Abstract:
A method for operating a reciprocating positive displacement pump for the simultaneous low-pulsation discharge of a plurality of liquids, having for each liquid at least two pump chambers and displacement devices capable of movement therein, of which the one displacement device takes in liquid during the actual discharge phase of the other displacement device, and at the end of its intake stroke reverses its direction of movement, and, in a pre-compression phase, pre-compresses the liquid taken into the associated pump chamber and when a predeterminable pre-compression pressure has been achieved comes to a standstill, and remains at the standstill until the other displacement device has ended its liquid discharge, and, subsequent to this discharge, begins its own discharge. The method includes the steps of carrying out a pressure compensation during the subsequent discharge stroke preventing a pressure compensation between the individual pump chambers during the subsequent discharge stroke.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a method for operating a reciprocating positive-displacement pump for the simultaneous low-pulsation discharge of a plurality of liquids, having for each liquid at least two pump chambers and displacement devices capable of movement therein, of which the one displacement device takes in liquid during the actual discharge phase of the other displacement device, and reverses its direction of motion at the end of the intake stroke, and, in a pre-compression phase, pre-compresses the liquid taken into the associated pump chamber, and comes to a standstill when a prespecifiable pre-compression pressure is reached, and remains at a standstill until the other displacement device has concluded its liquid discharge, and, subsequent to this discharge, begins its own discharge. Moreover, the present invention relates to a reciprocating positive displacement pump for the simultaneous low-pulsation discharge of a plurality of liquids, having for each liquid at least two pump chambers and displacement devices that are capable of movement therein, of which the one displacement device takes in liquid during the actual discharge phase of the other displacement device, and reverses its direction of motion at the end of the intake stroke, and the liquid taken into the associated pump chamber is pre-compressed in a pre-compression phase and comes to a standstill when a pre-specifiable pre-compression pressure is reached, and remains at a standstill until the other displacement device has concluded its liquid discharge, and subsequent to this discharge begins its own discharge.  
         [0002]     In known pumps of this type, the displacement devices for the individual pump chambers are rigidly connected to one another and have a common drive, usually in the form of a hydraulic cylinder.  
         [0003]     Extraordinarily low pulsation without the use of what are known as pulsation dampers is achieved by oscillating displacement pumps having two displacement devices for each liquid, of which the one displacement device takes in liquid during the actual discharge phase of the other displacement device, and reverses its direction of motion at the end of the intake stroke, and the liquid taken into the associated pump chamber is pre-compressed in a pre-compression phase and comes to a standstill when a pressure prespecified by the system is reached, and remains at a standstill until the other displacement device has concluded its liquid discharge, and subsequent to this discharge the displacement device that is at a standstill at the end of the pre-compression phase begins its discharge.  
         [0004]     In pumps of this type, the hydraulic pressure applied in the hydraulic drive cylinder is controlled for example by a control device according to DE 197 27 623 C1, dependent on the pressure of the current discharge side of the pump, in such a way that the pressure always remains at a safe distance below the pressure of the currently discharge side of the pump, so that liquid outlet valves of the pump chambers cannot open as a result of the higher pressure prevailing in the liquid line to the consumer.  
         [0005]     In pumps of this type, as a rule different pre-compression pressures arise in the two discharge cylinders until the opening of one of the outlet valves, and as a rule different pre-compression pressures also occur from one discharge stroke to the next discharge stroke. It is a fairly rare coincidence for the two pre-compression pressures to be equal. The differences in the pre-compression pressures are dependent on different degrees of filling of the pump chambers, differences in viscosity, different tightnesses at the valves, differences in the compressibility of the two liquids that are to be discharged, which for example can already vary strongly from stroke to stroke merely as a result of single air bubbles, and on different degrees of elasticity of the components.  
         [0006]     At the end of the pre-compression stroke, the hydraulic piston force of the drive cylinder is in equilibrium with the sum of the two axial forces on the displacement devices. The sum of the two displacement device forces (pre-compression pressure times surface) is always the same; i.e., the missing amount of pre-compression force at the one displacement device is added at the other displacement device. Thus, there is a double effect.  
