Industrial truck, hydraulic system for an industrial truck and method for operating a hydraulic system

Abstract

Industrial truck (2) with a lifting mast (4) with at least one mast lifting stage (41, 42) driven by at least one mast lifting cylinder (12) and with a free lifting stage driven by at least one free lifting cylinder (8), with which a load-handling means (6) can be moved along the lifting mast (4), comprising a hydraulic system (10) for supplying the at least one mast lifting cylinder (12) and the at least one free lifting cylinder (8) with a hydraulic fluid (14), wherein the hydraulic system (10) is designed for at least temporarily simultaneous actuation of the at least one mast lifting cylinder (12) and the at least one free lifting cylinder (8) in load lifting operation and / or in load lowering operation, wherein the hydraulic system (10) comprises a first hydraulic return line (181),which runs between the at least one free lift cylinder (8) and a reservoir (16) for the hydraulic fluid (14), and the hydraulic system (10) further comprises a second hydraulic return line (182) which runs between the at least one mast lift cylinder (12) and the reservoir (16), wherein a first lowering valve (21) is integrated into the first return line (181) and a second lowering valve (22) is integrated into the second return line (182), characterized by a controller (24) which is designed to open the lowering valves (21, 22) simultaneously during load lowering operation when lowering the load-carrying means (6).

Description

[0001] The invention relates to an industrial truck having a lifting mast with at least one mast lift stage driven by at least one mast lift cylinder and having a free lift stage driven by at least one free lift cylinder, with which a load handling device can be moved along the lifting mast. Furthermore, the invention relates to a hydraulic system for an industrial truck having a lifting mast with at least one mast lift stage and having a free lift stage, with which a load handling device can be moved along the lifting mast, wherein the hydraulic system comprises at least one mast lift cylinder for driving the at least one mast lift stage and at least one free lift cylinder for driving the at least one free lift stage.The invention also relates to a method for operating a hydraulic system of an industrial truck having a lifting mast with at least one mast lifting stage and with a free lift stage, with which a load handling means can be moved along the lifting mast, wherein the hydraulic system comprises at least one mast lifting cylinder for driving the at least one mast lifting stage and at least one free lift cylinder for driving the free lift stage.

[0002] Industrial trucks, such as forklifts, often have a mast with one or more mast lift stages, which are hydraulically operated by one or more mast lift cylinders. The mast comprises a stationary mast that is permanently attached to the vehicle and usually two extension masts, a center mast and an inner mast, which are extended by the mast lift cylinder. A free lift cylinder moves a free lift stage, with which a load handling device, such as a fork, can be moved along the inner mast of the mast. The free lift stage moves the load handling device along this mast stage and allows the operator of the industrial truck to adjust the height of the load handling device without extending the mast, which would otherwise change the overall height of the industrial truck.

[0003] Common forklift trucks have a common hydraulic lowering branch for the mast lift and the free lift, which includes an integrated lowering valve. To ensure that the mast lift stages and the free lift retract in the desired sequence when the load handling device is lowered, the individual mast lift stages and the free lift have hydraulic cylinders with different cross-sections. If several mast stages are extended, the mast stage with the smallest total effective hydraulic cross-section retracts first during load lowering of the industrial truck. This is because this hydraulic cylinder receives the greatest hydraulic pressure, so it retracts first when the hydraulic pressure drops. This is usually the topmost mast lift stage. As the hydraulic pressure continues to decrease, the mast stages are lowered serially, i.e., one after the other. Finally, after the mast stages have been fully retracted, the free lift stage retracts and lowers the load handling device.

[0004] It is an object of the invention to provide an industrial truck with a lifting mast with at least one mast lifting stage and with a free lift stage, a hydraulic system for such an industrial truck and a method for operating a hydraulic system of an industrial truck, wherein retraction of the at least one mast lifting stage and the free lift stage in load lowering operation should be possible more quickly than previously.

[0005] The object is achieved by an industrial truck with a lifting mast with at least one mast lifting stage driven by at least one mast lifting cylinder and with a free lifting stage driven by at least one free lifting cylinder, with which a load handling device can be moved along the lifting mast, wherein the industrial truck is further developed by a hydraulic system for supplying the at least one mast lifting cylinder and the at least one free lifting cylinder with a hydraulic fluid, wherein the hydraulic system is set up for at least temporarily simultaneous actuation of the at least one mast lifting cylinder and the at least one free lifting cylinder in load lifting operation and / or in load lowering operation.

