Abstract:
A hydraulic system with an adjustable hydrostatic machine is provided that includes an adjustment mechanism. The adjustment mechanism interacts with an actuating device which has at least one actuating piston delimiting an actuating pressure space. The system has a regulating valve with which, for the displacement of the actuating piston and therefore for the actuation of the adjustment device, the supply and removal of pressure medium into and from the actuating pressure chamber can be controlled, wherein the regulating valve has a valve piston which is subjected to a force of a feedback spring, which force is dependent on the position of the adjustment mechanism. The valve piston is additionally subjected to a counterforce acting counter to the force of the feedback spring, and an actuating component is provided, with which the valve piston can be subjected to a control force acting in the same direction as the counterforce or to a control force acting counter to the counterforce.

Description:
[0001]    This nonprovisional application is a continuation of International Application No. PCT/EP2008/006851, which was filed on Aug. 20, 2008, and which claims priority to German Patent Application No. 10 2007 039 173.2, which was filed in Germany on Aug. 20, 2007, and which are both herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a hydraulic system having an adjustable hydrostatic machine, an adjusting device which cooperates with the hydrostatic machine, and a regulating valve, wherein the adjusting device comprises at least a first adjusting piston which is influenced on an adjusting piston surface by an adjusting pressure, and wherein the regulating valve comprises a regulating element which is influenced in the direction of a first end position of the valve by a force dependent upon the position of the first adjusting piston, and which is influenced at a second end by an adjusting force for the purpose of adjusting a regulating valve position. 
         [0004]    2. Description of the Background Art 
         [0005]    A hydrostatic system having an adjustable hydraulic pump disposed in an open circuit is known from DE 199 49 169 C2, which corresponds to U.S. Pat. No. 6,725,658. The adjustment mechanism of the hydraulic pump is actuated by means of an adjusting device. The adjusting device comprises an adjusting piston which delimits an adjusting pressure chamber formed in an adjusting cylinder. The adjusting pressure prevailing in the adjusting pressure chamber is regulated by a regulating valve. In order to adjust an adjusting pressure, which acts upon the adjusting piston surface of the adjusting piston, the adjusting pressure chamber can be connected in a variable manner via the regulating valve to a delivery-side working line of the hydraulic pump or to a tank volume. 
         [0006]    The regulating valve comprises a valve piston as a valve element which is influenced at a first end by the force of a feedback spring. The feedback spring is supported on the one hand at one end of the valve piston and on the other hand on the piston surface of the adjusting piston which is influenced by the adjusting pressure. The force generated by the feedback spring on the valve piston is thus dependent upon the position of the adjusting piston. In the opposite direction the force of an electromagnet acts upon the valve piston. In dependence upon a control signal for the electromagnet a proportional adjustment of the adjusting piston is thus achieved in dependence upon a control signal of the proportional magnet. 
         [0007]    Although the proposed hydrostatic system permits convenient integration into a pump unit by reason of the arrangement of the feedback spring on the one hand and the proportional magnet on the other hand, it is, however, disadvantageous that a central position of the adjusting piston is not possible in the event of a disappearing control signal and a hydrostatic machine which can be adjusted in two opposite directions can only be moved to its neutral position by the generation of a control force by the proportional magnet. 
       SUMMARY OF THE INVENTION 
       [0008]    Therefore, it is the object of the invention to provide a hydrostatic system which starting from a rest position can be pivoted in two opposite directions in the event of a disappearing control signal. 
         [0009]    The hydraulic system in accordance with an embodiment of the invention comprises an adjustable hydrostatic machine, an adjusting device which cooperates with an adjustment mechanism of the hydrostatic machine, and a regulating valve. 
         [0010]    The adjusting device comprises at least one adjusting piston which is influenced at least on an adjusting piston surface by an adjusting pressure. The regulating valve comprises a valve piston as a regulating element which is influenced by a force dependent upon the position of the adjusting piston. In order to adjust the desired position of the adjustment mechanism, the regulating element can be influenced by a control force which can be generated by actuating means. 
