Industrial truck comprising a hydraulic system and at least one hydraulically operated component and method for operating a hydraulic system of an industrial truck

Abstract

The invention relates to a forklift truck (2) comprising a hydraulic system (10) and at least one hydraulically operated component (30), as well as a method for operating a hydraulic system (10) of the forklift truck (2). The hydraulic system (10) comprises a first hydraulic pump (11), a second hydraulic pump (12), and a hydraulic line system (15). The hydraulic line system (15) is designed and configured to connect the hydraulically operated component (30) selectively either exclusively to the first hydraulic pump (11), exclusively to the second hydraulic pump (12), or simultaneously to both the first hydraulic pump (11) and the second hydraulic pump (12). The first hydraulic pump (11) has a maximum efficiency at a first flow rate (102) and a first operating pressure (101). The second hydraulic pump (12) has a second maximum efficiency at a second flow rate (102) and a second operating pressure (101).The first operating pressure (101) is greater than the second operating pressure (101) and / or the first volume flow rate (102) is smaller than the second volume flow rate (102).

Description

[0001] The invention relates to a forklift truck comprising a hydraulic system and at least one hydraulically operated component. The invention also relates to a method for operating a hydraulic system of a forklift truck.

[0002] Many industrial trucks are equipped with a hydraulic system to operate various hydraulically driven components. A classic example is the forklift's load forks, which are raised and lowered hydraulically. However, there are also a number of other adjustment functions performed by hydraulically driven components. These include, for example, tilting the load forks, tilting the mast, lateral movement of the load forks, or, in the case of reach trucks or similar industrial trucks, mast extension. Furthermore, many industrial trucks utilize attachments with hydraulically driven components.

[0003] A key aspect of hydraulic system design is selecting a suitable hydraulic pump to power the hydraulically operated components. For example, external gear pumps and internal gear pumps are used as hydraulic pumps for industrial trucks. Axial piston pumps, radial piston pumps, and vane pumps can also be used as hydraulic pumps for industrial trucks. The different types of pumps vary in that they achieve high efficiency at different speeds, operating pressures, and temperature ranges. The noise emissions of hydraulic pumps also differ, depending on the required operating pressures and speeds.

[0004] If the hydraulic system operates only a single hydraulically driven component with a single adjustment function, the appropriate hydraulic pump can be selected based on the pump speed and operating pressure required for that adjustment function to optimize efficiency and noise emissions. However, if the industrial truck is designed to perform several different adjustment functions, the hydraulic pump cannot execute all adjustment functions at optimal efficiency.

[0005] The object of the invention is to improve the efficiency and noise emission of a hydraulic system of a forklift truck with several hydraulic adjustment functions.

[0006] This problem is solved by a forklift truck comprising a hydraulic system and at least one hydraulically operated component, wherein the hydraulic system has a first hydraulic pump, a second hydraulic pump and a hydraulic piping system, wherein the hydraulic piping system is designed and configured to connect the hydraulically operated component selectively either exclusively to the first hydraulic pump or exclusively to the second hydraulic pump or simultaneously to the first hydraulic pump and the second hydraulic pump, wherein the first hydraulic pump has a first efficiency maximum at a first flow rate and a first operating pressure and the second hydraulic pump has a second efficiency maximum at a second flow rate and a second operating pressure, wherein the first operating pressure is greater than the second operating pressure and / or the first flow rate is less than the second flow rate.

[0007] The efficiency and noise emissions of the hydraulic system can be advantageously improved by using two different hydraulic pumps, as the appropriate hydraulic pump is selected for each adjustment function. For example, the first hydraulic pump is chosen so that it achieves its maximum efficiency at a first flow rate that is lower than the second flow rate at which the second hydraulic pump achieves its maximum efficiency. The flow rate is the volume of hydraulic fluid that drives the hydraulically operated component. The first maximum efficiency is that of the first hydraulic pump, and the second maximum efficiency is that of the second hydraulic pump. Furthermore, the first operating pressure at which the first hydraulic pump reaches its baseline maximum efficiency is higher than the second operating pressure at which the second hydraulic pump reaches its maximum efficiency.In this way, based on the required operating pressures and volume flows of the adjustment function, it is possible to select which of the hydraulic pumps or combination of hydraulic pumps is better suited to perform this adjustment function.