[0006] Advantageously, in such an industrial truck, the mast lift stages and the load handling device are retracted and / or extended simultaneously. Synchronous, i.e., at least temporarily simultaneous, actuation of at least one mast lift cylinder and at least one free lift cylinder can reduce the lowering time of the industrial truck. This increases the handling capacity of the industrial truck. This applies if the industrial truck accesses heights that can only be reached with the mast at least partially extended. During lifting operation, a jerky transition between free lift and mast lift can be avoided.

[0007] An industrial truck or industrial conveyor is a means of transport for the transportation of goods, which is mostly used within a company and on the ground, for example a forklift.

[0008] Ideally, according to aspects of the invention, the industrial truck, when lowering the load handling device in load lowering mode from maximum height, is faster by the time period typically required to retract the load handling device with the free lift stage. The extension process, i.e., raising the load handling device to the maximum height, is more homogeneous and smoother than before. This improves the handling of the industrial truck.

[0009] According to an advantageous embodiment, the industrial truck is further developed in that the hydraulic system for relieving the at least one mast lift cylinder and the at least one free lift cylinder in load lowering operation comprises separate hydraulic return lines.

[0010] The separate hydraulic return lines allow the lowering of the load handling device to be accelerated by simultaneously retracting the mast stage(s) and the free lift stage. The separate hydraulic return lines allow the mast lift cylinder and the free lift cylinder to be relieved simultaneously. This also applies if the mast lift cylinder and the free lift cylinder, or the mast lift cylinders of the individual mast lift stages, have different cross-sections, so that they extend serially, i.e., one after the other, during load lifting. If only a single hydraulic line were used to relieve the lift cylinders, the retraction of the mast lift stage and the free lift would necessarily occur in the reverse order to their extension. This is now advantageously no longer the case.

[0011] The industrial truck is particularly developed in that the hydraulic system comprises a first hydraulic return line which runs between the at least one free lift cylinder and a reservoir for the hydraulic fluid and the hydraulic system further comprises a second hydraulic return line which runs between the at least one mast lift cylinder and the reservoir, wherein a first lowering valve is integrated into the first return line and a second lowering valve is integrated into the second return line.

[0012] In particular, it is provided that the first lowering valve and the second lowering valve can be controlled separately, i.e., independently of each other. This makes it possible to lower the mast lifting stage(s) and / or the load-handling device separately and independently of each other during load lowering operation.

[0013] According to a further advantageous embodiment, the industrial truck is further developed with a control system configured to simultaneously open the lowering valves during load lowering operation when lowering the load-handling device. By simultaneously opening both lowering valves, the mast lift stage(s) and the load-handling device are lowered simultaneously, at least at the beginning of the process. The load-handling device is lowered particularly quickly.

[0014] It is further provided in particular that the lowering valves are proportional valves and the control is further configured to control or regulate a first volume flow through the first lowering valve and a second volume flow through the second lowering valve such that in load lowering operation when lowering the load-carrying means, the at least one mast lifting stage and the load-carrying means reach a lower end position at least approximately simultaneously.

[0015] This design allows for consistent and comparable performance of the industrial truck when lowering the load lifting device from different lifting heights. This simplifies the truck's operation. Furthermore, a consistent lowering speed of the load handling device can be achieved throughout the entire lowering process.

[0016] To provide appropriate control, a position sensor is provided, for example, at the mast lift and the free lift, so that the respective speed at which the mast lift and the free lift are extended or retracted can be determined. Based on this measured value, a control system is possible that can achieve a smooth and homogeneous lowering process, in particular so that the mast lift stage and the load handling device reach their lower end position at least approximately simultaneously.

[0017] The industrial truck is further developed in particular in that the hydraulic system comprises a hydraulic pump which is integrated into a hydraulic supply line and is designed in load lifting operation to supply the at least one mast lift cylinder and the at least one free lift cylinder with pressurised hydraulic fluid, wherein the hydraulic supply line between the hydraulic pump and the lifting cylinders forks into a first and a second supply branch and the first supply branch runs to the free lift cylinder and the second supply branch runs to the mast lift cylinder, wherein a lift valve designed as a proportional valve is integrated into the hydraulic supply line, with which a ratio between the volume flows in the first and second supply branch can be changed.

