Method for operating a hydraulic system of an industrial truck, comprising a plurality of hydraulic pumps, and industrial truck

Abstract

The invention relates, inter alia, to a method for operating a hydraulic system (20) comprising several hydraulic pumps (40, 50), in particular a lifting hydraulic system (20), of a forklift truck (FFZ), especially for lifting a load by means of a lifting mast, wherein a hydraulic device comprising at least one hydraulic pump (40), which provides a minimum flow rate of a hydraulic medium (14), preferably incompressible, in particular hydraulic fluid, at a minimum speed of the at least one hydraulic pump (40), provides a flow rate of the hydraulic medium (14) for moving at least one hydraulic cylinder (20, 31, 32), wherein when a predetermined flow rate is reached which is greater than the minimum flow rate of the hydraulic device, a further hydraulic pump (50) having a minimum speed is switched on to increase the flow rate of the hydraulic medium (14),if the rotational speed of at least one hydraulic pump (40) of the hydraulic device is equal to or greater than the minimum rotational speed of at least one hydraulic pump (40) of the hydraulic device and the rotational speed of the further hydraulic pump (50) is equal to or greater than the minimum rotational speed of the further hydraulic pump (50).

Description

[0001] The invention relates to a method for operating a hydraulic system comprising several hydraulic pumps, in particular a lifting hydraulic system, of a forklift truck, especially for lifting a load by means of a lifting mast. The invention further relates to a forklift truck with a hydraulic system comprising several hydraulic pumps, in particular a lifting hydraulic system, especially for lifting a load by means of a lifting mast.

[0002] It is known in the prior art that industrial trucks are designed with movable lifting devices. For example, a vertically movable load-handling attachment is lifted by means of a lifting cylinder, which is hydraulically connected to a pump, such as a hydraulic pump, to supply a pressure medium, such as hydraulic oil, to the lifting cylinder during operation, thereby moving the lifting cylinder and thus the load-handling attachment.

[0003] For example, DE 10 2015 115 817 A1 describes a method for controlling a lifting hydraulic system on a forklift truck. Furthermore, EP 0 798 260 A2 discloses a method for monitoring and controlling the position of hydraulic devices, e.g., on a forklift truck.

[0004] Industrial trucks, such as forklifts, often have a mast with one or more mast lift stages, which are hydraulically operated by one or more mast lift cylinders. The mast comprises a fixed mast section connected to the vehicle and typically two extension sections: a center mast and an inner mast, which are extended by the mast lift cylinder. A free lift cylinder moves a free lift stage, which allows a load-handling attachment, such as forks, to travel along the inner mast of the main mast. The free lift stage moves the load-handling attachment along this mast stage and allows the operator to adjust the height of the load-handling attachment without extending the mast and thus without changing the overall height of the truck.

[0005] Common forklifts have a shared hydraulic lowering system for the mast lift and free lift, which incorporates a lowering valve. To ensure that the mast lift stages and free lift retract in the desired sequence when lowering the load-handling attachment, each stage has hydraulic cylinders with different cross-sections. If several mast stages are extended, the stage with the smallest total effective hydraulic cross-section retracts first during the forklift's load-lowering operation. This is because the hydraulic pressure is highest at this cylinder, so it retracts first as the hydraulic pressure decreases. This is typically the uppermost mast lift stage. As the hydraulic pressure continues to decrease, the mast stages are lowered sequentially. Finally, after the mast stages are fully retracted, the free lift stage retracts, lowering the load-handling attachment.

[0006] One object of the invention is to extend the service life of a hydraulic system comprising several hydraulic pumps of a forklift truck in a simple manner.

