Patent Publication Number: US-2015083002-A1

Title: Method for operating a hydraulic press, and hydraulic press

Description:
The invention relates to a method for operating a hydraulic press according to the preamble of Patent claim  1  and also a hydraulic press according to the preamble of Patent claim  22 . 
     DE 25 44 794 A1 describes a hydraulic press consisting of a press framework, a hydraulic drive, an oil pump, and two accumulators. The hydraulic drive consists of a working cylinder having a working piston of greater diameter and a movement cylinder having a movement piston of lesser diameter. The pistons are fixedly connected to one another with spacing. The working cylinder is fed from the first accumulator via a directional valve and the movement cylinder is fed from the second accumulator via a servo valve inside a closed control loop. The control loop consists of a target value generator, a regulating amplifier, the servo valve, a travel transducer, and a measurement amplifier. The directional valve is implemented as a valve having fixed switch positions. To secure the hydraulic system against excessively high pressures, a pressure limiting valve is arranged in the pressure line of the pump which leads to the accumulators. The quantity of oil required for the drive of the press is delivered by the pump from a tank via the accumulator. A compressible medium from a gas bottle is applied to the accumulator. 
     This document additionally describes the work sequence and operation of this press, in which the uppermost position of the hydraulic piston is the starting point. After the system is turned on and the accumulators are filled, a signal is given by the target value generator to the regulating amplifier. This actuates the servo valve into the position in which oil is released for feeding the movement cylinder. The movement piston and therefore the working piston connected fixedly thereto move downward. This downward movement is transmitted to the travel transducer. The output signal of the travel transducer is converted in the measuring amplifier into a signal proportional to the travel, which is compared in the regulating amplifier to the target value signal. Deviations of the two signals are processed in the regulating amplifier and provide correction signals to the servo valve. The regulating amplifier, the servo valve, the movement cylinder, the travel transducer, and the measuring amplifier thus form a closed control loop, which enables the movement of the movement piston in proportion to an electrical signal coming from the target value generator. In this manner, the working piston is moved into a precisely established position up to shortly before or on the workpiece. 
     During the second part of the working stroke, the pressing stroke, which is initiated by switching over the directional valve, the first accumulator, which was previously filled with a quantity of oil metered in accordance with the pressing stroke, is applied to the working piston via the directional valve. This quantity of oil causes a predetermined continuation of the movement of the working piston, which corresponds to the quantity of oil metered in the first accumulator. 
     After the working piston has carried out the pressing stroke, the directional valve is switched over again and the movement piston moves back into the starting position in the movement cylinder. The oil present in the working cylinder is conveyed back via the directional valve into the tank in this case. 
     During the working pause, the pump refills the first accumulator with the predetermined quantity of pressure oil, which can be predefined by a limit switch on the first accumulator, which switches an oil valve in the pressure line of the pump leading to the first accumulator. 
     In this known method and this known press, the filling of the first accumulator to the predetermined quantity of pressure oil is thus regulated during the working pause in that the charging volume stream is set and/or changed by switching the oil valve. Since a compressible medium from the gas bottle is applied to the first accumulator, this filling regulation corresponds to a regulation of the pressure prevailing in the first accumulator to a pressure reference variable, which is dependent on the limit switch and the compressible medium from the gas bottle. In the case of this known regulation, the charging volume stream is set to zero, in that the oil valve is switched into a shutoff position, in which it disconnects the first accumulator from the pressure fitting of the pump. It is disadvantageous in this case that then the pump, which is still driven by the still running motor, increases the pressure in the pressure line until the pressure limiting valve responds and connects the pressure fitting to the tank, so that the pump runs at full speed in idle. This results in an unnecessarily high power consumption. In addition, the pump must be designed sufficiently large that it can deliver the predetermined quantity of pressure oil into the accumulator during the working pause. 
     It is the object of the invention to reduce the energy consumption in a method for shaping workpieces by means of a hydraulic press and a hydraulic press for shaping workpieces. 
     This object is achieved by a method according to claim  1  and a press according to claim  22 . Further possible embodiments and variants are described in the dependent claims. 
     According to a first aspect, the invention proposes a method for operating a hydraulic press in cycles, in particular for shaping workpieces, wherein:
         each cycle has at least one phase, in which hydraulic fluid is pressed out of a hydraulic accumulator ( 15 ) into a chamber ( 11 . 1 ,  11 . 2 ) of a hydraulic cylinder ( 11 ) of the press ( 10 ) by the accumulator pressure p S  prevailing in the accumulator ( 15 ), in order to move a ram ( 12 ) of the press ( 10 ), which is coupled to the cylinder ( 11 ), and which can be coupled to a shaping tool ( 21 ) for shaping a workpiece, in relation to the cylinder ( 11 );   in at least one part of each cycle, a hydraulic pump ( 13 ) driven by a motor ( 14 ) conveys hydraulic fluid into the accumulator ( 15 ) with a charging volume stream and the accumulator pressure p S  is regulated to a pressure reference variable P SOLL , in that a speed of the motor ( 14 ) is set to a nominal speed n N  of the motor ( 14 ) and to at least one intermediate value n Z , for which 0&lt;n Z &lt;n N  applies.       

