An on-line switching system for hydraulic fixed and variable loading of a wheel type medium-speed coal mill

Abstract

The present invention belongs to the technical field of hydraulic control, and particularly relates to an on-line switching system for hydraulic fixed and variable loading of a wheeled medium-speed coal mill, which includes a hydraulic oil tank. There are two oil outlets and seven oil return ports on the hydraulic oil tank. The two oil outlets are respectively connected to a working oil circuit and a reaction oil circuit. A working double gear oil pump and a reaction double gear oil pump are provided in the hydraulic oil tank; the working oil circuit includes an oil pump motor, and the oil pump motor is sequentially connected to a working proportional overflow valve through a working double gear oil pump and a working fixed loading safety overflow valve and then connected to a working electromagnetic directional valve. The working electromagnetic directional valve is respectively connected to a first hydraulic cylinder, a second hydraulic cylinder, a third hydraulic cylinder and the oil return port of the hydraulic oil tank. The present invention can realize the switching of the fixed and variable loading modes of the system, reduce the failure rate of the hydraulic pressure, and at the same time quickly recover after a failure occurs in the hydraulic system, reducing the maintenance time.

Description

Technical Field

[0001] The present invention belongs to the technical field of hydraulic control, and particularly relates to an on-line switching system for hydraulic fixed and variable loading of a wheel type medium-speed coal mill. Background Art

[0002] As a device for providing grinding pressure of a vertical roller medium-speed coal mill, the hydraulic system has the characteristics of small floor area, stable pressure supply, high load force levels that can be provided, etc., and is widely used in fields such as thermal power generation, chemical industry, steel mills, mines, etc. The hydraulic system of the traditional vertical roller medium-speed coal mill adopts a variable loading working mode. That is, the grinding pressure provided by the hydraulic system changes in real time according to the required output. This working mode has the advantages of adapting to a variety of coal types and a wide pressure adjustment range.

[0003] In the initial stage of coal mill commissioning and equipment operation, the variable loading control mode of the hydraulic station is convenient to adjust, which is beneficial to equipment commissioning and during the initial stage of equipment startup when the load is unstable, and the pressure can be adjusted at any time. However, after the equipment operates stably, the hydraulic system will operate under a stable load all year round. At this time, the hydraulic station does not need to frequently adjust the pressure, but the motor of the hydraulic station runs under variable loading for a long time, there are risks of valve blockage, valve group jamming, motor burnout, and oil pump grinding. Once problems occur in the hydraulic oil station, such as proportional valve blockage, amplifier board damage, motor burnout, oil pump damage, valve jamming, etc., it will cause the entire coal mill equipment to shut down.

[0004] In view of the above problems, we have proposed an on-line switching system for hydraulic fixed and variable loading of a wheel type medium-speed coal mill. Summary of the Invention

[0005] The present invention provides an on-line switching system for hydraulic fixed and variable loading of a wheel type medium-speed coal mill, aiming to achieve the switching of the fixed and variable loading modes of the system, reduce the failure rate of the hydraulic system, and quickly recover after a failure occurs in the hydraulic system, reducing the maintenance time.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] An on-line switching system for hydraulic fixed and variable loading of a wheel type medium-speed coal mill includes a hydraulic oil tank. There are two oil outlets and seven oil return ports on the hydraulic oil tank. The two oil outlets are respectively connected to the acting oil circuit and the reaction oil circuit. An acting double gear oil pump and a reaction double gear oil pump are provided in the hydraulic oil tank;

[0008] The acting oil circuit includes an oil pump motor. The oil pump motor is sequentially connected to an acting double gear oil pump, an acting fixed loading safety overflow valve, and an acting proportional overflow valve, and then connected to an acting electromagnetic directional valve. The acting electromagnetic directional valve is respectively connected to a first hydraulic cylinder, a second hydraulic cylinder, a third hydraulic cylinder, and the oil return port of the hydraulic oil tank;

[0009] The reaction force circuit includes an oil pump motor, which is sequentially connected to a reaction force double gear oil pump, a reaction force fixed loading safety overflow valve, and a reaction force proportional overflow valve, and then connected to a reaction force electromagnetic directional valve. The reaction force electromagnetic directional valve is respectively connected to the oil return ports of the first hydraulic cylinder, the second hydraulic cylinder, the third hydraulic cylinder, and the hydraulic oil tank.

