Patent Publication Number: US-11035391-B2

Title: Hydrostatic linear drive system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority to DE 10 2019 110 917.5, filed Apr. 26, 2019, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a hydrostatic linear drive system, in particular for a closure unit of a blow mold installation 
     BACKGROUND OF THE INVENTION 
     Hydrostatic linear drive systems are used, for example, in hydraulic presses, deep-drawing or injection-molding machines. Such machines generally have a plurality of movement sequences. One of these movement sequences is a power mode in which a high force is applied at low speed to a workpiece which is intended to be processed or a component which is intended to be moved. Another of these movement sequences is a rapid mode in which a smaller force is applied but enables a more rapid movement. 
     Such a linear drive system is known, for example, from DE 10 2016 113 882 A1. The known linear drive system has a hydraulic pump which is driven by an electric motor and which can be reversed in terms of flow direction. The hydraulic pump provides a changeable volume flow of a hydraulic fluid in a closed hydraulic circuit which comprises a first differential cylinder as a main cylinder. The annular piston face at the rod side of the differential cylinder is smaller than the piston face at the piston side. The closed hydraulic circuit is closed with respect to the environment thereof and has during operation an excess pressure with respect to the environment. This excess pressure is produced in a manner known per se by a pretensioning source. In order to compensate for the different volumes of the differential cylinder when it is moved in the retraction and extension direction, the drive system requires an equalizing tank. The equalizing tank is preferably constructed as a second differential cylinder whose cylinder chamber is open with respect to the environment at the piston side and whose annular face corresponds to the difference between the piston face and the annular face of the main cylinder. The piston rods of the two differential cylinders are mechanically coupled. A 2/2-way valve is arranged in the connection line between the annular space of the second cylinder which functions as an equalizing tank and the annular space of the main cylinder. In another connection line between the annular space of the second cylinder which functions as an equalizing tank and the piston chamber of the main cylinder, another 2/2-way valve is arranged. In order to retract and extend the main cylinder in the power mode, the 2/2-way valve is opened between the two annular spaces of the two differential cylinders, whilst the additional 2/2-way valve is blocked. In order to retract and extend the main cylinder in the rapid mode, the 2/2-way valve is in the meantime blocked between the two annular spaces of the differential cylinders, whilst the additional 2/2-way valve is opened. 
     The equalizing tank which is constructed as a differential cylinder results in the extension and retraction movement of the main cylinder in rapid mode always being carried out counter to the resistance of the second cylinder which functions as an equalizing tank, whereby high displacement speeds in rapid mode with at the same time significant forces in the power mode cannot be produced. For the operation of the drive system in rapid or in power mode, two 2/2-way valves are in addition absolutely necessary. 
     JP 46 14 544 B2 discloses a hydrostatic linear drive system having a 3-face cylinder, wherein a hydraulically active face for a power mode and two hydraulically active faces for a rapid mode are provided. The retraction and extension of the 3-face cylinder in rapid mode is carried out by the two hydraulically active faces for the rapid mode, which can be selectively acted on by a closed hydraulic circuit comprising a hydraulic pump. The cylinder chamber which is associated with the hydraulically active face for the power mode, when extended in rapid mode, is filled with fluid by a pretensioned equalizing tank. In power mode, this cylinder chamber is connected in a fluid-conducting manner to a pump which is connected at the intake side to a tank and the pretensioned equalizing tank. At the same time, the pressure of the pretensioned equalizing tank acts counter to the extension direction on one of the two hydraulically active faces for the rapid mode. 
     DE 10 2010 051 140 A1 discloses a linear drive system for a drawing press having a pressing frame, in which a tappet carrying an upper tool and a drawing cushion are supported with a drawing cushion plate on which a workpiece holding-down member is supported. The drawing cushion plate is mechanically coupled to the tappet by a strut arrangement. Rapid mode cylinders are provided for accelerated activation of the tool holding-down member and for the hydraulically driven upward and downward movement thereof. Furthermore, clamping cylinders which are supported on the drawing cushion plate and which are constructed as plungers are provided; they serve to move and act with force on the workpiece holding-down member relative to the drawing cushion plate. The pistons of the clamping cylinders and the rapid mode cylinders are coupled. For the pre-acceleration operation, there is provision shortly before the upper tool is placed on the holding-down members for the rapid mode cylinders to pre-accelerate the holding-down members and the pistons of the clamping cylinders in the movement direction of the tappet. The clamping cylinder chambers are in this instance connected by non-return valves to a pretensioned low-pressure store. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the invention is to provide a linear drive system which has a simpler and more compact construction than the prior art, with at the same time higher retraction and extension speed in rapid mode, higher forces in power mode, and reduced energy consumption. 
     The object is met based on the notion of bringing about the retraction and extension movement in rapid mode by separate hydraulically active faces, which are independent of a larger hydraulically active face which is acted on with pressurized hydraulic fluid only in power mode. The mechanical and hydraulic resistances are reduced by the larger hydraulically active face in rapid mode not functioning counter to a pressurized hydraulic fluid, but instead by the hydraulic fluid which is intended to be displaced from the cylinder chamber being supplied to an equalizing tank which is open with respect to the environment, that is to say, is not pretensioned, or by the hydraulic fluid which is intended to be supplied to the cylinder chamber being drawn from the equalizing tank. During the extension movement in power mode, however, the hydraulically active faces cooperate with each other which contributes to high forces with a compact structure of the drive system. 
