Hydraulic system of reloading electric forklift

Abstract

The hydraulic system comprises an oil tank, a metering pump and a variable pump, a first motor drives the metering pump to work, a second motor drives the variable pump to work, an output port of the metering pump is connected with a lifting oil cylinder through a one-way overflow valve set, and an output port of the variable pump is connected with a steering gear and an oil inlet of a multi-way valve through a priority valve. A working oil port of the multi-way valve is connected with the lifting oil cylinder and the portal executing element, and the steering gear and an oil return port of the multi-way valve are connected with the oil tank. On one hand, the performance of the multi-way valve does not need to meet high pressure and large flow at the same time, control over the size and cost of the multi-way valve is obviously improved, on the other hand, descending is completed through oil return of the multi-way valve, accurate proportional control can still be achieved, the descending control effect of the portal frame is not reduced while the lifting performance of the forklift is improved, and the descending cost is reduced. The comprehensive performance is improved and the system loss is reduced.

Description

Technical Field

[0001] The present invention relates to the field of hydraulic control, in particular to a hydraulic system for a heavy-duty electric forklift. Background Art

[0002] Electric forklifts are widely used in various technical fields such as logistics and transportation. Among them, the oil pump of the hydraulic system of electric heavy-duty electric forklifts is driven by an electric motor, and a quantitative hydraulic system with a gear pump as the power source is often used. For small-tonnage electric forklifts, they are widely used due to their low cost, low noise and good maintainability.

[0003] However, for large-tonnage electric forklifts, especially those above 32 tons, due to the increase in tonnage, the oil pump displacement will also be larger when meeting the requirements of speed performance such as lifting. If the quantitative hydraulic system with a quantitative pump as the power source continues to be used, on the one hand, the gear pump will become noisier and more difficult to control as the displacement increases; on the other hand, the loss of the quantitative hydraulic system will also increase as the system flow increases; furthermore, the control valves such as large-flow multi-way valves adapted to the quantitative hydraulic system will have fewer options, and the overall cost will increase significantly.

[0004] Therefore, how to provide a heavy-duty electric forklift hydraulic system that adapts to different work requirements is a technical problem that those skilled in the art currently need to solve. Summary of the Invention

[0005] The purpose of the present invention is to provide a heavy-duty electric forklift hydraulic system, in which a fixed-displacement pump supplies oil to the lifting cylinder alone, and a variable-displacement pump supplies oil to the steering, lifting and mast actuators, thereby improving working performance and reducing system losses.

[0006] In order to solve the above technical problems, the present invention provides a heavy-duty electric forklift hydraulic system, including an oil tank, a fixed-displacement pump and a variable-displacement pump. The first motor drives the fixed-displacement pump to work, and the second motor drives the variable-displacement pump to work. The output port of the fixed-displacement pump is connected to the lifting cylinder through a one-way overflow valve group, and the output port of the variable pump is connected to the steering gear and the oil inlet of the multi-way valve through a priority valve. The working oil port of the multi-way valve is connected to the lifting cylinder and the mast actuator, and the return oil port of the steering gear and the multi-way valve is connected to the oil tank.

[0007] Preferably, the one-way relief valve group includes a one-way valve, a relief valve, a two-position two-way solenoid reversing valve and a manual stop valve. The output port of the metering pump is simultaneously connected to the inlets of the one-way valve, the relief valve and the two-position two-way solenoid reversing valve. The outlet of the one-way valve is connected to the lifting cylinder. The outlets of the relief valve and the two-position two-way solenoid reversing valve are connected to the oil tank. The outlet of the one-way valve is also connected to the oil tank through the manual stop valve. The two working positions of the two-position two-way solenoid reversing valve are a two-way stop type and a straight-through type respectively.

[0008] Preferably, the outlet of the one-way valve is connected to the rodless chamber interface of the lifting cylinder through a speed limiting valve, and the speed limiting valve includes a one-way component and a throttling component connected in parallel.

