Abstract:
A valve assembly for regulating the pressure of a pressure medium ( 2 ) of a pressure medium pump ( 3 ) to at least one first consumer ( 4 ) includes a pilot-controlled pressure control valve ( 5 ) with a main piston ( 6 ) acted on by the pressure medium ( 2 ) and a pilot piston ( 7 ). A pressure chamber ( 8 ) between a piston back side ( 9 ) of the main piston ( 6 ) and the pilot piston ( 7 ) can be relieved. A relief valve ( 10 ) is connected in a fluid-conducting manner to the pressure chamber ( 8 ), opens when pressure at the load sensor (LS) represents an out-of-operation mode of the consumer ( 4 ), and recirculates pressure medium ( 2 ) having a low pressure to a pressure medium reservoir ( 11 ) or to the pressure medium pump ( 3 ). The relief valve ( 10 ) closes when the pressure at the load sensor (LS) represents an in-operation mode of the consumer ( 4 ).

Description:
FIELD OF THE INVENTION 
     The invention relates to a valve assembly for pressure control of a pressure medium from a pressure medium pump to at least one first consumer, comprising a pilot-operated pressure control valve. The valve includes a main piston pressurized by the pressure medium and a pilot piston. A pressure chamber between a back of the main piston and the pilot piston can be relieved. 
     BACKGROUND OF THE INVENTION 
     Typically, pressure control valves are used if the travel speed of a hydraulic cylinder or the speed of a hydraulic motor is to be kept constant independently of the pressure difference prevailing on a flow valve, independently of the temperature or viscosity of a pressure medium used for this purpose and independently of the load to be moved. The pressure medium flow that has not been routed through the pressure control valve is drained via a pressure limiting valve for a pressure medium pump with relatively great losses in performance and pressure. 
     To minimize such performance losses, linking a pressure control valve that works as a pressure compensator to a load sensor on a consumer, for example, of a hydraulic cylinder, such that the LS (load sensing) pressure from the load sensor of the consumer prevails in a pressure chamber downstream of the pilot piston, is known. In particular, the pump pressure can be compared essentially to the spring pretensioning on the control piston plus the pressure on the consumer (LS). When the consumer is in the off-position, the pressure medium can be drained with less energy loss than in use with a pressure limiting valve. The performance loss of these known pilot-operated pressure control valves with a pressure compensator function, however, cannot be completely avoided. 
     DE 103 22 585 A1 describes, for example, a valve assembly for pressure control of a pressure medium from a pressure medium pump to a consumer, wherein a main control valve can be able to be hydraulically actuated by a pilot valve. In particular, the document describes a valve module system with at least one valve housing that, on its opposite ends both to the inside and to the outside on the periphery and in the housing interior, has standardized nominal sizes for mounting of other valve components. Such valve components can be a valve piston, an energy store, a pilot valve, and at least one fluid port for securing the valve assembly designed as a screw-in cartridge in the vicinity. 
     DE 10 2005 059 240 A1 shows and describes a hydrostatic drive system with a variable-stroke pressure medium pump that supplies a consumer with pressure medium via control valves. In idle operation of the hydrostatic drive system in which the control valves are not actuated, a pressure compensator used as a circulation device is set to a minimum control pressure difference. The pressure medium pump is set to a minimum delivery volume, with the pressure medium flow that comes from the pressure medium pump flowing via the pressure compensator to a pressure medium tank with low power loss. 
     The hydraulic drive system has a complex structure and does not have minimized pressure losses. 
     DE 689 08 317 T2 describes a pressure control valve whose main valve is pilot-operated by a pilot valve located in a common valve housing. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an improved valve assembly for pressure control of a pressure medium that enables further minimization of the pressure loss when a consumer is not connected. 
