Abstract:
A proportional pressure control valve includes a valve housing ( 38 ) with at least three fluid connections ( 1, 2, 3 ). The valve can be connected to a hydraulic drive system with a pre-determinable consumer pressure. The valve can perform reliable valve switching functions even for low-viscous fluid media, and can reduce its susceptibility to disturbance variables. A valve piston ( 56 ) actively connected to a pilot seat ( 5 ) by an energy accumulator ( 62 ) can be controlled by a control device ( 40 ). The consumer pressure on a fluid connection ( 1 ) acts on the valve piston ( 56 ) such that, according to the consumer pressure and the actuating force of the control device ( 40 ), a fluid can flow between the two other fluid connections ( 2  and  3 ) in both directions inside the valve housing ( 38 ).

Description:
FIELD OF THE INVENTION 
     The present invention relates to a valve, especially a proportional pressure control valve, including a valve housing with at least three fluid ports. The valve is connectable to a hydraulic drive system with a definable consumer pressure. 
     BACKGROUND OF THE INVENTION 
     Proportional pressure control valves (such as disclosed in U.S. Pat. No. 4,316,599) are known which among other things form control valves for oil hydraulic systems and deliver an essentially constant output pressure for a variable input pressure. The output pressure to be controlled is dictated by the current signal delivered by the corresponding trigger electronics and acting on the actuating magnet as the magnet system. The actuating magnet can be made as a pressure-tight oil bath magnet and has a long service life. 
     Proportional pressure control valves such as these can be directly controlled piston sliding valves in a three-way design, i.e., with output side pressure safeguarding. They are used among other things in oil hydraulic systems to control clutches in shift transmissions for controlled influencing of pressure build-up and pressure drop, for remote pressure setting, and for controlling pressure variations and for pilot control of hydraulic valves and logic elements. 
     These conventional proportional pressure control valves have poor stability especially for thin-liquid fluid media, i.e., they begin to “oscillate”. This problem is especially harmful when the known valves are designed to perform special functions, for example, in motor vehicle power steering systems, hydraulic drive units, and other safety engineering-relevant domains. It has generally been found that when pressure control valves are used, susceptibility to fault variables lies in the natural frequency region of the valve. The instabilities occurring can lead to failure of the valve and the pertinent parts of a hydraulic system. 
     In the prior art, multiaxle drive units for elevating work platforms are known which generally have a rear drive and optional all-wheel drive. To ensure safe operation with driving of only one axle and a free-running of the other axle and execution of braking processes with sudden stopping, in the known solution a plurality of valve components is necessary, such as two shock valves, two counter balance valve combinations. The valve combinations can include one pressure control valve and one 2/2-way valve each, and a 2/2-way valve as the recirculation valve. This functional structure is reliable in operation, but due to the plurality of valves its failure in operation must be expected. This failure shortens maintenance intervals. The known solution also requires a large amount of installation space and is expensive to manufacture due to the diversity of parts. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a valve, especially a proportional pressure control valve, which has good stability when thin-liquid fluid media are used, and which especially when used in safety-relevant domains including hydraulic drive unit helps reduce the diversity of components. 
     This object is basically achieved by a valve with a control means to trigger a valve piston dynamically connected to a pilot seat by an energy storage device. Because the prevailing consumer pressure at one fluid port acts at least on the valve piston, depending on the prevailing consumer pressure and on the actuating force of the control means, fluid can flow between the two other fluid ports in both directions within the valve housing. The valve is devised which, even for thin-liquid fluid media, can perform reliable valve operating functions. Susceptibility to fault quantities is also reduced. If the valve is used within completely hydraulic systems, such as hydraulic drive units, the shock valves, counter balance valves and directional control valves used in the past can be replaced by a base valve. Essentially the same switching and operating functions can then be implemented with only one valve according to the present invention. This arrangement reduces the production and maintenance costs. Since within the drive unit only one valve need be managed, operating reliability overall is increased. 
