Electro-hydraulic work vehicle with energy recovery

Abstract

Electro-hydraulic work vehicle system having a hydraulic lift mechanism comprising a first electric machine, a first hydraulic machine operatively connected to the first electric machine. A load holding valve is switchable between a first position in which it retains pressurized fluid in the hydraulic lift mechanism and a second position in which it enables pressurized fluid to flow between the first hydraulic machine and the hydraulic lift mechanism. A pressure relief valve is switchable between a first, initial position in which it prevents pressurized fluid from flowing from the first hydraulic machine to the hydraulic tank and a second position in which it enables pressurized fluid to flow from the first hydraulic machine to the hydraulic tank. An hydraulic energy storage is hydraulically connected between the first hydraulic machine and the relief valve, wherein in decent mode of hydraulic lift mechanism, the hydraulic system is configured to supply pressurized fluid to the hydraulic energy storage and when the load-holding valve is at its second position and the pressure relief valve is at its first position, wherein pressurized fluid from the hydraulic lift mechanism is capable of driving the first hydraulic machine which can drive the first electric machine to create electricity that can be stored in the electrical energy storage, and/or is capable of charging the hydraulic energy storage.

Claims

1. An electro-hydraulic work vehicle system having a hydraulic lift mechanism, the electro-hydraulic system comprising: a first electric motor/generator; a first hydraulic pump/motor operatively connected to the first electric motor/generator, wherein the first hydraulic pump/motor is configured to provide pressurized fluid to the hydraulic lift mechanism when driven by the first electric motor/generator; a load-holding valve hydraulically connected between the first hydraulic pump/motor and the hydraulic lift mechanism, wherein the load holding valve is switchable between a first position in which the load holding valve retains pressurized fluid in the hydraulic lift mechanism and a second position in which the load holding valve enables pressurized fluid to flow between the first hydraulic pump/motor and the hydraulic lift mechanism; a pressure relief valve hydraulically connected between the first hydraulic pump/motor and a hydraulic tank, wherein the pressure relief valve is switchable between a first, initial position in which the pressure relief valve prevents pressurized fluid from flowing from the first hydraulic pump/motor to the hydraulic tank and a second position in which the pressure relief valve enables pressurized fluid to flow from the first hydraulic pump/motor to the hydraulic tank; and an electric energy storage electrically connected to the first electric motor/generator; wherein an hydraulic energy storage is hydraulically connected between the first hydraulic pump/motor and the relief valve, wherein in descent mode of hydraulic lift mechanism, the electro-hydraulic system is configured to supply pressurized fluid to the hydraulic energy storage when the load-holding valve is at its second position and the pressure relief valve is at its first position, wherein pressurized fluid from the hydraulic lift mechanism is capable of driving the first hydraulic pump/motor which can drive the first electric motor/generator to create electricity that can be stored in the electrical energy storage, and/or charging the hydraulic energy storage; and wherein the pressurized fluid that drives the first hydraulic pump/motor for driving the first electric motor/generator is, at least at times, the pressurized fluid that is supplied to the hydraulic energy storage.

2. The electro-hydraulic work vehicle system according to claim 1, further comprising an electronic control unit to control a descent rate of a load by means of adjusting the displacement of the first hydraulic pump/motor.

3. The electro-hydraulic work vehicle system according to claim 2, wherein the first electric motor/generator, in descent mode of hydraulic lift mechanism, is capable to drive the first hydraulic pump/motor, in order to raise the descent rate of the load and/or to enhance the pressure in the hydraulic energy storage when the pressure relief valve is in its first position, or to discharge hydraulic fluid to the tank when the pressure relief valve is in its second position.

4. The electro-hydraulic system according to claim 2, further comprising: a second electric motor/generator; a second hydraulic pump/motor operatively connected to the second electric motor/generator; a lowering control valve hydraulically connected between the load-holding valve and the second hydraulic pump/motor, wherein the lowering control valve is switchable between a first position in which the lowering control valve prevents pressurized fluid from flowing from the load-holding valve to the second hydraulic pump/motor and a second position in which the lowering control valve enables pressurized fluid to flow from the load-holding valve to the second hydraulic pump/motor; and a pre-charge valve hydraulically connected between the second hydraulic pump/motor and the first hydraulic pump/motor, wherein the pre-charge valve is switchable between a first position in which the pre-charge valve prevents pressurized fluid from flowing from the second hydraulic pump/motor to the first hydraulic pump/motor and a second position in which the pre-charge valve enables pressurized fluid to flow from the second hydraulic pump/motor to the first hydraulic pump/motor; wherein the electro-hydraulic system comprises a first flow path; wherein the electro-hydraulic system provides for pressurized fluid from the hydraulic lift mechanism to the hydraulic energy storage via the first flow path and the first hydraulic pump/motor when the load holding valve is at its second position, the pressure relief valve is at its first position, and the pre-charge valve is at its first position; and wherein the second hydraulic pump/motor is configured to provide pressurized fluid to the first hydraulic pump/motor when driven by the second electric motor/generator when the pre-charge valve is in the second position.