[0008] The required flow rate for an adjustment function is achieved by selecting an appropriate pump speed for the hydraulic pumps. The pump speed necessary to achieve a specific flow rate is calculated by dividing the flow rate by the known displacement volume. The displacement volume is the volume displaced by all displacement elements of the hydraulic pump during one revolution. The displacement volumes of the first and second hydraulic pumps are not necessarily identical. If the displacement volumes of the two hydraulic pumps differ, they will require different pump speeds to achieve the same flow rate. Therefore, the flow rate at which a hydraulic pump reaches its maximum efficiency depends on the pump speed at which this maximum efficiency is reached.When several hydraulic pumps are used simultaneously, the volume flow generated by the hydraulic pumps usually adds up.

[0009] The at least one hydraulically operated component comprises, in particular, at least one hydraulically operated cylinder and / or one hydraulically operated motor. In particular, the at least one hydraulically operated component comprises all hydraulically operated components of the industrial truck. The first hydraulic pump is, in particular, a pump optimized for high operating pressure. The second hydraulic pump is, in particular, a noise-optimized pump and / or a pump optimized for high pump speed or high flow rate. Noise-optimized pumps, in particular, achieve their maximum efficiency at high pump speed or high flow rate, and especially at a comparatively low operating pressure.

[0010] According to one embodiment, the first hydraulic pump is an internal gear pump and / or the second hydraulic pump is an external gear pump. These pump types differ in that they achieve their maximum efficiency in different flow rate and pressure ranges, depending on the operating temperature. Internal gear pumps maintain good efficiency at high operating pressures and low pump speeds or flow rates over a wide temperature range compared to external gear pumps. External gear pumps, on the other hand, have the advantage of often operating significantly quieter than internal gear pumps and exhibiting good efficiency in operating ranges of medium to high speeds or flow rates.By incorporating these two types of hydraulic pumps into the hydraulic system, the system can be optimized for efficiency and noise emission according to the requirements of the respective hydraulic adjustment function. An axial piston pump, a radial piston pump, or a vane pump can also be used as the first and / or second hydraulic pump. Crucially, the two pumps differ in that they achieve their maximum efficiency at different operating pressures and / or pump speeds.

[0011] Preferably, the hydraulic piping system comprises at least one hydraulic valve, wherein the at least one hydraulic valve has a first position, a second position, and a third position, wherein in the first position the at least one hydraulically operated component is connected exclusively to the first hydraulic pump, wherein in the second position the at least one hydraulically operated component is connected exclusively to the second hydraulic pump, and wherein in the third position the at least one hydraulically operated component is connected simultaneously to the first hydraulic pump and the second hydraulic pump. By selecting the first position, the second position, or the third position, the at least one hydraulically operated component can thus be operated exclusively by the first hydraulic pump, exclusively by the second hydraulic pump, or simultaneously by the first hydraulic pump and the second hydraulic pump.Consequently, at least one hydraulic valve allows the hydraulically operated component to be operated with the appropriate hydraulic pump for the currently executed adjustment function. To select between the first, second, and third positions, either a single hydraulic valve or multiple hydraulic valves can be used. For example, a single hydraulic valve can be provided that connects a specific hydraulically operated component to either the first hydraulic pump, the second hydraulic pump, or both hydraulic pumps. Alternatively, a first hydraulic valve can open or close a connection between the component and the first hydraulic pump, and a second hydraulic valve can open or close a connection between the component and the second hydraulic pump, thus achieving the three positions.In particular, at least one hydraulic valve has a fourth position in which the hydraulically operated component is not driven by any hydraulic pump.

[0012] Preferably, the at least one hydraulically operated component is configured to perform at least two different hydraulically driven adjustment functions that differ with respect to a required pump speed and / or a required flow rate. The term "hydraulically driven adjustment functions" includes, in particular, all hydraulic adjustment functions of the industrial truck. This includes, in particular, various mast lifting functions with different speeds and / or tilting of the load-handling device and / or tilting of the mast and / or extension of the mast and / or similar adjustment functions. In particular, the at least one hydraulically operated component comprises at least two different hydraulically operated components.For example, the industrial truck includes a lifting cylinder for raising and lowering the load-handling attachment and another hydraulic cylinder or hydraulically driven motor, e.g., for tilting the load-handling attachment. The at least two different adjustment functions can be performed by a single hydraulically driven component or by several different hydraulically driven components. In particular, the first, second, and third positions of the at least one hydraulic valve can be set separately for each hydraulically driven component. In other words, it can be selected separately for each hydraulically driven component whether it is connected exclusively to the first hydraulic pump, exclusively to the second hydraulic pump, or simultaneously to both the first and second hydraulic pumps.

