HYDRAULIC SYSTEM FOR AN INDUSTRIAL TRUCK

Description

[0001] The invention relates to a hydraulic system for an industrial truck, in particular a battery-powered industrial truck, with a lifting drive for raising and lowering a load-carrying device and at least one auxiliary consumer, wherein the hydraulic system has a first hydraulic pump which is driven by a first electric drive machine, wherein the first hydraulic pump can be operated as a pump and as a motor, and a second hydraulic pump which can be driven by a second electric drive machine, wherein the second hydraulic pump can only be operated as a pump, wherein the hydraulic system is designed such that the first hydraulic pump supplies the lifting drive with pressure medium at all operating points during the lifting operation of the lifting drive and the second hydraulic pump supplies the auxiliary consumers with pressure medium.

[0002] Hydraulic systems of this type are usually used in battery-powered industrial trucks, such as counterbalance forklifts or reach trucks.

[0003] In battery-electric industrial trucks, for example counterbalance forklifts or reach trucks, hydraulic systems are known in which a single hydraulic pump is provided which supplies the lifting drive, with which a load handling device can be raised and lowered, and optionally other consumers of a working hydraulic system, for example a tilt drive of the load handling device and / or a side shift drive of the load handling device, and / or a hydraulic steering device with pressure medium.

[0004] In an industrial truck's hydraulic system with a single hydraulic pump, multiple hydraulic consumers are supplied with pressure fluid from a single hydraulic pump. This results in a simple hydraulic system design with just a single hydraulic pump. However, such a hydraulic system does not allow energy recovery from electrical energy during lowering of the lifting drive by motor operation of the hydraulic pump if one or more auxiliary consumers are to be supplied with pressure fluid from the hydraulic pump at the same time as the lowering of the lifting drive. However, such operating cases in which an auxiliary consumer, for example the hydraulic steering system, is actuated simultaneously during lowering of the lifting drive, frequently occur on industrial trucks.

[0005] Without energy recovery from electrical energy during lowering of the lifting drive by motor operation of the hydraulic pump, the pressure in the hydraulic system can only be reduced by throttling valves. This results in the hydraulic system's pressure fluid becoming significantly hot. Particularly when lowering a load during lowering of the lifting drive, a high level of heat is introduced into the hydraulic system's pressure fluid as the potential energy of the load is converted into heat by throttling valves.

[0006] In an industrial truck's hydraulic system with a single hydraulic pump that supplies multiple consumers with pressure fluid, the hydraulic pump's displacement must be designed for the largest consumer. However, a high displacement of the hydraulic pump results in a high required drive torque from the hydraulic pump's electric drive motor, even for consumers with lower flow requirements. Considered across the electric drive motor and the inverter that controls the electric drive motor, this leads to poorer energy efficiency for these functions.

[0007] In an industrial truck's hydraulic system with a single hydraulic pump that supplies multiple consumers with pressure fluid, the control valves for the consumers are combined in a control valve block. This results in high flow resistance in the control valve block and thus high energy losses, especially for consumers with high volume flows, such as the lifting drive during lifting and lowering operations.

[0008] To avoid these disadvantages, hydraulic systems for industrial trucks are already known, which feature two hydraulic pumps. A first hydraulic pump supplies the lifting drive with pressure fluid at all operating points. To achieve higher lifting speeds of the load handling device and enable energy recovery during the lowering operation of the lifting drive, a second hydraulic pump is provided, which can be switched on as needed to supply the lifting drive.

[0009] Such a hydraulic system for an industrial truck with two hydraulic pumps is known from DE 198 31 828 B1. In DE 198 31 828 B1, the lifting drive is supplied by a hydraulic pump during lifting operation at low to medium flow requirements, which also supplies the auxiliary consumers with pressure medium. A further hydraulic pump is only switched on to supply the lifting drive during lifting operation when the lifting drive has a high flow requirement during lifting operation. The disadvantage here is that in an operating state in which the lifting drive is actuated during lifting operation at low to medium flow requirements and an auxiliary consumer, for example a tilt drive of the load handling device, is simultaneously supplied by the hydraulic pump supplying the lifting drive, for example in the fine control range of the lifting drive and / or the auxiliary consumer, mutual influence of the controlled consumer functions can occur.