     Since therefore in the case of this proposed method, the setting of the charging volume stream is performed by setting the speed of the motor to the nominal speed n N  and to at least one intermediate speed n Z  and the energy requirement of the motor, for example, the fuel consumption of an internal combustion engine or the power consumption of an electric motor, is less at speeds below nominal speed n N  than at nominal speed n N , the energy consumption can be reduced in comparison to the method known from DE 25 44 794 A1. In this way, the efficiency of the method can also be increased. In addition, a reduction of the speed also results in a noise reduction. 
     The nominal speed is understood here as the maximum speed which the motor can provide for a longer time without damage, or for which the design of the motor is intended. 
     The proposed method can be implemented as needed in an arbitrary manner and can have, for example, the regulation of the accumulator pressure p S  in at least one additional phase. 
     The press can be, for example, one of the presses proposed according to the second aspect described hereafter. 
     The pump can be implemented as needed in an arbitrary manner, for example, as a gearwheel pump, axial piston pump, or radial piston pump. 
     The motor can be implemented as needed in an arbitrary manner and can be an asynchronous motor, for example, and the setting of its speed can be performed as needed in an arbitrary manner, for example, by means of a frequency converter. 
     In the closing phase, the moving or positioning or lowering or raising of the ram to the first stroke height is preferably performed proceeding from the third stroke height. 
     In the working phase, the ram can be kept at the second stroke height as needed after the lowering of the ram, for example, in that the press chamber is closed and/or is disconnected from accumulator and tank. 
     In the reset phase, the movement or positioning or lowering or raising of the ram to the third stroke height is preferably performed without intermediate step at the first stroke height. 
     For the pressure reference variable P SOLL , for example, the maximum pressure which the accumulator can withstand for a longer time or for which the design of the accumulator is intended, can simply be selected. 
     The speed is preferably set continuously to speeds from zero up to the nominal speed n N . 
     It can be provided that in at least one of the phases, the pressure reference variable P SOLL  is set as a function in dependence on at least one chamber pressure p K  prevailing in the chambers. 
     It can then be provided that for the function: P SOLL =p K +K P , wherein K P  is a pressure correction value with 0&lt;K. 
     In this way, it is ensured that the accumulator always provides sufficient overpressure in comparison to the chamber pressure pK, which often rises again and again during the phase, but on the other hand is not excessively high, so that the motor does not have to run unnecessarily rapidly or the pump does not have to deliver unnecessarily strongly. 
     The pressure correction value K P  can be selected arbitrarily as needed and can be constant at least during one part of the phase and/or at least during one part of the other phases, for example. Alternatively or additionally, it can be chronologically variable at least during one part of the phase and/or at least during one part of the other phases, for example. Alternatively or additionally, it can be selected in such a manner, for example, that the pressure reference variable P SOLL  is greater by a specific percentage than the chamber pressure pK. This percentage is, for example, at least 2% or at least 3% or at least 4% or at least 5% or at least 6% or at least 7% or at least 8% or at least 9% or at least 10% or at least 12% or at least 14% or at least 16% or at least 18% or at least 20%. Alternatively or additionally, this percentage is, for example, at most 2% or at most 3% or at most 4% or at most 5% or at most 6% or at most 7% or at most 8% or at most 9% or at most 10% or at most 12% or at most 14% or at most 16% or at most 18% or at most 20%. 
     It can be provided that the regulation of the accumulator pressure p S  is performed in at least one of the phases. 
     It can be provided that
         each cycle has a closing phase for closing the press, a working phase for shaping the workpiece, a reset phase for opening the press, and a charging phase for removing a shaped workpiece from the press and for inserting a workpiece to be shaped into the press in this sequence;   in the closing phase, the ram is moved to a first stroke height, so that the shaping tool touches the workpiece to be shaped or is located at a slight spacing to the workpiece to be shaped;   in the working phase, the ram is moved back to a second stroke height, so that the shaping tool presses against the workpiece;   in the reset phase, the ram is moved back to the first stroke height and further to a third stroke height, so that the shaping tool disengages and moves away from the shaped workpiece;   in the charging phase, the ram is kept at the third stroke height.       

