HYDRAULIC SYSTEM

Abstract: A hydraulic system with a pump (1) of fixed positive displacement, a tank (42) and a valve group connected via a tank line (118) to the tank (22) is disclosed, which has at least one load valve (105 to 107) and a control section (103) comprising a pilot overflow valve (16) for control of the hydraulic fluid supplied to the load valve in accordance with demand. In a system of that kind, it is desirable to be able to arrange an auxiliary valve arrangement in the tank line without the function of the system being adversely affected. For that purpose, besides the auxiliary valve arrangement (31) provided in the tank line (118) for controlling a further hydraulic load, provision is made for the discharge side of the pilot overflow valve (16) to be connected to a point of constant pressure.

Description: Hydraulic system The invention relates to a hydraulic system having a pump with fixed positive displacement, a tank and a valve group connected to the tank by way of a tank line, the valve group having at least one load valve and a control section containing a pilot overflow valve for control of the hydraulic fluid supplied on demand to the load valve. A system of that kind is known from the Danfoss range of proportional valves, for example from the company publication "Directional Valve, Proportional Valve, Type PVG", edition 2/88, page 4. In that system, several load valves, each of which controls a respective load, are combined to form a block. Hydraulic fluid not required by the loads is returned to the tank again. So that this fluid also can be used, it is known from US 4 262 580 to transfer hydraulic fluid not required by the loads by way of an auxiliary valve arrangement to a downstream hydraulic load, which has the advantage that the loads controlled by way of the valve group are given preference, that is to say, priority between the loads and further loads is guaranteed. In an arrangement of that kind, however, the pressure in the tank line may change, namely, as a function of the pressure requirement of the further loads. For the system known from US 4 262 580 this is of no great importance since the flow of liquid supplied to the load valves is not controlled according to demand. In the system described in the introduction, however, the simple connection of an auxiliary valve arrangement to the tank line (the so- called "power-beyond") is impossible, because the effect on the control of the hydraulic fluid supplied to the load valves would be intolerable. Moreover, parts connected to the tank line would have to be of sufficiently robust dimensions to cope with the high pressures. For that purpose a relatively robust or strong material would be required, which more often than not has to be ruled out for reasons of cost. It is therefore the aim of the present invention to be able to make use of the excess hydraulic fluid even in a hydraulic system of the kind mentioned in the introduction. This problem is solved in a hydraulic system of the kind mentioned in the introduction in that an auxiliary valve arrangement for controlling at least one downstream hydraulic load is arranged in the tank line, and that the discharge side of the pilot overflow valve is connected to a point of low pressure. The feature of disconnecting the discharge side of the pilot overflow valve from the tank line and connecting it to a point of constant pressure, which looks relatively simple, ensures that the valve group is able to continue operating virtually as before, but in addition, the power-beyond, that is, the loading of the tank line, is rendered possible. The valve group is consequently rendered suitable for loading the hitherto existing tank output, which was not previously possible. Admittedly, an additional line for the pilot overflow valve is necessary, but the considerably improved potential application of the valve group compensates for this additional complexity. Fluctuations in pressure in the tank line are no longer able to exert an adverse effect on the control of the hydraulic fluid supplied to the load valves. In this connection, it is especially preferred for the discharge side of the pilot overflow valve to be connected directly to the tank. The tank in itself is without pressure, that is to say, the pressure prevailing in it is the atmospheric or ambient pressure. A direct connection of the discharge side of the pilot overflow valve therefore ensures that the pressure on the discharge side of the pilot overflow valve is, as before, atmospheric pressure. Advantageously, at the point of low pressure the pressure is constant. This simplifies subsequent control of the hydraulic fluid through precision control of the load valves. It is also an advantage for the load valve to be in the form of a proportional valve. In this way the connected loads can be controlled relatively precisely with minimum expense. In an especially preferred embodiment, the load valve is hydraulically operable, in particular electro- hydraulically operable, and has a first supply connection which is connected to the pump by way of a pilot reducing valve; the pilot reducing valve is connected by way of a leakage oil outlet directly to the tank. Hydraulically operable load valves are known per se. They can advantageously be remote- controlled. For that purpose, using a remote-control device which can be operated, for example, electrically and/or mechanically, hydraulic pressure is applied to an end face of a slide in the load valve in