Hydraulic System for an Industrial Truck

Abstract

A hydraulic system for an industrial truck with a lifting frame has at least one telescoping mast that can be raised and lowered in a stationary mast and a load handling means that can be raised and lowered in the telescoping mast. The hydraulic system has a free lift cylinder for raising and lowering the load handling means and at least one mast lift cylinder for raising and lowering the telescoping mast. A control valve device is provided for the control of the lifting operation and the lowering operation of the free lift cylinder and of the mast lift cylinders. A mast transition damping device is provided that includes at least one electrically actuated proportional valve. The proportional valve of the mast transition damping device, in the unactuated state, has a throttling connection that effects a throttled volume flow, and in the event of an electrical control action, can be actuated toward an open position.

Claims

1. A hydraulic system for an industrial truck with a lifting frame, comprising at least one telescoping mast that can be raised and lowered in a stationary mast and a load handling device that can be raised and lowered in the telescoping mast, wherein the hydraulic system has a free lift cylinder for raising and lowering the load handling device and at least one mast lift cylinder for raising and lowering the telescoping mast, wherein a control valve device is provided for the control of the lifting operation and the lowering operation of the free lift cylinder and of the mast lift cylinders, wherein a mast transition damping device is provided that comprises at least one electrically actuated proportional valve, and wherein the proportional valve of the mast transition damping device, in the unactuated state, has a throttling connection that effects a throttled volume flow, and in the event of an electrical control action can be actuated toward an open position.

2. The hydraulic system according to claim 1, wherein the proportional valve in the unactuated state is actuated into a normal position which is provided with the throttling connection.

3. The hydraulic system according to claim 2, wherein the proportional valve in the normal position is provided with a throttling opening.

4. The hydraulic system according to claim 1, wherein the proportional valve is located in a housing, wherein the proportional valve in the unactuated state is actuated into a normal position which is in the form of a closed position, and the throttling connection is formed by a bypass line in the housing of the proportional valve in which a throttling device is located.

5. The hydraulic system according to claim 1, wherein the free lift cylinder is connected by a first branch line with the control valve device, and the at least one mast lift cylinder is connected by a second branch line with the control valve device, wherein there is a first proportional valve of the mast transition damping device in the first branch line and a second proportional valve of the mast transition damping device in the second branch line.

6. The hydraulic system according to claim 5, wherein located on the first proportional valve is a bypass line in which there is a stop valve that opens toward the control valve device.

7. The hydraulic system according to claim 5, wherein located on the second proportional valve is a bypass line in which there is a stop valve that opens toward the at least one mast lift cylinder.

8. The hydraulic system according to claim 1, wherein a mechanical line break safety device is attached to each mast lift cylinder.

9. The hydraulic system according to claim 1, wherein a mechanical line break safety device is attached to the free lift cylinder.

10. The hydraulic system according to claim 5, wherein the first proportional valve is attached to the free lift cylinder and has the function of an electrical line break safety device.

11. The hydraulic system according to claim 1, wherein the free lift cylinder is connected by a first branch line with the control valve device, and each mast lift cylinder is in communication by a connecting line with the control valve device, wherein a proportional valve of the mast transition damping device is located in each of the first branch line and the connecting line.

12. The hydraulic system according to claim 11, wherein attached to the free lift cylinder is a first proportional valve and/or attached to each mast lift cylinder is a second proportional valve, wherein the first and second proportional valves each have the function of an electrical line break safety device.

13. An industrial truck with a hydraulic system according to claim 1.

14. The hydraulic system according to claim 3, wherein the throttling opening is a throttling boring.

15. The hydraulic system according to claim 6, wherein the stop valve is a non-return valve.

16. The hydraulic system according to claim 7, wherein the stop valve is a non-return valve.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Additional advantages and details of the invention are described in greater detail below with reference to the exemplary embodiments illustrated in the accompanying schematic figures, in which:

[0032] FIG. 1 is a schematic circuit diagram of a first embodiment of a hydraulic system according to the invention,

[0033] FIG. 2 is a schematic circuit diagram of a second embodiment of a hydraulic system according to the invention,

[0034] FIG. 3 is a schematic circuit diagram of a third embodiment of a hydraulic system according to the invention, and

[0035] FIG. 4 is a schematic circuit diagram of a fourth embodiment of a hydraulic system according to the invention.

