Dosing pump hydraulic system of crane and control method thereof and crane

Abstract

The present disclosure relates to a fixed displacement pump hydraulic system for a crane, a control method thereof, and a crane. The fixed displacement pump hydraulic system includes a first fixed displacement pump, a second fixed displacement pump, and an engine for driving the first and second fixed displacement pumps. The first fixed displacement pump provides pressurized oil to a working mechanism via a first hydraulic oil circuit, and the second fixed displacement pump provides pressurized oil to a working mechanism via a second hydraulic oil circuit. The fixed displacement pump hydraulic system also includes a control device configured to: when the crane is operating in a rated load idle state and the engine speed is less than the idle speed, unload the pressure in the first hydraulic oil circuit to shut off the first fixed displacement pump; and when the engine speed is equal to or greater than the idle speed, gradually increase the pressure in the first hydraulic oil circuit until the first fixed displacement pump reaches normal operation. By unloading the pressure in the first hydraulic oil circuit, the pressure in the hydraulic system can be reduced, thereby reducing the required engine output torque and preventing engine stall.

Description

Technical Field

[0001] The present disclosure relates to the technical field of engineering machinery, and in particular to a quantitative pump hydraulic system of a crane, a control method for the quantitative pump hydraulic system of the crane, and a crane. Background Art

[0002] Crane design specifications require that the engine must be able to operate normally at rated load while idling. However, in crane hydraulic systems with fixed-load pumps, the engine is prone to stalling when idling and operating at rated load. This is because when the crane is operating at rated load and idling, the hydraulic system pressure rises rapidly. For example, it takes only 0.1 seconds for the hydraulic system pressure to reach maximum, while the engine's response time is typically 3-5 seconds. This slow engine response results in insufficient output torque, which can easily lead to engine stalling. Summary of the Invention

[0003] The purpose of the present disclosure is to provide a dosing pump hydraulic system for a crane, a crane equipped with the dosing pump hydraulic system, and a control method for the dosing pump hydraulic system of the crane, wherein the dosing pump hydraulic system can effectively prevent the engine from stalling when the crane is operating in a rated load idling state.

[0004] To achieve the above-mentioned object, the present disclosure provides a fixed-displacement pump hydraulic system for a crane, comprising a first fixed-displacement pump, a second fixed-displacement pump, and an engine for driving the first and second fixed-displacement pumps. The first fixed-displacement pump provides pressure oil to a working mechanism through a first hydraulic oil circuit, and the second fixed-displacement pump provides pressure oil to the working mechanism through a second hydraulic oil circuit. The fixed-displacement pump hydraulic system further comprises a control device for controlling the first fixed-displacement pump. The control device is configured such that: when the crane is operating in a rated load idle state,

[0005] When the engine speed is lower than the idle speed, the pressure of the first hydraulic oil circuit is unloaded to cut off the operation of the first fixed displacement pump; and

[0006] When the rotation speed of the engine is equal to or greater than the rotation speed in the idle state, the pressure of the first hydraulic oil circuit is gradually increased until the first fixed displacement pump reaches normal operation.

[0007] Optionally, the control device includes a hydraulically controlled reversing valve and a proportional pressure reducing valve for controlling the hydraulically controlled reversing valve, one end of the hydraulically controlled reversing valve is connected to the first hydraulic oil circuit, and the other end is connected to the oil tank. The proportional pressure reducing valve is a solenoid valve, which is used to control the hydraulically controlled reversing valve to open when power is supplied, and to control the hydraulically controlled reversing valve to close when power is off.

[0008] Optionally, the current value of the proportional pressure reducing valve gradually decreases after power is lost.

[0009] Optionally, the control device further includes a damping for controlling the oil source pressure of the proportional pressure reducing valve.

[0010] Optionally, the control device further comprises a filter for filtering the oil entering the proportional pressure reducing valve.

[0011] Optionally, the control device includes a detection element for detecting the pressure rise time of the first hydraulic oil circuit.

[0012] Optionally, the quantitative pump hydraulic system further includes a controller and a detection device for detecting the real-time rotational speed of the engine in an idle state, and the detection device and the control device are respectively connected to the controller signal.

[0013] According to a second aspect of the present disclosure, a crane is provided, comprising the above-described fixed displacement pump hydraulic system for the crane.