     It can then be provided that in the closing phase, the chamber is disconnected from the accumulator and connected to a tank, or the chamber is disconnected from a tank and is connected to the accumulator. 
     In the first alternative, the ram can be lowered or moved passively by its intrinsic weight and/or actively by a closing drive in the closing direction or can be raised or moved actively by a closing drive in the closing direction. This closing drive can be, in comparison to a hydraulic drive for the working phase, which is preferably formed by a press chamber in the cylinder and the accumulator, smaller and/or weaker and/or faster, for example, and/or can have an additional hydraulic drive, for example. 
     In the second alternative, the ram is actively lowered or raised or moved in the closing direction by the accumulator. 
     It can be provided that in the working phase, the movement of the ram is performed in that a press chamber forming the chamber is disconnected from a tank and connected to the accumulator. 
     It can then be provided that in the reset phase, the press chamber is disconnected from the accumulator and connected to the tank. 
     The ram can thus be raised or moved in the reset direction actively by a reset drive or passively by its intrinsic weight and/or can be lowered or moved actively by a reset drive in the reset direction. This reset drive can be, for example, in comparison to a hydraulic drive for the working phase, which is preferably formed by a press chamber in the cylinder and the accumulator, smaller and/or weaker and/or faster and/or can have an additional hydraulic drive, for example. This additional hydraulic drive preferably has a reset chamber in the cylinder, which is separated from the press chamber by a piston, for example, which is guided in the cylinder and coupled to the ram, and is disconnected from the tank and connected to the accumulator in the reset phase. 
     It can be provided that in the charging phase, the chamber is closed and/or is disconnected from the accumulator and a tank. 
     In this way, the ram can be kept at the third stroke height. 
     It can be provided that in the reset phase, the movement of the ram is performed in that a reset chamber forming the chamber is disconnected from a tank and is connected to the accumulator. 
     It can then be provided that in the working phase, the reset chamber is disconnected from the accumulator and is connected to the tank. 
     The ram can then be actively raised or moved further in the closing direction by a press drive. This press drive can be, for example, in comparison to the hydraulic drive formed by reset chamber and accumulator, larger and/or stronger and/or slower and/or can have an additional hydraulic drive, for example. This additional hydraulic drive preferably has a press chamber in the cylinder, which is separated from the reset chamber by a piston, which is guided in the cylinder and coupled to the ram, for example, and in the working phase is disconnected from the tank and is connected to the accumulator. 
     In the case of each proposed method, the charging volume stream can be set as needed in an arbitrary manner to zero, in particular for or during the regulation of the accumulator pressure p S . Thus, for example, it can be provided that the tank is connected to the pressure fitting, and the charging volume stream is set to zero, in particular for or during the regulation of the accumulator pressure p S , in that a pressure fitting of the pump is connected to a tank. Alternatively or additionally, it can be provided that the charging volume stream is set to zero, in particular for or during the regulation of the accumulator pressure p S , in that the speed of the motor is set to zero. 
     An excessively rapid rise of the accumulator pressure p S  can be decelerated or even ended by the setting to zero. 
     In the first variant, the setting to zero can be performed rapidly. 
     In the second variant, the setting to zero can be performed in an energy-saving manner. 
     It can be provided that:
         in each cycle, a charging duration T L  is ascertained, during which the charging volume stream is greater than zero;   the charging duration T L  is regulated to a charging duration target value T SOLL , in that the speed is set and/or changed accordingly.       

     In this way, the most uniform possible running of the motor and the pump can be achieved, during which speed peaks are avoided, if the charging duration target value T SOLL  is located as much as possible at or just below the cycle duration T Z . 
     The charging duration target value T SOLL  can be selected arbitrarily as needed, for example, such that it is less by a specific percentage than the cycle duration T Z . This percentage is, for example, at least 2% or at least 3% or at least 4% or at least 5% or at least 6% or at least 7% or at least 8% or at least 9% or at least 10% or at least 12% or at least 14% or at least 16% or at least 18% or at least 20%. Alternatively or additionally, this percentage is, for example, at most 2% or at most 3% or at most 4% or at most 5% or at most 6% or at most 7% or at most 8% or at most 9% or at most 10% or at most 12% or at most 14% or at most 16% or at most 18% or at most 20%. 
     It can be provided that:
         in a specific cycle, a cycle duration T Z  and a charging duration T L , during which the charging volume stream is greater than zero, and a charging speed n L , which is averaged over the charging duration T L , are ascertained;   for at least one following cycle, the speed is set to an intermediate value n Z , for which n Z =n L ·T L : T Z +K N , wherein K N  is a speed correction value with 0≦K N &lt;n L ·(1-T L :T Z ).       

     In this way, the most uniform possible running of the motor and the pump, during which speed peaks are avoided, can be achieved. The speed correction value K N  is used, if 0≦K N  applies, as a safety cushion, so that in the following cycle, a pressure and volume reserve can be built up in the accumulator. For example, if in one cycle T L : T Z =75% and n L =1600 RPM were ascertained, accordingly, 0≦K N &lt;1600 RPM·(1-75%)=400 RPM should apply. Therefore, for the following cycle, the third correction value K N  can accordingly be, for example, 0 RPM or 50 RPM or 100 RPM or 150 RPM or 200 RPM or 250 RPM or 300 RPM or 350 RPM and the intermediate value n Z  can accordingly be 1200 RPM or 1250 RPM or 1300 RPM or 1350 RPM or 1400 RPM or 1450 RPM or 1500 RPM or 1550 RPM. 
     It can then be provided that in the specific cycle:
         firstly the speed is set to the nominal speed n N  and   the accumulator pressure p S  is then regulated, in particular exclusively, in that the charging volume stream is set to zero and the speed is set to the nominal speed n N .       

