Hydraulic flushing valve arrangement

Abstract

Hydraulic flushing valve arrangement (15) comprising a flushing valve (30) for flushing out a flushing quantity with a flushing piston (36) which is longitudinally displaceable in a valve bore (46) of a housing (45) by means of two control collars (52) spaced apart from each other by a piston neck (53), by which, together with two circular housing control edges (47), the fluidic connections between two inlet channels (33, 34) opening into the valve bore (46) and an outlet channel (35) opening into the valve bore (46) between the two inlet channels (33, 35) are controllable, which is pre-tensioned by at least one spring (37) in a central position in which the outlet channel (35) is closed off from the inlet channels (33, 34), and which opens from a certain difference between the pressure prevailing in one inlet channel (33, 34) and the pressure prevailing in the other inlet channel (33, 34) prevails,is movable from the central position and thereby opens a fluidic connection between the inlet channel (33, 34), in which the lower pressure prevails, and the outlet channel (35) via a control collar (52), characterized in that the circular shape of a piston control edge (56) defining a control collar (52) towards the piston neck (53) of the flushing piston (36) is interrupted by at least one recess (60) and that the recess (60) has a second section (63) in which the cross-section in planes perpendicular to the axis of the flushing piston (36) is constant and of such a size that the flushing quantity is reduced compared to an end position of the flushing piston (36) when the second section (63) of the recess (60) is located in the region of the housing control edge (47).

Description

[0001] The invention relates to a hydraulic flushing valve arrangement comprising a flushing valve with a flushing piston, which is longitudinally displaceable in a valve bore of a housing by means of two control collars spaced apart from each other by a piston neck, by which, together with two circular housing control edges, the fluidic connections between two inlet channels opening into the valve bore and a drain channel opening into the valve bore between the two inlet channels can be controlled, which is pre-tensioned by at least one spring in a central position in which the drain channel is closed off from the inlet channels, and which, from a certain difference between the pressure prevailing in one inlet channel and the pressure prevailing in the other inlet channel, can be moved out of the central position and thereby opens a fluidic connection between the inlet channel, in which the lower pressure prevails, and the drain channel via a control collar.

[0002] Such a flushing valve arrangement is used, for example, in hydrostatic drive systems where two hydraulic machines operate in a closed hydraulic circuit. The flushing valve arrangement draws hydraulic fluid from the respective low-pressure branch of the closed hydraulic circuit to remove heat and dirt particles. The extracted hydraulic fluid is replaced by hydraulic fluid fed into the low-pressure branch, with the aim of maintaining a constant low pressure of, for example, 30 bar. The flushing piston typically moves when there is a pressure difference of approximately 5 bar between the high-pressure and low-pressure branches of the closed hydraulic circuit.

[0003] For example, DE 10 2005 051 324 A1 discloses a hydraulic flushing valve arrangement which shows a flushing valve with a flushing piston. Both control collars of the piston are provided with three flattened areas, apparently evenly distributed around the circumference of one control collar and open towards the piston neck. The flattened areas extend to the diameter of the piston neck and terminate axially in a surface perpendicular to the axis of the flushing piston. Axial webs, which are quite narrow at the diameter of one control collar, are located circumferentially between the flattened areas. It follows that the design of the control collars of the flushing piston known from DE 10 2005 051 324 A1 aims to ensure that, after the opening of a fluidic connection between an inlet channel and the outlet channel, the flushing piston is still guided by the corresponding control collar, namely by the webs located between the flattened areas.The control edge of a control collar is therefore located axially at the level of the closed ends of the flattened sections.

[0004] From DE 10 2011 119 427 A1, a flushing valve arrangement is known in which, at small pressure differences between the two inlet channels (up to, for example, 40 bar), the flushing piston of the flushing valve is moved against the force of a spring into a position where the circular control edge of a control collar is only a small distance from the corresponding housing control edge. At larger pressure differences, the flushing piston is moved further against the additional force of a second spring, which is greater than the force of the first spring, into a position where there is a large axial distance between the two control edges.

[0005] From DE 103 15 512 B4, a flushing valve arrangement is known in which the flushing piston is suspended between two springs, each with a limited extension in one direction. Thus, neglecting friction, the flushing piston moves even with the smallest pressure differences between the two branches of the closed hydraulic circuit. Up to the end of the extension path of one spring, the slope of the displacement / force characteristic of the flushing piston is twice the spring constant of that spring. The control collars of the flushing piston have a section with a diameter corresponding to the valve bore, ending at a first circular control edge, and a second section with a slightly reduced diameter and a second circular control edge extending inwards.

[0006] EP 2 613 058 A2 shows a flushing valve which has an annular gap between the housing and the flushing piston in the area of ​​the housing control edge, acting as a throttle.

[0007] DE 11 2014 000 101 T5 shows a flushing valve to which a flushing orifice is assigned, which is defined by an annular gap.