[0010] CN 108 502 816 A discloses a generic hydraulic system with the features of the preamble of patent claim 1.

[0011] A hydraulic system of an industrial truck is known from DE 10 2011 053 958 A1.

[0012] The object of the present invention is to provide a hydraulic system for an industrial truck of the type mentioned above which enables improved operation.

[0013] This object is achieved according to the invention in that the hydraulic system is designed to switch on the second hydraulic pump to supply the lifting drive with pressure medium during the lifting operation of the lifting drive if a lifting volume flow requirement of the lifting drive exceeds a lifting limit value.

[0014] According to the invention, the first hydraulic pump, which can be operated as a pump and as a motor, supplies the lifting drive with pressure medium at all operating points during lifting operation. The second hydraulic pump, which can only be operated as a pump and also supplies the auxiliary consumers with pressure medium, is only switched on during lifting operation of the lifting drive if the lifting volume flow requirement of the lifting drive exceeds a lifting limit value. The lifting limit value is preferably selected such that the lifting drive is only supplied by the first hydraulic pump during lifting operation when the volume flow requirement is low or medium, and the second hydraulic pump is switched on when the volume flow requirement is high, for example for quickly lifting the load-handling device.In an operating state in which the lifting drive is actuated in lifting mode with low and medium volume flow requirements and an auxiliary consumer, for example a tilt drive of the load-handling device, is simultaneously supplied, for example in the fine control range of the lifting drive and / or the auxiliary consumer, the lifting drive in the hydraulic system according to the invention is supplied with pressure medium exclusively by the first hydraulic pump and the auxiliary consumer is supplied with pressure medium exclusively by the second hydraulic pump. In the hydraulic system according to the invention, therefore, in such an operating state, no mutual influence of the controlled consumer functions can occur, so that the hydraulic system according to the invention enables improved operation.

[0015] According to an advantageous embodiment of the invention, the hydraulic system is preferably designed such that, during lowering operation of the lifting drive, the pressure medium flow flowing out of the lifting drive is discharged to a container via the first hydraulic pump, wherein the first hydraulic pump is operated as a motor and drives the first electric drive motor operating in generator mode. The first hydraulic pump operating as a motor is driven during lowering operation of the lifting drive by the pressure medium flow flowing out of the lifting drive and drives the electric drive motor operating as a generator. During generator operation, the electrical power generated by the electric drive motor can preferably be used to charge a traction battery of the industrial truck connected to the electric drive motor.This advantageously enables particularly effective energy recovery through the recuperation operation of the first hydraulic pump during lowering operation of the lifting drive.

[0016] According to an advantageous embodiment of the invention, the lifting drive can preferably be connected to the container by means of a control valve, wherein the hydraulic system is configured to actuate the control valve into a control position connecting the lifting drive to the container depending on a lowering volume flow requirement. With such a control valve, in particular in addition to or as an alternative to the recuperation operation of the first hydraulic pump, an additional pressure medium volume flow can be discharged to the container during the lowering operation of the lifting drive, thereby enabling a high lowering speed of the load-handling device.

[0017] According to an advantageous embodiment of the invention, the hydraulic system is preferably designed such that, during lowering operation of the lifting drive, the first hydraulic pump discharges the pressure medium flow flowing from the lifting drive to the container at all operating points. During lowering operation of the lifting drive, the control valve is actuated into the control position and additionally discharges the pressure medium flow flowing from the lifting drive to the container if the lowering volume flow requirement of the lifting drive exceeds a lowering limit value. The lowering limit value is preferably selected such that, during lowering operation, the lifting drive is discharged to the container only via the first hydraulic pump when the volume flow requirement is low or medium. During high volume flow requirement, for example, for quickly lowering the load-handling device, the control valve is actuated into the control position.By moving the control valve to the control position, an additional lowering volume flow can be directed to the container, particularly above the lowering limit of the lowering volume flow requirement, thus increasing the lowering speed. This enables particularly effective energy recovery through the recuperation operation of the first hydraulic pump during the lowering operation of the lifting drive, whereby a high lowering speed of the load handling device is enabled with the control valve moved to the control position.

