CH705960A1 - Hydraulic system with temperature-dependent hydraulic fluid leakage. - Google Patents

Abstract

The present invention relates to a hydraulic system with a closed hydraulic circuit of hydraulic pump (1) and hydraulic motor (2), with a feed pump (5) to compensate for volume losses in the closed circuit (1, 2, A, B) and with a flushing device (8 , 20) for discharging a part of the hydraulic fluid from the closed circuit (1, 2, A, B), wherein the flushing device (8, 20) is designed so that the discharge fluid flow depends on the temperature of the hydraulic fluid. As a flushing device (8), a two-stage flush valve (20) comprising a main piston (21) with a central throttle point (22) and downstream of a measuring point (23) as a viscosity-dependent throttle, be provided. The main piston (21) opens and closes the leading to the tank main opening (25) due to the pressure difference at the throttle point (22) and against the bias of the spring (24). A viscosity-dependent leakage current adjusts itself via the resistance, for example, of the annular gap of the measuring point (23). At higher temperature and thus lower viscosity of the hydraulic fluid, the leakage through the relief hole (26) increases. This results in a reduction of the pressure downstream of the orifice (22), a displacement of the main piston (21) against the spring and an increased Ausspeisefluidstrom through the main opening (25) to the tank result.

Description

The present invention relates to a hydraulic system with a closed hydraulic circuit of hydraulic pump and hydraulic motor, with a feed pump to compensate for volume losses in the closed circuit and with a flushing device for feeding a portion of the hydraulic fluid from the closed circuit.

In a closed hydraulic system, the hydraulic pump promotes fluid and thus drives the hydraulic motor. Its discharge oil quantity is returned directly to the hydraulic pump. The volumetric losses are compensated by the feed pump. The purge dispenses some of the oil out of the circuit to prevent overheating.

Flushing valves are usually used in the prior art, which are designed as constant systems and abruptly emit a constant Ausspeisefluidstrom from a certain pressure difference between the two sides of the hydraulic circuit.

The object of the present invention is to provide an improved flushing device for such a hydraulic system.

According to the invention this object is achieved by a hydraulic system according to claim 1 and a flushing device according to claim 14.

The inventive hydraulic system has a closed hydraulic circuit of hydraulic pump and hydraulic motor, wherein a feed pump to compensate for volume losses in the closed circuit and a flushing device are provided for feeding a portion of the hydraulic fluid from the closed circuit. According to the invention, the flushing device is designed so that the Ausspeisefluidstrom depends on the temperature of the hydraulic fluid. The present invention thus allows a need-based flushing of the hydraulic fluid in a closed circuit, and thus a more efficient operation of the hydraulic system. In particular, the Ausspeisefluidstrom is thereby fed to a hydraulic cooler and / or the tank.

Advantageously, increases with increasing temperature of the hydraulic fluid Ausspeisefluidstrom so as to prevent overheating of the hydraulic fluid in a closed circuit as needed. Conversely, at a low temperature of the hydraulic fluid of the Ausspeisefluidstroms be correspondingly low.

In a first advantageous embodiment of the present invention, the flushing device has a viscosity-dependent throttle. The present invention takes advantage of the physical fact that the viscosity of the hydraulic fluid decreases with increasing temperature. This increases with increasing temperature of the hydraulic fluid, the flow through the viscosity-dependent throttle. According to the invention, the Ausspeisefluidstrom thus falls by the use of a viscosity-dependent throttle variable, which is obtained in hot oil much Ausspeisefluidstrom, and thus the temperature is lowered in a closed circuit.

The use of a viscosity-dependent throttle has in addition to the temperature-dependent flushing the further advantage that the Ausspeisefluidstrom not abruptly changes when switching on the flushing device, but increases attenuated. As a result, pressure drops on the low pressure side can be prevented.

The viscosity-dependent throttle can be formed by a bore or a gap. In particular, an annular gap can be used.