[0013] According to one embodiment, the industrial truck comprises a control device, wherein the control device is configured to access a data storage device on which a function list is stored, in which the required operating pressure and the required flow rate are stored for each of the at least two different adjustment functions, wherein the control device is further configured to retrieve from the function list the operating pressure and the flow rate required for an adjustment function to be executed and to control the at least one hydraulic valve in order to move the at least one hydraulic valve into the first position or the second position or the third position depending on the operating pressure and the flow rate required for this adjustment function.The control device is advantageously designed to select a suitable hydraulic pump or pumps for a given adjustment function in order to optimize efficiency and noise emission depending on the application.

[0014] The pump speed affects the flow rate. The higher the pump speed, the higher the flow rate. With a higher flow rate, a hydraulic cylinder, for example, is deflected more quickly. Thus, the pump speed, or rather the flow rate, affects how quickly, for example, the load-handling attachment is raised or lowered. The operating pressure affects the force exerted by the hydraulically operated component. The higher the operating pressure, the greater the force exerted. Certain adjustment functions, such as the lifting of the load-handling attachment, require a significantly higher operating pressure than other adjustment functions, such as mast extension on a reach truck.

[0015] In particular, the function list specifies an optimal temperature range for the hydraulic fluid for each adjustment function. The control device is specifically designed to also consider the temperature of the hydraulic fluid for the adjustment function to be performed when controlling the hydraulic valve and to compare this temperature with a measured current temperature of the hydraulic fluid.

[0016] The control device is, in particular, a control system for the industrial truck. For example, the control device is a computer or similar data processing device configured to control the various functions of the industrial truck. The data storage device is, in particular, a component of the control device or connected to the control device. Alternatively, the data storage device is separate from the control device and, in particular, separate from the industrial truck. For example, the data storage device is an external data storage device in a goods logistics system, and the control device is configured to access the data storage device wirelessly.

[0017] The adjustment function to be executed is one of at least two different hydraulically driven adjustment functions that is currently being performed or is about to be performed. For example, a forklift operator activates a control element to raise the forklift's load-handling attachment at high speed. The control device then accesses the data storage to retrieve the required flow rate, operating pressure, and, in particular, a temperature range for this adjustment function.Subsequently, depending on the required flow rate and operating pressure, and in particular the operating temperature, the control device selects either the first position, the second position, or the third position to actuate the at least one hydraulic valve in such a way that the hydraulically operated component, which performs the adjustment function to be carried out, is driven by the appropriate hydraulic pump or the appropriate combination of hydraulic pumps.

[0018] Preferably, the control device is configured to compare the operating pressure required for the adjustment function to be performed with a limit pressure in order to select the position to which the at least one hydraulic valve is moved. In other words, the required operating pressure is compared with a limit pressure to select the appropriate hydraulic pump or combination of hydraulic pumps. If, for example, the required operating pressure is lower than the limit pressure, the at least one hydraulic valve is moved to the second position, for example, so that the at least one hydraulically operated component is driven exclusively by the second hydraulic pump. Otherwise, the at least one hydraulic valve is moved to either the first or the third position. In this way, the selection of the hydraulic pump or hydraulic pumps is made dependent on the power requirement of the adjustment function to be performed.

[0019] Preferably, the control device is configured to compare the volume flow required for the adjustment function with a first limiting volume flow and a second limiting volume flow in order to select the position to which the at least one hydraulic valve is moved. The second limiting volume flow is particularly greater than the first limiting volume flow. By comparing the required volume flow with the first limiting volume flow and the second limiting rotational volume flow, the appropriate hydraulic pump or combination of hydraulic pumps is selected, depending on the required volume flow for the adjustment function. For example, if the required volume flow is very low, the first hydraulic pump is selected; for a medium required volume flow, the second hydraulic pump is selected; and for a very high required volume flow, a combination of both hydraulic pumps is selected.In this way, for example, a different hydraulic pump or combination of hydraulic pumps is selected to operate the lifting cylinder at different speeds when lifting the load-handling device.

[0020] Preferably, the control device is configured to operate the at least one hydraulically driven component exclusively by the first hydraulic pump, provided the required operating pressure is greater than the limit pressure and the required flow rate is less than the first limit flow rate. If the required operating pressure is greater than the limit pressure and the required flow rate is less than the first limit flow rate, the control device moves the at least one hydraulic valve to the first position, so that only the first hydraulic pump operates the hydraulically driven component. This is useful, for example, for optimizing efficiency during slow lifting of the load-handling device, during tilting of the load-handling device, during tilting of the mast, or during lateral thrust of the load-handling device.

