Abstract:
A hydraulic system for a suspension system is disclosed. The system has a hydraulic cylinder, a hydraulic tank, a conveying means, a hydraulic accumulator, a control device, a load holding device disposed between the hydraulic accumulator and the hydraulic cylinder, and an on-off valve disposed between the hydraulic accumulator and the control device. In order to provide a suspension function for the hydraulic cylinder while simultaneously ensuring safeguarding of the hydraulic cylinder and of the hydraulic accumulator against a pressure drop in the case of a tube break, the present invention provides a system that controls the on-off valve and the load holding device synchronously through a control pressure device, such that when the suspension function is activated the load holding device is opened and the on-off valve is closed.

Description:
TECHNICAL FIELD 
     The present invention relates to a hydraulic system for a suspension system and to suspension systems for booms or rocker arms for construction equipment. 
     BACKGROUND 
     In machines used in agriculture, such as for example telescope loaders, wheel loaders, or front-end loaders on tractors, it is known to use a hydraulic suspension system that cushions the boom or the rocker arm in order to achieve an overall improvement of the vehicle suspension and riding comfort, especially when the vehicle is traveling. These types of hydraulic suspension systems use a suitable hydraulic system of valves, a hydraulic cylinder and a hydraulic accumulator. The lifting side of the hydraulic cylinder is connected to the hydraulic accumulator to achieve the required suspension by. In addition, the lowering side of the hydraulic cylinder is connected to a hydraulic tank to avoid cavitation and to enable free movement of the piston rod during the suspension process. 
     In order to increase safety against a sudden lowering of the boom or rocker arm, these suspension systems have a load holding device to prevent hose breaks. However, in order to lower the hydraulic cylinder it is necessary to close the tank connection of the lowering side of the hydraulic cylinder so that a required pressure can build up in order to open the load holding device. Oil will flow off from the lifting side of the hydraulic cylinder only when the load holding device has been opened. 
     A hydraulic system for a suspension system of this type is disclosed in EP 1 157 963 A2. The suspension system for the boom of a telescope loader is proposed. The suspension system provides a load holding device for safeguarding against a pressure drop in a hydraulic cylinder and in a hydraulic accumulator. The load holding device essentially comprises a check valve that, in combination with a controllable pressure relief valve, can be bypassed by bringing the pressure relief valve from a normally closed position into an opened position using control pressure lines. In order to avoid limiting the functionality of the suspension system, or to avoid hindering an exchange of hydraulic fluid between the hydraulic accumulator and the hydraulic cylinder, the load holding device is situated at the supply side before the hydraulic cylinder and the hydraulic accumulator. A disadvantage of this system is that it provides only one load holding device for safeguarding pressure-loaded hydraulic components. A hose break occurring between the hydraulic cylinder and the hydraulic accumulator would result in the boom falling downward, and is thus not safeguarded by the load holding device. 
     Therefore, a need exists to create a hydraulic system of the type described above that provides a separate safeguard for the hydraulic cylinder and the hydraulic accumulator, while at the same time providing a suspension function. 
     SUMMARY 
     In an aspect of the present invention, a hydraulic system having a first on-off valve disposed in a first line between the load holding device and the control device is provided. Further, a hydraulic accumulator between the load holding device and the first on-off valve is connected to the first hydraulic line, and the load holding device. The first on-off valve can be controlled synchronously independent of a filling pressure of the hydraulic cylinder. 
     The load holding device is capable of being switched from a closed position oriented in the direction of the control device into an open position, and the first on-off valve is switched from an open position into a closed position oriented in the direction of the control device. Because the first on-off valve is disposed between the hydraulic accumulator and the control device and the load holding device is disposed between the hydraulic accumulator and the hydraulic cylinder, a separate safeguard of the hydraulic cylinder and of the hydraulic accumulator is ensured. 
     The first on-off valve is fashioned in such a way that in the closing position it closes in the direction of the control device without leakage. In addition, due to the fact that the on-off valve and the load holding device can be switched synchronously in such a way that an opening of the load holding device is connected with a closing of the first on-off valve, it is ensured that a suspension function can take place through the hydraulic accumulator for the hydraulic cylinder. 
     The hydraulic system is provided with a hydraulic control pressure device that is set via a conveying means that produces a control pressure. A control pressure line for valves that are to be switched hydraulically is connected optionally to the conveying means or to the hydraulic tank, via a control pressure valve. The present invention contemplates providing an arrangement of a plurality of control pressure lines that can be operated independent of one another via additional control pressure valves, so that a plurality of hydraulically switchable on-off valves can be switched independent of one another. For example, the load holding device and the first on-off valve can then be controlled by mutually independent control pressure lines. 