[0018] According to an alternative and likewise advantageous embodiment of the invention, the hydraulic system is preferably designed such that, during lowering operation of the lifting drive, the control valve actuated into the control position discharges the pressure medium flow flowing from the lifting drive to the container, and the first hydraulic pump additionally discharges the pressure medium flow flowing from the lifting drive to the container if the lowering volume flow requirement of the lifting drive exceeds a lowering limit value. The lowering limit value is preferably selected such that, during lowering operation of the lifting drive, when the volume flow requirement is low, for example in a fine control range, the control valve is actuated into the control position and thus the volume flow from the lifting drive is discharged to the container only via the control valve, and when a higher volume flow requirement is required during lowering operation of the lifting drive, an additional volume flow is discharged to the container via the first hydraulic pump.This enables easy control of the lowering speed of the lifting drive in the fine control range by the control valve.

[0019] According to an advantageous embodiment of the invention, an electronic control device is preferably provided, which is connected on the input side to an operating device for controlling the lifting drive and the auxiliary consumer, and is connected on the output side to the electric drive motors for controlling them. With such an electronic control device, the electric drive motors drivingly connected to the hydraulic pumps can be easily controlled accordingly depending on the signals predetermined by the actuation of the operating device during the lifting and lowering operation of the lifting drive, as well as when controlling the auxiliary consumer.For this purpose, the electronic control device preferably stores the lifting limit value of the lifting volume flow requirement of the lifting drive, which is compared with a corresponding signal generated by the actuation of an operating device controlling the lifting drive. This makes it easy to supply the lifting drive with pressure medium exclusively from the first hydraulic pump during lifting operation if a signal is specified at the actuated operating device that is lower than the lifting limit value of the lifting volume flow requirement of the lifting drive, and to supply the lifting drive with pressure medium during lifting operation from the first hydraulic pump and additionally from the second hydraulic pump if a signal is specified at the actuated operating device that is higher than the lifting limit value of the lifting volume flow requirement of the lifting drive.

[0020] According to an advantageous embodiment of the invention, the electronic control device is preferably connected to the control valve for controlling the latter. For this purpose, the lowering limit value of the lowering volume flow requirement of the lifting drive is preferably stored in the electronic control device, which is compared with a corresponding signal generated by the actuation of an operating device controlling the lifting drive. This allows the control valve to be easily actuated into the control position connecting the lifting drive to the container if a high lowering speed of the lifting drive is to be achieved or if simple control of the lowering speed in the fine control range is to be achieved.

[0021] According to an advantageous embodiment of the invention, the electronic control device is preferably designed such that, during lifting operation of the lifting drive, the first hydraulic pump is operated as a pump that supplies the lifting drive with pressure medium at all operating points, and that, during lifting operation of the lifting drive, the second hydraulic pump is switched on to additionally supply the lifting drive with pressure medium if the lifting volume flow requirement of the lifting drive exceeds the predetermined lifting limit. The first hydraulic pump operated as a pump can thus supply the lifting drive with pressure medium at all operating points during lifting operation, and the second hydraulic pump is only switched on as needed during lifting operation of the lifting drive when, during lifting operation of the lifting drive, the lifting volume flow requirement of the lifting drive exceeds a lifting limit in order to achieve a high lifting speed of the load handling device.

[0022] According to an advantageous embodiment of the invention, the electronic control device is preferably designed such that, during lowering operation of the lifting drive, the first hydraulic pump is operated as a motor which, at all operating points, discharges the pressure medium flow flowing out of the lifting drive to the container. And, during lowering operation of the lifting drive, the control valve is actuated into the lowering position in order to additionally discharge the pressure medium flow flowing out of the lifting drive to the container if the lowering volume flow requirement of the lifting drive exceeds the predetermined lowering limit value. This enables particularly effective energy recovery through the recuperation operation of the first hydraulic pump during lowering operation of the lifting drive, with the control valve actuated into the control position enabling a high lowering speed of the load-handling device.