order to displace the slide, if desired against the force of a spring. The hydraulic pressure is supplied via the first supply connection. To ensure that this hydraulic pressure remains at a predetermined value, the pilot reducing valve is provided. The pilot reducing valve can have, for example, a slide, as explained in detail in DE 38 40 865 C2, which on one side is biased by pressure behind the pilot reducing valve, that is, by pressure at the first supply connection, and on the opposite side is biased by a spring and the pressure in the control outlet, which in the known case is formed by the tank outlet. If the pressure at the first supply connection now increases beyond a predetermined extent, the pilot reducing valve opens against the force of the spring and against the pressure at the control output and allows hydraulic fluid to flow away. If the control output were then to be subjected to changing pressures, the pressure at the first supply connection would also change correspondingly. By connecting the control output directly to the tank, fluctuations in pressure that could be caused by the further hydraulic loads can be avoided. Advantageously, the load valve has a second supply connection acting oppositely to the first supply connection and connected to the tank. A somewhat more complicated control of the movement of the slider of the load valve could also compensate for the changing pressures at the first supply connection. But it is still impossible to ensure that the load valve will not be subjected to pressures that are too high. Some materials, such as diecast aluminium, for example, would be unable to withstand such a loading. Connecting the second supply connection directly to the tank eliminates this problem. The pressures then at the load valve are the same as those that previously existed in the valve group. To avoid the further hydraulic loads having an effect on the loads connected to the load valves, in a further preferred embodiment provision can be made for the load valve to have a tank connection which is directly connected to the tank. In a system of this construction the auxiliary valve group is still supplied with the hydraulic fluid that is not taken up by the load valves, but because the hydraulic fluid flowing back through the load valve or valves generally has only a slight pressure or no pressure at all, the use of this fluid can be omitted without appreciable loss. The valve group is preferably constructed in a modular manner with elements flange-mounted on one another; a first end element (input module) comprises the pilot overflow valve and the tank line connected to the auxiliary valve arrangement and a second end element (final module) comprises the pilot reducing valve and a pilot tank connection connected directly to the tank. In this connection, in contrast to the known case, the pilot overflow valve and the pilot reducing valve are arranged separately from one another in different end elements. This simplifies the construction of the valve group and enables the connections, which can be connected to the auxiliary valve arrangement, to be separated more easily from the connections that have to be taken directly to the tank. Even unskilled operatives are able to assemble such a valve group relatively easily. The tank connection of the load valve advantageously terminates in one end element and is led through the other. Depending on the arrangement, in a simple manner the tank connection of the loads is therefore connected either still to the tank line connected to the auxiliary valve arrangement, or is led directly to the tank. To allow a simple adaptation, in a further preferred embodiment provision is made for the connection to the tank line and to the pilot tank connection to be provided in each end element, and to be interruptible. For interruption, it is sufficient, for example, for a plug or other barrier to be inserted in the corresponding connection. The valve group can therefore be switched over relatively easily from one function to another. The invention is described hereinafter with reference to preferred embodiments. In the drawings: Fig. 1 shows a state-of-the-art hydraulic system, Fig. 2 shows a first embodiment of a hydraulic system with an auxiliary valve arrangement and Fig. 3 shows a second embodiment. To make clear the problems, a state-of-the-art hydraulic system is illustrated in Fig. 1. A pump 1 of fixed positive displacement, that is to say, a constant volumetric displacement, is connected via a pump line 2 to a control section 3. The control section 3 forms an end element of a valve group 4. In addition to the control section 3, the valve group 4 comprises a series of proportional valves 5, 6, 7 which, in that order, control a rotary motor 8, a bidirectional hydraulic piston-cylinder unit 9 and a unidirectional hydraulic piston cylinder unit 10. Each proportional valve 5 to 7 has a pump connection 11 connected to the pump line 2 and a tank connection 12. Furthermore, a load-sensing line 13 is provided on which, by way of change-over valves 14, the largest pressure required from all loads is relayed to the control section 3. The load-sensing line 13 is connected to the tank connection 12 by an end element 15 flange-mounted on the valve group 4 at the opposite end to the control section 3. The control section 3 comprises a pilot overflow valve 16 which is connected at its inlet side to the load-sensing line 13 by way of a throttle 17 and on its output side is connected