Advantageously, the length of the bore or the gap is more than twice as large as the diameter of the bore or the height of the gap. Further advantageously, the length of the bore or the gap is more than three times, more advantageously more than five times the diameter of the bore or the height of the gap. In this way, a corresponding dependence of the flow rate through the throttle of the viscosity of the hydraulic fluid can be achieved.

Particularly advantageously, the throttle is designed as a cylindrical annular gap. In particular, the inner diameter of the cylinder can be between 0.5 times and 2 times the gap length, furthermore advantageously between 0.8 times and 1.2 times the gap length.

In a first embodiment, the exit fluid flow can flow directly through the viscosity-dependent throttle, and are controlled as a function of its viscosity directly through the throttle.

In a second embodiment, however, the flushing device comprises a two-stage valve with a main piston for controlling the Ausspeisefluidstroms and a control stage, wherein the viscosity-dependent throttle is arranged in the control stage. The increasing amount of the throttle due to the decreasing viscosity thus provides for a higher control current, by means of which the main piston is likewise deflected accordingly, in order to produce an increased outflow fluid flow.

Furthermore, it can be provided that a throttle piston is provided for setting a defined pressure difference across the viscosity-dependent throttle, which is acted upon by the inlet pressure of the flushing device. Advantageously, the throttle piston is arranged behind the viscosity-dependent throttle and controls the flow of hydraulic fluid to the tank.

In particular, while the throttle piston form a pressure control valve, which is acted upon on one side with the pressure on the input side of the flushing device and on the other side with the pressure behind the viscosity-dependent throttle. In particular, such a throttle piston can be provided when the viscosity-dependent throttle is arranged in the control stage of a two-stage valve.

Advantageously, a shuttle valve is further provided in the present invention, through which the Ausspeisefluidstrom is always removed from the side of the hydraulic circuit to which the lower load pressure is applied. This lower load pressure usually corresponds to the feed pressure.

According to the invention, the shuttle valve and the viscosity-dependent throttle can be carried out separately. In particular, a shuttle valve with the viscosity-dependent throttle or a two-stage valve, in whose control stage the viscosity-dependent throttle is arranged, are connected in series.

Alternatively, the viscosity-dependent throttle may be integrated into the switching valve. Advantageously, the switching valve in this case has a piston, and is designed so that the piston is acted on opposite sides with the pressure of the two sides of the closed hydraulic circuit. Depending on the pressure conditions, the piston releases the connection between the one or the other side with the tank, with the piston connecting the low-pressure side to the tank.

Advantageously, the connection between the low pressure side and the tank is in each case via a viscosity-dependent throttle, which thus according to the invention sets the Ausspeisefluidstrom depending on the temperature of the hydraulic fluid. Advantageously, two respectively one side of the closed hydraulic circuit associated viscosity-dependent throttles are provided.

Furthermore, at least one pilot piston may be provided in the piston, which prevents a purge flow below a certain minimum pressure on the low pressure side. In particular, the pilot piston can be biased against a spring force, which must be overcome by the pressure on the low pressure side only to allow a purge flow.

Furthermore, it can be provided according to the invention that the length of the viscosity-dependent throttle is adjusted by the pressure difference between the high and the low pressure side. Advantageously, the length of the throttle decreases with an increase in the pressure difference, so that at a higher pressure difference, a correspondingly higher volume flow is achieved.

If the throttle as described in detail above integrated in a switching valve, the viscosity-dependent throttle is advantageously formed by an annular gap between the piston and a valve housing, wherein the length of the viscosity-dependent throttle changes with a displacement of the piston.

In a further embodiment of the present invention, the flushing device has a temperature sensor, via which a control valve for controlling the Ausspeisefluidstroms is driven. This allows an even more flexible control of the Ausspeisefluidstroms based on the temperature. In particular, the Ausspeisefluidstrom can be increased with increasing temperature and reduced with decreasing temperature. Advantageously, the temperature of the hydraulic fluid is regulated by the flushing device to a predetermined temperature. In particular, this temperature can be adjustable.

