Abstract
A lifting device for an industrial truck comprises a lift frame with a moveably guided load carrier and at least one moveably guided mast stage. A free lift cylinder is configured to actuate the load carrier and at least one mast lift cylinder is configured to actuate the at least one mast stage. A hydraulic assembly supplies the free lift cylinder and the at least one mast lift cylinder with hydraulic fluid and further comprises at least one delivery valve and at least one recirculating valve.
Claims
1. A lifting device for an industrial truck comprising: a lift frame comprising a load carrier and at least one mast stage, wherein the load carrier and the at least one mast stage are moveably guided; a free lift cylinder configured to actuate the load carrier; at least one mast lift cylinder configured to actuate the at least one mast stage; a hydraulic assembly comprising a hydraulic tank and configured to provide the free lift cylinder and the at least one mast lift cylinder with hydraulic fluid from the hydraulic tank; only one delivery valve connected to the hydraulic assembly and to at least one of the free lift cylinder and the at least one mast lift cylinder, wherein the only one delivery valve is configured to supply hydraulic fluid from the hydraulic tank to at least one of the free lift cylinder and the mast lift cylinder; at least one recirculation valve connected to the hydraulic assembly and to at least one of the free lift cylinder and the at least one mast lift cylinder, wherein the at least one recirculation valve is configured to recirculate hydraulic fluid from at least one of the free lift cylinder and the mast lift cylinder to the hydraulic tank; a supply line connected to the hydraulic assembly; a first connecting line connected to the free lift cylinder; and a second connecting line connected to the at least one mast cylinder, wherein the only one delivery valve is positioned in one of the first connecting line and the second connecting line, wherein a first pressure is required to actuate the free lift cylinder and a second pressure is required to actuate the mast lift cylinder, wherein the first pressure is lower than the second pressure when the only one delivery valve is positioned in the first connecting line, and wherein the first pressure is higher than the second pressure when the only one delivery valve is positioned in the second connecting line, and wherein the only one delivery valve is configured to move between a blocked position and a flow-through position, and wherein the first pressure and the second pressure are adjusted through the movement of the only one delivery valve between the blocked position and the flow-through position.
2. The lifting device according to claim 1, wherein the only one delivery valve further comprises a proportional valve.
3. The lifting device according to claim 2, wherein the proportional valve comprises one of a 3/2 and a 2/2 proportional valve.
4. The lifting device of claim 1, wherein the at least one recirculation valve further comprises a proportional valve.
5. The lifting device according to claim 1, wherein the at least one recirculation valve connects to a first return line and a second return line to the hydraulic assembly independently of the at least one delivery valve.
6. The lifting device according to claim 5, wherein the first return line and the second return line are merged into a common third return line.
7. The lifting device according to claim 5, wherein the at least one recirculation valve comprises a recycling 3/2-way proportional valve, and wherein the first return line and the second return line are merged into a common third return line.
8. The lifting device according to claim 5, further comprising a 4/2-way proportional valve configured to selectively separate the first return line and the second return line from the at least one recirculation valve, wherein the at least one recirculation valve is a recirculating 3/2-way proportional valve.
9. The lifting device according to claim 5, further comprising a 4/2 way proportional valve configured to selectively connect the first return line and the second return line to the at least one recirculation valve, wherein the at least one recirculation valve is a recycling 3/2-way proportional valve.
10. The lifting device according to claim 5, wherein the first connecting line comprises a first check valve disposed between the at least one delivery valve and a branch connection of the first return line from the first connecting line and the second connecting line comprises a second check valve disposed between the delivery valve and a branch connection of the second return line from the second connecting line.
11. The lifting device according to claim 1, wherein the hydraulic assembly further comprises a hydraulic pump that is configured to conduct the hydraulic fluid out of the hydraulic tank via the supply line.
12. The lifting device according to claim 1, wherein at least one of the supply line comprises an isolation valve configured to separate hydraulic flow from the hydraulic assembly to the at least one delivery valve.
13. The lifting device according to claim 1, further comprising a control unit configured to actuate at least one of the only one delivery valve and the at least one recirculation valve.
