Abstract:
A suspension system, in particular for a working machine, such as a tractor or similar device, with a suspension cylinder ( 10 ) is subjected to various load pressures (m). The piston slide ( 12 ) and rod side ( 14 ) of the system may each be connected to a suspension reservoir ( 16, 18 ). A locking device ( 20 ) locks the suspension and, with an equalization device ( 28 ) having a switching valve ( 48 ) within the supply device, controls the piston side ( 12 ) of the suspension cylinder ( 10 ). A pressure supply (P) supplies the system pressure necessary for the suspension system. A pressure equalization of the system to the relevant load pressure can be performed before renewed operation by the equalization device. The equalization device ( 28 ) includes a further switching valve ( 50 ) within the supply device ( 30 ) for control of the rod side ( 14 ) of the suspension cylinder ( 10 ). A load monitoring device (LS) is connected to parts of the stop device ( 20 ). The associated suspension reservoir ( 16 ) for the piston side ( 12 ) of the suspension cylinder ( 10 ) is in a branch ( 38 ) of a line ( 40 ).

Description:
FIELD OF THE INVENTION 
   The present invention relates to a suspension system, in particular for a working machine such as a tractor or the like, with a suspension cylinder to which varying load pressures may be applied. A suspension cylinder whose piston and rod side may each be connected as required to a suspension reservoir by a locking device which locks the spring suspension system. An equalization device, having a switching valve, controls the piston side of the suspension cylinder inside the supply device. A pressure supply provides the system pressure needed in the suspension system. 
   BACKGROUND OF THE INVENTION 
   In certain spring suspension systems, for example for machines such as tractors or the like, it may be advisable to stop such machines, for example, at a speed below an assignable speed of the machine. This stopping is accomplished by locking the hydropneumatic suspension when the vehicle is moving at low speeds or is stationary. Operations, such as hoisting and loading and unloading loads or activation of harvesting or other implements, may accordingly proceed without disruptive vehicle spring suspension travel. 
   Since the load situation is unknown after such operations as the vehicle continues in movement or starts movement, in the conventional spring suspension systems movement of the vehicle or operating machine is not controlled when the suspension is unlocked. This lack of control is in principle an unacceptable situation. 
   DE-A-2 023 283 discloses a generic spring suspension system for a machine, such as a construction site vehicle, having two suspension cylinders to which different load pressures may be applied. Each of the piston sides of those suspension cylinders may be connected to a suspension reservoir of a stop device which locks the suspension. An equalization device controls the piston side of the respective suspension cylinder. The generic spring suspension system is used for a suspension device between frame and axletree of a vehicle. Two individual wheel beam supports of the axeltree are hinge-connected to the frame by a support element connected between the frame and each wheel beam support. Automatic level equalization for the vehicle is achieved with its spring suspension system. This adjustment may be referred to a central level. The level of a vehicle is understood to mean the position of the vehicle relative to the ground. The distance between a point of the vehicle frame and the ground indicates the level of the vehicle in approximation. Although the disclosed solution achieves an especially soft suspension in addition to the level adjustment, disruptive vehicle spring deflections occur under load, as indicated above. 
   DE-C-41 27 917 discloses a spring suspension system comprising a suspension reservoir with piston and rod sides. The rod side can also be connected to a suspension reservoir. As a type of suspension, a supporting force operates in the opposite direction by the suspension reservoir for the piston side as well as the rod side of the suspension cylinder. Even with this suspension system the disadvantages of the state of the art cannot be avoided. 
   SUMMARY OF THE INVENTION 
   Objects of the present invention are to provide an improved spring suspension system which eliminates the disadvantages described in conventional systems. The desired objects are basically achieved by a spring suspension system having an equalization device with an additional switching valve for control of the rod side of a suspension cylinder within the supply device. In a branch line, a load monitoring system is connected to a line with components of the locking device as well as to the associated suspension cylinder for the piston side of the suspension cylinder. The load monitoring system controls the pressure supply for the content of the required system pressure. By the equalization device, pressure equalization of the system can be affected on the currently prevailing load pressure before the spring suspension is reactivated. Hence, with the spring suspension system of the present invention, pressure equalization between the suspension cylinder and the associated suspension reservoir is effected. Thus, uncontrolled movement of the shaft is prevented when the suspension system is engaged, independently of the change in load. 
   In a preferred embodiment of the spring suspension system of the present invention, a sensor device monitors the current operating position of the actuating piston in the suspension cylinder. Even slight movement in the area of the suspension cylinder triggers pressure equalization independently of change in load. 
   Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the drawings which form a part of this disclosure: 
       FIG. 1  is a diagrammatic illustration of a fluid circuit for a spring suspension system, according to an embodiment of the present invention in a first state of operation; 
       FIG. 2  is a diagrammatic illustration of the fluid circuit of  FIG. 1  in a second state of operation; 
       FIG. 3  is a diagrammatic illustration of the fluid circuit of  FIG. 1  in a third state of operation; and 
       FIG. 4  is a diagrammatic illustration of the fluid circuit of  FIG. 1  in a fourth state of operation. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The spring suspension in  FIG. 1  is shown in its locked state, that is, the suspension feature of the suspension system is disabled. 
