Abstract:
A directional control valve for a hydraulic load has, in a piston bore of a valve housing, a control piston which can be displaced axially in opposite directions from a central position and which has first fine control grooves for opening a connection between a load chamber and a feed chamber. There are second fine control grooves for opening a connection between a load chamber and a discharge chamber of the piston bore under control. In its central position, there is a restricted relief connection from a load chamber to the discharge chamber via the control piston. The restricted relief connection is obtained via a recess in the control piston, independently of the second fine control grooves.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
   The invention is based on a directional control valve which is used for the load-independent control of a hydraulic load in terms of direction and speed, comprising a control piston ( 22 ) which can be displaced axially in opposite directions from a central position in a piston bore ( 21 ) of a valve housing ( 20 ). with which the pressure medium paths can be controlled between a feed chamber ( 27 ), to which pressure medium can be fed from a pressure medium source, a discharge chamber ( 32 ,  33 ), via which pressure medium can be led away to a tank, and two load chambers ( 28 ,  29 ), which has first fine control grooves ( 55 ) for opening a connection between a load chamber ( 28 ,  29 ) and the feed chamber ( 27 ) under control, and second fine control grooves ( 56 ) for opening a connection between a load chamber ( 28 ,  29 ) and the discharge chamber ( 3 ,  33 ) under control, and via which, in the central position, there is a restricted relief connection from a load chamber ( 28 ,  29 ) to the discharge chamber ( 32 ,  33 ). 
   In practice, in the control piston of the variant with a relieved central position, as compared with the control piston which, in the central position, shuts off the load chambers present, the second fine control grooves are lengthened to such an extent that, even in the central position of the control piston, they project into the load chambers. The disadvantage in this case is that, in the event of valve actuation, at the start of opening the first fine control grooves, the flow cross section between the feed chamber and one load chamber is very small as compared with the flow cross section between the other load chamber and a discharge chamber. Precise control and fully restrained displacement of the hydraulic load is not possible in this stroke region. If further shut-off valves or braking valves are connected between the directional control valve and the hydraulic load, then a great deal of effort has to be expended for coordinating these valves in relation to the flow cross sections at the directional control valve. It is also disadvantageous that, as compared with a control piston with a blocked central position, a completely new design of the second fine control grooves is necessary. 
   SUMMARY OF THE INVENTION 
   The invention is based on the object of developing a directional control valve which has the features of the introductory-mentioned type, in such a way that, even at the start of opening the first fine control grooves, precise control of the hydraulic load is possible, and wherein the effort for designing the fine control grooves is reduced. 
   In a directional control valve of the introductory-mentioned type according to the invention, the intended object can be achieved, wherein, the relief connection exists via a recess in the control piston, independently of the second fine control grooves, and, in the event of displacement of the control piston with the effect of connecting a load chamber to a discharge chamber, the relief connection is interrupted before the second fine control grooves open a flow cross section. Accordingly, in the case of the variant with a relieved central position, the second fine control grooves no longer have anything to do with relieving a load chamber. Their design can therefore be based only on the dimensions necessary for the desired control of the hydraulic load. At a predefined maximum flow rate, the second fine control grooves for the valve variant with a relieved central position and for the valve variant with a blocked central position can be the same. 
   According to a feature of the invention the recess in the central position of the control piston is directly open both to a load chamber and to a discharge chamber and, in the event of displacement of the control piston from the central position, is shut off with respect to one of the chambers. Here, therefore, the restricted relief is carried out in a short way merely via the recess. 
   The recess can be a recess which is provided in addition to the recesses already present on the control piston in the known directional control valve. In a particularly preferred way, however, a recess which known directional control valves already also have, namely a recess which serves to feed back a load pressure, is changed slightly in such a way that it is able to bring about the relief of the load chamber. 
   In this case, the relief can take place directly via the recess. This appears to be particularly expedient in the case of a directional control valve which, without a relieved central position, is disclosed by DE 38 41 507 C1 and in which the load feedback is carried out via radial ducts in the control piston, which are located in the region of the housing lands between the load chambers and the discharge chambers. There, the radial ducts in the outer opening region have to be made axially only so large that, in the central position of the control piston, they are open both to the load chamber and to the discharge chamber. 
