Abstract:
A suspension device is for at least one drive part ( 10 ) that is to be cushioned. The drive part can be connected to an accumulator device ( 12 ) by a first valve unit ( 28 ) and to a tank connection ( 46 ) by a second valve unit ( 34 ) to transport fluid. The device is provided with a pressure compensation device ( 50 ), which when actuated compares the respective prevailing pressures in the drive part ( 10 ) and the accumulator device ( 12 ) to generate a common pressure level. This comparison permits the pressure level to be rapidly compensated in advance when the actual suspension is enabled in the form of the accumulator device, in such a way that the suspension pressure employed exactly matches the pressure of the respective drive part in the form of a hydraulic or working cylinder that prevails in the later as a result of the preceding load actuation operations.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a suspension device for at least one drive part which is to be cushioned and which can be connected by a first valve unit to an accumulator device used for suspension to carry fluid. The drive part can be connected by a second valve unit to a tank connection to carry fluid. 
   BACKGROUND OF THE INVENTION 
   A suspension device is disclosed in EP-A-1 157 963. This known solution relates to a wheel loader with a swiveling extension arm, which arm can also be changed in its length. To move the extension arm, a hydraulic drive system with individual lifting or working cylinders, preferably one independently working cylinder as the drive part, is assigned to each movable extension arm part. With the working cylinders of the hydraulic system, the swiveling motion can be actuated for the extension arm, and retracting and extending in length changes can be accomplished. By another working cylinder unit, a tool located, for example, on the free end side of the extension arm, for example, in the form of a load fork, can be actuated. Swiveling motion for the extension arm can be accomplished by a parallelogram drive to change the working height of the extension arm relative to the vehicle chassis according to the given working conditions for the wheel loader. 
   In addition to these drive means in the form of working cylinders, the hydraulic system in the conventional wheel loader design has a suspension device. The suspension device has at least one hydraulic accumulator as an accumulator device and at least two actuatable valve units to connect the accumulator device to the respective hydraulic cylinder or to the ambient pressure or tank pressure in the form of a tank connection for the suspension device. With this suspension device the drive part can be stopped to damp its natural motion dictated, for example, by a load, and can undertake damping when the drive part is being actuated, that is to say, in the known solution when the piston of the hydraulic cylinder consequently is extended or retracted. Thus, for example, in the raised position of the extension arm under the load which is to be moved by a machine in the form of a wheel loader, the accompanying vibration movements on the extension arm can be damped by the known suspension device. The known solution provides for maintenance of the pertinent damping when the working cylinder, and therefore, the drive part are further actuated, for example, to raise or lower the extension arm in this way. 
   Although the pertinent known solution leads to good damping and suspension behavior overall and on the whole enables a reliable operation of the hydraulic system for a wheel loader, problems can arise when the working cylinder, which is to be cushioned as the drive part, is in a definable load situation accompanied by the corresponding pressure level, especially on the piston side of the respective working cylinder, and as outlined when the suspension is connected by the respective hydraulic accumulator as the accumulator device when it does not have the same pressure level as the drive part used in the form of the respective working cylinder. Depending on the different pressure level, distinct transient and decay effects can occur when the suspension is connected to the respective working cylinder. Such effects can be considerable depending on the difference in level, and lead to deflection movements on the drive part which are critical in terms of safety engineering and accordingly on the mechanical component which is to be moved, for example, in the form of the indicated extension arm. Furthermore, due to the vibration processes, rocking can unintentionally occur on the extension arm until the different pressure levels of the respective working cylinder and hydraulic accumulator have been equalized to one another. In addition to that safety risk, pressure spikes within the piping of the valve units can occur when the different pressure levels are matched, something that can damage the entire system of the tool. 
   SUMMARY OF THE INVENTION 
   Objects of the present invention are to provide an improved suspension system, retaining the advantages and avoiding disadvantages described above. Especially, safety risks and overloads in the hydraulic system are reliably avoided. 
   These objects are achieved by a suspension device with a pressure compensation device, which, when actuated, equalizes the respective fluid pressure prevailing between the drive part and the accumulator device for producing a common pressure level. When the actual suspension is connected in the form of the accumulator device, level compensation is induced for the pressure beforehand in an extremely short time so that the suspension pressure used “is applied” exactly to the pressure of the respective drive part in the form of a hydraulic or working cylinder. This pressure is applied to the working cylinder at that time due to the preceding load actuation processes. An unintentional extension and retraction movement of the piston rod part of the respective working cylinder, which presents a danger for the surrounding area, is reliably prevented. Unintentional rocking motions do not occur either on the drive part and accordingly on mechanical components such as an extension arm or the like, which motions are actuated and caused by the drive part. Since in the present invention the desired pressure level is immediately established, pressure spikes in the hydraulic system are avoided. This arrangement promotes a long service life of the overall hydraulic system. Furthermore, the present invention is operationally reliable and economical to produce. 