[0010] Further, a working force loading pressure transmitter and a reaction force loading pressure transmitter are respectively provided at the outlets of the working force electromagnetic directional valve and the reaction force electromagnetic directional valve.

[0011] Further, a working force variable loading local pressure gauge and a working force fixed loading pressure transmitter are provided at the inlet of the working force electromagnetic directional valve.

[0012] Further, a reaction force variable loading local pressure gauge and a reaction force variable loading pressure transmitter are provided at the inlet of the reaction force electromagnetic directional valve.

[0013] Further, the working force electromagnetic directional valve is also connected to the reaction force electromagnetic directional valve and is connected to the oil tank via a double-barrel filter.

[0014] Further, the first hydraulic cylinder, the second hydraulic cylinder, and the third hydraulic cylinder are respectively connected to the first accumulator, the second accumulator, and the third accumulator, and the first hydraulic cylinder, the second hydraulic cylinder, and the third hydraulic cylinder are respectively connected to the first pressure relief safety valve, the second pressure relief safety valve, and the third pressure relief safety valve.

[0015] Further, it also includes a standby motor, a standby working force oil pump, and a standby reaction force oil pump 23. The standby motor is connected to the outlet of the working force double gear oil pump through the standby working force oil pump, and the standby motor is connected to the outlet of the reaction force double gear oil pump through the standby reaction force oil pump.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] During the commissioning and initial operation of the coal mill, variable loading operation is adopted, which is convenient for experiments and load changes; due to the addition of the working force electromagnetic directional valve and the reaction force electromagnetic directional valve, when the load of the coal mill is stable, the hydraulic station can change the direction of the pressure oil through the electromagnetic directional valve, switching from the variable loading working mode to the fixed loading working mode. By adding the working force proportional overflow valve, the reaction force proportional overflow valve, the working force variable loading local pressure gauge, and the reaction force variable loading local pressure gauge, the pressure can be increased or decreased in real time to ensure the safe and stable operation of the equipment. During fixed loading, the oil pump motor can be stopped, and the hydraulic system can achieve automatic pressure holding, reducing the power consumption of the equipment. Once the equipment needs to adjust the pressure, the oil pump motor can be started at any time, and the electromagnetic valve can be switched to the variable loading working mode to meet various on-site working conditions.

[0018] When the hydraulic system operates under fixed load, if the oil pump motor continues to run, the hydraulic oil pumped by the oil pump will directly return to the fuel tank. At this time, the oil pump motor can be stopped. Since the oil pressure in the pipeline achieves self-holding, the stable operation of the equipment can also be ensured. To avoid accidents caused by pipeline blockage, a first pressure relief safety valve, a second pressure relief safety valve, and a third pressure relief safety valve are added to the hydraulic system to ensure the safety of the equipment. When the system needs to frequently adjust the coal mill equipment, the hydraulic system can be switched online from the fixed load working mode to the variable load working mode, which is flexible and convenient to operate.

[0019] When the hydraulic system operates under fixed load, the pressure transmitters for force loading and the pressure transmitters for variable loading of the reaction force can provide real-time feedback on the pressure in the oil pipeline, realizing online monitoring of the oil pressure. Once pressure leakage, rapid pressure change, or the deviation between the pressure and the corresponding curve is greater than 3 MPa, the system can start the oil pump motor and switch the electromagnetic directional valve to the proportional overflow valve to ensure the continuous operation of the equipment. Brief Description of the Drawings

[0020] Figure 1 It is the schematic diagram of the online switching system for fixed and variable loading of the hydraulic system of the wheel type medium-speed coal mill;

[0021] Figure 2 It is the schematic diagram of the variable loading oil circuit of the online switching system for fixed and variable loading of the hydraulic system of the wheel type medium-speed coal mill;

[0022] Figure 3 It is the schematic diagram of the fixed loading oil circuit of the online switching system for fixed and variable loading of the hydraulic system of the wheel type medium-speed coal mill;

[0023] Figure 4 It is the schematic diagram of the online switching hydraulic system for fixed and variable loading in the double-pump form of the online switching system for fixed and variable loading of the hydraulic system of the wheel type medium-speed coal mill.