5. The electro-hydraulic work vehicle system according to claim 2, wherein the electronic control unit is capable to switch the pressure relief valve into the second position when a predetermined pressure level is reached in the hydraulic energy storage.

6. The electro-hydraulic work vehicle system according to claim 5, wherein the first electric motor/generator, in descent mode of hydraulic lift mechanism, is capable to drive the first hydraulic pump/motor, in order to raise the descent rate of the load and/or to enhance the pressure in the hydraulic energy storage when the pressure relief valve is in its first position, or to discharge hydraulic fluid to the tank when the pressure relief valve is in its second position.

7. The electro-hydraulic system according to claim 5, further comprising: a second electric motor/generator; a second hydraulic pump/motor operatively connected to the second electric motor/generator; a lowering control valve hydraulically connected between the load-holding valve and the second hydraulic pump/motor, wherein the lowering control valve is switchable between a first position in which the lowering control valve prevents pressurized fluid from flowing from the load-holding valve to the second hydraulic pump/motor and a second position in which the lowering control valve enables pressurized fluid to flow from the load-holding valve to the second hydraulic pump/motor; and a pre-charge valve hydraulically connected between the second hydraulic pump/motor and the first hydraulic pump/motor, wherein the pre-charge valve is switchable between a first position in which the pre-charge valve prevents pressurized fluid from flowing from the second hydraulic pump/motor to the first hydraulic pump/motor and a second position in which the pre-charge valve enables pressurized fluid to flow from the second hydraulic pump/motor to the first hydraulic pump/motor; wherein the electro-hydraulic system comprises a first flow path; wherein the electro-hydraulic system provides for pressurized fluid from the hydraulic lift mechanism to the hydraulic energy storage via the first flow path and the first hydraulic pump/motor when the load holding valve is at its second position, the pressure relief valve is at its first position, and the pre-charge valve is at its first position; and wherein the second hydraulic pump/motor is configured to provide pressurized fluid to the first hydraulic pump/motor when driven by the second electric motor/generator when the pre-charge valve is in the second position.

8. The electro-hydraulic work vehicle system according to claim 1, wherein the first electric motor/generator, in descent mode of hydraulic lift mechanism, is capable to drive the first hydraulic pump/motor, in order to raise a descent rate of the load and/or to enhance the pressure in the hydraulic energy storage when the pressure relief valve is in its first position, or to discharge hydraulic fluid to the tank when the pressure relief valve is in its second position.

9. The electro-hydraulic system according to claim 1, further comprising: a second electric motor/generator; a second hydraulic pump/motor operatively connected to the second electric motor/generator; a lowering control valve hydraulically connected between the load-holding valve and the second hydraulic pump/motor, wherein the lowering control valve is switchable between a first position in which the lowering control valve prevents pressurized fluid from flowing from the load-holding valve to the second hydraulic pump/motor and a second position in which the lowering control valve enables pressurized fluid to flow from the load-holding valve to the second hydraulic pump/motor; and a pre-charge valve hydraulically connected between the second hydraulic pump/motor and the first hydraulic pump/motor, wherein the pre-charge valve is switchable between a first position in which the pre-charge valve prevents pressurized fluid from flowing from the second hydraulic pump/motor to the first hydraulic pump/motor and a second position in which the pre-charge valve enables pressurized fluid to flow from the second hydraulic pump/motor to the first hydraulic pump/motor; wherein the electro-hydraulic system comprises a first flow path; wherein the electro-hydraulic system provides for pressurized fluid from the hydraulic lift mechanism to the hydraulic energy storage via the first flow path and the first hydraulic pump/motor when the load holding valve is at its second position, the pressure relief valve is at its first position, and the pre-charge valve is at its first position; and wherein the second hydraulic pump/motor is configured to provide pressurized fluid to the first hydraulic pump/motor when driven by the second electric motor/generator when the pre-charge valve is in the second position.

