Hydraulic systems for construction machinery

Abstract

The present invention relates to a hydraulic system comprising a first actuator, a first variable displacement pump fluidly connected to the first actuator via a first circuit and adapted to drive the first actuator. The system further comprises a second actuator and a second pump fluidly connectable to the second actuator via a second circuit and adapted to drive the second actuator, wherein the second pump is fluidly connectable to the first actuator via a first control valve, and wherein the second pump is fluidly connectable to the second actuator via a second control valve.

Claims

1. A hydraulic system comprising: a first actuator; a first variable displacement pump fluidly connected to the first actuator via a first circuit and adapted to drive the first actuator; a second actuator; a second pump fluidly connectable to the second actuator via a second circuit and adapted to drive the second actuator, wherein the second pump is fluidly connectable to the first actuator via a first control valve, and wherein the second pump is fluidly connectable to the second actuator via a second control valve, wherein the second pump is also arranged to act as a charge pump maintaining the hydraulic system at an elevated fluid pressure.

2. The hydraulic system of claim 1, wherein the first circuit is a closed loop circuit.

3. The hydraulic system of claim 1, wherein the second pump is a variable displacement pump.

4. The hydraulic system of claim 1, wherein the first pump is directly connected or connectable to the first actuator, and wherein the first control valve is a first proportional control valve adapted to variably restrict a fluid flow from the second pump provided to the first actuator.

5. The hydraulic system of claim 4, wherein the first proportional control valve is a directional, proportional spool valve.

6. The hydraulic system of claim 4, wherein the first proportional control valve is an independent metering valve.

7. The hydraulic system of claim 6, wherein the independent metering valve is connected to a first chamber of the first actuator via a first fluid line and to a second chamber of the first actuator via a second fluid line, wherein a first pressure sensor is provided in the first fluid line and a second pressure sensor is provided in the second fluid line.

8. The hydraulic system of claim 7, wherein the hydraulic system comprises a control unit adapted to receive pressure information from the first and second pressure sensors, and wherein the control unit is configured to control the independent metering valve to connect one of the first or second chamber to a fluid return line, depending on the pressure information.

9. The hydraulic system of claim 1, wherein the second control valve is a second proportional control valve adapted to variably restrict the second fluid pressure of the second pump provided to the second actuator.

10. The hydraulic system of claim 9, wherein the second proportional control valve is a directional, proportional spool valve.

11. The hydraulic system of claim 1, further comprising a third actuator and a third pump connectable to the third actuator via a third circuit and adapted to drive the third actuator.

12. The hydraulic system of claim 11, wherein the second pump is fluidly connectable to the third actuator via a third control valve.

13. The hydraulic system of claim 12, wherein the third pump is directly connected or connectable to the third actuator, and wherein the system comprises a third proportional control valve adapted to variably restrict a fluid flow from the second pump provided to the third actuator.

14. The hydraulic system of claim 13, wherein the third proportional control valve is a directional, proportional spool valve, preferably a 4/3 spool valve.

15. The hydraulic system of claim 1, wherein the first pump is configured as a bidirectional variable displacement pump and the second pump is configured as a unidirectional pump, and wherein the first and second control valves are directional control valves.

16. The hydraulic system of claim 15, wherein the first pump comprises a first port connected or selectively connectable to a first chamber of the first actuator and a second port connected or selectively connectable to a second chamber of the first actuator.

17. The hydraulic system of claim 15, wherein the second circuit is an open circuit.

18. The hydraulic system of claim 17, wherein the second pump comprises a first port selectively connectable to the first or second chamber of the first actuator via the first control valve and a second port connected to a hydraulic fluid reservoir.

19. The hydraulic system of claim 18, wherein the first port of the second pump is connected to the hydraulic fluid reservoir via a bypass-valve, preferably a variable pressure relief valve.

20. A construction machine, comprising the hydraulic system of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described, by way of example only, with reference to the accompanying figure, in which:—

(2) FIG. 1a shows a schematic of a hydraulic system according to an embodiment of the present invention;

(3) FIG. 1b shows a schematic of a hydraulic system according to an embodiment of the present invention;

(4) FIG. 1c shows a schematic of a hydraulic system according to an embodiment of the present invention;

(5) FIG. 1d shows a schematic of a hydraulic system according to an embodiment of the present invention;

(6) FIG. 1e shows a schematic of a hydraulic system according to an embodiment of the present invention;

(7) FIG. 1f shows a schematic of a hydraulic system according to an embodiment of the present invention;

(8) FIG. 1g shows a schematic of a hydraulic system according to an embodiment of the present invention;

(9) FIG. 2 shows a schematic of a hydraulic system according to a sixth embodiment of the present invention;

(10) FIG. 3 shows a schematic of a hydraulic system according to a seventh embodiment of the present invention;

(11) FIG. 4 shows a schematic of a hydraulic system according to an eighth embodiment of the present invention;

(12) FIG. 5 shows a schematic of a hydraulic system according to a ninth embodiment of the present invention; and

(13) FIG. 6 shows the flow rate requirements of the first and second actuator during a typical duty cycle.

DETAILED DESCRIPTION OF THE INVENTION

(14) FIG. 1a shows a schematic of a hydraulic system according to an embodiment of the present invention. By way of example, this embodiment of the hydraulic system will be described below in connection with an earth moving device, such as an excavator. However, it should be understood that the hydraulic system shown in FIG. 1 is not restricted to this application and is suitable for a variety of different machinery.