[0018] According to this design, not only is it possible to retract the lift mast or the mast lift stages and the free lift stage at least temporarily simultaneously during load lowering operation, but also to extend the lift mast and the free lift stage at least temporarily simultaneously during load lifting operation. This not only further improves handling performance, but also the handling and operability of the industrial truck.

[0019] According to a further advantageous embodiment, the free-lift cylinder has a first cross-section and the lifting cylinder has a second cross-section, the first cross-section being larger than the second cross-section, and the lift valve is integrated into the first supply branch. The arrangement of the lift valve in the first supply branch is advantageous because the lift valve can be used to reduce the effective hydraulic flow cross-section of the first supply branch. Thus, by appropriately selecting or adjusting the effective hydraulic flow cross-section of this supply branch, the free-lift cylinder and the lifting cylinder can be extended simultaneously.

[0020] The industrial truck is further developed with a controller configured to control the lift valve in such a way that, during load lifting operation, the free lift cylinder and the mast lift cylinder are extended simultaneously, at least temporarily. In particular, the controller is configured to control the lift valve in such a way that a smooth transition between free lift and mast lift is achieved.

[0021] The control or control unit is in particular a part of the operating control or operating control unit of the industrial vehicle.

[0022] The object is also achieved by a hydraulic system for an industrial truck with a lifting mast with at least one mast lifting stage and with a free lift stage, with which a load-handling device can be moved along the lifting mast, wherein the hydraulic system comprises at least one mast lifting cylinder for driving the at least one mast lifting stage and at least one free lift cylinder for driving the at least one free lift stage, wherein the hydraulic system is further developed in that it is set up to at least temporarily simultaneously supply the at least one mast lifting cylinder and the at least one free lift cylinder with a hydraulic fluid in load lifting operation and / or in load lowering operation.

[0023] The hydraulic system offers the same or similar advantages as those already mentioned with regard to the industrial truck itself. The hydraulic system makes it possible to provide an industrial truck in which the load-handling device can be lowered more quickly than before. Thus, using the hydraulic system according to aspects of the invention, it is possible to equip an industrial truck so that it achieves higher handling performance. The hydraulic system is particularly interesting and advantageous with regard to the possible upgrading or conversion of existing industrial trucks.

[0024] According to an advantageous embodiment, it is provided that the hydraulic system is further developed in that it comprises a first hydraulic return line which runs between the at least one free lift cylinder and a reservoir for the hydraulic fluid and the hydraulic system further comprises a second separate hydraulic return line which runs between the at least one mast lift cylinder and the reservoir, wherein a first lowering valve is integrated into the first return line and a second lowering valve is integrated into the second return line.

[0025] Furthermore, it is particularly provided that the hydraulic system further comprises a hydraulic pump which is integrated into a hydraulic supply line and is set up in load lifting operation to supply the at least one mast lifting cylinder and the at least one free lifting cylinder with pressurised hydraulic fluid, wherein the hydraulic supply line between the hydraulic pump and the lifting cylinders forks into a first and a second supply branch and the first supply branch runs to the free lifting cylinder and the second supply branch runs to the mast lifting cylinder, wherein a lift valve designed as a proportional valve is integrated into the hydraulic supply line, with which a ratio between the volume flows in the first and second supply branch can be changed.

[0026] The object is also achieved by a method for operating a hydraulic system of an industrial truck with a lifting mast with at least one mast lifting stage and with a free lift stage, with which a load-handling device can be moved along the lifting mast, wherein the hydraulic system comprises at least one mast lifting cylinder for driving the at least one mast lifting stage and at least one free lift cylinder for driving the free lift stage, wherein the hydraulic system is further developed in that it is operated in such a way that in load lifting operation and / or in load lowering operation the at least one mast lifting cylinder and the at least one free lift cylinder are actuated at least temporarily simultaneously.

[0027] The same or similar advantages apply to the method for operating the hydraulic system as those already mentioned with regard to the industrial truck or the hydraulic system.

[0028] According to an advantageous embodiment, the method is further developed in that the hydraulic system comprises a first hydraulic return line which runs between the at least one free lift cylinder and a reservoir for the hydraulic fluid and the hydraulic system further comprises a separate second hydraulic return line which runs between the at least one mast lift cylinder and the reservoir, wherein a first lowering valve is integrated into the first return line and a second lowering valve is integrated into the second return line, and wherein in load lowering operation when lowering the load-carrying means, the first lowering valve and the second lowering valve are opened simultaneously.

