Hydraulic lifting system and tractor

Abstract

The utility model provides a hydraulic lifting system and a tractor. Comprising an oil tank, a main oil supply way, a main oil return way, a lifting cylinder, a first reversing valve, a switching valve and an oil supplementing way. Due to the arrangement of the oil supplementing oil way, when vacuum negative pressure is formed in the rod cavity of the lifting cylinder, hydraulic oil in the main oil return oil way can flow back to the rod cavity of the lifting cylinder, oil supplementing of the rod cavity of the lifting cylinder is achieved, it is guaranteed that the pressure in the whole hydraulic system is stable, and hydraulic elements are prevented from being damaged. Moreover, the high-pressure function and the non-high-pressure function are switched through the switching valve, the first reversing valve and the switching valve can be arranged separately, the arrangement of the whole scheme is not limited by the space of the whole machine, and the hydraulic system has the advantages of being easy to maintain, convenient to replace parts and the like.

Description

Technical Field

[0001] The utility model belongs to the technical field of tractors, in particular to a hydraulic lifting system and a tractor. Background Art

[0002] In the prior art, for the strong-pressure hydraulic lifting system of a tractor, when the descending speed of the lifting cylinder is too fast under the action of a load, a vacuum negative pressure will be formed in the rod chamber of the lifting cylinder, resulting in the forced return of the hydraulic oil in the hydraulic lifting system to the rod chamber of the lifting cylinder, thereby affecting the stability and safety of the hydraulic lifting system. Therefore, it is necessary to provide a hydraulic lifting system and a tractor to overcome the defects existing above. Summary of the Utility Model

[0003] The purpose of the utility model is to provide a hydraulic lifting system and a tractor.

[0004] According to one aspect of the utility model, there is provided a hydraulic lifting system and a tractor, comprising: an oil tank; a main oil supply circuit which is connected to the oil tank through an oil pump in series and can provide pressurized hydraulic oil; a main oil return circuit which is connected to the oil tank; a lifting cylinder which includes a rod chamber and a rodless chamber; a first reversing valve which includes a first oil inlet, a first oil return port, a first working oil port and a second working oil port, the first oil inlet is connected to the main oil supply circuit, the first oil return port is connected to the main oil return circuit, the first working oil port is connected to the rodless chamber, and the second working oil port is connected to the rod chamber; a switching valve which includes a third oil inlet connected to the first working oil port, a third oil return port connected to the main oil return circuit and / or the oil tank, and a fifth working oil port connected to the rodless chamber; in the strong-pressure working state, the third oil inlet is communicated with the fifth working oil port; in the non-strong-pressure working state, the third oil return port is communicated with the fifth working oil port; a supplementary oil circuit on which a supplementary oil valve is arranged, and the supplementary oil valve can unidirectionally return the main oil return circuit to the third oil inlet.

[0005] Preferably, a back-pressure valve is also connected in series on the main oil return circuit.

[0006] Preferably, the switching valve further includes a sixth working oil port connected to the oil tank, and the third oil return port is connected to the oil inlet of the supplementary oil valve; in the strong-pressure working state, the third oil return port is communicated with the sixth working oil port; in the non-strong-pressure working state, the third oil inlet is communicated with the sixth working oil port.

[0007] Preferably, a second reversing valve is further included. The second reversing valve includes a second oil inlet, a second oil return port, a third working oil port, and a fourth working oil port. The second oil inlet is connected to the main oil supply circuit, the second oil return port is connected to the main oil return circuit, and the third working oil port and the fourth working oil port are connected to a hydraulic output joint.

[0008] Preferably, the oil pump is a load-sensing piston pump.

[0009] Preferably, an oil pressure compensation valve is provided between the first oil inlet and / or the second oil inlet and the main oil supply circuit.