10. The electro-hydraulic system according to claim 9, wherein switching the pre-charge valve to its second position enables pressurized fluid flow towards the second electric motor/generator to create electricity that is stored in the electrical energy storage, and wherein hydraulic fluid is dumped to the hydraulic tank.

11. The electro-hydraulic system according to claim 9, wherein none of the load-holding valve, lowering control valve, and the pre-charge valve are a proportional control valve.

12. The electro-hydraulic system according to claim 9, wherein the electro-hydraulic system is configured to provide pressurized fluid to the hydraulic lift mechanism when there is insufficient pressurized fluid in the hydraulic energy storage to pre-charge the first pump/motor by switching the load-holding valve into its second position, switching the lowering control valve into its first position, switching the pre-charge valve into its second position, operating the second electric motor/generator to drive the second hydraulic pump/motor to supply pressurized fluid to the first hydraulic pump/motor, and operating the first electric motor/generator to drive the first hydraulic pump/motor to provide pressurized fluid to the hydraulic lift mechanism.

13. The electro-hydraulic system according to claim 9, wherein the hydraulic system is configured to provide pressurized fluid to the hydraulic lift mechanism when there is sufficient pressurized fluid in the hydraulic energy storage to pre-charge the first hydraulic pump/motor by the load-holding valve at its second position, switching the lowering control valve at its first position, switching the pre-charge valve at its first position, and operating the first electric motor/generator to drive the first hydraulic pump/motor to provide pressurized fluid to the hydraulic lift mechanism.

14. The electro-hydraulic system according to claim 9, wherein secondary hydraulic functions can be connected to the electro-hydraulic system between the lowering control valve and the second hydraulic pump/motor and can either be driven by the second hydraulic pump/motor with the lowering control valve at its first position or by pressurized fluid from the hydraulic lift mechanism when the load is lowered while the lowering control valve is at its second position.

15. The electro-hydraulic system according to claim 1, further comprising: a sensor configured to determine a load parameter for a load carried by the hydraulic lift mechanism; and a controller operatively connected to the sensor, the controller configured to receive the load parameter from the sensor and programmed to determine a load lowering quality based on the load parameter; wherein the controller is configured to raise the descent rate of a load from a current descent rate of the load when the controller determines that a load lowering quality indicates one-half, or less, of maximum lowering performance.

16. The electro-hydraulic system according to claim 15, wherein the descent rate of the load is controlled by the controller modifying the rate at which electricity is generated by the first electric motor/generator.

17. The electro-hydraulic system according to claim 15, wherein the controller is configured to lower the descent rate of a load from the current descent rate of the load when the controller determines that a load lowering quality indicates greater than one-half of maximum lowering performance.

18. The electro-hydraulic system according to claim 15, wherein the descent rate of the load is controlled by (a) the controller modifying the rate at which electricity is generated by the first electric motor/generator, and/or (b) the controller modifying the rate at which electricity is generated by a second electric motor/generator.

19. The electro-hydraulic system according to claim 1, wherein the hydraulic lift mechanism is a double acting hydraulic cylinder.

20. The electro-hydraulic system according to claim 1, wherein secondary hydraulic functions can be powered directly by the hydraulic energy storage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic circuit diagram showing a first embodiment of the electro-hydraulic system for a work vehicle according to the invention;

(2) FIG. 2 is a schematic circuit diagram showing another embodiment of the electro-hydraulic system according to the invention;

(3) FIG. 3 is a schematic circuit diagram showing an further embodiment of the electro-hydraulic system according to the invention;

(4) FIG. 4 is a schematic circuit diagram showing an further embodiment of the electro-hydraulic system according to the invention; and

(5) FIG. 5 is a schematic circuit diagram showing an further embodiment of the electro-hydraulic system according to the invention.