DESCRIPTION OF THE INVENTION

[0036] FIGS. 1 to 4 each show a schematic construction of a hydraulic system 1 of an industrial truck which is not illustrated in any further detail. Identical components are identified by identical reference numbers.

[0037] The industrial truck has a lifting frame which is not illustrated in any further detail, on which a load handling means or device is located so that it can be raised and lowered. The load handling means consist in particular of a lifting carriage that can move vertically on a lifting frame, to which is fastened, for example, a load fork formed by fork tines as an accessory device.

[0038] The lifting frame consists in particular of a stationary mast and at least one telescoping mast located so that it can be raised and lowered on the stationary mast, with load holding means located on the telescoping mast so that it can be raised and lowered.

[0039] The lifting frame has at least two lifting stages. The hydraulic system has a free lift cylinder 10 to raise and lower the load holding means relative to the telescoping mast. The free lift cylinder 10 forms a first lifting stage (free lift). For the raising and lowering of the load handling means, in particular flexible traction means or device which are not illustrated any further detail, for example, a lifting chain, are provided, which are fastened with a first end to the lifting carriage, are guided over a deflector roller on the telescoping piston rod of the free lift cylinder 10 and fastened with a second end to the telescoping mast. For raising and lowering the telescoping mast relative to the stationary mast, the hydraulic system 1 has at least one mast lift cylinder 11a, 11b. The mast lift cylinder 11a, 11b forms a second lifting stage (mast lift). In the illustrated exemplary embodiments, two mast lifting cylinders 11a, 11b are provided.

[0040] A control valve device 15 is provided for the control of the lifting and lowering operation of the free lift cylinder 10 and of the mast lift cylinders 11a, 11b.

[0041] The control valve device 15 in the illustrated exemplary embodiments is in the form of a proportional valve that throttles in intermediate positions, with a closed position 15a in the form of a normal position, a lifting position 15b, and a lowering position 15c. For this purpose, the control valve device 15 is connected to a delivery line 16 of a pump 17, which by a suction line 18 sucks pressure medium out of a reservoir 19 to a reservoir line 20 that leads to the reservoir 19, and to a consumer line 21 which is connected with the free lift cylinder 10 and the mast lift cylinders 11a, 11b.

[0042] In the closed position 15a of the control valve device 15, the connection of the consumer line 21 with the delivery line 16 and the reservoir line 20 is closed. In the lifting position 15b of the control valve device 15, the delivery line 16 is connected with the consumer line 21. In the lowering position 15c of the control valve device 15, the consumer line 21 is connected with the reservoir line 20.

[0043] In the illustrated exemplary embodiments the control valve device 15 is located on a multi-way control valve block 22.

[0044] The control valve device 15 can, for example, be actuated electrically by an electronic control device 25.

[0045] Alternatively, the control valve device 15 can have a separate lifting valve to control the lifting operation of the load handling means and a separate lowering valve to control the lowering operation of the load handling means.

[0046] The free lift cylinder 10 and the mast lift cylinders 11a, 11b in FIGS. 1 to 4 are realized in such a way that during lifting operations of the load handling means, first the free lift cylinder 10 is extended and then the mast lift cylinders 11a, 11b are extended, and during lowering operations of the load handling means, first the mast lift cylinders 11a, 11b are retracted and then the free lift cylinder 10 is retracted.

[0047] The hydraulic system 1 in FIGS. 1 to 4 also has a mast transition damping device 35 that comprises at least one electrically actuated proportional valve.