         [0011]    By virtue of the provision of a counter-force which acts upon the regulating element and against the force which is dependent upon the position of the adjusting piston, a balance of forces can be achieved on the regulating valve in a rest position of the first adjusting piston even in the event of disappearing control signals when no control force is generated onto the regulating element. Since the control force generated by the actuating means is directed in the same direction as the counter-force or in the opposite direction thereto, it is possible, starting from the rest position of the adjusting device determined by the counter-force in the event of a disappearing signal, for the adjusting device to be deflected in two opposite directions by the actuating means. The hydrostatic system in accordance with the invention thus has the advantage that not only can a hydraulic pump be adjusted between e.g. a zero delivery volume and its maximum delivery volume but also that hydrostatic machines which can be pivoted in opposite directions, such as e.g. pumps provided for two delivery devices or else a pump/motor unit, as used in braking energy recovery devices, can be controlled in a convenient manner. 
         [0012]    The counter-force is can be generated by a spring and the control force can be continuously adjusted in both force directions. 
         [0013]    The actuating means comprises preferably two proportional magnets which act in opposite direction upon the valve piston. Proportional magnets of this type are constructed in a simple manner and are available at low cost. 
         [0014]    In particular, it is advantageous that the control of both adjusting directions by means of only one actuator as the actuating means enables the regulating valve to be integrated in a simple manner into a hydrostatic machine, in which the side of the regulating element influenced by the position-dependent force is not accessible. 
         [0015]    In accordance with an alternative embodiment, the actuating means is designed preferably as a double lifting magnet having a first winding and a second winding. Each of the windings serves to generate a force in each case in one of the opposite force directions. With this type of actuator, it is possible by influencing the first or second winding for the force direction and also the magnitude of the force to be specified in a convenient manner by two individual control signals. In particular, it is possible to control the hydrostatic system in a convenient manner using an electronic control device. The use of only one actuator for both force directions also has the advantage that the line and connector outlay can be minimised. 
         [0016]    A particularly convenient structure is achieved if the feedback spring is disposed between the adjusting piston and the valve piston. 
         [0017]    It is also advantageous that, in the event of a disappearing control force and a balance of forces between the counter-force and the force onto the regulating element which is dependent upon the position of the adjusting piston, the hydrostatic machine is adjusted to a non-zero delivery volume. This type of adjustment in the rest position of the hydrostatic system to a non-zero delivery volume ensures that after initial operation and in the event of a disappearing control signal a minimum delivery volume is already delivered by the hydrostatic machine. By reason of this minimal delivery volume, a slightly increased pressure already exists in the hydraulic system and can be used in order to actuate the adjusting device of the hydrostatic machine. The said adjustment in the rest position also prevents intake problems from occurring in the hydrostatic machine by reason of an incorrect pivot side. 
         [0018]    In addition to the adjusting piston which influences the hydrostatic machine with an adjusting force starting from a rest position in the direction of a first end position, the adjusting device preferably comprises a further adjusting piston which is also referred to as a counter-piston and which influences the hydrostatic machine with an adjusting force in an opposite direction starting from the rest position in the direction of a second end position. This type of division of the adjusting device into a first adjusting piston for a first movement direction and a second adjusting piston for a second movement direction has the advantage that the available installation space can be utilised in a particularly efficient manner in e.g. a hydrostatic axial piston machine. Therefore, it is possible to dispose the two adjusting pistons and the adjusting cylinders, in which the adjusting pistons are disposed, on opposite sides of a pivot cradle or swash-plate. By virtue of the respective adjusting piston, only compressive forces are transmitted to the pivot cradle, which simplifies the mechanical coupling between the adjusting piston and the hydrostatic machine. 
         [0019]    The adjusting pressure chamber delimited by the piston surface of the adjusting piston can preferably be connected to an adjusting pressure source via the regulating valve. In particular, the adjusting pressure source is connected to the regulating valve via a connection line, in which a pressure-reducing valve is disposed. The connection line having the pressure-reducing valve provided therein has the advantage that by means of the pressure-reducing valve an input pressure which is reduced with respect to the working pressure is generated. Therefore, the adjusting pressure source used can be a pressure which is dependent e.g. upon different operating situations of the hydrostatic system. In particular, the delivery pressure of the hydrostatic machine can be provided as an adjusting pressure source. Furthermore, it is particularly advantageous when using the pressure-reducing valve in the connection line to produce the adjusting device by means of a first adjusting piston and a second adjusting piston. The second adjusting piston has a piston surface which is smaller in comparison with the first adjusting piston and is influenced directly by the working pressure of the hydrostatic machine. Reducing the adjusting pressure gained from the working pressure of the hydrostatic machine whilst at the same time having a larger piston surface of the first adjusting piston ensures that the adjusting mechanism of the hydrostatic machine is hydraulically clamped at each point in time. Nevertheless, the maximum possible adjusting pressures of the first adjusting piston are reduced with respect to the actual working pressure. In particular, the actuating means, but also the regulating valve, are thereby subjected only to a smaller pressure loading. 