   In the case of load feedback as is made in the directional control valve according to the data sheet or in the case of a directional control valve disclosed by EP 0 638 730 A1, relief of a load chamber is likewise obtained in a simple manner, according to features of the invention, by involving the fluid path for the load feedback and by adapting the radial duct tapping off the pressure in the load chamber. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Two exemplary embodiments of a directional control valve according to the invention are illustrated in the drawings. The invention will now be explained in more detail by using the figures of these drawings, in which: 
       FIG. 1  shows the first exemplary embodiment, in which the load chambers are relieved via the fluid path for load feedback, and 
       FIG. 2  shows the second exemplary embodiment, in which the load chambers are relieved via additional recesses in the control piston. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   According to the figures, a piston bore  21 , in which a control piston  22  can be moved axially, passes through a valve housing  20  of a continuously adjustable directional control valve used within a load-sensing control system for a hydraulic load. The length of the control piston  22  agrees with the length of the piston bore  21  from one end  23  as far as the other end  24  of the valve housing  20 . The piston bore  21  is surrounded by five control chambers which are spaced apart axially from one another, are used for the feed of pressure medium to and the discharge of pressure medium from a hydraulic load, and whose connections to one another are controlled by the control piston  22 . The central of the five control chambers is the feed chamber  27 . On either side of this feed chamber there is in each case a load chamber  28  and  29 , respectively. Each load chamber  28  and  29  is further adjoined by a discharge chamber  32  and  33 , respectively, which is open to a tank duct which passes through the valve housing  20  but is not specifically illustrated. 
   On account of two centering springs, which are not shown, are accommodated in covers which are fixed to the ends  23  and  24  of the valve housing  20  and close the piston bore  21  to the outside, the control piston  22  assumes a central position. In this central position, two piston collars  34  and  35  which belong to the control piston  22  and between which, in the region of the feed chamber  27 , there is a piston neck  36  with a circumferential annular groove  37  block off the two load chambers  28  and  29  with respect to the feed chamber  27 . The two piston collars  34  and  35  are so long that, in the central position of the control piston, they are intrinsically also able to isolate the load chambers  28  and  29  from the discharge chambers along the housing lands  38 . Each piston collar  34  and  35  is adjoined by a further piston neck  39  and  40  with an annular groove  41  and  42 , which is in each case located in the region of a discharge chamber. The piston neck  40  is adjoined by a further piston collar  43 , which seals off the spring chamber in one cover with respect to the discharge chamber  33 . 
   The distance between the discharge chamber  32  and the end  23  is greater than the distance between the discharge chamber  33  and the end  24  of the valve housing  20 . Between the discharge chamber  32  and the end  23 , in the piston bore  21 , there are three annular chambers  44 ,  45  and  46  which are spaced apart axially from one another, it being possible for the central one of these three chambers to be designated a load feedback chamber  45  since, from there, the load pressure is fed back to an individual pressure compensator, associated with the directional control valve but not shown, and, if the load pressure is the highest, is fed back to a pump controller. The annular chamber  44  located toward the end  23  of the valve housing  20 , as viewed from the load feedback chamber  45 , can be connected to the input of a pressure limiting valve. A pressure limiting valve can also be connected to the annular chamber  46  located between the load feedback chamber  45  and the discharge chamber  32 . In the illustrated central position of the control piston  22 , a piston section which follows the piston neck  39  reaches from the end  23  of the valve housing  20  into the region of the housing land  47  between the annular chamber  46  and the discharge chamber  32 . A flat annular groove  49  is machined into this piston section, via which groove all three annular chambers  44 ,  45  and  46  are connected to one another in the central position of the control piston  22 . Between the annular groove  49  and the piston neck  39  there is a piston collar  48 , whose effective sealing length is less than the width of the annular chamber  46 . At its outer edge facing the discharge chamber  32 , the piston collar  48  has two diametrically opposite milled recesses  50 , through which the annular chamber  46  and discharge chamber  32  are open to each other with a small opening cross section when the control piston  22  is in its central position. If the control piston  22  is moved out of its central position, then the load feedback chamber  45  is isolated from the annular chamber  44  or from the annular chamber  46 , depending on the direction of movement of the control piston. In addition, irrespective of the direction of movement, the connection existing in the central position of the control piston  22  via the annular chamber  46  between the load feedback chamber  45  and the discharge chamber  32  is closed, by the load feedback chamber  45  being isolated from the annular chamber  46  or the annular chamber  46  being isolated from the discharge chamber  32 . 