[0008] DE 10 2015 213 936 A1 shows a flushing valve in which two sleeve-shaped flushing valve slides interact with a changeover valve slide.

[0009] The invention is based on the objective of further developing a known hydraulic flushing valve arrangement in such a way that vibrations in the hydraulic system within which the flushing valve arrangement is installed are reduced, thereby improving comfort and controllability for the driver of a vehicle with a hydrostatic drive. The hydraulic flushing valve arrangement should also have a compact design.

[0010] The desired objective is achieved in a hydraulic flushing valve arrangement of the known type by interrupting the circular shape of a piston control edge, which delimits a control collar towards the piston neck, by at least one recess, and by the recess having a second section in which the cross-section in planes perpendicular to the axis of the flushing piston is constant and of such a size that the flushing volume is reduced compared to an end position of the flushing piston when the second section of the recess is located in the region of the housing control edge. To avoid assembly errors and because the operating principle should be the same regardless of whether flushing occurs from the first or second inlet channel, both control collars of the flushing piston are provided with one or more recesses in the same manner.

[0011] With the aid of a hydraulic flushing valve arrangement according to the invention, the tendency of a hydraulic system to oscillate can be reduced or eliminated, since the flushing volume no longer switches from zero to maximum within a short stroke of the flushing piston, but rather in at least two stages. If the difference between the high pressure in one inlet channel and the low pressure in the other inlet channel is just large enough that the second section of the recess on the piston control edge is located in the area of ​​the typically circular housing control edge, then a slight change in the difference and a corresponding change in the position of the flushing piston have no or only a slight effect on the flushing volume, thus preventing oscillations. The multi-stage flushing volume is achieved by the contour and stroke of the flushing piston. More than two springs are not necessary, resulting in a compact design.

[0012] A hydraulic flushing valve arrangement according to the invention can advantageously be further developed.

[0013] It is therefore advantageous if the recess has a first section whose cross-section, in planes perpendicular to the axis of the flushing piston, increases steadily with decreasing distance from the piston control edge, starting from the closed end of the recess, at least over a portion of the recess's axial extent, and transitions smoothly into the second section. Thus, the transition from the diameter of the flushing piston to the second section of the recess is not abrupt, but gradual, so that no abrupt change in the flushing volume occurs, however small.

[0014] In particular, the change in cross-section at the transition between the first section and the second section of the recess is zero.

[0015] Advantageously, a third section of the recess seamlessly follows the second section, in which the cross-section of the recess steadily increases in planes perpendicular to the axis of the flushing piston and extends to the piston control edge. This also ensures a smooth transition to the full flushing volume.

[0016] If the recess, with its second section, is located in the area of ​​the housing control edge and the closed end of the recess is located beyond the housing control edge, then an opening is present on the diameter of the flushing piston, which is bounded by the housing control edge and by the boundary line of the recess on the diameter of the flushing piston. Advantageously, the recess is designed such that the cross-section of the second section of the recess represents the minimum flow cross-section between an inlet channel and the outlet channel, i.e., the cross-section is smaller than the opening on the diameter of the flushing piston. Then, provided the second section of the recess remains in the area of ​​the housing control edge, the flushing volume will change only slightly when the position of the flushing piston is changed.

[0017] Advantageously, the depth of the recess at the piston control edge, measured perpendicular to the axis of the flushing piston, is less than the difference between the diameter of a control collar and the diameter of the piston neck of the flushing piston, so that a large travel distance of the flushing piston is available for the fine metering of the flushing quantity.

[0018] The recess is advantageously designed as a flat surface, which is easier to manufacture compared to a groove. The flat surface has a second section where the depth of the flat surface is constant and of such a size that the flushing volume is reduced compared to an end position of the flushing piston when the second section of the flat surface is located in the area of ​​the corresponding housing control edge.

[0019] The flattened section can also have a first section in which the depth of the flattened section, measured perpendicular to the axis of the flushing piston, increases continuously from the diameter of the control collar, at least over a portion of the axial extent of the flattened section, with decreasing distance from the piston control edge, and transitions smoothly into the second section. Preferably, the first and second sections of the flattened section merge seamlessly. Equally preferably, the first section merges seamlessly into the diameter of the flushing piston. This results in the flattened section being initially convex from the diameter of the flushing piston and concave towards the second section. The term "kink-free" can also be used instead of "edge-free."

[0020] The flattened section advantageously has a third section that terminates at the piston control edge and in which the depth of the flattened section, measured perpendicular to the axis of the flushing piston, increases continuously with decreasing distance to the piston control edge. The depth of the third section can initially increase along a curved, convex line and subsequently along a straight line towards the piston control edge. Furthermore, the third section can transition seamlessly into the annular surface that delimits the control collar towards the piston neck, following a curved, convex line.

[0021] Preferably, the first and / or third section are designed to be edgeless in the axial direction, meaning that there are no edges not only between the sections but also within the sections themselves. This design contributes to a gentle variation in the flushing volume.