[0014] According to a third aspect of the present disclosure, a control method for a fixed-displacement pump hydraulic system of a crane is provided. The fixed-displacement pump hydraulic system includes a first fixed-displacement pump, a second fixed-displacement pump, and an engine for driving the first and second fixed-displacement pumps. The first fixed-displacement pump provides pressurized oil to a working mechanism via a first hydraulic oil circuit, and the second fixed-displacement pump provides pressurized oil to the working mechanism via a second hydraulic oil circuit. The fixed-displacement pump hydraulic system also includes a detection device for detecting the real-time speed of the engine in an idle state, and a control device for controlling the first fixed-displacement pump. The detection device and the control device are signal-connected. When the crane is operating in a rated load idle state, the control method includes the following steps:

[0015] In the first step, the detection device detects the rotation speed of the engine in real time;

[0016] In the second step, when the detected speed is lower than the speed in the idle state, the control device unloads the pressure of the first hydraulic oil circuit to cut off the operation of the first fixed displacement pump;

[0017] In the third step, when the detected rotational speed is equal to or greater than the rotational speed in the idle state, the control device controls the pressure of the first hydraulic oil circuit to gradually increase until the first fixed displacement pump reaches normal operation.

[0018] Optionally, the control device includes a hydraulically controlled reversing valve and a proportional pressure reducing valve for controlling the hydraulically controlled reversing valve, one end of the hydraulically controlled reversing valve is connected to the first hydraulic oil circuit, and the other end is connected to the oil tank, and the proportional pressure reducing valve is a solenoid valve, wherein,

[0019] In the second step, when the detected speed is lower than the speed in the idle state, the proportional pressure reducing valve is controlled to be energized, the hydraulically controlled reversing valve is opened, and the pressure oil in the first hydraulic oil circuit flows back to the oil tank through the hydraulically controlled reversing valve;

[0020] In the third step, when the detected speed is equal to or greater than the speed in the idle state, the current value controlling the proportional pressure reducing valve is gradually reduced, the hydraulically controlled reversing valve is slowly closed, and the pressure of the first hydraulic oil circuit is gradually increased.

[0021] With the above technical solution, when the crane is operating at rated load in idle state, the first fixed displacement pump, through the first hydraulic oil circuit, and the second fixed displacement pump, through the second hydraulic oil circuit, jointly provide power to the working mechanism. When the engine speed falls below the idle value, the control device can unload the pressure in the first hydraulic oil circuit, thereby shutting off the first fixed displacement pump, reducing the pressure in the hydraulic system and, in turn, the required engine output torque, thereby avoiding engine stalling due to excessive torque increase. Simultaneously, when the engine speed returns to the idle value, to ensure that the final operating speed of the working mechanism is not affected, the control device can gradually increase the pressure in the first hydraulic oil circuit to a normal value, allowing the pressure oil from the first fixed displacement pump to enter the hydraulic system and jointly act on the working mechanism with the second fixed displacement pump. During this process, the time it takes for the first hydraulic oil circuit to gradually return to normal pressure exceeds the engine's response time, providing the engine with sufficient response time so that the actual torque output from the engine power take-off port meets the torque required by the hydraulic system.

[0022] Other features and advantages of the present disclosure will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings are used to provide a further understanding of the present disclosure and constitute a part of the specification. Together with the following detailed description, they are used to explain the present disclosure but do not constitute a limitation of the present disclosure. In the accompanying drawings:

[0024] Figure 1 is a working principle diagram of a quantitative pump hydraulic system of a crane provided by an exemplary embodiment of the present disclosure;

[0025] Figure 2 4 is a block diagram of a control method for a crane quantitative pump hydraulic system provided by an exemplary embodiment of the present disclosure.

[0026] Description of Reference Numerals

[0027] 10-first metering pump, 20-second metering pump, 30-engine, 40-working mechanism, 50-control device, 51-hydraulic controlled reversing valve, 52-proportional pressure reducing valve, 53-detection component, 54-damping, 55-filter, 60-oil tank, 70-main valve, 81-first one-way valve, 82-second one-way valve, 90-overflow valve. DETAILED DESCRIPTION

[0028] The following describes the specific embodiments of the present disclosure in detail with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present disclosure and are not intended to limit the present disclosure.

[0029] In this disclosure, unless otherwise indicated, directional terms such as "inner" and "outer" refer to the inner and outer parts of the corresponding component outlines. Furthermore, the terms "first," "second," and the like are used in this disclosure to distinguish one element from another and do not convey sequential or significant meanings. In the following description, when referring to the accompanying drawings, identical numerals in different drawings represent identical or similar elements unless otherwise indicated.