to a tank line 18. The pilot overflow valve 16 has a slide which is biased on one side by a spring and on the other side by the pressure upstream of the pilot overflow valve 16. The pilot overflow valve guarantees a constant pressure on its input side corresponding to the force of the spring 19. This pressure acts together with a spring 20 on a slide of a pressure balance valve 21, the other side of which is biased by the pressure of the pump 1. The pressure balance valve 21 connects the pump line 2 with the tank line 18. The tank line 18 is also connected to the tank connection 12. By means of the pressure falling at the pilot overflow valve 16, the pressure balance valve 21 can be controlled so that the proportion of hydraulic fluid not required by the loads 8 to 10 can be returned directly to the tank again. Control of the hydraulic fluid supplied to the loads 8 to 10, or rather to the proportional valves 5 to 7, is therefore effected in accordance with demand, the demand being communicated to the control section 3 by way of the load-sensing line 13. The proportional valves 5 to 7 each have a slide 23 which can be displaced by remote control. For that purpose, a remote control connection 24 is provided to which remote control signals can be applied. On the basis of the remote control signals, a pressure that is present at a first supply connection 25 can be passed to the end face of the slide 23 in order to displace this correspondingly against the force of a restoring spring 26. Displaced hydraulic fluid is then able to flow to the tank connection 12 via a line which forms a second supply connection 27. When the slide 23 is restored, hydraulic fluid can be suctioned subsequently via this second supply connection 27. The first supply connection 25 is connected via a pilot reducing valve 28 to the pump 1. The pilot reducing valve 28 ensures, in a manner known per se. that there is always a constant pressure at the first supply connection 25. For that purpose the pilot reducing valve 28 has a slide which is biased from one side with the pressure behind the pilot reducing valve 28, that is to say, the pressure at the first supply connection 25, and is biased on the other side by a spring 29 and by the pressure at a control output 30. If the pressure at the first supply connection 25 increases beyond an extent that is determined by the force of the spring 29 and the pressure at the control output 30, then the pilot reducing valve 28 opens and allows the hydraulic fluid supplied from the pump 2 to flow away to the tank 22 again. For the rest, the construction of the proportional valves 5 to 7 is known and requires no further details. If the tank line 18 is now connected to an auxiliary valve arrangement 31 (Fig. 2) for further loads, not illustrated in detail, the problem arises that neither the pilot overflow valve 16 nor the pilot reducing valve 28 nor the slide 23 of the proportional valves 5 to 7 works correctly. In that case, pressure fluctuations caused by the auxiliary valve arrangement 31 in fact occur on the tank line 18. Since the input pressure of the pilot overflow valve 16, and thus the control pressure of the pressure balance valve 21, is, however, also dependent on the output pressure of the pilot overflow valve 16, control of the hydraulic fluid in accordance with demand for the individual proportional valves 5 to 7 can no longer be carried out correctly. It is also no longer possible to guarantee that these valves will move into the correct position when the valve slide 23 is operated by remote control, since neither the pressure set by the pilot reducing valve 28 at the first supply connection 25 nor the pressure at the second supply connection 27 is predictable. Both pressures are far more dependent on the load situation at the auxiliary valve arrangement 31. Far more serious, however, is the fact that the control elements operated via the remote control connection 24 are not designed for such pressures. Normally, they consist of diecast aluminium and would become damaged very quickly on increases in pressure. To avoid this problem, that is to say, in order nevertheless to be able to connect an auxiliary valve 31 to the tank line 18, in a first embodiment shown in Fig. 2 certain changes have been made. In Fig. 2, identical elements are provided with the same reference numerals, and corresponding elements are provided with numerals increased by 100. A first modification compared with the construction shown in Fig. 1 relates to the control section 103. Here, a pilot tank line 32 has been added. The pilot tank line 32 is connected to the output of the pilot overflow valve 16. The pilot tank line 32 is led through the proportional valves 105, 106, 107 and opens directly into the tank 22. For reasons of clarity two tanks 22 are shown here. It is clear, however, that these can be embodied physically by a single container. A second modification in the control section 103 consists in that the pilot reducing valve 28 has been removed. It is now arranged in the end element 115. As before, it is arranged between the pump connection 11 and the first supply connection 25. Only the control output 30 opens into the pilot tank line 32. A third modification concerns the proportional valves 105, 106, 107. Here, the second supply connection 127 is no longer connected to the tank connection 12, but to the pilot tank line 32. A case in which the valve 23 can be controlled only in one direction is illustrated. In that instance, the second