Abstract:
Adjusting device which has a main cylinder ( 2 ) and a spring cylinder ( 3 ) arranged separately therefrom, wherein, during a pressure drop, the spring force acts hydraulically on the main cylinder ( 2 ). In order to design such an adjusting device more effectively, the effective spring piston area is variable.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in International Patent Application No. PCT/DK2007/000314 filed on Jun. 27, 2007 and German Patent Application No. 10 2006 029 523.4 filed Jun. 27, 2006. 
       FIELD OF THE INVENTION 
       [0002]    The invention concerns an adjusting device comprising a main cylinder with a main piston and a spring cylinder with a spring piston, the main piston being acted upon in its two movement directions and the spring piston being activated in one direction by the pressure of a spring and in the opposite direction by an operating pressure, a pressure chamber of the spring cylinder being connectable with a first pressure chamber of the main cylinder and a second pressure chamber of the main cylinder being connected to a supply pipe. 
       BACKGROUND OF THE INVENTION 
       [0003]    Such adjusting devices are, for example, used for activating valve flaps. They usually comprise a hydraulically acting cylinder, which is arranged in a housing, the piston rod emerging through an opening in one end of the housing. With linearly moving valve flaps, the linear movement of the piston rod is usually transferred directly. With revolving valve flaps, the linear movement of the piston must initially be converted to a rotation. This, for example, takes place by means of a steep pitch between the piston rod and the housing, which causes a rotation of the cylinder rod. Another possibility is provided by the use of a toothed rod with a corresponding pinion, the conversion of the linear movement to a rotary movement then typically occurring outside the housing. 
         [0004]    We distinguish between double-acting and single-acting adjusting devices. Double-acting adjusting devices are controlled via two pressure pipes, the valve flap opening when a control pressure is applied to a first pressure pipe and closing when a control pressure is applied to a second pressure pipe. Double-acting adjusting devices are usually dimensioned in accordance with the rotation torque and the rotation angle, or in accordance with the lift power and the lift length, respectively, which is required for a reliable activation of the valve flap. 
         [0005]    For single-acting adjusting devices, however, one single control pressure pipe is sufficient. Here, the piston is moved by the control pressure against the force of a spring, which ensures the reset movement of the piston in case of a reduction of the control pressure. This has the advantage that also in connection with a defect, for example a pressure loss in the control pipe, a reset movement of the piston can take place. In connection with valve flaps, the opening often occurs by applying a control pressure, whereas the closing takes place by means of spring force. This ensures that the valve will also be closed in connection with a defect in the system. However, the valve can also be controlled in the opposite manner, so that it closes when a control pressure is applied. 
         [0006]    The dimensioning of the spring takes place on the basis of the required closing force, both losses in the springs and the slackening of the spring force during reducing spring tension being considered. For example, cup springs or coil springs are used as springs, and they are suspended between one of the piston sides and the corresponding end of the housing. Also the use of various kinds of gas springs is known. 
         [0007]    One disadvantage of single-acting adjusting devices with this embodiment is that the piston is constantly exposed to the spring force. Further to the adjusting force for the activation of the valve flap, an additional force must be provided to balance the spring force. This force could be provided by an increase of the control pressure, which is often only possible with great effort. Therefore, usually the effective piston surface is enlarged. With unchanged control pressure, the larger cross-section will also increase the total tension in the housing, meaning that the housing has to be dimensioned to be more robust. 
         [0008]    This causes that single-acting adjusting devices require more space than double-acting adjusting devices with the same torque or the same lift force. This results in higher manufacturing and transport costs. Further, the larger dimensions are particularly disadvantageous in connection with the use onboard ships, as the additional weight makes a vibration safe mounting more difficult. 
         [0009]    The invention is based on an adjusting device as known from DE 195 43 237 A1. Here, a hydraulic adjusting device is disclosed, which comprises a main cylinder and a separately located spring cylinder. The separate location causes that the tensioning of the spring does not occur via the main piston, so that the main cylinder can be made smaller than it is usually the case with single-acting adjusting devices. 