   This spring suspension system has a suspension cylinder  10  to which varying load pressures m may be applied. The piston side  12  and piston rod side  14  may each be connected to a respective suspension reservoir  16  or  18  by a locking device  20 . The locking device  20  locks the spring suspension in the switched state illustrated. The suspension cylinder  10  is connected by its housing to a vehicle body (not shown in detail), and is linked at the free end of the piston rod  24 , connected to the piston  22 , to a vehicle wheel  26 . A plurality of vehicle wheels together with associated suspension cylinders (not shown) ensures operability of the machine, such as a tractor or the like. The spring suspension system also has an equalization device  28  which engages or disengages the cylinder with the aid of a supply device  30 . 
   As  FIG. 1  also shows, the two suspension reservoirs  16  and  18  are in the form of conventional hydraulic reservoirs or accumulators, such as membrane reservoirs or accumulators or the like. The locking device  20  is connected to supply device  30 , which is to be regarded as part of the equalization device  28  and which ensures connection at option to a pressure supply source or pump P and/or to a tank connection T by a switching device  32 . In addition, the two suspension reservoirs  16  and  18  are each separable from the supply device  30  by a non-return valve or one-way valve  34 ,  36 , each of which is held in a closed position by a spring under load and move into their open positions in the direction of the relevant suspension reservoir  16  or  18 . 
   A releasable non-return or one-way valve  43  is mounted in a branch line  38  of the line  40  leading to the locking device  20  of the suspension cylinder  10 . Releasable non-return valve  43  is connected by control line  41  to load monitoring device LS of the supply device  30  and a line  45  between the non-return valve  36  and the 3-way, 2-position equalization switching valve  50 . The fluid control connection for this purpose is made at connecting point  47 . A response to the load sensing system may be made over the connection for this purpose so that the hydropump P supplying the system may assume system pressure. The releasable non-return valve  43  itself is connected between lines  38  and  40 . In addition to the line  40  on the piston side, an additional line  42  is connected to suspension reservoir  18  and associated non-return valve  36  on the rod side  14  of the suspension cylinder  10 . 
   The locking device  20  has both for the piston side  12  and for the rod side  14  of the suspension cylinder a respective 2-way, 2-position locking switching valve  44 ,  46 . While in the inactive basic position shown in  FIG. 1 , each switching valve  44 ,  46  blocks the fluid conducting path from the suspension cylinder  10  toward the suspension reservoir  16 ,  18 . Under spring loading, each switching valve  44 ,  46  opens the respective path in the other direction. In the locked position shown in  FIG. 1 , the suspension cylinder  10 , with its piston side  12  and its rod side  14 , is separated from the associated suspension reservoirs  16  and  18 . Accordingly, the suspension system is locked so that spring deflection or rebound of the vehicle (not shown) is not possible. A change in load m cannot result in undesirable spring deflection or rebound of the total system. 
   The equalization device  28  has, both for the piston side  12  and for the rod side  14  of the suspension cylinder  10 , within the supply device  30 , a 3-way, 2-position equalization switching valve  48  or  50 . In their unactuated initial positions these switching valves are connected to each other by a connecting line  52  to the tank connection T. On their admission or input side, these switching valves are additionally joined together by another connecting line  54  to conduct fluid. In addition, the suspension cylinder  10  has a sensor device  11  permitting monitoring of the position of the piston rod  24  and/or of the piston  22 . This sensor device (shown only graphically) forwards its signals to a data interpretation unit  13  (shown only graphically), which in turn activates the switching valves  44 ,  46 ,  48 , and  50  for an actuation process. Activation for the purpose of effecting pressure equalization prior to activation of the spring suspension will now be explained. 
   First of all, reference is made once again to the state of the system as illustrated  FIG. 1 . In the situation shown, the spring suspension is locked and the suspension cylinder  10  is in the level position. Deflection or rebound of the piston  22  with piston rod  24  is consequently not possible, since discharge of fluid on the piston side  12  or the rod side  14  is prevented by the switching valves  44 ,  46  being in their closed positions. In the locked position as shown, locking by switching valves  44 ,  46  is achieved in that the non-return valves integrated into them can open in the direction of the suspension cylinder  10 , but are kept closed in the opposite direction, that is, in the direction of the suspension reservoir  16 ,  18 . Increased pressure on the suspension reservoir  16 ,  18  side opens the switching valves  44 ,  46  even in their locked position in the direction of the suspension cylinder  10 . If the load or load pressure m remains unchanged in the switching position shown in  FIG. 1 , the pressure on the piston side  12  corresponds to the fluid pressure of the piston suspension reservoir  16  and the fluid pressure on the rod side  14  corresponds to the fluid pressure of the rod suspension reservoir  18 , the pressure on the rod side corresponding to the pressure of the system. To prevent, in this situation, uncontrolled movement of the vehicle wheel  26  and accordingly of the movable parts of the suspension cylinder  10  when the suspension is activated, pressure equalization must first be carried out between the suspension cylinder  10  and the suspension reservoirs  16 ,  18 . 