     This object is basically achieved with a valve assembly for pressure control of a pressure medium from a pressure medium pump to a consumer and includes a pilot-operated pressure control valve with a spring-loaded main piston. A pilot piston that controls a valve seat for a fluid-carrying connection on a rear pressure chamber of the main piston is a component of the pressure control valve. The pressure chamber of the main piston on the piston back is pressurized via a first throttle in the main piston by the pump pressure so that the circulating pressure compensator formed in this way allows a comparison between the pump pressure and the pressure on the load sensor plus the spring pretensioning of the main control piston and of the pilot piston. A pressure of the pressure medium pump that is higher by the respective set spring tensions than the pressure on the load sensor of the consumer is established. Furthermore, according to the invention, a relief valve is provided for the space between the main piston and the pilot piston. 
     The relief valve is formed as a gate valve or seat valve, with a valve element of the relief valve being arranged such that at zero pressure of the load sensor, corresponding to the consumer in the off position, a flow of the pressure medium from the space between the main piston and pilot piston to a pressure medium vessel, tank, or into the LS line is enabled. During operation in unpressurized circulation, the relief valve is opened, and likewise the main valve can be opened. The pilot valve is closed in this case. 
     If the pressure on the load sensor rises above a set value at the relief valve, the relief valve closes the bypass formed in this way and enables a load sensing-controlled function of the pressure control valve according to the known prior art. The main valve and pilot valve are in the control position here. 
     The relief valve according to the invention thus enables a significant reduction of the pressure losses of the valve assembly compared to the known circuits of circulating pressure compensators. 
     In a travel position of the valve element of the relief valve, the pressure medium coming from the pressure medium port of the pressure control valve can be drained away via a first throttle and via the relief valve to the pressure medium tank. The pressure medium can be routed to the relief valve via a longitudinal channel between the pressure chamber of the main piston and another second pressure chamber that can be traversed by the pilot piston. 
     In one especially preferred exemplary embodiment of the valve assembly, the longitudinal channel has another second throttle. The second throttle then divides the longitudinal channel into two channel sections. A first channel section is assigned to the pressure chamber of the main piston in this case. A second channel section of the longitudinal channel is assigned fluidically to the second pressure chamber, which second pressure chamber can be traversed by the pilot piston. The second throttle can be used as a damping element for the relief valve. In one especially preferred exemplary embodiment, the relief valve is located in the housing of the pressure control valve. The valve element of the relief valve in this case is guided to be able to move axially in a longitudinal bore. A fluid-carrying connection to one channel section of the longitudinal channel or the other in at least one travel position of the valve element is established via at least one annular recess in the housing of the pressure control valve or in the valve element of the relief valve. 
     The valve element of the relief valve is preferably preloaded using an energy store (compression spring) in the direction of the second pressure chamber that can be traversed by the pilot piston. The pilot piston of the pilot valve can actuate a fluid-carrying connection between a load sensor LS and one free side of the valve element of the relief valve, which side is opposite the energy store. In partial load or full load operation of a consumer controlled using the pressure control valve and at a corresponding LS pressure, the valve element of the relief valve blocks a fluid-carrying connection between the pressure chamber of the main piston and the pressure medium tank. However, when the consumer is in the off position and at an LS pressure that approaches zero, the valve element of the relief valve conversely under the action of the energy store is moved into a travel position in which a direct fluid-carrying connection between the pressure chamber of the main piston is opened via the relief valve to the pressure medium tank. In this case, the pressure medium flows via the annular recess on the relief valve. 
     In a valve solution in which the annular recess in the housing or in the valve element discharges into the second channel section of the longitudinal channel between the pressure control valve and the pilot valve, the annular recess is linked at the second pressure chamber of the pilot piston or of the pressure chamber that can be traversed by the pilot piston to carry fluid. 
     Instead of an integrated construction of the pilot valve, main valve, and relief valve, a decentralized individual arrangement of the indicated valves into an overall valve assembly is possible. The relief valve can be located in a parallel arrangement to the pressure control valve between a pressure medium pump and the pressure medium tank. The relief valve can be pilot-operated directly. Alternatively and advantageously, the relief valve can be designed as an electrically actuatable 2/2-way valve that is actuated, for example, by a control and/or regulating device processing pressure signals of a pressure sensor. 