[0009] Preferably, it includes two lifting cylinders connected in parallel, the inlets of the two speed limiting valves are simultaneously connected to the oil outlet of the one-way relief valve group and the working oil port of the multi-way valve, and the rod chambers of the two lifting cylinders are simultaneously connected to the oil tank.

[0010] Preferably, a shuttle valve is further included, wherein the first inlet of the shuttle valve is connected to the load feedback oil port of the steering gear, the second inlet of the shuttle valve is connected to the load feedback oil port of the multi-way valve, and the outlet of the shuttle valve is connected to the load feedback oil port of the variable pump.

[0011] Preferably, the priority valve includes a two-position three-way hydraulically controlled reversing valve, the inlet of the two-position three-way hydraulically controlled reversing valve is connected to the output port of the variable pump, one outlet of the two-position three-way hydraulically controlled reversing valve is connected to the oil inlet of the steering gear, the other outlet of the two-position three-way hydraulically controlled reversing valve is connected to the oil inlet of the multi-way valve, and the hydraulic control port of the two-position three-way hydraulically controlled reversing valve is connected to the load feedback oil port of the steering gear.

[0012] Preferably, the multi-way valve is an electric proportional control load-sensitive pressure-compensated multi-way valve, and each working valve core of the electric proportional control load-sensitive pressure-compensated multi-way valve is a closed middle position structure.

[0013] Preferably, a brake pump is included, the second motor drives the brake pump, and the output port of the brake pump is connected to the brake mechanism.

[0014] Preferably, the metering pump and the brake pump are gear pumps, and the variable displacement pump is a load-sensitive variable displacement plunger pump.

[0015] Preferably, an oil suction filter is provided at the oil inlet of the metering pump, the variable displacement pump and the brake pump.

[0016] The present invention provides a hydraulic system for a heavy-duty electric forklift, comprising an oil tank, a metering pump and a variable pump, wherein a first motor drives the metering pump to work, and a second motor drives the variable pump to work, the output port of the metering pump is connected to a lifting cylinder via a one-way relief valve group, the output port of the variable pump is connected to an oil inlet of a steering gear and a multi-way valve via a priority valve, the working oil port of the multi-way valve is connected to the lifting cylinder and a mast actuator, and the oil return ports of the steering gear and the multi-way valve are connected to the oil tank.

[0017] During operation, the metering pump only works during lifting operation, while the variable pump is responsible for steering the forklift and various operations of the mast. Through the action of the one-way relief valve group, after the first motor receives the stop signal, the metering pump is unloaded from the time it receives the stop signal to the time it completely stops rotating, minimizing the delay in the metering pump outputting oil to stop the mast lifting during this process. During non-lifting operation, the metering pump does not work, and the hydraulic system saves energy to the maximum extent. When the mast is lifting, the flow of the metering pump and the variable pump merges outside the multi-way valve, which can effectively increase the lifting speed and improve work efficiency. When lowering, the oil is returned through the multi-way valve. On the one hand, the multi-way valve itself does not need to meet the requirements of high pressure and large flow at the same time, which has significantly improved the size and cost control of the multi-way valve. On the other hand, the descent is completed through the multi-way valve oil return, and proportional precise control can still be achieved. While improving the lifting performance of the forklift, the mast lowering control effect is not reduced, the overall performance is improved, and the system loss is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A hydraulic principle diagram of a specific embodiment of the hydraulic system for a heavy-duty electric forklift provided by the present invention;

[0019] Figure 2 This is a working block diagram of a specific embodiment of the hydraulic system for a heavy-duty electric forklift provided by the present invention;

[0020] Figure 3 This is a hydraulic principle diagram of a one-way relief valve group in a specific embodiment of the heavy-duty electric forklift hydraulic system provided by the present invention.