[0021] Preferably, the control device is configured to operate the at least one hydraulically driven component exclusively via the second hydraulic pump, provided the required operating pressure is lower than the limit pressure or the required flow rate is higher than the first limit flow rate but lower than the second limit flow rate. In this case, the at least one hydraulic valve is thus moved to the second position. This is advantageous, for example, when lifting the load-handling attachment at a medium speed, as a medium to high pump speed is required in this case, even if the operating pressure is comparatively high. The second position is also advantageous for mast extension, as implemented, for example, in reach trucks, since only a low operating pressure is necessary here, and a medium pump speed or flow rate is typically used.

[0022] Preferably, the control device is configured to allow the at least one hydraulically operated component to be operated simultaneously by the first hydraulic pump and the second hydraulic pump, provided the required flow rate is greater than the second limit flow rate. In this case, the at least one hydraulic valve is moved to the third position. This is advantageous, for example, when rapidly lifting the load-handling device, as this requires a very high pump speed or a very high flow rate, as well as a high operating pressure. The operating pressure is particularly higher than the limit pressure when the at least one hydraulically operated component is operated by both hydraulic pumps. At a lower operating pressure, the second hydraulic pump can also be used alone.

[0023] In cases where the operating pressure exactly matches the limit pressure, the pump can be selected based on preference: either for an operating pressure lower than the limit pressure or for an operating pressure higher than the limit pressure. This also applies to the flow rate compared to the first limit flow rate and the second limit flow rate. In an intermediate flow rate range where both hydraulic pumps could operate with similar efficiency, the pump with the lower noise level is generally used.

[0024] According to one embodiment, the at least one hydraulically operated component comprises at least one hydraulic lifting cylinder for raising and / or lowering a load-handling attachment of the industrial truck, wherein the control device is configured to operate the at least one lifting cylinder, depending on the deflection speed of the lifting cylinder, either exclusively by the first hydraulic pump, exclusively by the second hydraulic pump, or simultaneously by the first and second hydraulic pumps. The load-handling attachment is, for example, a fork or a transport platform of the industrial truck. By configuring the control device to move the hydraulic valve to either the first position, the second position, or the third position, depending on the deflection speed, the lifting cylinder is always driven with good efficiency and no unnecessary noise emissions are generated.

[0025] According to a further embodiment, the at least one hydraulically operated component comprises an auxiliary component, wherein the control device is configured to operate the auxiliary component exclusively by the second hydraulic pump. The auxiliary component performs a secondary function and differs from the hydraulic cylinder for raising and / or lowering the load-handling attachment. The auxiliary component comprises, for example, a hydraulic motor or a hydraulic cylinder, which is provided, for example, for tilting the load-handling attachment or for mast extension in a reach truck. In particular, the auxiliary component performs at least one adjustment function that requires a lower operating pressure than the lifting cylinder for raising and / or lowering the load-handling attachment. In particular, the auxiliary components are driven by the second hydraulic pump.

[0026] According to one embodiment, the control device is configured to regulate the efficiency of the first hydraulic pump and / or the second hydraulic pump when both are operating simultaneously, specifically by reducing pump power and / or flow rate. Regulating the efficiency is particularly effective in improving the efficiency of both hydraulic pumps during simultaneous operation. The efficiency is the portion of the force exerted by the hydraulically driven component that is attributable to the respective hydraulic pump. If the at least one hydraulic valve is in the third position, both hydraulic pumps simultaneously drive the at least one hydraulically driven component.In this case, it may be advantageous for one or both of these hydraulic pumps to operate at reduced capacity to improve efficiency and / or noise emissions. In other words, the hydraulic pumps are operated optimally with varying power outputs according to their efficiency. Specifically, the function list for at least one adjustment function includes a reduced power output for the first hydraulic pump and / or the second hydraulic pump. The control device is specifically configured to access the function list and control the first hydraulic pump and / or the second hydraulic pump to reduce the power output of the hydraulic pumps according to the stored power outputs.

[0027] The problem is further solved by a method for operating a hydraulic system of a forklift truck, wherein the hydraulic system comprises a first hydraulic pump, a second hydraulic pump and a hydraulic piping system, wherein the hydraulic piping system connects at least one hydraulically operated component optionally exclusively to the first hydraulic pump or exclusively to the second hydraulic pump or simultaneously to the first hydraulic pump and the second hydraulic pump, wherein the first hydraulic pump has a first efficiency maximum at a first flow rate and a first operating pressure and the second hydraulic pump has a second efficiency maximum at a second flow rate and a second operating pressure, wherein the first operating pressure is greater than the second operating pressure and / or the first flow rate is less than the second flow rate.