   To achieve this pressure equalization, in a first step, as illustrated in  FIG. 2 , the 3-way, 2-position rod equalization switching valve  50  is activated and the other line  42  is connected to the hydraulic pump P. Since the piston equalization switching valve  48  remains in its unactuated position as illustrated, the pump pressure is blocked by the switching valve  48  in the other connecting line  54  in the direction of the first line  40 . If the pressure on the piston side  12  corresponds to the load pressure required, movement begins immediately on the suspension cylinder  10  and the piston  22  with piston rod  24  begins to travel in the direction of the piston side  12  of the suspension cylinder  10 . The pressure in the piston suspension reservoir  16  may be even lower than the pressure on the piston side  12  of the suspension cylinder  10 . In particular, the excess amount of fluid in movement for the purpose as indicated by the arrow from the piston side  12  is expelled in the direction of the side branch. 
   If the pressure on the piston side  12  is above the required load pressure m, first fluid, especially hydraulic fluid, is drained from the piston suspension reservoir  16  by the 2-way, 2-position piston locking switching valve  44  as indicated by the arrow in  FIG. 3  to the tank T, until the load pressure m has been reached. Once the load pressure has been reached, movement of the suspension cylinder  10  begins and the cylinder travels in the direction of the arrow. When the suspension cylinder starts to travel, hydraulic fluid is drawn from the associated rod suspension reservoir  18  on the ring or rod side  14  of the suspension cylinder  10 . Pressure equalization is then established between cylinder pressure on the rod side  14  and reservoir pressure for the rod suspension reservoir  18  at system pressure. If the control system (sensor device  11  and data interpretation unit  13 ) detects movement at the system suspension cylinder  10 , the first step toward pressure equalization has been completed. The pressure on the rod side  14  of the suspension cylinder  10  then corresponds to the storage pressure of the associated rod suspension reservoir  18 , and both pressure values correspond to the system pressure. The rod locking switching valve  46  is subsequently actuated and then assumes a switch position as shown in  FIG. 4 . 
   The next or second step for pressure equalization involves switching of both the 3-way, 2-position rod switching valve  50  and the 3-way, 2-position piston switching valve  48  to the position allowing supply of pressure of line  40  or  42 . In the switching state shown in  FIG. 4  and achieved for this purpose, the piston  22  with piston rod  24  now emerges from the suspension cylinder  10  in the direction of the arrow. If the reservoir pressure of the piston suspension reservoir  16  now corresponds to the required load pressure m, movement at the suspension cylinder  10  begins immediately and the suspension cylinder  10  is extended. But if the reservoir pressure at the piston suspension reservoir  16  is initially lower than the required load pressure m, in a first step the suspension reservoir  16  is charged to load pressure by the associated return valve  34  in the line  40 . If the load pressure has been reached, movement begins at the suspension cylinder  10  and the piston  22  with piston rod  24  is extended. Since the system of piston  22  and piston rod  24  rests on the unsprung mass which customarily may be in contact with the ground, extension of the system of piston  12  and piston rod  24  is equivalent to lifting of the sprung mass. In the case of the extension movement of the suspension cylinder  10  referred to, the pressure on the piston side  12  equals the pressure in the piston suspension reservoir  16 , and this in turn equals the load pressure m. The control unit referred to above, consisting essentially of the sensor device and the data interpretation unit, detects the movement at the suspension cylinder  10  and the pressure equalization is then considered to be complete. Valves  48  and  50  in turn are shifted to their blocked position as shown in  FIG. 1 . After engagement of the 2-way, 2-position piston switching valve  44 , as seen in  FIG. 4 , the spring suspension is open, that is, both the piston side  12  and the rod side  14  are connected to the suspension reservoirs  16  and  18  associated with them in a fluid conducting state for a spring suspension process. 
   Equalization of pressure between suspension cylinder  10  and the associated reservoirs  16 ,  18  is effected by the circuitry shown and the switching process described. With slight movement at the suspension cylinder in one direction or the direction opposite it, independently of possible change load m, uncontrolled movement is prevented when the suspension is engaged. 
   In an embodiment of the present invention not shown, the sprung masses may be replaced by unsprung masses. The circuitry of the suspension system is more or less retained, that is, the hydraulic components described remain connected as illustrated both to the piston and to the rod cavity of the suspension cylinder  10 . The advantages referred to are retained as a result of the exchange made for the purpose. In another embodiment (not shown), it is also possible to detect the differential pressure present upstream and downstream from the switching valve  44  with conventional measurement devices. In the event that no differential pressure (permissible tolerance: minimum differential pressure) exists, the second step for pressure equalization as described in the foregoing may be dispensed with and accordingly the spring suspension may be released even more quickly. 
   Only the most important components of the spring suspension system of the present invention are shown in the illustrations. Corresponding throttles and return valves for damping of the system and the path-measuring system itself for measurement of movement of the state of the system at suspension cylinder  10  have been omitted from the circuit diagram. On the free end of the load monitoring device LS, a conventional variable capacity pump is mounted and controls the amount of fluids so that the level of the pressure reported by the load monitoring device LS is maintained. A closed control cycle is thereby obtained as a function of the pressure and load situation.