Method for operating hydraulic system of industrial vehicle having plurality of hydraulic pumps, and industrial vehicle

Abstract

The invention relates to a method for operating a hydraulic system (20) of an industrial vehicle (FFZ) having a plurality of hydraulic pumps (40, 50), a delivery volume flow of a hydraulic medium (14) being provided by a hydraulic device having at least one hydraulic pump (40) for moving at least one hydraulic cylinder (20, 31, 32), which hydraulic device, at a minimum rotational speed of the at least one hydraulic pump (40), drives the at least one hydraulic cylinder (20, 31, 32) to move the at least one hydraulic cylinder (20, 31, 32). A minimum delivery volume flow rate of a preferably incompressible hydraulic medium (14), in particular a hydraulic fluid, can be provided, when a predetermined delivery volume flow rate which is greater than the minimum delivery volume flow rate of the hydraulic device is reached, if a rotational speed of at least one hydraulic pump (40) of the hydraulic device is equal to or greater than a minimum rotational speed of the at least one hydraulic pump (40). If the rotational speed of the other hydraulic pump (50) is equal to or greater than the minimum rotational speed of the other hydraulic pump (50), the other hydraulic pump (50) with the minimum rotational speed is switched on in order to increase the delivery volume flow of the hydraulic medium (14).

Description

Technical Field

[0001] This invention relates to a method for operating an industrial vehicle (especially an industrial vehicle for raising and lowering loads by means of a lifting strut) with a hydraulic system having multiple hydraulic pumps (especially a lifting hydraulic system, particularly a hydraulic system for raising and lowering loads by means of a lifting strut). Furthermore, this invention also relates to an industrial vehicle having a hydraulic system having multiple hydraulic pumps (especially a lifting hydraulic system, particularly a hydraulic system for raising and lowering loads by means of a lifting strut). Background Technology

[0002] It is known in the prior art that industrial vehicles are equipped with movable lifting devices. Here, for example, the lifting of a vertically movable load-picking device is achieved by means of a lifting cylinder, which is hydraulically connected to a pump (e.g., a hydraulic pump) so that a pressure medium (e.g., hydraulic oil) is delivered to the lifting cylinder during operation, thereby moving the lifting cylinder and thus the load-picking device.

[0003] For example, DE102015115817A1 describes a method for controlling a lifting hydraulic device in an industrial vehicle. Furthermore, EP0798260A2 provides a method for position monitoring and control of a hydraulic device (e.g., in a forklift).

[0004] Industrial vehicles (such as forklifts) typically have lifting struts with one or more lifting stages, which are hydraulically operated by one or more lifting cylinders. The lifting strut includes a fixed strut permanently connected to the vehicle and typically two telescopic struts (an intermediate strut and an inner strut) that extend via the lifting cylinders. A free-lift cylinder drives a free-lift stage, by which a load-picking device (such as forks) can be moved along the inner strut of the lifting strut. The free-lift stage allows the load-picking device to move along the strut stage and allows the operator of the industrial vehicle to move the load-picking device vertically without extending the lifting strut and thus without changing the structural height of the industrial vehicle.

[0005] Known forklifts have a common hydraulic lowering branch for both the strut lift and free lift, which integrates a lowering valve. To ensure the strut lift and free lift retract in a desired sequence during load pickup, each strut lift and free lift has a hydraulic cylinder with a different cross-sectional area. If multiple strut lifts are extended, during the load lowering operation of the industrial vehicle, the strut lift with the smallest total effective hydraulic cross-sectional area retracts first. Because the hydraulic pressure acting on this cylinder is the highest, it retracts first as the hydraulic pressure decreases. This strut lift is typically the uppermost one. As the hydraulic pressure further decreases, the strut lifts descend sequentially. Finally, after the strut lifts are fully retracted, the free lift retracts, causing the load pickup to lower. Summary of the Invention

[0006] The objective of this invention is to extend the service life of hydraulic systems with multiple hydraulic pumps in industrial vehicles (ground transport vehicles) in a simple manner.

[0007] This task is solved by a method using a hydraulic system (especially a lifting hydraulic system) with multiple hydraulic pumps for operating industrial vehicles (especially industrial vehicles used for lifting loads by means of lifting struts).