[0023] According to an alternative and likewise advantageous embodiment of the invention, the electronic control device is preferably designed such that, during lowering operation of the lifting drive, the control valve is actuated into the control position in order to discharge the pressure medium flow flowing out of the lifting drive to the container, and during lowering operation of the lifting drive, the first hydraulic pump is operated as a motor in order to additionally discharge the pressure medium flow flowing out of the lifting drive to the container if the lowering volume flow requirement of the lifting drive exceeds the predetermined lowering limit value. This enables particularly simple lowering control of the lifting drive in the fine control range, whereby the additional recuperation operation of the first hydraulic pump during lowering operation of the lifting drive enables a high lowering speed of the load handling device.

[0024] According to an advantageous embodiment of the invention, the auxiliary consumer is preferably designed as a tilt drive of the load-carrying means and / or as a hydraulic steering device.

[0025] The invention offers a number of advantages.

[0026] A mutual influence of the lifting drive with an auxiliary consumer when the auxiliary consumer is actuated simultaneously in the fine control range of the lifting drive, for example when lifting the load-handling device and simultaneously actuating a tilt drive, is excluded in the hydraulic system according to the invention, since the lifting drive in the fine control range is supplied with pressure medium exclusively by the first hydraulic pump and the auxiliary consumer is supplied with pressure medium exclusively by the second hydraulic pump.

[0027] In lifting operation of the lifting drive below the lifting limit value and / or in lowering operation below the lowering limit value, the height position of the load handling device can be easily determined from the volume delivered or taken up by the first hydraulic pump, i.e. from the displacement volume of the first hydraulic pump and its number of revolutions, since in lifting operation the second hydraulic pump is not yet switched on or in lowering operation the control valve is not yet controlled in the control position.

[0028] The hydraulic system according to the invention enables high energy efficiency through recuperation during large portions of the lowering operation of the lifting drive, thus maintaining a low temperature of the pressure medium in the hydraulic system. Throttling of the pressure medium at the control valve, and the associated heat input into the pressure medium, preferably only occurs above the lowering limit. As a result, the hydraulic system according to the invention requires only a low cooling capacity to cool the pressure medium, thus reducing the power required for the fan of a radiator and corresponding smaller components of a cooling system for cooling the pressure medium.

[0029] Furthermore, significantly reduced flow resistances and thus high energy efficiency for the actuation of the lifting drive can be achieved as long as the lifting drive is supplied with pressure medium exclusively by the first hydraulic pump, since the flow supplied by the first hydraulic pump does not have to be guided to the lifting drive via a control directional valve device.

[0030] Furthermore, the displacement of the first hydraulic pump and the second hydraulic pump can be reduced compared to a hydraulic system with a single hydraulic pump. Small hydraulic pumps require a lower drive torque from the electric drive motors and thus lower electrical currents when the flow rate is low, resulting in fewer losses in the converter controlling the corresponding drive motor and in the electrical connecting cables.

[0031] The hydraulic system according to the invention can also be part of a modular system, in which the second hydraulic pump and a control valve block are used as the hydraulic system for smaller industrial trucks. By expanding the hydraulic system to include the first hydraulic pump, the hydraulic system according to the invention can be created in a modular manner for larger industrial trucks. The hydraulic system according to the invention can thus be manufactured from existing components with minimal construction effort.

[0032] Further advantages and details of the invention are explained in more detail by way of example with reference to the embodiment shown in the schematic figures.

[0033] The figure shows a circuit diagram of a hydraulic system 1 according to the invention of an industrial truck. The industrial truck preferably has a battery-electric drive.

[0034] The hydraulic system 1 has a hydraulic lifting drive 2 for raising and lowering a load-handling device (not shown in detail), for example, a load fork comprising forks. In the illustrated embodiment, the lifting drive 2 has one or more lifting cylinders 3. The hydraulic system 1 further has one or more hydraulic auxiliary consumers 5. The auxiliary consumer 5 can be a tilt drive 5a and / or a sideshift drive 5b of the load-handling device.