         [0007]     The surface ratio of the two displacement devices has a great influence on the difference in the pre-compression pressures. The greater the surface difference, the faster and farther the pre-compression pressure increases at the displacement device having the smaller surface, due to the small pump chamber volume, above the pre-compression pressure of the pump chamber having the larger-surface displacement device.  
         [0008]     The possible pressure difference is greater: 
        the greater the surface difference of the displacement devices is,     the tighter a suction valve of the smaller pump chamber is,     the less tight a suction valve of the larger pump chamber is,     the faster the suction valve of the smaller pump chamber closes,     the slower the suction valve of the larger pump chamber closes,     the greater the degree of filling of the smaller pump chamber is,     the lesser the degree of filling of the larger pump chamber is,     the smaller the volume of the smaller pump chamber is,     the larger the volume of the larger pump chamber is,     the less compressible the liquid in the smaller pump chamber is, and     the more compressible the liquid in the larger pump chamber is.        
 
         [0020]     From the above, it is clear that statements about the size of the differences in the pre-compression pressures are less reliable the greater the differences in the displacement device surfaces, and the greater the differences between the two liquids that are to be discharged. For the sake of simplicity, hereinafter only different degrees of filling will be referred to.  
         [0021]     If, given an displacement device surface ratio of 1:1, for example no pre-compression at all is achieved in one of the two pump chambers (e.g. due to insufficient filling or a leaky suction valve), the pre-compression pressure in the other cylinder increases to twice the pressure that would arise given a uniform distribution. If no pre-compression is achieved given an displacement device surface ratio of e.g. 49:1 in the cylinder having the larger displacement device, the pressure in the other cylinder increases to 49 times the pressure.  
         [0022]     If the unequally pre-compressed pump chamber fillings of one pump side are connected by lines to the consumer, e.g. a spray gun, at the beginning of the discharge stroke, the discharge of the two liquids begins nonuniformly and with a temporal offset. The discharge cylinder having the higher pressure relative to a consumer line pressure emits, when its pressure relaxes to consumer pressure, a corresponding liquid quantity into the consumer line in pulsed fashion, in addition to the target quantity. The other discharge cylinder, whose pre-compression pressure is lower than the consumer pressure, must first be further compressed to consumer pressure before material is pressed into the consumer line. For this purpose, a displacement device stroke, i.e. time, is required. While the one pump chamber, relative to one pump side, begins with surplus discharge, the discharge from the other pump chamber begins too late, so that at the consumer there is a brief lack of liquid in the insufficiently pre-compressed discharge cylinder.  
         [0023]     Therefore, for the present invention the problem arises of removing these disadvantages of discharge flow irregularities of the previous methods and devices, and to create a method and a device of the type indicated above that provide a discharge of the liquids that are to be discharged that is uniform at all times.  
       SUMMARY OF THE INVENTION  
       [0024]     In order to solve the first part of the problem, a method of the type named above is proposed that is characterized in that during the pre-compression phase a pressure compensation is carried out between the individual pump chambers, and during the subsequent discharge stroke a pressure compensation between the individual pump chambers is prevented.  
         [0025]     With the method according to the present invention, it is advantageously achieved that different pre-compression pressures are avoided, and that the liquid discharge begins simultaneously and is maintained from all pump chambers at the beginning of the discharge stroke, in quantities corresponding to the displacement device cross-sections.  
         [0026]     A first construction provides that, for each liquid to be discharged, pistons that are rigidly connected to a single drive piston transmit their force via a fluid to the displacement device allocated to the respective piston, fluid chambers between the individual pistons and the associated displacement devices being connected to one another during the pre-compression phase and being decoupled from one another during the discharge stroke. The fluid chambers allocated to the displacement devices are therefore interconnected during the pre-compression phase, so that the displacement devices can move relative to one another until pressure equality is achieved in the pump chambers. In contrast, during the discharge stroke the connection of the fluid chambers is interrupted, so that no pressure compensation can then take place, and the displacement devices execute the discharge stroke as if they were rigidly coupled to one another.  