[0008] The hydraulic device, having at least one hydraulic pump, provides a hydraulic medium with a delivery volume flow rate for moving at least one hydraulic cylinder. The hydraulic device is capable of providing a minimum delivery volume flow rate (preferably incompressible) of the hydraulic medium (especially hydraulic fluid) at the minimum speed of the at least one hydraulic pump.

[0009] When a predetermined flow rate greater than the minimum flow rate of the hydraulic equipment is reached, if the rotational speed of the at least one hydraulic pump of the hydraulic equipment is equal to or greater than the minimum rotational speed of the at least one hydraulic pump of the hydraulic equipment, and the rotational speed of another hydraulic pump is equal to or greater than the minimum rotational speed of the other hydraulic pump, then the other hydraulic pump having a minimum rotational speed (especially its own) is turned on to increase the flow rate of the hydraulic medium.

[0010] This invention is based on the concept that in a hydraulic system of an industrial vehicle with multiple hydraulic pumps (especially those used for lifting loads via lifting struts), each pump is activated as needed according to the current operating point of the hydraulic system. This allows for optimized control of the volumetric flow rate of the hydraulic pumps, taking into account their respective minimum operating speeds. Since the minimum operating speeds of each hydraulic pump may vary depending on their structure and / or operating mode, adhering to these minimum operating speeds can extend the service life or operating time of each pump.

[0011] Furthermore, by employing the control method of this invention and by connecting another hydraulic pump to one or more hydraulic pumps of the already operating hydraulic equipment as needed when the delivery volumetric flow rate increases, a sudden increase in the delivery volumetric flow rate can be avoided. This results in a generally gradual increase in the delivery volumetric flow rate, while the rotational speed of at least one operating hydraulic pump in the hydraulic equipment is equal to or greater than the minimum rotational speed of that at least one hydraulic pump, and the rotational speed of the other hydraulic pump is equal to or greater than the minimum rotational speed of the other connected hydraulic pump.

[0012] For example, a noise-optimized hydraulic pump can be used as one of the pumps in a hydraulic device, providing a high volumetric flow rate. Here, the efficiency loss of the noise-optimized hydraulic pump is small relative to the required pump power. Furthermore, the noise-optimized hydraulic pump has a higher operating speed compared to another hydraulic pump that can be switched on or is already switched on. Typically, the operating speed of the noise-optimized hydraulic pump is in the range of 300 rpm and above. These hydraulic pumps are used in hydraulic functions and work areas with medium to high speeds and low to high pressure requirements, such as for lifting or for prop pushing.

[0013] Another hydraulic pump, either connected or already connected, is configured as an efficiency-optimized pump and is used for functions where there is no significant power drop even under harsh operating conditions. Here, the efficiency-optimized pump is used for hydraulic functions and work areas with speeds below medium speed. Furthermore, the efficiency-optimized pump meets medium to high pressure requirements, such as tilting, lateral pushing, and various auxiliary equipment with corresponding work areas. Such pumps are particularly suitable for work areas where high precision of the hydraulic function is critical (e.g., when used in auxiliary systems or similar systems), such as precise positioning of the forks during lifting, lateral shifting, or tilting, where low speeds produce slight to moderate noise emissions.

[0014] Furthermore, the method is characterized in that: the hydraulic device has a single hydraulic pump with a minimum rotational speed to provide a minimum delivery volume flow rate, wherein when a predetermined delivery volume flow rate (the predetermined delivery volume flow rate is greater than the minimum delivery volume flow rate of the single hydraulic pump) is reached, the rotational speed of the single hydraulic pump is equal to or greater than the minimum rotational speed of the single hydraulic pump, and the rotational speed of another hydraulic pump is equal to or greater than the minimum rotational speed of the other hydraulic pump.

[0015] In another design of the method, the hydraulic equipment is specified to have a plurality of hydraulic pumps, particularly those connected in parallel with each other, wherein each of the hydraulic pumps has a minimum speed to provide a corresponding minimum delivery volume flow rate of the corresponding hydraulic pump, and when another hydraulic pump is turned on, the speed of each hydraulic pump of the hydraulic equipment is equal to or greater than the corresponding minimum speed of each hydraulic pump of the hydraulic equipment, and the speed of the other hydraulic pump is equal to or greater than the minimum speed of the other hydraulic pump.