[0007] This problem is solved by a method for operating a hydraulic system comprising several hydraulic pumps, in particular a lifting hydraulic system, of a forklift truck, especially for lifting a load by means of a lifting mast, wherein a hydraulic device comprising at least one hydraulic pump, which provides a minimum flow rate of a hydraulic medium, preferably incompressible, in particular hydraulic fluid, at a minimum speed of the at least one hydraulic pump, provides a flow rate of the hydraulic medium for the movement of at least one hydraulic cylinder, wherein, upon reaching a predetermined flow rate that is greater than the minimum flow rate of the hydraulic device, a further hydraulic pump, in particular having its own minimum speed, is switched on to increase the flow rate of the hydraulic medium, if the speed of the at least one hydraulic pump of the hydraulic device is equal to or greater than the minimum speed of the at least one hydraulic pump of the hydraulic device and the speed of the further hydraulic pump is equal to or greater than the minimum speed of the further hydraulic pump.

[0008] The invention is based on the idea that in a hydraulic system of a forklift truck with multiple hydraulic pumps, which are particularly intended for lifting a load by means of a lifting mast, the hydraulic pumps are switched on as needed, depending on the current operating point of the hydraulic system. This enables, in particular, volume flow-optimized control of the hydraulic pumps while taking into account the respective minimum speeds of the individual hydraulic pumps. By maintaining the minimum speeds of the hydraulic pumps, which can vary due to the design and / or function of the hydraulic pumps, the respective service life or operating time of the hydraulic pump is extended.

[0009] Furthermore, the control system according to the invention and the demand-based activation of the additional hydraulic pump(s) in addition to the hydraulic pump(s) of the already operating hydraulic device prevent a sudden increase in the delivery flow rate when the delivery flow rate increases. Overall, this results in a smooth increase in the delivery flow rate, whereby the rotational speed of the at least one operating hydraulic pump of the hydraulic device is equal to or greater than the minimum rotational speed of the at least one hydraulic pump of the hydraulic device, and the rotational speed of the additional hydraulic pump is equal to or greater than the minimum rotational speed of the additionally activated hydraulic pump.

[0010] For example, a noise-optimized hydraulic pump capable of delivering high flow rates is used as the primary hydraulic pump in a hydraulic system. In this case, the efficiency loss of the noise-optimized hydraulic pump is low relative to the required pumping capacity. Furthermore, a noise-optimized hydraulic pump operates at high speeds compared to a secondary or additional hydraulic pump. Typically, the speed of a noise-optimized hydraulic pump is in the range of 300 rpm and above. These hydraulic pumps are used for hydraulic functions and applications with medium to high speeds and low to high pressure requirements, such as lifting or mast thrust.

[0011] The switchable or integrated auxiliary hydraulic pump is, for example, designed as an efficiency-optimized pump and is used for functions where there is no significant drop in the hydraulic pump's performance, even under difficult operating conditions. In this context, efficiency-optimized hydraulic pumps are used for hydraulic functions and working ranges with speeds lower than the average speed. Furthermore, these efficiency-optimized hydraulic pumps meet medium to high pressure requirements, such as those encountered during tilting or sideshifting, as well as for various attachments with corresponding operating ranges.In particular, such hydraulic pumps are used in operating areas where high accuracy of hydraulic functions is required, for example for assistance systems or the like, such as fine positioning of a fork when lifting, or when side-shifting or tilting, whereby low to moderate noise emissions result from the low speeds.

[0012] Furthermore, the method is characterized in that the hydraulic device comprises only one hydraulic pump, wherein the only hydraulic pump has a minimum speed to provide a minimum delivery volume flow, wherein, upon reaching the predetermined delivery volume flow, which is greater than the minimum delivery volume flow of the only hydraulic pump, the speed of the only hydraulic pump is equal to or greater than the minimum speed of the only hydraulic pump and the speed of the other hydraulic pump is equal to or greater than the minimum speed of the other hydraulic pump.

[0013] In another embodiment of the method, it is provided that the hydraulic device has several hydraulic pumps, in particular connected in parallel to each other, wherein each of the hydraulic pumps has a minimum speed to provide a respective minimum delivery volume flow rate of the respective hydraulic pump, wherein when the further hydraulic pump is switched on, the speeds of the hydraulic pumps of the hydraulic device are each equal to or greater than the respective minimum speed of the respective hydraulic pump of the hydraulic device and the speed of the further hydraulic pump is equal to or greater than the minimum speed of the further hydraulic pump.