[0010] Preferably, the following are further included: a first shuttle valve, one oil inlet of the first shuttle valve is connected to the first oil inlet, and the other oil inlet of the first shuttle valve is connected to the third oil inlet; a second shuttle valve, one oil inlet of the second shuttle valve is connected to the oil outlet of the first shuttle valve, the other oil outlet of the second shuttle valve is connected to the second oil inlet, and the oil outlet of the second shuttle valve is connected to one oil inlet of the next second shuttle valve or the sensor end of the load-sensing piston pump.

[0011] Preferably, a throttle valve is connected in series between one oil inlet of the first shuttle valve and the first oil inlet, between the other oil inlet of the first shuttle valve and the third oil inlet, and between the other oil outlet of the second shuttle valve and the second oil inlet.

[0012] Preferably, a first safety valve is further provided between the main oil supply circuit and the main oil return circuit; a oil filtering device is connected in series on the main oil supply circuit and / or the main oil return circuit.

[0013] According to another aspect of the present invention, a tractor is provided, including the hydraulic lifting system.

[0014] Compared with the prior art, the hydraulic lifting system and the tractor provided by the present invention have the following beneficial effects:

[0015] By providing a supplementary oil circuit, when a vacuum negative pressure is formed in the rod chamber of the lifting cylinder, the hydraulic oil in the main oil return circuit can flow back to the rod chamber of the lifting cylinder to supplement the oil in the rod chamber of the lifting cylinder, ensuring the pressure stability in the entire hydraulic system and preventing damage to hydraulic components.

[0016] Moreover, the present invention realizes the conversion between the strong pressure and non-strong pressure functions through a switching valve. The load-sensing multi-way valve and the strong pressure-non-strong pressure switching valve can be arranged separately, and the layout of the entire scheme is not limited by the space of the whole machine. In addition, the hydraulic system has the characteristics of simple maintenance and convenient replacement of components. Description of the Drawings

[0017] The present utility model will be further described in detail below in conjunction with the accompanying drawings and specific embodiments:

[0018] Figure 1 It is a schematic structural diagram of a hydraulic lifting system in Embodiment 1 of the present utility model;

[0019] Figure 2 It is a schematic structural diagram of another hydraulic lifting system in Embodiment 1 of the present utility model;

[0020] Figure 3 It is a schematic structural diagram of the first reversing valve in Embodiment 1 of the present utility model;

[0021] Figure 4 It is a schematic structural diagram of the second reversing valve in Embodiment 1 of the present utility model;

[0022] Figure 5 It is a schematic structural and connection diagram of the switching valve in Embodiment 1 of the present utility model;

[0023] Figure 6 It is a schematic structural diagram of the hydraulic lifting system in Embodiment 2 of the present utility model;

[0024] Figure 7 It is a schematic structural diagram of the first reversing valve in Embodiment 2 of the present utility model;

[0025] Figure 8 It is a schematic structural diagram of the second reversing valve in Embodiment 2 of the present utility model;

[0026] Figure 9 It is a schematic structural and connection diagram of the switching valve in Embodiment 2 of the present utility model.

[0027] Explanation of reference numerals: 1, oil tank; 2, oil pump; 3, lifting cylinder; 31, rod chamber; 32, rodless chamber; 4, multi-way valve; 41, first reversing valve; P1, first oil inlet; T1, first oil return port; A1, first working oil port; B1, second working oil port; 42, second reversing valve; P2, second oil inlet; T2, second oil return port;

[0028] A2, third working oil port; B2, fourth working oil port; 43, overflow safety valve; 44, oil pressure compensation valve; 45, first shuttle valve; 46, second shuttle valve; 51, switching valve; P3, third oil inlet; T3, third oil return port; A3, fifth working oil port; 52, oil replenishing valve; 53, throttle valve; 6, back pressure valve; 7, hydraulic output joint; 81, suction oil filter; 82, pressure oil filter; 83, return oil filter;

[0029] L1, main oil supply circuit; L2, main oil return circuit; L3, oil replenishing circuit; L4, feedback circuit. Specific embodiments

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0031] To simplify the drawings, only the parts related to the present invention are schematically shown in each drawing, and they do not represent the actual structure of the product. Additionally, to simplify the drawings for easier understanding, in some drawings, for components with the same structure or function, only one of them is schematically shown, or only one of them is labeled. In this article, "one" not only means "only this one", but also means "more than one" situation.