[0016] According to the present invention, when another hydraulic pump is turned on or has been turned on to increase the delivery volumetric flow rate of the hydraulic medium, the rotational speed of one or more hydraulic pumps in the operating hydraulic equipment is reduced. By reducing the rotational speed of each hydraulic pump in the hydraulic equipment, a constant delivery volumetric flow rate can be achieved at the moment the other hydraulic pump is turned on.

[0017] According to another embodiment of the method, it is advantageous to reduce the speed of at least one hydraulic pump in the hydraulic equipment when another hydraulic pump is turned on, or to reduce the speed of each hydraulic pump in the hydraulic equipment when another hydraulic pump is turned on. This extends the service life or service period of the operating hydraulic pumps in the hydraulic equipment.

[0018] Therefore, in an improved version of the method, it is specified that the reduced speed of a hydraulic pump or the reduced speed of each hydraulic pump is determined based on the delivery volume that another hydraulic pump can provide and the additional delivery volume used to increase the delivery volume flow rate.

[0019] The method is further improved by providing a constant delivery volume flow rate while switching on another hydraulic pump, even when reducing the speed of one or all hydraulic pumps in the hydraulic system. This avoids abrupt increases in the delivery volume flow rate.

[0020] Preferably, according to another aspect, the hydraulic system enables the functional speed (especially the lifting speed) of the hydraulic system to be achieved, wherein the functional speed of the hydraulic system remains constant when another hydraulic pump is turned on. For example, when another hydraulic pump is turned on, the lifting speed of the hydraulic system used to move one or more hydraulic cylinders is maintained in this way.

[0021] This invention provides a standalone task solution or an improved method, implemented through a method of a hydraulic system (especially a lifting hydraulic system) with multiple hydraulic pumps for operating industrial vehicles (especially industrial vehicles for lifting loads by means of lifting struts), particularly the method according to any one of claims 1 to 7.

[0022] The hydraulic medium is supplied by a hydraulic device having at least one hydraulic pump to move at least one hydraulic cylinder. The hydraulic device provides a minimum supply volume flow of (preferably incompressible) hydraulic medium (especially hydraulic fluid) at the minimum speed of the at least one hydraulic pump.

[0023] Specifically, when a predetermined delivery volume flow rate is reached (the predetermined delivery volume flow rate is greater than the minimum delivery volume flow rate of the hydraulic equipment), if the rotational speed of at least one hydraulic pump of the hydraulic equipment is equal to or greater than the minimum rotational speed of the at least one hydraulic pump of the hydraulic equipment, then the other hydraulic pump with the minimum rotational speed is disconnected to reduce the delivery volume flow rate of the hydraulic medium.

[0024] According to the present invention, when reducing the volumetric flow rate of the hydraulic medium, one hydraulic pump is disconnected, wherein when another hydraulic pump is disconnected, the rotational speed of at least one hydraulic pump in the hydraulic equipment is equal to or greater than the minimum rotational speed of the corresponding hydraulic pump in the hydraulic equipment. This ensures that the operating hydraulic pumps do not run below their own minimum rotational speed.

[0025] The method is further improved in particular by the following: the hydraulic device has a single hydraulic pump with a minimum speed to provide a minimum delivery volume flow rate, and when a predetermined delivery volume flow rate (which is greater than the minimum delivery volume flow rate of the single hydraulic pump) is reached, the speed of the single hydraulic pump is equal to or greater than the minimum speed of the single hydraulic pump.

[0026] According to another improvement, the method is characterized in that the hydraulic device has a plurality of hydraulic pumps (especially connected in parallel with each other), each of the hydraulic pumps having a minimum rotational speed to provide a corresponding minimum delivery volume flow rate of the corresponding hydraulic pump, wherein, when another hydraulic pump is disconnected, the rotational speed of each hydraulic pump of the hydraulic device is equal to or greater than the corresponding minimum rotational speed of each hydraulic pump of the hydraulic device.

[0027] Furthermore, in one embodiment, it is advantageously specified that when another hydraulic pump is disconnected, the rotational speed of at least one hydraulic pump of the hydraulic equipment is increased, or when another hydraulic pump is disconnected, the rotational speed of each hydraulic pump of the hydraulic equipment is increased.