[0021] Among them, the oil tank 1, the metering pump 2, the variable pump 3, the first motor 4, the second motor 5, the one-way overflow valve group 6, the one-way valve 61, the overflow valve 62, the two-position two-way solenoid reversing valve 63, the manual stop valve 64, the lifting cylinder 7, the priority valve 8, the steering gear 9, the multi-way valve 10, the shuttle valve 11, the brake pump 12, the oil suction filter 13, and the speed limiting valve 14. DETAILED DESCRIPTION

[0022] The core of the present invention is to provide a heavy-duty electric forklift hydraulic system, in which a fixed-displacement pump supplies oil to the lifting cylinder separately, and a variable-displacement pump supplies oil to the steering, lifting and mast actuators, thereby improving working performance and reducing system losses.

[0023] In order to enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0024] Please refer to Figures 1 to 3 , Figure 1 A hydraulic principle diagram of a specific embodiment of the hydraulic system for a heavy-duty electric forklift provided by the present invention; Figure 2This is a working block diagram of a specific embodiment of the hydraulic system for a heavy-duty electric forklift provided by the present invention; Figure 3 This is a hydraulic principle diagram of a one-way relief valve group in a specific embodiment of the heavy-duty electric forklift hydraulic system provided by the present invention.

[0025] A specific embodiment of the present invention provides a heavy-duty electric forklift hydraulic system, comprising a fuel tank 1, a metering pump 2, and a variable displacement pump 3. A first motor 4 and a second motor 5 are also provided. The first motor 4 drives the metering pump 2, and the second motor 5 drives the variable displacement pump 3. The system also includes a one-way relief valve group 6, a priority valve 8, a steering gear 9, and a multi-way valve 10. The one-way relief valve group 6 is provided with an oil inlet, an oil outlet, and an oil return port. The oil inlet of the metering pump 2 is connected to the fuel tank 1, the output port of the metering pump 2 is connected to the oil inlet of the one-way relief valve group 6, the oil outlet of the one-way relief valve group 6 is connected to the lifting cylinder 7, and the oil return port of the one-way relief valve group 6 is connected to the fuel tank 1. The priority valve 8 is provided with an oil inlet, a first oil outlet, a second oil outlet and an oil return port, and the multi-way valve 10 is provided with an oil inlet, an oil return port and multiple working oil ports. The oil inlet of the variable pump 3 is connected to the oil tank 1, and the output port of the variable pump 3 is connected to the oil inlet of the priority valve 8. The first oil outlet of the priority valve 8 is connected to the steering gear, the second oil outlet of the priority valve 8 is connected to the multi-way valve 10, and the oil return port of the priority valve 8 is connected to the oil tank 1. At the same time, one working oil port of the multi-way valve is connected to the lifting cylinder 7, and the other working oil ports are connected to other gantry actuators.

[0026] During operation, the metering pump 2 only operates during the lifting operation, while the variable pump 3 is responsible for steering the forklift and various operations of the mast. Through the action of the one-way relief valve group 6, after the first motor 4 receives the stop signal, the metering pump 2 is unloaded from the time it receives the stop signal to the time it completely stops rotating, minimizing the delay in the metering pump 2 outputting oil to stop the mast lifting during this period. During the non-lifting operation, the metering pump 2 does not operate, maximizing energy conservation in the hydraulic system. When the mast is lifting, the flow of the metering pump 2 and the variable pump 3 merges outside the multi-way valve 10, which can effectively increase the lifting speed and improve work efficiency. When lowering, the oil is returned through the multi-way valve 10. On the one hand, the multi-way valve 10 itself does not need to meet both high pressure and high flow, which significantly improves the size and cost control of the multi-way valve 10. On the other hand, the descent is completed through the return oil of the multi-way valve 10, and the proportional precision control can still be achieved. While improving the lifting performance of the forklift, the descent control effect of the mast is not reduced, improving the overall performance and reducing system losses.