Advantageously, an electronic control system for evaluating the data of the temperature sensor and for controlling the control valve is used.

According to the invention may further be provided at least one pressure sensor, wherein the Ausspeisefluidstrom is driven based on the data of the pressure sensor.

Advantageously, the Ausspeisefluidstrom is driven in response to the pressure difference between high and low pressure side. In particular, the Ausspeisefluidstrom be increased with increasing pressure difference.

Further advantageously, the Ausspeisefluidstrom can be controlled in dependence on the pressure on the low pressure side. In particular, it may be provided that the Ausspeisefunktion uses only at a certain minimum pressure on the low pressure side. This can prevent the occurrence of sudden pressure drops by the onset of the outfeed.

According to the invention, for example, a pressure reducing valve may be provided in combination with a diaphragm or a proportional flow control valve as a control valve. Again, the control valve for controlling the exit fluid flow can be performed separately to a switching valve or combined with this. In particular, a 2-way proportional flow control valve can be provided, which simultaneously serves as a switching and control valve.

Particularly preferably, the present invention is used in a hydraulic system used, the feed pump is designed as a variable displacement pump. As a result, the power loss can be reduced by the feed pump, whereby the consumed feed quantity and thus the power loss is reduced by the demand-driven outfeed of the hydraulic fluid according to the present invention.

Advantageously, the feed pump is controlled as needed. In particular, the feed pump can be designed as a pressure regulator.

In addition to the hydraulic system, the present invention further includes a flushing device for a hydraulic system as described above.

In particular, the flushing device comprises a viscosity-dependent throttle. Further advantageously, the flushing device is constructed as described above.

Alternatively, the inventive flushing device may comprise a temperature sensor and a control valve, which is controlled in dependence on the temperature measured by the temperature sensor.

Furthermore, the inventive flushing device may comprise a shuttle valve. The shuttle valve can form a separate element or be combined with one of the other components of the flushing device, in particular the viscosity-dependent throttle or the control valve. Advantageous embodiments are also already described above.

The present invention further includes a method of operating a hydraulic system with a closed hydraulic circuit of hydraulic pump and hydraulic motor with a rinsing device for discharging a part of the hydraulic fluid from the closed circuit. The inventive method is characterized in that the Ausspeisefluidstrom is changed in dependence on the temperature of the hydraulic fluid. Advantageously, the outlet fluid flow is increased and / or the outlet fluid flow decreases as the temperature decreases.

Advantageously, the inventive method is used to operate a hydraulic system, as described above. Further advantageously, the hydraulic system is thereby operated, as has also already been shown above.

The present invention allows on the consideration of the circulation temperature a needs-based flushing. This improves the energetic design of the flushing function. Furthermore, a sudden switching on the flushing is prevented, which would otherwise lead to feed pressure drops as in the prior art.

Particularly advantageously, the present invention is combined with a variable feed, so that a corresponding energy savings can be realized.

The present invention will now be described in more detail with reference to embodiments and drawings. Showing: <Tb> FIG. 1: <sep> an embodiment of a hydraulic circuit according to the invention, <Tb> FIG. 2a: <sep> a first embodiment of a flushing device according to the invention with a two-stage flushing valve, <Tb> FIG. 2b shows a second embodiment of a flushing device according to the invention with a two-stage flushing valve, <Tb> FIG. 3: <sep> an embodiment of a flushing device according to the invention, which is combined with a shuttle valve, <Tb> FIG. 4: <sep> an embodiment of an inventive flushing device, which is also combined with a shuttle valve and equipped with a pilot piston and <Tb> FIG. 5: <sep> a hydraulic circuit with an embodiment of a flushing device according to the invention, which provides an electronic temperature control.