[0030] Reference Figure 1 The embodiment of the present disclosure provides a fixed-flow pump hydraulic system for a crane, comprising a first fixed-flow pump 10, a second fixed-flow pump 20, and an engine 30 for driving the first fixed-flow pump 10 and the second fixed-flow pump 20. When the hydraulic system is operating normally, the first fixed-flow pump 10 and the second fixed-flow pump 20 provide pressure oil to the working mechanism 40 through the first hydraulic oil circuit and the second fixed-flow pump 20 through the second hydraulic oil circuit. Specifically, Figure 1 In the shown metering pump hydraulic system, the working mechanism 40 can be a cylinder, and the first metering pump 10 and the second metering pump 20 can both be gear pumps. When the metering pump hydraulic system is operating normally, the engine 30 drives the first metering pump 10 and the second metering pump 20 to work together. The first hydraulic oil circuit and the second hydraulic oil circuit are connected to the cylinder through the main oil circuit after converging. A main valve 70 is provided on the main oil circuit. The hydraulic oil in the oil tank 60 enters the first hydraulic oil circuit from the first metering pump 10, flows through the first one-way valve 81 on the first hydraulic oil circuit and then enters the main oil circuit. The hydraulic oil in the oil tank 60 enters the second hydraulic oil circuit from the second metering pump 20, flows through the second one-way valve 82 on the second hydraulic oil circuit and then enters the main oil circuit. The converged hydraulic oil then enters the cylinder 40 through the main valve 70 on the main oil circuit. A relief valve 90 can also be connected to the main oil circuit. The other end of the relief valve 90 can be connected to the oil tank 60 to control the pressure of the hydraulic system and stabilize the system pressure.

[0031] Among them, reference Figure 1The metering pump hydraulic system may further include a control device 50 for controlling the first metering pump 10. The control device 50 may be configured to, when the crane is operating in a rated load idle state and the speed of the engine 30 is lower than the speed in the idle state, unload the pressure of the first hydraulic oil circuit to cut off the operation of the first metering pump 10; and when the speed of the engine 30 is equal to or higher than the speed in the idle state, gradually increase the pressure of the first hydraulic oil circuit until the first metering pump 10 reaches normal operation.

[0032] When the crane is operating at rated load in idle state, when the system pressure reaches the set pressure value, the output torque of the engine 30 power take-off port is the sum of the torques output by the first and second fixed displacement pumps 10 and 20. At this time, due to the rapid increase in system pressure, the actual output torque of the engine 30 power take-off port is less than the sum of the torques output by the first and second fixed displacement pumps 10 and 20, which may cause the engine 30 to slow down and even stall. Through the technical solution provided by the present disclosure, when the speed of the engine 30 is less than the idle value, the control device 50 can unload the pressure in the first hydraulic oil circuit, thereby shutting off the operation of the first fixed displacement pump 10 (i.e., shutting off the torque generated by the first fixed displacement pump 10), reducing the pressure in the hydraulic system, and thus reducing the required engine output torque, thereby avoiding engine stalling caused by excessive torque increase. At the same time, when the speed of the engine 30 returns to the idle value, in order to ensure that the final operating speed of the working mechanism 40 is not affected, the control device 50 can control the pressure in the first hydraulic oil circuit to gradually increase to a normal value, so that the pressure oil of the first fixed displacement pump 10 enters the hydraulic system and acts together with the second fixed displacement pump 20 on the working mechanism 40. During this process, the time it takes for the first hydraulic oil circuit to gradually return to normal pressure can be greater than the response time of the engine 30, thereby providing the engine 30 with sufficient response time so that the actual torque output from the power take-off port of the engine 30 meets the torque required by the hydraulic system. Here, the control device 50 controls the pressure in the first hydraulic oil circuit to increase when the speed of the engine 30 is equal to the idle speed value. When the speed of the engine 30 is greater than the idle speed value, the control device 50 can also control the pressure in the first hydraulic oil circuit to gradually increase.