         [0020]    The adjusting pressure source is preferably a working line of the hydrostatic machine or a further pressure medium source. The working line or the pressure medium source are connected to the connection line via a shuttle valve. The ability to also provide as an alternative a further pressure medium source as an adjusting pressure source in addition to the working line has the advantage that even in the event of a disappearing working line pressure an adjusting pressure sufficient to actuate the hydrostatic system is available. This is particularly advantageous if the hydrostatic system is used in the recovery of kinetic energy by the storage of energy in a hydrostatic accumulator. During operation the hydrostatic accumulator can be completely emptied. In this case, the further pressure medium source can be used to generate a sufficient adjusting pressure in the first adjusting pressure chamber, by means of which the hydrostatic machine can be adjusted to a higher delivery volume during subsequent energy storage. 
         [0021]    The regulating valve can be designed as a 3/3-port directional control valve. The use of a 3/3-port directional control valve renders it possible to provide in addition a neutral position of the regulating valve between the two end positions. This neutral position is determined by a defined regulating valve position. In this neutral position of the regulating valve, the adjusting pressure chamber delimited by the adjusting piston surface of the first adjusting piston can be connected in a restricted manner to a tank volume and is connected in a restricted manner to the adjusting pressure source. 
         [0022]    In the first end position of the regulating valve, the adjusting pressure chamber is connected to the adjusting pressure source. However, in the opposite second end position of the regulating valve, the adjusting pressure chamber delimited by the adjusting piston surface of the first adjusting piston is connected to a tank volume. The 3/3-port directional control valve can be adjusted variably between these two end positions. 
         [0023]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
           [0025]      FIG. 1  shows a schematic illustration of a first embodiment of the hydraulic system in accordance with the invention using the example of a device for recovering braking energy; 
           [0026]      FIG. 2  shows a schematic illustration to illustrate the proportional adjustment of the displaced volume of the hydrostatic machine in dependence upon a first and a second control signal; 
           [0027]      FIG. 3  shows a schematic illustration of a modified embodiment of the hydraulic system in accordance with the invention, having a regulating valve which is actuated by two proportional magnets; 
           [0028]      FIG. 4  shows a sectional illustration of an exemplary axial piston machine which is constructed in accordance with the system of the invention; and 
           [0029]      FIG. 5  shows a sectional illustration of a regulating valve which is actuated by two proportional magnets. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    In accordance with a first exemplified embodiment, the hydrostatic system  1  in accordance with the invention comprises a hydrostatic machine  2 . The hydrostatic machine  2  is designed as a pump/motor machine. The hydrostatic machine  2  is designed to be adjustable in terms of its displaced volume and is preferably a hydrostatic axial piston machine in a swash-plate construction which can be pivoted out of a neutral position in two directions. The displaced volume is adjusted by an adjustment of the swash-plate. In the illustrated example, the hydrostatic machine  2  is connected to a first working line  3  and a second working line  4 . The first working line  3  connects the hydrostatic machine  2  to a hydraulic accumulator  6 . In contrast, the second working line  4  is connected to a tank volume  7 . Alternatively, a high pressure accumulator can also be provided as a hydraulic accumulator  6  and a low pressure accumulator can be provided instead of the tank volume  7 . 
         [0031]    The hydrostatic machine  2  can be operated as a pump, so that it intakes pressure medium from the tank volume  7  via the second working line  4  and delivers it via the first working line  3  against the pressure prevailing in the hydraulic accumulator  6 . Therefore, pressure energy can be stored in the hydraulic accumulator  6 . In order to be able to operate the hydrostatic machine  2  as a pump, a drive shaft  5  is provided which connects the hydrostatic machine  2  to a drive train of a vehicle. By adjusting the delivery volume of the hydrostatic machine  2  operated as a pump whilst at the same time connecting the drive shaft  5  to the drive train of a vehicle, pressure medium is delivered into the hydraulic accumulator  6  during a braking procedure of the vehicle and the vehicle is thus braked. 