In Fig. 1, a closed hydraulic circuit according to the present invention is shown schematically. In this case, a hydraulic pump 1 is provided, which promotes hydraulic fluid and drives the hydraulic motor 2. Its outlet oil quantity is returned directly to the pump 1.

Volumetric losses in the closed hydraulic circuit are compensated by means of a feed pump 5, which is connected via check valves 6 with the two load sides A and B of the closed hydraulic circuit in combination.

According to the invention, in the present closed hydraulic circuit, this is a hydraulic circuit with two directions of flow, so that depending on the pumping direction of the pump 1, either the load side A or the load side B operates as the high pressure side while the other side operates as the low pressure side.

Depending on the embodiment, the pump 1 and / or the motor 2 may be designed as an adjustment. Furthermore, the pump 1 may be driven by a motor 4, while the hydraulic motor 2 drives a device 3.

About the shuttle valve 7 is now the lower load pressure (usually corresponds to the feed pressure) of a flushing device 8 according to the invention fed to auszuspeisen a portion of the hot oil from the circulation, and thus to avoid overheating. The inventive flushing device 8 is shown in Fig. 1 as a separate unit to the shuttle valve 7, but can also be combined with this. The shuttle valve 7 is designed so that from a certain pressure difference between the load side A and the load side B uses a purge function and a certain Ausspeisefluidstrom is dissipated.

According to the invention, the flushing device 8 is now designed so that this Ausspeisefluidstrom depends on the temperature of the hydraulic fluid. As a result, a needs-based flushing is achieved.

In a first variant, the present invention takes advantage of the fact that the viscosity of the hydraulic fluid decreases with increasing temperature. According to the invention, therefore, the Ausspeisefluidstrom fail by means of a viscosity-dependent throttle depending on the temperature variable, so that when hot oil much Ausspeisefluidstrom obtained, and thus the temperature can be reduced in a closed circuit.

In this case, the following physical property of a viscosity-dependent gap choke is used: Q gap ~ (h gap ^ 3 / l gap / v)

Q gap = volume flow through the gap, h gap = gap height, gap = gap length, v = kinematic viscosity of the fluid.

Therefore, as the viscosity of the fluid decreases with increasing temperature, the volume flow through the gap increases correspondingly. Similar conditions also apply to long calibration holes.

Advantageously, while the length of the gap is more than twice as large as its height, advantageously more than five times and further advantageously more than ten times. If a hole is used, its length is advantageously also more than twice as large as its diameter, furthermore advantageously more than five times and furthermore advantageously more than ten times.

Such a viscosity-dependent throttle point can be arranged in a first variant according to the invention as a flushing device 8 behind the shuttle valve 7. The throttle point must only be dimensioned so that sufficient Ausspeisefluidstrom is generated.

An embodiment of a flushing device according to the invention, in which a two-stage flushing valve 20 is used instead, is shown in FIG. 2. This comprises a main piston 21 with a central throttle point 22. The main piston 21 determines the Ausspeisefluidstrom by depending on its position, the connection between the input side of the valve and the main opening 25, which leads to the tank, opens or closes. The piston 21 is biased by a spring 24 against the pressure on the input side.

Through the throttle point 22 in the main piston 21, the fluid is guided to the viscosity-dependent measuring point 23, which is designed according to the invention as a viscosity-dependent throttle. In particular, the measuring point 23 can be designed as a long cylindrical annular gap, as shown in the exemplary embodiment. The geometric conditions are ideally such that the diameter of the cylindrical annular gap is in the range of the gap length. Alternatively, however, a long calibration bore could also be used, wherein advantageously the length of the calibration bore corresponds at least five times to the diameter of the bore.

An increasing by increasing viscosity (corresponding to increasing oil temperature) at the measuring point 23 in the direction of the relief hole 26 increases the pressure difference at the throttle point 22 and deflects the main piston 22 against the spring 24. As a result, the main opening 25 is getting bigger and more Ausspeisefluidstrom falls to. The main opening 25 can be designed so that a maximum Ausspeisefluidstrom is not exceeded.