     In a first embodiment according to the invention, the retraction and extension of the cylinders in the rapid mode is produced exclusively by synchronous cylinders which are incorporated in the closed hydraulic circuit. In rapid mode, hydraulic fluid is prevented from acting on the first hydraulically active face of the single-action cylinder. 
     The extension of the cylinders in power mode is produced first and foremost by the single-action cylinder whose first hydraulically active face is acted on with the hydraulic fluid in the extension direction. However, the single-action cylinder is supported during extension by the synchronous cylinder whose second hydraulically active face is also acted on with hydraulic fluid in the extension direction. In this embodiment, the mechanical coupling between the single-action cylinder and the synchronous cylinder is carried out by a coupling member between the piston rods of the two cylinders. 
     Another reduction of the mechanical resistances and the energy consumption is achieved by the single-action cylinder being a plunger cylinder, also referred to as a plunger piston cylinder. The piston rod of the plunger cylinder acts at the same time as a piston. Plunger cylinders have a better degree of mechanical efficiency than conventional, single-action cylinders. 
     A second embodiment of the present invention relates to a linear drive system having a cylinder—also referred to below as a 3-face cylinder—which integrates the hydraulically active faces of the single-action cylinder and the synchronous cylinder of the first embodiment in one component and which thereby contributes to a particularly compact construction type. 
     The retraction and extension of the 3-face-cylinder in rapid mode, as in the first embodiment, is brought about exclusively by acting on the second or third hydraulically active face with hydraulic fluid. In rapid mode, acting on the larger first hydraulically active face is prevented. 
     The extension of the 3-face cylinder in power mode is brought about first and foremost by acting on the larger, first hydraulically active face with hydraulic fluid. As in the first embodiment, however, the extension is supported by acting on the second hydraulically active face in the extension direction. 
     The low-pressure connection of the second hydraulic pump is connected in a fluid-conducting manner to the equalizing tank and the high-pressure connection is connected in a fluid-conducting manner to the first fluid connection of the single-action cylinder in the first embodiment or to the first fluid connection of the 3-face cylinder according to the second embodiment. 
     The fluid-conducting connection between the high-pressure connection of the second hydraulic pump and the first fluid connection can be carried out directly or indirectly. 
     In the case of a direct fluid-conducting connection, in rapid mode of the linear drive system, action on the first hydraulically active face simply by deactivation of the second hydraulic pump can be prevented. A blocking member is not required in this preferred embodiment since the high-pressure connection of the second hydraulic pump is exclusively connected in a fluid-conducting manner to the first fluid connection of the single-action cylinder or the 3-face cylinder. This embodiment therefore contributes to the simple and compact structure of the linear drive system. 
     In the case of an indirect fluid-conducting connection, the high-pressure connection of the second hydraulic pump is connected in a fluid-conducting manner to the first fluid connection indirectly via the second pressure connection of the first hydraulic pump. In such an embodiment of the invention, a non-return valve is arranged in the fluid-conducting connection between the second and first hydraulic pump such that a return flow of the hydraulic fluid in the direction of the second hydraulic pump is prevented. A fluid-conducting connection to a second shut-off member is provided between the first pressure connection of the first hydraulic pump and the first fluid connection. In rapid mode of the linear drive system acting on the first hydraulically active face is prevented using the shut-off member. In the power mode of the linear drive system, the first hydraulically active face is acted on with hydraulic fluid by the open shut-off member. 
     In the embodiment having the indirect fluid-conducting connection, the second hydraulic pump may be configured to be weaker than the first hydraulic pump since in principle it only has to provide the additional volume of hydraulic fluid, whilst the pressure build-up is carried out mainly via the first hydraulic pump. Of course, however, a second hydraulic pump with correspondingly greater dimensions may also contribute mainly to the pressure build-up in power mode. In addition, the second hydraulic pump in this embodiment may compensate for leakage oil in the closed hydraulic circuit and may pretension the closed hydraulic circuit. 
     In order to control the speed and force of the linear drive system in the power or rapid mode, the first and/or second hydraulic pump provides in an advantageous embodiment of the invention a changeable volume flow of the hydraulic fluid. To this end, the displacement volume and/or the drive speed of the first and/or second hydraulic pump may be changeable. 
     The driving of the hydraulic pump is carried out, for example, by an electric motor, whose speed and rotation direction can be changed in order to change the volume flow and the flow direction. If the driving is carried out by a speed-constant electric motor, the displacement volume of the hydraulic pump can be variable in order to change the volume flow. The displacement volume with a variable displacement pump, for example, is steplessly changed by adjusting a swash plate. When the swash plate is changed through the zero position, the flow direction of the volume flow changes so that the flow direction is reversed and the high-pressure and low-pressure side of the variable-displacement pump change over. Particularly energy-saving operation with an optimized overall degree of efficiency is preferably achieved with a combination of changeable speed of the electric motor and changeable displacement volume of the variable displacement pump. 
     In an advantageous embodiment of the invention, the pressure connections of the first and/or second hydraulic pump are connected to a pressure store. The connection to the connections is carried out by non-return valves which prevent a return flow from the pressure connections in the direction of the pressure store. The non-return valves open when there is a lower pressure at the pressure connections than in the pressure container. As a result of the pressure store, therefore, the dynamics of the linear drive system can be improved and/or energy can be saved. 
     If the pressure connections of the first hydraulic pump are connected to a pressure store, it can act at the same time as a pretensioning source for the closed hydraulic circuit. The pretensioning is, however, produced first and foremost by a hydraulic pump. 