[0022] A hydraulic flushing valve arrangement according to the invention advantageously comprises a pressure-holding valve arranged in the drain channel, which enters an open position when a certain minimum pressure exists between it and the flushing valve in the drain channel. If the pressure in the low-pressure branch of a closed hydraulic circuit is lower than a minimum pressure of, for example, 15 bar, the pressure-holding valve is closed, so that no hydraulic fluid is flushed out and the pressure in the low-pressure branch drops even further.

[0023] A flushing orifice plate is advantageously arranged in the drain channel between the flushing valve and the pressure-maintaining valve. The pressure required to actuate the pressure-maintaining valve is taken from the orifice plate upstream of the flushing orifice plate. The flushing orifice plate determines the maximum flushing volume. Specifically, when both the flushing valve and the pressure-maintaining valve are fully open and a sufficient quantity of hydraulic fluid is supplied, the low pressure is present at the inlet of the flushing orifice plate, and the tank pressure is present at the outlet. The maximum flushing volume results from this maximum pressure differential across the flushing orifice plate and its flow cross-section.

[0024] The pressure-reducing valve and the flushing orifice, in conjunction with a limited flow cross-section in the flushing valve, have a very special effect. As long as the effective flow cross-section in the flushing valve is smaller than the flow cross-section of the flushing orifice, if the pressure-reducing valve (which is functionally a pressure-operated directional control valve) were fully open, a pressure would develop between the flushing valve and the flushing orifice. Assuming a low pressure of 30 bar and an opening pressure of 15 bar for the pressure-reducing valve, this pressure would be lower than 15 bar. In that case, the pressure-reducing valve would close. This means that the pressure-reducing valve maintains a pressure of 15 bar between the flushing valve and the flushing orifice by further restricting the flushing flow, so that the pressure differential across the flushing valve remains a constant, low 15 bar, and the flushing volume is determined solely by the flow cross-section of the flushing valve.A constant flow cross-section in the flushing valve over a specific stroke of the flushing piston results in a constant flushing volume over that stroke. Due to the low pressure drop across the flushing valve, changes in the flushing volume with the stroke of the flushing piston are also small. This contributes to reducing vibrations in the hydraulic system.

[0025] If the flow cross-section of the flushing valve is larger than the flow cross-section of the flushing orifice, the pressure between the flushing valve and the orifice increases, and the pressure-holding valve opens fully. The flushing volume then results from the interaction between the flow cross-sections of the flushing valve and the orifice. If the flow cross-section of the flushing valve is significantly larger than that of the orifice, the full pressure differential of 30 bar is present across the orifice, and the flushing volume is determined by this pressure drop and the orifice's flow cross-section. Compared to this state, fine control of the flushing volume involves not only reducing the flow cross-section but also decreasing the pressure differential across that flow cross-section, making the control particularly sensitive.

[0026] The problem underlying the invention is thus solved in a particularly advantageous manner with regard to a smooth change in the flushing quantity with the stroke of the flushing piston by a hydraulic flushing valve arrangement comprising a flushing valve, a pressure-holding valve and a flushing orifice, wherein the flushing valve has a flushing piston which is guided longitudinally displaceable in a valve bore of a housing by means of two control collars spaced apart from each other by a piston neck, by which, together with two circular housing control edges, the fluidic connections between two inlet channels opening into the valve bore and an outlet channel opening into the valve bore between the two inlet channels can be controlled, which is pre-tensioned by at least one spring in a central position in which the outlet channel is closed off from the inlet channels, and which, from a certain difference between the pressure prevailing in one inlet channel and the pressure,The pressure in the other inlet channel is movable from its neutral position and, via a control collar, opens a fluidic connection between the inlet channel, where the lower pressure prevails, and the outlet channel. The pressure-holding valve is located in the outlet channel and enters an open position when a certain minimum pressure exists between it and the flushing valve in the outlet channel. The flushing orifice is located in the outlet channel between the flushing valve and the pressure-holding valve, and the pressure for actuating the pressure-holding valve is taken upstream of the flushing orifice. This allows for a small flushing volume and a smooth change in the flushing volume with the stroke of the flushing piston.that the circular shape of a piston control edge, which delimits a control collar of the flushing piston towards the piston neck, is interrupted by at least one recess, and that through the recess on the piston control edge, a flow cross-section is opened over a certain distance of the flushing piston, which is smaller than the opening cross-section of the flushing orifice. This certain distance of the flushing piston can certainly be in the range of a few millimeters, for example, in the range of 4 mm.

[0027] The invention is also manifested in a hydrostatic drive, in particular a hydrostatic drive, which has two hydraulic machines that are fluidically connected to each other in a closed hydraulic circuit via a first working line and a second working line, a feed pump for supplying pressure medium into the respective working line at low pressure, and a flushing valve arrangement with an inlet channel that is fluidically connected to the first working line and with an inlet channel that is fluidically connected to the second working line, wherein the flushing valve arrangement is designed according to the invention.