         [0010]    The main piston is moved to the opening position in that a control pressure is applied on a first pressure chamber of the main cylinder. At the same time, the control pressure is applied on a pressure chamber of the spring cylinder, so that the spring is compressed. A reduction of the operating pressure, whether caused by a defect or by an intended intervention, causes a release of the spring. This presses pressure means from the pressure chamber of the spring cylinder into a second pressure chamber of the main cylinder. As the first pressure chamber of the main cylinder has at least a reduced pressure, the main piston is moved to the closing position. This means that a pressure drop causes a hydraulic transfer of the spring force to the main piston. Thus, a transmission of the spring force may occur, meaning, for example, that a shorter spring length can be used, which causes a more compact embodiment. The hydraulic transfer also reduces the oscillation inclination of the adjusting device, as the hydraulic fluid serves as damping. 
         [0011]    EP 0 902 195 A1 shows an adjusting device, in which a spring cylinder is located outside the main cylinder. The spring cylinder is prestressed by the control pressure, which also activates the main cylinder. During a pressure drop, the spring force is mechanically transferred to the main piston, a transmission of the spring force not being provided. 
         [0012]    In the known adjusting devices, the adjusting force, which is generated by the spring and is supposed to move the main piston back to the closing position, is reduced with increasing release of the spring. 
         [0013]    If a valve flap is moved by the main piston, the valve flap must usually engage in the closing position to ensure a reliable fit of the closed valve flap. For this purpose, an additional force is required at the end of the closing movement. The spring is dimensioned in accordance with the force, which is required for a safe closing of the valve flap, which occurs at the end of the adjusting movement. This causes that over a large area the spring is overdimensioned, as the spring force is reduced with increasing release of the spring. 
         [0014]    At the beginning of the reset movement a similarly large force is often required as for the safe holding at the end of the movement, as initially the static friction of the stillstanding valve flap has to be overcome. During the largest part of the closing movement, however, only a small force is required. Thus, in a large movement area, the known adjusting devices are overdimensioned, which causes an additional weight and further basically increases the risk of overloading the piston rod or a spindle of the valve flap. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0015]    The invention is based on the task of making a single-acting adjusting device more efficient. 
         [0016]    With an adjusting device as mentioned in the introduction, this task is solved in that the active spring piston surface is variable. 
         [0017]    As the closing pressure, which can be generated by the spring piston, corresponds to the relation between the spring force and the active spring piston surface, the reduction of the spring force caused by the increasing expansion of the spring can be compensated by a reduction of the active spring piston surface. Thus, it is possible to generate the same pressures on the closing side of the main piston both at the beginning and at the end of the reset movement, whereas in the middle part of the closing movement smaller pressures are ruling. 
         [0018]    Due to the variable active spring piston surface, the spring cylinder can be dimensioned for the volume change required for carrying through the closing movement, the force requirement at the end of the closing movement mainly influencing the relation of the active spring piston surfaces. As the main piston must not contribute to the tension of the spring and the closing pressure being available at the end of the closing movement influences the main piston like the operating pressure at a corresponding dimensioning of the spring cylinder, the main piston can be dimensioned according to the same criteria as those being the basis of double-acting adjusting devices. The adjusting device will thus be smaller than know single-acting adjusting devices. In this connection, it can also be imagined to locate the spring housing spatially separated from the main housing. The size of the active spring piston surface can, for example, be dependent on the position of the main piston or on the pressure in individual pressure chambers and pressure pipes. 
         [0019]    Preferably, the active spring piston surface depends on the position of the spring piston. This ensures a simple design, as the spring piston can, for example, serve as mechanical activation element for valve arrangements. 
         [0020]    In a preferred embodiment, the spring cylinder has at least two separately arranged pressure chambers, which are located at the same side of the spring piston, the number of the active pressure chambers being variable. By means of a variable number of pressure chambers the active spring piston surface can be influenced in a simple manner. At the beginning of and during the largest part of the closing movement, for example two pressure chambers are pressurised, whereas at the end of the movement only one pressure chamber is pressurised. Thus, the active spring piston surface is smaller at the end of the closing movement than at the beginning of the closing movement, so that a higher pressure can be generated. An increase of the number of pressure chambers acting against the spring force can in principle be used to generate random force or torque characteristics, respectively. The number of active pressure chambers can, for example, be controlled in dependence of the position of the main piston. 
         [0021]    It is particularly preferred that the number of active pressure chambers of the spring cylinder depends on the position of the spring piston. Thus, it can be ensured that in each position of the spring piston the predetermined number of pressure chambers is active. At the same time, the spring piston can be used as adjusting member, for example for valve elements. Such valves can, for example be used for relieving individual pressure chambers to a tank. 