[0028] The method for operating a hydraulic system of a forklift truck embodies the same advantages, features and characteristics as the forklift truck described above.

[0029] Preferably, the hydraulic piping system comprises at least one hydraulic valve, wherein the at least one hydraulic valve is selectively set to a first position, a second position or a third position, wherein in the first position the at least one hydraulically operated component is connected exclusively to the first hydraulic pump, wherein in the second position the at least one hydraulically operated component is connected exclusively to the second hydraulic pump, wherein in the third position the at least one hydraulically operated component is connected simultaneously to the first hydraulic pump and the second hydraulic pump.

[0030] Preferably, the at least one hydraulically operated component performs at least two different adjustment functions that differ with regard to a required volume flow rate and / or a required operating pressure.

[0031] Preferably, a control device of the industrial truck accesses a data storage device on which a function list is stored, in which the required operating pressure and the required flow rate are stored for each of the at least two different adjustment functions, wherein the control device retrieves the required operating pressure and the required flow rate for an adjustment function to be executed from the function list and controls the at least one hydraulic valve in order to move the at least one hydraulic valve into the first position or the second position or the third position depending on the required operating pressure and the required flow rate for this adjustment function.

[0032] Preferably, the control device regulates a proportion of the efficiency of the first hydraulic pump and / or the second hydraulic pump when the first hydraulic pump and the second hydraulic pump are operated simultaneously, wherein the proportion of efficiency is regulated in particular by reducing a pump output and / or a flow rate.

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

[0034] Within the scope of the invention, features marked with "in particular" or "preferably" are to be understood as optional features.

[0035] The invention is described below, without limiting the general concept of the invention, with reference to exemplary embodiments and the drawings, whereby for all details of the invention not explained in detail in the text, explicit reference is made to the drawings. The drawings show: Fig. 1 a schematically simplified perspective view of a forklift truck, Fig. 2 a schematically simplified view of a hydraulic system of a forklift truck, Fig. 3 a schematically simplified cross-sectional view of an internal gear pump, Fig. 4 a schematically simplified cross-sectional view of an external gear pump, and Fig. 5 a schematically simplified flow diagram of a method for operating a hydraulic system of a forklift truck with two different hydraulic pumps.

[0036] In the drawings, identical or similar elements and / or parts are provided with the same reference numbers, so that a re-presentation is omitted.

[0037] In Fig. 1 A simplified schematic representation of a reach truck (2) is shown in perspective. The truck includes a driver's cab (3) and several wheels, of which in Fig. 1Only the drive wheel 4 is visible. The reach truck comprises two wheel arms 5, of which only one is visible. A load-handling device 7 in the form of a fork is arranged on one mast 6 of the industrial truck 2, which can be raised and lowered by means of a lifting cylinder that is not visible. With a reach truck, it is also possible to move the mast 6 together with the load-handling device 7 horizontally forwards and backwards between the wheel arms 5. The mast 6 and / or the load-handling device 7 of an industrial truck 2 can also be tilted or moved laterally in multiple positions. A hydraulic system, controlled by a control device 8, is generally provided to operate such adjustment functions. The control device 8 is located in Fig. 1 It is shown with dashed lines because it is hidden inside the forklift truck 2.

[0038] Fig. 2shows a schematically simplified exemplary embodiment of a hydraulic system 10, as used, for example, in the industrial truck 2 from Fig. 1The hydraulic system 10 is connected to several hydraulically operated components 30, which are driven by the hydraulic system 10. For this purpose, the hydraulic system 10 includes a tank 14 containing hydraulic fluid. A hydraulic line system 15 with several hydraulic lines connects the tank 14 to a first hydraulic pump 11 and a second hydraulic pump 12, each driven by a motor 13. The first hydraulic pump 11 differs from the second hydraulic pump 12 in that it achieves its maximum efficiency at a higher operating pressure and / or a lower pump speed than the second hydraulic pump 12. The hydraulic line system 15 connects the two hydraulic pumps 11 and 12 to a switching system 25, which controls several hydraulic valves 20.Specifically, the first hydraulic pump 11 and the second hydraulic pump 12 are connected to a 2 / 2-way valve 21, another 2 / 2-way valve 22, and two 4 / 3-way valves 23, 24, as well as several vent ports 16, which together form the hydraulic valves 20 of the hydraulic system. A lifting cylinder 31 is supplied with hydraulic fluid from the hydraulic pumps 11, 12 via the 2 / 2-way valve 22, while the 2 / 2-way valve 21 returns the hydraulic fluid. The 2 / 2-way valves 21, 22 can each either open or close the flow in one direction. In the illustrated embodiment, two auxiliary components 32 are each double-acting cylinders. One of the 4 / 3-way valves 23, 24 is provided between each of the hydraulic pumps 11, 12 and the auxiliary components 32.The 4 / 3 directional valves each regulate in which direction the hydraulic fluid flows into the double-acting cylinder and from which direction the hydraulic fluid flows into a drain line, which in turn connects the auxiliary components 32 to the tank 14.

