VALVE DEVICE FOR A FLOOR CONVEYOR

Description

[0001] The present application relates to a valve device for influencing a lifting process and a lowering process of a hydraulic lifting cylinder of an industrial truck according to the preamble of independent claim 1 and to an industrial truck according to the preamble of independent claim 15.

[0002] Forklifts with an extendable mast have a defined lifting sequence. During a lifting operation, the load-handling attachment is first raised in a free lift, followed by an extension of the mast's lifting mechanism. During the free lift, the load-handling attachment is raised, but the mast itself is not extended. Once the free lift is complete, the mast extension begins, raising the load-handling attachment beyond the height achievable through the free lift. The lowering process is reversed.

[0003] Hydraulic lifting cylinders are typically used on industrial trucks to perform lifting and lowering operations, with at least one hydraulic lifting cylinder each for the free lift and the mast lift. The stroke sequence of free lift and mast lift is predetermined by the load pressures of the lifting cylinders, determined by their load, or by the surface area ratios of the hydraulic lifting cylinders. The hydraulic lifting cylinder(s) responsible for the free lift, also known as the free lift cylinder(s), generally has a larger effective piston area than the hydraulic lifting cylinder(s) responsible for the mast lift, also known as the mast lift cylinder(s). Due to the lower load pressure of the free lift cylinder, under the influence of the hydraulic fluid pressure, only the free lift cylinder extends initially.When the free-lift cylinder reaches its end position (lift stop), the mast lift cylinder(s) are extended, triggering the mast lift.

[0004] During a lifting or lowering operation with a load on the lifting device, further boundary conditions must be met to ensure safe operation, as described above in the sequence of free lift and mast lift. Firstly, the extension and retraction speeds of the lifting cylinders must be reduced before they reach their respective lifting limits. This prevents load peaks in the hydraulic circuit and avoids vibrations of the load carried by the lifting device due to induced shocks. Secondly, a safety mechanism must be in place to prevent uncontrolled lowering of the load in the event of a hose rupture in one of the hydraulic lifting cylinders. This mechanism limits the maximum lowering speed to a standardized value (max. 0.1 m / s) in such a failure scenario. This safety device is typically designed as a pipe rupture protection system.

[0005] For example, EP 2 508 465 B1 discloses a forklift truck comprising a lifting frame, a load-bearing element, and a pressure source. The lifting frame has a mast movable along its length. The load-bearing element is movably mounted along the mast. The pressure source supplies a free-lift cylinder that interacts with the load-bearing element and a mast lift cylinder that interacts with the mast. The forklift truck also includes a sensor that detects the transition from mast lift to free lift during a lowering operation. Furthermore, a common throttle valve for the lowering operation of the free-lift cylinder and the mast lift cylinder is provided, along with a control unit. The control unit is configured, in response to a transition from mast lift to free lift detected by the sensor, to actuate the common throttle valve to a lowering speed of the free-lift cylinder corresponding to the lowering speed of the mast lift cylinder.Each free-lift cylinder and mast lift cylinder is equipped with a pipe rupture protection device that blocks the lowering process in the event of a pipe rupture.

[0006] A key parameter in the use of industrial trucks is the so-called throughput, which defines the number of goods handled per given unit of time. To achieve a high throughput, the operating speed of an industrial truck is therefore of considerable importance. A high operating speed is directly related to the maximum lifting and lowering speed of the industrial truck. For example, lifting and lowering speeds of ≥1 m / s are desirable. However, in known industrial trucks, such as the one described in EP 2 508 465 B1, the maximum lowering speed during normal operation is limited by the use of pipe rupture protection devices.Such pipe rupture safety devices typically trigger at lowering speeds between 0.6 m / s and 0.9 m / s, which does not allow high lowering speeds of ≥1 m / s in the proper operation of the industrial truck.

[0007] Document WO 2015 / 049828 A1 discloses a valve device according to the preamble of independent claim 1.

[0008] Based on this, the object of the present invention is therefore to provide a valve device for influencing a lifting and lowering process of a hydraulic lifting cylinder and thus for controlling load transitions during the lifting or lowering process of the hydraulic lifting cylinders of a lifting frame of a forklift truck, which allows lifting and lowering speeds of ≥1 m / s and at the same time enables lifting and lowering processes with and without load to be carried out in accordance with safety regulations.

[0009] This problem is solved according to the invention by the features of independent claim 1, wherein advantageous further developments of the valve device according to the invention are the subject of the corresponding dependent claims 2 to 14.

[0010] The present invention relates to a valve device for controlling the lifting and lowering processes of at least one hydraulic lifting cylinder of a forklift truck. The valve device is designed to be interposed between the hydraulic lifting cylinder and the hydraulic system of the forklift truck for supplying the hydraulic lifting cylinder with hydraulic fluid. The valve device comprises a fluid-controlled lifting stage and a fluid-controlled lowering stage, wherein the lifting stage and the lowering stage each have a working port that is fluidly connected to the hydraulic system of the forklift truck and to the hydraulic lifting cylinder, respectively. The valve device is characterized in that it includes an electrically controllable pilot valve that is fluidly connected to a control port of the lifting stage and a control port of the lowering stage.The pilot valve is designed and controllable in such a way as to increase a control pressure by a predefinable pressure value to control the lifting stage and the lowering stage during a lifting operation or during a lowering operation of the hydraulic lifting cylinder before reaching an end position of the hydraulic lifting cylinder.