     In order to enable hydraulic control of the load holding device from pressure prevailing in the hydraulic cylinder and by pressure provided by the control pressure device, a pressure-controlled means is provided in the form of a shuttle valve. The shuttle valve is connected at one inlet side to a first control pressure line of the control pressure device and is connected at another inlet side to a second control pressure line that is connected to a chamber of the hydraulic cylinder. Depending on the pressure occurring in the control pressure lines, the shuttle valve is fed either at the hydraulic cylinder side or at the control pressure device side. Preferably, the shuttle valve is connected to a second chamber of the hydraulic cylinder. 
     The first on-off valve is preferably connected to the control pressure device by a third control pressure line that branches from the first control pressure line. This ensures that the first on-off valve and the pressure-controlled means can be controlled essentially synchronously, or in parallel. The pressure-controlled means, fashioned as a shuttle valve, is connected at the outlet side to the load holding device via a fourth control pressure line, so that the load holding device can be controlled, or opened, for example, via the pressure acting in the second chamber of the hydraulic cylinder or via the pressure produced by the control pressure device. 
     A fifth control pressure line connected to the first chamber of the hydraulic cylinder enables the load holding device to be opened by the pressure acting in the first chamber. In this way, it is ensured that the load holding device opens when, for example, a pressure is reached that overloads the hydraulic cylinder (e.g., due to excessive loads), so that hydraulic fluid can flow out of the first chamber in a controlled manner. 
     Between the hydraulic accumulator and the first line there is provided a second on-off valve that can be brought into an open position or into a position that closes in one direction or in both directions. The second on-off valve connects the hydraulic accumulator to the hydraulic cylinder, so that the hydraulic cylinder can have a cushioning effect when the load holding device is opened. Preferably, the second on-off valve is fashioned in such a way that it closes without leakage, such that when it is in a position in which it closes in one direction, it closes only in the direction of the hydraulic accumulator. A design of the second on-off valve so as to close in one direction ensures that pressure compensation can take place at the hydraulic accumulator. 
     An additional safeguarding of the hydraulic accumulator is provided by connecting a line having a pressure relief valve to the hydraulic accumulator, which line is disposed between the second on-off valve and the hydraulic accumulator. Further, the line connects the hydraulic accumulator with the hydraulic tank. In this way, for example pressure peaks that occur in the hydraulic accumulator, which could arise when there are excessively strong cushioning movements of the hydraulic cylinder, are dissipated. The hydraulic accumulator is safeguarded against excess pressure by the pressure relief valve. Similar systems may be used in order for example to protect the conveying means against excess pressure. 
     In order to supply a second chamber of the hydraulic cylinder, the hydraulic system is provided with a second line that connects the control device to the second chamber. The second line allows the hydraulic cylinder to be charged with pressure at both sides, so that pressure-charged raising and lowering of the hydraulic cylinder, controlled by the control device, is provided. 
     A third line, which is provided with a third on-off valve and is connected to the second chamber and to the hydraulic tank, makes it possible for hydraulic fluid to flow off from the second chamber of the hydraulic cylinder independent of the setting of the control device. By opening the third on-off valve, for example, in a neutral setting of the control device (in which the first and second line are closed) a cushioning movement of the hydraulic cylinder in both directions can take place without a vacuum occurring in the second chamber (decreasing cushioning movement) and a suspension-inhibiting excess pressure occurring in the second chamber (increasing cushioning movement). For the pressure-charged lowering of the hydraulic cylinder, the third on-off valve can be closed. 
     The first chamber of the hydraulic cylinder can advantageously be brought into connection with the hydraulic accumulator via a fourth line. The fourth line is preferably provided with a check valve that closes in the direction of the hydraulic accumulator without leakage. In this way, pressure compensation can take place at the hydraulic accumulator, resulting in advantages in the suspension function (in that a bucking or jerking lifting of the hydraulic cylinder is avoided when the suspension function is switched active). In this exemplary embodiment, at the same time the second on-off valve should be fashioned in such a way that in the closed position it closes without leakage in both directions. In addition, it is possible to provide the fourth line with a throttle or choke instead of the check valve, so that constant but moderate pressure compensation can take place between the hydraulic cylinder and the hydraulic accumulator in both directions. 
     It is also conceivable to situate the throttle in a parallel circuit to the check valve. This would have the advantage that an unthrottled pressure compensation can always take place at the hydraulic accumulator in the direction of the hydraulic cylinder, so that even in the case of rapid load changes the same pressure can always arise in the hydraulic cylinder, and a brief slackening or slumping of the hydraulic cylinder is also avoided. 
     In order to check whether the hydraulic cylinder is situated in a position that is preferred for the activation of the suspension, for example, in a fully lowered position, a sensor can be provided. The sensor can be fashioned as a contact switch or as an angle sensor that is coupled to the movement of the hydraulic cylinder and thus to its position. Depending on the output of the sensor, the suspension function can then be activated or blocked. 
     In order to check whether the hydraulic cylinder is in a pressure state that is preferred for the activation of the suspension, for example, in a state charged with low pressure or in a pressureless state, a pressure sensor or pressure switch can be provided. Depending on the output of the sensor or switch, the suspension function can then be activated or blocked. 