[0027] Specifically, the one-way relief valve assembly 6 includes a one-way valve 61, a relief valve 62, a two-position, two-way electromagnetic reversing valve 63, and a manual shut-off valve 64. The output port of the metering pump 2 is simultaneously connected to the inlets of the one-way valve 61, the relief valve 62, and the two-position, two-way electromagnetic reversing valve 63. The outlet of the one-way valve 61 is connected to the lifting cylinder 7. The outlets of the relief valve 62 and the two-position, two-way electromagnetic reversing valve 63 are connected to the oil tank 1. The outlet of the one-way valve 61 is also connected to the oil tank 1 via the manual shut-off valve 64. The two working positions of the two-position, two-way electromagnetic reversing valve 63 are respectively a two-way shut-off type and a straight-through type. That is, the inlets of the one-way valve 61, the relief valve 62, and the two-position, two-way electromagnetic reversing valve 63 are simultaneously connected to the oil inlet of the one-way relief valve assembly 6, the outlet of the one-way valve 61 is connected to the oil outlet of the one-way relief valve assembly 6, and the outlets of the relief valve 62, the two-position, two-way electromagnetic reversing valve 63, and the manual shut-off valve 64 are simultaneously connected to the oil return port of the one-way relief valve assembly 6. The one-way valve 61 is used for oil shielding of the lifting cylinder 7, ensuring that the oil at the oil outlet of the one-way relief valve group 6 can only flow to the lifting cylinder 7 and cannot return, while reducing the leakage of the oil circuit; the relief valve 62 is used for overpressure safety protection of the oil outlet of the metering pump 2; the initial position of the two-position two-way electromagnetic reversing valve 63 is a two-way cut-off type, and the working position is a straight-through type. This two-position two-way electromagnetic reversing valve 63 does not open when the metering pump 2 is working, and is in a two-way cut-off position. It needs to cooperate with the control signal of the first motor 4 of the metering pump 2 to open. It is only when the first motor 4 of the metering pump 2 is working that the valve is opened. The status is opened after receiving the stop signal and completely stopping the rotation. The two-position two-way solenoid reversing valve 63 is in the straight-through position. This process is used for unloading. The output oil of the metering pump 2 from the rotation to the stop process flows back to the oil tank 1 through the two-position two-way solenoid reversing valve 63, avoiding the oil in this process from continuing to flow to the gantry lifting cylinder 7, thereby avoiding the lifting cylinder from having a delay in stopping after receiving the stop lifting control signal; the manual stop valve 64 can only be opened and closed manually, and is only used for emergency descent when the vehicle fails and cannot be lowered.

[0028] The outlet of the check valve 61 is connected to the rodless cavity of the lift cylinder 7 via the speed limiting valve 14. The speed limiting valve 14 comprises a parallel one-way assembly and a throttling assembly. Each valve connected to the lift cylinder 4 is connected to the rodless cavity of the lift cylinder 7 via the speed limiting valve 14. During lifting, hydraulic oil flows directly through the one-way assembly into the rodless cavity of the lift cylinder 7. During lowering, hydraulic oil flows out through the throttling assembly, reducing the speed of descent.

[0029] Specifically, it includes two lifting cylinders 7 connected in parallel, and the inlets of the two speed limiting valves 14 are simultaneously connected to the oil outlet of the one-way overflow valve group 6 and the working oil port of the multi-way valve 10 for pressure balance and flow sharing. The rod chambers of the two lifting cylinders 7 are simultaneously connected to the oil tank 1.

[0030] The heavy-duty electric forklift hydraulic system provided in the specific embodiment of the present invention also includes a shuttle valve 11 to implement load feedback control. The first inlet of the shuttle valve 11 is connected to the load feedback oil port of the steering gear 9, the second inlet of the shuttle valve 11 is connected to the load feedback oil port of the multi-way valve 10, and the outlet of the shuttle valve 11 is connected to the load feedback oil port of the variable pump 3. The first inlet and the second inlet are used to receive the load signal of the steering gear 9 and the working load signal of the multi-way valve 10 respectively, perform pressure selection, and feedback to the load feedback oil port of the variable pump 3 through the outlet, and finally used for displacement control of the variable pump. The load signal of the steering gear 9 is isolated from that of the multi-way valve 10, which can avoid the situation where the high-pressure load feedback signal of the multi-way valve 10 comes only from the multi-way valve 10 when the multi-way valve 10 is working but there is no working demand for steering. The high-pressure signal will not interfere with the steering gear 9, thereby avoiding damage to the steering gear 9 and the like, and improving the safety of the hydraulic system itself.