         [0022]    Preferably, the adjusting device has a control valve, which connects the pressure chambers of the spring cylinder to the supply pipe, a pressure equalisation of the pressure chambers of the spring cylinder to the supply pipe being prevented in a first position and released in a second position. Thus, this control ensures that via the control pressure a pressure can be generated in the pressure chambers of the spring cylinder, so that the spring piston is deflected against the spring force. If, at a reduced control pressure, the valve is moved to the second position, at least one pressure chamber of the spring cylinder is relieved to the supply pipe, which reduces the active spring piston surface. In this connection, a mechanical element that engages the spring piston can move the control valve against the force of a spring into the second position. 
         [0023]    Advantageously, a valve is arranged in the connection between a pressure chamber of the spring cylinder and the first pressure chamber of the main cylinder. This valve ensures that a control pressure applied on the second pressure chamber of the main cylinder does not also reach the first pressure chamber of the main cylinder via the pressure chamber of the spring cylinder, thus counteracting a movement of the main piston. 
         [0024]    Preferably, the position of the valve is dependent on the pressure in the supply pipe, the first pressure chamber of the main cylinder being relieved on application of a minimum pressure, and at least one of the pressure chambers of the spring cylinder being connected to the first pressure chamber of the main cylinder in the case of a too low pressure. The relief of the first pressure chamber ensures that the main piston can be moved by the control pressure. The relief usually takes place to a tank. As the valve depends on the pressure of the supply pipe, a connection of the pressure chamber of the spring cylinder to the first pressure chamber of the main cylinder automatically occurs, as soon as a pressure drop occurs in the supply pipe. Thus, the piston is automatically reversed without requiring an external influence. Thus, a safe closing is also ensured in the case of a breakdown. 
         [0025]    Advantageously, the second pressure chamber of the main cylinder can be relieved directly to the tank. Thus, it can be prevented that the main piston gets stuck in, for example, an interim position, if only a reduced control pressure is available. On the contrary, the relief of the second pressure chamber ensures that the main piston is reverted to its starting position by the spring cylinder. 
         [0026]    It is particularly preferred that the pressure pipes are arranged in walls of the cylinder. Such an embodiment results in a compact adjusting device which is insusceptible to faults, as no external pipes have to be mounted between the spring cylinder and the main cylinder. 
         [0027]    Preferably, the spring cylinder has at least one non-return valve, which is arranged between the pressure chambers of the spring cylinder. Such a valve prevents a pressure equalisation between the pressure chambers of the spring cylinder, if one of these pressure chambers is relieved, in order to reduce the active spring piston surface. If a control pressure is applied, this pressure opens the non-return valve, so that all pressure chambers of the spring cylinder can be pressurised. 
         [0028]    Preferably, the spring piston is made in two steps. A two-step embodiment of the spring piston permits a simple realisation of two separate pressure chambers. The sealing between these pressure chambers then occurs via the spring piston. 
         [0029]    Preferably, the spring piston has several spring piston surfaces, which overlap each other, at least partly, in the movement direction. In this connection, several serially arranged and mutually connected pistons can be concerned. The force transmission between the individual part pistons can take place both mechanically and hydraulically. 
         [0030]    Preferably, the main cylinder comprises a transmission element that rotates because of the movement of the main cylinder. For this purpose, in connection with rotating main pistons, the transmission element is unrotatably and axially movably connected to the main piston. If the main piston only makes translatory movements, the transmission element can, for example, be forced to rotate by means of a thread arranged between the transmission element and the main piston. A revolving valve flap can, for example, be connected directly to such a transmission element. Thus, an additional transmission gear can be avoided. 
         [0031]    It is particularly preferred that the transmission element comprises an accommodation, which enables a form-fitting connection to a drive element. The integration of an accommodation in the transmission element permits an easy connection of a spindle of the valve flap to the transmission element. By means of a standardisation of the accommodation, this also makes a mutual exchange of the valve flaps or the adjusting devices possible. 
         [0032]    Preferably, the adjusting device comprises a shaft, which extends throughout the whole adjusting device. The shaft can be an individual component or be a part of the transmission element. It rotates with the transmission element or the main piston and can serve as a position indicator and for the control of valves of the adjusting device. It is also possible, in dependence of the position of the shaft to control the number of active pressure chambers. Pressure pipes can be integrated in the shaft, so that additional pipes will not be necessary. 