In this case, the lower load pressure in the closed circuit is always supplied to the flush valve 20 via the shuttle valve 7.

The alternative embodiment without a control stage already described above could be constructed such that the main piston 21, the spring 24 and the main opening 25 are dispensed with in the device shown in FIG. 2a. The measuring point 23 would then have to be designed so that the same amount of outflow fluid flow is generated. The maximum Ausspeisefluidstrom could then be limited by means of the relief hole 26.

In Fig. 2b, a further embodiment is shown with a two-stage valve, in which the main stage and the measuring point is designed as in the variant shown in Fig. 2a. In addition, however, a pressure compensation is provided. In this case, a throttle piston 29 is provided, which controls the volume flow from the measuring point 23 to the relief bore 28. In this case, the inlet pressure Px at the flush valve 20 is guided to the rear side of the throttle piston 29, which is biased by a spring 27 against this pressure. The throttle piston 29 thus generates in combination with the spring 27, a throttling of the control flow relative to the relief bore 28. As a result, the pressure difference across the measuring point 23 regardless of the level of the inlet pressure Px be kept constant. The advantage is that no variation of the Ausspeisefluidstroms is caused by feed pressure variations.

An annular gap acting as a viscosity-dependent throttle according to the invention can, as shown in FIGS. 2a and 2b, be provided by a cylinder which is inserted into a correspondingly larger hollow cylinder, for example screwed in. In the variant shown in Fig. 2b, the throttle piston 29 is arranged in a bore within this cylinder, where the control current is conducted from the measuring point 23 through a bore 26 through the cylinder wall into the bore in the interior of the cylinder.

While in Figs. 2a and 2b each embodiments of a flushing device according to the invention have been shown, in which the shuttle valve and the purge valve were performed separately, the present invention further includes such flushing devices, in which the shuttle valve is formed combined with the purge valve. In particular, the viscosity-dependent throttle can be integrated into the shuttle valve.

In Fig. 3, an embodiment of such a shuttle valve is shown. In this case, a piston 31 is provided, which is deflected due to the pressure difference between the sides A and B against the springs 32. As a result, in each case the low-pressure side is connected to the tank connection T via an annular gap 33 acting as a viscosity-dependent throttle, so that a temperature-dependent outflow fluid flow is established.

This embodiment has the further advantage that with increasing pressure difference in the closed circuit, the piston 31 deflects more and more against the springs 32. As a result, the effective annular gap 33 becomes shorter and the Ausspeisefluidstrom increases inversely proportional to the length of the annular gap 33 at. Furthermore, an attenuation of the movement of the piston 31 can be provided. For example, the Ausspeisefluidstromzunahme be delayed in time over the bore 34 at sudden load pressure increase. As a result, supply pressure drops in the closed circuit can be prevented. Basically, however, more exit fluid flow is generated at high load pressures. Furthermore, the tank line T can be used as a limitation of the maximum Ausspeisefluidstroms, z. B. by combination with a limiting nozzle.

Also in the embodiment shown in FIG. 4, the shuttle valve is combined with the temperature-dependent rinsing function according to the invention, the basic structure of the shuttle valve corresponding to the principle already explained with reference to FIG. 3. However, a switching stage is additionally integrated, which shuts off the purge below a certain minimum value on the low pressure side.

In this case, in turn, a piston 47 is provided, which deflects due to a pressure difference between A and B against the springs 46 and thereby connects the respective low pressure side via the corresponding annular gap 44 with the tank port T.

However, the piston 47 is now designed as a socket, in each of which pilot pistons 41 and 41 are provided for the sides B and A. The pilot pistons 41 and 41 are biased by a centrally disposed spring 45 against the pressure on the side A and B, respectively, and close the connection point 43 to the respective annular gaps 44 via a switching edge when the pressure on the respective low pressure side under a corresponding Minimum pressure drops. On the other hand, if the pressure increases, the respective connection point 43 is opened, so that hydraulic fluid can flow from the measuring point 44 via the connection point 43 through the interior of the pilot piston 49 and the interior of the bushing 47 to the tank connection T. The bushing 47 may be damped to achieve a time delay, similar to the bore 34 in FIG. 3.