[0028] Preferably, the increased rotational speed of one hydraulic pump or the increased rotational speed of each hydraulic pump is determined based on the reduced delivery volume of another hydraulic pump and the smaller delivery volume used to reduce the delivery volume flow rate.

[0029] Furthermore, one design of the method is improved by providing a constant delivery volume flow rate while disconnecting another hydraulic pump, when increasing the speed of one hydraulic pump or the speed of each hydraulic pump in the hydraulic equipment.

[0030] The method specifically stipulates that the functional speed (especially the lifting speed) of the hydraulic system is achieved through the hydraulic system, wherein the functional speed of the hydraulic system remains constant when another hydraulic pump is disconnected.

[0031] Preferably, the minimum speed of each hydraulic pump and the minimum speed of another hydraulic pump in the hydraulic equipment depend on the corresponding efficiency characteristic curve of the respective hydraulic pump. By taking into account the corresponding efficiency characteristic curves, optimized control of each hydraulic pump can be achieved.

[0032] Furthermore, one design specifies that the hydraulic pump and the other hydraulic pump are controlled independently of each other using a control device. By independently controlling the hydraulic pumps using a control device (especially for industrial vehicles), abrupt increases in lifting speed (e.g., for lifting struts) can be avoided when this method is implemented, and advantageously, the minimum operating speed of each hydraulic pump is maintained.

[0033] Furthermore, the task is also accomplished by an industrial vehicle comprising a hydraulic system (particularly a lifting hydraulic system) with multiple hydraulic pumps, specifically for lifting loads using a lifting strut, wherein the industrial vehicle is configured to perform the method described above. To avoid repetition, explicit reference is made to the above description.

[0034] Other features of the invention can be learned from the description of the embodiments of the invention, as well as the claims and accompanying drawings. Embodiments of the invention may include a single feature or a combination of multiple features.

[0035] Within the scope of this invention, features marked with "especially" or "preferred" should be understood as optional features. Attached Figure Description

[0036] The present invention will now be described with reference to embodiments and the accompanying drawings, without limiting the overall inventive concept. All details of the invention not described in detail herein are explicitly referenced in the accompanying drawings. In the drawings:

[0037] Figure 1 A schematic diagram illustrating the wiring of the hydraulic system of an industrial vehicle;

[0038] Figure 2 A schematic diagram showing the time graph of rotational speed and volumetric flow rate of a hydraulic system;

[0039] Figure 3 The flowchart illustrating the hydraulic system control of the present invention is shown schematically.

[0040] In all the accompanying drawings, the same or similar elements and / or parts are labeled with the same reference numerals, and therefore will not be described again accordingly. Detailed Implementation

[0041] Figure 1 A hydraulic system 10 for an industrial vehicle FFZ, shown schematically, is illustrated. The hydraulic system 10 includes a lifting cylinder 20 for raising the frame and two hydraulic cylinders 31 and 32. For example, the hydraulic cylinders 31 and 32 can be used to tilt the forks of the lifting frame and / or move the struts.

[0042] The lifting cylinder 20 and hydraulic cylinders 31 and 32 are supplied with hydraulic fluid 14 from the reservoir 16, and the hydraulic fluid can also be returned to the reservoir 16. For this purpose, a hydraulic pump 40 (especially with noise reduction optimization) is provided to draw hydraulic fluid 14 from the reservoir 16 and deliver it to the lifting cylinder 20 and hydraulic cylinders 31 and 32 via supply lines 41. Each supply line 41 for the lifting cylinder 20 and hydraulic cylinders 31 and 32 is equipped with a corresponding check valve 42.

[0043] Hydraulic fluid is returned from the lifting cylinder 20 to the storage tank 16 via the return line 43. Here, the return line 43 is equipped with an operable two-position two-way valve 44.

[0044] In the supply lines 41 for the two hydraulic cylinders 31 and 32, there are operable three-position four-way valves 46. Hydraulic fluid is returned from the hydraulic cylinders 31 and 32 to the storage tank via the return line 45.

[0045] Depending on the position of the three-position four-way valve 46, the lines 41 and 45 between each hydraulic cylinder 31, 32 and the three-position four-way valve 46 connected thereto are respectively used as supply lines or return lines for hydraulic fluid.