[0011] The inventive design of the valve assembly allows the speed at which a hydraulic lifting cylinder is moved during a lifting or lowering operation to be reduced before reaching an end position (lifting stop), and simultaneously the speed of a subsequent hydraulic lifting cylinder (e.g., mast lifting cylinder) to be increased so that, on the one hand, no shocks or impacts occur when the end position is reached that could be transmitted to the load moved by the load-handling device and lead to unsafe situations, and on the other hand, a consistently large quantity of oil can continue to flow to or from the hydraulic system. This allows for continuous lifting or lowering without a reduction in hydraulic power.

[0012] Particularly when using such a valve device for lifting and lowering operations with free stroke and mast stroke, it is possible to achieve a smooth transition between the two strokes while simultaneously minimizing or eliminating any loss of speed. By increasing the control pressure of the lifting and lowering stages by a predefinable pressure value before reaching the end position, the speed of the lifting cylinder is reduced, while another lifting cylinder is simultaneously accelerated to its maximum speed and extended or retracted. The pressure value by which the control pressure is increased by the pilot valve preferably corresponds to the pressure difference between the load pressure acting on the free stroke cylinder and the load pressure acting on the mast stroke cylinder.During a lifting operation, the load pressure exerted by the free-lift cylinder is artificially increased before reaching the end position, reducing the speed of the free-lift cylinder while simultaneously extending the mast lift cylinder. Conversely, during a lowering operation, the speed of the mast lift cylinder is reduced before reaching the end position, while the free-lift cylinder retracts.

[0013] A further advantage is that, thanks to the pilot valve, no additional pipe rupture protection in the form of a separate valve upstream of the hydraulic lifting cylinder is required. Instead, the pilot valve fulfills the function of the pipe rupture protection by switching to a locked position in the event of a detected rupture. The load is then either held by the pilot-operated lifting and lowering stage or lowered in a controlled manner at a low speed, for example, approximately 100 mm / min. The absence of a conventional pipe rupture protection system, in turn, allows lifting and lowering speeds of ≥1 m / s, leading to a significant increase in the handling capacity of a forklift truck.

[0014] According to an advantageous further development of the invention, the pilot valve is designed and controllable in such a way that the control pressure for controlling the lifting stage and the lowering stage is increased steplessly by the predefinable pressure value.

[0015] This ensures that there is no sudden decrease in speed at the hydraulic cylinder, but rather that the speed decreases continuously. This significantly reduces the risk of induced shocks.

[0016] According to an advantageous embodiment of the invention, the pilot valve is designed and controllable in such a way as to increase the control pressure for controlling the lifting stage and the lowering stage beyond the predefinable pressure value immediately before reaching the end position of the hydraulic lifting cylinder.

[0017] Increasing the control pressure via the pilot valve beyond the preset pressure value ensures that the speed of the hydraulic lifting cylinder is gradually reduced until the end position is reached, so that the hydraulic lifting cylinder stops its movement when the end position is reached.

[0018] According to an advantageous embodiment of the invention, the pilot valve is designed and controllable in such a way that the pilot valve assumes a (fully) open position at the beginning of a lifting process and a lowering process.

[0019] This ensures that, until the control pressure of the lifting and lowering stage increases, the hydraulic lifting cylinder is moved with minimal hydraulic losses at the lifting and lowering stage and at maximum speed. This enables lifting or lowering speeds of ≥1 m / s over essentially the entire stroke achievable through the lifting sequence. Furthermore, it allows for faster lowering of very light loads, for example, with an empty load-handling attachment, where the maximum speed is limited by the low load pressure.

[0020] According to an advantageous embodiment of the invention, the pilot valve is an inversely proportional pressure relief valve.

[0021] This ensures that, in addition to controlled lowering or load holding in the event of a line break, a safety mechanism is also in place in case of electronic failure. If the pressure relief valve loses power, a pre-tensioned spring automatically switches it to the closed position, allowing the load to be lowered or held in a controlled manner even in such a case.

[0022] According to an advantageous embodiment of the invention, it is provided that one or the inlet of the pilot valve is fluidically connected to the control port of the lifting stage and the control port of the lowering stage.

[0023] The inlet or return line of the pilot valve is preferably still fluidically connected to an inlet (working connections) of the lifting and lowering stage.

[0024] Depending on the control signal of the pilot valve, the behavior of the lifting and lowering stage is directly influenced, resulting in fast switching times of the valve assembly. This, in turn, improves operating speed and prevents shocks during lifting and lowering operations.

[0025] Preferably, the valve assembly includes a control line connected in parallel to the lifting stage, which is fluidly connected to the control port of the lifting stage, the control port of the lowering stage and the inlet of the pilot valve, and can be fluidly connected to the hydraulic system of the industrial truck.

[0026] In addition or alternatively, it is provided that the valve assembly includes a control line connected in parallel to the lowering stage, which is fluidly connected to the control port of the lifting stage, the control port of the lowering stage and the inlet of the pilot valve and can be fluidly connected to the hydraulic lifting cylinder.

[0027] In particular, it is provided that the control line connected in parallel to the lifting stage and / or the control line connected in parallel to the lowering stage includes an orifice and a check valve.