     This is preferably performed after the startup of the press, wherein the specific cycle is in particular the first cycle after the startup of the press. 
     In this specific cycle, in particular the first cycle, the charging speed n L  averaged over the charging duration T L  will thus correspond to the nominal speed n N . 
     It can be provided that the charging volume stream is set and/or changed by the regulation of the accumulator pressure p S  such that, in particular in each phase or during the entire cycle, the accumulator pressure p S  does not fall below a lower operating pressure p U  and/or does not exceed an upper operating pressure p O . 
     Maintaining the lower operating pressure p U  can prevent, for example, in the case of an accumulator which has a gas is a compression medium, this gas from entering the hydraulic circuit. The upper operating pressure p O  can be the maximum pressure, for example, that the accumulator can withstand for a longer time without damage or for which the design of the accumulator is intended. 
     It can then be provided that p U ≦P SOLL ≦p O  applies. 
     It can be provided that:
         at least one of the pressure reference variables P SOLL  and/or at least one of the intermediate values n Z  and/or at least one of the charging duration target values T SOLL  and/or at least one of the correction values K P , K N  is stored;   the stored values are used as starting values during a later shaping task for identical or similar workpieces.       

     Therefore, during a later shaping task, the method can be carried out already with at least partially optimized values. 
     It can be provided that the accumulator pressure p S  is set by means of adaptive regulation. 
     It can then be provided that during the adaptive regulation, at least one of the pressure reference variables P SOLL  and/or at least one of the intermediate values n Z  and/or at least one of the charging duration target values T SOLL  and/or at least one of the correction values K P , K N  is changed. 
     In each proposed method, each disconnection, for example, the disconnection of the press chamber or the reset chamber from the tank or from the accumulator, and/or each connection, for example, the connection of the press chamber or the reset chamber to the accumulator or to the tank or the connection of the pressure fitting to the accumulator or to the tank, and/or each closing, for example, the closing of the press chamber or the reset chamber, can be performed with the aid of valves, for example. Preferably, at least one valve can be provided or seated between the accumulator and the press chamber and/or at least one valve can be provided or seated between the accumulator and the reset chamber and/or at least one valve can be provided or seated between the accumulator and the pressure fitting and/or at least one valve can be provided or seated between the tank and the press chamber and/or at least one valve can be provided or seated between the tank and the reset chamber and/or at least one valve can be provided or seated between the tank and the pressure fitting. 
     Each valve can be implemented as needed in an arbitrary manner, for example, as a proportional valve or regulating valve or slope valve or directional valve or check valve or pressure limiting valve. 
     Each proposed method can be implemented as needed in an arbitrary manner and can have at least one additional phase, for example. 
     Each press used in one of the proposed methods can be implemented as needed in an arbitrary manner and can have, for example, at least one additional hydraulic cylinder and/or at least one additional ram and/or at least one additional hydraulic pump and/or at least one additional motor and/or at least one additional hydraulic accumulator and/or at least one additional tank for hydraulic fluid. Each cylinder provided in this press can be implemented as needed in an arbitrary manner and can have, for example, at least one additional press chamber and/or at least one additional reset chamber. Each pump provided in this press can be implemented as needed in an arbitrary manner and can have, for example, at least one additional pressure fitting. 
     The proposed methods can be combined as needed in an arbitrary manner, in particular entirely or partially. 
     According to a second aspect, the invention proposes a hydraulic press, in particular for shaping workpieces, having:
         a hydraulic cylinder, which has at least one chamber,   a ram, which is coupled to the cylinder and on which a shaping tool for shaping a workpiece can be coupled;   a hydraulic pump, which has a pressure fitting,   a motor, which is coupled to the pump and has a nominal speed n N ,   a hydraulic accumulator, which is connected to at least one of the chambers and the pressure fitting,   a tank for hydraulic fluid, which is connected to at least one of the chambers,   an accumulator pressure sensor for registering the accumulator pressure p S  prevailing in the accumulator;   a control unit, which enables an operation of the press in cycles and is connected to the accumulator pressure sensor and the motor;
 
wherein:
   the motor is implemented such that its speed can be set to the nominal speed n N  and to at least one intermediate value n Z , for which 0&lt;n Z &lt;n N  applies;   the control unit is implemented such that:
           in at least one phase of each cycle, hydraulic fluid is pressed out of the accumulator into at least one of the chambers by the accumulator pressure p S , to move the ram in relation to the cylinder;   in at least one part of each cycle, the pump conveys hydraulic fluid into the accumulator with a charging volume stream and the control unit regulates the accumulator pressure p S  to a pressure reference variable P SOLL , in that it sets the speed of the motor to the nominal speed n N  and to at least one of the intermediate values n Z .   
               

     The proposed press can be implemented as needed in an arbitrary manner and can have, for example, the regulation of the accumulator pressure p S  in at least one additional phase. 
     The proposed press enables the execution of the methods proposed according to the first aspect. 
     It can be provided that the press additionally has:
         at least one chamber pressure sensor for registering the chamber pressure p K  prevailing in one of the chambers;
 
wherein:
   the control unit is connected to each chamber pressure sensor;   the control unit is implemented such that in at least one of the phases:
           it selects the pressure reference variable P SOLL  as a function in dependence on at least one of the chamber pressures p K .   
               