     The pretensioning pressure in the closed hydraulic circuit is higher than ambient pressure. Ambient pressure is the hydrostatic pressure of the air which is applied at the location of the installation of the linear drive system. The mean air pressure of the atmosphere (atmospheric pressure) is normally approximately 1 bar. The pretensioning pressure is 5 to 50 bar, preferably between 10 and 25 bar. 
     The shut-off members provided in the hydraulic lines serve to block or release the volume flow of the hydraulic fluid. The shut-off members are preferably non-return valves, in particular 2/2-way valves. The 2/2-way valve has two connections and two switch positions. In the first, closed switch position, the throughflow is blocked by the 2/2-way valve, in the second, open switch position, the throughflow is released by the 2/2-way valve. 
     In order to prevent damage to the equalizing tank in the decompression phase of the single-action cylinder, a throttle is arranged in the fluid-conducting connection between the equalizing tank and the first fluid connection on the single-action cylinder or the 3-face cylinder according to an another embodiment of the invention. As a result of the throttle, the pressure of the hydraulic fluid flowing through in the fluid-conducting connection is reduced. The throttle may be constructed as an integral component of the non-return valve. 
     Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, wherein like reference characters denote similar elements throughout the several views: 
         FIG. 1  is a schematic diagram of a first embodiment of the hydrostatic linear drive system having a single-action cylinder and a synchronous cylinder, 
         FIG. 1A  is a schematic diagram of the first embodiment during an extension in a rapid mode, 
         FIG. 1B  is a schematic diagram of the first embodiment during an extension in a power mode, 
         FIG. 1C  is a schematic diagram of the first embodiment during cancellation of a force, 
         FIG. 1D  is a schematic diagram of the first embodiment during a retraction in the rapid mode, 
         FIG. 2  is a schematic diagram of a second embodiment of the hydrostatic linear drive system with a single-action cylinder and a synchronous cylinder, 
         FIG. 2A  is a schematic diagram of the second embodiment during an extension in a rapid mode, 
         FIG. 2B  is a schematic diagram of the second embodiment during an extension in a power mode, 
         FIG. 2C  is a schematic diagram of the second embodiment during cancellation of the force, 
         FIG. 2D  is a schematic diagram of the second embodiment during the retraction in rapid mode, 
         FIG. 3  is a schematic diagram of the first embodiment of the hydrostatic linear drive system with a 3-face cylinder, 
         FIG. 3A  is a schematic diagram of the system of  FIG. 3  during the extension in the rapid mode, 
         FIG. 3B  is a schematic diagram of the system of  FIG. 3  during the extension in the power mode, 
         FIG. 3C  is a schematic diagram of the system of  FIG. 3  during cancellation of the force, 
         FIG. 3D  is a schematic diagram of the system of  FIG. 3  during the retraction in the rapid mode, 
         FIG. 4  is a schematic diagram of the second embodiment of the hydrostatic linear drive system having a 3-face cylinder, 
         FIG. 4A  is a schematic diagram of the system of  FIG. 4  during the extension in the rapid mode, 
         FIG. 4B  is a schematic diagram of the system of  FIG. 4  during the extension in the power mode, 
         FIG. 4C  is a schematic diagram of the system of  FIG. 4  during cancellation of the force, and 
         FIG. 4D  is a schematic diagram of the system of  FIG. 4  during the retraction in the rapid mode. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a first embodiment of a hydrostatic linear drive system  1  having a single-action cylinder  2  which is constructed as a plunger cylinder. The plunger cylinder comprises a piston rod  3 , a first hydraulically active face  4  and a first cylinder chamber  5  having a first fluid connection  6  for a hydraulic fluid  7 . As a result of the construction of the single-action cylinder  2  as a plunger cylinder, the piston rod  3  is at the same time the piston. The end side of the piston rod  3  facing the cylinder chamber  5  is the piston face and is hydraulically active. 
     The first hydraulically active face  4  of the single-action cylinder  2  can be acted on with the hydraulic fluid  7  via the first fluid connection  6  in an extension direction  8  (see  FIG. 1A ). 
     Referring back to  FIG. 1 , the linear drive system  1  further has a synchronous cylinder  9  which has at both sides of the piston  10  a piston rod  11 ,  12 . The annular piston faces which surround the piston rods  11 ,  12  form a second hydraulically active face  13  and a third hydraulically active face  14  which correspond in terms of size/area. The synchronous cylinder  9  has a second fluid connection  15  and a third fluid connection  16 , wherein the second fluid connection  15  opens in an annular second cylinder chamber  35  at the left side of the piston  10  and the third fluid connection  16  opens in an annular third cylinder chamber  36  at the right side of the piston  10 . 
     The second hydraulically active face  13  can be acted on with hydraulic fluid  7  via the second fluid connection  15  in the extension direction  8  (see  FIG. 1A ). The third hydraulically active face  14  can be acted on with hydraulic fluid  7  via the third fluid connection  16  in the retraction direction  17  (see  FIG. 1D ). 
     A closed hydraulic circuit  18  which is under pretensioning pressure comprises the synchronous cylinder  9  and a first hydraulic pump  19  with a first and a second pressure connection  21 ,  22 . An electric motor  20  drives the first hydraulic pump  19  at a constant or variable motor speed. The first hydraulic pump  19  is preferably an axial piston variable-displacement pump of the swash plate construction type. As a result of the adjustment of a pivot angle the swash plate, the volume flow of the first hydraulic pump  19  can be changed in a stepless manner. When the swash plate is adjusted through the zero position, the flow direction  23 . 1 ,  23 . 2  (see  FIGS. 1A, 1B, and 1D ) of the volume flow changes. The adjustment of the pivot angle of the swash plate is carried out in this instance hydraulically by an actuating piston. 