     Via the control pressure device, one or more on-off valves can be controlled. In an exemplary embodiment, the load holding device and the first on-off valve are controlled via the first control pressure line and via the shuttle valve, so that a synchronous switching is ensured. In additional exemplary embodiments, additional on-off valves, such as e.g. the second and the third on-off valve, can be controlled via the first control pressure line, so that by switching the first control pressure valve the load holding device and the first to third on-off valves can be switched synchronously. However, it is conceivable to carry out a controlling separate from the first control pressure line and to provide a second control pressure valve in the control pressure device, so that a controlling of additional on-off valves independent of the first control pressure valve can be carried out. For example, the load holding device and the first on-off valve can be controlled via the first control pressure line or via the first control pressure valve, and the second and third on-off valves can be controlled via a second control pressure line or via a second control pressure valve. In addition, other control combinations and variants are also conceivable. 
     In another exemplary embodiment, one or more on-off valves can be controlled electrically. For example, the load holding device and the first on-off valve are controlled via the control pressure device, the first control pressure valve and the second and third on-off valve being switched electrically. The suspension is then activated through electrical switching of the first control pressure valve and of the second and third on-off valves. The present invention also contemplates controlling only the load holding device via the control pressure device and to switch the first to third on-off valves electrically. For the monitoring and synchronization of electrical switching processes and switching states, an electronic control device or an electrical controller can be used. 
     Between the hydraulic accumulator and the first chamber, there can also be situated what is known as a pressure compensation device, which compensates the pressure in the hydraulic accumulator during a working cycle with the pressure of the first chamber. The pressure compensation device provides a pressure compensation of the hydraulic accumulator with the first chamber of the hydraulic cylinder by providing a line that has a check valve that opens in the direction of the hydraulic accumulator, this line being situated parallel to a pressure scale, pressure regulator or pressure-maintaining valve. The pressure scale is controlled dependent on the pressure prevailing in the hydraulic cylinder and in the hydraulic accumulator. Pressure compensation devices of this sort are known from the prior art and are offered for example by the firm HYDAC. 
     The conveying means that produces a control pressure and the conveying means that produces a filling pressure can be a joint conveying means or can form two or more separate conveying means. For example, a conveying means fashioned as a hydraulic pump can be designed and situated in such a way that it supplies on the one hand a filling pressure for the hydraulic accumulator and on the other hand also supplies a control pressure for the control pressure device via suitable pressure control means, for example via an accumulator that always supplies a constant control pressure and that is charged via the conveying means. However, the use of two separate conveying means or hydraulic pumps is also contemplated. 
     In the following, the present invention and its advantages, as well as advantageous developments and constructions of the present invention, are described and explained in more detail on the basis of the drawings, which indicate a plurality of exemplary embodiments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a first hydraulic system, in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of another hydraulic system, in accordance with an embodiment of the present invention; 
         FIG. 3  is a schematic diagram of another hydraulic system, in accordance with an embodiment of the present invention; 
         FIG. 4  is a schematic diagram of another hydraulic system, in accordance with an embodiment of the present invention; 
         FIG. 5  is a schematic diagram of another hydraulic system, in accordance with an embodiment of the present invention; 
         FIG. 6  is a schematic diagram of another hydraulic system, in accordance with an embodiment of the present invention; and 
         FIG. 7  is a schematic representation of a loading device, fashioned as a telescope loader, having a hydraulic system in accordance with an embodiment of the present invention. 
     
    
    
     DESCRIPTION 
     In accordance with an embodiment of the present invention, a hydraulic system  10  for use in a suspension system of a machine is shown in  FIG. 1 . Hydraulic system  10  contains a control device  12  that can be switched via an actuating device  11 . Control device  12  is, for example, a gate valve connected via hydraulic lines  14 ,  16  to a conveying means  18 , for example, a hydraulic pump, and to a hydraulic tank  20 . Control device  12 , preferably, is switchable between three operating positions: lifting position, neutral position, and lowering position. The switching of control device  12 , preferably, is actuated by mechanical means, but can also be actuated electrically, hydraulically, or pneumatically. 
     Control device  12  is connected to a hydraulic cylinder  26  by first supply line  22  in communication with a first chamber  28  of hydraulic cylinder  26  and second supply line  24  in communication with a second chamber  30  of hydraulic cylinder  26 . A piston  29  separates the two chambers  28 ,  30  from one another. First chamber  28  of hydraulic cylinder  26  represents the piston floor side, or lifting side chamber, whereas second chamber  30  represents the piston rod side, or lowering side chamber of hydraulic cylinder  26 . 