     It can then be provided that for the function P SOLL =p K +K P  applies, wherein K P  is a pressure correction value with 0&lt;K P . 
     It can be provided that the control unit is implemented such that the regulation of the accumulator pressure p S  occurs in at least one of the phases. 
     It can be provided that:
         each cycle has a closing phase for closing the press, a working phase for shaping the workpiece, a reset phase for opening the press, and a charging phase for removing a shaped workpiece from the press and for inserting a workpiece to be shaped into the press in this sequence;   the control unit is implemented such that it:
           in the closing phase, it moves the ram to a first stroke height, so that the shaping tool touches the workpiece to be shaped or is located at a slight spacing to the workpiece to be shaped;   in the working phase, it moves the ram further to a second stroke height, so that the shaping tool presses against the workpiece;   in the reset phase, it moves the ram back to the first stroke height and further to a third stroke height, so that the shaping tool disengages and moves away from the shaped workpiece;   in the charging phase, it keeps the ram at the third stroke height.   
               

     It can then be provided that the control unit is implemented such that, in the closing phase, it disconnects at least one of the chambers from the accumulator and connects it to the tank or disconnects at least one of the chambers from the tank and connects it to the accumulator. 
     It can be provided that the control unit is implemented such that it causes the movement of the ram in the working phase, in that it disconnects a press chamber, which forms the chamber, from the tank and connects it to the accumulator. 
     It can then be provided that the control unit is implemented such that, in the reset phase, it disconnects the press chamber from the accumulator and connects it to the tank. 
     It can be provided that the control unit is implemented such that, in the charging phase, it closes at least one of the chambers and/or disconnects it from the accumulator and tank. 
     It can be provided that the control unit is implemented such that, in the reset phase, it causes the movement of the ram in that it disconnects a reset chamber, which forms the chamber, from the tank and connects it to the accumulator. 
     It can then be provided that the control unit is implemented such that, in the working phase, it disconnects the reset chamber from the accumulator and connects it to the tank. 
     In the case of each proposed press, the charging volume stream can be set as needed in an arbitrary manner to zero, in particular for or during the regulation of the accumulator pressure p S . Thus, for example, it can be provided that the tank is connected to the pressure fitting, and the control unit is implemented such that it sets the charging volume stream to zero in that it connects the pressure fitting to the tank. Alternatively or additionally, it can be provided that the motor is implemented such that its speed can be set to zero, and the control unit is implemented such that it sets the charging volume stream to zero, in that it sets the speed to zero. 
     It can be provided that the control unit is implemented such that:
         in each cycle, it ascertains a charging duration T L , during which the charging volume stream is greater than zero;   it regulates the charging duration T L  to a charging duration target value T SOLL , in that it sets and/or changes the speed accordingly.       

     It can be provided that the control unit is implemented such that:
         in a specific cycle, it ascertains a cycle duration T Z  and a charging duration T L , during which the charging volume stream is greater than zero, and a charging speed n L , which is averaged over the charging duration T L ;   for at least one following cycle, it sets the speed to an intermediate value n Z , for which n Z =n L ·T L : T Z +K N , wherein K N  is a speed correction value with 0≦K N &lt;n L ·(1−T L : T Z ).       

     It can then be provided that the control unit is implemented such that, in the specific cycle:
         it firstly sets the speed to the nominal speed n N  and   it then regulates the accumulator pressure p S , in particular exclusively, in that it sets the charging volume speed to zero and sets the speed to the nominal speed n N .       

     It can be provided that the control unit is implemented such that it sets and/or changes the charging volume stream by way of the regulation of the accumulator pressure p S  such that the accumulator pressure p S  does not fall below a lower operating pressure p U  and/or does not exceed an upper operating pressure p O . 
     It can then be provided that p U ≦P SOLL ≦p O  applies. 
     It can be provided that the control unit is implemented such that: 
     it stores at least one of the pressure reference variables P SOLL  and/or at least one of the intermediate values n Z  and/or at least one of the charging duration target values T SOLL  and/or at least one of the correction values K P , K N ; it uses the stored values as starting values during a later shaping task for identical or similar workpieces. 
     It can be provided that the control unit is implemented such that it sets or can set the accumulator pressure p S  by means of adaptive regulation. 
     They can then be provided that the control unit is implemented such that in the case of the adaptive regulation, it changes or can change at least one of the pressure reference variables P SOLL  and/or at least one of the intermediate values n Z  and/or at least one of the charging duration target values T SOLL  and/or at least one of the correction values K P , K N . 
     It can be provided that it additionally has:
         at least one valve between the accumulator and the press chamber and/or   at least one valve between the accumulator and the reset chamber and/or   at least one valve between the accumulator and the pressure fitting and/or   at least one valve between the tank and the press chamber and/or   at least one valve between the tank and the reset chamber and/or   at least one valve between the tank and the pressure fitting;
 
wherein:
   the control unit is connected to the valves.       