In the following, the operation of the shuttle valve in Fig. 4 is summarized again for the situation in which the side B forms the high pressure side, while the side A forms the low pressure side are described:

By increasing the pressure on the load side B of the pilot piston 41 is moved to the left against the spring 45 to a stop with the socket 47. Now, the bushing 47 is deflected against the spring 46 and releases the viscosity-dependent throttle point 44. The higher the load pressure in B, the shorter in turn is the active length of the throttle body 44 and the higher the Ausspeisefluidstrom. Such a load pressure dependence can optionally be integrated.

So that now a pressure difference across the throttle point 44 between A and T may arise, the low pressure in A must be so large that the pilot piston 41 deflects against the spring 45 and releases the switching edge to the connection point 43. Only then can the Ausspeisefluidstrom flow from A through the throttle point 44 through the junction 43 and from there through the central bore 49 in the pilot piston 41 to T. The spring 45 thus prevents the purge from starting below a certain pressure value in A. This value is for example between 10 and 30 bar. This additional function can be combined as desired.

In summary, in the case of the embodiments, which are hydraulically adapted to the requirements described herein, the outflow fluid flow is thus adjusted to be viscosity-dependent. Such a viscosity-dependent Ausspeisefluidstromgenerierung can optionally be combined with the already described functions such as a change function, a switching function, a load pressure dependence, a damping and a setting of a maximum Ausspeisefluidstroms.

Alternatively, the outfeed can also be implemented via a temperature measurement and a correspondingly actuated control valve. In particular, a control loop with temperature measurement can be provided.

In Fig. 5, a corresponding embodiment of a hydraulic circuit is shown. In this case, the circuit temperature is measured at a suitable point in a closed circuit by means of a temperature sensor 51. However, this measurement could optionally take place after the shuttle valve 7. With the help of an electronic control circuit ICU (52), an actuator 54 is now driven depending on the temperature in the circuit, which extracts an adjustable Ausspeisefluidstrom from the circulation. This actuator, a shuttle valve 43 can be connected upstream, or the actuator 54 may already be equipped with the change function.

As actuators, the following valve types / systems are conceivable, for example:

On the one hand, a pressure reducing or reducing valve can be used, which can flow fluid with variable pressure via a diaphragm. Advantageously, such a valve is then combined with a shuttle valve.

Alternatively, a 2-way proportional flow control valve can be used, which simultaneously serves as a shuttle valve and over which the Ausspeisefluidstrom can be adjusted.

The electronic control also gives the possibility to integrate other monitoring and control functions. In particular, at least one pressure sensor for the pressure in the hydraulic circuit can be provided, advantageously one pressure sensor for the side A and the side B. Advantageously, the electronic control 52 monitors the feed pressure level in order to switch off the purging when the feed pressure level drops. As a result, the entire amount of food for the circulation filling is available.

The flushing devices of the present invention are advantageously used in a hydraulic system with a feed pump 5 designed as a variable displacement pump. In particular, such a feed pump can operate as required and, for example, be designed as a pressure regulator. As a result, a needs-based and thus energy-saving feed is realized.

If such a supply is available, all consumers who depend on this supply network, should be optimized. This also includes the flushing system of the circuit, which can be reduced by the need-based and optimized flushing according to the present invention, the amount of feed and thus the power loss.