[0029] In a further embodiment, it is provided that the lowering valves are proportional valves and a first volume flow through the first lowering valve and a second volume flow through the second lowering valve are controlled or regulated such that when the load-carrying means is lowered, the at least one mast lifting stage and the load-carrying means reach a lower end position at least approximately simultaneously.

[0030] Furthermore, the method is advantageously further developed in that the hydraulic system comprises a hydraulic pump which is integrated into a hydraulic supply line and with which the at least one mast lift cylinder and the at least one free lift cylinder are supplied with pressurised hydraulic fluid during load lifting operation, wherein the hydraulic supply line between the hydraulic pump and the lifting cylinders forks into a first and a second supply branch and the first supply branch runs to the free lift cylinder and the second supply branch runs to the mast lift cylinder, wherein a lift valve designed as a proportional valve is integrated into the hydraulic supply line, with which a ratio between a volume flow in the first and second supply branch can be changed, wherein the free lift cylinder has a first cross section and the mast lift cylinder has a second cross section,wherein the first cross-section is larger than the second cross-section and the lift valve is integrated into the first supply branch and wherein the lift valve is controlled such that the free lift cylinder and the mast lift cylinder are extended at least temporarily simultaneously.

[0031] According to a further embodiment, it is provided that, depending on an operating mode of the industrial truck and / or depending on a preselected lifting height of the load-handling device, the lifting valve is brought into a first position for sequential extension of the free lift cylinder and the mast lift cylinder or into a second position for at least temporarily simultaneous extension.

[0032] The corresponding operating mode can be selected manually, for example. However, it is also possible to select a corresponding operating mode in which the mast lift stage and the free lift stage are moved simultaneously, for example, if a lifting height is manually entered that lies outside the range achievable exclusively with the free lift.

[0033] Further features of the invention will become apparent from the description of embodiments of the invention together with the claims and the accompanying drawings. Embodiments of the invention may incorporate individual features or a combination of several features.

[0034] The invention is described below, without limiting the general inventive concept, using exemplary embodiments with reference to the drawings, whereby express reference is made to the drawings for all details of the invention not explained in more detail in the text. They show: Fig. 1 an industrial truck in a schematically simplified perspective view, Fig. 2 a schematic diagram of a hydraulic system, Fig. 3a to 3d show a lowering process of a load-handling device in an industrial truck according to the state of the art from a maximum height that can be reached with this industrial truck and Fig. 4a to 4c show a lowering process of a load-handling device from a maximum height achievable with the industrial truck in an industrial truck according to an embodiment.

[0035] In the drawings, identical or similar elements and / or parts are provided with the same reference numbers, so that a repeated presentation is omitted.

[0036] Fig. Figure 1 shows an industrial truck 2, for example a forklift truck, with a lifting mast 4, comprising, for example, a first mast lifting stage 41, the inner mast, and a second mast lifting stage 42, the middle mast. The lifting mast 4 is supported by a Fig. 1 mast lift cylinder (not shown) (several mast lift cylinders can also be provided). The lifting mast 4 comprises, in addition to the inner mast and the center mast, a stationary mast that is firmly connected to the vehicle chassis. The center mast is driven, for example, by the mast lift cylinder, and the inner mast is also coupled, for example, to the center mast via a chain, so that these two extension masts extend simultaneously. Furthermore, the industrial truck 2 comprises a free lift stage with a load handling device 6, for example a fork, which can be moved along the inner mast of the lifting mast 4. For this purpose, the free lift stage comprises a free lift cylinder 8. The free lift cylinder 8 can move the load handling device 6 along the first mast stage 41 of the lifting mast 4.

[0037] Fig. Figure 2 shows a schematic diagram of a hydraulic system 10, as integrated into the industrial truck 2 according to one exemplary embodiment. The hydraulic system 10 serves to supply a mast lift cylinder 12 and the free lift cylinder 8, which is used to move the load-handling device 6, with a hydraulic fluid 14. The hydraulic fluid is drawn from a reservoir 16 and returned thereto. The hydraulic system 10 is configured to simultaneously operate the mast lift cylinder 12 and the free lift cylinder 8, at least temporarily, in load lifting mode and / or in lowering mode.

[0038] In the illustrated embodiment, the hydraulic system 10 is designed to actuate the mast lifting cylinder 12 and the free lifting cylinder 8 simultaneously both in load lifting mode, ie when lifting the load handling device 6, and in load lowering mode, ie when lowering the load handling device 6.