[0033] According to some embodiments, reference Figure 1The control device 50 may include a hydraulically controlled reversing valve 51 and a proportional pressure reducing valve 52 for controlling the hydraulically controlled reversing valve 51. One end of the hydraulically controlled reversing valve 51 may be connected to the first hydraulic oil circuit, and the other end may be connected to the fuel tank 60. The proportional pressure reducing valve 52 may be a solenoid valve, configured to control the hydraulically controlled reversing valve 51 to open when energized and to control the hydraulically controlled reversing valve 51 to close when de-energized. Specifically, when the engine 30 speed falls below the idle speed, the proportional pressure reducing valve 52 may be energized. At this time, the pressurized oil passes through the proportional pressure reducing valve 52 and rapidly pushes the hydraulically controlled reversing valve 51 to open. The pressurized oil in the first metering pump 10 flows directly back to the fuel tank 60 through the hydraulically controlled reversing valve 51, reducing the power of the first metering pump 10 and thereby cutting off the torque generated by the first metering pump 10, allowing the engine 30 speed to return to the idle speed. When the speed of the engine 30 returns to the idle value, the proportional pressure reducing valve 52 can be controlled to lose power, thereby closing the hydraulically controlled reversing valve 51, and the first metering pump 10 can return to normal operation to provide power to the working mechanism 40 together with the second metering pump 20.

[0034] As described above, to provide sufficient response time for the engine 30, the proportional pressure-reducing valve 52 can be controlled to gradually decrease its current value after power is lost, thereby slowly closing the hydraulically controlled reversing valve 51. The total time from energizing the proportional pressure-reducing valve 52 to de-energizing it is greater than the response time of the engine 30. For example, the current value of the proportional pressure-reducing valve 52 can be slowly decreased from 650 mA to 0. Furthermore, to quickly restore the engine 30 speed to its idle value and prevent the engine 30 from stalling, the proportional pressure-reducing valve 52 can be controlled to energize quickly. In this way, the proportional pressure-reducing valve 52 can be controlled to energize quickly and de-energize slowly, causing the hydraulically controlled reversing valve 51 to open quickly and close slowly. This allows the pressure in the first hydraulic circuit to be quickly relieved when the engine 30 speed is below the idle value, and gradually increased when the engine 30 speed returns to the idle value, thereby quickly shutting off the torque of the first metering pump 10 or slowly increasing it.

[0035] As an implementation method, refer to Figure 1 The control device 50 may include a detection element 53 for detecting the pressure rise time of the first hydraulic oil circuit. The detected data can be used to provide a reference for the control time of the proportional pressure reducing valve 52, further ensuring that the engine 30 has sufficient response time. The detection element 53 may be, for example, a pressure sensor that can convert the detected pressure value of the first hydraulic oil circuit into a corresponding time value.

[0036] According to some embodiments, reference Figure 1The control device 50 may further include a damper 54 for controlling the oil source pressure of the proportional pressure reducing valve 52 so that the proportional pressure reducing valve 52 has a stable pressure oil source to avoid fluctuations. The damper 54 may be constructed in the form of a throttle hole, for example.

[0037] According to some embodiments, the control device 50 may further include a filter 55 for filtering the oil entering the proportional pressure reducing valve 52. Figure 1 The pressure oil flows into the proportional pressure reducing valve 52 after passing through the filter 55 and the damper 54 in sequence. The filter 55 can also prevent the oil from clogging the damper 54.

[0038] According to an embodiment provided by the present disclosure, the metering pump hydraulic system may further include a controller and a detection device for detecting the real-time speed of the engine 30 in the idle state. The detection device and the control device 50 may be respectively connected to the controller for signal transmission. In this way, the detection device detects the speed of the engine 30 in real time. When the speed of the engine 30 is detected to be less than the idle value, the signal may be transmitted to the controller, which outputs a command to energize the proportional pressure reducing valve 52 in the control device 50. When the speed of the engine 30 is detected to be equal to or greater than the idle value, the signal may be transmitted to the controller, which outputs a command to de-energize the proportional pressure reducing valve 52 in the control device 50. The detection device may be, for example, a speed sensor.

[0039] The present disclosure also provides a crane, wherein the crane can be equipped with the above-mentioned fixed displacement pump hydraulic system. The crane has all the beneficial effects of the above-mentioned fixed displacement pump hydraulic system, which will not be described in detail here.