     Depending on the flow direction  23 . 1 / 23 . 2 , the first pressure connection  21  is the high-pressure side and the second pressure connection  22  is the low-pressure side of the first hydraulic pump  19 , or vice versa. 
     The first pressure connection  21  is connected to the second fluid connection  15  and the second pressure connection  22  is connected to the third fluid connection  16  in a fluid-conducting manner by a hydraulic line  24 ,  25 , respectively. The pretensioning of the hydraulic fluid in the closed hydraulic circuit  18  may, for example, be produced by a pressure source which is not illustrated and which is connected to the pressure connections  21 ,  22  (for example, feed oil pump). Since the pretensioning of a closed hydraulic circuit  18  is known to the person skilled in the art, an illustration of the components required for this has been omitted for the sake of clarity. 
     In order to act on the first hydraulically active face  4  of the single-action cylinder  2  with hydraulic fluid  7 , a second hydraulic pump  26  is provided with a low-pressure connection  27  and a high-pressure connection  28 . The low-pressure connection  27  is connected in a fluid-conducting manner via a hydraulic line  29  to an equalizing tank  30  and the high-pressure connection  28  is connected in a fluid-conducting manner via a hydraulic line  31  to the first fluid connection  6  of the single-action cylinder  2 . 
     The equalizing tank  30  which is open towards the environment receives hydraulic fluid  7  and is further connected in a fluid-conducting manner to the first fluid connection  6  of the single-action cylinder  2  via a hydraulic line  32 . A 2/2-way valve  33 . 1  is arranged as a first shut-off member  33  in the hydraulic line  32  between the equalizing tank  30  and the single-action cylinder  2 . 
     Finally, the piston rod  3  of the single-action cylinder  2  and the piston rod  12  of the synchronous cylinder  9  are mechanically connected to each other by a coupling member  34  in such a manner that both cylinders move exclusively in a synchronous manner. 
     When the linear drive system  1  is extended in rapid mode as shown in  FIG. 1A , the second hydraulically active face  13  is acted on with hydraulic fluid  7  through the second fluid connection  15  by the first hydraulic pump  19  operating in the flow direction  23 . 1 , whereby the piston  10  of the synchronous cylinder  9  moves in the extension direction  8 . The piston rod  3  of the single-action cylinder  2  which is connected to the piston rod  12  of the synchronous cylinder  9  by the coupling member  34  is also moved in the extension direction  8  without the hydraulically active face  4  thereof being acted on with hydraulic fluid  7 . The hydraulic fluid  7  passes through the open 2/2-way valve  33 . 1  from the equalizing tank  30  via the first fluid connection  6  into the cylinder chamber  5  of the single-action cylinder  2  (extraction). The second hydraulic pump  26  which is also connected to the first fluid connection  6  is not active. 
     When the linear drive system  1  is retracted in rapid mode as shown in  FIG. 1D , the third hydraulically active face  14  is acted on with hydraulic fluid  7  through the third fluid connection  16  by the first hydraulic pump  19  which is now operating in the opposite flow direction  23 . 2 , whereby the piston  10  of the synchronous cylinder  9  moves in the retraction direction  17 . The piston rod  3  of the single-action cylinder  2  which is connected to the piston rod  12  of the synchronous cylinder  9  via the coupling member  34  is also moved in the retraction direction  17  without the hydraulically active face  4  thereof being acted on with hydraulic fluid  7 . The hydraulic fluid  7  is displaced through the open 2/2-way valve  33 . 1  from the cylinder chamber  5  of the single-action cylinder  2  into the equalizing tank  30 . The second hydraulic pump  26  which is connected to the first fluid connection  6  is not active. 
     When the linear drive system  1  is extended in power mode as shown in  FIG. 1B , the first hydraulically active face  4  of the single-action cylinder  2  is acted on with hydraulic fluid  7  through the first fluid connection  6  by the activated second hydraulic pump  26 , whereby the single-action cylinder  2  moves in the extension direction  8 . During extension in power mode, the first hydraulic pump  19  is further activated in the flow direction  23 . 1  so that through the second fluid connection  15  the second hydraulically active face  13  of the synchronous cylinder  9  is acted on with hydraulic fluid  7 . The synchronous cylinder moves synchronously with the single-action cylinder  2 , to which it is coupled via the coupling member  34 , in the extension direction  8 . A flow of the hydraulic fluid  7  via the hydraulic line  32  into the equalizing tank  30  is prevented by the closed 2/2-way valve  33 . 1 . The force produced during extension in power mode is produced by the cooperation of the single-action cylinder  2  and the synchronous cylinder  9  which is acted on in the extension direction  8 . 
     In order to end the power mode, the first hydraulic pump  19  and the second hydraulic pump  26  are deactivated and the 2/2-way valve  33 . 1  is opened so that the hydraulic fluid  7  can flow from the first cylinder chamber  5  via the open 2/2-way valve  33 . 1  in the hydraulic line  32  into the equalizing tank  30  as shown in  FIG. 1C . 