[0039] The hydraulic system 10 comprises separate hydraulic return lines 18. A first hydraulic return line 181 runs between the free lift cylinder 8 and the reservoir 16. Furthermore, a second hydraulic return line 182 is included, which runs between the mast lift cylinder 12 and the reservoir 16. A first lowering valve 21 is integrated into the first return line 181, and a second lowering valve 22 is integrated into the second return line 182. The lowering valves 21, 22 are, for example, proportional valves. These can be switched between a first switching position 21a, 22a, in which the lowering valves 21, 22 operate as check valves, and a second switching position 21b, 22b. In the second switching position 21b, 22b, the lowering valves 21, 22 are configured to control or regulate a first volume flow or a second volume flow.Thus, the first lowering valve 21 controls or regulates a first volume flow through the first return line 181, while the second lowering valve 22 controls or regulates a second volume flow through the second return line 182. The lowering valves 21, 22 can be controlled separately from one another. For controlling and / or regulating purposes, the hydraulic system 10 includes a controller 24, which controls the two lowering valves 21, 22 via connecting lines (not shown).

[0040] The control system 24 is configured or programmed such that during load lowering operation, i.e., when lowering the load-handling device 6, the lowering valves 21, 22 open simultaneously. Thus, the free-lift cylinder 8 of the free-lift stage and the mast lift cylinder 12 of the lifting mast 4 are retracted simultaneously. Consequently, the load-handling device 6, actuated by the free-lift stage, lowers along the first mast lift stage 41, while simultaneously the lifting mast 4, i.e., the first and second mast lift stages 41, 42, retract.

[0041] This novel process is explained by comparing the Fig. 3 and Fig. 4. The Fig. 3a to 3d show a lowering process of a load-handling device 6, as it takes place in an industrial truck according to the prior art. Fig. 3a shows the industrial truck 2 with the lift mast 4 fully extended. The load handling device 6 is located at the upper stop of the first mast lift stage 41. A conventional industrial truck 2 includes a common return line with which both the free lift cylinder 8 and the mast lift cylinder 12 are pressure-relieved.

[0042] The free lift cylinder 8 and the mast lift cylinder 12 have different cross-sections. These are selected so that at an initial pressure p1 (cf. Fig. 2) of the hydraulic fluid 14, the free lift cylinder 8 is extended first. When the free lift stage reaches the upper stop of the lifting mast 4, more precisely the first mast lifting stage 41 (this situation shows the Fig. 3c), the pressure of the hydraulic fluid 14 in the hydraulic system 10 continues to rise until it reaches the value p2, which is higher than p1. When the hydraulic pressure p2 is exceeded, the mast lift stages 41, 42 begin to extend. The individual mast lift cylinders 12 of the mast lift stages 41, 42 can in turn be designed such that their different cross-sections ensure that the first mast lift stage 41 extends first, followed by the second mast lift stage 42.

[0043] In Fig. In the embodiment shown in Figure 3, the two mast lifting stages 41, 42 retract approximately simultaneously. When the hydraulic fluid 14 is drained, the lifting mast 4 and the free lift stage retract in the reverse order. Starting from a situation with the lifting mast 4 fully extended and a load lifting device 6 at the upper stop of the first mast lifting stage 41 (see Fig. 3a) the lifting mast 4 retracts first (cf. Fig. 3b). Since the pressure is still above p1, the free lift stage and thus the load handling device 6 remain at the upper stop until the lifting mast 4 is fully retracted (cf. Fig. 3c). Only when the hydraulic pressure in the hydraulic system 10 subsequently drops further, namely below the value of p1, does the free lift stage retract and the load handling device 6 sink to the lower stop.

[0044] The Fig. 4a to 4c show a lowering process of a load-handling device 6 of an industrial truck 2 according to an embodiment.

[0045] Fig. Figure 4a shows the industrial truck 2 with the mast 4 fully extended, with the load handling device 6 also at the upper stop of the first mast lifting stage 41. This situation is identical to that in Fig. 3a. In load lowering mode, the Fig. 4, the free lift stage and the mast lift stages 41, 42 are activated by simultaneously opening the first and second lowering valves 21, 22 (cf. Fig. 2) lowered synchronously. Fig. 4b shows the industrial truck 2 after a first time interval, after which the conventional industrial truck 2 also Fig. 3b. In contrast to the conventional industrial truck 2 in Fig. 3b is in the industrial truck 2 according to an embodiment in Fig. 4b, the load handling device 6 has already reached the lower stop of the first mast lifting stage 41. It is therefore already significantly lower than the load handling device 6 in the conventional industrial truck 2. During a further time interval, the lifting mast 4 sinks completely and the load handling device 6 reaches the lowest stop ( Fig. 4c). In a conventional industrial truck 2 (cf. Fig. 3c) after this time interval, the load handling device 6 is still at the upper stop of the first mast lifting stage 41.

