output shaft. The clutch is adapted to separate the input shaft from the output shaft depending on the pressing force and/or the relative displacement position of the press ram. The shaft assembly comprises an input shaft and an output shaft. The input shaft extends as an inner shaft through the output shaft being configured as a hollow shaft. The clutch assembly has a first clutch designed as a stroke stop for immobilizing the input shaft with a housing of said press depending on pressing force and/or relative displacement position of the press ram, and a second clutch designed as an overload clutch for interrupting a flow of force between the input shaft and the output shaft.

Description:
CROSS REFERENCE TO OTHER APPLICATIONS 
       [0001]    The present application is a continuation of pending International patent application PCT/EP2006/006717, filed Jul. 10, 2006 which designates the United States and was published in German, and which claims priority of German patent application 10 2005 034 424.0, filed Jul. 13, 2005. The disclosure of the above applications is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is related to the field of manually operated presses which are conventionally used for pressing workpieces together. 
         [0003]    The invention, further, is related to a method for protecting a manually operated press against mechanical overload and for aborting an insufficiently executed pressing operation. 
         [0004]    More specifically, the invention is related to a manually operated press comprising an actuation member coupled to a shaft, an actuation of the actuation member being transformed into a stroke movement of a press ram coupled to the shaft and, accordingly, into a change of a relative displacement position of the press ram, and a clutch for interrupting a flow of force between the actuation member and the press ram. 
       BACKGROUND OF THE INVENTION 
       [0005]    U.S. Pat. No. 7,080,595 B2 discloses a manually operated press into which a backstroke inhibitor is electronically implemented, such as known e.g. from U.S. Pat. No. 6,755,124 B2. 
         [0006]    Manually operated presses of the kind described above are conventionally used for piece-work workplaces. As in manually operated presses the exerted force increases towards the end of the pressing stroke, some operating persons tend to exert too much force and, thereby, produce pressed workpieces of bad quality or even damaged workpieces. 
         [0007]    For many conventional manually operated presses no documentation is produced, in contrast to automatically executed pressing operations for which numerically controlled presses are conventionally used. The lack of documentation, however, is nowadays no longer acceptable for many fields of application, in particular when production processes are to be certified under the ISO 9000 standard. 
         [0008]    In order to make sure that one can distinguish between “good” and “bad” parts, European patent specification 0 960 017 B1 teaches to provide a press with a sensor for the pressing stroke (displacement position) as well as with another sensor for the pressing force for generating a displacement-force diagram for any pressing operation being characteristic for a good and for a bad pressing. If a measured displacement-force curve lies within a given tolerance band, then the respective part is identified as well pressed. If not, the part is identified as a bad part that must be disposed of. 
         [0009]    In order to be able to determine the pressing force exerted during a pressing operation, e.g. on a Seeger circlip ring, a bearing, a pinion, a sealing etc., a press ram of the press is configured as a force sensor. The force sensor has a force measuring system, for example a strain gauge strip, integrated therein. The strain gauge is connected to a press control unit of the press which, in turn, may be connected with a rotation sensor, for example, for sensing the rotation angle of the actuation lever and, hence, the ram displacement position. The control unit then processes the sensed data to enable the above-mentioned differentiation between good and bad parts once the pressing operation is executed. 
         [0010]    If the examination shows that a bad part was pressed, the press may switch off automatically when the bad part is still within the press. This is likewise made upon a respective command from the control unit. 
         [0011]    To start with, there is a first problem that during conventional manual pressing operations, in contrast to automatically executed pressing operations, there may unintentionally occur high pressing forces which, for example, are caused by negligence of the operating person. High pressing forces may likewise occur when the parts which are to be pressed together, are already sufficiently joined, however, the mechanical final position of the pressing stroke has not yet been reached. In such a situation, the operating person “feels” that the actuation lever may still be moved further in the pressing direction, and, therefore executes such movement to its end. In such a case a too high pressing force may be executed resulting in a “bad” pressing. 