[0040] Reference Figure 2The embodiment of the present disclosure also provides a control method for a crane metering pump hydraulic system. As described above, the metering pump hydraulic system may include a first metering pump 10, a second metering pump 20, and an engine 30 for driving the first metering pump 10 and the second metering pump 20. When the hydraulic system is working normally, the first metering pump 10 and the second metering pump 20 provide pressurized oil to the working mechanism 40 through the first hydraulic oil circuit and the second metering pump 20 through the second hydraulic oil circuit. The metering pump hydraulic system may further include a detection device for detecting the real-time speed of the engine 30 in an idle state and a control device 50 for controlling the first metering pump 10. The detection device and the control device 50 may be signal-connected. When the crane is operating in a rated load idle state, the control method may include the following steps: a first step S101, wherein the detection device detects the speed of the engine 30 in real time; a second step S102, wherein when the detected speed is less than the speed in the idle state, the control device 50 unloads the pressure in the first hydraulic oil circuit to shut off the operation of the first metering pump 10; and a third step S103, wherein when the detected speed of the engine 30 is equal to or greater than the speed in the idle state, the control device 50 controls the pressure in the first hydraulic oil circuit to gradually increase until the first metering pump 10 reaches normal operation. The detection device may be, for example, a speed sensor.

[0041] Thus, when the detection device detects that the engine 30 speed is less than the idle value, the control device 50 can be controlled to unload the pressure in the first hydraulic oil circuit, thereby shutting off the operation of the first metering pump 10 (i.e., shutting off the torque generated by the first metering pump 10), reducing the pressure in the hydraulic system, and thereby reducing the required engine output torque, thereby avoiding engine stalling due to excessive torque increase. Simultaneously, when the detection device detects that the engine 30 speed has returned to the idle value, to ensure that the final operating speed of the working mechanism 40 is not affected, the control device 50 can be controlled to gradually increase the pressure in the first hydraulic oil circuit to a normal value, thereby allowing the pressure oil from the first metering pump 10 to enter the hydraulic system and act together with the second metering pump 20 on the working mechanism 40. During this process, the time it takes for the first hydraulic oil circuit to gradually return to normal pressure can be controlled to be greater than the response time of the engine 30, thereby providing sufficient response time for the engine 30 to ensure that the actual torque output from the engine 30 power take-off port meets the torque required by the hydraulic system. This control method can effectively prevent engine stalling when the crane is operating at rated load and idle.

[0042] According to some embodiments, the control device 50 may include a hydraulically controlled reversing valve 51 and a proportional pressure reducing valve 52 for controlling the hydraulically controlled reversing valve 51. One end of the hydraulically controlled reversing valve 51 may be connected to the first hydraulic oil circuit, and the other end may be connected to the fuel tank 60. The proportional pressure reducing valve 52 may be a solenoid valve. In the second step S102, when the detected speed of the engine 30 is less than the speed at idle, the proportional pressure reducing valve 52 may be controlled to be energized. At this time, the pressure oil passes through the proportional pressure reducing valve 52 to rapidly push the hydraulically controlled reversing valve 51 to open, and the pressure oil in the first hydraulic oil circuit flows directly back to the fuel tank 60 through the hydraulically controlled reversing valve 51. In the third step S103, when the detected speed of the engine 30 is equal to or greater than the speed at idle, the current value of the proportional pressure reducing valve 52 may be controlled to gradually decrease, thereby slowly closing the hydraulically controlled reversing valve 51 and gradually increasing the pressure in the first hydraulic oil circuit. That is, by controlling the proportional pressure reducing valve 52 to be energized quickly and de-energized slowly, the hydraulically controlled reversing valve 51 can be made to open quickly and close slowly, so as to quickly unload the pressure of the first hydraulic oil circuit when the speed of the engine 30 is less than the idle value and gradually increase the pressure of the first hydraulic oil circuit when the speed of the engine 30 returns to the idle value, thereby quickly cutting off the torque of the first metering pump 10 or slowly increasing the torque of the first metering pump 10, preventing the engine 30 from stalling while providing sufficient response time for the engine 30.

[0043] Where the fixed-displacement pump hydraulic system includes a controller, the control device 50 and the detection device can each be signal-connected to the controller. In the aforementioned control method steps, commands can be output by the controller to implement corresponding operations. Furthermore, where the fixed-displacement pump hydraulic system is the aforementioned fixed-displacement pump hydraulic system for a crane, the control method exhibits all the benefits of the aforementioned fixed-displacement pump hydraulic system and, to avoid repetition, is not further described here.

[0044] The preferred embodiments of the present disclosure are described in detail above in conjunction with the accompanying drawings. However, the present disclosure is not limited to the specific details of the above embodiments. Within the technical concept of the present disclosure, various simple modifications can be made to the technical solutions of the present disclosure, and these simple modifications all fall within the scope of protection of the present disclosure.