       FIG. 2  shows a second embodiment of a hydrostatic linear drive system  1  having a single-action cylinder  2 , which is constructed as a plunger cylinder. The plunger cylinder comprises a piston rod  3 , a first hydraulically active face  4  and a first cylinder chamber  5  with a first fluid connection  6  for a hydraulic fluid  7 . As a result of the construction of the single-action cylinder  2  as a plunger cylinder, the piston rod  3  is at the same time the piston. The end side of the piston rod  3  facing the cylinder chamber  5  is the first hydraulically active face  4 . 
     The first hydraulically active face  4  of the single-action cylinder  2  can be acted on with the hydraulic fluid  7  in an extension direction  8  via the first fluid connection  6 . 
     The linear drive system  1  further has a synchronous cylinder  9  which has at both sides of the piston  10  a piston rod  11 ,  12 . The annular piston faces which surround the piston rods  11 ,  12  form a second hydraulically active face  13  and a third hydraulically active face  14  which correspond to each other in terms of size/area. The synchronous cylinder  9  has a second fluid connection  15  and a third fluid connection  16 , wherein the second fluid connection  15  opens in an annular second cylinder chamber  35  at the left side of the piston  10  and the third fluid connection  16  opens in a third annular cylinder chamber  36  at the right side of the piston  10 . 
     The second hydraulically active face  13  can be acted on in the extension direction  8  with hydraulic fluid  7  via the second fluid connection  15 . The third hydraulically active face  14  can be acted on with hydraulic fluid  7  in the retraction direction  17  via the third fluid connection  16 . 
     A closed hydraulic circuit  18  which is under a pretensioning pressure comprises the synchronous cylinder  9  and a first hydraulic pump  19  with a first and a second pressure connection  21 ,  22 . An electric motor  20  drives the hydraulic pump  19  at a constant or variable motor speed. The hydraulic pump  19  is preferably an axial piston variable-displacement pump of the swash plate construction type as in the embodiment according to  FIG. 1 . 
     The first pressure connection  21  is connected to the second fluid connection  15  and the second pressure connection  22  is connected to the third fluid connection  16  in a fluid-conducting manner by a hydraulic line  24 ,  25 , respectively. The pretensioning of the hydraulic fluid in the closed hydraulic circuit  18  may, for example, be produced by a pressure source which is not illustrated and which is connected to the pressure connections  21 ,  22 . 
     In order to act on the first hydraulically active face  4  of the single-action cylinder  2  with hydraulic fluid  7 , the first fluid connection  6  of the single-action cylinder  2  is connected by a hydraulic line  38  in a fluid-conducting manner to the first pressure connection  21  of the first hydraulic pump  19 . A second shut-off member  39  which is constructed as a 2/2-way valve  39 . 1  is arranged in the hydraulic line  38 . A high-pressure connection  28  of a second hydraulic pump  26  is connected in a fluid-conducting manner via a hydraulic line  31  to the second pressure connection  22  of the first hydraulic pump  19 . A non-return valve  40  in the hydraulic line  31  prevents a return flow of the hydraulic fluid  7  in the direction of the second hydraulic pump  26 . 
     A low-pressure connection  27  of the second hydraulic pump  26  is connected in a fluid-conducting manner by a hydraulic line  29  to the equalizing tank  30 . In contrast to the embodiment according to  FIG. 1 , however, the high-pressure connection  28  of the second hydraulic pump  26  is not directly connected to the first fluid connection  6  of the single-action cylinder  2  in a fluid-conducting manner, but instead indirectly by the flow path released by the open 2/2-way valve  39 . 1  and the first hydraulic pump  19  which is activated in the flow direction  23 . 1 . 
     The equalizing tank  30  which is open in the direction towards the environment receives hydraulic fluid  7  and is further connected in a fluid-conducting manner via a hydraulic line  32  to the first fluid connection  6  of the single-action cylinder  2 . A 2/2-way valve  33 . 1  is arranged in the hydraulic line  32  between the equalizing tank  30  and the single-action cylinder  2  as a first shut-off member  33 . 
     Finally, the piston rod  3  of the single-action cylinder  2  and the piston rod  12  of the synchronous cylinder  9  are mechanically connected to each other by a coupling member  34  in such a manner that both cylinders move exclusively in a synchronous manner. 
     When the linear drive system  1  is extended in the rapid mode, the second hydraulically active face  13  is acted on with hydraulic fluid  7  through the second fluid connection  15  by the first hydraulic pump  19  operating in the flow direction  23 . 1  as shown in  FIG. 2A , whereby the piston  10  of the synchronous cylinder  9  moves in the extension direction  8 . The piston rod  3  of the single-action cylinder  2  which is connected to the piston rod  12  of the synchronous cylinder  9  via the coupling member  34  is also moved in the extension direction  8  without the hydraulically active face  4  thereof being acted on with hydraulic fluid  7 . The hydraulic fluid  7  from the equalizing tank  30  reaches the cylinder chamber  5  of the single-action cylinder  2  by the open 2/2-way valve  33 . 1  through the first fluid connection  6  (extraction). The first hydraulically active face  4  of the single-action cylinder  2  is not acted on since the 2/2-way valve  39 . 1  is closed in the hydraulic line  38 . 
     When the linear drive system  1  is retracted in rapid mode, the third hydraulically active face  14  is acted on with hydraulic fluid  7  through the third fluid connection  16  by the first hydraulic pump  19  which is operated in the opposite flow direction  23 . 2  as shown in  FIG. 2D , whereby the piston  10  of the synchronous cylinder  9  moves in the retraction direction  17 . The piston rod  3  of the single-action cylinder  2 , which is connected to the piston rod  12  of the synchronous cylinder  9  by the coupling member  34 , is also moved in the retraction direction  17  without the hydraulically active face  4  being acted on with hydraulic fluid  7 . The hydraulic fluid  7  is displaced through the open 2/2-way valve  33 . 1  from the cylinder chamber  5  of the single-action cylinder  2  into the equalizing tank  30 . The 2/2-way valve  39 . 1  in the hydraulic line  38  is closed. 