     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 and which are schematic and not to scale: 
         FIG. 1  is a hydraulic circuit diagram, parts of a hydraulic elevating work platform according to the prior art; 
         FIG. 2  is a hydraulic circuit according to an exemplary embodiment of the present invention; 
         FIGS. 3 to 5  are side elevational views in section of first, second and third exemplary embodiments, respectively, of proportional pressure control valves according to the present invention, in  FIG. 3  the shuttle valve as shown in  FIG. 2  being included for the purposes of more detailed explanation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a circuit diagram part of a hydraulic drive unit for a scissors lift platform. These vehicles generally have two axles. Generally a rear drive is implemented, and the front axle can be separately actuated for implementation of optional all-wheel drive. The drives for the two axles are designed to be essentially the same.  FIG. 1  for the prior art shows a hydraulic motor  10  as the driving means for the axle of the pertinent vehicle, which axle is not detailed. Depending on the operating position of the 4/3-way valve  12  in both directions of rotation, the motor can be driven in both directions of rotation for forward or backward operation and for implementing a braking function. The 4/3-way valve  12  according to  FIG. 1  is shown in the neutral position, and otherwise is connected on the input side to the hydraulic pump  14  and to a tank  16 . 
     The supply line of the hydraulic pump  14  is safeguarded via a conventional pressure control valve  18 . Two parallel running supply lines  20  extend to the respective input sides of the hydraulic motor  10 . Between the two supply lines  20 , different valves such as a recirculation valve  22 , two counter balance valves  24  with each one pressure control valve  26  connected thereto, and two other pressure control valves  28  as shock valves are connected in parallel. In the region of the connection sites of the shock valve  28  located topmost in  FIG. 1 , two secondary lines  30  are connected oppositely to the two supply lines and lead back to the tank and safeguarded by check valves. These check valves are used as replenishing valves to prevent cavitation. Altogether the known solution has two shock valves  28 , two counter balance valves  24  with pressure control valves  26  and one recirculation valve  22 , the two counter balance valves  24  and the recirculation valve  22  in the form of 2/2-way valves. 
     To explain operation of the known drive unit shown in  FIG. 1  in greater detail, a typical operating sequence is reproduced below. If, for example, when only one axle (rear drive) is driven, the second axle is entrained in free running, the recirculation valve  22  is switched into its enabling position shown in  FIG. 1  so that the oil returning from the hydraulic or traveling motor  10  is routed back to the motor to avoid running dry. In traveling operation with only one drive axle, the 4/3-way valve  12  remains off, so that all the pump oil is routed to the active axle, and the speed is doubled. 
     If a braking process is to be carried out, the inactive axle can be engaged to support the braking process. To this end, the recirculation valve  22  is blocked, and depending on the braking direction, the other 2/2-valve  24  is switched as a counter balance valve to clear the fluid-carrying path via an additional pressure control valve  26  directly connected to it. Since for this braking possibility in two opposite traveling directions the arrangement shown in  FIG. 1  must be doubled, for a pressurizing or braking function there are accordingly twice the number of pressure control valves  26  in addition to the 2/2-valves  24 . 
     If sudden stopping (emergency braking) of the drive unit is to take place by turning off all directional control valves, or unintentionally high external loads on the oil or fluid circuit otherwise occur, fundamentally the pressure in the lines, for example, the supply lines  20 , or in the valve housing, could increase such that failure of a component can occur. To counteract this possible failure, in the circuit other pressure control valves  28  as shock valves are installed. When a definable pressure is reached, pressure control valves  28  enable circulation of the fluid or oil in its own circuit. The described functions need never be switched at the same time for reliable operation, and the valve means are present for optional all-wheel drive for each axle to be triggered according to the circuit diagram in  FIG. 1 . 