[0046] In addition to hydraulic pump 40, a second (especially efficiency-optimized) hydraulic pump 50 is provided; when the rotational speed of hydraulic pump 40 is equal to or greater than its minimum rotational speed, and simultaneously the rotational speed of the second hydraulic pump 50 is equal to or greater than the minimum rotational speed of hydraulic pump 50, the second hydraulic pump 50 is connected in addition to the first hydraulic pump 40. Thus, by taking into account the minimum rotational speeds of the two hydraulic pumps 40 and 50, optimized control of their volumetric flow rate is achieved. By adhering to the minimum rotational speeds of hydraulic pumps 40 and 50, the service life or operational life of the hydraulic pumps can be extended.

[0047] To allow hydraulic fluid 14 to be drawn from the reservoir 16 and delivered to the lifting cylinder 20 and hydraulic cylinders 31 and 32 when the second hydraulic pump 50 is connected, a supply line 51 is provided. When supplying hydraulic fluid to the lifting cylinder 20, a two-position two-way valve 52 is provided. Furthermore, in the supply line 51, a check valve 53 is provided before and after the two-position two-way valve 52, respectively, according to the flow direction of the hydraulic fluid in the supply line 51.

[0048] In order to control the hydraulic pumps 40 and 50 and operate the directional valves 44, 46 and 52, the industrial vehicle FFZ is equipped with (not shown here) control devices for switching or controlling the hydraulic pumps and directional valves as required.

[0049] exist Figure 2 In the diagram, the left-hand time graph schematically illustrates the time-varying trend of the hydraulic pump's speed at 40 and 50 rpm in one design scheme. The right-hand time graph schematically illustrates the linear increase in volumetric flow rate.

[0050] Initially, only hydraulic pump 40 is engaged, while the second hydraulic pump 50 remains off. Hydraulic pump 40 has a minimum speed d40. As the demand for volumetric flow rate increases, the speed of hydraulic pump 40 is increased. At time t0, when the volumetric flow rate exceeds a predetermined value, the speed of hydraulic pump 40 is reduced, and simultaneously, the second hydraulic pump 50 is engaged. Hydraulic pump 50 has a minimum speed d50, wherein, when engaged, in addition to the already operating hydraulic pump 40, the speed of hydraulic pump 50 is equal to or higher than the minimum speed d50. The speed of the first hydraulic pump 40 is also equal to or higher than its minimum speed d40.

[0051] Figure 3 A flowchart illustrating the operation of an industrial vehicle is provided. The hydraulic system of this vehicle, which has multiple hydraulic cylinders, has functional requirements 200. These functional requirements may include the lifting function of the lifting strut, and / or auxiliary functions of the hydraulic system (such as fork tilt adjustment).

[0052] With functional requirement 200, the following question is raised in subsequent step 201: Should a lifting function, such as that of a lifting column, be performed? If the lifting function should not be performed, the following question is asked in subsequent step 202: Should the lifting function be performed by, for example, hydraulic cylinders 31, 32 (see...) Figure 1The auxiliary function is performed. This auxiliary function could be, for example, a support column movement. If the auxiliary function should not be performed in step 202, then in the subsequent step 203, the rotational speed of hydraulic pump 40 is checked to see if it is greater than or equal to its minimum rotational speed d40, and the rotational speed of hydraulic pump 50 is equal to or greater than its minimum rotational speed d50. If the first condition is met, hydraulic pump 40 is activated in step 204, while hydraulic pump 50 is deactivated. If the second condition is met, hydraulic pump 50 is engaged (especially for efficiency optimization) in step 205, while the other hydraulic pump 40 is deactivated.

[0053] If the auxiliary function of the hydraulic system is to be performed in step 202, then the hydraulic pump 50 is activated.

[0054] If, according to the inquiry in step 201, the lifting function should be performed using a hydraulic system, then in subsequent inquiry 206, it is inquired whether the rotational speed of hydraulic pump 40 is greater than or equal to its minimum rotational speed d40, and whether the rotational speed of hydraulic pump 50 is equal to or greater than its minimum rotational speed d50. If both conditions are met, then in step 207, both hydraulic pumps 40 and 50 are activated. Otherwise, in step 205, only hydraulic pump 50 is connected.