         [0012]    It is, therefore, necessary that the exerted pressing force be measured as precisely as possible in order to be able to optimally execute the quality examination based thereon. For that purpose highly sensitive force measuring systems are used which, however, are destroyed or at least damaged at too high mechanical overloads. Moreover, high overloads may occur from time to time that exceed admissible overload specifications by 100 or 200%. 
         [0013]    As already mentioned above, the prior art teaches to record the force as a function of the displacement position for making a quality evaluation (good/bad) once the pressing operation is finished. As the pressing operation is effected manually, each pressing operation is effected with different pressing force. For that reason some work pieces which shall be pressed together, may already be joined sufficiently “well” before the press ram has reached its mechanical final position, or some work pieces may be sufficiently joined only at the final position. In the event that the sufficient joining has already been achieved prematurely, it would be desirable to abort the pressing operation before the final (mechanical) press ram position has been reached. 
       SUMMARY OF THE INVENTION 
       [0014]    It is, therefore, an object underlying the invention to improve a manually operated press of the type specified at the outset such that the above-discussed problems are avoided. In particular, a manually operated press shall be provided in which admissible overloads can be limited to a predetermined threshold value, and in which a force sensor is protected against overload. Moreover, the pressing operation shall be aborted prematurely in the event that the desired pressing force has been reached prematurely. “Bad” pressings shall be avoided. 
         [0015]    In a press of the type specified at the outset, this object is achieved according to the invention in that the lever shaft is at least a two-piece construction, namely configured with an input shaft and an output shaft, and that the clutch is adapted to separate the input shaft from the output shaft depending on the pressing force and/or the relative displacement position of the press ram. 
         [0016]    The object underlying the invention is, thereby, entirely solved. 
         [0017]    Due to the fact that in contrast to the prior art the shaft of the invention is configured two-piece, one now has, unlike before, the option to interrupt the flow of force between the actuation member and the press ram at will, namely independent from the effective actuation of the actuation member. The prior art until now only teaches to brake or to immobilize a one-piece shaft upon occurrence of a failure, in particular when the back stroke is initiated prematurely. 
         [0018]    According to the present invention the flow of force between the press ram and the actuation lever can be interrupted at any moment in time by means of the clutch, namely by separating the shafts from each other. In the event that a desired pressing force is exerted already before a (mechanical) final position of the pressing operation has been reached, the shafts may be separated from each other depending on that event. If an inadmissibly high pressing force is exerted during a pressing operation which would damage a force sensor or would result in a “badly” pressed work piece, the shafts could likewise be separated from each other, again—depending on these events. 
         [0019]    For that purpose it is advantageous to additionally provide a first sensor for sensing the pressing force and/or a sensor for sensing the relative position of the press ram. 
         [0020]    If only a pressing force sensor is provided, a force measuring system of the press can be protected against overload. Should only a sensor for sensing the relative position of the press ram be provided then one can determine from the executed stroke displacement which pressing force was exerted, provided that all required further parameters as needed therefore are known, as, for example, the transmission of the lever movement into the press stroke, properties of the work pieces to be pressed together, etc. 
         [0021]    If both sensors are used in combination, one can record a force vs. displacement curve for each pressing operation so that it is possible to make a good/bad distinction already during the course of the pressing operation. In particular, one may determine when a “good” pressing has occurred. If the force is recorded vs. the displacement, one can, for example, decide by means of a higher level control that the pressing operation shall be aborted already before a (mechanical) final position of the press ram has been reached, because a desired pressing force has been reached. In such a way “bad” pressings are generally avoided. 
         [0022]    Preferably, a control unit is provided for that purpose which is coupled to the first and/or the second sensor and also to the clutch for supplying respective clutch signals to the clutch. 
         [0023]    The control unit samples the force sensor(s) for outputting clutch signals to the clutch as a response to signals produced by the sensors. 
         [0024]    The control unit, preferably, outputs a clutch signal for keeping the clutch closed when the sensed pressing force or the sensed relative displacement position of the press ram is smaller than a predetermined threshold value. 