[0031] Furthermore, priority valve 8 comprises a two-position, three-way hydraulically controlled directional valve. Its inlet is connected to the output of variable displacement pump 3, one outlet is connected to the oil inlet of steering gear 9, the other outlet is connected to the oil inlet of multi-way valve 10, and the hydraulic port of the two-position, three-way hydraulically controlled directional valve is connected to the load feedback oil port of steering gear 9. The load feedback signal from steering gear 9 is transmitted to priority valve 8, controlling the switching of the two-position, three-way hydraulically controlled directional valve to ensure priority oil supply to steering gear 9.

[0032] The multi-way valve 10 is an electrically proportional, load-sensitive, pressure-compensated multi-way valve. Each working spool of the valve is in a closed-center position. Three or more working oil ports can be provided, one of which is connected to the lift cylinder 7, and the others to the mast actuators. The number of working oil ports can be adjusted based on the number of actuators.

[0033] Based on the hydraulic systems for heavy-duty electric forklifts provided in the aforementioned embodiments, the system includes a brake pump 12. A second motor 5 drives the brake pump 12, and the output of the brake pump 12 is connected to the brake mechanism. Specifically, the metering pump 2 and the brake pump 12 are gear pumps, and the variable displacement pump 3 is a load-sensing variable displacement plunger pump. Suction filters 13 are installed at the oil inlets of the metering pump 2, variable displacement pump 3, and the brake pump 12.

[0034] According to the actual working conditions of the forklift, it can be divided into the following situations: when the mast actuator has a working demand except for the lifting cylinder 7, the second motor 5 is controlled to work, and the multi-way valve spool corresponding to the working oil port of the actuator is reversed. The load signal of the actuator at these working oil ports is fed back to the load feedback oil port of the multi-way valve 10 through the internal oil channel of the multi-way valve 10, and leads to the oil outlet of the shuttle valve 11 through the second oil port of the shuttle valve 11, and finally fed back to the load feedback oil port of the variable pump 3. According to the feedback load pressure signal and the characteristics of the variable pump 3 itself, the variable pump 3 will output hydraulic pressure oil that meets the flow demand of the actuator of the working oil port. The hydraulic pressure oil flows through the output port of the variable pump 3 to the oil inlet of the priority valve 8, and flows from the working oil outlet of the priority valve 8 to the oil inlet of the multi-way valve 10, passes through the internal oil channel of the multi-way valve 10, and finally flows to the actuator through the working oil port to complete the work. When the actuator behind the working oil port stops working, through control, the second motor 5 stops working and the multi-way valve core corresponding to the working oil port is closed, the valve core is in the middle position, the work stops, and the load is maintained by the valve core.