   At both ends of each of the two piston collars  34  and  35  there are two diametrically opposite fine control grooves  55  and  56 , whose cross section becomes greater toward the front edges. The fine control grooves reach from the front edges into the piston collars only to such an extent that their inner end is still at a distance from the load chambers  28  and  29 , respectively, in the central position of the control piston  22 . In the central position of the control piston  22 , therefore, the load chambers have a fluidic connection via the fine control grooves neither to the feed chamber  27  nor to the adjacent discharge chamber  32  or  33 . 
   Through the control piston  22  there passes an axial bore  57 , which is closed at both ends. In the region of the annular groove  49 , the control piston  22  has a narrow radial bore  62  which reaches into the axial bore  57  from outside and which acts as a nozzle or restrictor. The axial bore  57  is additionally crossed by two radial bores  58  and  59 , of which the radial bore  58  passes through the piston collar  34  and the radial bore  59  passes through the piston collar  35  of the control piston  22 . As viewed in the axial direction, the radial bores  58  and  59  are still located within the extent of the fine control grooves  55 , but are offset by 90 degrees with respect to the latter. They have a diameter such that, and a distance from the front edge of the respective piston collar  34  and  35  that faces the feed chamber  27  such that, in the central position of the control piston  22 , they would intrinsically be at a short distance from a load chamber  28 ,  29 , covered by the wall of the piston bore  21 . This is the case in the exemplary embodiment according to  FIG. 2 . 
   In the exemplary embodiment according to  FIG. 1 , however, a blind hole  60  and  61  of low depth is introduced eccentrically at the two opposite openings of each radial bore  58  and  59  and covering the radial bores, its diameter being greater than the diameter of the radial bores. In the central position of the control piston  22 , there is a flow cross section between the blind holes  60  and the load chamber  28  and between the blind holes  61  and the load chamber  29 . 
   Thus, in the exemplary embodiment according to  FIG. 1 , in the central position of the control piston  22 , the load chamber  28  is connected fluidically to the discharge chamber  32  via the blind holes  60  and the radial bore  58 , and the load chamber  28  is connected fluidically to the discharge chamber  32  via the blind holes  61  and the radial bore  59  and, just like the load chamber  28 , further via the axial bore  57 , the radial bore  62 , via the annular groove  49  of the control piston  22 , via the annular chamber  46  of the piston bore  21  and via the milled recesses  50 . Because of the nozzle action of the radial bore  62 , the connection is a restricted connection. Via the connection, pressurized liquid can therefore flow away from the load chambers in a small stream to the discharge chamber  32  and therefore to a tank. As a result, the load chambers are relieved of pressure, so that, for example, shut-off or braking valves which are arranged between the load chambers and a hydraulic load function reliably. A load which is supported on a load chamber when a shut-off valve is not present moves only slowly. 
   Relieving the load chambers is carried out without involving the fine control grooves  56 , so that these can be designed in exactly the same way as in the case of a directional control valve without restricted relief of the central position. The control piston  22  of the exemplary embodiment of the invention according to  FIG. 1  differs from the control piston of such a directional control valve only in the additional blind holes  60  and  61 . 
   If the control piston  22  is moved out of the central position, for example to the right in the view according to  FIG. 1 , then the blind holes  60  are covered and the flow cross section between them and the load chamber  28  is closed, even before this load chamber is connected to the discharge chamber  32  via the fine control grooves  56  in the piston collar  34 . The connection between the annular groove  49  in the control piston  22  and the discharge chamber  32  is likewise closed. By contrast, the radial bore  59  remains open to the load chamber  29 , which is connected to the feed chamber  27  only via the fine control grooves  55 , so that load feedback is possible. In the event of displacement of the control piston  22  from the central position to the left, the blind holes  61  will be covered and the radial bores  58  will remain open to the load chamber  28 . 
   In the exemplary embodiment according to  FIG. 2 , the radial bores  58  and  59  reach as far as the outer side of the piston collars  34  and  35  and, in the central position of the control piston  22 , are covered by the piston bore  21 . For the restricted relief of the load chambers  28  and  29  in the central position, each piston collar  34  and  35  has two opposite, narrow grooves  70  and  71  which run axially and via which, in the central position of the control piston  22  along the housing lands  38 , there is a direct restricted connection between the load chambers and the discharge chambers. In the central position of the control piston, the grooves  70  and  71  extend into the load chambers and the discharge chambers only to such an extent that the fluid connection via them is also interrupted in the event of displacement of the control piston  22  out of the central position before fine control grooves  55  and  56  open toward the load chambers. Even in the exemplary embodiment according to  FIG. 2 , therefore, the restricted relief of the load chambers is carried out without involving the fine control grooves  56 .