[0045] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, the present disclosure will not further describe various possible combinations.

[0046] In addition, the various embodiments of the present disclosure may be arbitrarily combined, and as long as they do not violate the concept of the present disclosure, they should also be regarded as the contents disclosed by the present disclosure.

Claims

1. A fixed-displacement pump hydraulic system for a crane, comprising a first fixed-displacement pump (10), a second fixed-displacement pump (20), and an engine (30) for driving the first fixed-displacement pump (10) and the second fixed-displacement pump (20), wherein the first fixed-displacement pump (10) provides pressure oil to a working mechanism (40) through a first hydraulic oil circuit, and the second fixed-displacement pump (20) provides pressure oil to the working mechanism (40) through a second hydraulic oil circuit, and wherein: The quantitative pump hydraulic system further comprises a control device (50) for controlling the first quantitative pump (10), wherein the control device (50) is configured to: when the crane is operating in a rated load idle state, When the rotation speed of the engine (30) is lower than the rotation speed in the idle state, the pressure of the first hydraulic oil circuit is unloaded to cut off the operation of the first quantitative pump (10); as well as When the speed of the engine (30) is equal to or greater than the speed in the idle state, the pressure of the first hydraulic oil circuit is gradually increased until the first metering pump (10) reaches normal operation, The control device (50) includes a hydraulically controlled reversing valve (51) and a proportional pressure reducing valve (52) for controlling the hydraulically controlled reversing valve (51). One end of the hydraulically controlled reversing valve (51) is connected to the first hydraulic oil circuit, and the other end is connected to the oil tank (60). The proportional pressure reducing valve (52) is a solenoid valve, which is used to control the hydraulically controlled reversing valve (51) to open when power is supplied, and to control the hydraulically controlled reversing valve (51) to close when power is lost. The current value of the proportional pressure reducing valve (52) gradually decreases after power is lost.

2. The quantitative pump hydraulic system according to claim 1, characterized in that: The control device (50) further includes a damper (54) for controlling the oil source pressure of the proportional pressure reducing valve (52).

3. The quantitative pump hydraulic system according to claim 1, characterized in that: The control device (50) further comprises a filter (55) for filtering the oil entering the proportional pressure reducing valve (52).

4. The quantitative pump hydraulic system according to claim 1, characterized in that: The control device (50) includes a detection component (53) for detecting the pressure rise time of the first hydraulic oil circuit.

5. The metering pump hydraulic system according to any one of claims 1 to 4, characterized in that: The quantitative pump hydraulic system further comprises a controller and a detection device for detecting the real-time rotation speed of the engine (30) in an idle state, wherein the detection device and the control device (50) are respectively connected to the controller signal.

6. A crane, characterized in that: A fixed displacement pump hydraulic system for a crane comprising the method according to any one of claims 1 to 5.

7. A control method for a hydraulic system of a crane quantitative pump, characterized in that: The quantitative pump hydraulic system is a quantitative pump hydraulic system according to any one of claims 1 to 5, and the quantitative pump hydraulic system further comprises a detection device for detecting the real-time rotational speed of the engine (30) in an idle state, the detection device and the control device (50) are connected by signal. When the crane is operating in a rated load idle state, the control method comprises the following steps: In the first step, the detection device detects the rotation speed of the engine (30) in real time; In the second step, when the detected rotational speed is lower than the rotational speed in the idle state, the control device (50) unloads the pressure of the first hydraulic oil circuit to cut off the operation of the first fixed displacement pump (10); In the third step, when the detected rotational speed is equal to or greater than the rotational speed in the idle state, the control device (50) controls the pressure of the first hydraulic oil circuit to gradually increase until the first fixed displacement pump (10) reaches normal operation.

8. The control method according to claim 7, characterized in that: In the second step, when the detected rotational speed is lower than the rotational speed in the idle state, the proportional pressure reducing valve (52) is controlled to be energized, the hydraulically controlled reversing valve (51) is opened, and the pressure oil in the first hydraulic oil circuit flows back to the oil tank (60) through the hydraulically controlled reversing valve (51); In the third step, when the detected rotational speed is equal to or greater than the rotational speed in the idle state, the current value of the proportional pressure reducing valve (52) is controlled to gradually decrease, the hydraulically controlled reversing valve (51) is slowly closed, and the pressure of the first hydraulic oil circuit gradually increases.

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