         [0027]     In another embodiment of the present invention, it is proposed that the pressure compensation be brought about by the displacement of arrestable pistons that are functionally connected and are exposed to the liquids that are to be pre-compressed. Preferably, for this purpose a compensating piston is used that is capable of being displaced in a housing and that divides the housing into two chambers and has two identical piston surfaces, each chamber being connected to one of the pump chambers. When there is a pressure difference between the pump chambers, the compensating piston moves, under the influence of the greater pressure, in the direction of the pump chamber having the lower pressure until the pressure difference is reduced to zero. In at least one of two lines between each pump chamber and its allocated chamber of the pressure compensation device, a shutoff device should be present that is open during the pre-compression phase for the purpose of pressure compensation between the pump chambers and is closed during the discharge stroke, so that the displacement devices carry out the discharge stroke as if they were connected rigidly to one another. During the intake stroke, and while the shutoff device is simultaneously open, the compensating piston should be compelled to move to its center position by at least one centering spring, preferably by two centering springs. In this way, the entire pressure compensation capacity of the pressure-compensating device is available for the subsequent pre-compression phase. Instead of the one compensating piston acted on at two sides by the two liquids that are to be conveyed, it is also possible to use two compensating pistons acted on at one side by the liquids, and functionally connected to one another via a lever system.  
         [0028]     According to an embodiment that is preferred in particular for cases in which the displacement devices have extremely different cross-sections, it is provided that during the pre-compression phase the displacement devices are driven independently of one another, and come to a standstill independently of one another when the desired pre-compression pressure is achieved, and are coupled to one another during the discharge stroke. In other words, therefore, the smaller-surface displacement device is to be rigidly coupled to the larger-surface displacement device during the discharge stroke. During the pre-compression phase, the smaller-surface displacement device should be capable of being displaced relative to the larger-surface displacement device, and should preferably produce, by a hydraulic cylinder having a corresponding size that is allocated thereto, approximately the same pre-compression pressure in the liquid enclosed in the associated pump chamber as is produced in the liquid in the pump chamber having the larger-surface displacement device.  
         [0029]     Usefully, with the aid of a cylinder, preferably a pneumatic cylinder, it should be possible to bring the smaller-surface displacement device into a zero position during the intake stroke, from which position the full provided pressure compensation capacity is available in both directions for the following pre-compression phase.  
         [0030]     In another preferred embodiment, the present invention proposes that the pre-compression be carried out by pre-compression displacement device that is capable of being arrested during the discharge stroke, is driven in a hydraulically decoupled manner, is allocated to the second displacement device and to all additional displacement devices of a pump side, and acts in each case on the same liquid that is to be discharged. Thus, differing from the embodiment indicated above, here instead of the one small-surface displacement device two displacement devices are to be used, each acting on the same quantity of liquid enclosed in the discharge cylinder. However, the one displacement device should be constantly coupled rigidly to the large-surface displacement device and should take over the liquid discharge function, while the other, smaller-surface displacement device takes over the pressure compensation during the pre-compression phase. This additional small-surface displacement device should therefore be capable of displacement relative to the other small-surface displacement device, and should be connected to a cylinder, preferably a hydraulic cylinder, of a corresponding size, that produces in the enclosed liquid approximately the same pre-compression pressure as is produced in the pump chamber having the large-surface displacement device. During the discharge stroke, the displaceability of this additional small-surface displacement device should be disabled, e.g. by closing a stop valve situated in a hydraulic line to the associated hydraulic cylinder.  
         [0031]     In this variant as well, during the intake stroke the additional small-surface displacement device should usefully be brought into a zero position, from which position the full provided capability of movement in both directions is available for the following pre-compression phase.  