(15) The hydraulic system comprises a first actuator 101 which is connected to a first pump 102 via a first circuit 103. The first actuator may be a linear actuator, such as a hydraulic cylinder. The first circuit 103 of FIG. 1a is depicted as a closed loop circuit, containing the first pump 102 connectable to the first actuator 101. The first pump 102 is connectable to the first actuator 101 via first and second fluid lines 110, 111.

(16) The first pump 102 is shown as a bi-directional, variable displacement pump, which is connectable to a first chamber 104 of the first actuator 101 via the first fluid line 110. A second outlet port of the first pump 102 is connected to a second chamber 105 of the first actuator 101 via second fluid line 111. Since the first pump 102 is a bi-directional pump, pressurized fluid may be provided to the first chamber 104 via fluid line 110 or, alternatively, to chamber 105 via second fluid line 111. By changing the displacement of the first pump 102, the first actuator 101 may be operated at different speeds.

(17) FIG. 1a further shows a second pump 202, which is connectable to a second actuator 201 in a second fluid circuit 203. The second pump 202 is selectively connectable to the first actuator 101 by means of a first control valve 701. The second pump 202 is further selectively connectable to the second actuator 201 by means of a second control valve 702. In particular, the first and second control valves 701, 702 are part of a valve arrangement 700, as depicted in FIG. 1a. Both control valves 701 and 702 are constructed as solenoid actuated proportional spool valves. In more detail, both of the spool valves of the control valves 701 and 702 are 4/3 directional spool valve, which are biased towards their closed position.

(18) The second pump 202 is a uni-directional variable displacement pump, which is connectable via the second control valve 702 to the second actuator 201. The uni-directional second pump 202 comprises a first high pressure port, which is connected to the second control valve 702 of the valve arrangement 700 via first fluid line 210 of the second circuit 203. The low pressure port of the second pump 202 is connected to the second control valve 702 via the second fluid line 211 of the second fluid circuit 203. At its rest position, the second control valve 702 is closed, that is, the connection between the second pump 202 and the second actuator 201 is shut off. In a first position (downwards in FIG. 1a), the valve 702 connects the high pressure port of the second pump 202 to a first chamber 204 of the second actuator via fluid line 210 and the second chamber 205 of the second actuator 201 with the low pressure port of the second pump 202 via fluid line 211, thus retracting the second actuator 201. In its second position (upwards in FIG. 1a), the second control valve 702 connects the high pressure port of the second pump 202 with the second chamber 205 of the second pump 201 via fluid line 210 and the low pressure port of the second pump 202 with the first chamber 204 of the second actuator via fluid line 211, thus extending the second actuator 201.

(19) The second pump 202 is connectable to the first pump 102 in a similar manner by means of the first control valve 701. In detail, the second pump 202 is disconnected from the first actuator 101, when the first control valve 701 is in its rest position. In the first position of the first control valve 701 (downwards in FIG. 1a), the high pressure port of the second pump 202 is connected with the second chamber 105 of the first actuator 101 and the low pressure port of the second pump 202 is connected to the first chamber 104 of the first actuator 101. This first position of the first control valve 701 can be used to assist the first pump 102 with extending the first actuator 101. When the first control valve 701 is in its second position (upwards in FIG. 1a), the high pressure port of second pump 202 is connected to the first chamber 104 of the first actuator 101 and the low pressure port of the second pump 202 is connected to the second chamber 105 of the first actuator 101, thus assisting the first pump 102 with retracting the first actuator. It will be appreciated that the first and second pumps 102, 202 as well as the first control valve 701 are controlled in such a way that the high pressure port of the first pump 102 and the high pressure port of the second pump 202 are always connected to the same chamber of the first actuator 101. Of course, the same applies to the low pressure ports of the first and second pumps 101, 202, which will also be connected to the same chamber.

(20) The valve arrangement 700 is connected to a controller (not shown), which will regulate positioning of the first and second control valves 701 and 702 in response to demands for actuation speed of the first, second actuators 101, 201. Under normal/average conditions, the first pump 102 will independently provide the first actuator 101 with pressurized fluid in a displacement controlled manner. As such, the high pressure flow of the first pump 102 will be connected to the second chamber 105 if the piston rod of the first actuator 101 (linear actuator, such as hydraulic cylinder) shall be extended out of the cylinder housing (to the left in FIG. 1a). In order to retract the linear actuator, the pumping direction of the first pump 102 is reversed such that the high pressure port of the first pump 102 is connected to the first chamber 104 and low pressure port is connected to the second chamber 105 of the first actuator 101. If the maximum fluid output flow of the first pump 102 is not sufficient to extend the first actuator 101 at the desired speed, the controller may transfer the first control valve 701 into its first position (downwards in FIG. 1a), such that the high pressure outlet of the second pump 202 is connected to the second chamber 105 in order to assist the first pump 102 in extending the ram of the first actuator 101. If the maximum fluid output flow of the first pump 102 is not sufficient to retract the first actuator 101 at the desired speed, the controller may transfer the first control valve 701 into its second position (upwards in FIG. 1a), such that the high pressure outlet of the second pump 202 is connected to the first chamber 104 in order to assist the first pump 102 in retracting the ram of the first actuator 101.