[0028] This design ensures that the higher pressure in the hydraulic circuit is always present at the pilot valve's inlet during both lifting and lowering operations. During lifting, this pressure is provided by the forklift's system to supply the hydraulic lifting cylinder(s) (control block). During lowering, it is the pressure exerted by the hydraulic cylinder itself.

[0029] According to an advantageous embodiment of the invention, the valve assembly comprises a shut-off valve which is connected between the control connections of the lifting and lowering stages and the inlet of the pilot valve, wherein the shut-off valve comprises two control connections, one control connection being fluidically connectable to the hydraulic lifting cylinder and the other control connection to the hydraulic system of the industrial truck.

[0030] Preferably, the switching pressure of the shut-off valve is lower than the upper limit of a control range of the pilot valve.

[0031] The use of an additional shut-off valve becomes necessary when more than two hydraulic cylinders are controlled by a single valve assembly. If a hydraulic line were to break in one of the cylinders, the entire load would then be supported by only one hydraulic cylinder. This would cause significant pressure spikes in the hydraulic circuit, which the valve assembly would be unable to compensate for without a shut-off valve. The load would then be lowered uncontrollably. The shut-off valve, which normally operates in an open position due to spring force, ensures that a line break prevents the load from lowering uncontrollably, as the valve automatically switches to the closed position.

[0032] Designing the shut-off valve with a lower switching pressure than the upper limit of the pilot valve's control range ensures that no load case can occur that would lead to an uncontrolled reduction of the load in the event of a line break.

[0033] According to an advantageous embodiment of the invention, it is provided that a control range of the pilot valve lies between 0 bar and a pressure value above the load pressure of the hydraulic lifting cylinder, for example 250 bar.

[0034] When using the additional shut-off valve, it is preferably provided that the control range of the pilot valve is between 0 bar and a pressure value above the switching pressure of the shut-off valve, for example 100 bar.

[0035] It is further preferably provided that the pilot valve is designed and controllable in such a way that the pilot valve is adjusted to a differential pressure, for example 20 bar to 40 bar, before the end position of the hydraulic lifting cylinder is reached.

[0036] A pilot valve with a limited control range has the advantage of higher control accuracy, which in turn leads to greater operational reliability.

[0037] According to an advantageous embodiment of the invention, the valve assembly comprises a changeover valve which is fluidly connected to one or the return line of the pilot valve, wherein the changeover valve is furthermore fluidly connectable to the hydraulic system of the industrial truck and the hydraulic lifting cylinder.

[0038] This design ensures that during both lifting and lowering operations, the lower pressure in the hydraulic circuit is always present at the pilot valve's return line. During a lifting operation, this is the pressure exerted by the hydraulic cylinder. During a lowering operation, it is the pressure exerted by the forklift's system supplying the hydraulic lifting cylinder(s) (control block).

[0039] According to an advantageous embodiment of the invention, the valve assembly comprises a bypass that can be fluidically connected to the hydraulic lifting cylinder and the hydraulic system of the industrial truck, wherein the bypass comprises an orifice.

[0040] This design allows, in the event of a line break or electronic failure, a lowering process to be carried out at a controlled speed, for example ≈ 100 mm / min, thus enabling the load to be lowered in compliance with all safety regulations.

[0041] The problem according to the invention is further solved by a forklift truck according to independent claim 15.

[0042] Accordingly, the present invention further relates to a forklift truck with a lifting mast comprising at least one mast lifting stage driven by at least one mast lifting cylinder, preferably two mast lifting cylinders, 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, and a hydraulic system for supplying the at least one free lifting cylinder and the at least one mast lifting cylinder with hydraulic fluid. At least one valve assembly is interposed between the hydraulic system and the at least one free lifting cylinder, as well as between the hydraulic system and the at least one mast lifting cylinder. The forklift truck includes a control unit for controlling the valve assemblies. A position measuring system, which is connected to the control unit for data transmission, is assigned to each of the at least one free lifting cylinder and the at least one mast lifting cylinder.The industrial truck is characterized in that the valve devices are each designed as a valve device according to one of claims 1 to 14.

[0043] The present invention is described in more detail below with reference to the embodiments illustrated in the figures.

[0044] They show: FIG. 1 a schematic and exemplary representation of an industrial truck according to the invention in the form of a forklift truck; FIG. 2 a schematic and exemplary circuit diagram of a hydraulic circuit for supplying a hydraulic lifting cylinder of the industrial truck made of FIG. 1 with a first embodiment of a valve device according to the invention; and FIG. 3 a schematic and exemplary circuit diagram of a hydraulic circuit for supplying a hydraulic lifting cylinder of the industrial truck made of FIG. 1 with a second embodiment of the valve device according to the invention.

[0045] FIG. 1 Figure 1 shows a schematic and exemplary representation of a forklift truck according to the invention, the basic structure of which is known. The forklift truck comprises a lifting mast 2 arranged or formed in the front area of ​​the forklift truck 1. The lifting mast 2 serves to lift and lower a load 4 located on a load handling device 3. A non-liftable part of the lifting mast 2 is arranged on a frame 5 of the forklift truck 1, which is also referred to as the base mast 6. A liftable or extendable part of the lifting mast 2 is referred to as the extension mast 7 and is slidably guided on or within the base mast 6.