supply connection 127 is connected directly to the tank. If it is desirable to operate the valve 23 in the other direction as well, the connections would have to be interchanged, if desired using suitable auxiliary valves. For the rest, the hydraulic system contains no further modifications. Whilst the effect of the first modification, that is, connecting the output of the pilot overflow valve 16 directly to the tank 22, is that the proportional valves 105, 106, 107 can continue to be supplied with hydraulic fluid on demand, without having to take pressure fluctuations in the tank line 118 into account, the two other modifications essentially serve to protect the proportional valves 105, 106, 107 against pressures that are too great, so that, as before, they can be manufactured from materials that withstand only a limited pressure stress. Of course, isolating the proportional valves 105, 106, 107 from the pressures by way of the slide 23 also simplifies remote control. Fig. 3 shows a further embodiment in which parts that correspond to parts in Fig. 2 are provided with the same reference numerals and corresponding parts are provided with reference numerals increased by 100. The single modification to the embodiment shown in Fig. 3 compared with the embodiment of Fig. 2 is that the tank connection 12 is no longer, as in Fig. 2, connected to the tank line 118 by way of the control section 103. On the contrary, this connection is interrupted in the control section 203. The tank connection 12 is instead connected in the end element 215 to the pilot tank line 32. As a consequence, the hydraulic fluid flowing back by way of the proportional valves 105, 106, 107 from the loads 8, 9, 10 no longer flows through the auxiliary valve arrangement 31, but directly to the tank 22. The loads 8, 9, 10 are also isolated from pressure changes because of the auxiliary valve arrangement 31. The tank connection 12 is therefore connected to the tank 22 either only by way of the control section 103, which forms an end element of the valve group 104, or only by way of the other end element 215 of the valve group 204. To ensure a high degree of flexibility, provision can be made for the connection between the tank connection 12 and the tank line 118 and between the tank connection 12 and the pilot tank line 32 to be arranged respectively both in the control section 103 and in the control section 203 and both in the end element 115 and in the end element 215. This connection can be interrupted however, if this should be necessary for the construction of the hydraulic system.

Claims: Patent Claims 1. A hydraulic system having a pump with fixed positive displacement, a tank and a valve group connected to the tank by way of a tank line, the valve group having at least one load valve and a control section containing a pilot overflow valve for control of the hydraulic fluid supplied to the load valve according to demand, characterized in that an auxiliary valve arrangement (31) for controlling at least one downstream hydraulic load is arranged in the tank line (118) , and that the discharge side of the pilot overflow valve (16) is connected to a point of low pressure. 2. A system according to claim 1, characterized in that the discharge side of the pilot overflow valve (16) is connected directly to the tank (22) . 3. A system according to claim l or 2, characterized in that at the point of low pressure the pressure is constant. 4. A system according to one of claims 1 to 3, characterized in that the load valve (105, 106, 107) is in the form of a proportional valve. 5. A system according to one of claims 1 to 4, characterized in that the load valve (105, 106, 107) is hydraulically operable, in particular electro- hydraulically operable, and has a first supply connection (25) which is connected to the pump (1) by way of a pilot reducing valve (28) , the pilot reducing valve (28) being connected by way of a leakage oil outlet (30) directly to the tank (22) . 6. A system according to one of claims 1 to 5, characterized in that the load valve (105 to 107) has a second supply connection (127) acting oppositely to the first supply connection (25) and connected to the tank (22). 7. A system according to one of claims 1 to 6, characterized in that the load valve (105, 107) has a tank connection (12) which is directly connected to the tank (22) . 8. A system according to one of claims 1 to 7, characterized in that the valve group (104, 204) is constructed in a modular manner with elements flange- mounted on one another, a first end element (103, 203) comprising the pilot overflow valve (16) and the tank line (118) connected to the auxiliary valve arrangement (31), and a second end element (115, 215) comprising the pilot reducing valve (28) and a pilot tank connection connected directly to the tank (22) . 9. A system according to claim 8, characterized in that the tank connection (12) of the load valve (105, 107) terminates in one end element (115, 203) and is led through the other (103, 215). 10. A system according to claim 9, characterized in that the connection to the tank line (118) and to the pilot tank connection (33) is provided in each end element (103, 115; 203, 215) but is interruptible.

Inventors: CHRISTENSEN THORKILD, ZENKER SIEGFRIED, CHRISTENSEN, THORKILD, ZENKER, SIEGFRIED
Applicants: CHRISTENSEN THORKILD, DANFOSS AS, ZENKER SIEGFRIED, CHRISTENSEN, THORKILD, DANFOSS A/S, ZENKER, SIEGFRIED
Application year: 1993
Main classification: F15B13
IPC codes: F15B11