(21) The first and second control valves 701 and 702 may be proportional spool valves such that the fluid flow/pressure supplied by the second pump 202 to the first and second actuators 101 and 201 can be distributed according to demand. That is, if only a small amount of additional flow/pressure is required to extend the first actuator 101 at the desired speed, the controller will adjust valve 701 such that only a small part of the second fluid flow supplied by the second pump 202 is diverted to the first or second chamber 104, 105 of the first actuator 101. The remaining flow provided by the second pump 202 may therefore be used to drive the second actuator 201 simultaneously.

(22) In the embodiment shown in FIG. 1a, the first and second pumps 102, 202 are driven by a common drive shaft 801, which connects each of the pumps 102, 202 to a single prime mover, shown as drive motor 800, such as a combustion engine or electric motor. The drive motor 800 is also connected to a charge pump 902 via the common drive shaft 801, as will be described in more detail below. The invention is not limited to this particular drive arrangement. For example, any prime mover could be used to drive the pumps and the pumps maybe connected to a plurality of prime movers via a plurality of drive shafts, examples of which are described below.

(23) Turning to FIG. 1b, there is shown another embodiment of the present hydraulic system. Parts of the embodiment shown in FIG. 1b, which are identical to the embodiment in the drawing of FIG. 1a are labeled with identical reference signs. The embodiment of FIG. 1b differs from the embodiment of FIG. 1a in that the second fluid circuit 203 is an open circuit. While the uni-directional second pump 202 still comprises a first high pressure port, which is connected to the first and second control valves 701, 702 via a first fluid line 210, the low pressure port of the second pump 202 is now connected to the hydraulic fluid reservoir 901. The return ports of the first and second control valves 701, 702 are now connected to the hydraulic fluid reservoir 901, via second fluid line 212 and relief valve 904.

(24) An inlet port of a bypass-valve, in this embodiment a variable pressure relief valve 207, is connected to the high pressure outlet port of the second pump 202 via fluid line 210. An outlet port of the variable pressure relief valve 207 is connected to an inlet port of relief valve 904 and an inlet port of the accumulator 903 via the second fluid line 212.

(25) During actuation of the first and/or second actuators 101, 201, the variable pressure relief valve 207 is set to a first relief value at a predetermined maximum operating pressure of the first and/or second actuator 101, 201. In other words, the variable pressure relief valve 207 acts as a safety relief valve if pressure in the respective chambers of the first and/or second actuators exceed a pre-determined threshold. During operation of the first and/or second actuator 101, 201, return flow from the first and/or second actuators 101, 201 is directed towards the hydraulic fluid reservoir 901 via the relief valve 904. As such, during use of the first and/or second actuator 101, 201, the return flow charges the system.

(26) When neither the first nor the second actuator 101, 201 is in use, that is, when the first and second control valves 701, 702 are closed, the variable pressure relief valve 207 is set to a second relief value. The second relief value may be a fully open state in which the second pressure relief valve does not restrict the fluid flow between fluid lines 210 and 212 significantly. The second pump 202 then solely acts as a charge pump and will set the system pressure by filling accumulator 903 up to a pressure value set by relief valve 904.

(27) The variable pressure relief valve 207 may be a solenoid actuated relief valve or any other suitable valve that allows a rapid interchange between two pre-determined relief values.

(28) Yet another embodiment of the present hydraulic system is shown in the schematic drawing depicted in FIG. 1c. Parts of the embodiment shown in FIG. 1c, which are identical to the embodiment in the drawing of FIG. 1a are labeled with identical reference numbers. As will be appreciated, the embodiment according to FIG. 1c only differs from the embodiment of FIG. 1a in that the valve arrangement 710 comprises first and second control valves 711, 712, which are constructed as bridge valves. Each of the bridge control valves 711, 712 comprises four independently controllable metering valves 711a, 711b, 711c, 711d, 712a, 712b, 712c, 712d. Each of the independent metering valves 711a, 711b, 711c, 711d, 712a, 712b, 712c, 712d is constructed as a normally closed 2/2 proportional solenoid valve. The independent metering valves 711a, 711b, 711c, 711d, 712a, 712b, 712c, 712d can be poppet or spool valves or any other kind of metering valve the skilled person would see fit. If the second pump 202 is used to assist the first pump 102 in driving the first actuator 101 to extend the piston rod, the controller moves the first metering valve 711a into its second position (towards the right in FIG. 1c) to connect the high pressure outlet of pump 202 with the chamber 105 of the first actuator 101, via the first fluid line 210. At the same time, the controller opens independent solenoid valve 711d such that the first chamber 104 of the first actuator 101 is connected to the low pressure port of the second pump 202, via the second fluid line 211. If, on the other hand, the second pump 202 is used to retract the piston of the first actuator 101, the high pressure fluid port of pump 202 is connected to the first chamber 104, while the low pressure fluid port is connected to the second chamber 105. To this end, the controller opens independent valves 711c and 711b, while valves 711a and 711d remain closed.

