Hydraulic control system and control method of travel cylinder of precision forging machine operator

Abstract

The present invention discloses a hydraulic control system for a travel cylinder of a precision forging machine operator and a control method thereof, comprising a servo proportional control valve group, the servo proportional control valve group comprising a valve body differential valve, a servo valve, a travel back pressure regulating proportional valve and a torque control valve, the travel back pressure regulating proportional valve being connected to a pressure sensor, the torque control valve being connected to a rod chamber of a travel cylinder, the differential valve, the A port of the servo valve and the A port of the travel back pressure regulating proportional valve being connected to a rodless chamber of the travel cylinder, and the P port of the servo valve being connected to the torque control valve. In the present invention, the torque control valve and the travel back pressure regulating proportional valve continuously control the oil inlet and outlet pressure through a control system according to the rodless chamber pressure of the travel cylinder and the outlet pressure of the torque control valve collected by the pressure sensor, so as to prevent the dynamic change of the friction force between the static friction and the dynamic friction between the operator and the operator track during the low-speed operation of the operator, causing pressure fluctuations in the travel cylinder.

Description

Technical Field

[0001] The invention relates to the technical field of forging equipment, in particular to a hydraulic control system for a travel cylinder of a precision forging machine operator and a control method thereof. Background Art

[0002] The precision forging machine is a fast and precise forging equipment. In order to ensure the dimensional accuracy, surface quality and production efficiency of the stepped shaft after forging, the precision forging machine needs to have a stable speed stepless adjustment function, precise positioning accuracy and fast response. At present, the hydraulic proportional variable pump is used to control the speed and positioning of the manipulator. Its low-speed working stability is poor, the positioning accuracy cannot meet the requirements, and the speed will fluctuate greatly when the large and small switch valves are switched. The slow response of the manipulator directly affects the quality and efficiency of forging.

[0003] The hydraulic control system of the existing precision forging machine operator is as follows: Figure 1 As shown in the figure, when the precision forging machine operator moves quickly and works at low speed, the proportional variable pump is mainly driven by a three-phase asynchronous motor. The remote control center inputs the set speed signal to the proportional variable pump. The proportional variable pump changes the swing angle according to the signal, and realizes the closed-loop control of the swing angle with its built-in swing angle sensor. The required flow oil is output through the high-speed and walking direction control valve or the low-speed switch valve, so that the walking cylinder drives the operator to walk. At the same time, the displacement sensor calculates the actual walking speed according to the time-displacement, and feeds back to the remote control center to correct the output flow of the proportional variable pump to ensure that the operator moves at the set speed. However, due to the poor stability of the proportional variable pump at the lowest flow output, the low-speed switch valve cannot adjust the valve core opening and can only output a fixed flow, and the bipolar back pressure valve cannot be adjusted in real time according to the pressure fluctuation, resulting in unstable speed at low-speed working.

[0004] When the manipulator needs to control the position of the workpiece, the remote control center controls the proportional variable pump to control the speed and position of the travel cylinder. When the high-speed and travel direction control valves are switched to the low-speed switch valves for deceleration, the speed fluctuates greatly, and it takes a long time to adjust the position to reach the specified position, and the error range is large.

[0005] When the manipulator stops rotating and moving, the pressure-building process to overcome the static friction is slow, resulting in a slow response of the manipulator. The above problems lead to a low qualified rate of forging products, a large amount of finished parts processing, and low production efficiency. Summary of the invention

[0006] The purpose of the present invention is to provide a hydraulic control system for the travel cylinder of a precision forging machine operator and a control method thereof, which adopts the throttling control of a servo proportional valve and the volume control of a proportional variable pump to achieve precise control of the speed, position and pressure of the operator, and ensure that the travel of the operator has a stepless adjustment function with stable speed, precise positioning accuracy and rapid responsiveness, so that the equipment can meet the requirements of the precision forging process, thereby ensuring the dimensional accuracy and surface quality of the stepped shaft after the forging is formed.