14. The lifting device according to claim 13, wherein at least one of the free lift cylinder and the mast lift cylinder further comprises a sensor configured to communicate with the control unit to determine a lifting height, a lifting speed, and a lowering speed of the load carrier.
15. The lifting device according to claim 14, wherein the control unit is configured to activate the only one delivery valve and the at least one recirculation valve in order to control the lifting speed and lowering speed of the load carrier.
16. The lifting device according to claim 14, wherein the control unit is configured to, activate the only one delivery valve and wherein the only one delivery valve is configured to adjust a target lifting speed, activate the recirculation valve to set a target lowering speed of at least one of the load carrier and the at least one mast stage, calculate a control deviation between the target lifting speed and actual lifting speed detected by the sensor and between the target lowering speed and actual lowering speed detected by the sensor, and activate at least one of the only one delivery valve and the at least one recirculation valve to control a supply of hydraulic fluid to at least one of the free lift cylinder and mast lift cylinder based on the calculated control deviation.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The invention is explained in below according to the drawings. The following is shown:
(2) FIG. 1 illustrates an embodiment of a lifting device;
(3) FIG. 2 illustrates another embodiment of the lifting device;
(4) FIG. 3 illustrates an embodiment of the lifting device;
(5) FIG. 4 illustrates an embodiment of a control flow diagram for the control of the lifting speed;
(6) FIG. 5 illustrates an embodiment of a control flow diagram for the control of the lowering speed;
(7) FIG. 6 illustrates another embodiment of the lifting device;
(8) FIG. 7 illustrates another embodiment of the lifting device;
(9) FIG. 8 illustrates another embodiment of the lifting device;
(10) FIG. 9 illustrates another embodiment of the lifting device;
(11) FIG. 10 illustrates another embodiment of the lifting device; and
(12) FIG. 11 illustrates a further embodiment of the lifting device.
DETAILED DESCRIPTION OF THE INVENTION
(13) FIG. 1 shows an embodiment of the lifting device. The lifting device has a schematically represented lift frame 10 with a load carrier 12 and a mast stage 14 that are both moveably guided. As shown, the load carrier 12 comprises a lift fork. The free lift cylinder 13 is configured to actuate the load carrier 12 while the mast lift cylinder 15 is configured to actuate the mast stage 14. The load carrier 12 can be raised and/or lowered in free lift mode by activating the free lift cylinder 13, and the load carrier 12 can be raised and/or lowered in mast lift mode by activating the mast lift cylinder 15. When the mast lift cylinder 15 is actuated, the load carrier 12 is moved together with the free lift cylinder 13. The free lift cylinder 13 comprises a schematically represented piston rod, wherein a sensor 17 is arranged on the piston rod or in the vicinity of the piston rod. The mast lift cylinder 15 also has a corresponding piston rod, on or near which a sensor 18 is arranged.
(14) A hydraulic tank 16 and a hydraulic pump 28 together form a hydraulic assembly.
(15) The hydraulic tank 16 provides hydraulic fluid to supply the free lift cylinder 13 and the mast lift cylinder 15 by means of the hydraulic pump 28. A delivery valve 20 connects the hydraulic tank 16 to the free lift cylinder 13 and to the mast lift cylinder 15. Said delivery valve 20 is a 3/2-way proportional valve having three (3) line connectors and two (2) valve positions. The hydraulic tank 16 is connected to a connector of the delivery valve 20 via a supply line 24, while the free lift cylinder 13 is connected via a first connecting line 25 and the mast lift cylinder 15 is connected via a second connecting line 26 to the rest of the connectors in the delivery valve 20. The two possible valve positions of the delivery valve 20 are identified with reference signs 20a and 20b, wherein valve position 20a connects the supply line 24 to connecting line 26 and thus to the mast lift cylinder 15, while valve position 20b connects the supply line 24 to first connecting line 25 and then on to the free lift cylinder 13. Since the delivery valve 20 is a proportional valve, any desired intermediate positions are possible between valve positions 20a and 20b, and so the supply line 24 can also be connected to first and second connecting lines 25 and 26 at the same time. The delivery valve 20 is electrically actuated by a control unit or control device 70. A first and a second check valve 40, 42 are provided in the first and second connecting lines 25, 26, respectively, and they prevent the back-flow of hydraulic fluid from the cylinders 13, 15 to the delivery valve 20.