[0048] In FIG. 1, the free lift cylinder 10 is connected with the control valve device 15 by a first branch line 30 branching off the consumer line 21, and the at least one mast lift cylinder 11a, 11b is connected with the control valve device 15 by a second branch line 31 branching off the consumer line 21. The branch line 31 is in connection by a first connecting line 31a with the mast lift cylinder 11a and by a first connecting line 31b with the mast lift cylinder 11b.

[0049] In FIG. 1, the mast transition damping device 35 has two electrically actuatable, first and second, proportional valves 36, 37, wherein the first proportional valve 36 is located in the first branch line 30 and the second proportional valve 37 is located in the second branch line 31.

[0050] Located on the first proportional valve 36 is a bypass line 40, in which there is a stop valve 41, in particular a non-return valve, which opens toward the control valve device 15.

[0051] Located on the second proportional valve 37 is a bypass line 42, in which there is a stop valve 43, in particular a non-return valve, which opens toward the mast lift cylinders 11a, 11b.

[0052] In FIG. 1, the proportional valves 36, 37 of the mast transition damping device 35 are located with the bypass lines 40, 42 and the stop valves 41, 43 located in them, in a valve block 45 of the mast transition damping device 35.

[0053] In FIG. 1, a mechanical line break safety device 60a, 60b is attached to each mast lift cylinder 11a, 11b. The line break safety device 60a attached to the mast lift cylinder 11a comprises a stop valve 62a that opens toward the mast lift cylinder 11a, for example a non-return valve, and a bypass line 63a with a throttle device 64a located in the bypass line 63a. The line break safety device 60b attached to the mast lift cylinder 11b comprises a stop valve 62b, for example a non-return valve, that opens toward the mast lift cylinder 11b, and a bypass line 63b with a throttle device 64b located in the bypass line 63b.

[0054] In FIG. 1, a mechanical line break safety valve device 60c is also attached to the free lift cylinder 10. The line break safety device 60c attached to the free lift cylinder 10 comprises a stop valve 62c that opens toward the free lift cylinder 10, for example a non-return valve, and a bypass line 63c with a throttle device 64c located in the bypass line 63c.

[0055] The electrically actuatable proportional valve 36 or 37 of the mast transition damping device 35, in the unactuated state, has a throttling connection 50 that effects a throttled volume flow, and can be actuated by an electrical control action toward an open position 51a.

[0056] In FIG. 1, the proportional valve 36 or 37 is actuated in the unactuated state into a normal position 51b which is provided with the throttling connection 50. The throttling connection 50 is, for example, formed by a throttling opening 52, for example a throttling boring, that acts in the normal position 51b. The normal position 51b is, therefore, realized as a throttling position.

[0057] Each proportional valve 36 or 37 is actuated by a spring device 55 into the normal position 51b and can be actuated toward the open position 51a by an electrical actuator device 56, such as a proportional magnet, for example. The actuator devices 56 are in connection with the electronic control device 25 for their control.

[0058] The hydraulic system 1 in FIG. 1, therefore, has a mast transition damping device 35 formed by the two electrically actuated proportional valves 36, 37, in combination with mechanical line break safety devices 60a, 60b, 60c on the free lift cylinder 10 and on the mast lift cylinders 11a, 11b.

[0059] In FIGS. 1 to 4 the electronic control device 25 is in communication with a sensor device not shown in any further detail, for example a lift height sensor or a lift height switch, with which the mast transitional range between free lift and mast lift can be determined.

[0060] The hydraulic system 1 illustrated in FIG. 1 functions as follows.