(29) The function of the second bridge control valve 712 of the valve arrangement 710 is substantially identical to the function of the first bridge control valve 711. Of course, in contrast to the first bridge control valve 711, the second bridge control valve 712 selectively connects the second pump 202 to the second actuator 201. It will be appreciated that the valve arrangements 710 of the embodiment shown in FIG. 1c allows for individual metering of the high pressure and low pressure fluid lines of the second circuit 203. For example, the first bridge control valve 711 allows for the high pressure fluid flow of the second pump to be metered via independent metering valve 711a when extending the first actuator 101, while fluid being pushed out of the first chamber 104 of the first actuator 101 can be connected to the low pressure port of the second pump, without any metering along valve 711d. That is, the bridging valve arrangement of the embodiment shown in FIG. 1c allows for differential metering of the fluid flows in the first and second fluid lines 210, 211.

(30) In FIG. 1d there is shown another embodiment of a hydraulic system according to the present invention. Parts of the embodiment shown in FIG. 1d, which are identical to parts of the embodiment according to FIG. 1c are labeled with identical reference signs. In contrast to the anti-cavitation system 130 of FIG. 1c, the embodiment shown in FIG. 1d shows an anti-cavitation system 131, which no longer requires pilot operated check valves. Instead, the embodiment of FIG. 1d includes first and second pressure sensors 730, 731 which are provided in the fluid lines that connect the first control valve 711 with the first actuator 101. In particular, a first pressure sensor 730 is arranged in a first fluid line between the first control valve 711 and the first chamber 104 of the first actuator 101. A second pressure sensor 731 is provided in the fluid line between the first control valve 711 and the second chamber 105 of the first actuator 101.

(31) According to the embodiment in FIG. 1d, the first control valve, which is constructed as a bridge valve, may be used to compensate for differences in volume between the first chamber 104 and the second chamber 105 of the first actuator 101. To this end, the first and second pressure sensors 730, 731 may be connected to a control unit, which in turn controls actuation of the independent metering valves 711a, 711b, 711c, 711d of the first control valve 711. The first and second pressure sensors 730, 731 measure the pressure across the first actuator 101 to determine which of the first and second chambers 104, 105 are loaded and unloaded respectively. The first control valve 711 may then connect the unloaded chamber to the fluid return line, i.e. to the second fluid line 211 of the second fluid circuit 203. In more detail, if the first chamber 104 is resistively loaded, the piston will move towards the second chamber 105, which is then unloaded and hydraulic fluid will be expelled from the second chamber 105. Due to the difference in volume between the rod side first chamber 104 and the head side second chamber 105, the first fluid circuit 103 will be provided with an excess of hydraulic fluid which can be released via the first control valve 711. In particular, in the above scenario, the control unit may open metering valve 711b in order to connect the second chamber 105 with the fluid return line, i.e. with second fluid line 211. If the first actuator 101 is extended, i.e. if the second chamber 105 is resistively loaded, the unloaded first chamber 104 may be connected to the fluid return line, i.e. the second fluid line 211 via the first control valve 711. In detail, the control unit may open metering valve 711d in order to connect the first chamber 104 of the first actuator 101 with the second fluid line 211. The skilled person will appreciate that the opposite is the case if the first actuator is over-running.

(32) Another embodiment of the present hydraulic system is shown in FIG. 1e. Parts of the embodiment shown in FIG. 1e, which are identical to parts of the embodiment according to FIG. 1a are labeled with identical reference signs. The embodiment according to FIG. 1e shows another valve arrangement 720, which differs from the valve arrangements 700 and 710 shown in FIGS. 1a and 1 c. The valve arrangement 720 shown in FIG. 1e has first and second control valves 721, 722, each of which include first and second independent metering spool valves 721a, 721b, 722a and 722b. Similar to the embodiment of FIG. 1c, the independent metering valves 721a and 721b can be used to meter the fluid flow in the first and second fluid lines 210, 211, between the second pump 202 and the first actuator 101, separately. Similarly, the first and second spool valves 722a, 722b of the second control valve 722 can be used to independently meter the fluid flow between the first and second fluid flow lines 210, 211 and the chamber 204, 205 of the second actuator 201.

(33) As mentioned previously, the first and second pumps 102, 202 can be driven by any kind of prime mover such as an electric or fuel motor 800, which is connected to each of the pumps via a common connector shaft 801. In another embodiment of the present invention, shown in FIG. 1e, each of the pumps 122, 222 and 902 is connected to a separate prime mover 810, 820, and 830. In a particular embodiment of FIG. 1f, the prime movers 810, 820, 830 are connected to their respective pump 102, 202, 902 via connector shafts 811, 821, 831. The prime movers or motors 810, 820, 830 are preferably adapted to drive the connector shaft 811, 821 or 831 at varying revolution speeds, thereby varying the output flow rate of their respective pumps 122, 222, 902. It will be appreciated that the first and second pumps 122, 222 of this embodiment may thus be fixed displacement pumps, as the output flow rate is controllable by varying the revolution speed of the individual connector shafts 811, 821 via prime movers or motors 810, 820. Alternatively, the motors 810, 820 may be single speed motors and comprise an adjustable gearing mechanism, which connects the output of the motor 810, 820, 830 with the connector shafts 811, 821, 831 so as to drive the connector shafts 811, 821, 831 at varying revolution speeds.