[0035] When the gantry has a lifting demand, through control, the first motor 4 and the second motor 5 work and rotate at the same time, and the metering pump 2 and the variable pump 3 both output pressurized oil; the pressure oil of the metering pump 2 flows from the output port to the oil inlet of the one-way valve relief valve group 6, and flows from the oil outlet of the one-way valve relief valve group 6 to the speed limiting valve 14 through the one-way valve 61 inside the one-way valve relief valve group 6; the oil output process of the working oil port of the multi-way valve 10 is consistent with the above content. The two-way pressure oil from the metering pump 2 and the variable pump 3 finally merge in front of the two speed limiting valves 14. Since two-way pipes are set at the two speed limiting valves 14, the two-way flow finally flows evenly to the two lifting cylinders 7 to complete the lifting action. When there is a demand to stop lifting, the first motor 4 and the second motor 5 stop rotating at the same time through control; the valve core corresponding to the multi-way valve 10 is closed, and no pressure oil flows to the speed limiting valve 14. The pressure oil of the lifting cylinder 7 will be maintained through the valve core of the working oil port corresponding to the multi-way valve 10 and the one-way valve 61 of the one-way relief valve group 6; during the period from the first motor 4 receiving the stop rotation signal to the complete stop, because the metering pump 2 is still rotating during this process, it is still outputting pressure oil. In order to avoid hydraulic oil being delivered to the lifting cylinder 7 after receiving the stop signal, at the same time as the first motor 4 receives the stop signal, the two-position two-way electromagnetic reversing valve 63 inside the one-way relief valve group 6 is instantly energized and closed through control. At this time, the oil output by the metering pump flows to the oil tank 1 through the two-position two-way electromagnetic reversing valve 63 inside the one-way relief valve group 6, avoiding the delay of the gantry lifting action compared to the stop signal, thereby ensuring the precise control of the gantry lifting; the above work occurs synchronously. When the mast is lowering, neither the first motor 4 nor the second motor 5 rotates, and the valve core corresponding to the working oil port of the multi-way valve 10 switches to the descending position. The oil from the lifting cylinder 7 flows through the working oil port and the return oil port of the multi-way valve 10 to the fuel tank 1, thereby lowering the mast. The rate of descent can be controlled by controlling the corresponding valve core. If a vehicle valve fails and the mast is at a certain height and cannot be lowered, and the mast needs to be lowered to ensure safety inspection, the manual shut-off valve 64 inside the one-way relief valve group 6 can be manually opened to drain the oil from the lifting cylinder 7, achieving emergency lowering of the lifting cylinder 7 and improving maintainability.

[0036] When the vehicle is only turning, the second motor 5 rotates, and the steering gear 9 feeds back the load pressure signal from the steering cylinder, which flows from the load feedback oil port of the steering gear 9 to the load feedback oil port of the priority valve 8 and the first oil inlet of the shuttle valve 11, and then to the oil outlet of the shuttle valve 11, and finally to the load feedback oil port of the variable pump 3. According to the fed-back load pressure signal and the characteristics of the variable pump 3 itself, the variable pump 3 outputs hydraulic pressure oil that adapts to the steering flow demand. The hydraulic pressure oil flows through the output port of the variable pump 3 to the oil inlet of the priority valve 8, and flows from the steering working oil outlet of the priority valve 8 to the oil inlet of the steering gear 9, and finally flows to the steering cylinder to realize the steering work.

[0037] When the steering and multi-way valve 10 require simultaneous operation, the load feedback signals of the two operating conditions are pressure-selected by the shuttle valve assembly 11. The higher-pressure load signal is input to the load feedback port of the variable pump 3. Based on its own characteristics, the variable pump 3 outputs the required flow rate according to the load flow demand. The oil output by the variable pump 3 first passes through the priority valve 8 before flowing to the multi-way valve 10. After being distributed by the priority valve 8, the oil will first flow to the steering gear 9, giving priority to the steering demand. The remaining oil flows to the multi-way valve 10 to meet the operating requirements of the multi-way valve 10. In the load pressure signal oil circuit, due to the selective characteristics of the shuttle valve 11, the load pressure signal from the multi-way valve 10 and the pressure signal from the steering gear 9 are not directly compared. Instead, they are isolated by the shuttle valve 11's selective characteristics. This prevents the high-pressure load pressure signal from being fed back to the steering gear 9 when the multi-way valve 10 is operating under high-pressure conditions, causing damage to the steering gear 9 and components such as the priority valve 8.

[0038] The above describes in detail the heavy-duty electric forklift hydraulic system provided by the present invention. This article uses specific examples to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is intended only to facilitate understanding of the method and core concept of the present invention. It should be noted that those skilled in the art may make various improvements and modifications to the present invention without departing from the principles of the present invention, and such improvements and modifications also fall within the scope of protection of the claims of the present invention.