         [0032]     The solution of the second part of the problem is achieved according to the present invention by a reciprocating positive displacement pump of the type cited above that is characterized by means that effect a pressure compensation between the individual pump chambers during the pre-compression phase, and that prevent a pressure compensation between the individual pump chambers during the subsequent discharge stroke. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]     In the following, exemplary embodiments of the present invention are explained on the basis of a drawing.  
         [0034]      FIG. 1  shows a reciprocating positive displacement pump in a first embodiment, in which two additional hydraulic pistons connected rigidly to a hydraulic drive piston transmit their force to displacement devices via an enclosed fluid,  
         [0035]      FIG. 2  shows the positive displacement pump in an embodiment having a pressure compensation device that is acted on by the liquids enclosed in two pump chambers,  
         [0036]      FIG. 3  shows the positive displacement pump in an embodiment in which during a pre-compression phase both displacement devices of each pump side bring the liquids enclosed in the pump chambers to pre-compression pressure independently of one another,  
         [0037]     FIGS.  4  to  6  each show a segment from  FIG. 3  with the pump part having the smaller-surface displacement device in various positions,  
         [0038]      FIG. 7  shows the positive displacement pump in an embodiment in which during the pre-compression phase the one liquid is brought to pre-compression pressure by its associated displacement device, and the other liquid is brought to pre-compression pressure by an additional displacement device that is not entrained with a discharge stroke and that is provided separately for a pre-compression, and  
         [0039]     FIGS.  8  to  10  each show a segment from  FIG. 7 , with the pump part having the smaller-surface displacement device in various positions. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]     All positive displacement pumps shown as examples are used for the pulsation-free discharge, which is constant and uniform with respect to the discharged quantity ratio, of two liquids  10  and  11  by the positive displacement pump, from respectively allocated supply reservoirs  10 ′ and  11 ′ via intake lines  10 ″ and  11  ″ to a consumer (not shown) to which discharge lines  19  and  19 ′ lead that are connected after the positive displacement pump.  
         [0041]     Liquids  10  and  11  can for example be the two components of a two-component coating material for roadway markings, in which case the consumer is then for example an application nozzle or a comparable device for applying the coating material onto a roadway that is to be marked.  
         [0042]     In the following, only the process of the building up of identical pre-compression pressure in the two pump chambers  1  and  2  of the first pump part (shown at left in FIGS.  1  to  3  and  7 ) will be considered; in the other, second pump part (shown at the right in the named Figures) the same process takes place identically, in push-pull fashion relative to the process in the first pump part.  
         [0043]     In the example according to  FIG. 1 , the force of a drive piston  3 , here a hydraulic piston, is transmitted via two pistons  6  and  7 , via a fluid (here hydraulic oil) enclosed in two fluid chambers  8  and  9 , and via two displacement devices  4  and  5 , to two liquids  10  and  11  enclosed in pump chambers  1  and  2 . Fluid chambers  8  and  9  are connected by a line  12  in which there is situated a stop valve  13 .  
         [0044]     If, given a backwards movement for example of displacement device  4  in pump chamber  1 , e.g. due to better pump chamber filling, the pressure increases in pump chamber  1  faster than in the other pump chamber  2 , then the pressure in the corresponding fluid chamber  8  also increases, and fluid is pressed via line  12  and opened stop valve  13  into fluid chamber  9  of the other displacement device  5 . Displacement device  4  therefore remains in its backwards movement while displacement device  5  is accelerated until approximately the same pressure, dependent on the hydraulic oil pressure regulated by a regulating device (not shown) in a drive cylinder chamber  14  of drive piston  3 , has established itself.  
         [0045]     Before the beginning of a subsequent discharge stroke, stop valve  13  is closed, and a displacement of fluid between fluid chambers  8  and  9  during the discharge stroke is therefore prevented.  