[0039] Through the in Fig. 2The hydraulic system 10 shown can be operated by switching the hydraulic valves 20, i.e., valves 21, 22, 23, 24 and the vent ports 16, either exclusively with the first hydraulic pump 11, exclusively with the second hydraulic pump 12, or simultaneously with the first hydraulic pump 11 and the second hydraulic pump 12. Thus, by appropriately adjusting the hydraulic valves 20, a first position (exclusively the first hydraulic pump 11), a second position (exclusively the second hydraulic pump 12), or a third position (both hydraulic pumps 11, 12 simultaneously) can be set for each component 30. In addition, a fourth position can be set for each hydraulically operated component 30, in which the hydraulically operated component 30 is not driven by either hydraulic pump 11 or 12.

[0040] Through the in Fig. 2In the hydraulic system 10 shown, it is possible to select an optimal hydraulic pump 11, 12 or combinations of hydraulic pumps 11, 12 for each adjustment function of the industrial truck 2 performed by the hydraulically operated components 30, so that the hydraulic pumps 11, 12 have good efficiency and noise emissions are minimized. Likewise, the control device 8 can be provided to access the hydraulic pumps 11, 12, and in particular the motor 13 of the hydraulic pumps 11, 12, to reduce the efficiency contribution of the hydraulic pumps 11, 12 when both hydraulic pumps 11, 12 are simultaneously connected to a specific hydraulically operated component 30. In this way, the efficiency of the hydraulic pumps 11, 12 can be further improved.

[0041] Fig. 3Figure 1 shows a schematically simplified cross-section of an exemplary embodiment of the first hydraulic pump 11. In this example, the hydraulic pump 11 shown is an internal gear pump 40 in the form of a so-called crescent pump. The internal gear pump 40 has an outer toothed ring 41 with internal teeth into which the teeth of an eccentrically arranged inner gear 43 engage. A crescent 45 is also arranged between the gear 43 and the toothed ring 41. The direction of rotation 42 of the toothed ring 41 is identical to the direction of rotation 44 of the gear 43. Hydraulic fluid enters the space between the toothed ring 41 and the gear 43 from below in the inlet direction 60. The hydraulic fluid is guided from bottom to top along two paths to the left and right of the crescent 45 and exits the internal gear pump 40 in the outlet direction 61.Such internal gear pumps 40 exhibit high efficiency at high operating pressures and low pump speeds or volume flows, even over a wide temperature range.

[0042] Fig. 4 Figure 1 shows a schematically simplified cross-sectional view of a second hydraulic pump 12 in the form of an external gear pump 50.

[0043] The external gear pump 50 comprises a first gear 51 and a second gear 53, whose teeth mesh. The direction of rotation 52 of the first gear 51 is opposite to the direction of rotation 51 of the second gear 53. When hydraulic fluid enters the external gear pump 50 from below in the inlet direction 60, the hydraulic fluid is conveyed by the two gears 51, 53 along the outside of the gears 51, 53 in the direction of rotation 52, 54 and finally exits the external gear pump 50 at the top in the outlet direction 61. Such external gear pumps 50 are often significantly quieter than internal gear pumps 40 and have good efficiency at medium to high pump speeds or flow rates.

[0044] Fig. 5Figure 1 shows a schematically simplified flowchart of a procedure for operating a hydraulic system 10 of a forklift truck 2. First, an adjustment function 100 is selected to be executed. For example, the operator of the forklift truck 2 activates a control element to execute the adjustment function 100. Next, it is checked whether the operating pressure 101 is lower or higher than a previously defined limit pressure. If the operating pressure 101 is lower than the limit pressure, meaning a lower pressure 110 is required for the adjustment function 100, the hydraulic valves 20 are moved to the second position, in which the hydraulically driven component 30, which performs the adjustment function 100, is connected exclusively to the second hydraulic pump 12. If the operating pressure 101 is higher than the limit pressure, meaning a high pressure 111 is present, the flow rate 102 required to execute the adjustment function 100 is then checked.For the required flow rate 102, a first limiting flow rate and a higher, second limiting flow rate are defined in the illustrated embodiment. If the flow rate 102 is lower than the first limiting flow rate, i.e., a low flow rate 120, the hydraulically operated component 30 is driven exclusively by the first hydraulic pump 11. If the flow rate 102 is higher than the first limiting flow rate but lower than the second limiting flow rate, i.e., a medium to high flow rate 121, the hydraulically operated component 30 is driven exclusively by the second hydraulic pump 12. At a very high flow rate 122, where the flow rate 102 is higher than the second limiting flow rate, the hydraulically operated component 30 is driven by both hydraulic pumps 11 and 12.In this way, depending on the required volume flow and the required operating pressure 101 of the adjustment function 100 to be performed, the hydraulic pump 11, 12 or combination of hydraulic pumps 11, 12 is selected with which an optimal efficiency and a reduced noise emission is achieved.