         [0032]    Conversely, it is possible to empty the hydraulic accumulator  6  via the hydrostatic machine  2 . In this case, the hydrostatic machine  2  is pivoted out in the opposite direction and is operated as a motor having a displacement which is adjusted in a defined manner. An output torque which is supplied to the drive train in order to accelerate the vehicle is then produced on the drive shaft  5  via the hydrostatic machine  2  as the hydraulic accumulator  6  is being emptied. The hydraulic accumulator  6  is emptied via the first working line  3 , the hydrostatic machine  2  and the second working line  4  into the tank volume  7 . 
         [0033]    An adjusting device is provided for the purpose of adjusting the delivery volume or the displacement of the hydrostatic machine  2 . The adjusting device comprises a first adjusting piston  8  and a second adjusting piston  9 . The first adjusting piston  8  is disposed in a displaceable manner in a first adjusting cylinder  10  and is mechanically coupled to the adjustment mechanism of the hydrostatic machine  2 . By virtue of the first adjusting piston  8 , shear forces can be transmitted to e.g. the swash-plate as the adjustment mechanism of the hydrostatic machine  2 . In a corresponding manner, the second adjusting piston  9  is disposed in a displaceable manner in a second adjusting cylinder  11 . The second adjusting piston  9  can transmit shear forces to the adjustment mechanism of the hydrostatic machine  2 , but with the opposite effect in relation to the pivoting direction. To this end, the adjusting pistons  8  and  9  act upon opposite sides of the swash-plate of the hydrostatic machine  2 , so that in the event of a balance of forces of the two forces transmitted by the adjusting pistons  8  and  9  to the swash-plate, a further adjustment of the hydrostatic machine  2  is omitted. 
         [0034]    In order to generate the adjusting forces by the first adjusting piston  8  or the second adjusting piston  9 , a first adjusting pressure chamber  12  is formed in the first adjusting cylinder  10  and a second adjusting pressure chamber  13  is formed in the second adjusting cylinder  11 . The two adjusting pressure chambers  12 ,  13  are delimited on one side by the first adjusting piston  8  or the second adjusting piston  9 . Therefore, according to the adjusting pressure prevailing in the first adjusting pressure chamber  12  a hydraulic force is generated on a first adjusting piston surface  14  of the first adjusting piston  8 , which surface delimits the first adjusting pressure chamber  12 . This hydraulic force is transmitted via a corresponding linkage to the adjustment mechanism of the hydrostatic machine  2 . In the same manner, a force which acts in the opposite direction upon the adjustment mechanism of the hydrostatic machine  2  is generated onto the second adjusting piston surface  15  of the second adjusting piston  9  by means of the pressure prevailing in the second adjusting pressure chamber  13 . 
         [0035]    A regulating valve  16  is provided for the purpose of adjusting the adjusting pressure prevailing in the first adjusting pressure chamber  12 . A first connection of the regulating valve  16  is connected to the first adjusting pressure chamber  12  via an adjusting pressure line  17 . In dependence upon the position of a regulating element of the regulating valve  16 , the adjusting pressure line  17  is connected to an adjusting pressure source or the tank volume  7 . For this purpose, a second connection of the regulating valve  16  is connected to a shuttle valve  19  via a connection line  18 . With the aid of the shuttle valve  19 , either the first working line  3  or else a further pressure medium source is connected as the adjusting pressure source to the connection line  18  via the further connection line  20 . Pressure medium can be fed from an external pressure medium source via the further connection line  20 . This can be practical e.g. if the residual pressure available in the hydraulic accumulator  6  is not sufficient to actuate the adjusting device. 
         [0036]    A pressure-reducing valve  21  is provided in the connection line  18 . The pressure-reducing valve  21  comprises a measuring surface which is influenced by the output pressure of the pressure-reducing valve  21  which is supplied to the regulating valve  16 . In the opposite direction the force of a spring acts upon the pressure-reducing valve  21  and the commencement of regulation of the pressure-reducing valve  21  is adjusted by this spring force. A constant input pressure is thus supplied to the regulating valve  16  as long as sufficient pressure can be provided by the adjusting pressure source on the input side of the pressure-reducing valve  21 . 
         [0037]    The regulating valve  16  comprises a regulating valve element which is designed preferably as a valve piston. This valve piston is disposed in an axially displaceable manner in a valve housing and comprises a first end and a second end remote therefrom. The first end of the valve piston is influenced by the adjusting pressure prevailing in the first adjusting pressure chamber  12  and delimits the first adjusting pressure chamber  12 . Disposed between the first adjusting piston  8  and the valve piston of the regulating valve  16  is a feedback spring  22 . Therefore, in dependence upon the position of the first adjusting piston  8  a force acts upon the valve piston of the regulating valve  16 , said force being generated by the feedback spring  22  and being dependent upon the position of the first adjusting piston  8 . 