   In one preferred embodiment of the suspension device of the present invention, the pressure compensation device is mounted in a parallel connection to the two valve units and the pressure compensation device has a compensator with control inputs connected on the input side of the first valve unit to carry fluid, and to the output side of the first valve unit. 
   In another preferred embodiment of the suspension device of the present invention, the respective hydraulic accumulator of the accumulator device can be pressurized from the working cylinder by a check valve, which is preferably integrated in the indicated first valve unit. 
   In this way, the hydraulic accumulator, for example, in the form of a piston accumulator or comparable accumulator designs, is immediately charged with the initial pressure of the drive part. The pertinent pressure level in the hydraulic cylinder can be delivered for a suspension process on the working cylinder as soon as the necessary pressure compensation between the two partial systems is produced beforehand by the compensator. The respective hydraulic cylinder then immediately is applied to the compensated pressure level of the hydraulic accumulator, and dangerous displacement or rocking motions for the drive part in the form of the working cylinder are thus reliably precluded. 
   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 schematic diagram of a suspension device according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A suspension device according to the present invention is designed for at least one drive part  10  to be cushioned and has at least one accumulator device  12  used for suspension. The drive part  10  is a conventional working or hydraulic cylinder  14  with a piston part  16  and rod part  18 . The piston part divides the working cylinder  14  into two fluid-carrying working spaces  20 ,  22 . Depending on the amount of fluid held in the working space  20 ,  22 , a definable position for the piston rod part  16 ,  18  arises therefrom. 
   The first working or piston space  20  is connected by a first connecting line  24  to the input  26  of a first valve unit  28  to carry fluid. The other, second working or rod space  22  is connected to the input  32  of the second valve unit  34  by a second connecting line  30  to carry fluid. The indicated two valve units  28  and  34  represent conventional, electrically actuatable 2/2-way switching valves. In the unactuated, spring-loaded or biased state, each valve assumes a blocked position in at least in one fluid direction of the respective valve. The first valve unit  28  in its unactuated, spring-loaded initial position is blocked in the direction toward its input  26  by an integrated check valve  36  which can be moved in the direction toward the output  38  of the first valve unit  28  into its open position. As  FIG. 1  shows, the two check valves  40  and  42  are blocked against one another, and are arranged flat such that in the unactuated initial position of the second valve unit  34 , fluid blocking takes place in two directions, that is, in the direction of the input  32  to the output  44  of the second valve unit  30  and in the reverse direction. The output  44  of the second valve unit  34  is connected to carry fluid to a tank connection  46  leading to the tank system of the hydraulic system for which the suspension device of the present invention is designed. Instead of the integrated check valve  36  in the first valve unit  28 , this check valve in an embodiment of the connection, which will not be detailed, can be connected parallel to the valve unit  28  and may be provided with one fluid-carrying connection each in front of the input  26  and the output  38 , respectively. The check valve in turn assumes its open position in the direction of the output  38 . In the pertinent case, the first valve unit  28  is provided with two check valves which can be unblocked in opposite directions, like the illustrated second valve unit  34 . This arrangement has the advantage that valve units  28 ,  34  with the same configuration can be used to implement the switching. 
   In the parallel connection  48  to the two indicated valve units  28 ,  34 , there is a pressure compensation device designated as a whole as  50 , which in the actuated state equalizes the respective fluid pressure prevailing between the drive part  10  (hydraulic cylinder) and the accumulator device  12  (hydraulic cylinder). The accumulator device  12  generally includes a hydraulic accumulator or hydraulic cylinder  52 , for example, in the form of a piston accumulator or the like. The use of hydraulic accumulator assemblies for cushioning and damping of movements in working cylinders is known in the prior art (cf. EP-A-1 157 963). The pertinent assemblies can also be used for “energy recovery”, in which hydraulic energy is delivered into the hydraulic accumulator in one working motion of the working cylinder and is then intentionally retrieved from there if the hydraulic cylinder is moved in the opposite direction. In this way, energy savings can be achieved to the relevant extent in the operation of machinery with hydraulic working cylinder devices. 