     In the altered embodiment as shown in  FIG. 2  with the valve according to the present invention, the same components as in  FIG. 1  are also labeled with the same reference numbers. The plurality of illustrated individual valves according to the solution as shown in  FIG. 1  is replaced in the circuit diagram of  FIG. 2  by a valve according to the present invention which interacts with a shuttle valve  32  as another valve. The three independent fluid ports  1 ,  2 , and  3  are indicated, and the control means or valve control of the valve is labeled  4 ′. Furthermore, the valve has another, fourth fluid port  4  connected to the tank  16  via the connecting line  34  and used for unpressurized diversion of the pilot oil. The valve arrangement with the shuttle valve  32  is switched between the two supply lines  20  via the connection sites  36 . The proportional valve used in the circuit diagram of  FIG. 2  is shown in particular in different embodiments as illustrated in  FIGS. 3 to 5 . In the embodiment of  FIG. 3  the shuttle valve  32  is shown again by a symbol. The shuttle valve is not shown in  FIGS. 4 and 5  for simpler illustration. 
     Before the function of the hydraulic circuit as shown in  FIG. 2  is detailed, first the proportional control valve as shown in  FIGS. 3 to 5  will be detailed below. 
     The valve, especially the proportional pressure control valve shown in  FIG. 3 , has a valve housing  38  in the mariner of a screw-in cartridge. On the free face end of the valve housing  38  there is one fluid port  1 . Along the outer periphery of the valve housing  38 , the other fluid ports  2 ,  3 , and  4  penetrate the pertinent valve housing  38  in the radial direction. The individual fluid ports  1 ,  2 ,  3  and  4  are accordingly separated fluid-tight from one another by seals located on the outer peripheral side on the valve housing  38 . Furthermore, as shown symbolically in  FIG. 3 , the shuttle valve  32 , on its input sides, is connected to the fluid ports  1 ,  2  and  3  and to the connection sites  36 , as shown in  FIG. 2 , discharging into the supply lines  20  (not shown in  FIG. 3 ). 
     The valve housing  38 , as shown, is made as a screw-in cartridge and can be screwed into the connection unit of the drive unit (not shown) with its all-hydraulic system. On its end opposite the fluid port  1 , the valve housing  38  is provided with a control means or valve control  40  made in the manner of a magnet system having a magnet armature  42  energized by a coil (not shown) to move back and forth within a pole tube  44 . In moving back and forth, armature  42  triggers a closing part  48  in the form of a closing cone by acting on the actuating rod  46 . The closing part  48  and actuating rod  46  are separated from one another via a compression spring  50  as an energy storage device. Both the closing part  48  and the compression spring  50 , as well as the front end of the actuating rod  46  facing away from the magnet armature  42 , are guided in a fluid space  52  which can be connected to carry fluid via the fluid port  4  to the connecting line  34  as shown in  FIG. 2 , which discharges with its free end into the tank  16 . 
     Within the valve housing  38 , a valve piston  56  is guided and is made in the manner of a primary stage. The component  58  constitutes the pilot seat of the valve in the manner of a pre-stage. The valve piston  56  has one face end bordering another fluid space  60  receiving another energy storage device in the form of a compression spring  62  which engages a hollow-cylindrical recess in the valve piston  56 . The valve piston  56  is furthermore provided on the outer peripheral side with a radial recess  64  having an axial length, viewed in the lengthwise direction of the valve, dimensioned such that in definable switching or displacement positions of the valve piston  56  a fluid-carrying path between the fluid ports  2  and  3  is partially cleared, and, as shown in  FIG. 3 , can be blocked. Both the valve piston  56  and the pilot seat  58  have fluid channels  66 ,  68  which pass in the lengthwise direction and which viewed in the direction of  FIG. 3  on their right end have one orifice  70  each which choke the fluid flow present on the fluid port  1 . 