[0035] The hydraulic system 1 has a first hydraulic pump 10, which can be driven by a first electric drive unit 11. The first hydraulic pump 10 is designed as a two-quadrant pump, which can be operated as a pump and as a motor. In the illustrated embodiment, the hydraulic pump 10 has a different direction of rotation in pump mode and in motor mode. The electric drive unit 11 can be operated as a motor and as a generator. In pump mode, the hydraulic pump 10, which operates as a pump, is driven by the electric drive unit 11, which operates as a motor. In motor mode, the hydraulic pump 10, which operates as a motor, is driven by the pressure medium flowing out via the hydraulic pump 10 and drives the electric drive unit 11, which operates as a generator.

[0036] The first hydraulic pump 10 is connected to a reservoir 13 via a reservoir line 12 and to the lifting drive 2 via a delivery line 14. A valve device 15 is arranged in the delivery line 14 of the first hydraulic pump 12. The valve device 15 has a blocking position 15a and a flow position 15b. In the illustrated embodiment, the valve device 15 can be actuated electrically by means of an electrical actuating device 16, for example a magnet. In the illustrated embodiment, the blocking position 15a is provided with a check valve 17 that blocks flow in the direction of the hydraulic pump 10.

[0037] The hydraulic system 1 has a second hydraulic pump 20, which can be driven by a second electric drive motor 21. The second hydraulic pump 20 is designed as a single-quadrant pump that can only be operated as a pump. In pump mode, the hydraulic pump 20, which functions as a pump, is driven by the electric drive motor 21, which functions as a motor.

[0038] The second hydraulic pump 20 is connected to the tank 13 via a tank line 22 and to a control directional valve block 25 via a delivery line 24.

[0039] A connecting line 26 leading to the lifting drive 2 is connected to the control directional valve block 25 and is connected to the delivery line 14 between the lifting drive 2 and the valve device 15.

[0040] A connecting line 27 leading to the auxiliary consumer 5a and a connecting line 28 leading to the auxiliary consumer 5b are also connected to the control directional valve block 25.

[0041] A drain line 29 leading to the container 13 is also connected to the control valve block 25.

[0042] The control directional valve block 25 is provided with a control valve (not shown in detail) with which the connecting line 26 can be connected to the conveying line 24 during lifting operation of the lifting drive 2 and with which the connecting line 26 can be connected to the discharge line 29 during lowering operation of the lifting drive 2.

[0043] The control directional valve block 25 is further provided for each auxiliary consumer 5a or 5b with a control directional valve device (not shown in detail), with which the connecting line 27 or 28 can be connected to the delivery line 24 in a first control position and the connecting line 27 or 28 can be connected to the discharge line 29 in a second control position.

[0044] The second hydraulic pump 20 can also be provided to supply a secondary consumer (not shown in detail) designed as a hydraulic steering system of the industrial truck. For this purpose, a priority valve can be provided in the delivery line 24 to provide preferential supply to the hydraulic steering system.

[0045] The hydraulic system 1 has an electronic control device 30 which is connected on the input side to an operating device 31, for example one or more operating levers, with which an operator can control the lifting drive 2 and the auxiliary consumers 5a, 5b.

[0046] The electronic control device 30 is connected on the output side to the first electric drive motor 11, the second electric drive motor 21, and the valve device 15 for controlling them. Furthermore, the electronic control device 30 can be connected to the control valve and the control directional valve devices of the control directional valve block 25 for controlling them.

[0047] In the hydraulic system 1 according to the invention, the lifting drive 2 and the auxiliary consumers 5a, 5b are thus operated by two hydraulic pumps 10, 20. The second hydraulic pump 20, which is designed as a single-quadrant pump, operates the lifting drive 2 and the auxiliary consumers 5a, 5b via the control directional valve block 25. The first hydraulic pump 10, which is designed as a two-quadrant pump and can be operated as a pump and as a motor, is directly connected to the lifting drive 2 via the delivery line 14 and, through motor operation, offers the possibility of recuperation during lowering operation of the lifting drive 2. In lifting operation with low and medium volume flow requirements, the lifting drive 2 is supplied with pressure medium only by the first hydraulic pump 10. In lifting operation with high volume flow requirements, the second hydraulic pump 20 is switched on, so that the lifting drive 2 is supplied with pressure medium by both hydraulic pumps 10, 20.

[0048] The hydraulic system 1 according to the invention operates as follows: An operator specifies an actuation of the lifting drive 2 and / or one or more of the auxiliary consumers 5a, 5b by actuating the operating device 31. The control device 30 receives the corresponding signals from the operating device 31.