[0007] To achieve the above object, the technical solution adopted by the present invention is:

[0008] A hydraulic control system for a travel cylinder of a precision forging machine operator comprises a servo proportional control valve group, an oil inlet of the servo proportional control valve group is connected to an oil outlet of a proportional variable pump, an oil inlet of the proportional variable pump and an oil return port of the servo proportional control valve group are both connected to an oil port of a hydraulic oil tank, a proportional variable pump is connected to a three-phase asynchronous motor, a working oil port of the servo proportional control valve group is connected to a working oil port of a travel cylinder of the precision forging machine operator, the proportional variable pump and the servo proportional control valve group are both electrically connected to a control system, the servo proportional control valve group comprises a valve body and a differential valve, a servo valve, a travel back pressure regulating proportional valve and a torque control valve which are installed in series on an internal oil circuit on a surface of the valve body, the travel back pressure regulating proportional valve is connected to a pressure sensor, the torque control valve is connected to a rod cavity of a travel cylinder, the differential valve, the A port of the servo valve and the A port of the travel back pressure regulating proportional valve are connected to a rodless cavity of the travel cylinder, the B port of the travel back pressure regulating proportional valve is connected to an oil tank, the T port of the servo valve is connected to an oil tank, and the P port of the servo valve is connected to the torque control valve.

[0009] Preferably, a displacement sensor is installed at one end of the traveling cylinder.

[0010] Preferably, a one-way valve is installed on the connecting pipeline between the torque control valve and the travel cylinder.

[0011] Preferably, an electromagnetic overflow valve is installed on the connecting pipeline between the proportional variable pump and the differential valve.

[0012] A hydraulic control method for a travel cylinder of a precision forging machine operator, the hydraulic control system of the travel cylinder of the precision forging machine operator as described above, includes the following processes:

[0013] a. When the manipulator moves quickly:

[0014] The three-phase asynchronous motor drives the proportional variable pump to start. The control system inputs the set speed signal to the proportional variable pump. The proportional variable pump changes the swing angle according to the speed signal, and realizes the closed-loop control of the swing angle with its built-in swing angle sensor, outputs the required flow oil, and the travel cylinder drives the manipulator to move forward or backward quickly. At the same time, the displacement sensor calculates the actual travel speed of the manipulator according to the time-displacement relationship, and feeds it back to the control system to correct the output flow of the proportional variable pump oil to ensure that the manipulator moves quickly at the set speed.

[0015] The direction of oil when the manipulator moves quickly:

[0016] When the travel cylinder moves backward quickly, the differential valve and servo valve are in the closed state; the oil flows out from the oil outlet of the proportional variable pump, one of which is input into the rod chamber of the travel cylinder through the torque control valve and the one-way valve, and the oil in the rodless chamber of the travel cylinder flows back to the oil tank through the travel back pressure regulating proportional valve;

[0017] When the travel cylinder moves forward quickly, the servo valve is in a closed state, the electromagnetic overflow valve and the pilot electromagnetic reversing valve of the differential valve are energized, the travel back pressure regulating proportional valve is in a state far higher than the travel pressure, and the travel back pressure regulating proportional valve is closed; the rod chamber of the travel cylinder generates low back pressure through the electromagnetic overflow valve; the oil flows out from the oil outlet of the proportional variable pump, and one path of the oil is transported to the rod chamber of the travel cylinder through the torque control valve and the one-way valve; the oil in the rod chamber of the travel cylinder is affected by the different pressures caused by the area ratio, overcomes the low-pressure overflow of the electromagnetic overflow valve, and enters the rodless chamber of the travel cylinder through the differential valve, realizing the differential advancement of the travel cylinder;

[0018] b. When the manipulator is moving at low speed, the speed is 0-7m / min, the proportional variable pump is set to a fixed flow output, the control system inputs a set speed signal to the servo valve, the servo valve changes the valve core opening according to the speed signal, and realizes closed-loop control of the valve core with its internal valve core displacement sensor, outputs the required flow oil, and the travel cylinder drives the manipulator to move at low speed. At the same time, the actual travel speed of the manipulator is calculated based on the displacement and time of the travel cylinder detected by the displacement sensor, and fed back to the control system to correct the oil output flow of the servo valve to ensure that the manipulator moves at the set speed;

[0019] The oil flow direction of the operating machine at low speed is as follows:

[0020] When the operating machine moves backward at low speed: the differential valve is closed, the electromagnetic overflow valve is energized and in a low-pressure state, the travel back pressure regulating proportional valve is in a state far higher than the travel pressure, the travel back pressure regulating proportional valve is closed, and the oil flows out from the oil outlet of the proportional variable pump, of which the first oil path is delivered to the rod chamber of the travel cylinder through the torque control valve and the one-way valve; the second oil path flows to the P port of the servo valve through the torque control valve, at which time the P port is connected to the B port and is blocked; the third oil path passes through the torque control valve and the electromagnetic overflow valve to the differential valve and is blocked; the oil in the rodless chamber of the travel cylinder flows to the hydraulic tank through the servo valve, and the travel cylinder moves backward;