[0014] According to the invention, the rotational speed of one or at least one hydraulic pump of the operated hydraulic device is reduced when the additional hydraulic pump is switched on to increase the flow rate of the hydraulic fluid. By reducing the rotational speed of the hydraulic pumps of the hydraulic device, a constant flow rate is achieved at the time the additional hydraulic pump is switched on.

[0015] According to a further embodiment of the method, it is advantageous that the rotational speed of at least one hydraulic pump of the hydraulic device is reduced when the additional hydraulic pump is switched on, or that the rotational speeds of the hydraulic pumps of the hydraulic device are reduced when the additional hydraulic pump is switched on. This extends the service life or operating time of the hydraulic pumps of the hydraulic device.

[0016] In a further development of the procedure, it is provided that the reduced speed of the hydraulic pump or the reduced speeds of the hydraulic pumps are determined depending on a available delivery volume of the additional hydraulic pump and on an additional delivery volume to increase the delivery volume flow.

[0017] The method is further enhanced by ensuring that a constant flow rate is maintained when the additional hydraulic pump is engaged, even if the speed of the hydraulic pump in the hydraulic device or the speeds of the hydraulic pumps in the hydraulic device are reduced. This prevents a sudden increase in the flow rate.

[0018] Preferably, according to a further aspect, the hydraulic system provides a functional speed, in particular a stroke speed, whereby the functional speed of the hydraulic system remains constant when the additional hydraulic pump is activated. For example, when the additional hydraulic pump is activated, the stroke speed of the hydraulic system for moving one or more hydraulic cylinders is maintained.

[0019] An independent solution to the problem or a further development of the method is provided by a method for operating a hydraulic system comprising several hydraulic pumps, in particular a lifting hydraulic system, of a forklift truck, in particular for lifting a load by means of a lifting mast, in particular according to one of claims 1 to 7. wherein a hydraulic device comprising at least one hydraulic pump, which provides a minimum flow rate of a hydraulic medium, preferably incompressible, in particular hydraulic fluid, at a minimum speed of the at least one hydraulic pump, provides a flow rate of the hydraulic medium for the movement of at least one hydraulic cylinder, wherein, upon reaching a predetermined flow rate that is greater than the minimum flow rate of the hydraulic device, a further hydraulic pump having a minimum speed is switched off to reduce the flow rate of the hydraulic medium, if the speed of the at least one hydraulic pump of the hydraulic device is equal to or greater than the minimum speed of the at least one hydraulic pump of the hydraulic device.

[0020] According to the invention, when the flow rate of the hydraulic medium is reduced, a hydraulic pump is switched off, whereby it is provided that when the other hydraulic pump is switched off, the rotational speed of the at least one hydraulic pump of the hydraulic device is equal to or greater than the minimum rotational speed of the corresponding hydraulic pump of the hydraulic device. This ensures that the operated hydraulic pumps are not operated below their own minimum rotational speed.

[0021] The method is further developed in particular by the fact that the hydraulic device comprises exclusively one hydraulic pump, wherein the exclusively one hydraulic pump has a minimum speed to provide a minimum delivery volume flow, wherein when the predetermined delivery volume flow, which is greater than the minimum delivery volume flow of the exclusively one hydraulic pump, is reached, the speed of the exclusively one hydraulic pump is equal to or greater than the minimum speed of the exclusively one hydraulic pump.

[0022] According to another embodiment, the method is characterized by the fact that the hydraulic device has several hydraulic pumps, in particular connected in parallel to each other, wherein each of the hydraulic pumps has a minimum speed to provide a respective minimum delivery volume flow rate of the respective hydraulic pump, wherein when the next hydraulic pump is switched off the speeds of the hydraulic pumps of the hydraulic device are each equal to or greater than the respective minimum speed of the respective hydraulic pump of the hydraulic device.