[0049] If a lifting operation of the lifting drive 2 is specified by a corresponding actuation of the operating device 31, the control device 30 compares the lifting volume flow requirement of the lifting drive 2 specified by the signal of the actuated operating device 31 with a lifting limit value stored in the control device 30.

[0050] If the lifting volume flow requirement of the lifting drive 2 specified by the actuated control device 31 is less than the lifting limit value, the control device 30 controls the electric drive unit 11 such that the first hydraulic pump 10 is operated as a pump and supplies the lifting drive 2 with pressure medium. By appropriately controlling the speed of the electric drive unit 11, a lifting speed of the load-handling device specified on the control device 31 can be controlled. The first hydraulic pump 10, operating as a pump, draws pressure medium from the container 13 and conveys the pressure medium into the delivery line 14 and, via the opening check valve 17 of the valve device 15 located in the blocking position 15a, directly to the lifting drive 2. Up to the lifting limit value, the lifting drive 2 is thus supplied with pressure medium during lifting operation only by the first hydraulic pump 10.If an auxiliary consumer 5a, 5b is activated by a corresponding actuation of the operating device 31, the control device 30 controls the electric drive motor 21 of the second hydraulic pump 20 and the control directional valve device of the control directional valve block 25 of the auxiliary consumer 5a, 5b that controls the corresponding auxiliary consumer in such a way that the auxiliary consumer 5a, 5b is actuated in accordance with the actuation of the operating device 31.

[0051] If the lifting volume flow requirement of the lifting drive 2 specified by the actuated operating device 31 is greater than the lifting limit value, the control device 30 additionally controls the electric drive motor 21 of the second hydraulic pump 20 and the control valve of the control directional valve block 25 controlling the lifting drive 2, so that the lifting drive 2 is supplied with pressure medium by both hydraulic pumps 10, 20.

[0052] The lifting limit is selected such that, during lifting operation at low and medium flow requirements, the lifting drive 2 is supplied with pressure medium only by the first hydraulic pump 10. Since the first hydraulic pump 10 delivers pressure medium directly to the lifting drive 2 via the delivery line 14, no losses occur in the control directional valve block 25.

[0053] The lifting limit value is further selected such that the lifting drive 2 is supplied with pressure medium by the first hydraulic pump 10 and the second hydraulic pump 20 during lifting operation when there is a high volume flow requirement.

[0054] The first hydraulic pump 10, which is operated as a pump, thus supplies the lifting drive 2 with pressure medium at all operating points during the lifting operation of the lifting drive 2, and the second hydraulic pump 20 is switched on during the lifting operation of the lifting drive 2 to additionally supply the lifting drive 2 with pressure medium if the lifting volume flow requirement of the lifting drive 2 exceeds the lifting limit value.

[0055] If a lowering operation of the lifting drive 2 is specified by a corresponding actuation of the operating device 31, the control device 30 compares the lowering volume flow requirement of the lifting drive 2 specified by the signal of the actuated operating device 31 with a lowering limit value stored in the control device 30.

[0056] The lowering operation of the lifting drive 2 can be controlled according to the following two variants.

[0057] If the lowering volume flow requirement specified by the actuated control device 31 is less than the lowering limit value, according to a first variant, the control device 30 controls the valve device 15 to the flow position 15b and the electric drive motor 11 such that the first hydraulic pump 10 is operated as a motor and is driven by the pressure medium flowing out of the lifting drive 2. The hydraulic pump 10 operated as a motor drives the electric drive motor 11 operated as a generator, which generates electrical energy and feeds it into a traction battery (not shown in detail) of the industrial truck. The hydraulic pump 10 thus operates in recuperation mode, in which energy is recovered during the lowering operation of the lifting drive 2. Up to the lowering limit value, the motor operation of the first hydraulic pump 10 thus results in recuperation mode for energy recovery.By appropriately controlling the speed of the electric drive motor 11, a lowering speed of the load-handling device specified on the operating device 31 can be controlled.