     With the aid of these valves, the control unit can, for example, cause or carry out the disconnection of the press chamber from the tank or from the accumulator and/or the connection of the press chamber or the reset chamber to the accumulator or to the tank or the connection of the pressure fitting to the accumulator or to the tank and/or the closing of the press chamber or the reset chamber. 
     Each valve can be implemented as needed in an arbitrary manner, for example, as a proportional valve or regulating valve or slope valve or directional valve or check valve or pressure limiting valve. 
     Each proposed press can be implemented as needed in an arbitrary manner and can have, for example, at least one additional hydraulic cylinder and/or at least one additional ram and/or at least one additional hydraulic pump and/or at least one additional motor and/or at least one additional hydraulic accumulator and/or at least one additional tank for hydraulic fluid and/or at least one additional control unit and/or at least one additional pressure sensor. Each cylinder can be implemented as needed in an arbitrary manner and can have, for example, at least one additional press chamber and/or at least one additional reset chamber. Each pump can be implemented as needed in an arbitrary manner and can have, for example, at least one additional pressure fitting. 
     The statements on one aspect of the invention, in particular on individual features of this aspect, also similarly apply accordingly for the other aspects of the invention, in particular for corresponding individual features of this aspect. 
    
    
     
       Embodiments and exemplary embodiments of the invention are explained in greater detail hereafter on the basis of the appended drawings. The individual features resulting therefrom are not restricted to the individual embodiments an exemplary embodiments, however, but rather can be combined with further above-described individual features and/or with individual features of other embodiments and exemplary embodiments. The details in the drawings are only to be interpreted as explanatory, but not as restrictive. The reference signs contained in the claims are not to restrict the scope of protection of the invention in any manner, but rather merely refer to the embodiments shown in the drawings. In the figures of the drawings: 
         FIG. 1  shows an overview plan of a preferred embodiment of a hydraulic press, wherein the press is located in a state according to a closing phase of a cycle of a preferred embodiment of a method for operating the press; 
         FIG. 2  shows a graph of the time curve of the accumulator pressure in the accumulator of the press of  FIG. 1 , the travel of the ram of the press, and the speed of the motor of the press over three cycles of the method. 
     
    
    