[0021] When the operating machine moves forward at low speed: the differential valve is closed, the electromagnetic relief valve is energized and is in a low-pressure state, the travel back pressure regulating proportional valve is in a state much higher than the travel pressure, the travel back pressure regulating proportional valve is closed, and the oil flows out from the oil outlet of the proportional variable pump, wherein the first oil path passes through the torque control valve and the one-way valve and is input into the rod chamber of the travel cylinder; the second oil path passes through the torque control valve and flows to the A port via the P port of the servo valve and enters the rodless chamber of the travel cylinder; the third oil path passes through the torque control valve and the electromagnetic relief valve to the differential valve, and is blocked; the oil in the rod chamber of the travel cylinder enters the rodless chamber of the travel cylinder due to the different pressures caused by the area ratio, thereby realizing the differential advancement of the travel cylinder.

[0022] During the working process of the present invention, in order to ensure the stability of the low-speed working advance of the operating machine, the torque control valve and the travel back pressure regulating proportional valve continuously control the oil inlet and outlet pressure through the control system according to the rodless chamber pressure of the travel cylinder and the torque control valve outlet pressure collected by the pressure sensor, so as to prevent the dynamic change of the friction force between the static friction and the dynamic friction between the operating machine and the operating machine track during the low-speed working advance of the operating machine, causing pressure fluctuations in the travel cylinder.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) The present invention adopts a proportional servo control system, which can realize one-key operation and adaptive adjustment of the manipulator's movement during the entire forging process, improve the automation and intelligence of the entire system, greatly reduce the technical threshold and workload of operators, and improve the operating environment;

[0025] (2) By adopting the closed-loop deviation control of the servo proportional valve, the speed and position of the entire operating machine can be accurately controlled, thereby ensuring that the forging machine operating machine has a stable speed stepless adjustment function, precise positioning accuracy and fast follow-up.

[0026] (3) By controlling the proportional pressure valve, servo proportional valve and pressure sensor, the back pressure of the operating machine cylinder is dynamically adjusted in a closed loop, which improves the stability of the cylinder under low-speed movement and eliminates the creeping phenomenon.

[0027] (4) By introducing proportional servo control technology in the entire system, the equipment can meet the requirements of high-precision precision forging process, thereby ensuring the dimensional accuracy and surface quality of the stepped shaft after forging, achieving near-net forming of the forging, reducing the production cost of the equipment user, improving product qualification rate, and improving production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is the principle diagram of the hydraulic control system for the travel of the existing precision forging machine operator;

[0029] Figure 2 It is a schematic diagram of the travel hydraulic control system of the precision forging machine operator of the present invention.

[0030] In the figure: 1. operating machine track, 2. operating machine, 3. traveling cylinder, 4. displacement sensor, 5. control system, 6. one-way valve, 7. proportional variable pump, 8. three-phase asynchronous motor, 9. servo proportional control valve group, 10. differential valve, 11. low-speed switching valve, 12. hydraulic oil tank, 13. bipolar back pressure valve, 14. servo valve, 15. pressure sensor, 16. traveling back pressure regulating proportional valve, 17. torque control valve. DETAILED DESCRIPTION

[0031] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0032] like Figure 2 The hydraulic control system of the travel cylinder of the precision forging machine operator shown in the figure includes a servo proportional control valve group 9, the oil inlet of the servo proportional control valve group 9 is connected to the oil outlet of the proportional variable pump 7, the oil inlet of the proportional variable pump 7 and the return oil port of the servo proportional control valve group 9 are both connected to the oil port of the hydraulic oil tank 12, the proportional variable pump 7 is connected to the three-phase asynchronous motor 8, the working oil port of the servo proportional control valve group 9 is connected to the working oil port of the travel cylinder 3 of the precision forging machine operator, the proportional variable pump 7 and the servo proportional control valve group 9 are both electrically connected to the control system 5, and the servo proportional control valve group 9 includes a valve body and The differential valve 10, the servo valve 14, the travel back pressure regulating proportional valve 16 and the torque control valve 17 are installed in series in the internal oil circuit on the surface of the valve body. The travel back pressure regulating proportional valve 16 is connected to the pressure sensor 15, and the torque control valve 17 is connected to the rod chamber of the travel cylinder 3. The differential valve 10, the A port of the servo valve 14, and the A port of the travel back pressure regulating proportional valve 16 are connected to the rodless chamber of the travel cylinder 3, the B port of the travel back pressure regulating proportional valve 16 is connected to the oil tank 12, the T port of the servo valve 14 is connected to the oil tank 12, and the P port of the servo valve 14 is connected to the torque control valve 17.