[0032] It should be further understood that the term "and / or" used in the description of the present application and the appended claims refers to any combination and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0033] In this article, it should be noted that unless otherwise clearly specified and defined, the terms "installation", "connection", and "connection" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0034] In addition, in the description of the present application, the terms "first", "second", etc. are only used for distinguishing descriptions and cannot be understood as indicating or implying relative importance.

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will describe the specific implementation manners of the present invention with reference to the drawings. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings, and other implementation manners can be obtained.

[0036] Embodiment 1

[0037] See Figures 1 to 5 , this embodiment provides a hydraulic lifting system for a tractor. The hydraulic lifting system includes: a fuel tank 1, a main oil supply circuit L1, a main oil return circuit L2, a lifting cylinder 3, a multi-way valve 4, a switching valve 51, and a supplementary oil circuit L3.

[0038] Refer to the appendix Figures 1 to 3 , a fuel pump 2 is connected in series on the main fuel supply oil path L1 and is connected to the fuel tank 1 through the fuel pump to provide pressurized hydraulic oil for the hydraulic system. The fuel pump 2 can be, but is not limited to, a vane pump, a gear pump, a piston pump or a screw pump. The main oil return path L2 is connected to the fuel tank 1 to redirect the hydraulic oil back to the fuel tank 1. The lifting cylinder 3 includes a rod chamber 31 and a rodless chamber 32. The piston rod in the rod chamber 31 is connected to the lifter to realize the lifting of the load.

[0039] The first reversing valve 41 includes a first oil inlet P1, a first oil return port T1, a first working oil port A1 and a second working oil port B1. The first oil inlet P1 is connected to the main fuel supply oil path L1, the first oil return port T1 is connected to the main oil return path L2, the first working oil port A1 is connected to the rodless chamber 32, and the second working oil port B1 is connected to the rod chamber 31. The first reversing valve 41 includes a first state and a second state. In the first state, the first oil inlet P1 is in communication with the first working oil port A1, and the first oil return port T1 is in communication with the second working oil port B1, thereby realizing the lifting of the lifting cylinder 3. In the second state, the first oil inlet P1 is in communication with the second working oil port B1, and the first oil return port T1 is in communication with the first working oil port A1, thereby realizing the lowering of the lifting cylinder 3. Of course, for those skilled in the art, the first reversing valve 41 may also include a neutral state and a unloading state.

[0040] Refer to the appendix Figure 1 , 2 , 5, the switching valve 51 includes a third oil inlet P3 connected to the first working oil port A1, a third oil return port T3 connected to the main oil return path L2 and / or the fuel tank 1, and a fifth working oil port A3 connected to the rodless chamber 32. In the strong pressure working state, the third oil inlet P3 is in communication with the fifth working oil port A3; in the non-strong pressure working state, the third oil return port T3 is in communication with the fifth working oil port A3.

[0041] A make-up oil valve 52 is provided on the make-up oil path L3. Preferably, the make-up oil valve 52 is a first one-way valve. The inlet end of the first one-way valve is connected to the main oil return path L2, and the outlet end of the first one-way valve is connected to the third oil inlet P3, and can unidirectionally return the hydraulic oil in the main oil return path L2 and / or the fuel tank 1 to the third oil inlet P3. That is to say, the third oil inlet P3 can be connected to the fuel tank 1, can also be connected to the main oil return path L2, or can be connected to both the fuel tank 1 and the main oil return path L2. It can be understood that the above three methods can all achieve the make-up oil function.