(16) Furthermore, two recirculation valves 30, 30 can be seen in FIG. 1, wherein the first recirculation valve 30 is connected via a first return line 31 to the free lift cylinder 13 and to the hydraulic tank 16, while the second recirculation valve 30 is connected via a second return line 32 to the mast lift cylinder 15 and to the hydraulic tank 16. The first return line 31 and the second return line 32 are merged by a common third return line 33. The first return line 31 branches off from the first connecting line 25 upstream of check valve 40, while the second return line 32 branches off from the second connecting line 26 upstream of check valve 42. The two recirculation valves 30, 30 are 2/2-way proportional valves that have two connectors and two valve positions. In a first valve position 30a, the first recirculation valve 30 permits the back-flow of hydraulic fluid out of the free lift cylinder 13 into the hydraulic tank 16. In a second valve position 30b, the first recirculation valve 30 blocks the back-flow of hydraulic fluid out of the free lift cylinder 13. The second delivery valve 30 is designed similarly and thus has a flow-through position 30a and a blocked position 30b. Since the recirculation valves 30, 30 are proportional valves, any desired intermediate positions are also possible here. In this way, it is possible to control the volume flow of the back-flow from the free lift cylinder 13 or the mast lift cylinder 15 by means of the valve position. The recirculation valves 30, 30 are also electrically actuated by means of the control device 70.
(17) The embodiment of lifting device shown in FIG. 2 further comprises isolation valve 60, which can control and interrupt the hydraulic flow from the hydraulic tank 16 to the delivery valve 20. The isolation valve 60 may be a 2/2-way proportional valve with a flow-through position and a blocked position. However, the isolation valve 60 can also be configured as a switching valve. When the isolation valve 60 is a proportional valve it can also assume any desired intermediate positions to control the flow of hydraulic fluid. The isolation valve 60 can choke or interrupt the supply of hydraulic fluid to the free lift cylinder 13 and/or the mast lift cylinder 15 in order to make a part of the volume flow of the hydraulic fluid available for further functions of the industrial truck via a branch line 62.
(18) FIG. 3 shows a further embodiment of the lifting device. This embodiment differs from the embodiment shown in FIG. 1 by the use of valves other than recirculation valves. The branching to supply the hydraulic fluid to the cylinders is the same as in FIG. 1. In the embodiment in FIG. 3, a 3/2-way proportional valve is provided as the first circulation valve 30, by which the first return line 31 and the second return line 32 are merged into a common third return line 33. Additionally, a 4/2-way proportional valve 50 is provided as a recirculation valve, by which the first return line 31 and the second return line 32 can be separated from the 3/2-way proportional valve 30 or connected to it.
(19) Referring to FIGS. 4 and 5, in order to lift the load carrier 12 (FIGS. 1-3, 6-11), hydraulic fluid is conducted out of the hydraulic tank 16 (FIGS. 1-3, 6-11) by the hydraulic pump 28 (FIGS. 1-3, 6-9) through the supply line 24 (FIGS. 1-3, 6-11) and the delivery valve 20 (FIGS. 1-3) in valve position 20b (FIG. 1) as well as through the first connecting line 25 (FIGS. 1-3, 6-11) and into the free lift cylinder 13. The free lift is carried out in this way. The position of the piston rod of the free lift cylinder 13 in this case is monitored by a position sensor and is transmitted to the control unit 70. The mast position is monitored in this way. Shortly before the free lift cylinder 13 reaches its end position, the delivery valve 20 (FIGS. 1-4) is gradually switched into valve position 20a (FIG. 1) by the control unit 70. Thus the volume flow to the free lift cylinder 13 is reduced and the volume flow to the mast lift cylinder 15 is initiated. In this way, the piston rod of the free lift cylinder 13 makes contact slowly and gently. Hydraulic fluid is now conducted out of the hydraulic tank 16 (FIGS. 1-3, 6-11) by means of the hydraulic pump 28 (FIGS. 1-3, 6-9) via the supply line 24 (FIGS. 1-3, 6-11) through the delivery valve 20 (FIGS. 1-4) and into the second connecting line 26 (FIGS. 1-3, 6-11) and thus into the mast lift cylinder 15. This results in the extension of the piston rod of the mast lift cylinder 15 and thus to the start of the mast lift. In mast lift mode, the load carrier 12 (FIGS. 1-3, 6-11) is raised along with the free lift cylinder 13. By appropriately positioning the delivery valve 20 (FIGS. 1-4), however, it is likewise possible to carry out the mast lift first and then the free lift. It is also possible to carry out both at the same time.