(34) According to another embodiment shown in FIG. 1g, the hydraulic system again comprises a single prime mover or motor 800 adapted to drive a common shaft 801, similar to the embodiment of FIG. 1a. Again, identical parts of the embodiment shown in FIG. 1g, are labeled with identical reference numbers. In contrast to the embodiment of FIG. 1a, the embodiment of FIG. 1g shows variable ratio mechanisms 840, 850 arranged between the common drive shaft 801 and the first or second pump 122, 222 respectively. The variable ratio mechanism 840 connects a drive shaft 841 of the first pump 122 to the common drive shaft 801 of the motor 800. A second variable ratio mechanism 850 connects a second drive shaft 851 of the second pump 222 to the common shaft 801. The variable ratio mechanisms 840 and 850 are adapted to convert the revolution speed of the common drive shaft 801 into a revolution speed of the first and second drive shaft 841, 851 desired to drive the first or second pumps 122, 222 respectively. As such, the variable ratio mechanisms 840, 850 can have any commonly available form, such as gearing, belt or chain mechanisms. Similar to the embodiment of FIG. 1f, it is thus not required to provide variable displacement pumps, such as swash plate pumps, and hence the first and second pumps 122, 222 are illustrated as fixed displacement pumps. Of course, it will be appreciated that variable displacement pumps could still be implemented as the first and second pumps.

(35) Another embodiment of the hydraulic system according to the present invention is shown in FIG. 2. The embodiment of FIG. 2 mostly corresponds to the embodiment of FIG. 1a and corresponding parts are labeled with identical reference signs. As can be derived from FIG. 2, this embodiment further comprises a third actuator 301 connected to a third pump 302 in a third closed loop circuit 303, and a third control valve 703.

(36) The third actuator 301 shown in FIG. 2 is again depicted as a linear actuator (particularly a hydraulic cylinder). The third actuator 301 may be used to move the dipper or arm of an excavator. The third actuator 301 is connected to a third pump 302 in a closed loop circuit 303. The third circuit 303 is substantially identical to the first circuit 103 and corresponding parts are labeled with reference numbers corresponding to the first circuit and increased by “200”. Similar to the first circuit 102, the second pump 202 can be connected to the third circuit 303 via a third control valve 703 of the valve arrangement 700. As such, the second pump 202 can also be used to assist the movement of the third actuator 301, if the third pump 302 is not sufficient under high speed conditions, i.e. to achieve a predetermined minimal cycle time for the third actuator.

(37) A typical duty cycle of the first and third actuators 101, 301 is shown in FIG. 6. In particular, FIG. 6 shows a duty cycle of an excavator performing a 180 degree loading process. In this example, the first actuator is a boom actuator, whereas the third actuator is an arm/dipper actuator of the excavator. The chart shows the flow requirements of the first and third actuators 101, 301 at different times during the 180 degree loading duty cycle. The solid line represents the flow provided to the first actuator 101, whereas the dashed line refers to the flow provided to the third actuator 301. It will be appreciated by the skilled person that different flow rates are required at different times of the duty cycle. In this particular example, the flow rates required by the first actuator (solid line in FIG. 6) shows two distinct peaks, while for most of the duty cycle, the flow requirements are relatively low. A very similar behavior is shown for the third actuator (dashed line in FIG. 6), which only comprises a single distinct peak.

(38) In particular, the chart of FIG. 6 shows a percentage of the peak flow required by the first and second actuators at any point during the 180 degree loading duty cycle. It should be understood that the 100% horizontal line refers to a peak flow that can be provided to the first or third actuators respectively by combining the fluid flows of the first and second or third and second pumps respectively. As such, the 100% relates to the peak flow rate required to achieve the minimal cycle time as defined hereinbefore.

(39) Evidently, the first and third actuators 101, 301 only require less than 50% of the peak flow rate during most of the duty cycle shown in FIG. 6. As mentioned previously, the first and third pumps 102, 302 can be sized such that their maximum output flow equals 25 to 75%, more preferably 45 to 55%, of the peak flow rate necessary to drive the first actuator at said minimal cycle time. If, as an example only, the maximum fluid output rate of the first and third pump 102, 302 equals 50% of the peak flow rate required to actuate the first and third actuators 101, 301 at a speed sufficient to obtain the minimal cycle time, then any fluid flow requirement below the 50% horizontal line shown in FIG. 6 can be provided by only using the first or third pump 102, 302.

(40) With particular reference to the graph of the first actuator (solid line), this means that during time intervals T1, T3, and T5 shown in FIG. 6, the first actuator can be supplied exclusively with fluid flow from the first pump 102, without the need of extra fluid flow from the second pump 202. Only during time intervals T2 and T4, that is when the first actuator is moved at higher speeds (i.e. higher flow rates and shorter cycle times are required), is assistance needed from the second pump 202. In other words, the fluid flow of the first pump 102 is assisted by fluid flow from the second pump 202 only during intervals T2 and T4. It should be understood that the duty cycle shown in FIG. 6 only refers to a typical 180 degree loading cycle, and thus other duty cycles may have substantially higher or lower flow requirements. However, it has generally been found that peak flow in the respective actuators is only rarely requested by the operator, and thus most of the duty cycle is performed at flow rates relating to 25 to 75% of the peak flow. Accordingly, sizing the first and third pumps to produce a maximum output flow, which relates to 25 to 75% of the peak flow was found to increase the energy efficiency of the system significantly.

(41) While the embodiment of FIG. 2 shows a motor 800 and spool valves 701, 702, 703 equivalent to FIG. 1a, it will be appreciated that the alternative valve arrangements and prime movers shown in FIGS. 1b to 1g could also be utilized in the hydraulic system shown in FIG. 2.