[0046] As a comparison of the Fig. 3 and Fig. As shown in Figure 4, in the industrial truck 2 according to one embodiment, the load-handling device 6 is lowered significantly faster. This saves exactly the time interval that the load-handling device 6 needs in a conventional industrial truck 2 to be lowered along a mast lift step 41, 42.

[0047] According to a further embodiment, the controller 24 is configured such that the lowering of the lifting mast 4 and the free-lift stage, which moves the load-handling device 6, is controlled or regulated such that the mast lifting stages 41, 42 and the load-handling device 6 reach the lower stop at least approximately simultaneously. This allows for a homogeneous lowering process, which simplifies the operation of the industrial truck 2 for the operator.

[0048] In order to lift the load-handling device 6 during load-lifting operation, the hydraulic system 10 of the industrial truck 2 comprises a hydraulic pump 26, which draws hydraulic fluid 14 from the reservoir 16 via a hydraulic supply line 28. The hydraulic pump 26 is integrated into the hydraulic supply line 28. During load-lifting operation, the hydraulic pump 26 serves to supply the mast lift cylinder 12 and the free lift cylinder 8 with pressurized hydraulic fluid 14.

[0049] The hydraulic supply line 28 forks between the hydraulic pump 26 and the lifting cylinders, i.e., the free-lift cylinder 8 and the mast lift cylinder 12, into a first supply branch 31 and a second supply branch 32. The first supply branch 31 leads to the free-lift cylinder 8, and the second supply branch 32 leads to the mast lift cylinder 12. The two supply branches 31, 32 are also considered part of the hydraulic supply line 28. A lift valve 34, which can be configured as a proportional valve, is integrated into the first supply branch 31. The lift valve 34, like the hydraulic pump 26, can be controlled or regulated via the controller 24.

[0050] The settings of the lift valve 34 can be used to change the ratio between the volume flows in the first and second supply branches 31, 32. The free lift cylinder 8 has a first cross-section and the mast lift cylinder 12 has a second cross-section, wherein the first cross-section is larger than the second cross-section. For this reason, the free lift cylinder 8 is activated at a first pressure p1, wherein the pressure p1 is lower than the pressure p2 at which the mast lift cylinder 12 is activated. The settings of the lift valve 34 can be used to variably adjust the effective hydraulic flow cross-section of the lift cylinders 8, 12, making it possible to extend both lift cylinders 8, 12 simultaneously. This occurs in the first switching position 34a of the lift valve 34. In the second switching position 34b, the free lift can be blocked, so that only the mast lift cylinder 12 is actuated.Furthermore, the dynamic adjustment of the lift valve 34 makes it possible to achieve a smooth transition between a lift of the load-handling device 6 effected by the free-lift cylinder 8 and a lift of the load-handling device 6 effected by the mast lift cylinder 12.

[0051] In order to prevent backflow of the hydraulic fluid 14 in the first and second supply branch 31, 32, a check valve 36 is integrated into each supply branch.

[0052] All mentioned features, including those that can be inferred from the drawings alone, as well as individual features disclosed in combination with other features, are considered essential to the invention, both individually and in combination. Embodiments according to the invention may be fulfilled by individual features or a combination of several features. Within the scope of the invention, features marked "in particular" or "preferably" are to be understood as optional features. List of reference symbols 2 industrial trucks 4 lifting mast 6 load handling equipment 8 free lift cylinders 10 hydraulic system 12 mast lift cylinders 14 Hydraulic fluid 16 storage containers 18 hydraulic return line 21 first lowering valve 22 second lowering valve 21a, 22a, 34a first switching position 21b, 22b, 34b second circuit 24 Control 26 Hydraulic pump 28 supply line 31 first supply branch 32 second supply branch 34 Lift valve 36 Check valve 41 first mast lifting stage 42 second mast lifting stage 181 first hydraulic return line 182 second hydraulic return line