       FIG. 1  schematically shows a preferred embodiment of a hydraulic press  10 , which can be operated in cycles, each one of which has a closing phase, a working phase, a reset phase, and a charging phase in this sequence. The press  10  has a hydraulic cylinder  11 , a ram  12 , a charging pressure pump or hydraulic pump  13 , a motor  14 , a hydraulic accumulator  15 , a pre-filling container or tank  16  for hydraulic fluid, a control unit  17 , three pressure sensors  18 . 1  to  18 . 3 , three valves  19 . 1  to  19 . 3 , and a frequency converter  20 . 
     The cylinder  11  has two chambers, namely a press chamber  11 . 1  and a reset chamber  11 . 2 , and a piston  11 . 3 , which is guided in the cylinder  11  and which separates a press chamber  11 . 1 , which borders its upper side, from a reset chamber  11 . 2 , which borders its lower side. The ram  12  is fastened with its upper end on the lower side of the piston  11 . 3  and is therefore coupled to the cylinder  11  and holds on its lower end a shaping tool  21 , which is coupled thereto, for shaping a workpiece. The pump  13  has a suction fitting  13 . 1  and a pressure fitting  13 . 2 . The motor  14  is coupled as a drive to the pump  15 . The accumulator  15  is connected to the press chamber  11 . 1 , the reset chamber  11 . 2 , and the pressure fitting  13 . 2  and is implemented, for example, as a hydraulic accumulator having a pressure container filled with nitrogen. The tank  16  is connected to the press chamber  11 . 1 , the reset chamber  11 . 2 , and the suction fitting  13 . 1 . 
     The motor  14  is an asynchronous motor, for example, and has a nominal speed n N , which is 2000 RPM, for example. The frequency converter  20  is connected, on the one hand, to the motor  14  and, on the other hand, to the control unit  17 . The control unit  17  is implemented such that it can set the speed of the motor  14 , by suitable activation of the frequency converter  20 , continuously or nearly continuously from zero up to the nominal speed n N , and therefore to zero, to the nominal speed n N , and to at least one intermediate value n Z , for which 0&lt;n Z &lt;n N . 
     The control unit  17  is additionally connected to the pressure sensors  18 , of which an accumulator pressure sensor  18 . 1  is used for registering the accumulator pressure p S  prevailing in the accumulator  15 , a first chamber pressure sensor  18 . 2  is used for registering the working pressure p A  prevailing in the press chamber  11 . 1 , and a second chamber pressure sensor  18 . 3  is used for registering the reset pressure p R  prevailing in the reset chamber  11 . 2 . 
     The control unit  17  is additionally connected to the valves  19 , which are directional valves, for example, and of which a first valve  19 . 1  is seated between the press chamber  11 . 1  and the accumulator  15  and between the press chamber  11 . 1  and the tank  16 , a second valve  19 . 2  is seated between the reset chamber  11 . 2  and the accumulator  15  and between the reset chamber  11 . 2  and the tank  16 , and a third valve  19 . 3  is seated between the pressure fitting  13 . 2  and the accumulator  15  and the pressure fitting  13 . 2  and the tank  16 . The first valve  19 . 1  is a  3 / 3  directional valve, i.e., it has three fittings for hydraulic fluid lines and three switch positions, and can alternately disconnect the press chamber  11 . 1  from the tank  16  and connect it to the accumulator  15  or disconnect it from the accumulator  15  and connect it to the tank  16  or disconnect it from the accumulator  15  and the tank  16 . The second valve  19 . 2  is a  3 / 3  directional valve and can alternately disconnect the reset chamber  11 . 2  from the tank  16  and connect it to the accumulator  15  or disconnect it from the accumulator  15  and connect it to the tank  16  or disconnect it from the accumulator  15  and the tank  16 . The third valve  19 . 3  is a  3 / 2  directional valve, i.e., it has three fittings for hydraulic fluid lines and two switch positions, and can alternately disconnect the pressure fitting  13 . 2  from the tank  16  and connect it to the accumulator  15  or disconnect it from the accumulator  15  and connect it to the tank  16 . 
       FIG. 2  schematically shows three cycles of a preferred embodiment of a method for operating the press  10  of  FIG. 1  and for shaping workpieces by means of the press  10  from  FIG. 1  on the basis of the accumulator pressure p S  in the accumulator  15 , the movement H of the ram  12 , and the speed n of the motor  14  over time. 
     The control unit  17  enables a cyclic operation of the press  10  according to this preferred embodiment of the method. It is implemented such that, in each cycle, it lowers the ram  12  and the shaping tool  21  coupled thereto in the closing phase to a first stroke height H1, lowers it in the working phase further to a second stroke height H2 and keeps it there, raises it in the reset phase back beyond the first stroke height H1 and further to a third stroke height H3, and keeps it in the charging phase at the third stroke height H3. In  FIG. 2 , the closing phase can be recognized at the steeply dropping segment of the H line, the working phase can be recognized at the flatly falling and then horizontal segment adjoining thereon, the reset phase can be recognized at the flat and then steeply rising segment adjoining thereon, and the charging phase can be recognized at the horizontal segment adjoining thereon. 
     The lowering of the ram  12  and the shaping tool  21  in the closing phase is achieved or caused by the control unit  17  in that, by appropriate activation of the first valve  19 . 1  and the second valve  19 . 2 , it disconnects the press chamber  11 . 1  and the reset chamber  11 . 2  in each case from the accumulator  15  and connects them to the tank  16 . Therefore, the piston  11 . 3 , the ram  12 , and the shaping tool  21  are drawn downward by their intrinsic weight. In this case, hydraulic oil is suctioned from the tank  16  into the press chamber  11 . 1  and pressed out of the reset chamber  11 . 2  into the tank  16 . 
     The lowering of the ram  12  and the shaping tool  21  in the working phase is achieved or caused by the control unit  17  in that, by appropriate activation of the first valve  19 . 1 , it disconnects the press chamber  11 . 1  from the tank  16  and connects it to the accumulator  15 . The accumulator  15  is nearly fully charged after the startup of the press  10  and therefore at the beginning of the first cycle, so that the accumulator pressure p S  is just below an upper operating pressure p O , which corresponds to the maximum pressure which the accumulator  15  can withstand for a longer time without damage or for which its design is intended. Therefore, the piston  11 . 3 , the ram  12 , and the shaping tool  21  are pressed downward by the hydraulic oil, which is under the accumulator pressure p S  in the accumulator  15 , against the shaping force or the shaping pressure. In this case, hydraulic oil is pressed out of the accumulator  15  into the press chamber  11 . 1  and out of the reset chamber  11 . 2  into the tank  16 . 