   If the pressure compensation means  50  is mounted in a parallel connection  48  to the two valve units  28  and  34 , two other connecting lines  54  and  56  lead away from the output  38  of the first valve unit  23  and from the output  44  of the second valve unit  34 . Between the two pertinent connecting points  58  in a branch line  60  connecting the connecting points  58 , a pressure compensation device  50  is provided essentially in the form of a compensator  62 . A compensator is conventionally defined as a valve component designed to keep the volumetric flow which is given by a metering choke constant, even when the load pressure fluctuates. To do this, the compensator operates in an internal control circuit as a differential pressure regulator with a movable control edge which can be changed such that the pressure difference on the metering orifice is always constant, regardless of the prevailing load pressure. Here, the compensator  62  can be connected upstream of the metering choke  64 , as shown in the illustrated embodiment. Moreover, the compensator  62  has two control inputs  66  and  68 . The first control input  66  can be actuated by another choke  70  by the fluid pressure at the input  26  of the first valve unit  28 . The second control input  68  on the opposite side of the compensator  62  can be actuated by the fluid pressure prevailing in the third connecting line  54  to which the hydraulic accumulator  52  is also connected by an orifice  72  to carry fluid. The input side  74  to which the second control input  68  is connected is consequently connected by the third connecting line  54  to the output side  38  of the first valve unit  28 . In turn, the output side  76  of the compensator  62  is connected preferably by the metering choke  64  to the input  78  of the third valve unit  80  with an output  82  connected in turn to the tank connection  46  to carry fluid. 
   The three valve units  28 ,  34  and  80 , together with the compensator  62  and a pressure limitation valve  84 , which can be set to a definable pressure and which in turn in a parallel connection to the compensator  62  protects the respective hydraulic accumulator  52  against an overpressure, form a suspension control block  86  as a retrofittable functional unit. The control block can be used even for existing, delivered machinery of any type, if for different systems their suspension comfort and safety are to be improved. Between the branch line  60  and the pressure limitation valve  84  mounted in a parallel connection, a shutoff valve  88  is connected parallel to the pressure compensation means  50 . By this shutoff valve, the pressure from the hydraulic accumulator  52  can be released, for example, for carrying out maintenance. 
   For the respective drive part  10 , a control block designated as a whole as  90 , is provided, as is conventional in the prior art (EP-A-1 157 963). By two control lines  92 ,  94 , control block  90  ensures the primary function for the drive part  10 , specifically actuating the extension and retraction of the piston rod unit  16 ,  18  of the working cylinder  14 . For the corresponding actuation of the control block  90 , pressurized fluid is moved into the working space  20  of the working cylinder  14  to raise the piston rod unit  16 ,  18 . The excess fluid in the other working space  22  is returned with the pertinent lifting motion to the control block  90  by the other control line  92 . For the lowering motion of the piston rod unit  16 ,  18 , the pertinent process can be reversed, and the excess fluid on the working space side  20  travels by the control line  94  back into the control block  90 . Depending on the applied load situation, the pertinent return process can also be supported by the corresponding fluid pressure prevailing in the first control line  92 . 
   For better understanding, the operation of the suspension device of the present invention will be detailed below using one working example. 
   With the suspension disengaged or blocked, the loading of the extension arm-lifting cylinder  14  can be changed by altering the length of the telescoping extension arm and by loading or unloading a load shovel or load fork (not shown). In the subsequent activation of the suspension, undefined movements caused by the different level pressure which is blocked in the respective hydraulic accumulator  52  relative to the working cylinder  14  are avoided. 
   For this reason, the hydraulic accumulator  52  is permanently connected by the check valve  36  to the working space  20  of the piston side  16  of the working cylinder  14 , and is thereby charged with the maximum pressure occurring in the respective working play with the working cylinder  14 . Before activation of the suspension, the third valve unit  80  is briefly opened at this time, so that at this instant the compensator  62  can release the pressure from the hydraulic accumulator  52  to the level of the current pressure on the piston side of the working cylinder  14 . Immediately afterwards, the valve  80  is in turn closed. 
   To activate the suspension, by opening the first valve unit  28 , can be regarded as a type of suspension-shutoff valve, the latter is moved into the opened, fluid-carrying valve position and thus the piston side of the working cylinder  14  is connected to the hydraulic accumulator  52 . The rod side, that is, the working space  22  of the working cylinder  14 , is then connected to the tank connection  46  and accordingly to the tank by simultaneous opening of the second valve unit  34  configured as a prefill valve. The suspension activated in this way can then also be maintained in operation of the working cylinder  14  in which the piston rod unit  16 ,  18  within the cylinder housing changes in its position. If the prefill valve  34  is closed, to activate the suspension, the suspension shutoff valve  28  is opened. By the control block  90 , the working cylinder  14  can be moved for the retraction and extension processes of its piston rod part  16 ,  18 . The fluid pressure then prevailing in the two control lines  92 ,  94  provides a fluid flow between the working space  20  of the hydraulic cylinder  14  and the control block  90 , by the second control line  94 . 
   In the present invention, the hydraulic accumulator  52  is pressurized from the working cylinder  14  by a check valve  36 . Because before activation of the suspension the pressure between the hydraulic accumulator  52  and the working cylinder  14  is compensated by the compensator  62 , undefined movements of the actuated mechanical parts of a working device or machine, such as a telescoping loader, which movements adversely affect safety, are reliably prevented. Pressure spikes which damage the hydraulic system are likewise avoided by the gentle unblocking and connection of the suspension pressure. 
   While one embodiment has been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.