Claims

[1] Industrial truck (2) with a lifting mast (4) with at least one mast lifting stage (41, 42) driven by at least one mast lifting cylinder (12) and with a free-lift stage driven by at least one free-lift cylinder (8), with which a load-handling means (6) can be moved along the lifting mast (4), comprising a hydraulic system (10) for supplying the at least one mast lifting cylinder (12) and the at least one free-lift cylinder (8) with a hydraulic fluid (14), wherein the hydraulic system (10) is designed for at least temporarily simultaneous actuation of the at least one mast lifting cylinder (12) and the at least one free-lift cylinder (8) in load lifting operation and / or in load lowering operation, wherein the hydraulic system (10) comprises a first hydraulic return line (181),which runs between the at least one free lift cylinder (8) and a reservoir (16) for the hydraulic fluid (14), and the hydraulic system (10) further comprises a second hydraulic return line (182) which runs between the at least one mast lift cylinder (12) and the reservoir (16), wherein a first lowering valve (21) is integrated into the first return line (181) and a second lowering valve (22) is integrated into the second return line (182), , characterized by a control system (24) which is designed to open the lowering valves (21, 22) simultaneously during load lowering operation when lowering the load-carrying means (6). [2] Industrial truck (2) according to claim 1, characterized by in that the hydraulic system (10) for relieving the at least one mast lifting cylinder (12) and the at least one free lifting cylinder (8) in load lowering operation comprises a first hydraulic return line (181) and a separate second hydraulic return line (182). [3] Industrial truck (2) according to claim 1 or 2, characterized by that the control (24) is set up or programmed in such a way that in load lowering operation when lowering the load-carrying device (6), the lowering valves (21, 22) open simultaneously, so that the free-lift cylinder (8) of the free-lift stage and the mast lifting cylinder (12) of the lifting mast (4) are retracted simultaneously. [4] Industrial truck (2) according to claim 1, characterized by that the lowering valves (21, 22) are proportional valves and the controller (24) is further configured to control or regulate a first volume flow through the first lowering valve (21) and a second volume flow through the second lowering valve (22) such that in load lowering operation when lowering the load-carrying means (6), the at least one mast lifting stage (41, 42) and the load-carrying means (6) reach a lower end position at least approximately simultaneously. [5] Industrial truck (2) according to one of claims 1 to 4, characterized byin that the hydraulic system (10) comprises a hydraulic pump (26) which is integrated into a hydraulic supply line (28) and is designed, in load lifting operation, to supply the at least one mast lifting cylinder (12) and the at least one free lifting cylinder (8) with pressurised hydraulic fluid (14), wherein the hydraulic supply line (28) between the hydraulic pump (26) and the lifting cylinders (8, 12) forks into a first and a second supply branch (31, 32), and the first supply branch (31) runs to the free lifting cylinder (8) and the second supply branch (32) runs to the mast lifting cylinder (12), wherein a lifting valve (34) designed as a proportional valve is integrated into the hydraulic supply line (28), with which a ratio between the volume flows in the first and second supply branch (31, 32) can be changed. [6] Industrial truck (2) according to claim 5, characterized bythat the free-lift cylinder (8) has a first cross-section and the lifting cylinder (12) has a second cross-section, wherein the first cross-section is larger than the second cross-section and the lift valve (34) is integrated into the first supply branch (31). [7] Industrial truck (2) according to claim 5 or 6, characterized by a control system (24) which is designed to control the lifting valve (34) in such a way that, during load lifting operation, the free lift cylinder (8) and the mast lift cylinder (12) are extended at least temporarily simultaneously. [8] Hydraulic system (10) for an industrial truck (2) with a lifting mast (4) with at least one mast lifting stage (41, 42) and with a free lift stage with which a load-carrying means (6) can be moved along the lifting mast (4), wherein the hydraulic system (10) comprises at least one mast lifting cylinder (12) for driving the at least one mast lifting stage (41, 42) and at least one free lift cylinder (8) for driving the at least one free lift stage, wherein the hydraulic system (10) is designed to at least temporarily simultaneously supply the at least one mast lifting cylinder (12) and the at least one free lift cylinder (8) with a hydraulic fluid (14) in load lifting operation and / or in load lowering operation, wherein the hydraulic system (10) comprises a first hydraulic return line (181),which runs between the at least one free lift cylinder (8) and a reservoir (16) for the hydraulic fluid (14), and the hydraulic system (10) further comprises a second separate hydraulic return line (182) which runs between the at least one mast lift cylinder (12) and the reservoir (16), wherein a first lowering