     When the linear drive system  1  is extended in power mode, the first hydraulically active face  4  of the single-action cylinder  2  is acted on with hydraulic fluid  7  through the first fluid connection  6  as shown in  FIG. 2B , whereby the single-action cylinder  2  moves in the extension direction  8 . The 2/2-way valve  39 . 1  in the hydraulic line  38  is now open. Both the first hydraulic pump  19  and the second hydraulic pump  26  are activated and convey the hydraulic fluid  7  in a corresponding flow direction  23 . 1  in the direction of the first fluid connection  6  of the single-action cylinder  2 . The activated second hydraulic pump  26  provides the required additional volume of hydraulic fluid  7  from the equalizing tank  30  in order to act with hydraulic fluid  7  on the first hydraulically active face  4  of the single-action cylinder  2  through the first fluid connection  6  and to act with hydraulic fluid  7  on the second hydraulically active face  13  of the synchronous cylinder  9  through the second fluid connection  15 . The synchronous cylinder  9  moves synchronously in the extension direction  8  with the single-action cylinder  2 , to which it is coupled via the coupling member  34 . A flow of the hydraulic fluid  7  via the hydraulic line  32  into the equalizing tank  30  is prevented by the closed 2/2-way valve  33 . 1 . 
     In order to end the power mode, the first hydraulic pump  19  and the second hydraulic pump  26  are deactivated, the 2/2-way valve  33 . 1  is opened, and the 2/2-way valve  39 . 1  in the hydraulic line  38  is closed as shown in  FIG. 2C  so that the hydraulic fluid  7  flows from the first cylinder chamber  5  via the open 2/2-way valve  33 . 1  in the hydraulic line  32  into the equalizing tank  30 . 
       FIG. 3  shows a third embodiment of a hydrostatic linear drive system  1  having a 3-face cylinder  42 , which integrates the functions of the single-action cylinder  2  and the synchronous cylinder  9  of the embodiments according to  FIGS. 1 and 2  in a sub-assembly. Components of the 3-face cylinder  42  which correspond in terms of function to the embodiments according to  FIGS. 1 and 2  are given corresponding reference numerals. The 3-face cylinder  42  has a cylinder pipe  43 , a cylinder base  44  which terminates the cylinder pipe  43  at an end side and a piston rod guide  45  which is arranged at the opposite end side. The piston rod guide  45  guides a piston rod  46  in an axial direction. An annular piston  47  is arranged at one end of the piston rod  46 . From the cylinder base  44 , a guiding pin  48  extends into the cylinder pipe  43 . The annular piston  47  surrounds the guiding pin  48  and is moved in a sliding manner along the guiding pin  48  in an extension direction  8  (see  FIGS. 3A and 3B ) and a retraction direction  17  (see  FIG. 3D ). 
     The piston rod  46  has a hollow space  49  in the form of a blind hole which extends from the central passage in the annular piston  47  into the piston rod  46  and which surrounds the guiding pin  48 . 
     The 3-face cylinder has first, second, and third hydraulically active faces  4 ,  13 ,  14 . The first hydraulically active face  4  is formed by the first annular piston face  47 . 1  facing the cylinder base  44  and delimits a first annular cylinder chamber  5 . The second hydraulically active face  13  is formed by a partial surface  50  of the hollow space  49 , which is opposite the end side of the guiding pin  48 . The third hydraulically active face  14  is formed by the second annular piston face  47 . 2  facing the piston rod guide  45 . 
     A second annular cylinder chamber  51  is formed by the annular piston face  47 . 2 , the cover (outer surface) of the piston rod  46 , the inner face of the cylinder pipe  43  and at the end side by the piston rod guide  45 . 
     The second and third hydraulically active faces  13 ,  14  correspond to each other in terms of size/area. 
     The first cylinder chamber  5  has a first fluid connection  6 , through which the first hydraulically active face  4  can be acted on with a hydraulic fluid  7  in the extension direction  8  of the 3-face cylinder  42 . 
     The second hydraulically active face  13  can be acted on with the hydraulic fluid  7  through a second fluid connection  15  in the extension direction  8 . The second fluid connection  15  is located on the cylinder base  44 . From the second fluid connection  15 , the hydraulic fluid  7  reaches a discharge opening arranged at the end side of the guiding pin  48  through a fluid channel  52 . 
     The third hydraulically active face  14  can be acted on with the hydraulic fluid  7  through the third fluid connection  16  in a retraction direction  17 . The third fluid connection  16  opens in the second annular cylinder chamber  51 . 
     The first hydraulically active face  4  can be acted on with the hydraulic fluid  7  in the extension direction  8  through the first fluid connection  6 . 
     The second hydraulically active face  13  can be acted on with the hydraulic fluid  7  in the extension direction  8  through the second fluid connection  15 . 
     The third hydraulically active face  14  can be acted on with hydraulic fluid  7  in the retraction direction  17  Via the third fluid connection  16 . 