     A load holding device  32 , or a hose breakage safety device, is provided in first supply line  22 . Load holding device  32  has a pressure and spring controlled pressure relief valve  34  as well as a check valve  36  that opens towards the hydraulic cylinder side and is parallel connection with respect to pressure relief valve  34  via a bypass line  38 . A pressure connection is created from pressure relief valve  34  to the hydraulic-cylinder-side segment of first supply line  22  by a control pressure line  40 . Another control pressure line  42  creates a pressure connection between pressure relief valve  34 , which is held in the closed position by an adjustment spring  43 , to a shuttle valve  44 . Shuttle valve  44  is connected at its outlet side to control pressure line  42  and at its inlet side to an additional control pressure line  45 . Control pressure line  45  connects shuttle valve  44  to second supply line  24 . 
     A hydraulic line  46  connects first supply line  22  to a hydraulic accumulator  48 , the end  50  of hydraulic line  46  not connected to hydraulic accumulator  48  is connected between load holding device  32  and switching device  12 . 
     An on-off valve  52  is situated in hydraulic line  46 . On-off valve  52  represents an electrically switchable seat or globe valve that is held in the closed position by an adjustment spring  54  and can be brought into an open position by a magnetic coil  56 . However, on-off valve  52  can also be fashioned so as to be hydraulically switchable. 
     In the closed position, on-off valve  52  closes in the direction of hydraulic accumulator  48 . On-off valve  52  can also be fashioned so that it seals in both directions without leakage. In the open position, a hydraulic flow is ensured in both directions in order to create a suspension function between hydraulic cylinder  26  and hydraulic accumulator  48 . 
     Between end  50  of hydraulic line  46  and control device  12 , an additional on-off valve  60  is provided in first supply line  22 . In its normal position, on-off valve  60  is held in an open position by an adjustment spring  62 . Via a control pressure line  64 , second on-off valve  60  can be brought into a closed position, the on-off valve closing in the direction of control device  12  without leakage. Here it is also possible to fashion on-off valve  60  so as to be electrically switchable. 
     Second supply line  24  is connected to hydraulic tank  20  via an additional line  66 , an on-off valve  68  is situated in line  66 . On-off valve  68  is preferably fashioned as a seat valve and can be switched electrically into an open position or can be brought into a position that closes in the direction of hydraulic tank  20 . Alternatively, on-off valve  68  may be configured to be hydraulically or pneumatically switchable. 
     Hydraulic system  10  includes a control pressure device  70 . Control pressure device  70  has an additional conveying means  72  that is connected to hydraulic tank  20 . In addition, control pressure device  70  has a control pressure valve  74  that is connected via a supply line  76  to conveying means  72  and via an additional supply line  78  to hydraulic tank  20 . Control pressure valve  74  can be switched in such a way that a control pressure line  80  can be connected either to conveying means  72  or to hydraulic tank  20 . Control pressure line  80  is connected at the inlet side to shuttle valve  44 . In addition, control pressure line  80  is connected to control pressure line  64  of on-off valve  60 . 
     In addition, pressure relief valves  81  are provided in hydraulic system  10 , via which conveying means  18 ,  72 , as well as hydraulic accumulator  48 , are connected to the hydraulic tank in order to prevent a pressure overload. 
     In the embodiments shown in  FIGS. 1 ,  2 ,  3 ,  4 , and  6 , a sensor  82  is provided that detects the position of hydraulic cylinder  26 . For example, sensor  82  can be combined as a contact switch that signals a predetermined position of piston  29 . Alternatively, this sensor  82  can also be combined as a pressure sensor or pressure switch (see  FIG. 1 ), the pressure sensor or pressure switch produces a signal at a predetermined pressure of first chamber  28 . 
     Control device  12  is connected to a switch or sensor  84  that detects the position of control device  12  and emits a control signal to an electronic control device  86 . In addition, an activation switch  88  is provided that is connected to control unit  86 . Control unit  86  is configured to switch electrically switchable on-off valves  52 ,  68 , or control pressure valve  74 . 
     When suspension is not activated (i.e., on-off valves  52 ,  68  are in the closed position and control pressure valve  74  is switched such that control pressure line  80  is connected to hydraulic tank  20 ), the operating states “lifting,” “lowering,” and “neutral position” for hydraulic cylinder  26  are controlled as follows via control device  12  in corresponding operating positions. As is shown in  FIGS. 1 to 6 , control device  12  is held in the neutral position; i.e., it is closed and no flow of hydraulic fluid takes place. As shown in  FIGS. 1 to 6 , control device  12  is brought from the neutral position into the lifting or lowering position by means of actuating device  11  after the receipt of a control signal or by manual actuation. Actuating device  11  can also be operated electrically, hydraulically, or pneumatically. 