(20) Using the delivery valve 20 (FIGS. 1-4), the target speed provided by the control unit 70 for the movement of the load carrier 12 (and thus the load) can be translated into a volume flow of the hydraulic fluid to the free lift cylinder and/or mast lift cylinder. As depicted in FIG. 4, the person operating a control unit 70 can enter a preset speed , for example. In accordance with this preset target speed , the control unit 70 controls the valve position of the delivery valve 20 by means of a control current i1. The delivery valve 20 then divides the volume flow of hydraulic fluid coming from the hydraulic pump 28 (FIGS. 1-3, 6-9) into two (2) volume flows q.sub.m and q.sub.f, wherein volume flow q.sub.m moves the mast lift cylinder 15 and volume flow q.sub.f moves the free lift cylinder 13. The desired target lifting speed is controlled by the pump speed of the hydraulic pump 28 (FIGS. 1-3, 6-9), while the delivery valve 20 distributed the hydraulic fluid to the two cylinders 13, 15. The sensors 17, 18 provided on the free lift cylinder 13 and/or the mast lift cylinder 15 additionally detect the actual lifting speed v.sub.f of the load carrier and/or the actual lifting speed v.sub.m of the mast stage 14. This can be carried out, for example, by measuring the movement speed of the piston rod of the respective valve relative to the respective piston housing. The actual speeds v.sub.f, v.sub.m can deviate from the preset target speed =v.sub.f+v.sub.m as a result of disturbances, such as different loads, oil viscosities or pump efficiency as well as mechanical losses. For this reason, the control unit 70 calculates this deviation of the actual speed v.sub.f of the free lift and the actual speed v.sub.m of the mast lift into the control variable and adapts the valve stream i.sub.1 and thus the valve position of the delivery valve 20. Therefore, the actual speeds are continuously corrected to the target speed. This leads to a significantly more precise control of the movement of the load.
(21) To lower a load located on the load carrier 12 (FIGS. 1-3, 6-11), hydraulic fluid can be conducted via the recirculation valves 30, 30 from the free lift cylinder 13, from the mast lift cylinder 15 or from both back to the hydraulic tank 16 (FIGS. 1-3, 6-11). For lowering in free lift mode, only the first recirculation valve 30 (FIGS. 1, 2, 5-11) is actuated; in other words, it is switched to valve position 30a (FIGS. 1, 6-9). For lowering in mast lift mode, only recirculation valve 30 (FIGS. 1, 6-9) is actuated; in other words, it is switched to valve position 30a (FIGS. 1, 6-9). Hydraulic fluid streaming out of the free lift cylinder 13 flows via the first connecting line 25 (FIGS. 1-3, 6-11) through the branch connection into the first return line 31 (FIGS. 1-3, 6-11) and via the first recirculation valve 30 (FIGS. 1,2, 5-11) into the hydraulic tank 16. Hydraulic fluid streaming out of the mast lift cylinder 15 flows via the connecting line 26 (FIGS. 1-3, 6-11) through the branch connection via the second return line 32 (FIGS. 1-3, 6-11) through the second recirculation valve 30 into the hydraulic tank 16. As can be seen in FIG. 5, the control unit 70 provides a preset lowering speed as the electrical control currents i4, i5 to the two recirculation valves 30, 30. The valve position of the first recirculation valve 30 is controlled by the electric control current i4, and so a volume flow of hydraulic fluid q.sub.m reaches the mast lift cylinder 15. Accordingly, the valve position of the second recirculation valve 30 is controlled by the electric control current i5, and so a volume flow q.sub.r of hydraulic fluid reaches the free lift cylinder 13. The actual lowering speeds v.sub.f of the free lift and v.sub.m of the mast lift are calculated by the sensors 17, 18 (FIGS. 1, 6-11) and transmitted to the control unit 70. The control unit 70 calculates the control deviation of the actual lowering speeds v.sub.f, v.sub.m into control variable and computes from it the necessary adaptation of the electrical control currents i4, i5. As with the lifting process, disturbances can also be eliminated and the control of the lowering process is performed with greater precision.