[0055] All features mentioned, including those available only from the accompanying drawings and individual features disclosed in combination with other features, whether present individually or in combination, are considered essential to the invention. Embodiments of the invention may include a single feature or a combination of multiple features.

[0056] List of reference numerals

[0057] 10 Hydraulic System

[0058] 14 Hydraulic Fluid

[0059] 16 storage tanks

[0060] 20 lifting cylinder

[0061] Hydraulic cylinders 31 and 32

[0062] 40 hydraulic pump

[0063] 41 Supply Pipeline

[0064] 42 Check Valve

[0065] 43 Return Pipeline

[0066] 44 Two-way valve

[0067] 45 Return Pipeline

[0068] 46 Three-position four-way valve

[0069] 50 hydraulic pump

[0070] 51 Supply Pipeline

[0071] 52 Two-way valve

[0072] 53 Check Valve

[0073] 200 functional requirements

[0074] 201 steps

[0075] 202 steps

[0076] 203 steps

[0077] 204 steps

[0078] 205 steps

[0079] 206 Inquiries

[0080] 207 steps

[0081] FFZ Industrial Vehicles

Claims

1. A method for operating an industrial vehicle (FFZ) with a hydraulic system (20) having multiple hydraulic pumps (40, 50), said industrial vehicle being particularly used for lifting loads by means of a lifting strut, said hydraulic system being particularly a lifting hydraulic system (20), wherein, - A hydraulic medium (14) is supplied by a hydraulic device having at least one hydraulic pump (40) for moving at least one hydraulic cylinder (20, 31, 32), the hydraulic device providing the minimum supply volume flow of the hydraulic medium (14) at the minimum speed of the at least one hydraulic pump (40), the hydraulic medium preferably being an incompressible hydraulic medium, the hydraulic medium being in particular a hydraulic fluid; When a predetermined flow rate greater than the minimum flow rate of the hydraulic equipment is reached, if the rotational speed of the at least one hydraulic pump (40) of the hydraulic equipment is equal to or greater than the minimum rotational speed of the at least one hydraulic pump (40) of the hydraulic equipment and the rotational speed of another hydraulic pump (50) is equal to or greater than the minimum rotational speed of the other hydraulic pump (50), then the other hydraulic pump (50) with the minimum rotational speed is turned on to increase the flow rate of the hydraulic medium (14).

2. The method according to claim 1, characterized in that, The hydraulic equipment has a single hydraulic pump (40) with a minimum rotational speed to provide a minimum delivery volume flow rate, wherein when a predetermined delivery volume flow rate greater than the minimum delivery volume flow rate of the single hydraulic pump (40) is reached, the rotational speed of the single hydraulic pump (40) is equal to or greater than the minimum rotational speed of the single hydraulic pump (40), and the rotational speed of the other hydraulic pump (40) is equal to or greater than the minimum rotational speed of the other hydraulic pump (40).

3. The method according to claim 1, characterized in that, The hydraulic equipment has a plurality of hydraulic pumps (40) connected in parallel with each other, each of the hydraulic pumps (40) having a minimum speed to provide a corresponding minimum delivery volume flow rate of the corresponding hydraulic pump (40), wherein, when the other hydraulic pump is turned on, the speed of each hydraulic pump (40) of the hydraulic equipment is equal to or greater than the corresponding minimum speed of the corresponding hydraulic pump (40) of the hydraulic equipment, and the speed of the other hydraulic pump (50) is equal to or greater than the minimum speed of the other hydraulic pump (50).

4. The method according to any one of claims 1 to 3, characterized in that, When the other hydraulic pump (50) is turned on, the speed of at least one hydraulic pump (40) of the hydraulic equipment is reduced, or when the other hydraulic pump (50) is turned on, the speed of each hydraulic pump (40, 50) of the hydraulic equipment is reduced.

5. The method according to claim 4, characterized in that, The reduced speed of the hydraulic pump (40) or the reduced speed of each hydraulic pump (40, 50) is determined based on the delivery volume that the other hydraulic pump (50) can provide and the additional delivery volume used to increase the delivery volume flow rate.

6. The method according to claim 4 or 5, characterized in that, When the speed of one hydraulic pump (40) of the hydraulic equipment is reduced or the speed of each hydraulic pump (40) of the hydraulic equipment is reduced, a constant delivery volume flow rate is provided during the process of turning on the other hydraulic pump (50).