     In the lifting position, the connection of first supply line  22  to conveying means  18  and the connection of second supply line  24  to hydraulic tank  20  are created. Conveying means  18 , connected to hydraulic tank  20 , fills first chamber  28  of hydraulic cylinder  26  via first supply line  22  and via on-off valve  60 , which is in the open position, as well as via check valve  36  of load holding device  32  (pressure relief valve  34  of load holding device  32  is in the closed position). As a result, piston  29  moves in the direction of second chamber  30 , and presses the oil situated there through second supply line  24  into hydraulic tank  20 . If switching now takes place back into the neutral position, control device  12  interrupts the connections to conveying means  18  and to hydraulic tank  20 , so that the pressure in the two chambers  28 ,  30  of hydraulic cylinder  26  is maintained, and the movement of piston  29  is stopped and piston  29  remains stationary. 
     In the lowering position, the connection of first supply line  22  to hydraulic tank  20  and the connection of second supply line  24  to conveying means  18  are created. The conveying means conveys oil into second chamber  30  of hydraulic cylinder  26 , and the pressure building up in second supply line  24  opens pressure relief valve  34  of load holding device  32  via control pressure line  45 , and also opens shuttle valve  44  and control pressure line  42 . At the same time, piston  29  is moved in the direction of first chamber  28 , so that the oil flowing out of first chamber  28  moves into hydraulic tank  20  via first supply line  22  and via the open pressure relief valve  34 . 
     Load holding device  32 , thus, ensures that in the neutral position hydraulic cylinder  26  maintains its position, in the lifting and neutral position, no oil can escape from pressure-charged first chamber  28 , and that in the lowering position the oil can flow off from first chamber  28  via the opened pressure relief valve  34 . As depicted, load holding device  32  is located at the lifting side of hydraulic cylinder  26 , the lifting side being the side of hydraulic cylinder  26  in which a pressure is built up in order to lift a load. In the exemplary embodiments shown in  FIGS. 1 to 6 , the lifting side is first chamber  28  of hydraulic cylinder  26 , however, by rotating hydraulic cylinder  26 , second chamber  30  could also act as the lifting side. Control pressure line  40  represents an overload safety device, so that when there are excessive operating pressures in first chamber  28  of hydraulic cylinder  26 , which could result for example from excessive carried loads or from heating of hydraulic cylinder  26 , a threshold limit pressure is reached that opens pressure relief valve  34  in order to reduce the pressure. In these states, which deviate from the normal case, pressure relief valve  34  can also be opened in the lifting and neutral positions via control pressure line  40 . 
     It should be noted that on-off valve  60  is open in its normal position, and permits a free flow in both directions. On-off valve  60  is closed by a hydraulic control pressure that moves on-off valve  60  into a switching position in which only a flow in the direction of lifting cylinder  26  is permitted. In the opposite direction, on-off valve  60  is leak proof such that required standards concerning the lowering of a load can be met. Shuttle valve  44  connects control pressure lines  45 ,  80 , which come from control pressure valve  74  and the rod side of lifting cylinder  26 , with control pressure line  42  of pressure relief valve  34  in such a way that pressure relief valve  34 , which represents a part of load holding device  32 , can be opened. Control pressure valve  74  serves to conduct the control pressure of conveying means  72  to pressure relief valve  34  and to on-off valve  60  in order to open or close these. In the normal position, shown in  FIGS. 1 to 6 , of control pressure valve  74 , control pressure lines  42 ,  64 ,  80  are relieved of stress towards hydraulic tank  20 , so that pressure relief valve  34  and on-off valve  60  are in the normal position (pressure relief valve  34  is closed and on-off valve  60  is open). If control pressure valve  74  is switched, hydraulic fluid flows via control pressure lines  42 ,  64 ,  80  to pressure relief valve  34  and to on-off valve  60 . In this way pressure relief valve  34  is opened and on-off valve  60  is closed. 
     In order to ensure optimal protection when a hose breaks, hydraulic cylinder  26 , load holding device  32 , on-off valve  52 , hydraulic accumulator  48 , and on-off valve  60 , as well as the connecting lines, are preferably parts of an assembly made of steel. This can be a valve block fastened to hydraulic cylinder  26  together with hydraulic accumulator  48 , or can be an assembly of valves connected to steel lines. Other parts of the hydraulic system can also be integrated into the named assemblies. 
     The suspension function having the specific embodiments shown in  FIGS. 1 to 6  for a hydraulic system according to the present invention can be realized as described below. Shuttle valve  44 , on-off valve  52 , on-off valve  68 , and control pressure valve  74 , shown in  FIGS. 1 to 6 , are utilized for the suspension function. 
     With reference to the exemplary embodiment shown in  FIG. 1 , an activation of the suspension function is enabled by sensor  82 , connected to control unit  86 , which sensor detects a lowered position (given the use of a contact or position sensor) or a low-pressure operating state (given use of a pressure sensor) of hydraulic cylinder  26 . In order to activate the suspension function, activation switch  88  connected to control unit  86  is actuated. Control unit  86  actuates on-off valve  52  and brings this valve into an open position, through which hydraulic accumulator  48  is connected to supply line  22 . At the same time, control pressure valve  74  is controlled, which releases a control pressure and, via control pressure lines  42 ,  80  in connection with shuttle valve  44 , opens pressure relief valve  34  and, via pressure control lines  64 ,  80 , brings on-off valve  60  into the leakage-free closed position. In addition, on-off valve  68  connected to control unit  86  is simultaneously opened. 