[0045] All features mentioned, including those discernible 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. Inventive embodiments may be fulfilled by individual features or by a combination of several features. Reference symbol list

[0046] 2 Forklift 3 Driver's cab 4 Drive wheel 5 Wheel arm 6 Mast 7 Load handling device 8 Control device 10 Hydraulic system 11 First hydraulic pump 12 Second hydraulic pump 13 Motor 14 Tank 15 Hydraulic line system 16 Vent connection 20 Hydraulic valve 21 2 / 2-way valve 22 2 / 2-way valve 23 4 / 3-way valve 24 4 / 3-way valve 25 Switching system 30 Hydraulically operated component 31 Lifting cylinder 32 Auxiliary component 40 Internal gear pump 41 Gear ring 42 Direction of rotation 43 Gear 44 Direction of rotation 45 Sickle 50 External gear pump 51 First gear 52 Direction of rotation 53 Second gear 54 Direction of rotation 60 Inlet direction 61 Outlet direction 100 Adjustment function 101 Operating pressure 102 Volume flow rate 110 Low pressure 111 High pressure 120 Low volume flow rate 121 High volume flow rate 122 Very high volume flow rate

Claims

1. Industrial truck (2) comprising a hydraulic system (10) and at least one hydraulically operated component (30), wherein the hydraulic system (10) comprises a first hydraulic pump (11), a second hydraulic pump (12) and a hydraulic piping system (15), wherein the hydraulic piping system (15) is designed and configured to connect the hydraulically operated component (30) optionally exclusively to the first hydraulic pump (11) or exclusively to the second hydraulic pump (12) or simultaneously to the first hydraulic pump (11) and the second hydraulic pump (12), wherein the first hydraulic pump (11) has a first efficiency maximum at a first flow rate (102) and a first operating pressure (101) and the second hydraulic pump (12) has a second efficiency maximum at a second flow rate (102) and a second operating pressure (101),where the first operating pressure (101) is greater than the second operating pressure (101) and / or the first volume flow rate (102) is less than the second volume flow rate (102).

2. Industrial truck (2) according to claim 1, characterized by the fact that the first hydraulic pump (11) is an internal gear pump (40) and / or the second hydraulic pump (12) is an external gear pump (50).

3. Industrial truck (2) according to claim 1 or 2, characterized by the fact thatthe hydraulic piping system (10) comprises at least one hydraulic valve (20), wherein the at least one hydraulic valve (20) has a first position, a second position and a third position, wherein in the first position the at least one hydraulically operated component (30) is exclusively connected to the first hydraulic pump (11), wherein in the second position the at least one hydraulically operated component (30) is exclusively connected to the second hydraulic pump (12), wherein in the third position the at least one hydraulically operated component (30) is simultaneously connected to the first hydraulic pump (11) and the second hydraulic pump (12).

4. Industrial truck (2) according to one of claims 1 to 3, characterized by the fact thatthe at least one hydraulically operated component (30) is designed to perform at least two different hydraulically driven adjustment functions (100) which differ with regard to a required volume flow (102) and / or a required operating pressure (101).

5. Industrial truck (2) according to claims 3 and 4 comprising a control device (8), wherein the control device (8) is configured to access a data storage device on which a function list is stored, in which the required operating pressure (101) and the required flow rate (102) are stored for each of the at least two different adjustment functions (100), wherein the control device (8) is further configured to retrieve from the function list the operating pressure (101) and the flow rate (102) required for an adjustment function (100) to be performed and to control the at least one hydraulic valve (30),to move at least one hydraulic valve (30) into the first position, the second position, or the third position depending on the operating pressure (101) and the volume flow rate (102) required for this adjustment function (100).