(22) Moreover, the lifting height (i.e. the mast position of the load carrier 12 (FIGS. 1-3, 6-11)) is used during the lowing process, as well, to control the lowering speed in particular ranges. As with the lifting process, this makes it possible to reduce the lowering speed in the end ranges of the free lift cylinder 13 and/or the mast lift cylinder 15 so that dampened contact is achieved during lowering. The electrical currents i4, i5 are calculated using the control loop depicted in FIG. 5 such that the lowering speed of the load also remains constant in the transitional range between the mast lift and the free lift. During both the lifting process and the lowering process, the free lift cylinder travels at a speed where v.sub.f<v.sub.m in its lift stop. This results in a very gentle transition between free lift and mast lift. The lifting height of the load carrier and/or of the mast stage is entered into the control unit 70 as the mast position, as is shown in FIGS. 4 and 5. A corresponding control can also occur for the mast lift cylinder 15. If, for instance, a lowing process is initiated from the mast lift, then the second recirculation valve 30 is moved toward valve position 30a (FIGS. 1, 6-9) until the desired lowering speed is achieved. Shortly before the mast lift cylinder 15 is fully retracted, the volume flow of the mast lift cylinder 15 is gradually reduced in that the second recirculation valve 30 is gradually moved into the blocked position 30b (FIGS. 1, 6-9). While the recirculation valve 30 is being closed, the first recirculation valve 30 is opened, i.e. it is moved into valve position 30a (FIGS. 1, 6-9), and the lowering process is thereby ensured by the free lift. As was mentioned above, the two recirculation valves 30, 30 are controlled in such a way that the lowering speed remains constant despite the changing valve positions.
(23) However, it is also entirely possible to achieve the aforementioned functions of the lifting device according to the description without a 3/2-way proportional valve. Referring to the embodiment shown in FIG. 6, a 2/2-way proportional delivery valve 100 is arranged in the first connecting line 25 leading to the free lift cylinder 13 and is configured to act as the delivery valve instead of the 3/2-way proportional valve. The 2/2-way proportional delivery valve 100 has a blocked position 100a and a flow-through position 100b, wherein the 2/2-way proportional delivery valve 100 can also assume any desired intermediate positions. The supply line 24 splits into the first and second connecting lines 25,26 upstream of the hydraulic pump, wherein connecting line 26 does not have a delivery valve. Required here is that the pressure p.sub.1 necessary to actuate the free lift cylinder 13 is always lower than the pressure p.sub.2 necessary to actuate the mast lift cylinder 15. Thus p.sub.1<p.sub.2 must be true. This can be achieved in particular by selecting the effective piston surface of the free lift cylinder 13 to be larger than the effective piston surface of the mast lift cylinder 15.
(24) To lift the load carrier 12, hydraulic fluid is conducted out of the hydraulic tank 16 by the hydraulic pump 28 through the supply line 24 and the 2/2-way proportional delivery valve 100 in valve position 100b as well as through the first connecting line 25 and into the free lift cylinder 13. Moreover, hydraulic fluid is also conducted through connecting line 26 to the mast lift cylinder 15. As long as the prevailing system pressure p is lower than the pressure p.sub.2 required to actuate the mast lift cylinder 15 (i.e., as long as p<p.sub.2) initially only the free lift cylinder 13 is moved and thus the free lift is carried out. When the free lift cylinder 13 reaches its lift stop, the system pressure p rises until p.sub.2 is reached. Then the mast lift begins with the actuation of the mast lift cylinder 15. Thus the free lift is carried out first and subsequently the mast lift.