DETAILED DESCRIPTION

(6) In FIG. 1 a schematic circuit diagram a first embodiment of the electro-hydraulic system 100 according to the invention is shown. The present schematic illustrates that an exemplary hydraulic lift mechanism 11 can be supplied with hydraulic fluid under high pressure by means of a first hydraulic pump/motor 2 in order to lift a load. In the hydraulic line 16 connecting the first hydraulic pump/motor 2 with a hydraulic lift mechanism 11a load holding valve 9 is located, which is shown in a first position in which the load holding valve 9 retains the pressure in the hydraulic lift mechanism 11. The load holding valve 9 is switchable into a second position in which pressurized hydraulic fluid is enabled to flow between the first hydraulic pump/motor 2 and the hydraulic lift mechanism 11. The first hydraulic pump/motor 2 can be charged/pre-charged by a hydraulic energy storage 5 arranged at the low pressure side of the first hydraulic pump/motor 2. Thus, when a load has to be lifted by the hydraulic lift mechanism 11, only the delta pressure between the pressure necessary at the high pressure side of the first hydraulic pump/motor 2 to lift the load and the pressure in the hydraulic energy storage 5 has to be provided by a first electric motor/generator 1 operatively connected preferably vie a clutch 40 to the first hydraulic pump/motor 2. Thus the power consumption of primary pump 2 will be reduced compared to systems without hydraulic energy storage 5 at the low pressure side. This power consumption is compensated by an electric energy storage 14 powering the first motor/generator 1. Lifting speed can be controlled thereby by first hydraulic pump/motor 2 and first electric motor/generator 1, which has the function of controlling pump displacement and rotational speed respectively, or can be controlled by a electronic control unit 15 which is capable to control also the electric energy storage 14 and command the position of load holding valve 9. The inventive electro-hydraulic system 100 further comprises at least one sensor 25 to determine load parameters for a load carried by the hydraulic lift mechanism (primary hydraulic function) and/or sensing rotational speeds or descent/lifting speed of this load. The control unit 15 can further be configured to receive the load parameter from the sensor 25 and is programmed to determine a load lowering quality based on the load parameters.

(7) In descent mode pressurized hydraulic fluid flows from the hydraulic lift mechanism 11 via the load holding valve 9 to the first hydraulic pump/motor 2 operating the same as hydraulic motor. Hence with the mechanical output of the first hydraulic motor 2 first electric motor/generator 1 can be driven which generates electric energy that can be stored in the electric energy storage 14. The hydraulic output of the first hydraulic pump/motor 2 can either be stored in the hydraulic energy storage 5 or guided via a pressure relief valve 8 to a tank 6.

(8) In FIG. 2 a schematic circuit diagram a further embodiment of the electro-hydraulic system 200 according to the invention is shown. Elements that are the same between the embodiments illustrated in the FIGS. 1 to 5 have the same reference numbers. Here, a pre-charge line 17 is branched-off of the hydraulic connection line connecting the hydraulic energy storage 5 with the first hydraulic pump/motor 2, and leads to a second hydraulic pump/motor 3. In this pre-charge line 17 a pre-charge valve 4, e.g. of the check valve type, is arranged, which opens when the second hydraulic pump/motor 3 is energized to supply pressurized hydraulic fluid to the first hydraulic pump/motor 2. In case the second hydraulic pump/motor 3 has to supply pressurized hydraulic fluid, the same will be driven by a second electric motor/generator 12 being energized by the electric energy storage 14 and operatively also preferably coupled to the second hydraulic pump/motor 3 via a clutch 41. Electronic control unit 15 is also capable of controlling the second electric motor/generator 12 as well as the second hydraulic pump/motor 3.

(9) In case of insufficient pressure present at the hydraulic energy storage 5, the second hydraulic pump/motor 3 can charge the hydraulic energy storage 5 via the pre-charge line 17 and/or provide the first hydraulic pump/motor 2 with pressurized hydraulic fluid. In addition, pressure sensors are integrated in the system to define, e.g., the states of hydraulic energy storage 5 and the load pressure, which will determine in lifting mode the power consumption of the first hydraulic pump/motor 2. Aside from supporting the lifting, the hydraulic energy storage 5 and the secondary pump 3 are able to provide energy into secondary/auxiliary functions 20.

(10) During lowering, depending on the state of vehicle, energy recovery can be achieved by either running the first electric motor/generator 1 as a generator storing electric energy in the electric energy storage 14, charging the hydraulic energy storage 5, or a combination of both of approaches. When hydraulic energy storage 5 is filled up, the lowering energy will drive the first hydraulic pump/motor 2 and motor 1 to generate electricity; the returning flow will be dumped into hydraulic tank 6 when pressure relief valve 8 is in its second position, i.e. in the open position. As long as pressure relief valve 8 is in its first (closed) position the returning flow will be guided towards the hydraulic energy storage 5. Thereby an over-pressure limitation of the hydraulic energy storage 5 can be realized with pressure relief valve 8. Further functions of second hydraulic pump/motor 3 are, compensating leakage in the circuit and maintaining emergency lifting flow. The size and pre-charge pressure of the hydraulic energy storage 5 will determine energy recovery distribution to electric energy storage 14 or (and) to hydraulic energy storage 5.