         [0033]    It is particularly preferred that the shaft is a multipart shaft. A multipart embodiment of the shaft simplifies the mounting or dismounting of the spring cylinder, which can only be dismounted by means of heavy machines because of the required spring preload. This makes a division of the shaft in the axial direction particularly favourable, even though also a concentric division is possible. 
         [0034]    Preferably, the spring has a preload in any position of the spring piston. Thus, it is ensured that also in the closing position, that is, with maximum expansion, the spring can provide a force. 
         [0035]    In a preferred embodiment, the main cylinder and the spring cylinder are arranged on a common axis. Thus, on the one side, material can be saved, and on the other side a valve arrangement can favourably be arranged between the two cylinders, as here mechanical signals, which reflect the positions of the two pistons and the valve flap, can be made accessible in a particularly easy manner. 
         [0036]    Preferably, at least one of the pressure chambers of the spring piston comprises a pressure sensor. By means of such a sensor, a signal can be generated, which can, for example, be passed on to an overriding safety arrangement, as soon as an undesired pressure drop occurs. 
         [0037]    Preferably, the adjusting device has at least one mechanical element that is arranged between the main piston and the spring piston, axial forces being transmittable between the main piston and the spring piston via the at least one mechanical element, if the spring is relieved to the stop, and axial forces not being transmittable between the main piston and the spring piston, if the spring is stressed to the stop. In this connection, such a mechanical element can be used independently of the embodiment of the spring cylinder, that is, also with spring cylinders, whose active spring piston surface is not variable. Through these mechanical elements, the force applied on the spring piston by the spring can also be transmitted to the main piston, if the connection pipe between a pressure chamber of the spring cylinder and the first pressure chamber of the main cylinder is leaky, which relieves the spring. The closing or at least the non-opening of the valve flap is thus also ensured in the case of leakages inside the adjusting device. 
         [0038]    Preferably, the length of the at least one mechanical element is smaller than the proper distance between the main piston and the spring piston. Whereas, during proper operation, no force transmission is possible through the mechanical element from the spring piston to the main piston, a leakage will reduce the distance between the main piston and the spring piston, and a mechanical force transmission from the spring piston to the main piston will take place by means of the mechanical elements. If, in the normal case, the movement of the spring piston is transmitted to the main piston by means of a liquid pressure means, the use of the mechanical elements causes a sudden pressure drop inside the pressure chambers of the spring cylinder. Thus, the mechanical elements improve the safety of the occurrence of a reset movement. 
         [0039]    Preferably, the mechanical element is pressure-tight and supported to be axially displaceable in openings in a wall between the main cylinder and the spring cylinder. This enables a particularly simple embodiment of the adjusting device, when the main cylinder and the spring cylinder are arranged on a common axis. Here, the wall firstly serves as a guide for the mechanical element, secondly as a separation between the pressure chamber of the spring cylinder and the first pressure chamber of the main cylinder. The pressure-proof support thus prevents a pressure equalisation between the pressure chamber of the spring cylinder and the first pressure chamber of the main cylinder. 
         [0040]    Preferably, the mechanical element is floatingly supported. Thus, it is moved during the opening movement by the main piston, causing that a slightly larger force must be applied on the main piston. In the opposite direction, the mechanical element is then moved by the spring piston. Thus, a fixing of the mechanical element on one of the pistons can be avoided, so that the number of required components is kept small. 
         [0041]    In a preferred embodiment, the mechanical element is fixed on the spring piston. Thus, it is not necessary for the main piston to provide an additional force during the opening movement. 
         [0042]    Preferably, the mechanical element is arranged eccentrically opposite the spring piston. Thus, it also serves as distortion protection for the spring piston. 
         [0043]    Preferably, the mechanical element is made as a cylindrical rod. Such an element is easily manufactured. At the same time, the sealing of the mechanical element in the wall between the main cylinder and the spring cylinder can be made with simple sealing rings. Thus, the manufacturing remains cost effective. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]    In the following, the invention is explained on the basis of preferred embodiments in connection with the drawings, showing: 
           [0045]      FIG. 1  is a schematic view of a preferred embodiment, 
           [0046]      FIG. 2  is a valve arrangement that can be arranged between the main cylinder and the spring cylinder, 
           [0047]      FIG. 3  is an embodiment of a control valve, 
           [0048]      FIG. 4  is a hydraulic circuit diagram of a preferred embodiment with the pressure states during the opening, 
           [0049]      FIG. 5  is the hydraulic circuit diagram with the pressure states at the beginning of the closing movement, 
           [0050]      FIG. 6  is the hydraulic circuit diagram with the pressure states at the end of the closing movement, 
           [0051]      FIG. 7  is an alternative control of the valve; and 
           [0052]      FIG. 8  is a hydraulic circuit diagram of a preferred embodiment with a multipart spring piston. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0053]      FIG. 1  shows an adjusting device  1  in the form of a rotary drive. A main cylinder  2  and a spring cylinder  3  are arranged on a common axis and connected to each other at a front side. 