[0061] To raise the load handling means, which is done by actuating the control valve device 15 into the lifting position 15b, pressure medium is transported from the control valve device 15 into the consumer line 21 to the valve block 45 of the mast transition damping device 35. The proportional valve 36 of the mast transition damping device 35 is actuated into the open position 51a by the control device 25 so that pressure medium flows via the proportional valve 36 actuated into the open position 51a and the branch line 30 into the free lift cylinder 10. On account of the ratio of the surface area of the free lift cylinder 10 to the mast lift cylinders 11a, 11b, initially no pressure medium flows via the stop valve 43 into the mast lift cylinders 11a, 11b. As soon as the free lift cylinder 10 approaches the mast transition range, which is detected by the control device 25 by the sensor device, the electronic control device 25 begins to actuate the proportional valve 36 into the normal position 51b which is in the form of a throttling position. Consequently, the volume flow upstream of the proportional valve 36 is backed up to the extent that pressure medium flows from the control valve device 15 into the mast lift cylinders 11a, 11b via the opening stop valve 43.

[0062] To lower the load handling means, for which purpose the control valve device 15 is actuated into the lowering position 15c, pressure medium is transported out of the mast lift cylinders 11a, 11b via the branch line 31 into the valve block 45 of the mast transition damping device 35. The proportional valve 37 of the mast transition damping device 35 is actuated by the control device into the open position 51a, so that pressure medium flows out of the proportional valve 37 actuated into the open position 51a into the consumer line 21, and via the control valve device actuated into the lowering position 15c to the reservoir 19. On account of the ratio of the surface area of the free lift cylinder 10 to the mast lift cylinders 11a, 11b, initially no pressure medium flows out of the free lift cylinder 10 via the stop valve 41 to the control valve device 15. As soon the mast lift cylinders 11a, 11b approach the mast transition range, which is detected by the control device 25 by the sensor device, the electronic control device 15 begins to actuate the proportional valve 37 into the normal position 51b which is in the form of a throttling position. Consequently, the volume flow upstream of the proportional valve is backed up to the extent that pressure medium flows via the opening stop valve 41 out of the free lift cylinder 10 to the control valve device 15.

[0063] In FIG. 2, the free lift cylinder 10 is connected by a first branch line 30 that branches off the consumer line 21 with the control valve device 15, the first mast lift cylinder 11a is connected by a second branch line 31a that branches off the consumer line 21 with the control valve device 15, and the second mast lift cylinder 11b is connected by a third branch line 31b that branches off the consumer line 21 with the control valve device 15.

[0064] In FIG. 2, the mast transition damping device 35 has three electrically actuatable proportional valves 36, 37a, 37b, wherein in each branch line 30, 31a, 31b there is a proportional valve 36, 37a, 37b of the mast transition damping device 35. The first proportional valve 36 is located in the first branch line 30, the second proportional valve 37a is located in the second branch line 31a and the third proportional valve 37b is located in the third branch line 31b.

[0065] The proportional valve 36 is attached to the free lift cylinder 10, wherein the proportional valve 36 of the mast transition damping device 35 also has the function of an electrical line break safety device 60c of the free lift cylinder 10. The proportional valve 37a is attached to the mast lift cylinder 11a, wherein the proportional valve 37a of the mast to transition damping device 35 also has the function of an electrical line break safety device 60a of the mast lift cylinder 11a. The proportional valve 37b is attached to the mast lift cylinder 11b, wherein the proportional valve 37b of the mast transition damping device 35 also has the function of an electrical line break safety device 60b of the mast lift cylinder 11b.

[0066] The electrically actuatable proportional valves 36, 37a, 37b of the mast transition damping device 35, in the unactuated state, each have a throttling connection 50 that effects a throttled volume flow, and can be actuated toward an open position 51a in the event of an electrical control action.

[0067] In FIG. 2, the proportional valves 36, 37a, 37b are each actuated in the unactuated state into a normal position 51b, which is provided with the throttling connection 50. The throttling connection 50 is, for example, formed by a throttling opening 52, for example a throttling boring, that acts in the normal position 51b. The normal position 51b is therefore realized as a throttling position.

[0068] The proportional valve 36, 37a, 37b is actuated by a spring device 55 into the normal position 51b and can be actuated by an electrical actuator device 56, such as a proportional magnet, toward the open position 51a. The actuator devices 56 are in connection with the electronic control device 25 for driving.