6. Industrial truck (2) according to claim 5, characterized by the fact that the control device (8) is configured to compare the operating pressure (101) required for the adjustment function (100) to be performed with a limit pressure in order to select the position to which the at least one hydraulic valve (30) is moved.

7. Industrial truck (2) according to claim 5 or 6, characterized by the fact that the control device (8) is configured to compare the volume flow (102) required for the adjustment function (100) to be performed with a first limit volume flow and a second limit volume flow in order to select the position to which the at least one hydraulic valve (30) is transferred.

8. Industrial truck (2) according to claim 6 and 7, characterized by the fact that the control device (8) is configured to operate the at least one hydraulically operated component (30) exclusively by the first hydraulic pump (11) if the required operating pressure (101) is greater than the limit pressure and the required volume flow (102) is less than the first limit volume flow and / or to operate the at least one hydraulically operated component (30) exclusively by the second hydraulic pump (12) if the required operating pressure (101) is less than the limit pressure or the required volume flow (102) is greater than the first limit volume flow and less than the second limit volume flow and / or to operate the at least one hydraulically operated component (30) simultaneously by the first hydraulic pump (11) and the second hydraulic pump (12) if the required volume flow (102) is greater than the second limit volume flow.

9. Industrial truck (2) according to one of claims 5 to 8, characterized by the fact that the at least one hydraulically operated component (30) comprises at least one hydraulic lifting cylinder (31) for raising and / or lowering a load handling device (7) of the industrial truck (2), wherein the control device (8) is configured to allow the at least one lifting cylinder (31) to be operated exclusively by the first hydraulic pump (11) or exclusively by the second hydraulic pump (12) or simultaneously by the first hydraulic pump (11) and the second hydraulic pump (12), depending on a deflection speed of the lifting cylinder (31).

10. Industrial truck (2) according to one of claims 5 to 9, characterized by the fact thatthe control device (8) is configured to regulate the efficiency of the first hydraulic pump (11) and / or the second hydraulic pump (12) when the first hydraulic pump (11) and the second hydraulic pump (12) are operated simultaneously, in particular by regulating the efficiency by reducing a pump power and / or a flow rate.

11. Method for operating a hydraulic system (10) of a forklift truck (2), wherein the hydraulic system (10) comprises a first hydraulic pump (11), a second hydraulic pump (12) and a hydraulic piping system (15), wherein the hydraulic piping system (15) connects at least one hydraulically operated component (30) optionally exclusively to the first hydraulic pump (11) or exclusively to the second hydraulic pump (12) or simultaneously to the first hydraulic pump (11) and the second hydraulic pump (12), wherein the first hydraulic pump (11) has a first efficiency maximum at a first flow rate (102) and a first operating pressure (101) and the second hydraulic pump (12) has a second efficiency maximum at a second flow rate (102) and a second operating pressure (101), wherein the first operating pressure (101) is greater than the second operating pressure (101) and / or the first flow rate (102) is less than the second flow rate (102) is.

12. Method according to claim 11, characterized by the fact that the hydraulic piping system (15) comprises at least one hydraulic valve (20), wherein the at least one hydraulic valve (20) is selectively set to a first position, a second position or a third position, wherein in the first position the at least one hydraulically operated component (30) is exclusively connected to the first hydraulic pump (11), wherein in the second position the at least one hydraulically operated component (30) is exclusively connected to the second hydraulic pump (12), wherein in the third position the at least one hydraulically operated component (30) is simultaneously connected to the first hydraulic pump (11) and the second hydraulic pump (12).

13. Method according to claim 11 or 12, characterized by the fact thatthe at least one hydraulically operated component (30) performs at least two different adjustment functions (100) which differ with regard to a required volume flow (102) and / or a required operating pressure (101).

14. Method according to claims 12 and 13, characterized by the fact thata control device (8) of the industrial truck (2) accesses a data storage device on which a function list is stored, in which the required operating pressure (101) and the required flow rate (102) are stored for each of the at least two different adjustment functions (100), wherein the control device (8) retrieves from the function list the operating pressure (101) and the flow rate (102) required for an adjustment function (100) to be performed and controls the at least one hydraulic valve (20) in order to move the at least one hydraulic valve (20) into the first position or the second position or the third position depending on the operating pressure (101) and the flow rate (102) required for this adjustment function (100).

15. Method according to any one of claims 11 to 14, characterized by the fact thatthe control device (8) regulates an efficiency share of the first hydraulic pump (11) and / or the second hydraulic pump (12) when the first hydraulic pump (11) and the second hydraulic pump (12) are operated simultaneously, in particular by regulating the efficiency share by reducing a pump power and / or a flow rate.

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