         [0054]    On its other front side, the main cylinder  2  has a mounting flange  4 . Inside the main cylinder  2  is arranged a main piston  5 , which is guided via a non-self-restricting thread  6 . Due to this thread  6 , the main piston  5  rotates during an axial displacement. This rotation is transmitted via form-fitting to a transmission element  7 , the transmission element  7  being fixed in the axial direction, thus being unable to perform an axial movement. For this purpose, it is fixed in the axial direction by a ring  8 , which is screwed into the main cylinder  2 . 
         [0055]    The transmission element  7  has an inner hollow  9 , which is made so that a further transmission element, for example a spindle, can be accommodated in a form-fitting manner. Such an accommodation can, for example, be made as a triangle, a rectangle or a hexagon. 
         [0056]    On the rotation axis of the main cylinder  2 , a shaft  10  is arranged, which also extends through the spring cylinder  3  and is fixed on the transition element  7 . Thus, the shaft  10  performs a rotation in dependence of the movement of the main piston  5 , and can thus be used as a position indicator or for the control of valves of the adjusting device  1 . 
         [0057]    The embodiment shown comprises an alternative position indicator  11 . It is pressed by a spring against an eccentric surface at the transmission element  7  and thus changes its radial position in dependence of the rotation of the transmission element  7 . 
         [0058]    The main cylinder  2  comprises a first pressure chamber  19  and a second pressure chamber  12 . Two annular hollows formed between the main piston  5  and the transmission element  7  belong to the pressure chamber  12 . Via an inlet connection (not shown), a control pressure is applied on the main cylinder  2 , the inlet connection being connected to the second pressure chamber  12  of the main cylinder  2 . The same control pressure is also applied to a first pressure chamber  13  and a second pressure chamber  14  of the spring cylinder  3 , which are formed by the cascade embodiment of the spring piston  15 . The two pressure chambers  13 , 14  of the spring cylinder  3  are separated from each other in a pressure-tight manner. Via a non-return valve  16 , not shown in  FIG. 1 , a pressure equalisation can take place from the first pressure chamber  13  to the second pressure chamber of the spring cylinder  3 . On the side opposite the two pressure chambers  13 ,  14 , the force of a spring  17  is applied on the spring piston  15 , the spring  17  being formed by an interconnection of spring washers. The spring  17  is mounted under tension, so that in any position a certain minimum force is applied on the spring piston  15 . In this connection, the spring  17  is guided on the inside by the spring piston  15  over a part of its length. For the guiding of the other part, a guide pipe  18  is provided, which guides the outside of the spring  17 . 
         [0059]    The first pressure chamber  19  of the main cylinder  2  is closed by a cover-shaped wall  20 . This wall  20  limits the movement of the main piston  5  and the first pressure chamber  19  of the main cylinder  2 . The wall  20  is provided with an outer thread  26  that engages a mating inner thread  27  of the main cylinder  2 . Thus, the wall  20  is retained in the main cylinder  2 . 
         [0060]    The wall  20  has three openings  21 ,  22 ,  23 , through which the shaft  10  and two rotation-symmetrical, mechanical elements  24 ,  25  are guided. In this connection, sealings are provided, which prevent a pressure loss through the wall  20 . 
         [0061]    The mechanical elements  24 , 25  are fixed on the spring piston  15  and arranged so that in connection with a pressure loss in the second pressure chamber  14  of the spring cylinder  3  the spring piston  15  presses them against the main piston  5 , so that the latter is sufficiently influenced to at least retain a valve flap, which can be connected to the transmission element  7  via a spindle, in the present position or to reset it completely. Thus, it is prevented that in the case of a leakage inside the adjusting device  1 , the valve flap moves to an undefined position. In the embodiment shown, the mechanical elements  24 ,  25  will transmit no forces between the pistons  5 , 15  during normal operation, as the pistons  5 ,  15  are controlled so that the minimum distance between them is larger than the length of the mechanical elements  24 ,  25 . However, other embodiments can be imagined, in which the mechanical elements  24 ,  25 , for example at the end of the normal closing movement, are used for a force transmission between the pistons  5 ,  15 . 