     Through the actuation of control pressure valve  74 , a connection between the lifting side of hydraulic cylinder  26  and hydraulic accumulator  48  is accomplished and, which is sealed in leak-free fashion towards the hydraulic tank. A sudden pressure increase in first chamber  28  of hydraulic cylinder  26  (bouncing up) is not possible, because in the closed position of on-off valve  52  a flow can take place through this valve in the direction of hydraulic cylinder  26 , so that at hydraulic accumulator  48  a pressure compensation in the direction of hydraulic cylinder  26  can always take place. Hydraulic cylinder  26  can be lifted or held via the already-described operating positions, and an exchange of hydraulic fluid can take place via the open connection (on-off valve  52  is open) between hydraulic cylinder  26  and hydraulic accumulator  48 , so that a suspension function is provided. The rod side, or second chamber  30 , of hydraulic cylinder  26  is connected to hydraulic tank  20  (on-off valve  68  is open), in order to enable a free oscillation of cylinder piston  29 , or to prevent a cavitation effect in chambers  28 ,  30 . If the suspension function is deactivated via activation switch  88 , on-off valves  52 ,  68 , as well as control pressure valve  54 , are switched without flow. Control pressure lines  42 ,  64 ,  80  are here switched pressureless, whereby pressure relief valve  34  is again brought into the closed position and on-off valve  60  is again brought into the open position. It is conceivable to control pressure relief valve  34 , as well as on-off valve  60 , via separate pressure control valves (not shown) in accordance with control pressure valve  74 , in order to compensate or to take into account time delays in the response characteristic of pressure relief valve  34  or of on-off valve  60 , so that hydraulic cylinder  26  does not change its position due to an overlapping of the switching times. However, this can also be achieved hydraulically using a throttle (not shown) in control pressure line  64 . 
     The lowering operating state of hydraulic cylinder  26  (moving cylinder piston  29 ) is not possible when the suspension function is activated with the hydraulic system shown in  FIG. 1 , because on-off valve  60  is closed in the direction of control device  12 . In order to ensure a frictionless transition from an operating state (lifting or normal position) with suspension function into the lowering operating state, the position of control device  12  is detected via sensor  84 . If control device  12  is brought into the lowering position, a control signal is automatically sent by sensor  84  to control unit  86 , and the suspension function is deactivated by switching on-off valve  52  and pressure control valve  74 . At the same time, on-off valve  68  is closed. When the suspension function is deactivated, hydraulic accumulator  48  empties via opened on-off valve  60 , and must be charged again for a new activation of the suspension function. Preferably, for this purpose hydraulic cylinder  26  is brought into a fully lowered position, so that the pressure in hydraulic cylinder  26  can build up together with the pressure in the hydraulic accumulator. A new activation of the suspension function can then take place after a new release by sensor  82 , because hydraulic cylinder  26  has been brought into its fully lowered position. 
     When the suspension function is activated, cylinder piston  29  can cushion freely in the lifting operating position and in the normal position. If this piston moves downward due to an impact transmitted to it, the oil is pressed from first chamber  28  into hydraulic accumulator  48 . The pressure building up in hydraulic accumulator  48  causes the oil to flow back into first chamber  28 , so that piston  29  moves upward again. This cushioning movement repeats as necessary until the impact has been completely compensated. 
       FIG. 2  shows an alternative exemplary embodiment in which a deactivation of the suspension function for the lowering of hydraulic cylinder  26  takes place only at times during the lowered state. Subsequently, the suspension function can be resumed without moving again into a release position detected by sensor  82 . The difference from the hydraulic system shown in  FIG. 1  is that in its closed position, on-off valve  52  of hydraulic accumulator  48  closes in leak-free fashion at both sides, so that in the closed position hydraulic accumulator  48  cannot empty in the direction of hydraulic tank  20 . In addition, a pressure compensation is nonetheless ensured between hydraulic accumulator  48  and the hydraulic cylinder (this compensation is provided in the exemplary embodiment shown in  FIG. 1  via on-off valve  52  in the closed position in combination with check valve  36 ), in that an additional line  92  having a check valve  90  is provided that creates a connection from second chamber  28  of hydraulic cylinder  26  to hydraulic accumulator  48 , check valve  90  closing in the direction of hydraulic accumulator  48 . Via line  92 , on the one hand a pressure compensation can take place, and on the other hand hydraulic accumulator  48  can empty via pressure relief valve  34  in a controlled fashion, so that at all times during the lowering the pressure in hydraulic accumulator  48  is equal to the pressure in hydraulic cylinder  26 . Thus, sudden pressure discharges between hydraulic accumulator  48  and hydraulic cylinder  26  when switching from one operating state into another operating state are avoided. Also, in the exemplary embodiment shown in  FIG. 2 , at the beginning of a suspension cycle, i.e., at the beginning of a planned working cycle in which the suspension function is to be used, hydraulic cylinder  26  must be brought into its lowest position so that hydraulic accumulator  48  and hydraulic cylinder  26  are exposed to a common (calibrating) pressure. In the exemplary embodiment shown in  FIG. 2 , in addition to the exemplary embodiment shown in  FIG. 1  it is possible to switch hydraulic cylinder  26  from the lifting operating state or the neutral position into the lowering operating state, and subsequently again to move directly into the suspension state. 