[0058] If the sink volume flow requirement specified by the actuated control device 31 is higher than the sink limit value, which results, for example, at maximum speed of the drive motor 11, the control device 30 additionally controls the control valve of the control directional valve block 25 controlling the lifting drive 2 in such a way that the connecting line 26 is connected to the drain line 29.

[0059] The lowering limit value is selected such that as much energy as possible can be recovered during lowering operation of the lifting drive 2. Furthermore, the lowering limit value is selected such that only at a high lowering speed of the lifting drive 2, due to the additional control of the control valve of the control directional valve block 25, does an additional volume flow from the lifting drive 2 flow out via the control directional valve block 25 to the container 13 and is throttled off at the control valve.

[0060] In the lowering operation of the lifting drive 2, the first hydraulic pump 10, which is operated as a motor, thus discharges the pressure medium flow flowing out of the lifting drive 2 to the container 13 at all operating points, and in the lowering operation of the lifting drive 2, the pressure medium flow flowing out of the lifting drive 2 is additionally discharged to the container 13 via the control valve of the control directional valve block 25 actuated into the control position if the lowering volume flow requirement exceeds the lowering limit value.

[0061] If the lowering volume flow requirement specified by the actuated control device 31 is less than the lowering limit value, the control device 30 actuates - according to a second variant - the control valve of the control directional valve block 25, which controls the lifting drive 2, into the control position in which the connecting line 26 is connected to the drain line 29. Up to the lowering limit value, the lowering operation of the lifting drive 2 thus takes place exclusively via the control valve of the control directional valve block 25. By appropriately controlling the control valve, a lowering speed of the load handling device specified on the control device 31 can be controlled.

[0062] If the lowering volume flow requirement specified by the actuated control device 31 is higher than the lowering limit value, the control device 30 controls the valve device 15 to the flow position 15b and the electric drive motor 11 such that the first hydraulic pump 10 is operated as a motor and driven by the pressure medium flowing out of the lifting drive 2. The hydraulic pump 10, operated as a motor, drives the electric drive motor 11, operated as a generator, which generates electrical energy and feeds it into a traction battery (not shown in detail) of the industrial truck. The hydraulic pump 10 thus operates in recuperation mode, in which energy is recovered during the lowering operation of the lifting drive 2.

[0063] The lowering limit value is selected such that in the lowering operation of the lifting drive 2 in the fine control range, lowering takes place exclusively via the control valve and only at a higher lowering speed of the lifting drive 2 is an additional volume flow discharged from the lifting drive 2 to the container 13 by the recuperation operation of the first hydraulic pump 10.

[0064] The second hydraulic pump 20 can supply one or more auxiliary consumers 5a, 5b with pressure medium. Alternatively, the second hydraulic pump 20 can supply only one auxiliary consumer designed as a hydraulic steering device. The second hydraulic pump 20 thus represents a separate, hydraulic steering drive, which is activated with the lifting drive 2 during peak lifting operation. This can provide particular advantages for industrial trucks, since industrial trucks generally do not steer at maximum speed and lift the load-handling device at the same time, allowing the hydraulic pump of the lifting drive and its electric drive motor to be smaller.

Claims

1. Hydraulic system (1) for an industrial truck, in particular a battery-electrically operated industrial truck, having a lifting drive (2) for raising and lowering a load-bearing means and having at least one auxiliary consumer (5a; 5b), wherein the hydraulic system (1) has a first hydraulic pump (10), which is driven by a first electric drive machine (11), wherein the first hydraulic pump (10) is operable as a pump and as a motor, and has a second hydraulic pump (20), which is driven by a second electric drive machine (21), wherein the second hydraulic pump (20) is operable only as a pump, wherein the hydraulic system is configured in such a way that the first hydraulic pump (10), in the raising mode of the lifting drive (2), supplies pressure medium to the lifting drive (2) at all operating points and the second hydraulic pump (20) supplies pressure medium to the auxiliary consumers (5a; 5b), characterized in that the hydraulic system is configured such that, in the raising mode of the lifting drive (2), it switches on the second hydraulic pump (20), for supplying pressure medium to the lifting drive (2), if a raising volume-flow requirement of the lifting drive (2) exceeds a raising limit value.