[0023] Furthermore, in one embodiment it is advantageously provided that the rotational speed of at least one hydraulic pump of the hydraulic device is increased when the other hydraulic pump is switched off, or that the rotational speeds of the hydraulic pumps of the hydraulic device are increased when the other hydraulic pump is switched off.

[0024] Preferably, the increased speed of the hydraulic pump or the increased speeds of the hydraulic pumps are determined as a function of a reducible delivery volume of the further hydraulic pump and of a lower delivery volume to reduce the delivery volume flow.

[0025] Furthermore, the method is further developed by ensuring that a constant delivery volume flow is provided when the speed of the hydraulic pump of the hydraulic device is increased or when the speeds of the hydraulic pumps of the hydraulic device are increased, when the other hydraulic pump is switched off.

[0026] In particular, the method provides that the hydraulic system causes a functional speed of the hydraulic system, especially a lifting speed, whereby the functional speed of the hydraulic system remains constant when the other hydraulic pump is switched off.

[0027] Preferably, the minimum speed of the hydraulic pumps in the hydraulic device and the minimum speed of the other hydraulic pump depend on the respective efficiency curves of the respective hydraulic pumps. Taking these efficiency curves into account ensures optimal control of the hydraulic pumps.

[0028] Furthermore, one embodiment provides for the independent control of the hydraulic pumps and the additional hydraulic pump by means of a control device. This independent control of the hydraulic pumps by means of the control device, particularly of the industrial truck, prevents a sudden increase in the lifting speed of, for example, a lifting mast during the execution of the method, advantageously ensuring that the minimum speeds of the respective hydraulic pumps are maintained.

[0029] Furthermore, the problem is solved by a forklift truck with a hydraulic system comprising several hydraulic pumps, in particular a lifting hydraulic system, especially for lifting a load by means of a lifting mast, wherein the forklift truck is configured to carry out a method as described above. To avoid repetition, explicit reference is made to the above statements.

[0030] 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.

[0031] Within the scope of the invention, features marked with "in particular" or "preferably" are to be understood as optional features.

[0032] The invention is described below, without limiting the general concept, 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: Figure 1 schematically shows a circuit diagram of a hydraulic system of a forklift truck; Figure 2 schematically shows time diagrams for rotational speeds and the volume flow rate of a hydraulic system; and Figure 3 schematically shows a flow diagram of a control of the hydraulic system according to the invention.

[0033] In the drawings, identical or similar elements and / or parts are provided with the same reference numbers, so that a re-presentation is omitted.

[0034] Figure 1Figure 1 shows a schematic view of a hydraulic system 10 for a schematically designated industrial truck (FFZ). The hydraulic system 10 has a lifting cylinder 20 for a lifting frame and two hydraulic cylinders 31 and 32. The hydraulic cylinders 31 and 32 are used, for example, to tilt a fork of a lifting frame and / or to move a mast.

[0035] The lifting cylinder 20 and the hydraulic cylinders 31, 32 are supplied with hydraulic fluid 14 from a reservoir 16, to which the hydraulic fluid is also returned. For this purpose, a hydraulic pump 40, particularly one optimized for noise reduction, is provided to draw the hydraulic fluid 14 from the reservoir 16 and supply it via a supply line 41 to the lifting cylinder 20 and the hydraulic cylinders 31, 32. Corresponding check valves 42 are arranged in the respective supply lines 41 for the pressure cylinder 20 and the hydraulic cylinders 31, 32.

[0036] Hydraulic fluid from the lifting cylinder 20 is returned to the reservoir 16 via the return line 43. An actuated 2 / 2-way valve 44 is provided in the return line 43.

[0037] Each of the two hydraulic cylinders 31 and 32 is equipped with a 4 / 3-way valve 46 that can be actuated. The hydraulic fluid from the hydraulic cylinders 31 and 32 is returned to the reservoir via return lines 45.