[0042] In this embodiment, the third oil inlet P3 is connected to both the fuel tank 1 and the main oil return circuit L2. Specifically, the switching valve 51 further includes a sixth working oil port B3 connected to the fuel tank 1, and the third oil return port T3 is connected to the oil inlet of the oil replenishing valve 52. In the strong pressure working state, the third oil return port T3 is in communication with the sixth working oil port B3. That is to say, in the strong pressure working state, on the one hand, the oil replenishing valve 52 is connected to the main oil return circuit L2, and on the other hand, it is connected to the fuel tank 1 through the third oil return port T3 and the sixth working oil port B3, enabling the hydraulic oil in the main oil return circuit L2 and the fuel tank 1 to flow back to the third oil inlet P3 unidirectionally, achieving a better oil replenishing effect.

[0043] To further improve the oil replenishing effect of the entire system, a back pressure valve 6 is also provided on the main oil return circuit L2. The back pressure valve 6 is preferably a second one-way valve. The inlet side of the second one-way valve is connected to the fuel tank 1, increasing the valve resistance at the end of the main oil return circuit L2 close to the fuel tank 1, preventing the hydraulic oil at the first oil return port T1 from flowing directly back to the fuel tank 1 during the oil replenishing process, and enabling the hydraulic oil at the first oil return port T1 to preferentially return to the oil replenishing circuit L3, improving the oil replenishing effect on the rodless cavity 31 of the lifting cylinder 3.

[0044] The switching valve 51 is an electromagnetic reversing valve, and this electromagnetic reversing valve is controlled by an electric control switch to realize the switching of the strong pressure and non-strong pressure working states of the lifting cylinder 3. The first reversing valve 41 is controlled by a joystick to realize the switching of the lifting and lowering working conditions of the lifting cylinder 3.

[0045] In the strong pressure working state, the switching valve 51 is in the right position. The third oil inlet P3 is in communication with the fifth working oil port A3, and the third oil return port T3 is in a cut-off state. The rodless cavity 32 of the lifting cylinder 3 is connected to the first working oil port A1 of the first reversing valve 41, and the rodless cavity 31 of the lifting cylinder 3 is connected to the second working oil port B1 of the first reversing valve 41. The lifting cylinder 3 is a double-acting lifting cylinder 3, and the switching of the lifting and lowering states of the lifting cylinder 3 is realized under the control of the first reversing valve 41.

[0046] The oil inlet path of the hydraulic oil circuit under the strong pressure lifting working condition is: fuel tank 1 → oil pump 2 → main oil supply circuit L1 → first inlet P1 of the first reversing valve 41 → first working oil port A1 of the first reversing valve 41 → rodless cavity 32 of the lifting cylinder 3. The oil return path of the hydraulic oil circuit under the strong pressure lifting working condition is: rodless cavity 31 of the lifting cylinder 3 → fifth working oil port A3 of the switching valve 51 → third oil inlet P3 of the switching valve 51 → second working oil port B1 of the first reversing valve 41 → first oil return port T1 of the first reversing valve 41 → main oil return circuit L2 → fuel tank 1.

[0047] Under the strong pressure descent condition, the oil inlet path of the hydraulic oil circuit is as follows: oil tank 1 → oil pump 2 → main oil supply circuit L1 → first oil inlet P1 of the first reversing valve 41 → second working oil port B1 of the reversing valve → third oil inlet P3 of the switching valve 51 → fifth working oil port A3 of the switching valve 51 → rod chamber 31 of the lifting cylinder 3. Under the strong pressure descent condition, the oil return path of the hydraulic oil circuit is as follows: rodless chamber 32 of the lifting cylinder 3 → first working oil port A1 of the first reversing valve 41 → first oil return port T1 of the first reversing valve 41 → main oil return circuit L2 → oil tank 1.

[0048] In the strong pressure descent state, when the load is too large, the lifting cylinder 3 descends rapidly. The actual descent speed of the lifting cylinder 3 will be greater than the descent speed provided by the hydraulic lifting system. A vacuum negative pressure will be formed in the rod chamber 31 of the lifting cylinder 3. Part of the hydraulic oil in the main oil return circuit L2 will flow back to the rodless chamber 32 of the lifting cylinder 3 to replenish the oil in the rod chamber 31 of the lifting cylinder 3, reduce the impact on the oil circuit on the side of the first reversing valve 41, ensure the pressure stability in the entire hydraulic system, and avoid damage to hydraulic components.