         [0038]    In the opposite direction a counter-force which is generated by a spring  23  acts upon the valve piston. The spring  23  is designed to be adjustable, so that the position of the first adjusting piston  8 , in which the valve piston is situated in a balance of forces between the feedback spring  22  and the spring  23 , can be adjusted. The preferred adjustment will be explained hereinafter with reference to  FIG. 2 . 
         [0039]    The regulating valve  16  is designed to be pressure-compensated. For this purpose, a line branch which influences the valve piston at its second end in the same direction as the spring  23  by means of a hydraulic force branches off from the adjusting pressure line  17 . The hydraulic force which acts in the adjusting pressure chamber  12  upon the first end of the valve piston is thus compensated for by a correspondingly large hydraulic force at the second end. The regulating valve  16  can therefore be disposed in an integrated manner in the adjusting device. The pressure reduction is also integrated into the adjusting device. 
         [0040]    In  FIG. 1 , the regulating valve  16  is illustrated in its neutral position. In this neutral position the first connection of the regulating valve  16  is connected in each case in a restricted manner to the second connection of the regulating valve  16  and to a third connection of the regulating valve  16 . The third connection is connected to the tank volume  7 . The regulating valve  16  is designed with negative overlap. 
         [0041]    From this neutral position of the regulating valve  16 , the regulating valve  16  can be adjusted in the direction of a first end position and in the direction of a second end position. The regulating valve  16  can assume any intermediate position between the two end positions. In the first end position of the regulating valve  16 , the first connection is connected in an unrestricted or virtually unrestricted manner to the second connection of the regulating valve  16 . As a consequence, a connection is established between the connection line  18  and the adjusting pressure line  17 . However, in the second end position of the regulating valve  16 , the first connection and thus the adjusting pressure line  17  are connected in an unrestricted or virtually unrestricted manner to the tank volume  7  via the third connection of the regulating valve  16 . As a consequence, the first adjusting pressure chamber  12  is connected in the second end position of the regulating valve  16  to the tank volume  7  and the pressure in the first adjusting pressure chamber  12  is relieved into the tank volume  7 . 
         [0042]    Moreover, the connection line  18  is connected to the second adjusting pressure chamber  13  in a portion between the shuttle valve  19  and the pressure-reducing valve  21 . The adjusting pressure prevailing in this manner in the second adjusting pressure chamber  13  and the adjusting pressure prevailing in the first adjusting pressure chamber  12  serve to adjust the hydrostatic machine  2  until a balance of forces prevails. The adjusting piston surface  15  of the second adjusting piston  9  is smaller than the adjusting piston surface  14  of the first adjusting piston  8 . 
         [0043]    An actuator  24  is provided at the second end of the valve piston of the regulating valve  16 . The pressure reduction effected by means of the pressure reducing valve  21  serves to reduce the loading on the actuator  24 . This actuator  24  is formed as a double lifting magnet having two windings. When a control signal is applied to the first winding, this first winding is able to generate a compressive force as a control force at the second end of the valve piston. Therefore, when the actuator  24  is controlled a control force is generated by means of the first winding in a first force direction which acts upon the valve piston in the same direction as the counter-force of the spring  23 . In addition, a force can be generated against the force of the spring  23  in an opposite second force direction by supplying a second winding of the actuator with current. The resultant from the force of the spring  23  and the force of the actuator  24  acts upon the second end of the valve piston. 
         [0044]    If, on the basis of the rest position of the hydraulic system  1  illustrated in  FIG. 1 , the actuator  24  is controlled in the direction of the first force direction, then the valve piston is adjusted in the direction of its second end position. As a consequence, the first adjusting pressure chamber  12  is relieved via the adjusting pressure line  17  into the tank volume  7 . As a result, the hydraulic force on the first adjusting piston  8  is reduced. Since an unchanged pressure prevails at the same point in time in the second adjusting pressure chamber  13 , the force on the second adjusting piston  9  is not reduced. By reason of the unbalance of forces, the first adjusting piston  8  is adjusted to the right in  FIG. 1 . As a result, the force of the feedback spring  22  on the first end of the valve piston increases until a balance of forces then prevails between the force of the feedback spring  22  and the resultant of the control force of the actuator  24  and the counter-force of the spring  23 . 