[0046] A hydraulic circuit 8 is provided for raising and lowering the load-handling device 3 with or without load 4, which is located in the FIGS. 2 and 3The hydraulic circuit 8 comprises hydraulic lifting cylinders 9 and a hydraulic system 10 fluidically connected to the hydraulic lifting cylinders 9 for supplying the hydraulic lifting cylinders 9 with hydraulic fluid. The hydraulic system 10 includes at least one pump (not shown in the figures) fluidically connected to a tank (also not shown in the figures), and a control block (not shown in the figures) that controls the supply of hydraulic fluid to the hydraulic lifting cylinders 9.

[0047] The extendable lifting mast 2 can be extended according to a defined lifting sequence. During a lifting operation using the industrial truck 1, the load handling device 3 is first raised in a free lift. This lifting stage of the sequence is also referred to as the free lift stage. Subsequently, the extension mast 7 of the lifting mast 2 is extended in a mast lift. This lifting stage of the sequence is also referred to as the mast lift stage. During the free lift, the load handling device 3 is raised, but the extension mast 7 is not extended. At the end of the free lift, the mast lift begins, during which the extension mast 7 of the lifting mast 2 is extended. This allows the load handling device 3, and thus also the load 4 placed on it, to be raised beyond the lifting height HF achievable by the free lift, up to a lifting height HM. The procedure is reversed for a lowering operation. First, the extension mast 7 is retracted.At the end of the lowering process of the extension mast 7, the load handling device 3 is retracted. The industrial truck 1 includes one so-called free-lift cylinder 9a for the free lift and two so-called mast lift cylinders 9b for the mast lift. It is equally possible for the industrial truck 1 to include two or more free-lift cylinders 9a or one or more than two mast lift cylinders 9b.

[0048] During a lifting or lowering operation with a load 4 on the load-handling device 3, certain boundary conditions must be met during the previously described lifting sequence of free lift and mast lift to ensure safe and reliable operation. As already described, it is necessary, on the one hand, that the extension and retraction speed of the hydraulic lifting cylinders 9—here, the free lift cylinder 9a and the two mast lift cylinders 9b—is reduced before each lifting end is reached. This ensures a smooth transition during the extension and retraction of the hydraulic lifting cylinders 9, thereby preventing load peaks in the hydraulic circuit 8 and preventing wobbling or falling of the load 4 carried by the load-handling device 3.Furthermore, a safety mechanism must be in place to prevent uncontrolled lowering of the load 4 in the event of a line rupture in one of the hydraulic lifting cylinders 9. It is also desirable to avoid a reduction in throughput during lifting or lowering operations using the industrial truck 1 by extending the mast lifting cylinders 9b in parallel with the free lifting cylinder 9a before the end position is reached during a lifting operation. The reverse should occur during a lowering operation. Generally, achieving high lifting and lowering speeds of ≥1 m / s is also desirable.

[0049] To meet or reconcile all these boundary conditions and objectives, a valve assembly 11 is provided for influencing the lifting and lowering processes of the hydraulic lifting cylinder(s) 9 of a forklift truck 1. The valve assembly 11 is described below with reference to the FIGS. 2 and 3 described in more detail.

[0050] The valve assembly 11 is designed to be interposed between the hydraulic lifting cylinder 9 and the hydraulic system 10 for supplying the hydraulic lifting cylinder 9. When the valve assembly 11 is used in the industrial truck 1, it is therefore interposed between the hydraulic lifting cylinder 9 and the hydraulic system 10, so that the hydraulic lifting cylinder 9 is fluidically connected to the hydraulic system 10 by means of the valve assembly 11. The hydraulic fluid flowing in the hydraulic circuit 8 thus passes through the valve assembly 11 during its flow from the hydraulic system 10 to the hydraulic lifting cylinder 9 and vice versa.

[0051] The valve assembly 11 comprises a lifting stage 12 and a lowering stage 13, each of which is fluid-controlled. In other words, actuation or switching of the lifting stage 12 and the lowering stage 13 is achieved via the hydraulic fluid in the hydraulic circuit 8. As the name suggests, the lifting stage 12 is switched during a lifting operation of the lifting cylinder 9, while the lowering stage 13 is switched during a lowering operation. Both the lifting stage 12 and the lowering stage 13 are preferably designed as flow control valves with a piston actuated by spring force. The lifting stage 12 and the lowering stage 13 each comprise two working ports 12.1, 12.2, 13.1, 13.2, wherein one working port 12.1, 13.2 is fluidically connected to the hydraulic system 10 and the other working port 12.2, 13.1 is fluidly connected to the hydraulic lifting cylinder 9. When the valve assembly 11 is used in the industrial truck 1, one working port 12.1, 13.The lifting stage 12 and the lowering stage 13 are thus connected to the hydraulic system 10, and the other working port 12.2, 13.1 is fluidically connected to the hydraulic lifting cylinder 9. In addition to the two working ports 12.1, 12.2, 13.1, 13.2, the lifting stage 12 and the lowering stage 13 each include a control port 12.3, 13.3, which serves to switch the lifting stage 12 and the lowering stage 13.