[0033] Preferably, a displacement sensor 4 is installed at one end of the travel cylinder 3 .

[0034] Preferably, a one-way valve 6 is installed on the connecting pipeline between the torque control valve 17 and the travel cylinder 3 .

[0035] Preferably, an electromagnetic overflow valve is installed on the connecting pipeline between the proportional variable pump 7 and the differential valve 10 .

[0036] A hydraulic control method for a travel cylinder of a precision forging machine operator, wherein the hydraulic control system of the travel cylinder of the precision forging machine operator is as described above, and comprises the following processes:

[0037] a. When the manipulator 2 moves quickly:

[0038] The three-phase asynchronous motor 8 drives the proportional variable pump 7 to start, and the control system 5 inputs a set speed signal to the proportional variable pump 7. The proportional variable pump 7 changes the swing angle according to the speed signal, and realizes the closed-loop control of the swing angle with its built-in swing angle sensor, outputs the required flow oil, and the travel cylinder 3 drives the manipulator 2 to move forward or backward quickly. At the same time, the displacement sensor 4 calculates the actual travel speed of the manipulator 2 according to the time-displacement relationship, and feeds back to the control system 5, and corrects the output flow of the proportional variable pump 7 oil to ensure that the manipulator 2 moves quickly at the set speed;

[0039] The direction of the oil when the manipulator 2 moves quickly:

[0040] When the travel cylinder 3 moves backward quickly, the differential valve 10 and the servo valve 14 are in the closed state; the oil flows out from the oil outlet of the proportional variable pump 7, one of which is input into the rod chamber of the travel cylinder 3 through the torque control valve 17 and the one-way valve 6, and the oil in the rodless chamber of the travel cylinder 3 flows back to the oil tank 12 through the travel back pressure regulating proportional valve 16;

[0041] When the travel cylinder 3 moves forward quickly, the servo valve 14 is in a closed state, the electromagnetic overflow valve and the pilot electromagnetic reversing valve of the differential valve 10 are energized, the travel back pressure regulating proportional valve 16 is in a state far higher than the travel pressure, and the travel back pressure regulating proportional valve 16 is closed; the rod chamber of the travel cylinder 3 generates low back pressure through the electromagnetic overflow valve; the oil flows out from the oil outlet of the proportional variable pump 7, and one path of the oil is delivered to the rod chamber of the travel cylinder 3 through the torque control valve 17 and the one-way valve 6; the oil in the rod chamber of the travel cylinder 3 is affected by the different pressures caused by the area ratio, overcomes the low-pressure overflow of the electromagnetic overflow valve, and enters the rodless chamber of the travel cylinder 3 through the differential valve 10, realizing the differential advancement of the travel cylinder 3;

[0042] b. When the manipulator 2 is working at a low speed, the speed is 0-7m / min, the proportional variable pump 7 is set to a fixed flow output, the control system 5 inputs a set speed signal to the servo valve 14, the servo valve 14 changes the valve core opening according to the speed signal, and realizes the closed-loop control of the valve core with the valve core displacement sensor installed therein, outputs the required flow oil, the travel cylinder 3 drives the manipulator 2 to travel at a low speed, and at the same time, the actual travel speed of the manipulator 2 is calculated according to the displacement and time of the travel cylinder 3 detected by the displacement sensor 4, and the actual travel speed of the manipulator 2 is fed back to the control system 5, and the oil output flow of the servo valve 14 is corrected to ensure that the manipulator 2 is working at the set speed;

[0043] The oil flow of the operating machine 2 at low speed:

[0044] When the operating machine 2 moves backward at a low speed: the differential valve 10 is closed, the electromagnetic overflow valve is energized and is in a low-pressure state, the travel back pressure regulating proportional valve 16 is in a state far higher than the travel pressure, the travel back pressure regulating proportional valve 16 is closed, and the oil flows out from the oil outlet of the proportional variable pump 7, wherein the first oil path passes through the torque control valve 17 and the one-way valve 6 to be delivered to the rod chamber of the travel cylinder 3; the second oil path passes through the torque control valve 17 to the P port of the servo valve 14, at which time the P port is connected to the B port and is blocked; the third oil path passes through the torque control valve 17 and the electromagnetic overflow valve to the differential valve 10 and is blocked; the oil in the rodless chamber of the travel cylinder 3 flows through the servo valve 14 to the hydraulic oil tank 12, and the travel cylinder 3 moves backward;