         [0025]    When the clutch is open, no flow of force may occur between the actuation member and the press ram. If the actuation member is not in its predetermined initial position, a pressing operation is not allowed at all due to the open clutch. This enhances pressing safety. 
         [0026]    It is, further, preferred, when the control unit supplies a clutch signal for opening the clutch when the pressing force or the relative displacement position of the press ram is greater than or equal to a predetermined threshold value. The threshold value may be an admissible maximum pressing force, at which a force sensor is not damaged, and/or may be a minimum pressing stroke displacement at which a “good” pressing is obtained. 
         [0027]    The control unit is, in particular, provided with means for determining whether a predetermined pressing force limit was exceeded or a desired pressing force has been reached. If the limit is exceeded, then a signal for opening the clutch is generated. Thereby, the flow of force between the actuation member and the press ram is interrupted. 
         [0028]    According to a preferred embodiment of the invention, the manually operated press is provided with a stroke stop for immobilizing the input shaft, wherein the stroke stop, in particular, comprises a brake disc and a brake magnet. 
         [0029]    With a stroke stop so configured the actuation of the actuation member may be immobilized in the forward as well as in the backward direction. The actuation lever is rigidly connected with the input shaft, such that an immobilization of the input shaft results in an immobilization of the actuation lever. 
         [0030]    In a preferred configuration of the stroke stop using a brake disc cooperating with a brake magnet, the brake disc is, preferably, secured against rotation to the input shaft, and the brake magnet is secured in a stationary manner to the press. Considering that the stroke stop in that case is an electrically operated brake assembly, the brake assembly may likewise be controlled by the above-mentioned control unit, by sending corresponding signals from the control unit to the stroke stop or its elements. 
         [0031]    Further, it is preferred when a third sensor is provided for sensing the relative displacement position of the input shaft, wherein the brake disc may be configured such that the third sensor senses the relative displacement position in cooperation with the brake disc. 
         [0032]    By means of the third sensor one can generate a signal according to which the clutch is closed, provided that the actuation member is in its corresponding initial position. The initial position may be sensed by means of the brake disc or a disc flange, being connected to the input shaft for rotation therewith and, hence, also to the actuation member. Thereby it is always guaranteed that the flow of force is only established between the actuation lever and the press ram when the actuation lever is in its initial position. Hence, it is guaranteed that the displacement that can be made by the actuation lever is sufficient to effect the press stroke required for making a sufficient pressing operation. In particular, the clutch is closed only when also the press ram is in its initial position. 
         [0033]    Further, it is advantageous when a return assembly, in particular a spring, is provided being coupled to the input shaft. 
         [0034]    By this measure one may effect that the actuation member is automatically moved back into its initial position, in particular when the clutch is separated, i.e. the flow of force between the actuation member and the press ram is opened and the operating person may have released the actuation lever. For an automatic return movement of the actuation lever it is, of course, necessary that the stroke stop is not immobilized. 
         [0035]    According to another preferred embodiment of the invention, the actuation member is a manually operable lever, the input shaft is an inner manual lever shaft and the output shaft is an outer hollow shaft. 
         [0036]    By this measure one can obtain a manually operated press with short dimensions because the input shaft constitutes an inner shaft being arranged coaxially to the outer hollow shaft. 
         [0037]    In particular, the second sensor may be a linear incremental displacement position measuring system sensing displacement position marks coupled to the press ram. 
         [0038]    By coupling the position marks to the press ram, the measurement of the effected displacement is made on the press ram without any inaccuracies caused e.g. by transmissions, in contrast to the prior art where the displacement is sensed by means of a rotary sensor at the input shaft. 
         [0039]    Here, too, it is advantageous when the two shafts are interconnected by the clutch in a form-fitting manner for transforming the stroke movement. By the additional form-fitting connection one can constitute a mechanical overload protection in which spontaneous overloads of (very) high intensity can be absorbed before they destroy a force sensor. 
         [0040]    For that purpose one preferably uses a toothing having a latch position being configured such as to open automatically from a closed state at a predetermined torque that is effected via the actuation member. The latch position is, preferably, reached when the shafts are in their respective initial positions. 