         [0045]    Conversely, by supplying the second winding of the actuator  24  with current a shear force can be generated against the spring  23  and the pre-stressing of the spring  23  on the valve piston is relieved. The valve piston is adjusted in the direction of its first end position by virtue of the thus decreasing resultant acting upon the second end of the valve piston. In the first end position of the regulating valve  16 , the connection line  18  is connected to the adjusting pressure line  17 , so that pressure medium is supplied to the first adjusting pressure chamber  12  from the adjusting pressure source. By reason of the area ratios, the hydraulic force which acts upon the first adjusting piston  8  is thus greater than the hydraulic force acting upon the second adjusting piston, so that the first adjusting piston  8  is adjusted to the left in  FIG. 1 . As a result, the force generated by the feedback spring  22  onto the valve piston at the first end is reduced. 
         [0046]    The first adjusting piston  8  is moved in each case until a balance of forces between the resultant and the force of the feedback spring  22  has been restored and the regulating valve is again located in the rest position illustrated in  FIG. 1 . Therefore, the position of the first adjusting piston  8  and thus the adjusted displaced volume of the hydrostatic machine  2  are in each case proportional to the control signals, which are supplied to the actuator  24 , for a respective movement direction. 
         [0047]    The maximum possible pivot angle of the hydrostatic machine  2  is adjusted by means of mechanical stops which are formed on the adjustment mechanism of the hydrostatic machine  2 . In order to move the hydrostatic machine  2 , when the system is pressureless, to a rest position, in which it is adjusted to any delivery volume, in particular a delivery volume slightly different from zero, a first return spring  25  and a second return spring  26  are provided. With the linkage, which connects the first adjusting piston  8  to the adjustment mechanism of the hydrostatic machine  2 , a pair of entrainment elements  27 ,  27 ′ are provided. The entrainment elements  27 ,  27 ′ pre-stress the first return spring  25 . During an adjustment of the hydrostatic machine  2  by virtue of a displacement of the first adjusting piston  8  to the right in  FIG. 1 , the first return spring  25  is supported on a spring plate  28  which is disposed in a displaceable manner. The spring plate is displaceable on the linkage and is supported on the housing side in the event of a movement to the right. During an adjusting movement, in which the first adjusting piston  8  is adjusted to the right in  FIG. 1 , the first return spring  25  is thus compressed beyond its pre-stressing. In the same manner, the second return spring  26  is supported on the housing side via a further spring plate  29 , which is disposed in a displaceable manner, in the event of an opposite adjustment. The second return spring  26  is pre-stressed between a pair of further entrainment elements  30 ,  30 ′. Depending upon the adjustment of the hydrostatic machine  2 , either the first return spring  25  or the second return spring  26  is thus compressed for this purpose outside a small adjustment range around the rest position. The respective other return spring  25  or  26  is moved at the same time with the linkage. 
         [0048]    The entrainment elements  27 ,  27 ′ and  30 ,  30 ′ are disposed on the linkage such that the return springs  25  and  26  are pre-stressed and lie against the displaceable spring plates  28 ,  29 . Even if it is not absolutely necessary, the displaceable spring plates  28 ,  29  are disposed at a spaced interval with respect to the housing when the hydrostatic machine  2  is in a rest position. In principle, it is also possible for there to be no spaced interval with respect to the housing. The spaced interval enables component tolerances to be easily compensated for. The pre-stressing of the return springs  25 ,  26  between the entrainment elements  27 ,  27 ′ and  30 ,  30 ′ serves to simplify the assembly significantly, as a preassembly can be performed. 
         [0049]    The adjustment of the hydrostatic machine  2  is illustrated once again in  FIG. 2  with the aid of a graph. The y-axis represents the currents I 1  of a first control signal and a second control signal I 2  for supplying current to the first winding or the second winding. In contrast, the x-axis represents the delivery volume V P  during operation of the hydrostatic machine  2  as a pump or the displacement V M  during operation of the hydrostatic machine  2  as a motor. It is apparent that the hydrostatic machine  2  can be adjusted from a currentless neutral position in both directions to the 100% end position. The spring  23  of the regulating valve  16  is adjusted such that the neutral position of the regulating valve  16  is achieved in a position of the first adjusting piston  8  which corresponds to a deflection of the hydrostatic machine  2  from its rest position to a minimal delivery volume V P, min . This type of adjustment of the hydrostatic machine  2  in the event of a disappearing control signal for the actuator  24  ensures that when the drive shaft  5  is connected to the drive train of a vehicle a minimum pressure is built up by the hydrostatic machine  2  in the first working line  3  by the delivery of pressure medium into the hydraulic accumulator  6 . This ensures that the hydraulic system  1  provides at all times a pressure sufficient to actuate the adjusting device and the hydrostatic machine does not have any intake problems by reason of an incorrect pivot side. 