[0052] To reconcile the previously described boundary conditions and objectives, it is essential that the valve assembly 11 includes a so-called pilot valve 14 as a central component for controlling the lifting and lowering process of the hydraulic lifting cylinder 9. The pilot valve 14 is designed as an electrically actuated valve that is fluidically connected to the control ports 12.3 and 13.3 of the lifting stage 12 and the lowering stage 13. In particular, an inlet 15 of the pilot valve 14 is fluidically connected to the control ports 12.3 and 13.3 of the lifting stage 12 and the lowering stage 13. In other words, the behavior of the lifting stage 12 and the lowering stage 13 can be directly influenced by actuating the pilot valve 14.

[0053] The pilot valve 14 is a proportionally adjustable pressure relief valve. By actuating or energizing the pilot valve 14, the closing force of a valve piston of the pilot valve 14 can be continuously adjusted from an open position to maximum force. The pilot valve 14 can be actuated by a control unit of the industrial truck 1 (not shown in the figures) depending on the position of the hydraulic lifting cylinder 9. The position of the hydraulic lifting cylinder 9 is detected by a displacement measuring system 16 and transmitted to the control unit, where it is evaluated. Preferably, the pilot valve 14 is designed as an inversely proportional pressure relief valve. In such a configuration, the pilot valve 14 assumes a closed position with maximum closing force due to a restoring force, which is applied, for example, by a spring, when there is no energization or actuation. With maximum energization or actuation, the pilot valve 14 assumes a closed position with maximum closing force.Control, on the other hand, sets an open position. Any switching position between maximum closing force and open position can be set via the current level. The control range of the pilot valve 14 is between 0 bar and a pressure value above the load pressure of the hydraulic lifting cylinder 9, for example, 250 bar.

[0054] The pilot valve 14 is designed and controllable, in particular by means of the control unit of the industrial truck 1, such that a control pressure for controlling the lifting stage 12 and the lowering stage 13 is increased by a predefinable pressure value during a lifting or lowering operation of the hydraulic lifting cylinder 9 before the hydraulic lifting cylinder 9 reaches its end position (lifting stop). The position that triggers the increase of the control pressure by the predefinable pressure value can be determined by means of the displacement measuring system 16. In other words, before the end position of the hydraulic lifting cylinder 9 is reached, the closing force of the pilot valve 14 is changed by actuating it, whereupon the control pressure of the lifting stage 12 and the lowering stage 13 is affected.The pressure value by which the control pressure is increased can be stored in a memory unit of the forklift truck's control system and preset during the production of forklift truck 1, or it can be set by an operator of forklift truck 1 via a driver assistance system (not shown in the figures). To increase the control pressure by the preset or predetermined pressure value before reaching the end position, the pilot valve 14 must be adjusted to a differential pressure, for example, to a value between 20 bar and 40 bar.

[0055] The control of the pilot valve 14 according to the invention, before the end position of the hydraulic lifting cylinder 9 is reached, ensures that the load pressure acting on the hydraulic lifting cylinder 9 is artificially increased, thereby reducing the speed at which the hydraulic lifting cylinder 9 is extended or retracted before reaching its end position. This ensures that no or only negligible shocks occur during a lifting or lowering operation. In the industrial truck 1 with free-lift cylinder 9a and mast lifting cylinder 9b, it can also be ensured that, during a lifting operation, the mast lifting cylinder 9b is extended even before the end position is reached by the free-lift cylinder 9a, without any loss of speed. The pressure value by which the control pressure, and thus also the load pressure acting on the free-lift cylinder 9, is increased corresponds to the pressure difference between the load pressure of the free-lift cylinder 9a and the load pressure of the mast lifting cylinder 9b.During a lowering operation, the free-lift cylinder 9a is retracted before the mast lifting cylinder 9b reaches its end position. Furthermore, the combination of lifting stage 12, lowering stage 13, and pilot valve 14 creates the necessary safety mechanism for a pipe rupture without requiring any additional pipe rupture protection devices in the hydraulic circuit 8.

[0056] Preferably, the pilot valve 14 is designed and controllable such that the control pressure for controlling the lifting stage 12 and the lowering stage 13 is increased successively by the predefinable pressure value. Preferably, the control pressure is increased in a ramp-like manner. This ensures that the speed of the hydraulic lifting cylinder 9 changes continuously rather than abruptly. Furthermore, the pilot valve 14 is designed and controllable such that the control pressure for controlling the lifting stage 12 and the lowering stage 13 is increased beyond the predefinable pressure value immediately before the end position of the hydraulic lifting cylinder 9 is reached. This increase also preferably occurs successively until the pilot valve 14 has reached the (fully) closed position.This ensures that the hydraulic lifting cylinder 9 does not move unbraked into the end position or has to be braked abruptly immediately before reaching the end position.

[0057] To initiate the lifting or lowering process, the pilot valve 14 is designed and controllable such that it assumes the (fully) open position at the beginning of a lifting or lowering process. In an inversely proportional pressure relief valve configuration, the pilot valve 14 is therefore energized to its maximum extent. This activation continues until the position of the hydraulic lifting cylinder 9 is detected by the displacement measuring system 16, at which point the pilot valve 14 is activated to increase the control pressure.