[0069] Therefore, the hydraulic system 1 in FIG. 2 has a mast transition damping device 35 formed by the three electrically actuated proportional valves 36, 37a, 37b, in combination with electrical line break safety devices 60a, 60b, 60c on the free lift cylinder 10 and on the mast lift cylinders 11a, 11b, wherein the proportional valves 36, 37a, 37b of the mast transition damping device 35 attached to the corresponding lift cylinders each have the function of a line break safety device 60a, 60b, 60c.

[0070] The hydraulic system 1 in FIG. 2 functions as follows.

[0071] To raise the load handling means, for which purpose the control valve device 15 is actuated into the lifting position 15b, pressure medium is transported from the control valve device 15 into the consumer line 21 and into the branch lines 30, 31a, 31b connected to the consumer line 21. The proportional valve 36 of the mast transition damping device 35 is actuated into the open position 51a by the control device 25, so that pressure medium flows via the proportional valve 36 actuated into the open position 51a and the branch line 30 into the free lift cylinder 10. On account of the ratio of surface area of the free lift cylinder 10 to the mast lift cylinders 11a, 11b, initially no pressure medium flows into the mast lift cylinder 11a, 11b. As soon as the free lift cylinder 10 approaches the mast transition range, which is detected by the control device 25 by the sensor device, the electronic control device 25 begins to actuate the proportional valve 36 into the normal position 51b which is in the form of a throttling position, and to actuate the proportional valves 37a, 37b each into the open position 51a. Consequently, the volume flow upstream of the proportional valve 36 is backed up to the extent that pressure medium flows from the control valve device 15 into the mast lift cylinders 11a, 11b via the proportional valves 37a, 37b actuated toward the open position 51a.

[0072] To lower the load handling means, for which purpose the control valve device 15 is actuated into the lowering position 15c, the proportional valves 37a, 37b are each actuated by the control device 25 into the open position 51a, as a result of which pressure medium flows out of the mast lift cylinders 11a, 11b via the opened proportional valves 37a, 37b into the consumer line 21 and via the control valve device 15 actuated into the lowering position 15c to the reservoir 19. On account of the ratio of the surface area of the free lift cylinder 10 to the mast lift cylinders 11a, 11b, initially no pressure medium flows out of the free lift cylinder 10 and the proportional valve 36, which is in the normal position 51b, to the control valve device 15. As soon as the mast lift cylinder 11a, 11b approaches the mast transition range, which is detected by the control device 25 by the sensor device, the electronic control device 15 begins to actuate the proportional valves 37a, 37b into the normal position 51b which is in the form of a throttling position, and to actuate the proportional valve 36 into the open position 51a. Consequently, the volume flow upstream of the proportional valves 37a, 37b is backed up to the extent that the pressure in the consumer line 21 is progressively reduced, so that an increasing quantity of pressure fluid flows out of the free lift cylinder 10 via the proportional valve 36 actuated into the open position 51a to the control valve device 15.

[0073] The electrically actuated proportional valves 36, 37a, 37b in FIG. 2 also have the function of line break safety devices 60a, 60b, 60c. If no lifting or lowering of the load handling means is underway and the proportional valves 36, 37a, 37b are not actuated, the proportional valves 36, 37a, 37b are in the normal position 51b which is in the form of a throttling position. This corresponds to the state of an engaged line break safety device. In the event of a line break, therefore, the lowering speed of the load handling means is limited to an allowable value by the throttling connection 50 of the proportional valve 36, 37a, 37b acting in the normal position 51b.

[0074] If, during a lifting or lowering of the load handling means, an anomaly or a fault such as a line break is detected by the control device 25, the control device 25 actuates the proportional valves 36, 37a, 37b into the normal position 51b which is in the form of a throttling position. It is thereby ensured that the lowering speed of the load handling means is limited to an allowable value by the throttling connection 50 acting in the normal position 51b.