     A closed hydraulic circuit  18  which is under a pretensioning pressure comprises the first hydraulic pump  19 , wherein the first pressure connection  21  is connected to the second fluid connection  15  via the hydraulic line  24  and the second pressure connection  22  is connected via the hydraulic line  25  to the third fluid connection  16  in a fluid-conducting manner. 
     An electric motor  20  drives the first hydraulic pump  19  at a constant or variable motor speed. The hydraulic pump  19  is preferably an axial piston variable-displacement pump of the swash plate construction type. As a result of the adjustment of the swash plate, the volume flow of the first hydraulic pump can be steplessly changed and reversed. 
     The pretensioning of the hydraulic fluid in the closed hydraulic circuit  18  may, for example, be produced by a pressure container which is not illustrated and which is connected to the pressure connections  21 ,  22  or by an external hydraulic pump. 
     In order to act on the first hydraulically active face  4  of the 3-face cylinder  42  with hydraulic fluid  7 , a second hydraulic pump  26  with a low-pressure connection  27  and a high-pressure connection  28  is provided. The low-pressure connection  27  is connected to an equalizing tank  30  in a fluid-conducting manner by a hydraulic line  29  and the high-pressure connection  28  is connected in a fluid-conducting manner to the first fluid connection  6  by a hydraulic line  31 . 
     The equalizing tank  30  which is open towards the environment receives hydraulic fluid  7  and is further connected in a fluid-conducting manner to the first fluid connection  6  of the 3-face cylinder  42  by a hydraulic line  32 . In the hydraulic line  32  between the equalizing tank  30  and the first fluid connection  6 , a 2/2-way valve  33 . 1  is arranged. When the linear drive system  1  is extended in rapid mode, the second hydraulically active face  13  is acted on with hydraulic fluid  7  through the second fluid connection  15  by the first hydraulic pump  19  which operates in the flow direction  23 . 1  according to  FIG. 3A , whereby the annular piston  47  with the piston rod  46  moves in an extension direction  8 . The hydraulically active face  4  is not acted on with hydraulic fluid  7  in rapid mode. The hydraulic fluid  7  from the equalizing tank  30  reaches the first cylinder chamber  5  of the 3-face cylinder  42  through the open 2/2-way valve  33 . 1  through the first fluid connection  6  (extraction). The second hydraulic pump  26  which is also connected to the first fluid connection  6  is not active. 
     When the linear drive system  1  is retracted in rapid mode, the third hydraulically active face  14  is acted on with hydraulic fluid  7  through the third fluid connection  16  by the first hydraulic pump  19  which now operates in the opposite flow direction  23 . 2  according to  FIG. 3D , whereby the annular piston  47  moves with the piston rod  46  in the retraction direction  17 . The hydraulically active face  4  is not acted on with hydraulic fluid  7 . The hydraulic fluid  7  is displaced through the open 2/2-way valve  33 . 1  from the cylinder chamber  5  of the first cylinder chamber  5  into the equalizing tank  30 . The second hydraulic pump  26  which is connected to the first fluid connection  6  is not active. 
     When the linear drive system  1  is extended in power mode, the first hydraulically active face  4  of the 3-face cylinder  42  is acted on with hydraulic fluid  7  through the first fluid connection  6  by the activated second hydraulic pump  26 , whereby the 3-face cylinder  42  moves in the extension direction  8  as shown in  FIG. 3B . When extending in power mode, the first hydraulic pump  19  is further activated in the flow direction  23 . 1  so that, through the second fluid connection  15 , the second hydraulically active face  13  is acted on with hydraulic fluid  7 . A flow of the hydraulic fluid  7  from the first cylinder chamber  5  via the hydraulic line  32  into the equalizing tank  30  is prevented by the closed 2/2-way valve  33 . 1 . The force produced during extension in power mode is produced by the cooperation of the larger first hydraulically active face  4  and the smaller second hydraulically active face  13 . 
     In order to end the power mode, the first hydraulic pump  19  and the second hydraulic pump  26  are deactivated and the 2/2-way valve  33 . 1  is opened as shown in  FIG. 3C  so that the hydraulic fluid  7  can flow from the first cylinder chamber  5  via the open 2/2-way valve  33 . 1  in the hydraulic line  32  into the equalizing tank  30 . 
       FIG. 4  shows a fourth embodiment of a hydrostatic linear drive system  1  with a 3-face cylinder  42  which integrates the functions of the single-action cylinder  2  and the synchronous cylinder  9  of the embodiments according to  FIGS. 1 and 2  in a sub-assembly. The 3-face cylinder  42  is constructed in accordance with the 3-face cylinder  42  of the third embodiment so that, in order to prevent repetition, reference may be made to the explanations of  FIG. 3 . 
     The fourth embodiment is different from the third embodiment with regard to the hydraulic supply of the 3-face cylinder  42  and will be explained in greater detail below. The differences correspond to the differences of the second embodiment compared with the first embodiment. 
     In order to act on the first hydraulically active face  4  with hydraulic fluid  7 , the first fluid connection  6  of the 3-face cylinder  42  is connected via a hydraulic line  38  in a fluid-conducting manner to the first pressure connection  21  of the first hydraulic pump  19 . In the hydraulic line  38 , there is arranged a second shut-off member  39  which is constructed as a 2/2-way valve  39 . 1 . A high-pressure connection  28  of a second hydraulic pump  26  is connected via a hydraulic line  31  in a fluid-conducting manner to the second pressure connection  22  of the first hydraulic pump  19 . A non-return valve  40  in the hydraulic line  31  prevents a return flow of the hydraulic fluid  7  in the direction of the second hydraulic pump  26 . 