[0049] Under the strong pressure descent condition, the oil replenishing path of the hydraulic oil circuit is as follows: rodless chamber 32 of the lifting cylinder 3 → first working oil port A1 of the first reversing valve 41 → first oil return port T1 of the first reversing valve 41 → main oil return circuit L2 → oil replenishing valve 52 → third oil inlet P3 of the switching valve 51 → fifth working oil port A3 of the switching valve 51 → rod chamber 31 of the lifting cylinder 3; and / or, oil tank 1 → sixth working oil port B3 of the switching valve 51 → third oil return port T3 of the switching valve 51 → oil replenishing valve 52 → third oil inlet P3 of the switching valve 51 → fifth working oil port A3 of the switching valve 51 → rod chamber 31 of the lifting cylinder 3.

[0050] Under the non-strong pressure lifting condition, the oil inlet path of the hydraulic oil circuit is as follows: oil tank 1 → oil pump 2 → main oil supply circuit L1 → first oil inlet P1 of the first reversing valve 41 → first working oil port A1 of the first reversing valve 41 → rodless chamber 32 of the lifting cylinder 3; under the non-strong pressure lifting condition, the oil return route of the hydraulic oil circuit is as follows: rod chamber 31 of the lifting cylinder 3 → fifth working oil port A3 of the switching valve 51 → third oil return port T3 of the switching valve 51 → main oil return circuit L2 → oil tank 1.

[0051] In the non-strong pressure state, the lifting cylinder 3 naturally descends under the action of the load until it reaches load balance. Under the non-strong pressure descent condition, the oil return and oil inlet paths of the hydraulic oil circuit are as follows: rodless chamber 32 of the lifting cylinder 3 → first working oil port A1 of the first reversing valve 41 → first oil return port T1 of the first reversing valve 41 → main oil return circuit L2 → third oil return port T3 of the switching valve 51 → fifth working oil port A3 of the switching valve 51 → rod chamber 31 of the lifting cylinder 3.

[0052] Refer to the appendix Figure 4, the second reversing valve 42 includes a second oil inlet P2, a second oil return port T2, a third working oil port A2, and a fourth working oil port B2. The second oil inlet P2 is connected to the main oil supply line L1, the second oil return port T2 is connected to the main oil return line L2, and the third working oil port A2 and the fourth working oil port B2 are connected to the hydraulic output joint 7. It is connected to the attached farm implement through the hydraulic output joint 7 to control the attached farm implement. The implementation method is similar to the principle of the first reversing valve 41 and will not be elaborated here. The number of the second reversing valves 42 is selected according to actual requirements, preferably 2 to 6 groups.

[0053] In addition, an overflow safety valve 43 is provided between the main oil supply line L1 and the main oil return line L2 to ensure that the working pressure of the hydraulic system does not exceed the specified value; a filter device is connected in series on the main oil supply line L1. Refer to the appendix Figure 1 , 2 , and the filter device includes a suction filter 81 on the oil inlet side of the oil pump 2 and a pressure filter 82 provided on the oil outlet side of the oil pump 2.

[0054] Embodiment 2

[0055] Based on Embodiment 1, this embodiment improves the structures of the oil pump 2 and the multi-way valve 4 (i.e., the first reversing valve 41 and the second reversing valve 42 in Embodiment 1). It should be noted that the structures and connection methods of the hydraulic components are the same as those in Embodiment 1 without special description in this embodiment.

[0056] Refer to the appendix Figures 6 to 9 , in this embodiment, the oil pump 2 is a load-sensitive piston pump. Correspondingly, the multi-way valve 4 in this embodiment adopts a load-sensitive valve with a load feedback function. The load-sensitive piston pump is equipped with a sensor that can monitor the feedback pressure of the hydraulic lifting system and adjust the oil flow and pressure according to the feedback pressure of the load-sensitive valve to meet the pressure requirements of the entire hydraulic lifting system. A pressure feedback oil line L4 is provided in the hydraulic lifting system to obtain the maximum pressure of the entire hydraulic lifting system.