     in order to enable the lowering operating position during the suspension state, here as well the switching position of control device  12  is acquired via sensor  84 . Analogous to the exemplary embodiment shown in  FIG. 1 , on the basis of the signal from sensor  84  on-off valves  52 ,  68  and control pressure valve  74  are controlled by control unit  86  so as to deactivate the suspension state for the duration of the lowering operating state. If control device  12  is switched from the lowering operating position into a different operating position, via control unit  86  on-off valves  52 ,  68  and control pressure valve  74  are again switched and the suspension function is activated. A sudden bouncing up of hydraulic cylinder  26  is prevented by check valve  90  ensuring the same pressure prevails in hydraulic accumulator  48  as in hydraulic cylinder  26 . This function is important for the case in which the load has changed during the lowering of the boom (simultaneous emptying and putting down of a pallet). A temporal sequence of switching processes introduced by control unit  86  can be controlled as necessary using throttles, additional valves, or electronic time delay elements. 
       FIG. 3  shows another exemplary embodiment in accordance with the present invention. On-off valves  52 ,  60 ,  68  are fashioned as pressure-controlled on-off valves  52 ,  60 ,  68 , and are switched in common via control pressure valve  74 . Temporal sequences are controlled for example via throttles (not shown). The exemplary embodiment shown in  FIG. 3  additionally corresponds to the exemplary embodiment shown in  FIG. 1 , and such a pressure-controlled situation and design of on-off valves  52 ,  60 ,  68  is also suitable for the exemplary embodiment shown in  FIG. 2 . 
       FIG. 4  shows an additional exemplary embodiment corresponding essentially to the exemplary embodiment shown in  FIG. 1 . However, pressure compensation device  94  is additionally provided. Pressure compensation device  94  has a line  96  that extends between hydraulic accumulator  48  and hydraulic cylinder  26 , and includes a check valve  98  that closes in the direction of hydraulic cylinder  26 . In addition, a line  99  is provided that extends between hydraulic accumulator  48  and an on-off valve  100  connected to hydraulic tank  20 , and that is provided with a pressure scale  102 . Pressure scale  102  is controlled via pressure lines  104 ,  106  on the one hand by the pressure acting in line  96 , and is controlled on the other hand by the pressure acting in line  99 . Depending on the ratio of these two pressures, pressure scale  102  switches into a closed position or into an open position to on-off valve  100 . On-off valve  100  has a closed position oriented in the direction towards hydraulic tank  20  and an open position. In comparison with the exemplary embodiments shown in  FIGS. 1 to 3 , here a sensor  82  for determining the position of hydraulic cylinder  26  can be omitted. Hydraulic accumulator  48  is here always charged with at least the highest load pressure of hydraulic cylinder  26  during a particular operating state. Here it is not required to lower hydraulic cylinder  26  before activating the suspension state; rather, the suspension state can be activated at any time after a pressure compensation has taken place between hydraulic accumulator  48  and hydraulic cylinder  26 . Such a pressure compensation can be effected by switching on-off valve  100  so as to open it briefly. This can, for example, take place automatically upon actuation of activation switch  88  for the suspension state by control unit  86 . However, a manual actuation is also contemplated by the present invention. In other respects, the functioning of the hydraulic system shown in  FIG. 4  resembles that of the previously described hydraulic system from  FIG. 1 . 
     With reference to  FIG. 5 , another exemplary embodiment is illustrated. The hydraulic system shown in  FIG. 5  corresponds essentially in its functioning and design to the exemplary embodiment shown in  FIG. 2 . However, in the present embodiment, on-off valve  52  has a leak-free closing position in the direction of control device  12 , and check valve  90  of line  92  is situated parallel to a throttle  107  that is situated in line  92 . Here as well, a sensor  82  that detects the position of the hydraulic cylinder is not necessary, because due to the described location of check valve  90  and throttle  107  and the design of on-off valve  52 , a pressure compensation between hydraulic accumulator  48  and hydraulic cylinder  26  can take place at all times. As a result, when the suspension function is activated hydraulic cylinder  26  cannot lower and also cannot raise or bounce up. The suspension function can, thus, be activated at any time, independent of the position of hydraulic cylinder  26 . As in the previously described exemplary embodiments relating to  FIGS. 1 to 4 , in the lowering operating position of control device  12  a deactivation of the suspension function is introduced by control unit  86  for the duration of the lowering operating state, in order to avoid an emptying of hydraulic accumulator  48 . In this exemplary embodiment, it is conceivable to omit check valve  90 , but this will have an adverse effect on the functionality of the pressure compensation. Hydraulic accumulator  48  would then experience a delayed pressure compensation, which would result in some decrease in comfort, but the functionality of the suspension function or of the hydraulic system would not be limited. In the handling of larger loads, a brief lagging or recoil of hydraulic cylinder  26  could occur due to the delayed pressure relief of hydraulic accumulator  48  in the direction of hydraulic cylinder  26 . 