         [0041]    If, suddenly, an inadmissibly high torque appears at the shafts being coupled by the toothing, an automatic separation of the coupling is effected by this kind of coupling. The closing force exerted by the clutch is no more sufficient for compensating a decoupling force caused by the toothing. In that case the clutch opens spontaneously, i.e. without the intervention of a higher level control, for interrupting the flow of force. 
         [0042]    It will be understood that the features of the invention mentioned above and those yet to be explained below can be used not only in the respective combination indicated, but also in other combinations or in isolation, without leaving the scope of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]    Exemplary embodiments of the invention are explained in more detail in the following description and are represented in the drawings, in which: 
           [0044]      FIG. 1  shows a highly schematical side elevational view (partially broken away) of an embodiment of a manually operated press according to the present invention; 
           [0045]      FIG. 2  shows a latch toothing of a first and of a second shaft section according to the present invention; and 
           [0046]      FIG. 3  shows a schematic force ratio at the location of the toothing according to  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0047]    In  FIG. 1  reference numeral  10  as a whole designates a manually operated press of essentially known design. 
         [0048]    Press  10  has a base member  12  standing on an appropriate base, for example a work bench. Posts  14  extend upwardly from base member  12  to a head member  16 , also referred to as shifting member because head member  16  is adapted to be adjustable in the direction of posts  14  depending on the desired stroke. An actuation lever  18  is arranged laterally at head member  16  and is connected to a shaft  22  journalled within shifting member  16  by means of bearings  20 . Shaft  22  is adapted to be rotated about its axis  24  by actuation lever  18 , as indicated by an arrow  26 .  FIG. 1  shows the initial position of lever  18 . 
         [0049]    A transmission  28  shown extremely schematically is provided within shifting member  16 . Transmission  28 , in the simplest case, may be a pinion-rack assembly. The assembly is provided for transmitting the rotary movement of shaft  22  into a vertical stroke movement of a press ram  32  as indicated by an arrow  30 . 
         [0050]    During a pressing operation, the lower front surface  34  of press ram  32  comes to rest on an upper work piece  36  of a pair of work pieces  36 ,  38  which shall be pressed together. 
         [0051]    Press  10  is provided with a brake disc  40  being rigidly connected for rotation with shaft  22 . Immediately adjacent brake disc  40  there is provided a second disc  42  configured as a ring for allowing shaft  22  to be guided through second disc  42 . Second disc  42  is rigidly connected to shifting member  16  and, therefore, is hereinafter referred to as stationary. 
         [0052]    Reference numeral  44  designates a first clutch having a brake magnet being electrically connected to an electronic control unit  46 . Control signals may be fed from outside, for example from a numerical control unit, to electronic control unit  44  via inputs  48  for constituting an electronic back stroke stop as disclosed in U.S. Pat. No. 7,080,595 B2. This function is likewise possible with the present press  10 . For further details reference is made to U.S. Pat. No. 7,080,595 incorporated herein by way of reference. 
         [0053]    The stroke stop configured by first clutch  44  is essentially characterized by the magnet clutch effect of electro magnet exerted on clutch discs  40  and  42 . This “magnetic brake” may, however, be likewise constituted by a pneumatic or a hydraulic brake interconnecting discs  40  and  42  in a frictional manner for preventing a rotation of shaft  22  about its axis  24 . 
         [0054]    Press ram  32  may be provided with a force sensor  49  by which the actually effective pressing force may be sensed. Force sensor  49 , too, is connected to electronic control unit  46 . Press ram  32  may insofar serve as a force sensor for a strain gauge strip integrated therein. Sensor  49 , however, may likewise be provided at another location, for example at base member  12 . 
         [0055]    Other force measuring systems, for example inductive force sensors or magnetoelastic force sensors or piezo-electric sensors etc. may likewise be used. 
         [0056]    Insofar, press  10  is of conventional design. 