[0057] Refer to the appendix Figure 7, the first reversing valve 41 includes a first oil inlet P1, a first oil return port T1, a first working oil port A1, and a second working oil port B1. The first oil inlet P1 is connected to the main oil supply line L1, the first oil return port T1 is connected to the main oil return line L2, the first working oil port A1 is connected to the rodless cavity 32, and the second working oil port B1 is connected to the rod cavity 31. The first reversing valve 41 includes a first state and a second state. In the first state, the first oil inlet P1 is in communication with the first working oil port A1, and the first oil return port T1 is in communication with the second working oil port B1, thereby realizing the lifting of the lifting cylinder 3. In the second state, the first oil inlet P1 is in communication with the second working oil port B1, and the first oil return port T1 is in communication with the first working oil port A1, thereby realizing the lowering of the lifting cylinder 3. The second reversing valve 42 in this embodiment has the same structure as the first reversing valve 41, and for the specific structure, refer to the appendix Figure 8 , which will not be elaborated here.

[0058] To ensure the oil pressure stability of the first reversing valve 41 and the second reversing valve 42, an oil pressure compensation valve 44 is provided between the first oil inlet P1 and / or the second oil inlet P2 and the main oil supply line L1. In this embodiment, the oil pressure compensation valve 44 is a pre-valve compensation, but for those skilled in the art, the oil pressure compensation valve 44 can also be a post-valve compensation.

[0059] Refer to the appendix Figure 6 , the feedback oil line L4 includes a first shuttle valve 45 and a second shuttle valve 46. One oil inlet of the first shuttle valve 45 is connected to the first oil inlet P1, and the other oil inlet of the first shuttle valve 45 is connected to the third oil inlet P3; the number of the second shuttle valves 46 is the same as the number of the second reversing valves 42. One oil inlet of the second shuttle valve 46 is connected to the oil outlet of the first shuttle valve 45, and the other oil outlet of the second shuttle valve 46 is connected to the second oil inlet P2. The oil outlet of the second shuttle valve 46 is connected to one oil inlet of the next second shuttle valve 46 until the oil outlet of the last second shuttle valve 46 is connected to the sensor end of the load-sensitive piston pump, transmitting the pressure of the hydraulic oil circuit of the multi-way valve 4 to the sensitive piston pump. Throttle valves 53 are connected in series between one oil inlet of the first shuttle valve 45 and the first oil inlet P1, between the other oil inlet of the first shuttle valve 45 and the third oil inlet P3, and between the other oil outlet of the second shuttle valve 46 and the second oil inlet P2.

[0060] It should be noted that, in order to simplify the connection circuit of the switching valve 51, refer to the appendix Figure 6 、 9 , in this embodiment, the sixth working oil port B3 is in a throttling state, the third oil return port T3 is connected to the oil replenishing line L3, and the third oil return port T3 is connected to the oil inlet of the oil replenishing valve 52, realizing the oil replenishing function through the main oil return line L2.

[0061] However, for those skilled in the art, in this embodiment, the switching valve 51 can also adopt the same connection method as that in Embodiment 1, that is, the sixth working oil port B3 is directly connected to the oil tank 1, and the third oil return port T3 is connected to the oil inlet of the oil replenishing valve 52. In the strong pressure working state, the third oil return port T3 is communicated with the sixth working oil port B3, and the main oil return oil path L2 and the hydraulic oil of the oil tank 1 flow back to the third oil inlet P3 unidirectionally, achieving a better oil replenishing effect.

[0062] In addition, an overflow safety valve 43 is provided between the main oil supply oil path L1 and the main oil return oil path L2 to ensure that the working pressure of the hydraulic system does not exceed the specified value. A filter device is connected in series on the main oil supply oil path L1 and / or the main oil return oil path L2. Refer to the appendix Figure 6 , the filter device includes a suction filter 81 located on the oil inlet side of the oil pump 2, a pressure filter 82 provided on the oil outlet side of the oil pump 2, and a return oil filter 83 connected in series on the main oil return oil path L2. The return oil filter 83 can also play a certain back pressure role to prevent the hydraulic oil at the first oil return port T1 from flowing directly back to the oil tank 1 during the oil replenishing process.