valve (21) is integrated into the first return line (181) and a second lowering valve (22) is integrated into the second return line (182), , characterized by a control system (24) which is designed to open the lowering valves (21, 22) simultaneously during load lowering operation when lowering the load-carrying means (6). [9] Hydraulic system (10) according to claim 8, characterized bythat the control (24) is set up or programmed in such a way that in load lowering operation when lowering the load-carrying device (6), the lowering valves (21, 22) open simultaneously, so that the free-lift cylinder (8) of the free-lift stage and the mast lifting cylinder (12) of the lifting mast (4) are retracted simultaneously. [10] Hydraulic system (10) according to claim 8 or 9, characterized byin that the hydraulic system (10) further comprises a hydraulic pump (26) which is integrated into a hydraulic supply line (28) and is designed, in load lifting operation, to supply the at least one mast lifting cylinder (12) and the at least one free lifting cylinder (8) with pressurised hydraulic fluid (14), wherein the hydraulic supply line (28) between the hydraulic pump (26) and the lifting cylinders (8, 12) forks into a first and a second supply branch (31, 32), and the first supply branch (31) runs to the free lifting cylinder (8) and the second supply branch (32) runs to the mast lifting cylinder (12), wherein a lift valve (34) designed as a proportional valve is integrated into the hydraulic supply line (28), with which a ratio between the volume flows in the first and second supply branch (31, 32) can be changed. [11] Method for operating a hydraulic system (10) of an industrial truck (2) with a lifting mast (4) with at least one mast lifting stage (41, 42) and with a free lift stage, with which a load-carrying means (6) can be moved along the lifting mast (4), wherein the hydraulic system (10) comprises at least one mast lifting cylinder (12) for driving the at least one mast lifting stage (41, 42) and at least one free lift cylinder (88) for driving the free lift stage, characterized bythat the hydraulic system (10) is operated in such a way that, in load lifting operation and / or load lowering operation, the at least one mast lifting cylinder (12) and the at least one free-lift cylinder (8) are actuated at least temporarily simultaneously, wherein the hydraulic system (10) comprises a first hydraulic return line (181) which runs between the at least one free-lift cylinder (8) and a reservoir (16) for the hydraulic fluid (14), and the hydraulic system (10) further comprises a separate second hydraulic return line (182) which runs between the at least one mast lifting cylinder (12) and the reservoir (16), wherein a first lowering valve (21) is integrated into the first return line (181) and a second lowering valve (22) is integrated into the second return line (182), characterized by that in load lowering operation, when lowering the load-carrying device (6), the first lowering valve (21) and the second lowering valve (22) are opened simultaneously. [12] Method according to claim 11, characterized by that the lowering valves (21, 22) are proportional valves and a first volume flow through the first lowering valve (21) and a second volume flow through the second lowering valve (22) are controlled or regulated such that when the load-carrying means (6) is lowered, the at least one mast lifting stage (41, 42) and the load-carrying means (6) reach a lower end position at least approximately simultaneously. [13] Method according to claim 11 or 12, characterized bythat the hydraulic system (10) comprises a hydraulic pump (26) which is integrated into a hydraulic supply line (28) and with which, in load lifting operation, the at least one mast lifting cylinder (12) and the at least one free lift cylinder (8) are supplied with pressurised hydraulic fluid (14), wherein the hydraulic supply line (28) between the hydraulic pump (26) and the lifting cylinders (8, 12) forks into a first and a second supply branch (31, 32), and the first supply branch (31) runs to the free lift cylinder (8) and the second supply branch (32) runs to the mast lifting cylinder (12), wherein a lift valve (34) designed as a proportional valve is integrated into the hydraulic supply line (28), with which a ratio between a volume flow in the first and in the second supply branch (31, 32) can be changed, wherein the free lift cylinder (8) has a first cross section and the mast lifting cylinder (12) have a second cross-section,wherein the first cross-section is larger than the second cross-section and the lift valve (34) is integrated into the first supply branch (31) and wherein the lift valve (34) is controlled such that the free lift cylinder (8) and the mast lift cylinder (12) are extended at least temporarily simultaneously. [14] Method according to claim 13, characterized by that depending on an operating mode of the industrial truck (2) and / or depending on a preselected lifting height of the load-handling device (6), the lifting valve (34) is brought into a first position for sequential extension of the free-lift cylinder (8) and the mast lifting cylinder (12) or into a second position for at least temporarily simultaneous extension.

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