(11) FIG. 3 shows a further embodiment of the electro-hydraulic system 300 according to the invention, in which, compared to the embodiment of FIG. 2, a high pressure line 21 branching-off of the pre-charge line 17 between the second hydraulic pump/motor 3 and the pre-charge valve 4, here in form of a switching valve, and leads to the high pressure side of first hydraulic pump/motor 2 connecting the hydraulic line 16 between the first hydraulic pump/motor 3 and load holding valve 9. This high pressure line 21 provides for two flow paths for (high) pressurized fluid from the hydraulic lift mechanism 11 and the load-holding valve 9. A first flow path leads via the first hydraulic pump/motor 2 to the hydraulic energy storage 5 or to the tank 6, and a second flow path leads via the lowering control valve 10 to the second hydraulic pump/motor 3 and ongoing to the tank 6.

(12) For excess lowering flow more than the capacity of the hydraulic energy storage 5 the pre-charge valve 4, for instance controlled by the electronic control unit 15, will close and the excessive lowering flow will go via the second hydraulic pump/motor 3, running the second electric motor/generator 12 as a generator and recovering electric energy, e.g., in the electric energy storage 14.

(13) When the hydraulic energy storage 5 is filled up or excess lowering flow is guided to the second hydraulic pump/motor 3, the lowering energy of the load is capable of driving the first hydraulic pump/motor 2 and the first electric motor/generator 1 as well, in order to generate electricity for storing in the electric energy storage 14, exemplified in the above description and illustrated in the embodiment of FIG. 1; the returning flow will be dumped into the hydraulic tank 6 when the hydraulic energy storage 5 is filled up.

(14) Another possibility is to recover energy during the lowering of the load via the second flow path and the lowering control valve 10. Here, the first hydraulic pump/motor 2 can be driven via the first flow path and, additionally, the second hydraulic pump/motor 3 can be driven via the second flow path, when the pre-charge valve 4 in its first position closing pre-charge line 17. Additionally, returning flow from the first hydraulic pump/motor 2 can charge the hydraulic energy storage 5 or can be dumped into the tank 6, when, e.g., the pressure level in the returning flow is too low to charge the hydraulic energy storage 5.

(15) As can be seen from FIGS. 2 and 3 secondary/auxiliary hydraulic functions 20, such as a fan drive, horizontal fork movement, inclination adjustment, or the like can be driven directly by second hydraulic pump/motor 3 and hydraulic energy storage 5. Thereby the secondary/auxiliary hydraulic functions 20 are preferably connected to the hydraulic energy storage 5 or to the pre-charge line 17 branching-off between the second hydraulic pump/motor 3 and the pre-charge valve 4.

(16) FIG. 4 is a further embodiment of the electro-hydraulic system 400 according to the invention which differs from the embodiment of FIG. 1 in that a directional charge control valve 7 is arranged on the low pressure side of the first hydraulic pump/motor 2. In the first, shown position, during lifting process the first hydraulic pump/motor 2 will be charged by hydraulic energy storage 5. Thus, the delta pressure and delta power consumption of primary pump 2 still needed when lifting the load will be reduced, as the pressure in the hydraulic energy storage 5 supports the lifting. When it comes to lifting speed, it is controlled by hydraulic pump 2 and electric motor 1, which have the function of pump displacement and rotational speed control respectively, which in turn can be controlled for instance by electronic control unit 15.

(17) If the energy supply from hydraulic energy storage 5 is not sufficient, first electric motor/generator 1 will supply more driving energy into the first hydraulic pump/motor 2 as and when it is required. During lowering process, first hydraulic pump/motor 2 is turned as hydraulic motor to drive first electric motor/generator 1 to generate electricity, feeding back the electricity to electric energy storage 14. Simultaneously, returning hydraulic fluid flow will be charge to the hydraulic energy storage 5 until the charging pressure of hydraulic energy storage 5 is equal to the load pressure, or reaches its maximum allowable load pressure. At this moment the charge control valve 7 will be switched into the second position, in which it guides returning hydraulic fluid flow to the tank 6. In case the hydraulic pressure in the returning hydraulic fluid flow downstream the first hydraulic pump/motor 2 is lower than the pressure in the hydraulic energy storage 5, however lower than the nominal load pressure, the first electric motor/generator 1 first electric motor/generator 1 will also consume energy to drive hydraulic pump 2 to charge the hydraulic energy storage 5 until load pressure and, if desired, to maintain the desired lowering speed constant, as lowering speed would decrease with increasing pressure in the hydraulic energy storage 5.