     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 preferred embodiments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the drawings which form a part of this disclosure: 
         FIG. 1   a  is a schematic diagram of a valve assembly with a side elevational view in section, not to scale, of a pressure control valve assembly according to an exemplary embodiment of the invention with a relief valve in the opened operating position in a pressure control valve with linkage to a constant flow-pressure medium pump and to two consumers; 
         FIG. 1   b  is an enlarged side elevational view in section of detail I in  FIG. 1   a;    
         FIG. 1   c  is a schematic side elevational view in section, not to scale, of the pressure control valve assembly of  FIG. 1  with the relief valve in the control position of the main valve and of the pilot valve with the closed operating position of the relief valve; 
         FIG. 1   d  is an enlarged side elevational view in section of detail I in  FIG. 1   c;    
         FIG. 1   e  is a side elevational view in section of a pressure control valve according to a second exemplary embodiment of the invention with a relief valve; 
         FIG. 1   f  is a side elevational view in section of detail I in  FIG. 1   e;    
         FIG. 2  is a hydraulic circuit diagram of the valve assembly according to a third exemplary embodiment of the invention; 
         FIG. 3  is a hydraulic circuit diagram of a valve assembly with a connection of a relief valve downstream of a first throttle and upstream of a second throttle between the main control valve and the pilot valve of the pressure control valve according to a fourth exemplary embodiment of the invention; 
         FIG. 4  is a hydraulic circuit diagram of a valve assembly according to a fifth exemplary embodiment of the invention; and 
         FIG. 5  is a hydraulic circuit diagram of a valve assembly according to a sixth exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1   a  shows a closed hydraulic circuit of a valve assembly  1 , comprising a constant pressure medium delivery pump  3  for supplying a consumer  4  with pressure medium  2 . The consumer  4  is shown as a hydraulic motor with two possible flow directions. The consumer  4  is actuated via an electrically actuated 4/3-way valve  19 . The pressure prevailing on the consumer  4  is signaled to an LS line by a selector valve  18 . A 2/2-way valve  20  with a pressure limiting function in the opened operating position is located upstream of this valve control. 
     In the exemplary embodiment of a hydraulic system shown in  FIG. 1   a , two consumers  4 , each with identical valve control engineering, are connected in parallel and can be supplied by a constant pressure medium delivery pump  3 . The manner of operation of the valve control block formed in this way for the consumers  4  will not be detailed here since it is adequately known from the prior art. The hydraulic system calls for a constant pressure medium delivery pump  3  as a more economical alternative to a variable delivery pump, but requires a control of its volumetric flow to be able to operate the consumer with a definable speed. A flow valve, especially a pressure control valve, is required, constituting altogether a simpler overall solution that is more economical than the one that results when using a variable delivery pump. 
     As  FIG. 1   a  further shows, to display a load-independent constant speed of the two consumers  4 , a single pressure control valve  5  with piloting, while taking into consideration a load sensor LS provided for the two consumers  4 . The load sensor LS proceeds first separately on each selector valve  18  for each consumer  4  to display or indicate the consumer in the off position and in the operating position. Upstream of each selector valve  18  one check valve  22  at a time is connected to the hydraulic circuit into the control lines that can also be referred to as “load sensing” control lines  21 . Each check valve  22  has the same set opening pressure and opens in the direction of the pressure control valve  5 , especially in the direction to its load sensing port LS. This parallel connection of the control lines  21  with respect to the pressure control valve  5  enables a comparison of the two load pressures on the consumers  4 , with the higher of the two possible load pressures being taken into account. In the pressure control valve  5 , also shown in  FIG. 1   c  and in another embodiment in  FIG. 1   e , a valve assembly  1  is provided with an additional relief valve  10 , which additional relief valve in this respect is an important component of the solution according to the invention. 