[0028] A hydrostatic drive system according to the invention, as well as two embodiments of a hydraulic flushing valve arrangement according to the invention, are shown in the drawings. The invention will now be explained in more detail with reference to the figures in these drawings.

[0029] They show Fig. 1 the circuit diagram of a hydrostatic drive according to the invention, Fig. 2 except for the flushing piston shown in the view, a longitudinal section through the first embodiment of a hydraulic flushing valve arrangement with a flushing valve, a pressure holding valve and a flushing orifice for determining the maximum flushing quantity, Fig. 3 an enlarged section from Fig. 2 in the area of ​​a control edge of the flushing piston, Fig. 4 the enlarged section from Fig. 3 in a section plane rotated by ninety degrees and Fig. 5 a longitudinal section through the second embodiment in the area of ​​the pressure holding valve.

[0030] The hydrostatic drive according to Fig. The system comprises a first hydraulic machine 10, whose stroke volume is adjustable and which primarily functions as a hydraulic pump and can be driven by a drive motor (not shown), for example, a diesel engine, and a second hydraulic machine 11, also with adjustable stroke volume, which primarily functions as a hydraulic motor and can drive at least one wheel of a vehicle. During coasting, the hydraulic motor 11 can also function as a hydraulic pump and the hydraulic pump 10 as a hydraulic motor. The hydraulic pump 10 and the hydraulic motor 11 are fluidically connected in a closed hydraulic circuit via a first working line 12 and a second working line 13. During operation, one of the two working lines forms the low-pressure side and the other the high-pressure side of the closed hydraulic circuit.

[0031] A hydraulic flushing valve arrangement 15 has the task of flushing heated hydraulic fluid and dirt particles from the closed hydraulic circuit into a tank 16.

[0032] The amount of hydraulic fluid flushed out, as well as the amount lost from the closed hydraulic circuit due to leakage, is replaced by a feed into the low-pressure side. For this purpose, a feed pump 17 is provided, which is usually assembled with the hydraulic pump 10 and driven by the drive motor. The feed pump 17 draws hydraulic fluid from the tank 16 via a filter 18 and discharges the hydraulic fluid into a feed line 19. A first pressure feed valve 20 is arranged between this feed line and the first working line 12. This valve is a combination of a feed valve 21, designed as a check valve opening from the feed line 19 to the first working line 12, and a pressure relief valve 22, which is connected with its inlet to the first working line 12 and with its outlet to the feed line 19.The pressure relief valve 22 limits the pressure in the first working line to a maximum high pressure of, for example, 300 bar. A second pressure feed valve 23 is arranged between the feed line 19 and the second working line 13. This valve is a combination of a feed valve 24, designed as a check valve opening from the feed line 19 to the first working line 12, and a pressure relief valve 25. The pressure relief valve 25 is connected with its inlet to the second working line 13 and with its outlet to the feed line 19. The pressure relief valve 25 limits the pressure in the second working line to a maximum high pressure of, for example, also 300 bar.Furthermore, a feed pressure limiting valve 26 is connected to the feed line 19, through which the pressure in the feed line 19 and thus on the respective low-pressure side of the closed hydraulic circuit is limited to the maximum low pressure of, for example, 30 bar.

[0033] The flushing valve arrangement 15 comprises a flushing valve 30, a pressure-maintaining valve 31, and a flushing orifice 32. The flushing valve 30 has a first inlet channel 33, which is connected to the first working line 12, and a second inlet channel 34, which is connected to the second working line 13, and an outlet channel 35. A flushing piston 36 of the flushing valve 30, the design of which is described in more detail in the Fig. 2, Fig. 3 to Fig. As shown in Figure 4, the flushing piston is centered in a neutral position by two pre-tensioned compression springs 37. From a certain pressure differential between the pressure in one inlet channel and the pressure in the other inlet channel, it can be moved from this neutral position to a position in which the working line carrying the low pressure to the outlet channel 35 is open. The pressure differential at which the flushing piston moves from its neutral position is, for example, 5 bar. In the outlet channel 35, the flushing orifice 32 is located first, and downstream of the flushing orifice, the pressure-holding valve 31 is arranged. This valve has a piston 38 which, under the action of a compression spring 39, tends to assume a closed position and is acted upon in the opening direction by a pressure taken upstream of the flushing orifice 32 between the orifice and the flushing valve. The pressure-holding valve 31 begins to open when the taken pressure is, for example, 15 bar.The flushing oil flowing through the flushing valve arrangement flows back to tank 16 via a return line 40.

[0034] As from the Fig. 2, Fig. 3 to Fig. As shown in Figure 4, the flushing valve arrangement 15 comprises a housing 45 common to the flushing valve 30, the pressure-holding valve 31, and the flushing orifice 32. A valve bore 46 passes through this housing, in which the flushing piston 36 is guided for longitudinal movement. The first inlet channel 33 and, at an axial distance therefrom, the second inlet channel 34 open into the valve bore 46. The opening regions of these channels are designed as circumferential control chambers, each with a circular housing control edge 47. The outlet channel 35 opens vertically into the valve bore 46 as a simple bore, centrally located between the two inlet channels.