[0058] In addition to the hydraulic components described above, the valve assembly 11 further comprises a control line 17 connected in parallel to the lifting stage 12. This control line is fluidically connected to the control port 12.3 of the lifting stage 12, the control port 13.3 of the lowering stage 13, and the inlet 15 of the pilot valve 14, and can be fluidly connected to the hydraulic system 10 of the industrial truck 1, or is connected when used in the industrial truck 1. The valve assembly 11 also comprises a control line 18 connected in parallel to the lowering stage 13. This control line is fluidly connected to the control port 12.3 of the lifting stage 12, the control port 13.3 of the lowering stage 13, and the inlet 15 of the pilot valve 14, and can be fluidly connected to the hydraulic system 10 of the industrial truck 1, or is connected when used in the industrial truck 1. The control line 17 parallel to the lifting stage 12 includes an orifice 19 and a check valve 20.The control line 18, running parallel to the lowering stage 13, also includes an orifice 21 and a check valve 22.

[0059] The valve assembly 11 further comprises a changeover valve 23, which is fluidically connected to a return line 24 of the pilot valve 14. The changeover valve 23 is also fluidly connected to the hydraulic system 10 of the industrial truck 1 and the hydraulic lifting cylinder 9. When the valve assembly 11 is used in the industrial truck 1, the changeover valve 23 is therefore fluidly connected to the hydraulic system 10 of the industrial truck 1 and the hydraulic lifting cylinder 9. The valve assembly 11 may also include two further orifices 25, 26, which are located downstream or upstream of the changeover valve 23, depending on the flow direction of the hydraulic fluid.

[0060] Furthermore, the valve assembly 11 includes a bypass 27, which can be fluidically connected to the hydraulic lifting cylinder 9 and the hydraulic system 10. Here too, when the valve assembly 11 is used in the industrial truck 1, the bypass 27 is fluidly connected to the hydraulic lifting cylinder 9 and the hydraulic system 10. The bypass 27 includes an orifice 28.

[0061] If the valve assembly 11 described above is used in the industrial truck 1 described above, which has one free-lift cylinder 9a and two mast lift cylinders 9b, then such a valve assembly 11 is connected between the free-lift cylinder 9a and the hydraulic system 10, and at least one further such valve assembly 11 is connected between the two mast lift cylinders 9b and the hydraulic system 10. Preferably, such a valve assembly 11 described above is connected between each hydraulic lift cylinder 9a, 9b.

[0062] However, if only one such valve assembly 11 is connected between the two mast lifting cylinders 9b, this valve assembly 11 includes an additional hydraulic component. The additional hydraulic component is a shut-off valve 29, which is connected between the control port 12.3 of the lifting stage 12, the control port 13.3 of the lowering stage 13, and the inlet 15 of the pilot valve 14. The shut-off valve 29 comprises two control ports 29.1, 29.2, wherein one control port 29.1 is fluidically connected to the hydraulic lifting cylinder 9 and the other control port 29.2 is fluidically connected to the hydraulic system 10 of the industrial truck 1. Therefore, it also applies here that when this embodiment of the valve assembly 11 is used in the industrial truck 1, the FIGS. 2As shown, one control port 29.1 is fluidically connected to the hydraulic lifting cylinder 9 and the other control port 29.2 to the hydraulic system 10 of the industrial truck 1. Preferably, the switching pressure of the shut-off valve 29 is lower than an upper limit of a control range of the pilot valve 14. A further orifice 30, 31 can be arranged in each control line to the control ports 29.1, 29.2.

[0063] The use of an additional shut-off valve 29 comes into play when a line breaks in one of the two mast lifting cylinders 9b. The shut-off valve 29, which normally assumes an open position due to spring force, ensures that no uncontrolled lowering of the load occurs in the event of a line break, as the shut-off valve 29 automatically switches to the closed position. The design of the shut-off valve 29 with a lower switching pressure than the upper limit of the control range of the pilot valve 14 ensures that no load case can occur that would lead to an uncontrolled lowering of the load 4 in the event of a line break. When the additional shut-off valve 29 is used, the control range of the pilot valve 14 can be between 0 bar and a pressure value above the switching pressure of the shut-off valve 29, for example, 100 bar.

[0064] When the valve device(s) 11 are used in the industrial truck 1, they are preferably mounted directly on the hydraulic lifting cylinder 9.

[0065] In summary, the operation of the lifting and lowering process using the valve device(s) 11 in the industrial truck 1 described above, with free-lift cylinder 9a and mast lift cylinders 9b, can therefore be described as follows:

[0066] The control of the hydraulic system 10, in particular via the control block of the hydraulic system 10, first determines whether a load 4 on the load-handling device 3 is to be raised or lowered. The pilot valve 14 always regulates the differential pressure between the hydraulic lifting cylinders 9 via the lifting stage 12 and lowering stage 13, regardless of whether the lifting or lowering process is taking place, irrespective of the absolute pressure acting on the load 4.