(42) Another embodiment of the present invention is shown in FIG. 3. FIG. 3 mostly corresponds to the embodiment shown in FIG. 2 and corresponding parts are labeled with identical reference signs.

(43) The hydraulic system shown in FIG. 3 further comprises a fourth actuator 401, which is connected to a fourth variable displacement pump 402 in a fourth closed loop circuit 403. The fourth actuator 401 may be a rotary actuator, such as a slew motor that can be used to slew the excavator about a vertical axis. The fourth pump 402 of this embodiment is a bi-directional variable displacement pump which is connected to first and second inlet ports of the fourth actuator 401 via first and second fluid lines 410, 411. As can be derived from FIG. 3, the fourth circuit 403 is not connected to any of the first, second and third circuits 103, 203, and 303. However, it is also feasible to arrange the second pump 202 of the second circuit 203 connectable to the fourth actuator 401 via valve arrangement 700.

(44) As depicted in another embodiment in FIG. 4, the first and third pumps 102, 302 can further be connectable to fifth and sixth actuators 501, 601. In more detail, the first pump 102 can be connected to inlet ports of the fifth actuator 501 via third and fourth fluid lines 510, 511. The connection between the first pump 102 and the fifth actuator 501 may be shut off by diverter valve 150, when the first actuator is in use. Similarly, the diverter valve 150 may be used to shut off the connection between the first pump 102 and the first actuator 101, when the first pump is used to drive the fifth actuator. The fifth actuator 501 may be a rotary actuator, which is used as a travel motor for one of the tracks of the excavator (i.e. left track). Accordingly, the first pump 102 is not only configured to supply the first actuator 101 with pressurized fluids, but can also supply the fifth actuator 501 sequentially to drive the left track of the excavator.

(45) When the first pump 102 is connected to the fifth actuator 501 via the diverter valve 150 (state not shown), the first actuator 101 is shut off from the first pump 102. Yet, it is still feasible to drive the first actuator 101 via the second pump 202 when the first pump 102 is used to drive the fifth actuator 501. As such, the system of FIG. 4 can be used to drive the fifth actuator 501 by means of pump 102 and, at the same time, activate the linear first actuator 101 by means of the second pump 202, which is connected to the first actuator 101 via the first control valve 701.

(46) The third pump 302 is, in turn, connectable to the sixth actuator 601 via third and fourth fluid lines 610, 611 and diverter valve 350. Accordingly, the third pump 302 can be used to sequentially provide the third actuator 301 and the sixth actuator 601 with pressurized fluid. The sixth actuator 601 is configured as a rotary actuator, such as a travel motor for driving the remaining track of the excavator (i.e. right track). Similar to the first actuator 101, the third actuator 301 can be actuated at the same time as the sixth actuator 601 by connecting the second pump 202 to the third actuator 301.

(47) In conclusion, when tracking the excavator via the fifth and sixth actuator 501, 601, the first and second pump 102, 302 of the embodiment shown in FIG. 4 are exclusively used for tracking purposes. If the first, second or third actuators 101, 201, 301 shall be used during tracking, the respective fluid flow is exclusively provided by second pump 202 via valve arrangement 700.

(48) The embodiment of FIG. 5 is very similar to the embodiment of FIG. 4. Corresponding parts in this embodiment have been labeled with the same reference numbers as in FIG. 4. As can be seen, the first circuit 110 according to this embodiment comprises first and second on/off valves 120, 121. The first on/off valve 120 selectively connects the first outlet port of the first pump 102 with the first chamber 104 of the first actuator 101 via first fluid line 110. The second on/off valve of the first circuit 103 connects the second outlet port of the first pump 102 with a second chamber 105 via the second fluid line 111 of the first circuit 103. The first pump 102 is further connected to the fifth actuator 501 via third and fourth on/off valves 520, 521. In particular, a first fluid port of the first pump 102 can be connected to the fifth actuator 501 via a third fluid line 510 if the third on/off valve 520 is in its open state. The second fluid port of pump 102 can be connected to the fifth actuator via fourth fluid line 511 if the fourth on/off valve 521 is opened. It will be appreciated, that the third and fourth on/off valves 520, 521 are preferably closed when the first and second on/off valves 120, 121 are opened and vice versa.

(49) Similar to the embodiment of FIG. 4, the first actuator 101 can be driven by the second pump 202 when the first pump 102 is used for tracking, i.e. actuating the fifth actuator 501. It will be appreciated that the first and second on/off valves 320/321 of the third circuit 303 function in an identical manner to the first and second on/off valves 120, 121 of the first circuit 103. The same is true for the third and fourth on/off valves 620, 621, which correspond to third and fourth on/off valves 520, 521. In other words, if the first and second on/off valves 320/321 of the third circuit 303 are closed, the third pump 302 can be used to drive the sixth actuator 601, by connecting the third pump 302 to the sixth actuator 601 via third and fourth on/off valves 620, 621.