[0075] In FIG. 3, analogous to FIG. 1, the free lift cylinder 10 is connected by a first branch line 30 branching off the consumer line 21 with the control valve device 15, and the at least one mast lift cylinder 11a, 11b is connected by a second branch line 31 branching off the consumer line 21 with the control valve device 15. The branch line 31 is in connection by a first connecting line 31a with the mast lift cylinder 11a and by a first connecting line 31b with the mast lift cylinder 11b.

[0076] In FIG. 3, the mast transition damping device 35 has two electrically actuatable proportional valves 36, 37, wherein the first proportional valve 36 is located in the first branch line 30 and the second proportional valve 37 is located in the second branch line 31.

[0077] The proportional valve 36, analogous to FIG. 2, is attached to the free lift cylinder 10, wherein the proportional valve 36 of the mast transition damping device 35 also has the function of an electrical line break safety device 60c of the free lift cylinder 10.

[0078] Located on the second proportional valve 37, analogous to FIG. 1, is a bypass line 42 in which there is a stop valve 43, in particular a non-return valve, which opens toward the mast lift cylinders 11a, 11b.

[0079] In FIG. 3, analogous to FIG. 1, a mechanical line break safety device 60a, 60b is attached to each mast lift cylinder 11a, 11b. The line break safety device 60a attached to the mast lift cylinder 11a comprises a stop valve 62a that opens toward the mast lift cylinder 11a, for example a non-return valve, and a bypass line 63a with a throttle device 64a located in the bypass line 63a. The line break safety device 60b attached to the mast lift cylinder 11b comprises a stop valve 62b, for example a non-return valve that opens toward the mast lift cylinder 11b, and a bypass line 63b with a throttle device 64b located in the bypass line 63b.

[0080] The electrically actuatable proportional valve 36 or 37 of the mast transition damping device 35, in the unactuated state, has a throttling connection 50 that effects a throttled volume flow, and can be actuated by an electrical control action toward an open position 51a.

[0081] In FIG. 3, the proportional valve 36 or 37 is actuated in the unactuated state into a normal position 51b which is provided with the throttling connection 50. The throttling connection 50 is, for example, formed by a throttling opening 52, for example a throttling boring, that acts in the normal position 51b. The normal position 51b is therefore realized as a throttling position.

[0082] Each proportional valve 36 or 37 is actuated by a spring device 55 into the normal position 51b and can be actuated toward the open position 51a by an electrical actuator device 56, such as a proportional magnet, for example. The actuator device 56 is in connection with the electronic control device 25 for their actuation.

[0083] The hydraulic system 1 in FIG. 3, therefore, has a mast transition damping device 35 actuated by the two electrically actuated proportional valves 36, 37, in combination with the mechanical line break safety devices 60a, 60b on the mast lift cylinders 11a, 11b and with the electrical line break safety device 60c on the free lift cylinder 10.

[0084] The hydraulic system 1 in FIG. 3 functions as follows.

[0085] To raise the load handling means, which is done by actuating the control valve device 15 into the lifting position 15b, pressure medium is transported by the control valve device 15 into the consumer line 21 and the branch lines 30, 31. The proportional valve 36 of the mast transition damping device 35 located on the free lift cylinder 10 is actuated into the open position 51a by the control device 25, so that pressure medium flows into the free lift cylinder 10 via the proportional valve 36 actuated into the open position 51a and the branch line 30. On account of the ratio of the surface area of the free lift cylinder 10 to the mast lift cylinders 11a, 11b, initially no pressure medium flows via the stop valve 43 into the mast lift cylinders 11a, 11b. As soon as the free lift cylinder 10 approaches the mast transition range, which is detected by the control device 25 by the sensor device, the electronic control device 25 begins to actuate the proportional valve 36 located on the free lift cylinder 10 into the normal position 51b which is in the form of a throttling position. Consequently, the volume flow upstream of the proportional valve 36 is backed up to the extent that pressure medium flows from the control valve device 15 into the mast lift cylinders 11a, 11b via the opening stop valve 43.