[0067] During a lifting operation, the hydraulic fluid first flows from the control block of the hydraulic system 10 to the valve assembly 11 of the free-lift cylinder 9. The control line 17, running parallel to the lifting stage 12 and equipped with an orifice plate 19 and a check valve 20, ensures that the higher pressure prevailing in the hydraulic circuit 8—which, during the lifting operation, is the pressure supplied by the control block—is always present at the inlet 15 of the pilot valve 14. Simultaneously, the changeover valve 23 ensures that the lower pressure in the hydraulic circuit 8 is present at the return line 24 of the pilot valve 14. This is the cylinder-side pressure during the lifting operation. The pilot valve 14 is then switched to the fully open position (0 bar setting) by applying maximum current. The hydraulic fluid flows via the lifting stage 12 to the free-lift cylinder 9a, while the lowering stage 13 remains in its closed position.Before the free-lift cylinder 9a reaches its end position (lift stop), the valve is adjusted to a value (20 to 40 bar setting) that increases the control pressure of the lifting stage 12 and the lowering stage 13 by this predefinable or predetermined pressure value. This increase preferably occurs in a ramped manner. By modifying the control of the pilot valve 14, the load pressure acting on the free-lift cylinder 9a during the lifting process is artificially increased via the lifting stage 12 to the level of the mast lifting cylinders 9b. The mast lifting cylinders 9b then also extend, with the valve assembly(s) 11 of the mast lifting cylinders 9b being configured to allow hydraulic fluid to flow to them.The flow rate provided by the control block of the hydraulic system 10 remains constant, but is now distributed across all three hydraulic lifting cylinders 9a and 9b, thus reducing the extension speed of the free-lift cylinder 9a. By further increasing the load pressure via control of the pilot valve 14, the free-lift cylinder 9a stops upon reaching its end position, and only the two mast lifting cylinders 9b continue to extend until they too reach their end positions and the load-handling device 3, with or without a load 4, is at height HM.

[0068] During a lowering operation, the hydraulic fluid flows from the mast lifting cylinders 9b to the control block of the hydraulic system 10 via the valve assembly(s) 11 of the mast lifting cylinders 9b. The control line 18 with orifice plate 21 and check valve 22, running parallel to the lowering stage 13, ensures that the higher pressure in the hydraulic circuit 8 is always present at the inlet 15 of the pilot valve 14. During the lowering operation, this is the cylinder-side pressure. The changeover valve 23 ensures that the lower pressure in the hydraulic circuit 8 is present at the return line 24 of the pilot valve 14. During the lowering operation, this is the pressure present at the control block. The pilot valve 14 is then switched to the fully open position (0 bar setting), which is achieved by maximum energization. The hydraulic fluid flows via the lowering stage 13 to the control block, while the lifting stage 12 remains in its closed position.Before the mast lift cylinders 9b reach their end position (lift stop), the valve is adjusted to a value (20 to 40 bar setting) that increases the control pressure of the lowering stage 13 and the lifting stage 12 by this predefinable or predetermined pressure value. This increase preferably also occurs in a ramp-like manner. The modified control of the pilot valve 14 reduces the load pressure acting on the mast lift cylinders 9b to the level of the load pressure acting on the free lift cylinder 9a. The load pressure acting on the control block is therefore reduced. The free lift cylinder 9a is thus also retracted until the load-handling device 3, with or without load 4, reaches a ground position HB, whereby the valve assembly 11 of the free lift cylinder 9a is switched accordingly so that the hydraulic fluid can flow from the free lift cylinder 9a to the control block.

[0069] If a line break is detected during a lifting or lowering operation, for example due to an impermissibly high lowering speed detected by suitable sensors, the pilot valve 14 is immediately de-energized. The maximum pressure setting determined by the spring force (e.g., 250 bar) then ensures that the load 4 is held securely or lowered in a controlled, slow manner, provided that the bypass 25 with orifice 26 is provided. Furthermore, the design of the pilot valve 14 as an inversely proportional pressure relief valve in a preferred embodiment ensures that a load-holding function or emergency lowering is also implemented in the event of an electronics failure.

[0070] Finally, it should be noted that the embodiments described above serve only to describe the claimed teaching, but are by no means to be regarded as limiting or exhaustive. Reference symbol list 1 forklift 13.3 Control connection 2 mast 14 pilot valve 3 Load-bearing equipment 15 Inflow 4 load 16 Position measuring system 5 Frame 17 Control line 6 Standing mast 18 Control line 7 Extendable mast 19 Aperture 8 hydraulic circuit 20 non-return valve 9 hydraulic lifting cylinder 21 Aperture 9a Free-lift cylinder 22 non-return valve 9b Mast lift cylinder 23 Changeover valve 10 hydraulic system 24 Return 11 Valve assembly 25, 26 Aperture 12 Lifting stage 27 bypass 12.1, 12.2 Work connection 28 Aperture 12.3 Control connection 29 shut-off valve 13 Lowering step 29.1, 29.2 Control connection 13.1, 13.2 Work connection 30, 31 Aperture

Claims

1. A valve device (11) for influencing a lifting procedure and a lowering procedure of at least one hydraulic lifting cylinder (9) of an industrial truck (1), wherein the valve device (11) is constructed so as to be interposed between the hydraulic lifting cylinder (9) and a hydraulic system (10) of the industrial truck (1) in order to supply the hydraulic lifting cylinder (9) with a hydraulic fluid, wherein the valve device (11) comprises a medium-controlled lifting stage (12) and a medium-controlled lowering stage (13), wherein the lifting stage (12) and the lowering stage (13) respectively comprise a working port (12.1, 12.2, 13.1, 13.2) which can be fluidically connected to the hydraulic system (10) of the industrial truck (1) and to the hydraulic lifting cylinder (9), wherein the valve device (11) comprises an electrically controllable pilot valve (14) which is fluidically connected to a control port (12.3) of the lifting stage (12) and to a control port (13.3) of the lowering stage (13), characterized in that the pilot valve (14) is constructed and can be controlled in a manner such that it increases a control pressure for controlling the lifting stage (12) and the lowering stage (13) during a lifting procedure or during a lowering procedure of the hydraulic lifting cylinder (9) by a predeterminable pressure value before reaching of an end position of the hydraulic lifting cylinder (9).