(25) The lifting sequence and the lifting speed of the mast stage and load carrier 12 in this embodiment can also be controlled in accordance with the control method explained above. So the position of the piston rod of the free lift cylinder 13 can be monitored by a position sensor and transmitted to the control unit 70. Shortly before the free lift cylinder 13 reaches its end position, the delivery valve 2/2-way proportional 100 is gradually switched into blocked valve position 100a by the control unit 70. The volume flow to the free lift cylinder 13 is thus reduced. In this way, the piston rod of the free lift cylinder 13 makes contact gently at a lower speed. At the same time, the system pressure p in the supply line 24 and in the connecting line 26 increases, which leads to an actuation of the mast lift cylinder 15 as soon as pp.sub.2. Thus the volume flow coming from the hydraulic pump 28 is gradually conducted to the mast lift cylinder 15. In particular, the lifting movement of the load carrier 12 remains at least approximately constant even during this rerouting process between the valve positions. At the end of the rerouting process, the 2/2-way proportional delivery valve 100 is entirely in its blocked position 100a and the free lift cylinder 13 is fully extended.
(26) The control of the lifting sequence and lifting speed can take place in accordance with the control method explained above. For instance, using the delivery valve 100, the target speed provided by the control unit 70 for the movement of the load carrier 12 can be translated into a volume flow of the hydraulic fluid to the free lift cylinder and/or mast lift cylinder. As depicted in FIG. 4, the person operating a control unit 70 can enter a preset speed , for example. In accordance with this preset target speed , the control unit 70 controls the valve position of the 2/2-way proportional delivery valve 100 by means of a control current i1. In this embodiment, as well, the 2/2-way proportional delivery valve 100 divides the volume flow of hydraulic fluid coming from the hydraulic pump 28 (FIGS. 1-3, 6-9) into the two volume flows q.sub.m and q.sub.f. Although a volume flow q.sub.m is always flowing to the mast lift cylinder 15, the volume flow q.sub.m does not have an effect as long as the pressure generated by this volume flow in the mast lift cylinder 15 does not meet the condition pp.sub.2. Accordingly, the lifting sequence and lifting speeds of the cylinders 13, 15 is controlled here by the 2/2-way proportional delivery valve 100 and by the different area ratios of the pistons of the free lift cylinder 13 and mast lift cylinder 15. The desired target lifting speed can also be controlled here by the pump speed of the hydraulic pump 28.
(27) Still referring to FIG. 6 and as was described above, the actual lifting speeds of the cylinders 13, 15 can be controlled by changing the valve position of the 2/2-way proportional delivery valve 100 using the control unit 70. The lowering process takes place via the recirculation valves 30, 30. In particular, the two recirculation valves can also be activated completely independently of each other here, and the movements of the lifting stages (i.e., load carrier and mast stage) take place completely independently of each other. Additionally, a gentle transition between the lifting stages can be achieved during lowering.
(28) Referring to the embodiment shown in FIG. 7, a proportional pre-charge valve is used as the delivery valve 110 instead of a 2/2-way proportional valve. Similar to the embodiment in FIG. 6, the delivery valve 110 is completely open during the free lift. During the transition from free lift to mast lift, the delivery valve 110 is activated and the pressure in the connecting line 26 that leads to the mast lift cylinder 15 is thereby gradually increased.
(29) Referring to the embodiment shown in FIG. 8, a proportional pre-charge valve with a choke position 120a and a flow-through position 120b is used as the delivery valve 120 instead of a 2/2-way proportional valve with a blocked position and a flow-through position. The lifting is carried out essentially as has already been explained with regard to FIG. 6. However, the delivery valve 120 cannot be completely closed, but it instead still permits a flow-through to the free lift cylinder 13 even in the choke position 120a. Said cylinder thus moves slowly in it stop without requiring any additional measures, such as the aforementioned position sensor for measuring the piston position. In this way, as well, it is possible to reroute from the free lift to the mast lift in a controlled manner.