[0035] At each end, the valve bore 46 transitions at a step into a spring chamber 48, which has a larger diameter than the other sections of the valve bore 46 and than the flushing piston 36. The axial distance between the two steps is the same as the axial distance between two stop surfaces on the flushing piston 36. A sealing screw 49 with a blind hole is screwed into each spring chamber 48, and the respective spring 37 is largely received by this screw. The spring 37 protrudes from the blind hole and acts upon a disc 50, which can simultaneously bear against one stop surface of the flushing piston 36 and against one step of the valve bore 46. Thus, if no other forces act upon it, the flushing piston 36, under the action of the springs 37, draws the fluid in the Fig. 2 the middle position shown, from which it only moves when the external force overcomes the preload force of a spring 37.

[0036] The flushing piston 36 has two axially spaced control collars 52, in which the diameter of the flushing piston 36, taking into account smooth operation, is equal to the diameter of the valve bore 46. These collars are connected by a piston neck 53, the diameter of which is significantly smaller than the diameter of the valve bore 46 and which, in every position of the flushing piston 36, is always located in the region of the outlet channel 35. The flushing piston is completely symmetrical with respect to a plane 54 passing centrally through the flushing piston 36 and perpendicular to the axis of the flushing piston. Therefore, the following discussion will focus only on one control collar and its interaction with an inlet channel and the outlet channel.

[0037] Each control collar 52 comprises a section 55 located towards the piston neck 53, which is bounded towards the piston neck by a piston control edge 56, and a section 57 located towards the spring chamber 48, which guides the scavenging piston between an inlet channel and a spring chamber in the valve bore 46. The two sections 55 and 57 are clearly delineated from each other by a circumferential groove 58, which is located in the region of the inlet channel opening in every position of the scavenging piston. Starting from the groove 58, two flattened surfaces 59, having a constant cross-section along their axial length, extend diametrically opposite each other on the outside of section 56. These flattened surfaces connect the inlet channel to the spring chamber in a throttled manner. The two flattened surfaces 59 form the Fig. 1 visible damping throttle in a control line leading from an inlet channel to a spring chamber, via which the pressure in a working line is tapped to actuate the flushing piston 36.

[0038] The control edge 56 on section 55 of a piston control collar 52 is interrupted by two diametrically opposed recesses, which are formed as flattened surfaces 60 and are open to the clearance 61 around the piston neck 53. The piston control edge 56 runs between the two flattened surfaces 60 as two circular arcs. The four flattened surfaces 60 are identical in design. In sections with planes perpendicular to the longitudinal axis of the flushing piston 36, the flattened surfaces 60 are, as already indicated by the term "flattened surface," planar; however, their depth varies in the axial direction of the flushing piston 36.In an axial plane of the flushing piston 36 passing centrally through a flattened section 60, the flattened section 60 has a first section 62 in which the depth of the flattened section, measured perpendicular to the axis of the flushing piston, increases continuously from the diameter of a control collar 52 over a portion of the axial extent of the flattened section to a second section 63 with decreasing distance from the piston control edge, and which transitions smoothly into the second section. The first section 62, more precisely, the contour of the control piston 36 in the region of a flattened section 60, is initially convex and then concave, starting from the diameter of a piston control collar 56, and transitions smoothly into the diameter of a control collar 52 and into the second section 63.

[0039] In the second section 63, the depth of a flattening 60 is constant and is about one fifteenth of the diameter of a control collar 52 of the control piston 36.

[0040] Each flattened section 60 has a third section 64, which follows the second section 63 without any edges and ends at the piston control edge 56. In the third section 64, the depth of the flattened section increases continuously with decreasing distance to the piston control edge. Specifically, the third section 64 is initially convexly curved and then straight until just before the piston control edge 56, before transitioning convexly into the annular surface at a very small radius, which defines a control collar 52 towards the piston neck 53.

[0041] The contour of a recess 60 in an axial plane passing through it centrally of the flushing piston 36 is particularly clear from Fig. 4. In particular, it is evident that the contour is edge-free even within sections 62 and 64, i.e., differentiable everywhere.

[0042] If the flushing piston can achieve a total stroke of 6.5 mm from its central position, the following has proven advantageous: With a positive overlap of 1 mm, a flow cross-section between the two flattened surfaces 60 of a control collar 52 of the flushing piston 36 begins to open after a stroke of 1 mm. The depth of the flattened surfaces 60 then increases over a further stroke of approximately 2.2 mm (which can range from 2.1 to 2.4 mm), remains constant over a stroke of approximately 1.2 mm (which can range from 1.1 to 1.3 mm), and then increases again over a further stroke of 1.6 mm (which can range from 1.5 to 1.8 mm). The piston control edge 56 then passes over the housing control edge 47, and the flushing valve is fully opened over a further 0.5 mm stroke of the flushing piston 36.