     Flow through the proportional valve shown in  FIG. 3  can take place in both directions, that is, from fluid port  2  to fluid port  3  or vice versa. The direction of the throughflow is among other things determined by where the highest consumer pressure prevails in the supply lines  20 . This consumer pressure is reported by the shuttle valve  32  via a signaling line  72  to the fluid port  1  of the proportional valve. If the closing pressure which can be set by the magnet system of the control means or valve control  4 ′ is exceeded by the consumer pressure prevailing on the fluid connection  1  to raise the closing part  48  off the pilot seat  58  as the pre-stage, the pilot control opens in this way, and the valve piston  56  designed as the primary stage moves against the force of the energy storage device in the form of a compression spring  62  to the left. In this way the fluid-carrying connection is opened from fluid port  2  to fluid port  3 , and fluid (oil) can flow from the higher to the lower pressure level. In the de-energized state, that is, when the magnet system has not been activated, the proportional valve meets the requirement for unpressurized oil circulation of one axle (axle free running), for which in the past an additional 2/2-way valve  22  (cf.  FIG. 1 ) was necessary according to the prior art. 
     For partially energized setting of the magnet system and therefore partial activation of the magnet armature  42 , with the valve according to the present invention the function of the pressurizing is implemented in which the hydraulic motor  10  can be braked by a defined pressure. With the solution according to the present invention, the brake action can be proportionally set via the magnetic force of the magnet system. This valve function then replaces the existing combination of the pressure control valve  26  with the 2/2-way valve  24  as the overall pressurizing system (cf.  FIG. 1 ). For complete energization and therefore the highest magnet closing force, the proportional valve according to the present invention performs the function of the previously known shock valves  28  as independent pressure control valves. Another advantage of the solution according to the present invention is that the reference of the proportional valve can be placed against the pressure in the tank  16  so that the pressures on the outflow side can no longer be added to the set pressure. 
     The altered embodiments as, shown in  FIGS. 4 and 5  will be explained in that they differ essentially from the valve solution as shown in  FIG. 3 . In terms of their actual valve structure, the three designs as shown in  FIGS. 3 to 5  correspond to one another and the essential differences can be seen only in the scope of execution of the respective control means or valve control  4 ′. 
     In the embodiment as shown in  FIG. 4 , the magnet system  4 ′ is replaced by mechanical presetting with which via a spindle  74 . The force of spring  76  the closing pressure for the closing part  48  can be dictated or set by hand. The valve is made as a double-acting, pilot-controlled shock valve with tank reference via the connecting line  34 . The double action is based on the decoupling of the control port  1  from the consumer ports  3  and  4 . This valve version is both defined (black/white—switching behavior) and can also be proportionally triggered. 
     While the embodiment as shown in  FIG. 3  has a pushing magnet system in which in the energized state the magnet armature  42  seeks to keeps the closing part  48  in its closed position to block the channel  68  via the actuating rod  46 , in the solution as shown in  FIG. 5  the magnetic system with the magnet armature  42  is made as a pulling system in which the magnet armature  42  under the influence of an energized winding coil (not shown) viewed in the direction of  FIG. 5  moves from right to left against the action of the force spring  76  as part of mechanical presetting according to the solution shown in  FIG. 4 . The embodiment shown in  FIG. 5  therefore combines a pulling magnetic system with mechanical presetting according to the solution shown in  FIG. 4 . If the assignable pole tube  44  is made as a fail-safe pole tube, this enables other versions. Thus, at full energization unpressurized circulation is enabled and in the de-energized state of the magnetic system as the switching means  4 ′ there would be a type of fail-safe valve with the function of a double-acting, pilot controlled shock valve with a tank reference. The maximum pressure can then be manually set, similarly to as in a normal pressure control valve. This de-energized state is shown in this way in  FIG. 5 . 
     If the proportional drive according to the present invention is used in the described hydraulic vehicle drive, a clear reduction of the number of required valves is possible, and altogether the installation space for the valve design is reduced. Since flow takes place through the proportional valve according to the present invention in both directions, twice the number of valves according to the known solution as shown in  FIG. 1  is eliminated. On the basis of the consideration that no two functions need proceed at the same time, in this way alternative switching of the proportional valve according to the hydraulic switching design as shown in  FIG. 2  can be used and thus a plurality of valves can be saved, which helps to reduce production and maintenance costs overall. The proportional valve, however, need not be limited to the use of vehicle drives, but can also be used in other hydraulic circuits based on its stable switching behavior, especially wherever increased safety requirements are emphasized. 
     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.