(30) Referring to the embodiment illustrated in FIG. 9, a 2/2-way proportional valve is the delivery valve 130 and is disposed in connecting line 26, which leads to the mast lift cylinder 15, instead of in the first connecting line 25. The 2/2-way proportional delivery valve 130 has a blocked position 130a, which acts in the direction of the connecting line 26 and is implemented by a check valve, and a flow-through position 130b. However, the same delivery valve as in FIG. 6 could also be provided here. In addition, a requirement of this lifting device is that the pressure p.sub.1 necessary to actuate the free lift cylinder 13 is always higher than the pressure p.sub.2 necessary to actuate the mast lift cylinder 15. The condition p.sub.1>p.sub.2 must be fulfilled. This can be achieved in particular in that the effective piston surface of the free lift cylinder 13 is smaller than the effective piston surface of the mast lift cylinder 15.
(31) At the beginning of the lifting process, hydraulic fluid is conducted out of the hydraulic tank 16 by the hydraulic pump 28 through the supply line 24 and the first connecting line 25 to the free lift cylinder 13. The delivery valve 130 is in the blocked position 130a in this instance. The system pressure p is increased until the pressure p.sub.1 required to actuate the free lift is reached. Before the free lift cylinder 13 reaches its end position, the delivery valve 130 is gradually opened, i.e. gradually switched into flow-through valve position 130b. As a result, the system pressure p falls to the level of the mast lift cylinder 15. The lifting speed is likewise reduced. Additionally, the volume flow to the mast lift cylinder 15 is released, and so it is actuated. It is therefore possible in this embodiment, as well, that the free lift is carried out first and then the mast lift.
(32) Referring to the embodiment of the lifting device shown in FIG. 10, a 2/2 way valve 140 is arranged in the lift branch upstream of the division of the supply line 24 into the first and second connecting lines 25, 26. Two recirculation valves 150, 152, are configured as 2/2-way proportional valves arranged in return lines 35, 36 leading to the hydraulic pump 28, by virtue of two check valves 44, 46. The two check valves 44, 46 are each arranged in one of the return lines 35, 36 and the hydraulic pump 28 can also function regeneratively.
(33) The lifting process takes place here as with the lifting device according to FIG. 6, wherein the supply line 24 must first be unblocked by the 2/2-way valve 140. The 2/2-way valve 140 assumes its flow-through position 140b here. The check valves 44, 46 can prevent the flow of hydraulic fluid to the valves 150, 152.
(34) During the lowering process, however, the embodiment of FIG. 10 includes the possibility of driving the hydraulic pump 28, which in this case functions generatively, with the hydraulic fluid that is flowing back to the hydraulic tank 16. To do so, the hydraulic fluid is not conducted via the recirculation valves 30, 30 to the hydraulic tank 16 from the free lift cylinder 13 and/or from the mast lift cylinder 15 during the lowering process. Instead, the hydraulic fluid is recirculated from the free lift cylinder 13 to the hydraulic pump 28 via the return lines 31, 35 through the recirculation valve 150, which is now in the flow-through position 150a, and through the check valve 44. The recirculation of hydraulic fluid from the mast lift cylinder 15 to the pump 28 similarly occurs via the return lines 32, 36 through the recirculation valve 152, which is now in the flow-through position 152a, and through the check valve 46. The hydraulic pump 28 is driven by the recirculated fluid. If the hydraulic pump 28 is not operating generatively, the recirculation valves 150, 152 are switched to their blocked positions 150b, 152b, and the recirculation occurs via the recirculation valves 30, 30 directly to the hydraulic tank 16 in the manner already described.
(35) Referring to the embodiment of the lifting device illustrated in FIG. 11, a 2/2-way proportional valve 130 is provided in connecting line 26, which leads to the mast lift cylinder 15, instead of in the first connecting line 25. This corresponds to the embodiment illustrated in FIG. 9 with the additional features of the embodiment of FIG. 10, which aid in the generative operation. Accordingly, generative operation of the hydraulic pump 28 is possible in this embodiment in a similar manner as was described above.