       FIG. 6  shows another exemplary embodiment that essentially resembles the exemplary embodiment according to  FIG. 2 . The difference is that on-off valve  52  is controlled electrically. In other respects, the manner of functioning in the present embodiment is the same as the manner of functioning of the exemplary embodiment in  FIG. 2 . In the present embodiment, control pressure valve  74  is used only to control pressure relief valve  34 . On-off valves  52 ,  60 ,  68 , and control pressure valve  74 , but in particular on-off valve  60  and control pressure valve  74 , are controlled and monitored by control unit  86 . This is important in order to ensure that control pressure valve  74  closes pressure relief valve  34 , should on-off valve  60  spring into its open position during the suspension state, for example due to an electrical defect (cable breakage, burned-out coil, etc.). Should this not take place, in the suspension state hydraulic cylinder  26  would be capable of lowering in an uncontrolled manner. Such an electrical controlling of on-off valve  60  is also contemplated for the other depicted exemplary embodiments. 
     An application for the exemplary embodiments shown in  FIGS. 1 to 6  is shown in  FIG. 7 .  FIG. 7  shows a mobile telescope loader  108  having a boom  110  that can be extended telescopically and that is coupled in pivotable fashion to a frame  109  of telescope loader  108 . A hydraulic cylinder  26  for raising and lowering boom  110  is situated between boom  110  and frame  109 . Further, hydraulic cylinder  26  is coupled in pivotable fashion to a first and to a second bearing point  112 ,  114 , the piston rod side being coupled to second bearing point  114  on boom  110  and the piston floor side being coupled to first bearing point  112  on frame  109 . In addition, hydraulic tank  20 , conveying means  18 , and control device  12  are positioned at or in a housing  115 , and are connected to one another via hydraulic lines  14 ,  16 ,  116 . In addition, in  FIG. 7  supply lines  22 ,  24  are disposed between control device  12  and hydraulic cylinder  26 . Load holding device  32 , as well as on-off valve  52  and on-off valve  60 , are situated in a common valve module directly on hydraulic cylinder  26 . On-off valve  68  is positioned in housing  115  together with control device  12 . Hydraulic accumulator  48  is, preferably, likewise situated directly on hydraulic cylinder  26 , so that hydraulic line  46  can be fashioned as a rigid connection between the common valve module and hydraulic accumulator  48 , requiring no separate breakage safeguarding device. Control unit  86  generates control or switching signals which switch or control on-off valves  52 ,  60 ,  68 , as well as control pressure valve  74 , depending on the status of sensors  82 ,  84  or activation switch  88 . 
     In  FIG. 7 , control unit  86 , sensors  82 ,  84 , activation switch  88 , control pressure device  70  are not shown. The location of such components is well known in the prior art and can be executed by someone skilled in the art in a known manner. 
     Corresponding to the above-described switching positions, hydraulic cylinder  26  can be actuated in such a way that boom  110  can be lifted, held steady, or lowered, and a suspension state can be set or activated for the individual operating states, as described above and illustrated in  FIGS. 1 to 6 . When the suspension function is activated, it is ensured that during an excitation, for example due to the traveling mechanism of telescope loader  108 , impact-type accelerations due to a free oscillation of boom  110  are damped, so that traveling comfort is increased, in particular if a work tool  108  is used to pick up and move loads. 
     Although the present invention has been described on the basis of some exemplary embodiments, in the light of the foregoing description and the drawings someone skilled in the art will infer a large number of different alternatives, modifications, and variants falling within the scope of the present invention. Thus, for example, the hydraulic system can also be applied to other vehicles, for example to wheel loaders or front-end loaders or to baggers or cranes having components that can be actuated hydraulically and that can be lifted or lowered and in which a suspension is considered desirable. 
     The foregoing disclosure is the best mode devised by the inventor for practicing this invention. It is apparent, however, that methods incorporating modifications and variations will be obvious to one skilled in the art of motor vehicle clutches and lubrication thereof. Inasmuch as the foregoing disclosure is intended to enable one skilled in the pertinent art to practice the instant invention, it should not be construed to be limited, thereby but should be construed to include such aforementioned obvious variations and be limited only by the spirit and scope of the following claims.