(50) In the embodiment shown in FIGS. 1a, 1b, 1c, 1d, 1e, 2, 3, 4 and 5, the first, second, third and fourth pumps 102, 202, 302, 402 are driven by a common drive shaft 801 which connects each of the pumps 102, 202, 302, 402 to a single prime mover or drive motor 800, such as a combustion engine or electric motor. The drive motor 800 is also connected to a charge pump 902 via the common drive shaft 801. As mentioned previously in connection with FIGS. 1f and 1g, the invention is not limited to this particular drive arrangement. For example, any prime mover could be used to drive the pumps and the pumps maybe connected to a plurality of prime movers via a plurality of drive shafts, as shown in FIG. 1f. Alternatively, the pumps could be connected to a common drive shaft via variable ratio mechanisms as depicted in FIG. 1g.

(51) The charge pump 902 is configured to maintain the system pressure of the hydraulic system by supplying pressurized fluid from a hydraulic reservoir 901 to the fluid circuits. To this end, each of the fluid circuits comprises an anti-cavitation arrangement 130, 230, 330, 430, 530, 630 with check valves that allow the charge pump 902 to maintain a slightly elevated pressure. Each of the anti-cavitation systems 130, 230, 330, 430, 530 and 630 further comprises pressure relief valves to avoid high pressure damages during operation of the respective fluid circuits.

(52) The invention is not restricted to the particular embodiments described with reference to the embodiment shown in the attached illustration. In particular, the first, second, third and fourth pumps 102, 202, 302, 402 may be fixed or variable displacement, uni- or bi-directional and/or reversible/non-reversible pumps. Similarly the first, second, third, fourth, fifth and sixth actuators 101, 201, 301, 401, 501, 601 are not restricted to the particular applications shown but may be any type of actuator suitable for moving respective parts of a construction machine.

(53) The following numbered clauses, which are not the claims, refer to examples of the hydraulic system and construction machinery described hereinbefore.

(54) 1. A hydraulic system comprising:

(55) a first actuator;

(56) a first variable displacement pump fluidly connected to the first actuator via a first circuit and adapted to drive the first actuator;

(57) a second actuator;

(58) a second pump fluidly connectable to the second actuator via a second circuit and adapted to drive the second actuator,

(59) wherein the second pump is fluidly connectable to the first actuator via a first control valve, and wherein the second pump is fluidly connectable to the second actuator via a second control valve.

(60) 2. The hydraulic system of clause 1, wherein the first circuit is a closed loop circuit.

(61) 3. The hydraulic system of clause 1 or 2, wherein the second circuit is a closed loop circuit.

(62) 4. The hydraulic system of any of clauses 1 to 3, wherein the second pump is a variable displacement pump.

(63) 5. The hydraulic system of any of clauses 1 to 4, wherein the first pump is directly connected or connectable to the first actuator, and wherein the first control valve is a first proportional control valve adapted to variably restrict a fluid flow from the second pump provided to the first actuator.

(64) 6. The hydraulic system of clause 5, wherein the first proportional control valve is a directional, proportional spool valve, preferably a 4/3 spool valve.

(65) 7. The hydraulic system of clause 5, wherein the first proportional control valve is an independent metering valve.

(66) 8. The hydraulic system of clause 7, wherein the independent metering valve is connected to a first chamber of the first actuator via a first fluid line and to a second chamber of the first actuator via a second fluid line, wherein a first pressure sensor is provided in the first fluid line and a second pressure sensor is provided in the second fluid line.

(67) 9. The hydraulic system of clause 8, wherein the hydraulic system comprises a control unit adapted to receive pressure information from the first and second pressure sensors, and wherein the control unit is configured to control the independent metering valve to connect one of the first or second chamber to a fluid return line, depending on the pressure information.

(68) 10. The hydraulic system of any of clauses 1 to 9, wherein the second control valve is a second proportional control valve adapted to variably restrict the second fluid pressure of the second pump provided to the second actuator.

(69) 11. The hydraulic system of clause 10, wherein the second proportional control valve is a directional, proportional spool valve, preferably a 4/3 spool valve.

(70) 12. The hydraulic system of any of clauses 1 to 11, further comprising a third actuator and a third pump connectable to the third actuator via a third circuit and adapted to drive the third actuator.

(71) 13. The hydraulic system of clause 12, wherein the second pump is fluidly connectable to the third actuator via a third control valve.

(72) 14. The hydraulic system of clause 13, wherein the third pump is directly connected or connectable to the third actuator, and wherein the system comprises a third proportional control valve adapted to variably restrict a fluid flow from the second pump provided to the third actuator.

(73) 15. The hydraulic system of clause 14, wherein the third proportional control valve is a directional, proportional spool valve, preferably a 4/3 spool valve.

(74) 16. The hydraulic system of any of clauses 1 to 15, wherein the first pump is configured as a bidirectional variable displacement pump and the second pump is configured as a unidirectional pump, and wherein the first and second control valves are directional control valves.

(75) 17. The hydraulic system of clause 16, wherein the first pump comprises a first port connected or selectively connectable to a first chamber of the first actuator and a second port connected or selectively connectable to a second chamber of the first actuator.

(76) 18. The hydraulic system of clause 16, wherein the second pump comprises a first port selectively connectable to the first or second chamber of the first actuator via the first control valve and a second port selectively connectable to the first or second chamber of the first actuator via the first control valve.

(77) 19. The hydraulic system of clause 15 or 16, wherein the second pump is arranged to selectively act as a charge pump maintaining the hydraulic system at an elevated fluid pressure.

(78) 20. The hydraulic system of clause 19, wherein the second circuit is an open circuit.