         [0046]     Between each of the pistons  6 ,  7  and one of the pressure springs  15  and  16  allocated to displacement devices  4 ,  5 , when stop valve  13  is open, during the intake stroke (upward stroke) displacement devices  4  and  5  move back into a central initial position relative to pistons  6  and  7 , which position ensures the complete capability of movement upward and downward for the following pre-compression phase.  
         [0047]     However, when the displacement devices are displaced from their initial position relative to pistons  6  and  7 , the pressure springs cause a slight inequality of the pressures in liquids  10  and  11 . This is because one of the springs is always shortened slightly, causing a pressure increase in the corresponding discharge medium, while the other spring is lengthened, causing a reduction in pressure in the other discharge medium. However, these differences do not disturb the functioning of the pump.  
         [0048]     In the embodiment according to  FIG. 2 , displacement devices  4  and  5  are connected rigidly to drive piston  3 , and execute the axial movements in common, without displacement relative to one another. First pump chamber  1  is connected to a first chamber  20 , and second pump chamber  2  is connected to a second chamber  22 , of a pressure compensation device  21 , via lines  12 . 1  and  12 . 2  respectively. Pressure compensation device  21  has a displaceable compensating piston  24  that extends into oppositely situated chambers  20  and  22 . A stop valve  13 ′ is installed in one of lines  12 . 1 ,  12 . 2  (here in line  12 . 1 ).  
         [0049]     If, given a backward movement of displacement devices  4  and  5 , the pressure for example in first pump chamber  1  increases faster than in second pump chamber  2  due to a higher degree of filling, then, given a now opened stop valve  13 ′, compensating piston  24  moves toward the chamber  22  having the lower pressure (here into chamber  22 ) due to the higher pressure that also arises in chamber  20 , until approximately the same pressure, determined by the pressure in drive cylinder chamber  14  of drive piston  3 , has established itself in both chambers  20  and  22 , and in both pump chambers  1  and  2  connected thereto.  
         [0050]     Before the beginning of the subsequent discharge stroke, stop valve  13 ′ is closed, thus preventing a further displacement of the liquids in pressure compensation device  21 .  
         [0051]     Compensating piston  21  is held between two axially acting pressure springs  25  that, given an opened stop valve  13 ′, guide compensating piston  21  back to its center position during the intake stroke, said position ensuring the complete provided capability of movement in both directions for the following pre-compression phase. The same holds for the influence of springs  25  on the liquid pressures, as was stated above in relation to  FIG. 1 .  
         [0052]     In the example according to FIGS.  3  to  6 , only first displacement device  4  is connected rigidly to drive piston  3 , while second displacement device  5  is capable of being displaced relative to displacement device  4 . The pressure from drive cylinder chamber  14  acts via a line  12 ″, in which a stop valve  13 ″ is situated, in a cylinder  42  connected rigidly to drive piston  3 , on a piston  43  allocated to second displacement device  5 .  
         [0053]     The ratio of the effective surface of piston  43  to the effective surface of second displacement device  5  corresponds to the surface ratio of drive piston  3  and displacement device  4 .  
         [0054]     Under the influence of the pressure prevailing in drive cylinder chamber  14 , when stop valve  13 ″ is open the same pre-compression pressures arise in both pump chambers  1  and  2  even given different degrees of filling, as a result of the displaceability relative to one another of drive piston  3  and piston  43 .  
         [0055]      FIG. 4  shows a segment from  FIG. 3  with small-surface displacement device  5 , in which this displacement device  5  is shown in its initial position relative to displacement device  4 , in which it has the full provided movement capability upward and downward relative to displacement device  4 .  
         [0056]     If, at the beginning of the pre-compression phase, second pump chamber  2  has a degree of filling less than that of first pump chamber  1 , then second displacement device  5  moves until the desired pre-compression pressure is achieved in a position that is advanced, i.e. here displaced downward, relative to first displacement device  4 , said position being shown in  FIG. 5 , which shows the same segment as  FIG. 4 . The magnitude of the relative displacement is dependent on the size of the difference in the degree of filling in pump chambers  1  and  2 .  