         [0050]    The dimensioning of the area ratios of the first adjusting piston  8  to the surface of the second adjusting piston  9  is preferably about 3/1. At the same time, the pressure-reducing valve  21  is preferably adjusted such that a reduction to about 2/3 of the working pressure is effected. The working pressure is the maximum storage pressure of the hydraulic accumulator  6 . In the case of the preferred area ratio, a pressure of ca. 1/3 of the working pressure is required in order to achieve a balance of forces between the two adjusting pistons  8 ,  9 . Therefore, by adjusting the pressure-reducing valve to 2/3 of the working pressure a sufficient pressure reserve is provided for pivoting the hydrostatic machine  2  in the direction of motor operation. 
         [0051]      FIG. 3  illustrates a modified exemplified embodiment. The function corresponds substantially with the function explained with reference to  FIG. 1 , for which reason only the changes in comparison with the first exemplified embodiment will be discussed hereinafter. In contrast to the system illustrated in  FIG. 1 , the regulating valve  16 ′ is in this case not disposed in an axial extension of the first adjusting cylinder  10 . The free arrangement of the regulating valve  16 ′ renders it possible to provide a first proportional magnet  24 . 1  and a second proportional magnet  24 . 2  as an actuating means instead of the double lifting magnet  24  used in  FIG. 1 . The two proportional magnets  24 . 1  and  24 . 2  are each suitable for transmitting shear forces to the valve piston of the regulating valve  16 ′. Since the first adjusting piston  8  and the second adjusting piston  9  are mechanically coupled to each other via the adjustment mechanism of the hydraulic pump  2 , generally a pivot cradle, the position of the adjustment mechanism can also be tapped via the connection between the second adjusting piston  9  and the adjustment mechanism of the hydraulic machine  2 . In the illustrated exemplified embodiment this is performed via a linkage  31 . 
         [0052]    Supported on the linkage  31  is the feedback spring  22 ′ which influences the valve piston at one end by the force dependent upon the position of the adjustment mechanism. As already explained with reference to  FIG. 1 , a counter-force which is generated by a spring  23 ′ acts in the opposite direction. In the event of a disappearing control force, i.e. without any control signal being applied to the first proportional magnet  24 . 1  and the second proportional magnet  24 . 2  the force of the feedback spring  22 ′ and the counter-force of the spring  23 ′ are situated in a balance of forces if the hydraulic machine  2  is in its rest position. The valve piston of the regulating valve  16 ′ is then no longer deflected via the double lifting magnet  24  which is disposed on one side of the valve piston, but rather is deflected via either a generation of a shear force by the first proportional magnet  24 . 1  or else a shear force by the second proportional magnet  24 . 2  on opposite end sides of the valve piston. 
         [0053]    In a similar manner to the previously described function, the hydraulic system is also adjusted in this case such that the resultant from the control force which is now generated by the two proportional magnets  24 . 1  and  24 . 2 , and the counter-force  23 ′ is in balance with the force of the feedback spring  22 ′. In this case, the regulating valve  16 ′ is in its rest position illustrated in  FIG. 3 . If the control signal is changed for the proportional magnet  24 . 1  or  24 . 2 , then by reason of the unbalance of forces the valve piston is displaced either in the direction of a first end position or the second end position. Consequently, as already explained with reference to  FIG. 1 , the adjusting pressure line  17 ′ is connected either to the tank volume  7  or else the adjusting pressure source. In the illustrated exemplified embodiment, the connection line  18  does not comprise a pressure-reducing valve. However, the pressure-reducing valve can be provided in the same manner as in the exemplified embodiment as shown in  FIG. 3 . 
         [0054]    Moreover, the arrangement of the second exemplified embodiment has the advantage that only leakage oil pressure can pass to the proportional magnets. A construction of the proportional magnets which is pressure-tight with respect to high pressure is thus not required. 
         [0055]    Like elements are designated by like reference numerals. In order to illustrate the equal effect of the hydraulic system, the changed elements have been provided with reference numerals having an apostrophe. 