         [0057]    In contrast to prior art presses, press  10  is provided with a second (outer) hollow shaft  50  being journalled coaxially to shaft  22 . First shaft  22  is hereinafter referred to as the inner manual lever shaft because it is guided through outer hollow shaft  50 . Outer hollow shaft  50  is directly connected to a toothed wheel or pinion constituting the transmission designated  28 . Press ram  32  has corresponding teeth  52  meshing with the pinion or toothed wheel of outer hollow shaft  50 . Preferably, the teeth of outer hollow shaft  50  and of press ram  32  engage one another directly. However, one could also provide further transmission elements therebetween. 
         [0058]    Outer hollow shaft  50  can be connected to inner manual lever shaft  22  in a form-fitting or a frictional manner via a second clutch  54 , preferably an electromagnetic clutch. Second clutch  54  is also connected to electronic control unit  46 . 
         [0059]    Inner manual lever shaft  22  constitutes an input shaft that is coupled to outer hollow shaft  50  via second clutch  54 . 
         [0060]    The operation of press  10  according to the present invention shall now be explained in further detail. 
         [0061]    By means of a third sensor  56  being, for example, located near brake disk  40  and cooperating with the latter, one can determine the relative position of manual lever shaft  22 . For that purpose, sensor  56  may likewise be connected to electronic control unit  46 . As actuation lever  18  is connected to shaft  22  (for rotation therewith), as is also disc  40 , one can, therefore, also draw conclusions on the position of lever  18 . 
         [0062]    If it is determined that manual lever shaft  22  as well as press ram  32  are in their respective initial positions, from which on a pressing operation may be initiated, a signal is outputted from control unit  46  to second clutch  54 , such that second clutch  54  closes, i.e. inner shaft  22  and outer shaft  50  are connected with one another at least frictionally. Thereby, a flow of force is possible between lever  18  and press ram  32 . 
         [0063]    Thereupon, lever  18  is rotated in the direction of arrow  26  for pressing work pieces  36  and  38  together. 
         [0064]    Under normal conditions, i.e. when no inadmissibly high pressing force occurs, that can be measured with force measuring system  49 , lever  18  and, hence, also press ram  32  eventually reaches its (electronic or mechanical) final position. The mechanical final position is reached when press ram  32  has run through the maximum possible press stroke, or when lever  18  has been moved against a corresponding mechanical stop. The electronic final position has been reached when either lever  18  has been rotated about a predetermined angle or when press ram  32  has run through a predetermined (stroke) displacement. 
         [0065]    The electronic final position could on the one hand be detected by sensor  56  by configuring brake disc  40 , for example, in an area corresponding to the final position such that stationary sensor  56  could detect the final position. If an inductive sensor is used as sensor  56 , brake disc  40  in this area could be configured more or less thick in axial direction  24 . 
         [0066]    The electronic final position could also be defined such that a desired pressing force (depending on the displacement of press ram  32 ) has been reached, i.e. work pieces to be pressed together have been sufficiently “well” be pressed together. For that purpose a distance or displacement measuring system  58  can be provided depicted schematically in  FIG. 1  as a dashed line. The displacement measuring system detects displacement position marks  59  coupled to press ram  32 . 
         [0067]    When the final position has been reached, control unit  46 —depending on the pressing force and/or the effected stroke displacement—can cause second clutch  54  to open, whereby the flow of force between lever  18  and press ram  32  is interrupted. Ram  32  can, in particular, be returned to its initial position corresponding to the initial position of lever  18 , by means of a gas spring not shown in  FIG. 1 . It is advantageous when press ram  32  is coupled with displacement mark  59  for determining the relative position of press ram  32  because conclusions may be drawn from that information with regard to the pressing force that has been reached. By determining the relative position of press ram  32  one can, moreover prevent that a subsequent pressing operation is effected before press ram  32  is in its initial position. 
         [0068]    This additional displacement measuring system  58  could be configured as a linear incremental measuring system having, for example, a resolution of 5 μm. Displacement marks  59  can be sensed by a measuring head being, preferably, positioned within head member  16  and being likewise connected to control unit  46 . 