         [0057]     If, in contrast, the degree of filling in second pump chamber  2  is greater than in first chamber  1 , then second displacement device  5  moves into a position that is retarded, i.e., here displaced upward, relative to first displacement device  4 , said position being shown in  FIG. 6 , which shows the same segment as  FIG. 4 .  
         [0058]     On its side facing away from displacement device  5 , piston  43  has a piston rod  44  that is led out from the cylinder chamber of cylinder  42  and is led through a subsequent cylinder chamber  46 . A piston  45  that is capable of axial displacement is situated on piston rod  44 . On the side facing away from displacement device  5 , piston rod  44  has an enlargement in its diameter that forms a collar  47 . In addition, on the side facing away from displacement device  5  cylinder chamber  46  has a taper that forms a collar  48 .  
         [0059]     The initial position of displacement device  5  according to  FIG. 4 , in which displacement device  5  has the provided capability of movement in both directions, is defined in that piston  45  is adjacent to collar  48  of cylinder chamber  46  and piston rod collar  47  is adjacent to piston  45 .  
         [0060]     If, during the pre-compression phase, displacement device  5  moves out of the initial position into a position that is displaced downward relative to displacement device  4 , piston  45  is carried along in the same direction by piston rod collar  47 . Through the pressure charging of cylinder chamber  46  during the intake stroke of the two displacement devices  4  and  5 , piston  45  and (via collar  47 ) displacement device  5  are guided upward back into the initial position.  
         [0061]     If, in contrast, during the pre-compression phase displacement device  5  moves out of the initial position relative to displacement device  4  into a position that is displaced upwardly, piston rod  44  moves through piston  45  adjacent to cylinder collar  48 , so that piston rod collar  47  moves away from piston  45 . During the intake stroke, displacement device  5  is guided back by the downward-directed frictional force at the sealing point of pump chamber  2  and by a slight remaining hydraulic pressure in the cylinder chamber of cylinder  42  until piston rod collar  47  on piston  45 , which is pressed against cylinder collar  48  in pressure-charged cylinder chamber  46 , comes to be seated and thus again assumes its initial position.  
         [0062]     During the discharge stroke, stop valve  13 ′ is closed, so that as a result a displacement of displacement device  5  relative to displacement device  4  is no longer possible, so that both displacement devices  4  and  5  execute the discharge stroke as if they were connected rigidly to one another.  
         [0063]     In the example according to FIGS.  7  to  10 , the functions “discharge” and “pre-compression pressure buildup” are divided between two displacement devices  5  and  51 . A second, smaller-surface displacement device  5  that is connected rigidly to the first, larger-surface displacement device  4  takes over the liquid discharge; the other, freely movable displacement device  51  takes over the buildup of the pre-compression pressure. The other displacement device  51  and its actuating device are not allocated to large-surface displacement device  4 , as is the case in the embodiment according to FIGS.  4  to  7 ; rather, they are allocated to second pump chamber  2 . Displacement devices  4  and  5 , rigidly connected to one another, can be displaced via common drive piston  3 . Drive cylinder chamber  14 , situated above drive piston  3 , is connected, via a hydraulic line  12 ′″ having a stop valve  13 ′″ situated therein, to a cylinder chamber  53  for a displacement of additional displacement device  51 .  
         [0064]      FIG. 8  shows a segment with smaller-surface displacement device  5  from  FIG. 7 , with additional displacement device  51  in its initial position, in which it has the complete provided capability of movement upward and downward for the compensation of differences in the degrees of filling in pump chambers  1  and  2 . The operating principle of the actuating device corresponds to the operating principle of the actuating device according to  FIG. 4 .  
         [0065]     If, at the beginning of the pre-compression phase, second pump chamber  2  has a degree of filling that is less than that of first pump chamber  1 , then the additional displacement device  51  moves into a position below the initial position, as is shown in  FIG. 9 . The magnitude of the displacement is dependent on the size of the difference in the degree of filling.  