         [0062]    In the shown embodiment, the mechanical elements  24 ,  25  are fixed on the spring piston  15  by means of retaining rings  28 ,  29 . However, they can also be floatingly supported, meaning that during the opening they must be moved away from the main piston  5  in stead of from the spring piston  15 , so that the load on the main piston  5  increases somewhat. In both cases, sealings are required between the second pressure chamber  14  of the spring cylinder  3  and the first pressure chamber  19  of the main cylinder  2 . If more than one mechanical element  24 ,  25  is provided, or if the only mechanical element  24 ,  25  is arranged eccentrically to the spring piston axis, the spring piston  15  is at the same time prevented from rotating. In this case, also the rotation of the shaft  10  in relation to the spring piston  15  can be used to control a valve arrangement. 
         [0063]    In the shown embodiment, the pressure in the first pressure chamber  14  of the spring cylinder  3  drops rapidly because of the relative incompressibility of the pressure means, when the mechanical elements  24 ,  25  rest on the main piston  5 . As, in all other situations, the first pressure chamber  14  of the spring cylinder  3  is pressurised, a simple pressure transmitter or pressure switch can be used to indicate a possible leakage. This can take place either optically, mechanically or electrically. 
         [0064]      FIG. 2  shows a valve arrangement of the kind, which could be arranged between the main cylinder  2  and the spring cylinder  3 . A valve  30  of the valve arrangement has a valve slide  31 , which is loaded in the closing direction by the pressure in the second chamber  12  of the main cylinder  2  and in the opening direction by a spring, which is not shown. In the opening position shown, the second pressure chamber  14  of the spring cylinder  3  is connected to the first pressure chamber  19  of the main cylinder  2 , so that pressure means can flow from the pressure chamber  14  into the pressure chamber  19 , so that the main piston  5  is moved. 
         [0065]    A non-return valve  32 , which is arranged between a low-pressure connection  33  and the first pressure chamber  19  of the main cylinder  2 , is activated in the closing direction by the pressure in the first pressure chamber  19 . The non-return valve  32  is opened by the valve slide  31 , when the valve slide is in the closing position. 
         [0066]    Further, a non-return valve  16  is provided, which connects the first pressure chamber  13  of the spring cylinder  3  and the second pressure chamber  14  of the spring cylinder to each other. The non-return valve  16  ensures that a pressure equalisation con only take place from the first pressure chamber  13  to the second pressure chamber  14  of the spring cylinder  3 , and not in the opposite direction. 
         [0067]      FIG. 3  shows a control valve  34 , through which a supply pipe  35  is connected to the first pressure chamber  13  of the spring cylinder  3 . In this connection the control valve  34  is pressurised in the opening direction by the pressure in the supply pipe  35  and a tappet  36  and in the closing direction by the pressure in the first pressure chamber  13  and a spring. A spring loaded element  39 , which is provided merely for this purpose, and which is connected to the spring piston  15 , moves the tappet  36  in the opening direction, as soon as the spring piston  15  has reached a corresponding position. Hereby the first pressure chamber  13  of the spring cylinder  3  is relieved to the supply pipe  35 , so that the active spring piston surface is reduced. It is also possible that the shaft  10  is made accordingly, for example in the form of a camshaft, thus activating the tappet  36 , or that the activation takes place via one of the mechanical elements  24 ,  25  or any other internal or external mechanical signal. 
         [0068]      FIG. 4  shows the hydraulic circuit diagram of a preferred embodiment, the state during the opening being shown. For the opening, the inlet connection  11  of the adjusting device  1  is, for example by means of a pump, pressurised by a control pressure via the supply connection  35 . The control pressure is then available in the second pressure chamber  12  of the main cylinder  2 . Hereby the main piston  5  is displaced to the right in the  FIGS. 1 and 4 . 
         [0069]    The control pressure moves the valve  30  to the position shown in  FIG. 4 , in which it relieves the first pressure chamber  19  of the main cylinder  2  to the tank  37 . In this connection, the valve  30  is made with differently sized activation flaps, so that it can also be moved reliably by the control pressure, if the closing pressure is still ruling in the second pressure chamber  14  of the spring cylinder  3 . 