         [0069]    Brake disc  40  may be connected to a return assembly, in particular a spring (not shown). The spring is then connected to stationary head member  16 . In the initial position, the spring is biased. In the final position it is tensed such that, if an operating person should let lever  18  loose, lever  18  is returned automatically into its respective initial position. For that purpose, second clutch  54  should be open. 
         [0070]    As soon as press ram  32  and lever  18  have reached their respective initial positions, a new pressing operation can be performed. 
         [0071]    In the event that during a pressing operation the admissible pressing force is exceeded so that there is the risk of a damage on the force measuring system  49 , the invention allows to open second clutch  54  before the final position of the pressing operation has been reached. In that case the flow of force between lever  18  and press ram  32  is interrupted. The force may no more act on force sensor  49 . Force sensor  49  is, hence, protected against overload. 
         [0072]    Similarly, a prematurely completed pressing operation that has been classified “good”, may be terminated. This means that the flow of force is also interrupted if the pressing operation has been completed before the final position has been reached. The pressing force exerted via actuation lever  18  can be registered by means of sensor  49  by (higher level) control  46 . Control  46  may, for example, comprise an appropriately prepared microprocessor. In the event that control unit  46 , on the basis of a force-displacement measurement, determines that the work pieces  36  and  38  to be pressed together have actually been combined “well”, control unit  46  interrupts the flow of force between lever  18  and ram  32  by means of an appropriate signal for second clutch  54 . 
         [0073]    The displacement measurement in this case is, preferably, effected via linear incremental displacement measuring system  58 . 
         [0074]    The pressing operation may also be aborted solely depending on the relative position of press ram  32  without actually measuring the pressing force. For that purpose, however, it is necessary that the force-displacement characteristics of the press be known so that one can determine solely on the basis of the stroke displacement whether or when a “good” pressing has been obtained. 
         [0075]    In order to avoid the operating person moving lever  18  “into emptiness”, which could result in injury to the operating person, the first clutch  44  is, preferably, actuated first. 
         [0076]    More specifically, this is effected as follows: Force sensor  49  senses the pressing force exerted via lever  18 ; the sensed pressing force is sampled in predetermined time intervals by control unit  46 ; subsequently, control unit  46  determines, whether there is an inadmissibly high pressing force that would damage force sensor  49 , by determining, for example, whether the sensed pressing force is greater as or equal to a predetermined threshold value, or, when a “good” pressing of work pieces  36  and  38  has occurred (for example a desired pressing force has been reached); if the sensed pressing force exceeds the predetermined threshold value or if the desired pressing force has been reached, control unit  46 , preferably, first outputs a signal for first clutch  40  to stop the movement of lever  18  more or less abruptly; subsequently a clutch signal is outputted by control unit  46  for second clutch  54  for opening second clutch  54 ; second clutch  54  opens; the flow of force between lever  8  and press ram  32  is, hence, interrupted; as an option, the brake may be released again. 
         [0077]    Similar considerations apply when only the stroke displacement is measured. 
         [0078]    Depending on whether the operating person still operates lever  18 , lever  18  can be moved further to the mechanical stop, without, however, being in frictional connection with press ram  32 , so that there is no danger of damaging force sensor  49 . Or, the operating person has already let lever  18  go. If the operating person has let lever  18  go, and if there is the above-mentioned return assembly between brake disc  40  and head member  16 , then lever  18  will automatically return into its initial position. 
         [0079]    In the switched-off condition of press  10  there is, preferably, no connection between lever  18  and press ram  34  which results in a higher process safety. For making a connection, second clutch  54  must first be energized with current. It goes, however, without saying that second clutch  54  could operate just the other way round, i.e. second clutch  54  could also be closed in the non-activated condition, wherein control unit  46  first interrupts such connection before a pressing operation can be effected and then makes the above-mentioned check on the initial position. In such a way it is always guaranteed that the respective initial positions of lever  18  and of press ram  32  are assumed at the beginning of a pressing operation. 