[0043] The valve piston 38 of the pressure-holding valve 31 is inserted into the drain channel 35, which is designed as a bore, and is guided therein for axial movement. It has an external circumferential shoulder 69, with which it can be pressed by the compression spring 39 against a step 70 of the housing 45, and is designed as a hollow piston with a blind hole 71 that is open towards the valve bore 46. The flushing orifice 32 is inserted into the blind hole 71. Between the flushing orifice and the bottom of the blind hole 71, six radial bores 72 extend outwards into a very shallow annular groove that circumferentially surrounds the valve piston 38 at a distance from the shoulder 70. Thus, the valve piston 38 is acted upon in the opening direction by the pressure acting between the flushing orifice 32 and the flushing valve 30 in the drain channel 35 against the force of the compression spring 39.

[0044] The compression spring is typically pre-tensioned to a pressure equivalent of 15 bar. The pressure-holding valve 31 therefore only opens when the pressure upstream of the flushing orifice 32, and thus in the low-pressure branch of the closed hydraulic circuit, reaches 15 bar, and closes when the pressure drops below 15 bar. This ensures that only a very small amount of flushing oil is drawn from the low-pressure branch below a pressure of 15 bar. The small amount of flushing oil is drawn through a nozzle 73, a fine bore in the valve piston 38 that leads from the blind hole 71 into the spring chamber 74, where the compression spring 39 is located. If the pressure rises above 15 bar, the opening characteristic of the valve piston 38 means that it first regulates the pressure upstream of the flushing orifice 32 to 15 bar before a flow cross-section that is very large compared to the flow cross-section of the flushing orifice 32 is reached.

[0045] When a hydraulic flushing valve arrangement according to the invention is used in a closed hydraulic circuit, the flushing piston 36 is moved from the into the Fig. 2, Fig. 3 to Fig. The center position shown in Figure 4 is shifted when the pressure difference between the two working lines exceeds 5 bar. As the pressure difference increases, the flushing piston 36 opens a flow cross-section with the first section 62 of the two flattened sections 60 on the corresponding control collar 52. This flow cross-section steadily increases until, due to the second section 63 of the flattened sections 60, the flow cross-section remains constant over a certain stroke of the flushing piston 36. In the third section 64 of the flattened sections 60, the flushing piston 36 slowly opens until the cross-section is fully open. The flow cross-section at the flushing piston 36 is initially smaller than the flow cross-section of the flushing orifice 32. Depending on the design, this can also be the case if the second section 63 of the flattened sections 60 is located in the area of ​​the housing control edge 47.As long as the flow cross-section at the flushing piston 36 is smaller than the flow cross-section of the flushing orifice 32, the pressure-maintaining valve 31 regulates a pressure of 15 bar between the flushing valve 30 and the flushing orifice 32. The flushing oil quantity is then determined by the flow cross-section at the flushing piston 36 and the pressure difference of 15 bar between the low pressure and the opening pressure of the pressure-maintaining valve 31. If the flow cross-section at the flushing valve 30 becomes larger than the flow cross-section of the flushing orifice 32, the pressure-maintaining valve 31 opens fully, and the flushing oil quantity results from the series connection of the two flow cross-sections. If the second section 63 of the flattened surfaces 60 is located in the region of a housing control edge 47, the flow cross-section at the flushing valve 30 remains constant over a specific stroke of the flushing piston 36.The flushing oil quantity then remains constant at a small value, regardless of whether the flow cross-section at the flushing valve 30 is larger or smaller than the flow cross-section of the flushing orifice 32. With the further stroke of the flushing piston 36, the piston control edge 56 finally passes over the housing control edge 47, and the flow cross-section in the flushing valve is at its maximum and significantly larger than the flow cross-section of the flushing orifice 32. Now the flushing oil quantity is determined by the flow cross-section of the flushing orifice 32 and the level of the low pressure, provided this exceeds the opening pressure of the pressure-holding valve 31.

[0046] It becomes clear that the pressure-holding valve here does not only serve to prevent the withdrawal of flushing oil when the low pressure falls below 15 bar. If the pressure difference between high and low pressure is small, but the low pressure exceeds the opening pressure of the pressure-holding valve, it additionally influences the amount of flushing oil by making the pressure difference across the flushing valve smaller than the low pressure, provided the flow cross-section of the flushing valve 30 is smaller than the flow cross-section of the flushing orifice 32.