[0045] When the operating machine 2 moves forward at a low speed: the differential valve 10 is closed, the electromagnetic overflow valve is energized and is in a low-pressure state, the travel back pressure regulating proportional valve 16 is in a state far higher than the travel pressure, the travel back pressure regulating proportional valve 16 is closed, and the oil flows out from the oil outlet of the proportional variable pump 7, wherein the first oil path passes through the torque control valve 17 and the one-way valve 6 and is input into the rod chamber of the travel cylinder 3; the second oil path passes through the torque control valve 17 and the P port of the servo valve 14 to the A port and enters the rodless chamber of the travel cylinder 3; the third oil path passes through the torque control valve 17 and the electromagnetic overflow valve to the differential valve 10, and is blocked; the oil in the rod chamber of the travel cylinder 3 enters the rodless chamber of the travel cylinder 3 due to the different pressures caused by the area ratio, thereby realizing the differential advancement of the travel cylinder 3.

[0046] The present invention adopts a proportional servo control system, realizes one-key operation and adaptive adjustment of the manipulator's movement during the entire forging process, improves the automation and intelligence of the entire system, greatly reduces the technical threshold and workload of operators, and improves the operating environment.

[0047] The above are only preferred embodiments of the present invention. It should be noted that for those skilled in the art, under the technical enlightenment provided by the present invention, other equivalent variations and improvements can also be made, which should also be regarded as the protection scope of the present invention.

Claims

1. A hydraulic control system for a travel cylinder of a precision forging machine operator, comprising a servo proportional control valve group (9), the oil inlet of the servo proportional control valve group (9) being connected to the oil outlet of a proportional variable pump (7), the oil inlet of the proportional variable pump (7) and the oil return port of the servo proportional control valve group (9) being connected to the oil port of a hydraulic oil tank (12), the proportional variable pump (7) being connected to a three-phase asynchronous motor (8), the working oil port of the servo proportional control valve group (9) being connected to the working oil port of a travel cylinder (3) of the precision forging machine operator, the proportional variable pump (7) and the servo proportional control valve group (9) being electrically connected to a control system (5), characterized in that: The servo proportional control valve group (9) comprises a valve body and a differential valve (10), a servo valve (14), a travel back pressure regulating proportional valve (16) and a torque control valve (17) which are installed in series in an internal oil circuit on the surface of the valve body. The travel back pressure regulating proportional valve (16) is connected to the pressure sensor (15), the torque control valve (17) is connected to the rod chamber of the travel oil cylinder (3), the differential valve (10), the A port of the servo valve (14) and the A port of the travel back pressure regulating proportional valve (16) are connected to the rodless chamber of the travel oil cylinder (3), the B port of the travel back pressure regulating proportional valve (16) is connected to the oil tank (12), the T port of the servo valve (14) is connected to the oil tank (12), and the P port of the servo valve (14) is connected to the torque control valve (17).

2. The hydraulic control system for the travel cylinder of the precision forging machine operator according to claim 1 is characterized in that: A displacement sensor (4) is installed at one end of the travel cylinder (3).

3. The hydraulic control system for the travel cylinder of the precision forging machine operator according to claim 2 is characterized in that: A one-way valve (6) is installed on the connecting pipeline between the torque control valve (17) and the travel cylinder (3).

4. The hydraulic control system for the travel cylinder of the precision forging machine operator according to claim 3 is characterized in that: An electromagnetic overflow valve is installed on the connecting pipeline between the proportional variable pump (7) and the differential valve (10).