         [0070]    Via the control valve  34 , the control pressure is also available in the first pressure chamber  13  of the spring cylinder  3  and further via the valve  16 , the control pressure is available in the second pressure chamber  14  of the spring cylinder  3 . This will move the spring piston  15  upwards in the drawing, thus loading the spring  17 . In this connection, the connection of the second pressure chamber  14  of the spring cylinder  3  to the first pressure chamber  19  of the main cylinder  2  is blocked via the valve  30 . 
         [0071]    If, now, the control pressure drops, either to initiate a closing process or because of an interference, the conditions in  FIG. 5  are ruling. The supply pipe  35  is pressureless, so that the second pressure chamber  12  of the main cylinder  2  is relieved. The pressure in the second pressure chamber  14  of the spring cylinder  3  moves the valve  30  to the position shown, as the counter-pressure from the supply pipe  35  is missing. Hereby, the second pressure chamber  14  of the spring cylinder  3 , which is connected to the first pressure chamber  13  via the valve  16 , is connected to the first pressure chamber  19  of the main cylinder  2 . As the first pressure chamber  13  and the second pressure chamber  14  of the spring cylinder are pressurised by the spring  17 , a pressure equalisation takes place from here to the first pressure chamber  19  of the main cylinder  2 , and the main piston  5  moves to the left in the drawing. 
         [0072]    If the spring piston  15  reaches a predefined position, the valve  34  is moved to the position shown in  FIG. 6 . This, for example, takes place via one of the mechanical elements  24 ,  25 . Hereby, the first pressure chamber  13  of the spring cylinder  3  is relieved to the pressureless supply pipe  35 . Then, only a small spring piston surface is active against the spring  17 , through which a higher pressure can be transmitted to the main piston  5 . The non-return valve  16  prevents a pressure equalisation from the second pressure chamber  14  of the spring cylinder  3  to the first pressure chamber  13  of the spring cylinder  3 , which is relieved to the supply pipe  35  via the control valve  34 . 
         [0073]      FIG. 7  shows a simplified control of the valve  30 . The valve  30  is moved by the pressure in the supply pipe  35  against a spring  38  into the position, in which it relieves the first pressure chamber  19  of the main cylinder  2  to the tank  37 . If the pressure in the supply pipe  35  drops, the valve  30  is moved by the spring  38  into the position, in which it connects the second pressure chamber  14  of the spring cylinder  3  to the first pressure chamber  19  of the main cylinder. In this embodiment, the valve  30  connects, in its initial position, the second pressure chamber  14  of the spring cylinder  3  to the first pressure chamber  19  of the main cylinder  2 . 
         [0074]    In  FIG. 8  the spring piston  15  is made as a multipart piston, that is, in the movement direction the two spring piston surfaces overlap each other partly. This also forms two pressure chambers  13 , 14 , the first pressure chamber  13  being relieved via the valve  34  to the control pipe  35  from a certain position of the spring piston  15 . 
         [0075]    In the embodiment shown, surplus pressure means from the second pressure chamber  12  of the main cylinder  2  and from the first pressure chamber  13  of the spring cylinder are returned to the supply pipe  35  of the adjusting device  1  during closing. If, in this connection, a residual pressure is available in the supply pipe  35  of the adjusting device  1 , this pressure may under certain circumstances counteract the movement of the main piston  5 , so that the main piston  5  and thus also the valve flap are stuck in an intermediate position. As usually an increased force is required to overcome the static friction, and as the spring cylinder  3  according to the invention transmits less force in intermediate positions as in the end positions, a further pressure drop in this situation may cause that the reset movement of the main piston  5  is no longer completely performed. If it is required, also in this case, to ensure a reliable closing, the valve arrangement can be changed so that, during closing, surplus pressure means from the second pressure chamber  12  of the main cylinder  2  and from the first pressure chamber  13  of the spring cylinder  3  are led via a pressureless return pipe into a tank  33  instead of into the supply pipe  35  of the adjusting device  1 . 
         [0076]    For the same reason, the valve flap may remain in a half-open position, if, during opening, the control pressure drops. This may be prevented in that the valve arrangement is changed so that the second pressure chamber  12  of the main cylinder  2  is not pressurised until the spring piston  15  has loaded the spring stack  17  completely. 
         [0077]    While the present invention has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this invention may be made without departing from the spirit and scope of the present invention.