Claims

1. A heavy-duty electric forklift hydraulic system, characterized in that: The invention comprises an oil tank (1), a metering pump (2) and a variable displacement pump (3), wherein a first motor (4) drives the metering pump (2) to work, and a second motor (5) drives the variable displacement pump (3) to work, the output port of the metering pump (2) is connected to the lifting cylinder (7) via a one-way overflow valve group (6), the output port of the variable displacement pump (3) is connected to the oil inlet of the steering gear (9) and the multi-way valve (10) via a priority valve (8), the working oil port of the multi-way valve (10) is connected to the lifting cylinder (7) and the mast actuator, and the oil return ports of the steering gear (9) and the multi-way valve (10) are connected to the oil tank (1).

2. The heavy-duty electric forklift hydraulic system according to claim 1, characterized in that: The one-way overflow valve group (6) includes a one-way valve (61), an overflow valve (62), a two-position two-way electromagnetic reversing valve (63) and a manual stop valve (64). The output port of the metering pump (2) is simultaneously connected to the inlets of the one-way valve (61), the overflow valve (62) and the two-position two-way electromagnetic reversing valve (63). The outlet of the one-way valve (61) is connected to the lifting cylinder (7). The outlets of the overflow valve (62) and the two-position two-way electromagnetic reversing valve (63) are connected to the oil tank (1). The outlet of the one-way valve (61) is also connected to the oil tank (1) through the manual stop valve (64). The two working positions of the two-position two-way electromagnetic reversing valve (63) are a two-way stop type and a straight-through type, respectively.

3. The heavy-duty electric forklift hydraulic system according to claim 2, characterized in that: The outlet of the one-way valve (61) is connected to the rodless chamber interface of the lifting cylinder (7) through a speed limiting valve (14), and the speed limiting valve (14) includes a one-way component and a throttling component connected in parallel.

4. The heavy-duty electric forklift hydraulic system according to claim 3, characterized in that: It comprises two lifting cylinders (7) connected in parallel, the inlets of the two speed limiting valves (14) are simultaneously connected to the oil outlet of the one-way overflow valve group (6) and the working oil port of the multi-way valve (10), and the rod chambers of the two lifting cylinders (7) are simultaneously connected to the oil tank (1).

5. The heavy-duty electric forklift hydraulic system according to claim 1, characterized in that: It also includes a shuttle valve (11), wherein a first inlet of the shuttle valve (11) is connected to the load feedback oil port of the steering gear (9), a second inlet of the shuttle valve (11) is connected to the load feedback oil port of the multi-way valve (10), and an outlet of the shuttle valve (11) is connected to the load feedback oil port of the variable pump (3).

6. The heavy-duty electric forklift hydraulic system according to claim 4, characterized in that: The priority valve (8) includes a two-position three-way hydraulically controlled reversing valve, the inlet of the two-position three-way hydraulically controlled reversing valve is connected to the output port of the variable pump (3), one outlet of the two-position three-way hydraulically controlled reversing valve is connected to the oil inlet of the steering gear (9), the other outlet of the two-position three-way hydraulically controlled reversing valve is connected to the oil inlet of the multi-way valve (10), and the hydraulic control port of the two-position three-way hydraulically controlled reversing valve is connected to the load feedback oil port of the steering gear (9).

7. The heavy-duty electric forklift hydraulic system according to claim 1, characterized in that: The multi-way valve (10) is an electric proportional control load-sensitive pressure-compensated multi-way valve, and each working valve core of the electric proportional control load-sensitive pressure-compensated multi-way valve is a closed middle position structure.

8. The heavy-duty electric forklift hydraulic system according to any one of claims 1 to 7, characterized in that: It comprises a brake pump (12), the second motor (5) drives the brake pump (12), and the output port of the brake pump (12) is connected to a brake mechanism.

9. The heavy-duty electric forklift hydraulic system according to claim 8, characterized in that: The metering pump (2) and the brake pump (12) are gear pumps, and the variable displacement pump (3) is a load-sensitive variable displacement plunger pump.

10. The heavy-duty electric forklift hydraulic system according to claim 9, characterized in that: Oil suction filters (13) are provided at the oil inlets of the metering pump (2), the variable displacement pump (3) and the brake pump (12).

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