Claims (15)

Hydraulic system with a closed hydraulic circuit comprising a hydraulic pump (1) and a hydraulic motor (2), with a feed pump (5) for compensating closed-loop volume losses and with a flushing device (8) for venting a portion of the hydraulic fluid from the closed circuit, characterized in that the flushing device (8) is designed so that the Ausspeisefluidstrom depends on the temperature of the hydraulic fluid. 2. Hydraulic system according to claim 1, wherein the flushing device has a viscosity-dependent throttle (23, 33, 44). 3. Hydraulic system according to claim 2, wherein the viscosity-dependent throttle (23, 33, 44) is formed by a bore or a gap, in particular an annular gap, wherein advantageously the length of the bore or the gap more than twice as large as the diameter the bore or the height of the gap is, advantageously more than three times and further advantageously more than five times as large. 4. Hydraulic system according to claim 2 or 3, wherein the flushing device comprises a two-stage valve (20) having a main piston (21) for controlling the Ausspeisefluidstroms and a control stage, wherein the viscosity-dependent throttle (23) is arranged in the control stage. 5. Hydraulic system according to one of claims 2 to 4, wherein for adjusting a defined pressure difference across the viscosity-dependent throttle (23) a throttle piston (29) is provided, which is acted upon by the inlet pressure of the flushing device, wherein the throttle piston (29) advantageously behind the Viscosity-dependent throttle (23) is arranged and controls the flow of hydraulic fluid to the tank. 6. Hydraulic system according to one of the preceding claims, wherein the viscosity-dependent throttle (33, 44) is integrated in a switching valve which connects the low-pressure side of the closed hydraulic circuit with the tank. 7. Hydraulic system according to claim 6, wherein the switching valve has a piston (31, 47) and is designed so that the piston is acted upon on opposite sides with the pressure of the two sides (A, B) of the closed hydraulic circuit, wherein the piston ( 31, 47) advantageously each of the low pressure side via a viscosity-dependent throttle (33, 44) connects to the tank, wherein advantageously two each one side of the closed hydraulic circuit associated with viscosity-dependent throttles (33, 44) are provided. 8. A hydraulic system according to claim 6 or 7, wherein in the piston (47) at least one pilot piston (41) is provided which prevents a Ausspeisefluidstrom below a certain minimum pressure on the low pressure side. 9. Hydraulic system according to one of claims 2 to 10, wherein the length of the viscosity-dependent throttle (33, 44) is adjusted by the pressure difference between the high and the low pressure side, wherein advantageously in a hydraulic system according to claim 7, the viscosity-dependent throttle or throttling an annular gap between the piston (31, 47) and a valve housing is or are formed, wherein the length of the viscosity-dependent throttle changes with a displacement of the piston. 10. Hydraulic system according to claim 1, wherein the flushing device has a temperature sensor (51) via which a control valve (54) is driven to control the Ausspeisefluidstroms, advantageously an electronic controller (52) for evaluating the data of the temperature sensor (51) and Control of the control valve (54) is provided and / or wherein the flushing device controls the temperature of the hydraulic fluid to a predetermined temperature. 11. A hydraulic system according to claim 10, wherein further at least one pressure sensor is provided, wherein advantageously the exit fluid flow is controlled in dependence on the pressure difference between high and low pressure side and / or in dependence of the pressure on the low pressure side. 12. A hydraulic system according to claim 10 or 11, wherein a pressure reducing valve is provided in combination with a diaphragm or a proportional flow control valve as a control valve (54), wherein advantageously a 2-way proportional flow control valve is provided, which also serves as a switching valve. 13. Hydraulic system according to one of the preceding claims, wherein the feed pump (5) is designed as a variable displacement pump and is further advantageously driven as needed. 14. flushing device for a hydraulic system according to one of the preceding claims. 15. A method for operating a hydraulic system with a closed hydraulic circuit of hydraulic pump (1) and hydraulic motor (2) and with a flushing device (8) for discharging a part of the hydraulic fluid from the closed circuit, in particular for operating a hydraulic system according to one of the preceding claims , characterized in that the Ausspeisefluidstrom is changed in dependence on the temperature of the hydraulic fluid, wherein advantageously the Ausspeisfluidstrom is increased with increasing temperature.