(18) FIG. 5 shows a further embodiment of the inventive electro-hydraulic system 500, in which the first hydraulic pump/motor 2 and the second hydraulic pump/motor 3 are operatively connected via clutches 40 and 41 to the first electric motor/generator 1 electrically connected to electric energy storage 14. Wherein during lifting process the second hydraulic pump/motor 3 will be driven by the hydraulic energy storage 5. If the energy supply from hydraulic energy storage 5 is not sufficient, electric motor 1 will supply more energy into the first hydraulic pump/motor 2 and/or second hydraulic pump/motor 3 to satisfy the desired lifting demands. Thereby, like in all other embodiment according to FIGS. 1 to 4, the first hydraulic pumps/motors 2 and 3 are mechanically coupled by means of commonly known couplings 40 to the first electric motor/generator 1 in order to be driveable independently from each other and the first electric motor/generator 1.

(19) Besides, energy regeneration is achieved by opening directional control valve 24, closing directional control valve 23, and oil is pushed from rod chamber directly flowing into piston chamber by which a higher lifting speed is gained. During the lowering process the hydraulic pump/motor 2 is turned as hydraulic motor to drive electric motor/generator 1 to generate electricity feeding back to electric energy storage 14. Hydraulic pump 3 will charge hydraulic energy storage 5 by load pressure. Until the charging pressure of hydraulic energy storage 5 is equal to the load pressure, hydraulic pump/motor 2 partially keep generating energy back to electric energy storage 14 and some part is to utilize charging the hydraulic energy storage 5.

(20) Return flow after crossing hydraulic pump/motor 2 is able to flow back to rod chamber of cylinders 22 or 11. The amount of return flow will be re-used for energy regeneration in the next lifting cycle. When the hydraulic energy storage 5 is filled up, the lowering energy will drive hydraulic pump/motor 2 and 3 to generate electricity; the returning flow will be dumped into hydraulic tank 6.

(21) Beneficial effects of the above systems are described as follows: 1. In the hydraulic system of the electro hydraulic system according to the invention the peak energy recovery is realised as a natural part of the inventive circuit. 2. In the hydraulic system of the electro hydraulic machine the circuit essentially show a low number of valves and in effect this improves efficiency over conventional hydraulic systems.

(22) The embodiment of FIG. 5 shows the possibility of using two hydraulic machines and only one electric machine for the supply of a plurality of hydraulic work functions with pressurized hydraulic fluid. This embodiment according to the invention shows also how hydraulic energy can be recovered not only by charging the hydraulic energy storage 5 connected to the first hydraulic pump/motor 2 and charging the electric energy storage 14 by means of driving/operating the first electric motor/generator 1, since, when a load at one hydraulic work functions 11 or 22 is lowered, how pressurized hydraulic fluid can be conducted by the help of direction control valves 23 and 24 to support the lowering speed and/or the lifting or lowering of another subsequent hydraulic work function. If, for instance, the load at lift mechanism 11 is to be lowered the correspondent load holding valve 9 is switched into its second position and pressurized hydraulic fluid flow is enabled to flow towards the first and second hydraulic pumps/motors 2 and 3. If these remain in neutral position, hydraulic flow over these hydraulic machines is prevented. However opening direction control valve 24 enables hydraulic flow from the piston chamber to the rod chamber of hydraulic lift mechanism 11 and thereby enhancing the lowering speed of the load at hydraulic lift mechanism 11.

(23) As can be seen in FIG. 5 as well, the energy recovering concepts shown in the embodiments of FIGS. 1 to 4 are also implemented in the embodiment of FIG. 5. In particular the charging embodiment for the hydraulic energy storage 5 according to FIG. 4, which a person with skills in the relevant art easily converts to the embodiments shown in the FIGS. 1 to 4.

(24) The foregoing is a detailed description of illustrative embodiments of the invention using specific terms and expressions. Various modifications and additions can be made without departing from the spirit and scope thereof. Therefore, the invention is not limited by the above terms and expressions, and the invention is not limited to the exact construction and operation shown and described. On the contrary, many variations and embodiments are possible and fall within the scope of the invention.