2. The valve device (11) according to claim 1, characterized in that the pilot valve (14) is constructed and can be controlled in a manner such that an increase in the control pressure for controlling the lifting stage (12) and the lowering stage (13) by the predeterminable pressure value is carried out gradually.

3. The valve device (11) according to claim 1 or claim 2, characterized in that the pilot valve (14) is constructed and can be controlled in a manner such as to increase the control pressure for controlling the lifting stage (12) and the lowering stage (13) above the predeterminable pressure value immediately before reaching of the end position of the hydraulic lifting cylinder (9).

4. The valve device (11) according to one of claims 1 to 3, characterized in that the pilot valve (14) is constructed and can be controlled in a manner such that the pilot valve (14) takes up an open position at the beginning of a lifting procedure and a lowering procedure.

5. The valve device (11) according to one of claims 1 to 4, characterized in that the pilot valve (14) is an inverse proportional pressure relief valve.

6. The valve device (11) according to claims 1 to 5, characterized in that an intake (15) of the pilot valve (14) is fluidically connected to the control port (12.3) of the lifting stage (12) and to the control port (13.3) of the lowering stage (13).

7. The valve device (11) according to claim 6, characterized in that the valve device (11) comprises a control line (17) which is connected in parallel to the lifting stage (12), the control line being fluidically connected to the control port (12.3) of the lifting stage (12), to the control port (13.3) of the lowering stage (13) and to the intake (15) of the pilot valve (14) and being capable of being fluidically connected to the hydraulic system (10) of the industrial truck (1).

8. The valve device (11) according to claim 6 or claim 7, characterized in that the valve device (11) comprises a control line (18) which is connected in parallel to the lowering stage (13), the control line being fluidically connected to the control port (12.3) of the lifting stage (12), to the control port (13.3) of the lowering stage (13) and to the intake (15) of the pilot valve (14) and being capable of being fluidically connected to the hydraulic lifting cylinder (9).

9. The valve device (11) according to claim 7 or claim 8, characterized in that the control line (17) which is connected in parallel to the lifting stage (12) and / or the control line (18) which is connected in parallel to the lowering stage (13) comprises an orifice plate (19, 21) and a check valve (20, 22).

10. The valve device (11) according to one of claims 6 to 9, characterized in that the valve device (11) comprises a shut-off valve (29) which is interposed between the control ports (12.3, 13.3) of the lifting stage (12) and lowering stage (13) and the intake (15) of the pilot valve (14), wherein the shut-off valve (29) comprises two control ports (29.1, 29.2), wherein one control port (29.1) can be fluidically connected to the hydraulic lifting cylinder (9) and the other control port (29.2) can be fluidically connected to the hydraulic system (10) of the industrial truck (1), wherein preferably, a switching pressure of the shut-off valve (29) is lower than an upper limiting value of a control range of the pilot valve (14).

11. The valve device (11) according to one of claims 1 to 10, characterized in that a control range of the pilot valve (14) lies between 0 bar and a pressure value above the load pressure of the hydraulic lifting cylinder, wherein preferably, the control range of the pilot valve (14) lies between 0 bar and a pressure value above the switching pressure of the shut-off valve (29).

12. The valve device (11) according to claim 11, characterized in that the pilot valve (14) is constructed and can be controlled in a manner such that the pilot valve (14) is adjusted to a differential pressure value before reaching of the end position of the hydraulic lifting cylinder (9).

13. The valve device (11) according to one of claims 1 to 12, characterized in that the valve device (11) comprises a shuttle valve (23) which is fluidically connected to a return line (24) of the pilot valve (14), wherein furthermore, the shuttle valve (23) can be fluidically connected to the hydraulic system (10) of the industrial truck (1) and to the hydraulic lifting cylinder (9).

14. The valve device (11) according to one of claims 1 to 13, characterized in that the valve device (11) comprises a bypass (27) which can be fluidically connected to the hydraulic lifting cylinder (9) and to the hydraulic system (10) of the industrial truck (1), wherein the bypass (27) comprises an orifice plate (28).

15. An industrial truck (1) with a lift mast (2) with at least one mast lifting stage which is driven by at least one mast lifting cylinder (9b), preferably two mast lifting cylinders (9b), with a free lifting stage which is driven by at least one free lifting cylinder (9a), with which free lifting stage a load-carrying means (3) can be moved along the lift mast (2), and with a hydraulic system (10) for supplying the at least one free lifting cylinder (9a) and the at least one mast lifting cylinder (9b) with a hydraulic fluid, wherein respectively, at least one valve device (11) is interposed between the hydraulic system (10) and the at least one free lifting cylinder (9a) as well as between the hydraulic system (10) and the at least one mast lifting cylinder (9b), wherein the industrial truck (1) comprises a control device for controlling the valve devices (11), wherein a distance measuring system (16) is respectively associated with the at least one free lifting cylinder (9a) and the at least one mast lifting cylinder (9b), the distance measuring system being connected to the control device for the transmission of data, characterized in that the valve devices (11) are respectively configured as a valve device (11) according to one of claims 1 to 14.