     The holding of the ram  12  and the shaping tool  21  in the working phase is achieved or caused by the control unit  17  in that, by appropriate activation of the first valve  19 . 1 , it disconnects the press chamber  11 . 1  from the accumulator  15  and the tank  16  and thus closes it. Since therefore neither can the hydraulic oil enclosed in the press chamber flow out nor can hydraulic oil flow back into the press chamber  11 . 1 , the piston  11 . 3 , the ram  12 , and the shaping tool  21  are kept motionless. 
     The raising of the ram  12  and the shaping tool  21  in the reset phase is achieved or caused by the control unit  17  in that, by appropriate activation of the first valve  19 . 1 , it disconnects the press chamber  11 . 1  from the accumulator  15  and connects it to the tank  16  and, by appropriate activation of the second valve  19 . 2 , it disconnects the reset chamber  11 . 2  from the tank  16  and connects it to the accumulator  15 . Therefore, the piston  11 . 3 , the ram  12 , and the shaping tool  21  are pressed upward by the hydraulic oil, which is under the accumulator pressure p S  in the accumulator  15 . In this case, hydraulic oil is pressed out of the accumulator  15  into the reset chamber  11 . 2  and out of the press chamber  11 . 1  into the tank  16 . 
     The control unit  17  is additionally implemented such that in all phases, it charges the accumulator  15  in accordance with the demand, i.e., in dependence on the respective presently required working pressure p A  and reset pressure p R , using a charging volume stream. 
     The charging of the accumulator  15  is achieved or caused by the control unit  17 , in that, by appropriate activation of the frequency converter  20 , it sets the speed of the motor  14  so that it drives the pump  13  and, by appropriate activation of the third valve  19 , it disconnects the pressure fitting  13 . 2  from the tank  16  and connects it to the accumulator  15 . Therefore, the pump  13  suctions hydraulic oil out of the tank  16  and presses it into the accumulator  15  using a charging volume stream, which is dependent on the speed of the motor  14  set by means of the frequency converter  20 . 
     In this preferred embodiment, the control unit  17  is additionally implemented such that, in all phases, it regulates the accumulator pressure p S  to a pressure reference variable P SOLL , in that it sets the speed and therefore the charging volume stream accordingly, as described in greater detail hereafter. 
     The setting of the speed for the pressure regulation in accordance with demand is achieved or caused by the control unit  17  in that, by suitable activation of the frequency converter  20 , it sets the speed continuously from zero to the nominal speed n N  and therefore to zero, to the nominal speed n N , and to intermediate values n Z , for which 0&lt;n Z &lt;n N . 
     The regulation of the accumulator pressure p S  is achieved or caused by the control unit  17  in that, in the first cycle after startup of the press  10  shown in  FIG. 2 , it firstly sets the speed to the nominal speed n N  and then exclusively regulates the accumulator pressure p S  in that either it sets the charging volume stream to zero, in that it sets the speed to zero, or it sets the speed to the nominal speed n N , and also ascertains a cycle duration T Z  and a charging duration T L , during which the charging volume stream is greater than zero, and also ascertains a charging speed n L , which is averaged over the charging duration T L . 
     In  FIG. 2 , the control unit  17  has set for this cycle, for example, the speed n in the closing phase to 0% of the nominal speed n N , in a starting section of the working phase to 100% of the nominal speed n N , in a subsequent end section of the working phase to 20% of the nominal speed n N , in the reset phase and in a starting section of the charging phase to 100% of the nominal speed n N , and then in a subsequent end section of the charging phase to 0% of the nominal speed n N . It has ascertained in this example for the charging duration T L  the value 75% of the cycle duration T Z  and for the charging speed n L , the value 100% of the nominal speed n N . 
     In this preferred embodiment, the control unit  17  is additionally implemented such that, for the second cycle shown in  FIG. 2 , during the regulation, it sets the speed from zero up to a maximum intermediate value n Z , for which n Z =n L ·T L : T Z +K N  applies, wherein K N  is a speed correction value with 0≦K N &lt;n L ·(1−T L : T Z ). For example, for the speed correction value K N =5%·n N  applies. Since the control unit  17  in the first cycle has ascertained T L : T Z =75% and n L =100%·n N =n N , for the second cycle it calculates n L ·T L : T Z =n N ·75% and n Z =n N ·75%+5%·n N =80%·n N . 
     In this preferred embodiment, the control unit is additionally implemented such that, in the second cycle, similarly to the first cycle, it again ascertains the cycle duration T Z , the charging duration T L , and the charging speed n L . 
     In  FIG. 2 , the control unit  17  has set for this cycle, for example, the speed n in a starting section of the closing speed to 0% of the nominal speed n N , in a subsequent end section of the closing phase and the working phase to 60% of the nominal speed n N , in the reset phase and in a starting section of the charging phase to the maximum intermediate value n Z , i.e., 80% of the nominal speed n N , and then in a subsequent end section of the charging phase to 0% of the nominal speed n N . It has ascertained in this example for the cycle duration T Z  the same value as in the first cycle, for the charging duration T L  a greater value than in the first cycle, and for the charging speed n L  a smaller value than in the first cycle. 
     In this preferred embodiment, the control unit  17  is additionally implemented such that, for the third cycle shown in  FIG. 2  and each following cycle, similarly to the first and second partial traction means during the regulation, it sets the speed from zero up to a maximum intermediate value n Z , for which n Z =n L ·T L : T Z +K N  applies, wherein the cycle duration T Z , the charging duration T L , and the charging speed n L  originate from the respective preceding cycle. (1435) 
     LIST OF REFERENCE NUMERALS: P1435 
     
         
           10  press 
           11  cylinder 
           11 . 1  press chamber of  11   
           11 . 2  reset chamber of  11   
           11 . 3  piston of  11   
           12  ram 
           13  pump 
           13 . 1  suction fitting of  13   
           13 . 2  pressure fitting of  13   
           14  motor 
           15  accumulator 
           16  tank 
           17  control unit 
           18  pressure sensors 
           19  valve 
           20  frequency converter 
           21  shaping tool