         [0080]    Instead of the type of transmission mentioned at the outset in which a rack meshes with a pinion or a toothed wheel, one might also use a planetary gear train, a worm drive, a chain drive, a belt drive a conical wheel drive, a toggle lever, a shoe lever, a hydraulic transmission or the like. 
         [0081]    According to another embodiment of the present invention, inner manual lever shaft  22  and outer hollow shaft  50  are not only interconnected frictionally but also in a form-fitting manner. 
         [0082]      FIG. 2  shows a highly schematic cross-sectional view perpendicularly to a coupling plane between inner manual lever shaft  22  and outer hollow shaft  50 . 
         [0083]    The drawing plane of  FIG. 2  corresponds to the plane extending perpendicular to the drawing plane of  FIG. 1 . The tooth pair  60  shown in  FIG. 2 , preferably, comprises one (latch) tooth  62  only which, in the embodiment shown is configured with outer hollow shaft  50 , and a corresponding recess  64  in inner manual lever shaft  22 .  FIG. 2  shows a condition, in which second clutch  54  (cf.  FIG. 1 ) is open, such that shafts  22  and  50  may freely be rotated with respect to each other. Should second clutch  54  close, shafts  22  and  50  will move relatively towards each other along axis  24  such that tooth  62  comes into engagement with recess  64 . 
         [0084]    It goes without saying that tooth  62 , as an alternative, can also be configured with inner manual lever shaft  22  and recess  64  at outer hollow shaft  50 . Instead of one tooth pair only, several such pairs  62 ,  64  could also be provided. Embodiments with one (latch) tooth, however, are preferred as will be explained further below. 
         [0085]    Tooth pair  60  may, additionally, be used for determining the initial position of press ram  32 . This means that only if shafts  22  and  50  are correctly oriented relative to one another, i.e. if hollow shaft  50  and, hence, press ram  32  are in their initial position, then tooth  62  and recess  64  may engage. If press ram  32  is not (yet) in its initial position, no coupling between shafts  22  and  50  is possible. 
         [0086]    Further, in  FIG. 3  there are schematically shown forces acting on shafts  22 ,  50  and their respective tooth, recess  62 ,  64 , pair along a tooth flange extending parallel to an imaginary line  66 . 
         [0087]    Assuming that for closing second clutch  54  (cf.  FIG. 1 ), a magnetic force F M  ( FIG. 3 ) is required for, for example, moving outer hollow shaft  50  or its tooth  62 , in the direction of inner hollow shaft  22  or its recess  64 . The (closing) force F M  of the clutch magnet may be resolved with the help of a force parallelogram into two force components F E  and F S1 , wherein F E  represents the coupling force acting along imaginary line  66  and F S1  represents the force acting perpendicularly to the toothing flange. 
         [0088]    If both shafts  22 ,  50  are coupled with each other and actuation lever  18  is actuated by an operating person, inner manual lever shaft  22  will transmit a rotary force F D  onto outer hollow shaft  50  as is also shown in  FIG. 3 . Rotary force F D  may likewise be resolved into two force components F A  and F S2 , wherein F A  represents the decoupling force and F S2  represents the force acting perpendicularly to the tooth flange. 
         [0089]    As long as the rotary force does not exceed a certain threshold value, decoupling force component F A  is smaller than coupling component F E . If, however, the operating person (spontaneously) exerts a very high force onto shaft  22 , rotary force F D  will increase abruptly, resulting in an increase of decoupling force FA. If force component F A  becomes greater than force component F E , an opening of tooth pair  60  results even if clutch  54  is closed or not yet opened. The force at which tooth pair  60  opens automatically depends on its design parameters, in particular on the flange angle α. Second clutch  54  then acts as an overload clutch. 
         [0090]    With this measure one can effect that, if a spontaneous torques occur which cannot be compensated for at that speed by the control unit, the coupling between shafts  20 ,  50  opens automatically. This, in turn, means that the flow of force between actuation lever  18  and press ram  32  is separated such that a force sensor is again protected against overload. 
         [0091]    Instead of a tooth pair one could likewise use rollers or the like.