[0038] Depending on the position of the 4 / 3-way valves 46, the lines 41 and 45 between the respective hydraulic cylinders 31, 32 and the associated 4 / 3-way valve 46 are used as supply lines or return lines for the hydraulic fluid accordingly.

[0039] In addition to hydraulic pump 40, a second, efficiency-optimized hydraulic pump 50 is provided. This second pump is activated alongside the first hydraulic pump 40 when the speed of hydraulic pump 40 is equal to or greater than its minimum speed and simultaneously the speed of the second hydraulic pump 50 is equal to or greater than the minimum speed of hydraulic pump 50. This ensures optimal flow rate control of both hydraulic pumps 40 and 50 while respecting their respective minimum speeds. Maintaining the minimum speeds of hydraulic pumps 40 and 50 extends their service life.

[0040] To draw hydraulic fluid 14 from the reservoir 16 when the second hydraulic pump 50 is activated and supply it to the lifting cylinder 20 and the hydraulic cylinders 31, 32, a supply line 51 is provided, wherein a 2 / 2-way valve 52 is provided at the point where the hydraulic fluid is supplied to the lifting cylinder 20. Furthermore, a check valve 53 is arranged in the supply line 51, both upstream and downstream of the 2 / 2-way valve 52, with respect to the flow direction of the hydraulic fluid in the supply line 51.

[0041] A control device (not shown here) of the industrial truck is provided for controlling the hydraulic pumps 40, 50 and for actuating the directional control valves 44, 46 and 52, in order to switch or control the hydraulic pumps and the directional control valves according to the requirements.

[0042] In the representation of Figure 2The right-hand time diagram schematically shows the time course of the rotational speeds of hydraulic pumps 40 and 50 in one configuration. The right-hand time diagram schematically depicts the linear increase in the volume flow rate.

[0043] Initially, only hydraulic pump 40 is switched on, while the second hydraulic pump 50 remains switched off. Hydraulic pump 40 has a minimum speed d40. As the required flow rate increases, the speed of hydraulic pump 40 is increased. If, at time t0, the flow rate exceeds a predetermined value, the speed of hydraulic pump 40 is reduced, and simultaneously, the second hydraulic pump 50 is switched on. Hydraulic pump 50 has a minimum speed d50, and upon activation, hydraulic pump 50, in addition to the already operating hydraulic pump 40, has a speed equal to or above this minimum speed d50. The speed of the first hydraulic pump 40 is also equal to or above its minimum speed d40.

[0044] Figure 3The diagram schematically shows a flowchart for the operation of a forklift truck with a function requirement of 200 for the hydraulic system with multiple hydraulic cylinders. The function requirement can be the function of lifting a mast and / or a function requirement concerning secondary functions of the hydraulic system, such as adjusting the fork tilt, etc.

[0045] In the subsequent step 201, function requirement 200 asks whether a lifting function, for example of a lifting mast, should be performed. If no lifting function is to be performed, the subsequent step 202 asks whether an auxiliary function, for example by a hydraulic cylinder 31, 32, should be performed (see [reference]). Figure 1The secondary function can, for example, be a mast thrust. If no secondary function is to be performed in step 202, the subsequent query in step 203 checks whether the speed of hydraulic pump 40 is greater than or equal to its minimum speed d40 and whether the speed of hydraulic pump 50 is equal to or greater than its minimum 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, which is particularly efficiency-optimized, is switched on in step 205, while the other hydraulic pump 40 is deactivated.

[0046] In the event that a secondary function of the hydraulic system is to be performed in step 202, the hydraulic pump 50 is activated.

[0047] If, after the query in step 201, a lifting function is to be performed using the hydraulic system, the subsequent query 206 checks whether the speed of hydraulic pump 40 is greater than or equal to its minimum speed d40 and whether the speed of hydraulic pump 50 is equal to or greater than its minimum speed d50. If both conditions are met, both hydraulic pumps 40 and 50 are activated in step 207. Otherwise, only hydraulic pump 50 is switched on in step 205.