     The low-pressure connection  27  of the second hydraulic pump  26  is connected to the equalizing tank  30  in a fluid-conducting manner via the hydraulic line  29 . In contrast to the embodiment according to  FIG. 3 , however, the high-pressure connection  28  of the second hydraulic pump  26  is not directly connected in a fluid-conducting manner to the first fluid connection  6  of the 3-face cylinder  42 , but instead indirectly via the flow path which has been released by the open 2/2-way valve  39 . 1  and the first hydraulic pump  19  which is activated in the flow direction  23 . 1 . 
     The equalizing tank  30  which is open in the direction towards the environment receives hydraulic fluid  7  and is connected in a fluid-conducting manner to the first fluid connection  6  of the 3-face cylinder  42  by a hydraulic line  32 . A 2/2-way valve  33 . 1  is arranged in the hydraulic line  32 . 
     When the linear drive system  1  is extended in rapid mode, the second hydraulically active face  13  is acted on with hydraulic fluid  7  via the second fluid connection  15  by the first hydraulic pump  19  which functions in the flow direction  23 . 1  as shown in  FIG. 4A , whereby the annular piston  47  moves with the piston rod  46  in the extension direction  8 . 
     The hydraulic fluid  7  from the equalizing tank  30  reaches the cylinder chamber  5  of the 3-face cylinder  42  through the open 2/2-way valve  33 . 1  via the first fluid connection  6  (extraction). The first hydraulically active face  4  is not acted on since the 2/2-way valve  39 . 1  in the hydraulic line  38  is closed. 
     When the linear drive system  1  is retracted in rapid mode, the third hydraulically active face  14  is acted on with hydraulic fluid  7  through the third fluid connection  16  by the first hydraulic pump  19  which is now operating in the opposite flow direction  23 . 2  as shown in  FIG. 4D , whereby the annular piston  47  together with the piston rod  46  moves in the retraction direction  17 . The hydraulic fluid  7  is displaced through the open 2/2-way valve  33 . 1  from the cylinder chamber  5  of the 3-face cylinder  42  into the equalizing tank  30 . The 2/2-way valve  39 . 1  in the hydraulic line  38  is closed. 
     When the linear drive system  1  is extended in power mode, the first hydraulically active face  4  is acted on with hydraulic fluid  7  via the first fluid connection  6  as shown in  FIG. 4B , whereby the 3-face cylinder  42  moves in the extension direction  8 . The 2/2-way valve  39 . 1  in the hydraulic line  38  is now open. Both the first hydraulic pump  19  and the second hydraulic pump  26  are activated and convey the hydraulic fluid  7  in a corresponding flow direction  23 . 1  in the direction of the first fluid connection  6 . The activated second hydraulic pump  26  provides the required additional volume of hydraulic fluid  7  from the equalizing tank  30  in order to act with hydraulic fluid  7  on the first hydraulically active face  4  of the 3-face cylinder  42  via the first fluid connection  6  and to act with the hydraulic fluid on the second hydraulically active face  13  via the second fluid connection  15 . A flow of the hydraulic fluid  7  via the hydraulic line  32  into the equalizing tank  30  is prevented by the closed 2/2-way valve  33 . 1 . 
     In order to end the power mode, the first hydraulic pump  19  and the second hydraulic pump  26  are deactivated, the 2/2-way valve  33 . 1  is opened and the 2/2-way valve  39 . 1  in the hydraulic line  38  is closed as shown in  FIG. 4C  so that the hydraulic fluid  7  flows from the first cylinder chamber  5  via the open 2/2-way valve  33 . 1  in the hydraulic line  32  into the equalizing tank  30 . 
     Thus, while there has been shown and described and pointed out the fundamental novel features of the invention is applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  Linear drive system 
           2  Single-action cylinder 
           3  Piston rod of single-action cylinder 
           4  First hydraulically active face 
           5  First cylinder chamber 
           6  First fluid connection 
           7  Hydraulic fluid 
           8  Extension direction 
           9  Synchronous cylinder 
           10  Piston 
           11  Piston rod synchronous cylinder 
           12  Piston rod synchronous cylinder 
           13  Second hydraulically active face 
           14  Third hydraulically active face 
           15  Second fluid connection 
           16  Third fluid connection 
           17  Retraction direction 
           18  Closed hydraulic circuit 
           19  First hydraulic pump 
           20  Electric motor 
           21  First pressure connection 
           22  Second pressure connection 
           23 . 1  Flow direction 
           23 . 2  Opposite flow direction 
           24  Hydraulic line 
           25  Hydraulic line 
           26  Second hydraulic pump 
           27  Low-pressure connection 
           28  High-pressure connection 
           29  Hydraulic line 
           30  Equalizing tank 
           31  Hydraulic line 
           32  Hydraulic line 
           33  First shut-off member 
           33 . 1  2/2-way valve 
           34  Coupling member 
           35  Second cylinder chamber 
           36  Third cylinder chamber 
           38  Hydraulic line 
           39  Second shut-off member 
           39 . 1  2/2-way valve 
           40  Non-return valve 
           42  3-face cylinder 
           43  Cylinder pipe 
           44  Cylinder base 
           45  Piston rod guide 
           46  Piston rod 
           47  Annular piston 
           47 . 1  First piston face 
           47 . 2  Second piston face 
           48  Guiding pin 
           49  Hollow space 
           50  Partial surface 
           51  Second cylinder chamber 
           52  Fluid channel