[0047] In the exemplary embodiment according to Fig.5. The purging of a closed hydraulic circuit can be deactivated. For this purpose, a holding piston 80 is provided, via which the valve piston 38 of the pressure-holding valve 31 can be actuated with a force in the closing direction. The holding piston 80 is arranged behind the compression spring 39, has the same diameter as the valve piston 38, and has an actuating rod 81 with which it can engage the valve piston 38 through the compression spring 39. The rounded end face of the actuating rod 81 can also close the bore / nozzle 73 in the valve piston 38. The holding piston can be pressurized via a control port 82. This pressure can be tapped, for example, via a switching valve (not shown) between the purge valve and the purge orifice 32.When the switching valve is open, the pressure forces acting on the valve piston 38 are balanced, allowing the compression spring 39 to hold the valve piston 38 in its closed position. This allows the flow of purging oil to be switched on and off depending on the situation. An example of this is a situation where the hydraulic pump is rapidly adjusted above zero, but the purging valve cannot keep up. As a result, a large quantity of fluid would be briefly flushed from the new high-pressure side, leading to a significant pressure drop on the new low-pressure side. This situation can occur particularly when the hydraulic pump is operated with an oscillating, synchronous cylinder in a closed hydraulic circuit. Reference symbol list 10 Hydraulic pump 11 Hydraulic motor 12 first work order 13 second work management 15 Flush valve arrangement 16 Tank 17 Feed pump 18 filters 19 Food Management 20 first pressure feed valve 21 Feed valve 22 Pressure relief valve 23 second pressure feed valve 24 Feed valve 25 Pressure relief valve 26 Feed pressure limiting valve 30 flush valve 31 Pressure holding valve 32 flush plate 33 first inlet channel 34 second inlet channel 35 Drainage channel 36 flushing pistons 37 compression springs 38 valve pistons out of 31 39 Compression spring 40 Return line 45 cases of 15 46 Valve bore 47 housing control edges 48 spring chambers 49 Locking screw 50 discs 52 Tax Federation 53 Piston neck 54 Plane of symmetry Section 55 on 52 56 Piston control edge at 52 Section 57 on 52 58 circumferential grooves 59 flattenings in 57 60 flattenings in 55 61 free space around 53 62 first section 60 63 second section of 60 64 third section of 60 69 shoulder at 38 70 level at 45 71 blind hole in 38 72 radial bores in 38 73 Nozzle 74 Spring space 80 locking pistons 81 Actuating rod of 80 82 Control connection