[0063] It will be apparent to those skilled in the art that various modifications and variations can be made to the above-described exemplary embodiments of the present invention without departing from the spirit and scope of the present invention. Therefore, it is intended that the present invention cover modifications and variations of the present invention falling within the scope of the appended claims and their equivalent technical solutions.

Claims

1. A hydraulic lifting system, characterized in that, Comprising: Fuel tank; Main oil supply circuit, the main oil supply circuit is connected in series with an oil pump and is connected to the fuel tank through the oil pump, and can provide pressurized hydraulic oil; Main oil return circuit, the main oil return circuit is connected to the fuel tank; Lifting cylinder, the lifting cylinder includes a rod chamber and a rodless chamber; First reversing valve, the first reversing valve includes a first oil inlet, a first oil return port, a first working oil port and a second working oil port, the first oil inlet is connected to the main oil supply circuit, the first oil return port is connected to the main oil return circuit, the first working oil port is connected to the rodless chamber, and the second working oil port is connected to the rod chamber; Switching valve, the switching valve includes a third oil inlet connected to the first working oil port, a third oil return port connected to the main oil return circuit and / or the fuel tank, and a fifth working oil port connected to the rodless chamber; in the strong pressure working state, the third oil inlet is in communication with the fifth working oil port; in the non-strong pressure working state, the third oil return port is in communication with the fifth working oil port; Make-up oil circuit, a make-up oil valve is provided on the make-up oil circuit, and the make-up oil valve can unidirectionally return the hydraulic oil in the main oil return circuit and / or the fuel tank to the third oil inlet.

2. The hydraulic lifting system according to claim 1, characterized in that, A back pressure valve is also connected in series on the main oil return circuit.

3. The hydraulic lifting system according to claim 1, characterized in that, The switching valve further includes a sixth working oil port connected to the fuel tank, and the third oil return port is connected to the oil inlet of the make-up oil valve; in the strong pressure working state, the third oil return port is in communication with the sixth working oil port.

4. The hydraulic lifting system according to claim 1, characterized in that, It further includes a second reversing valve, the second reversing valve includes a second oil inlet, a second oil return port, a third working oil port and a fourth working oil port, the second oil inlet is connected to the main oil supply circuit, the second oil return port is connected to the main oil return circuit, and the third working oil port and the fourth working oil port are connected to a hydraulic output joint.

5. The hydraulic lifting system according to claim 4, wherein The oil pump is a load-sensitive piston pump.

6. The hydraulic lifting system according to claim 5, wherein An oil pressure compensation valve is provided between the first oil inlet and / or the second oil inlet and the main oil supply circuit.

7. The hydraulic lifting system according to claim 6, characterized in that, It further includes: First shuttle valve, one oil inlet of the first shuttle valve is connected to the first oil inlet, and the other oil inlet of the first shuttle valve is connected to the third oil inlet; Second shuttle valve, one oil inlet of the second shuttle valve is connected to the oil outlet of the first shuttle valve, the other oil outlet of the second shuttle valve is connected to the second oil inlet, and the oil outlet of the second shuttle valve is connected to one oil inlet of the next second shuttle valve or to the sensor end of the load-sensitive piston pump.

8. The hydraulic lifting system according to claim 7, wherein, Throttle valves are connected in series between one oil inlet of the first shuttle valve and the first oil inlet, between the other oil inlet of the first shuttle valve and the third oil inlet, and between the other oil outlet of the second shuttle valve and the second oil inlet.

9. The hydraulic lifting system according to claim 8, characterized in that A first safety valve is also provided between the main oil supply circuit and the main oil return circuit; a filter device is connected in series on the main oil supply circuit and / or the main oil return circuit.

10. A tractor, characterized in that, Comprising the hydraulic lifting system according to any one of claims 1 to 9.

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