         [0056]      FIG. 4  illustrates a partial sectional view of an axial piston machine  40 , in which the hydraulic system in accordance with the invention is used. Elements which have already been illustrated and explained in the schematic illustrations are referenced accordingly in the sectional illustration. In order to avoid unnecessary repetition, they will not be explained again. The axial piston machine  40  comprises a pivot cradle  41  in order to adjust its delivery volume or displacement. This pivot cradle can be tilted in terms of its angle of inclination with respect to the rotational axis of a drive shaft of the axial piston machine  40 . In order to adjust the angle of inclination, the pivot cradle  41  which forms the adjustment mechanism of the hydraulic machine can be influenced by means of shear forces generated by the first adjusting piston  8  or the second adjusting piston  9 . The position of the pivot cradle  41  and thus of the adjusted delivery volume or displacement of the axial piston machine  40  is fed back to the regulating valve  16  by means of the feedback lever  31 . For this purpose, the feedback lever  31  acts upon a movably disposed spring bearing  42 . The feedback spring  22 ′ is supported on the spring bearing  42  and acts upon the regulating valve  16 ′ in the manner already described. The spring bearing  42  has a transmission element  43  passing through it which transmits the shear force generated by the second proportional magnet  24 . 2  to the valve piston of the regulating valve  16 ′ 
         [0057]    The spring bearing  42  and the transmission element  43  do not contact one another. 
         [0058]    An enlarged illustration of the regulating valve  16 ′ is illustrated in  FIG. 5 . 
         [0059]    The illustrated sectional view shows the valve piston  44  of the regulating valve  16 ′ which is disposed in a longitudinally displaceable manner in a valve sleeve. Disposed at its first end is a spring plate, on which the feedback spring  22 ′ is supported. The valve piston  44  is thus influenced by the force of the feedback spring  22 ′ via the spring plate. In the opposite direction the counter-force generated by the spring  23 ′ acts upon the valve piston  44 . As already explained, a pair of proportional magnets  24 . 1  and  24 . 2  are provided as the actuating means in this exemplified embodiment. The proportional magnets act in each case via actuating rods upon the two ends of the valve piston  44  which are remote from one another. Whereas the actuating rod  43  of the second proportional magnet  24 . 2  is supported on the spring plate of the feedback spring  22 ′ or in parallel with the spring  22 ′ on one side of the valve piston, the tappet of the proportional magnet  24 . 1  acts in parallel with the spring  23 ′ upon the opposite end side of the valve piston  44 . In the rest position of the valve piston  44  as illustrated in  FIG. 5 , the adjusting pressure line  17 ′ is connected in a restricted manner to the connection line  18  and to the tank volume  7 . Therefore, an average pressure is effective in the first adjusting pressure chamber  12 . In order to discharge pressure medium into the tank volume, bores  45 ,  46  are provided in the valve sleeve which are connected to each other and to the spring chamber  47 . At the site where the feedback arm  38  is guided through, the spring chamber  47  is connected to the housing volume of the axial piston machine  40  which thus forms the tank volume. In the event of a movement of the valve piston  44  in the axial direction, control edges release an unrestricted or virtually unrestricted connection between the adjusting pressure line  17 ′ and in each case one of two annular chambers formed around the valve piston  44 . The annular chamber which is oriented more closely to the first proportional magnet  24 . 1  is permanently connected to the bore  45  and thus to the tank volume  7 . However, the annular chamber which is disposed on the side facing towards the second proportional magnet  24 . 2  is permanently connected to the connection line  18  via further connection bores. 
         [0060]    The spring  23 ′ which generates the counter-force is received in a sleeve  51  which is open towards the valve piston  44  and whose outer side is conical in formation and lies against an adjusting pin  52  which is screwed into the valve sleeve. By screwing-in the adjusting pin to varying extents the sleeve  51  can be displaced axially and therefore the force of the spring  23 ′ can be adjusted. A counter-nut  53  secures the position of the adjusting pin  52 . Therefore, in the event of a disappearing control force the rest position of the adjustment mechanism can be adjusted. 
         [0061]    As an alternative to proportional valves, the regulating valve can also be actuated hydraulically, wherein forces of differing magnitude can be exerted upon the valve piston by means of control pressures of differing magnitude. 
         [0062]    The invention is not limited to the illustrated exemplified embodiment. On the contrary, individual features of the hydrostatic system in accordance with the invention can also be combined with one another in an advantageous manner. 
         [0063]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.