Claims

[1] Hydraulic flushing valve arrangement (15) comprising a flushing valve (30) for flushing out a flushing quantity with a flushing piston (36) which is longitudinally displaceable in a valve bore (46) of a housing (45) by means of two control collars (52) spaced apart from each other by a piston neck (53), by which, together with two circular housing control edges (47), the fluidic connections between two inlet channels (33, 34) opening into the valve bore (46) and an outlet channel (35) opening into the valve bore (46) between the two inlet channels (33, 35) are controllable, which is pre-tensioned by at least one spring (37) in a central position in which the outlet channel (35) is closed off from the inlet channels (33, 34), and which, from a certain difference between the pressure prevailing in one inlet channel (33, 34) and the pressure, which prevails in the other inlet channel (33, 34),is movable from the central position and thereby opens a fluidic connection between the inlet channel (33, 34), in which the lower pressure prevails, and the outlet channel (35) via a control collar (52), , characterized by , that the circular shape of a piston control edge (56) limiting a control collar (52) to the piston neck (53) of the flushing piston (36) is interrupted by at least one recess (60) and that the recess (60) has a second section (63) in which the cross-section in planes perpendicular to the axis of the flushing piston (36) is constant and of such size that the flushing quantity is reduced compared to an end position of the flushing piston (36) when the second section (63) of the recess (60) is located in the area of ​​the housing control edge (47). [2] Hydraulic flushing valve arrangement (15) according to claim 1, wherein the recess (60) has a first section (62) which has a cross-section in planes perpendicular to the axis of the flushing piston (36) which, starting from the closed end of the recess (60) with decreasing distance from the piston control edge (56), continuously increases at least over a part of the axial extent of the recess (60) and continuously transitions into the second section. [3] Hydraulic flushing valve arrangement (15) according to claim 2, wherein the change in cross-section at the transition between the first section (62) and the second section (63) of the recess (60) is zero. [4] Hydraulic flushing valve arrangement (15) according to claim 2 or 3, wherein a third section (64) of the recess (60) is continuously connected to the second section (63), in which the cross-section of the recess (60) increases continuously in planes perpendicular to the axis of the flushing piston (36) and extends to the piston control edge (56). [5] Hydraulic flushing valve arrangement (15) according to claim 2, 3 or 4, wherein the cross-section of the second section (63) of the recess (60) represents the minimum flow cross-section between an inlet channel (33, 34) and the outlet channel (35) when the flushing piston (36) is in a position in which the recess (60) with its second section (63) is located in the region of the housing control edge (47). [6] Hydraulic flushing valve arrangement (15) according to a previous claim, wherein the depth of the recess (60) at the piston control edge (56) measured perpendicular to the axis of the flushing piston (36) is less than the difference between the diameter of a control collar (52) and the diameter of the piston neck (53) of the flushing piston (36). [7] Hydraulic flushing valve arrangement (15) according to a preceding claim, wherein the circular shape of a piston control edge (56) limiting a control collar (52) towards the piston neck (53) is interrupted by at least one flattening (60) forming the recess, and wherein the flattening (60) has a second section (63) in which the depth of the flattening (60) is constant and of such a size that the flushing quantity is reduced compared to an end position of the flushing piston (36) when the second section (63) of the flattening (60) is located in the area of ​​the corresponding housing control edge (47). [8] Hydraulic flushing valve arrangement (15) according to claim 7, wherein the flattening (60) has a first section (62) in which the depth of the flattening (60) measured perpendicular to the axis of the flushing piston (36) increases continuously from the diameter of the control collar (52) at least over a part of the axial extent of the flattening (60) to the second section (63) with decreasing distance from the piston control edge (56) and which transitions continuously into the second section (63). [9] Hydraulic flushing valve arrangement (15) according to claim 8, wherein the first section (62) and the second section (63) of the flattening (60) merge seamlessly into one another. [10] Hydraulic flushing valve arrangement (15) according to claim 8 or 9, wherein the first section (62) transitions seamlessly into the diameter of the flushing piston (36). [11] Hydraulic flushing valve arrangement (15) according to claim 8, 9 or 10, wherein the flattening (60) has a third section (64) which terminates at the piston control edge (56) and in which the depth of the flattening (60) measured perpendicular to the axis of the flushing piston (36) increases continuously with decreasing distance to the piston control edge (56). [12] Hydraulic flushing valve arrangement (15) according to claim 11, wherein the depth of the third section (64) first increases according to a curved, convex line and subsequently increases towards the piston control edge (56) according to a straight line. [13] Hydraulic flushing valve arrangement (15) according to claim 11 or 12, wherein the third section (64) transitions seamlessly into the annular surface bounding the control collar (52) to the piston neck (53) of the flushing piston (36) according to a curved, convex line. [14] Hydraulic flushing valve arrangement (15) according to claim 11 or 12, wherein the first section (62) and / or third section (64) are progressively edgeless in the axial direction. [15] Hydraulic flushing valve arrangement (15) according to a preceding claim, comprising a pressure holding valve (31) which is arranged in the drain channel (35) and enters an open position when a certain minimum pressure is present between it and the flushing valve (30) in the drain channel (35). [16] Hydraulic flushing valve arrangement (15) according to claim 15, wherein a flushing orifice (32) is arranged in the drain channel (35) between the flushing valve (30) and the pressure holding valve (31) and wherein the pressure for actuating the pressure holding valve (31) is taken upstream of the flushing orifice (32). [17] Hydraulic flushing valve arrangement (15) comprising a flushing valve (30), a pressure-holding valve (31) and a flushing orifice (32), wherein the flushing valve (30) has a flushing piston (36) which is longitudinally displaceable in a valve bore (46) of a housing (45) by means of two control collars (52) spaced apart from each other by a piston neck (53), by which, together with two circular housing control edges (47), the fluidic connections between two inlet channels (33, 34) opening into the valve bore (46) and an outlet channel (35) opening into the valve bore (46) between the two inlet channels (33, 34) are controllable, which is pre-tensioned by at least one spring (37) in a central position in which the outlet channel (35) is closed off from the inlet channels (33, 34), and which, from a certain difference between the pressure, the pressure prevailing in one inlet channel (33, 34), and the pressure prevailing in the other inlet channel (33, 34),is movable from the central position and thereby opens a fluidic connection between the inlet channel (33, 34), in which the lower pressure prevails, and the outlet channel (35) via a control collar (52), wherein the pressure-holding valve (31) is arranged in the outlet channel (35) and enters an open position when a certain minimum pressure is present between it and the flushing valve (30) in the outlet channel (35), wherein the flushing orifice (32) is arranged in the outlet channel (35) between the flushing valve (30) and the pressure-holding valve (31) and wherein the pressure for actuating the pressure-holding valve (31) is taken upstream of the flushing orifice (32), characterized by, that the circular shape of a piston control edge (56) limiting a control collar (52) of the flushing piston (36) towards the piston neck (53) is interrupted by at least one recess (60) and that the recess (60) on the piston control edge (56) opens a flow cross-section over part of a maximum possible travel distance of the flushing piston (36) which is smaller than the opening cross-section of the flushing orifice (32). [18] Hydrostatic drive, in particular hydrostatic drive, with two hydraulic machines (10, 11) which are fluidically connected to each other in a closed hydraulic circuit via a first working line (12) and via a second working line (13), with a feed pump (17) for supplying pressure medium into the respective working line (12, 13) at low pressure and with a hydraulic flushing valve arrangement (15) with an inlet channel (33) which is fluidically connected to the first working line (12) and with an inlet channel (34) which is fluidically connected to the second working line (13), characterized by , that the hydraulic flushing valve arrangement (15) is designed according to one of claims 1 to 17.

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