     The operating principle of the pressure control valve  5  corresponds to a pilot-operated circulating pressure compensator  17 , with three valves that are different in terms of operation being combined in a common housing  12 . The fundamental functional linkage of the valves is also shown in a schematic circuit diagram in a detached construction. In particular, the three valves are the following:
         a main valve  23  with main piston  6  and a compression spring  24  that preloads it,   a pilot valve  25  with a pilot piston  7  and a compression spring  26  that preloads it, and   a relief valve  10  made as a miniature valve with a valve element  27  or relief valve piston and an energy store  28  that keeps it in the direction of a closed position.       

     In the cartridge-shaped housing  12  of the pressure control valve  5 , which housing is designed as a cartridge valve, in a main valve control section, the main piston  6  is guided to move longitudinally in a bore  29  of the housing  12  in a main valve control section. The main piston  6  actuates or operates in a pressure medium inlet  30 , by opening and blocking the fluid communication between inlet  30  and port  31 , extending centrally and axially into the housing  12 . A possible fluid-carrying connection can be established to a pressure medium port  31  extending radially out of the housing  12 , optionally including several radially arranged passage bores in the housing  12  and able to be connected to a pressure medium tank  11  from which the pressure medium pump  3  takes pressure medium for the hydraulic circuit. 
     The main valve  23  is designed with reference to its effective cross section such that the entire volumetric flow of the pressure medium  2  can be conveyed to the pressure medium tank  11  by the constant pressure medium delivery pump  3 . In the main piston&#39;s  6  piston bottom, a first throttle  13  has the form of a through opening or bore with a definable diameter. This throttle  13  enables the pressure on the piston back  9  of the main piston  6  to be signaled, which pressure is prevailing on the pump side. The main piston  6  is designed essentially as a cylindrical sleeve with a piston bottom as fluid separation so that on the back  9  of the piston a cup-shaped pressure chamber  8  is formed and is used for centering and accommodating the compression spring  24  and for accommodating the pressure medium  2 . In the axial direction of the pressure control valve  5 , a bore  32  with a diameter of roughly ⅕ of the main piston  6  in the valve housing  12  is made in the center. The bore  32  in roughly its axial center has another second throttle  14 . The second throttle  14  divides the bore  32  into a first channel section  32 ′ and a second channel section  32 ″. As  FIGS. 1   b, d , and  f  each show in respective details I, the first channel section  32 ′ is assigned to the pressure chamber  8  of the main valve  23 , whereas the second channel section  32 ″ is assigned to a second pressure chamber  35  that can be traversed by the pilot piston  7 . 
     The pilot piston  7  in turn is formed as a flat disk with a centering aid  33  in the form of a truncated cone for a compression spring  26 . The pilot piston  7  is exposed to the force of the compression spring  26  supported with radial play in a bore  34  for the pilot piston  7  and the compression spring  26 . The second pressure chamber  35 , on the front side of the pilot piston  7 , is the same pressure chamber as the space  34  in which the compression spring  26  is placed. Hence, a seal is not required. A bore  36  traversing the wall of the housing  12  for the load sensor LS of the consumer  4  discharges into the space  34  of the pilot piston  7 . The flow pressure of the pilot valve  25  arises from the pressure defined by the compression spring  26  plus the pressure on the load sensor LS. If the pump pressure is greater than the pressure from LS and the pressure set by the spring  24  of the main piston and set by the compression spring  26  of the pilot piston  7 , the pilot valve  25  and consequently the main valve  23  open and the pressure medium can flow out via the main valve  23  to the pressure medium tank  11 . 