[0086] To lower the load handling means, for which purpose the control valve device 15 is actuated into the lowering position 15c, the proportional valve 37 is actuated by the control device 25 into the open position 51a, as a result of which pressure medium flows out of the mast lift cylinders 11a, 11b via the opened proportional valve 37 into the consumer line 21 and via the control valve device 15 actuated into the lowering position 15c to the reservoir 19. On account of the surface area ratio of the free lift cylinder 10 to the mast lift cylinders 11a, 11b, initially no pressure medium flows out of the free lift cylinder 10 and the proportional valve 36, which is in the normal position 15b and which is attached to the free lift cylinder 10, to the control valve device 15. As soon as the mast lift cylinders 11a, 11b approach the mast transition range, which is detected by the control device 25 by the sensor device, the electronic control device 15 begins to actuate the proportional valve 37 into the normal position 51b which is in the form of a throttling position, and to actuate the proportional valve 36 located on the free lift cylinder 10 into the open position 51a. Consequently, the volume flow upstream of the proportional valve 37 is backed up to the extent that the pressure in the consumer line 21 progressively decreases, so that increasingly more pressure medium flows out of the free lift cylinder 10 via the proportional valve 36 actuated into the open position 51a to the control valve device 15.

[0087] FIG. 4 illustrates a variant of FIG. 2.

[0088] In FIG. 4, the proportional valves 36, 37a, 37b each have a housing 70, into which the corresponding proportional valve 36, 37a, 37b is incorporated. Each proportional valve 36, 37a, 37b is actuated into a normal position 51b which is in the form of a closed position The throttling connection 50 of each proportional valve 36, 37a, 37b is formed by a bypass line 71 in which a throttling device 72 is located, which is realized in the housing 70 of the corresponding proportional valve 36, 37a, 37b.

[0089] Each proportional valve 36, 37a, 37b in FIG. 4 is actuated by a spring device 55 into the normal position 51b and can be actuated by an electrical actuator device 56, such as a proportional magnet, toward the open position 51a. The actuator devices 56 are in communication with the electronic control device 25 for their control.

[0090] In FIGS. 1 to 4, the corresponding proportional valves 36, 37a, 37b of the mast transition damping device 35 are actuated by the control device 25 so that during lifting operation of the load handling means, at the mast transition from the free lift into the mast lift, and in lowering operation of the load handling means, at the mast transition from the mast lift into the free lift, a uniform movement of the load handling means is always achieved without a change in speed or discontinuities in the mast transition.

[0091] For this purpose, during lifting operation the volume flow into the free lift cylinder 10 in the mast transition range is increasingly throttled by the corresponding actuation of the proportional valve 36 of the mast transition damping device 35, as a result of which a continuous decrease of the speed of extension of the free lift cylinder 10 is achieved. The proportional valves 37a, 37b are each actuated into the open position 51a. The dynamic pressure increased as a result of the throttling leads to a continuously faster extension of the mast lift cylinders 11a, 11b. The overlapping movement of the free lift cylinder 10 and of the mast lift cylinders 11a, 11b is preferably executed so that the lifting speed of the load remains constant in the mast transition range.

[0092] During lowering operation, for this purpose, by a corresponding control of the proportional valves 37a, 37b into the normal position 51b of the mast transition damping device 35, the volume flow out of the mast lift cylinders 11a, 11b in the mast transition range is progressively throttled, as a result of which a continuous decrease in the speed of retraction of the telescoping mast lift cylinders 11a, 11b is achieved. The proportional valve 36 is actuated into the open position 51a. The pressure in the consumer line 31 reduced by the throttling results in a continuously faster retraction of the free lift cylinder 10. The overlapping movement of the free lift cylinder 10 and of the mast lift cylinders 11a, 11b is preferably executed so that the lowering speed of the load remains constant in the mast transition range.

[0093] As a result of the mast transition damping, the free lift cylinder 10 and the mast lift cylinders 11a, 11b reach their mechanical terminal position at a very low speed both during lifting operation and during lowering operation.