         [0066]     If, in contrast, the degree of filling in second pump chamber  2  is greater than in first pump chamber  1 , then additional displacement device  51  moves into a position (shown in  FIG. 10 ) above the initial position, as can be seen in  FIG. 10 .  
         [0067]     A hydraulic piston  52 , connected fixedly to additional displacement device  51 , has on its side facing away from displacement device  51  a piston rod  55  that is led out from the associated cylinder chamber  53  and is led through a subsequent cylinder chamber  54 . A piston  56  capable of axial displacement is situated on piston rod  55 . On the side facing away from additional displacement device  51 , piston rod  55  has an enlargement in its diameter that forms a collar  58 . In addition, on its side facing away from additional displacement device  51 , cylinder chamber  54  has a taper that forms a collar  57 .  
         [0068]     The initial position of additional displacement device  51 , as is shown in  FIG. 8  and in which additional displacement device  51  has the complete provided capability of movement in both directions, is defined in that piston  56  is adjacent to collar  57  of cylinder chamber  54  and piston rod collar  58  is adjacent to piston  56 .  
         [0069]     When there is a displacement of additional displacement device  51  during the pre-compression phase into a position below the initial position, piston  56  is carried along in the same direction by piston rod collar  58 . Through pressure charging of cylinder chamber  54  during the intake stroke of displacement devices  4  and  5 , piston  56  and, via collar  58 , additional displacement device  51  are guided upward back into the initial position.  
         [0070]     When there is a displacement of additional displacement device  51  into a position above the initial position, piston rod  55  moves through piston  56  adjacent to cylinder collar  57 , so that piston rod collar  58  moves away from piston  56 . During the intake stroke of displacement devices  4  and  5 , additional displacement device  51  is guided back by a slight remaining hydraulic pressure in cylinder chamber  53  until piston rod collar  58  on piston  56 , which is pressed against cylinder collar  57  in pressure-charged cylinder chamber  54 , comes to be seated and thus again assumes its initial position.  
         [0071]     During the discharge stroke of displacement devices  4  and  5 , stop valve  13 ′″ is closed, so that a displacement of additional displacement device  51  is no longer possible.  
         [0072]     As is apparent from the foregoing specification, the invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description. It should be understood that I wish to embody within the scope of the patent warranted hereon all such modifications as reasonably and properly come within the scope of my contribution to the art.  
       LIST OF REFERENCE CHARACTERS  
       [0000]    
       
           1  first pump chamber  
           2  second pump chamber  
           3  drive piston  
           4  first displacement device  
           5  second displacement device  
           6  first piston  
           7  second piston  
           8  first fluid chamber  
           9  second fluid chamber  
           10  first liquid to be discharged  
           10 ′ supply reservoir for  10   
           10 ″ intake line for  10   
           11  second liquid to be discharged  
           11 ′ supply reservoir for  11   
           11 ″ intake line for  11   
           12  line between  8  and  9   
           12 ″ line between  14  and  42   
           12 ″ line between  14  and  53   
           12 . 1  line between  1  and  20   
           12 . 2  line between  2  and  22   
           13 - 13 ′″ stop valves  
           14  drive cylinder chamber  
           15  spring in  4   
           16  spring in  5   
           17 . 1 ,  17 . 2  suction valves  
           18 . 1 ,  18 . 2  outlet valves  
           19 ,  19 ′ discharge lines  
           20  first chamber in  21   
           21  pressure compensation device  
           22  second chamber in  21   
           24  compensating piston between  20  and  22   
           25  springs in  20  and  22   
           42  hydraulic cylinder  
           43  hydraulic piston  
           44  piston rod  
           45  displaceable piston  
           46  cylinder chamber  
           47  collar on  44   
           48  collar on  46   
           51  additional displacement device  
           52  hydraulic piston  
           53  cylinder chamber  
           54  cylinder chamber  
           55  piston rod  
           56  displaceable piston  
           57  collar in  54   
           58  collar on  55