(79) 21. The hydraulic system of clause 20, wherein the second pump comprises a first port selectively connectable to the first or second chamber of the first actuator via the first control valve and a second port connected to a hydraulic fluid reservoir.

(80) 22. The hydraulic system of clause 21, wherein the first port of the second pump is connected to the hydraulic fluid reservoir via a bypass-valve, preferably a variable pressure relief valve.

(81) 23. The hydraulic system of any of clauses 16 to 22, further comprising a third actuator and a third pump connectable to the third actuator via a third closed loop circuit and adapted to drive the third actuator.

(82) 24. The hydraulic system of clause 23, wherein the third pump comprises a first port connected or selectively connectable to a first chamber of the third actuator and a second port selectively connectable to a second chamber of the third actuator.

(83) 25. The hydraulic system of clause 24, wherein the second pump comprises a first port selectively connectable to the first or second chamber of the third actuator via a third control valve and a second port selectively connectable to the first or second chamber of the third actuator via the third control valve.

(84) 26. The hydraulic system of any of clauses 16 to 25, wherein the second pump comprises a first port selectively connectable to a first or second chamber of the second actuator via the second control valve and a second port selectively connectable to the first or second chamber of the second actuator via the second control valve.

(85) 27. The hydraulic system of any of clauses 16 to 26, wherein the first and second pumps are connected to a single prime mover via a common drive shaft.

(86) 28. The hydraulic system of any of clauses 23 to 25 and clause 27, wherein the third pump is connected to the prime mover via the common drive shaft.

(87) 29. The hydraulic system of clause 27 or 28, wherein the prime mover is a single speed motor or an internal combustion engine.

(88) 30. The hydraulic system of any of clauses 1 to 29, wherein the first pump is sized such that a maximum output flow rate of the first pump equals 25% to 75%, preferably 40% to 60%, more preferably 45% to 55%, of a peak flow rate necessary to drive the first actuator at a predetermined minimal cycle time.

(89) 31. The hydraulic system of clause 30, wherein the hydraulic system comprises a controller connected to the first control valve and adapted to control the first control valve to selectively connect the second pump to the first circuit, if the maximum fluid output flow of the first pump is not sufficient to move the first actuator at a speed necessary to obtain the minimal cycle time for the first actuator.

(90) 32. The hydraulic system of Clause 30 or 31, wherein the first control valve is a proportional control valve.

(91) 33. The hydraulic system of Clause 32, wherein the proportional control valve is a directional spool valve.

(92) 34. The hydraulic system of any of clauses 30 to 33, further comprising a third actuator and a third pump connectable to the third actuator via a third circuit and adapted to drive the third actuator.

(93) 35. The hydraulic system of clause 34, wherein the third pump is sized such that a maximum output flow rate of the third pump equals 25% to 75%, preferably 40% to 60%, more preferably 45% to 55%, of a peak flow rate necessary to drive the third actuator at a predetermined minimal cycle time.

(94) 36. The hydraulic system of clause 35, wherein the second pump is fluidly connectable to the third actuator via a third control valve.

(95) 37. The hydraulic system of clause 36, wherein the hydraulic system comprises a controller connected to the third control valve and adapted to control the third control valve to selectively connect the second pump to the third circuit, if the maximum fluid output flow of the third pump is not sufficient to move the third actuator at a speed necessary to obtain the minimal cycle time for the third actuator.

(96) 38. The hydraulic system of any of clauses 1 to 37, wherein the first pump is sized to exhibit a maximum output flow which is 50% to 150%, preferably 75% to 125%, more preferably 95% to 105%, of a maximum output flow of the second pump.

(97) 39. The hydraulic system of any of clauses 1 to 38, wherein the third pump is sized to exhibit a maximum output flow which is 50% to 150%, preferably 75% to 125%, more preferably 95% to 105%, of the maximum output flow of the second pump.

(98) 40. The hydraulic system of one of clauses 1 to 39, wherein the first actuator is a linear actuator.

(99) 41. The hydraulic system of clause 40, wherein the first actuator is a hydraulic cylinder for displacement of an excavator boom.

(100) 42. The hydraulic system of one of clauses 1 to 41, wherein the second actuator is a linear actuator.

(101) 43. The hydraulic system of clause 42, wherein the second actuator is a hydraulic cylinder for displacement of an excavator bucket.

(102) 44. The hydraulic system of one of clauses 1 to 43, wherein the third actuator is a linear actuator.

(103) 45. The hydraulic system of clause 44, wherein the third actuator is a hydraulic cylinder for displacement of an excavator arm.

(104) 46. The hydraulic system of any of clauses 1 to 45, further comprising a fourth actuator and a fourth pump connectable to the fourth actuator via a fourth circuit and adapted to drive the fourth actuator.

(105) 47. The hydraulic system of clause 46, wherein the fourth actuator is a rotary actuator.

(106) 48. The hydraulic system of clauses 46 or 47, wherein the fourth actuator is a hydraulic motor for slewing.

(107) 49. The hydraulic system of any of clauses 1 to 48, wherein the system further comprises a fifth actuator, wherein the first pump is selectively connectable to the fifth actuator.

(108) 50. The hydraulic system of any of clauses 1 to 49, wherein the system further comprises a sixth actuator, wherein the third pump is selectively connectable to the sixth actuator.

(109) 51. A construction machine, comprising the hydraulic system of any of clauses 1 to 50.