7. The method according to any one of claims 1 to 6, characterized in that, The hydraulic system (20) enables the function speed, especially the lifting speed, of the hydraulic system (20), wherein the function speed of the hydraulic system (20) remains constant when the other hydraulic pump (50) is turned on.

8. A method for operating an industrial vehicle (FFZ) with a hydraulic system (20) having multiple hydraulic pumps (40, 50), said method particularly according to any one of claims 1 to 7, said industrial vehicle particularly for lifting loads by means of a lifting strut, said hydraulic system particularly a lifting hydraulic system (20), wherein, - A hydraulic medium (14) is supplied by a hydraulic device having at least one hydraulic pump (40) for moving at least one hydraulic cylinder (20, 31, 32), the hydraulic device providing the minimum supply volume flow rate of the hydraulic medium (14) at the minimum speed of the at least one hydraulic pump (40), the hydraulic medium preferably being an incompressible hydraulic medium, the hydraulic medium being in particular a hydraulic fluid; When a predetermined flow rate greater than the minimum flow rate of the hydraulic equipment is reached, if the rotational speed of the at least one hydraulic pump (40) of the hydraulic equipment is equal to or greater than the minimum rotational speed of the at least one hydraulic pump (40) of the hydraulic equipment, then another hydraulic pump (50) with the minimum rotational speed is disconnected to reduce the flow rate of the hydraulic medium (14).

9. The method according to claim 8, characterized in that, The hydraulic equipment has a single hydraulic pump (40) with a minimum rotational speed to provide a minimum delivery volume flow rate, wherein the rotational speed of the single hydraulic pump (40) is equal to or greater than the minimum rotational speed of the single hydraulic pump (40) when a predetermined delivery volume flow rate greater than the minimum delivery volume flow rate of the single hydraulic pump (40) is reached.

10. The method according to claim 9, characterized in that, The hydraulic equipment has a plurality of hydraulic pumps (40) connected in parallel with each other, each of the hydraulic pumps (40) having a minimum speed to provide a corresponding minimum delivery volume flow rate of the corresponding hydraulic pump (40), and when the other hydraulic pump is disconnected, the speed of each hydraulic pump (40) of the hydraulic equipment is equal to or greater than the corresponding minimum speed of the corresponding hydraulic pump (40) of the hydraulic equipment.

11. The method according to any one of claims 8 to 10, characterized in that, When the other hydraulic pump (50) is disconnected, the rotational speed of at least one hydraulic pump (40) of the hydraulic equipment is increased, or when the other hydraulic pump (50) is disconnected, the rotational speed of each hydraulic pump (40) of the hydraulic equipment is increased.

12. The method according to claim 11, characterized in that, The increased rotational speed of a hydraulic pump (40) or the increased rotational speed of each hydraulic pump (40) is determined based on the reduced delivery volume of the other hydraulic pump (50) and the smaller delivery volume used to reduce the delivery volume flow rate.

13. The method according to claim 11 or 12, characterized in that, When increasing the speed of one hydraulic pump (40) of the hydraulic equipment or increasing the speed of each hydraulic pump (40) of the hydraulic equipment, a constant delivery volume flow rate is provided during the disconnection of the other hydraulic pump (50).

14. The method according to any one of claims 8 to 13, characterized in that, The hydraulic system (20) enables the function speed, especially the lifting speed, of the hydraulic system (20), wherein the function speed of the hydraulic system (20) remains constant when the other hydraulic pump (50) is disconnected.

15. The method according to any one of claims 1 to 14, characterized in that, The minimum speed of each hydraulic pump (40) and the minimum speed of the other hydraulic pump (50) of the hydraulic equipment depend on the corresponding efficiency characteristic curve of the respective hydraulic pump.

16. The method according to any one of claims 1 to 15, characterized in that: The hydraulic pump (40) and the other hydraulic pump (50) are controlled independently of each other by means of a control device.

17. An industrial vehicle (FFZ) comprising a hydraulic system having multiple hydraulic pumps (40, 50), the hydraulic system being particularly a lifting hydraulic system (20), the industrial vehicle being particularly used for lifting loads by means of a lifting strut, wherein, The industrial vehicle (FFZ) is configured to perform the method according to any one of claims 1 to 16.

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