[0048] 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

[0049] 10 Hydraulic system 14 Hydraulic fluid 16 Reservoir 20 Lifting cylinder 31, 31 Hydraulic cylinder 40 Hydraulic pump 41 Supply line 42 Check valve 43 Return line 44 2 / 2-way valve 45 Return line 46 4 / 3-way valve 50 Hydraulic pump 51 Supply line 52 2 / 2-way valve 53 Check valve 200 Function request 201 Step 202 Step 203 Step 204 Step 205 Step 206 Query 207 Step FFZ forklift truck

Claims

1. Method for operating a hydraulic system (20) comprising several hydraulic pumps (40, 50), in particular a lifting hydraulic system (20), of a forklift truck (FFZ), especially for lifting a load by means of a lifting mast, wherein a hydraulic device comprising at least one hydraulic pump (40), which provides a minimum flow rate of a hydraulic medium (14), preferably incompressible, in particular hydraulic fluid, at a minimum speed of the at least one hydraulic pump (40), provides a flow rate of the hydraulic medium (14) for moving at least one hydraulic cylinder (20, 31, 32), wherein when a predetermined flow rate is reached which is greater than the minimum flow rate of the hydraulic device, a further hydraulic pump (50) having a minimum speed is switched on to increase the flow rate of the hydraulic medium (14),if the rotational speed of at least one hydraulic pump (40) of the hydraulic device is equal to or greater than the minimum rotational speed of at least one hydraulic pump (40) of the hydraulic device and the rotational speed of the further hydraulic pump (50) is equal to or greater than the minimum rotational speed of the further hydraulic pump (50).

2. Method according to claim 1, characterized by the fact thatthe hydraulic device comprises exclusively one hydraulic pump (40), wherein the exclusively one hydraulic pump (40) has a minimum speed to provide a minimum delivery flow rate, wherein when the predetermined delivery flow rate is reached, which is greater than the minimum delivery flow rate of the exclusively one hydraulic pump (40), the speed of the exclusively one hydraulic pump (40) is equal to or greater than the minimum speed of the exclusively one hydraulic pump (40) and the speed of the further hydraulic pump (40) is equal to or greater than the minimum speed of the further hydraulic pump (40).

3. Method according to claim 1, characterized by the fact thatThe hydraulic device comprises several hydraulic pumps (40), in particular connected in parallel to each other, wherein each of the hydraulic pumps (40) has a minimum speed to provide a respective minimum delivery volume flow rate of the respective hydraulic pump (40), wherein when the further hydraulic pump is switched on the speeds of the hydraulic pumps (40) of the hydraulic device are each equal to or greater than the respective minimum speed of the respective hydraulic pump (40) of the hydraulic device and the speed of the further hydraulic pump (50) is equal to or greater than the minimum speed of the further hydraulic pump (50).

4. Method according to any one of claims 1 to 3, characterized by the fact thatthe speed of at least one hydraulic pump (40) of the hydraulic device is reduced when the additional hydraulic pump (50) is switched on, or the speeds of the hydraulic pumps (40, 50) of the hydraulic device are reduced when the additional hydraulic pump (50) is switched on.

5. Method according to claim 4, characterized by the fact that the reduced speed of the hydraulic pump (40) or the reduced speeds of the hydraulic pumps (40, 50) are determined depending on a available delivery volume of the additional hydraulic pump (50) and on an additional delivery volume to increase the delivery volume flow.

6. Method according to claim 4 or 5, characterized by the fact that a constant delivery volume flow is provided when the speed of the hydraulic pump (40) of the hydraulic device is reduced or when the speeds of the hydraulic pumps (40) of the hydraulic device are reduced when the additional hydraulic pump (50) is switched on.