2. Hydraulic system according to Claim 1, characterized in that the hydraulic system is configured such that, in the lowering mode of the lifting drive (2), it discharges to a container (13) via the first hydraulic pump (10) the pressure-medium flow flowing away from the lifting drive (2), wherein the first hydraulic pump (10) is operated as a motor and drives the first electric drive machine (11), which is working in the generator mode.

3. Hydraulic system according to Claim 2, characterized in that the first hydraulic pump (10) is connected to the container (13) by means of a container line (12) and is connected to the lifting drive (2) by means of a conveying line (14), wherein, in the conveying line (14) of the first hydraulic pump (12), there is arranged a valve device (15) which has a blocking position (15a) and a throughflow position (15b), wherein the lifting drive (2) is connectable to the container (13) by means of a control valve, wherein the hydraulic system is configured to actuate the control valve into a control position, in which the lifting drive (2) is connected to the container (13), in a manner dependent on a lowering volume-flow requirement of the lifting drive (2).

4. Hydraulic system according to Claim 3, characterized in that the hydraulic system is configured in such a way that the first hydraulic pump (10), in the lowering mode of the lifting drive (2), discharges to the container (13) the pressure-medium flow flowing away from the lifting drive (2) at all operating points and, in the lowering mode of the lifting drive (2), the control valve is actuated into the control position and additionally discharges to the container (13) the pressure-medium flow flowing away from the lifting drive (2) if the lowering volume-flow requirement of the lifting drive exceeds a lowering limit value.

5. Hydraulic system according to Claim 3, characterized in that the hydraulic system is configured in such a way that, in the lowering mode of the lifting drive (2), the control valve actuated into the control position discharges to the container (13) the pressure-medium flow flowing away from the lifting drive (2) and the first hydraulic pump (10) additionally discharges to the container (13) the pressure-medium flow flowing away from the lifting drive (2) if the lowering volume-flow requirement of the lifting drive exceeds a lowering limit value.

6. Hydraulic system according to one of Claims 3 to 5, characterized in that provision is made of an electronic control device (30) which is connected at the input side to an operator control device (31) for control of the lifting drive (2) and of the auxiliary consumer (5a; 5b) and is connected at the output side to the electric drive machines (11; 21) for control thereof.

7. Hydraulic system according to Claim 6, characterized in that the electronic control device (30) is connected to the control valve for control thereof.

8. Hydraulic system according to Claim 6 or 7, characterized in that the electronic control device (30) is configured in such a way that, in the raising mode of the lifting drive (2), the first hydraulic pump (10) is operated as a pump which supplies pressure medium to the lifting drive (2) at all operating points, and that, in the raising mode of the lifting drive (2), the second hydraulic pump (20) is switched on for additional supply of pressure medium to the lifting drive (2) if the raising volume-flow requirement of the lifting drive (2) exceeds the predefined raising limit value.

9. Hydraulic system according to one of Claims 6 to 8, characterized in that the electronic control device (30) is configured in such a way that, in the lowering mode of the lifting drive (2), the first hydraulic pump (10) is operated as a motor which discharges to the container (13) the pressure-medium flow flowing away from the lifting drive (2) at all operating points, and that, in the lowering mode of the lifting drive (2), the control valve is actuated into the control position so as to additionally discharge to the container (13) the pressure-medium flow flowing away from the lifting drive (2) if the lowering volume-flow requirement of the lifting drive exceeds the predefined lowering limit value.

10. Hydraulic system according to one of Claims 6 to 8, characterized in that the electronic control device (30) is configured in such a way that, in the lowering mode of the lifting drive (2), the control valve is actuated into the control position so as to discharge to the container (13) the pressure-medium flow flowing away from the lifting drive (2) and, in the lowering mode of the lifting drive (2), the first hydraulic pump (10) is operated as a motor so as to additionally discharge to the container (13) the pressure-medium flow flowing away from the lifting drive (2) if the lowering volume-flow requirement of the lifting drive exceeds the predefined lowering limit value.

11. Hydraulic system according to one of Claims 1 to 10, characterized in that the auxiliary consumer (5a; 5b) is configured as a tilt drive of the load-bearing means and / or as a hydraulic steering device.