5. A hydraulic control method for a travel cylinder of a precision forging machine operator, wherein the hydraulic control system of the travel cylinder of the precision forging machine operator is as described in claim 4, characterized in that: The process includes: a. When the manipulator (2) moves quickly: The three-phase asynchronous motor (8) drives the proportional variable pump (7) to start, and the control system (5) inputs a set speed signal to the proportional variable pump (7). The proportional variable pump (7) changes the swing angle according to the speed signal, and realizes closed-loop control of the swing angle with its built-in swing angle sensor, outputs the required flow rate of oil, and the travel cylinder (3) drives the manipulator (2) to move forward or backward quickly. At the same time, the displacement sensor (4) calculates the actual travel speed of the manipulator (2) according to the time-displacement relationship, and feeds back to the control system (5), and corrects the output flow rate of the oil of the proportional variable pump (7) to ensure that the manipulator (2) moves quickly at the set speed; The direction of the oil when the operating machine (2) moves quickly: When the travel cylinder (3) moves backward quickly, the differential valve (10) and the servo valve (14) are in a closed state; oil flows out from the oil outlet of the proportional variable pump (7), one path of the oil passes through the torque control valve (17) and the one-way valve (6) and is input into the rod chamber of the travel cylinder (3); the oil in the rodless chamber of the travel cylinder (3) passes through the travel back pressure regulating proportional valve (16) and flows back to the oil tank (12); When the travel oil cylinder (3) moves forward quickly, the servo valve (14) is in a closed state, the electromagnetic overflow valve and the pilot electromagnetic reversing valve of the differential valve (10) are energized, the travel back pressure regulating proportional valve (16) is in a state far higher than the travel pressure, and the travel back pressure regulating proportional valve (16) is closed; the rod chamber of the travel oil cylinder (3) generates a low back pressure through the electromagnetic overflow valve; the oil flows out from the oil outlet of the proportional variable pump (7), and one path of the oil is delivered to the rod chamber of the travel oil cylinder (3) through the torque control valve (17) and the one-way valve (6); the oil in the rod chamber of the travel oil cylinder (3) overcomes the low-pressure overflow of the electromagnetic overflow valve due to the different pressures caused by the area ratio, and enters the rodless chamber of the travel oil cylinder (3) through the differential valve (10), thereby realizing the differential movement of the travel oil cylinder (3); b. When the operating machine (2) is moving at a low speed, the speed is 0-7 m / min, the proportional variable pump (7) is set to a fixed flow output, the control system (5) inputs a set speed signal to the servo valve (14), the servo valve (14) changes the valve core opening according to the speed signal, and realizes closed-loop control of the valve core with the valve core displacement sensor installed therein, outputs the required flow rate of oil, the travel cylinder (3) drives the operating machine (2) to move at a low speed, and at the same time, the actual travel speed of the operating machine (2) is calculated based on the displacement and time of the travel cylinder (3) detected by the displacement sensor (4), and the actual travel speed of the operating machine (2) is fed back to the control system (5), and the oil output flow of the servo valve (14) is corrected to ensure that the operating machine (2) moves at a low speed according to the set speed; The oil flow of the operating machine (2) at low speed is as follows: When the operating machine (2) moves backward at a low speed: the differential valve (10) is closed, the electromagnetic overflow valve is energized and is in a low-pressure state, the travel back pressure regulating proportional valve (16) is in a state much higher than the travel pressure, the travel back pressure regulating proportional valve (16) is closed, and the oil flows out from the oil outlet of the proportional variable pump (7), wherein the first oil path passes through the torque control valve (17) and the one-way valve (6) and is delivered to the rod chamber of the travel cylinder (3); the second oil path passes through the torque control valve (17) and flows to the P port of the servo valve (14), at which time the P port is connected to the B port and is blocked; the third oil path passes through the torque control valve (17) and the electromagnetic overflow valve to the differential valve (10), and is blocked; the oil in the rodless chamber of the travel cylinder (3) passes through the servo valve (14) and flows to the hydraulic oil tank (12), and the travel cylinder (3) moves backward; When the operating machine (2) moves forward at a low speed: the differential valve (10) is closed, the electromagnetic overflow valve is energized and is in a low-pressure state, the travel back pressure regulating proportional valve (16) is in a state far higher than the travel pressure, the travel back pressure regulating proportional valve (16) is closed, and oil flows out from the oil outlet of the proportional variable pump (7), wherein the first oil path passes through the torque control valve (17) and the one-way valve (6) and is input into the rod chamber of the travel cylinder (3); the second oil path passes through the torque control valve (17) and flows to the port A of the servo valve (14) and enters the rodless chamber of the travel cylinder (3); the third oil path passes through the torque control valve (17) and the electromagnetic overflow valve to the differential valve (10), and is blocked; the oil in the rod chamber of the travel cylinder (3) enters the rodless chamber of the travel cylinder (3) due to the different pressures caused by the area ratio, thereby realizing the differential movement of the travel cylinder (3).

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