7. Method according to any one of claims 1 to 6, characterized by the fact that The hydraulic system (20) causes a functional speed of the hydraulic system (20), in particular a lifting speed, whereby when the additional hydraulic pump (50) is switched on, the functional speed of the hydraulic system (20) remains constant.

8. Method for operating a hydraulic system (20) comprising several hydraulic pumps (40, 50), in particular a lifting hydraulic system (20), of a forklift truck (FFZ), in particular for lifting a load by means of a lifting mast, in particular according to one of claims 1 to 7, wherein a hydraulic device comprising at least one hydraulic pump (40), which provides a minimum flow rate of a hydraulic medium (14), preferably incompressible, in particular hydraulic fluid, at a minimum speed of the at least one hydraulic pump (40), provides a flow rate of the hydraulic medium (14) for moving at least one hydraulic cylinder (20, 31, 32), wherein when a predetermined flow rate greater than the minimum flow rate of the hydraulic device is reached, a further hydraulic pump (50) having a minimum speed is switched off to reduce the flow rate of the hydraulic medium (14).if the rotational speed of the at least one hydraulic pump (40) of the hydraulic device is equal to or greater than the minimum rotational speed of the at least one hydraulic pump (40) of the hydraulic device.

9. Method according to claim 8, characterized by the fact thatThe hydraulic device comprises exclusively one hydraulic pump (40), wherein the exclusively one hydraulic pump (40) has a minimum rotational speed to provide a minimum delivery flow rate, wherein, upon reaching the predetermined delivery flow rate, which is greater than the minimum delivery flow rate of the exclusively one hydraulic pump (40), the rotational speed of the exclusively one hydraulic pump (40) is equal to or greater than the minimum rotational speed of the exclusively one hydraulic pump (40), wherein, in particular, the hydraulic device comprises several hydraulic pumps (40), in particular connected in parallel to one another, wherein each of the hydraulic pumps (40) has a minimum rotational speed to provide a respective minimum delivery flow rate of the respective hydraulic pump (40).wherein, when the other hydraulic pump is switched off, the rotational speeds of the hydraulic pumps (40) of the hydraulic device are each equal to or greater than the respective minimum rotational speed of the respective hydraulic pump (40) of the hydraulic device.

10. Method according to claim 8 or 9, characterized by the fact that the speed of at least one hydraulic pump (40) of the hydraulic device is increased when the other hydraulic pump (50) is switched off, or the speeds of the hydraulic pumps (40) of the hydraulic device are increased when the other hydraulic pump (50) is switched off.

11. Method according to claim 10, characterized by the fact thatthe increased speed of the hydraulic pump (40) or the increased speeds of the hydraulic pumps (40) are determined depending on a reducible delivery volume of the further hydraulic pump (50) and on a lower delivery volume to reduce the delivery flow rate and / or when the speed of the hydraulic pump (40) of the hydraulic device is increased or when the speeds of the hydraulic pumps (40) of the hydraulic device are increased a constant delivery flow rate when the further hydraulic pump (50) is switched off.

12. Method according to any one of claims 8 to 11, characterized by the fact that The hydraulic system (20) causes a functional speed of the hydraulic system (20), in particular a lifting speed, whereby the functional speed of the hydraulic system (20) remains constant when the additional hydraulic pump (50) is switched off.

13. Method according to any one of claims 1 to 12, characterized by the fact thatThe minimum speed of the hydraulic pumps (40) of the hydraulic device and the minimum speed of the other hydraulic pump (50) depend on the respective efficiency characteristics of the respective hydraulic pump.

14. Method according to any one of claims 1 to 13, characterized by the fact that The hydraulic pumps (40) and the other hydraulic pump (50) can be controlled independently of each other by means of a control device.

15. Industrial truck (FFZ) with a hydraulic system comprising several hydraulic pumps (40, 50), in particular a lifting hydraulic system (20), in particular for lifting a load by means of a lifting mast, wherein the industrial truck (FFZ) is equipped to carry out a method according to one of claims 1 to 14.

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