     As  FIGS. 1   a  to  1   f  further show, the relief valve  10  with a valve element  27 , located in an axial region A in a longitudinal bore  40  is able to move between the pilot valve  25  and the main valve  23 , and is connected, in particular, in parallel to the pressure control valve  5 . The relief valve  10 , shown enlarged in  FIGS. 1   b, d , and  f  is incorporated into the housing  12 , is located radially offset laterally to a longitudinal axis  37  of the valve housing  12  and has a diameter roughly identical to the load bore formed by the bore  32  above and below the second throttle  14 . A valve element  27  or a relief valve piston is shown striking an upper stop on which it terminates more or less flush with the end of the bore  34  for the accommodation of the pilot piston  7 . The positions of the relief valve piston which are shown in  FIGS. 1   a ,  1   b ,  1   e , and  1   f  correspond to an opened operating position of the relief valve  10 . The relief valve piston is sprung or biased by a smaller energy store  28 , a compression spring that, for example, applies a flow pressure of 0.5 bar on its opposite face side in the sense of an opened position. 
     In the axial vicinity to the compression spring-side end of the relief valve piston, an annular recess  41  is formed as an annular groove  38  in the periphery of the relief valve piston. In the exemplary embodiment of the valve assembly  1  shown in  FIGS. 1   a ,  1   b ,  1   c , and  1   d , the annular recess  41  communicates with the first channel section  32 ′ of the bore  32 . If, at this point, there is no longer any pressure on the load sensor LS on the side facing away from the load sensor side of the pilot valve piston  7  and thus facing away from the compression spring  28 , the relief valve piston assumes the position shown in  FIGS. 1   a ,  1   e , and  1   f . The annular groove  38  overlaps an assigned opening  39  of the bore  34 . The pressure medium can thus be routed or conveyed from the pressure chamber  8  on the back  9  of the main piston  6  via the bore  32 , the opening  39  and a connecting line  42  communicating with the opening in the housing  12  via the annular groove  38  to a discharge  15  of the relief valve  10 . The pressure medium  2  then flows out unpressurized without the pressure medium pump  3  having to deliver against the set pressure on the pilot valve  25 . This design measure saves considerable energy in the operation of the hydraulic system equipped with a valve assembly  1  according to the invention when the consumer  4  is shut off. If the pressure on the load sensor LS rises when the consumer  4  is restarted, the relief valve piston travels against the spring force of its compression spring  26  into the position shown in  FIGS. 1   c  and  1   d  closing the opening  39  and the connecting line  42  with the annular groove  38 . The pressure control valve  5  in its above-described control operation is not influenced by the relief valve  10 . 
       FIGS. 1   e  and  1   f  in turn show in a schematic longitudinal section (not to scale) a version of a valve assembly  1  modified relative to  FIGS. 1   a ,  1   b ,  1   c , and  1   d , in turn combined in a housing  12  of the pressure control valve  5  with an offset opening  39  to the extent that the drainage of the pressure medium  2  out of the pressure chamber  8  into the LS line is ensured to take place. In this exemplary embodiment, the opening  39  is assigned to the second channel section  32 ″. The second throttle  14  thus acts in a damping manner on the entire operation of the valve assembly  1 , especially on the main piston  6 . 
       FIGS. 2 and 3  show the interconnection of the three valves  10 ,  23 , and  25  with a pressure medium sensor according to the solutions shown in  FIGS. 1   a  and  1   c . In this way, the valve assembly  1  according to the invention can also be implemented in an unattached valve design. 
       FIGS. 4 and 5  in turn show a circuit diagram comparable to  FIG. 3 , with the relief valve  10  being able to be designed as 2/2-way valve  16 , implemented for the entire volumetric flow of the pressure medium pump  3 . 
     The relief valve can generally be integrated into an existing pressure control valve as a valve of compact size. Advantageously, the relief valve can be arranged axially between the pilot valve and the main valve with a valve piston of the relief valve being insertable into the housing of the pressure control valve from the pilot valve side. In this way, the main bore for the relief valve can be produced from the same valve side as a throttle between the pressure chamber and the pilot valve. 
     While various embodiments have 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.