[0024] Wherein: 1 - oil pump motor; 2 - double gear oil pump for driving force; 3 - double gear oil pump for reaction force; 4 - fixed loading safety overflow valve for driving force; 5 - fixed loading safety overflow valve for reaction force; 6 - double - barrel filter; 7 - proportional overflow valve for reaction force; 8 - proportional overflow valve for driving force; 9 - local pressure gauge for variable loading of reaction force; 10 - pressure transmitter for variable loading of reaction force; 11 - local pressure gauge for variable loading of driving force; 12 - pressure transmitter for fixed loading of driving force; 13 - solenoid directional valve for driving force; 14 - solenoid directional valve for reaction force; 15 - pressure transmitter for loading of reaction force; 16 - pressure transmitter for loading of driving force; 171 - first hydraulic cylinder; 172 - second hydraulic cylinder; 173 - third hydraulic cylinder; 18 - hydraulic oil tank; 191 - first accumulator; 192 - second accumulator; 193 - third accumulator; 201 - first pressure relief safety valve; 202 - second pressure relief safety valve; 203 - third pressure relief safety valve; 21 - standby motor; 22 - standby oil pump for driving force; 23 - standby oil pump for reaction force. Detailed implementation manners

[0025] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0026] As Figure 1 shown, a hydraulic fixed - and variable - loading on - line switching system for a wheel - type medium - speed coal mill includes a hydraulic oil tank 18. The hydraulic oil tank 18 is provided with two oil outlets and seven oil return ports. The two oil outlets are respectively connected to the driving - force oil circuit and the reaction - force oil circuit. The hydraulic oil tank 18 is internally provided with a double gear oil pump 2 for driving force and a double gear oil pump 3 for reaction force.

[0027] The driving - force oil circuit includes an oil pump motor 1. The oil pump motor 1 is sequentially connected to a solenoid directional valve 13 for driving force through a double gear oil pump 2 for driving force, a fixed loading safety overflow valve 4 for driving force, and a proportional overflow valve 8 for driving force. The solenoid directional valve 13 for driving force is respectively connected to the first hydraulic cylinder 171, the second hydraulic cylinder 172, the third hydraulic cylinder 173, and the oil return port of the hydraulic oil tank 18.

[0028] The reaction - force circuit includes an oil pump motor 1. The oil pump motor 1 is sequentially connected to a solenoid directional valve 14 for reaction force through a double gear oil pump 3 for reaction force, a fixed loading safety overflow valve 5 for reaction force, and a proportional overflow valve 7 for reaction force. The solenoid directional valve 14 for reaction force is respectively connected to the first hydraulic cylinder 171, the second hydraulic cylinder 172, the third hydraulic cylinder 173, and the oil return port of the hydraulic oil tank 18.

[0029] The first hydraulic cylinder 171, the second hydraulic cylinder 172, and the third hydraulic cylinder 173 are respectively connected to the first accumulator 191, the second accumulator 192, and the third accumulator 193. The first hydraulic cylinder 171, the second hydraulic cylinder 172, and the third hydraulic cylinder 173 are respectively connected to the first pressure relief safety valve 201, the second pressure relief safety valve 202, and the third pressure relief safety valve 203.

[0030] At the outlets of the acting force electromagnetic directional valve 13 and the reaction force electromagnetic directional valve 14, there are respectively an acting force loading pressure transmitter 16 and a reaction force loading pressure transmitter 15. At the inlet of the acting force electromagnetic directional valve 13, there is an acting force variable loading local pressure gauge 11 and an acting force fixed loading pressure transmitter 12. At the inlet of the reaction force electromagnetic directional valve 14, there is a reaction force variable loading local pressure gauge 9 and a reaction force variable loading pressure transmitter 10.

[0031] The acting force electromagnetic directional valve 13 is also connected to the reaction force electromagnetic directional valve 14 and is connected to the oil tank 18 via a duplex filter 6.

[0032] As a further option of this embodiment, the present invention further includes a standby motor 21, a standby acting force oil pump 22, and a standby reaction force oil pump 23. The standby motor 21 is connected to the outlet of the acting force double gear oil pump 2 through the standby acting force oil pump 22, and the standby motor 21 is connected to the outlet of the reaction force double gear oil pump 3 through the standby reaction force oil pump 23.

[0033] As Figure 2 , Figure 3 and Figure 4 shown, the working principle of the present invention is as follows:

[0034] I. The process of lifting the grinding roller: The process of lifting the grinding roller is that the coal mill is adjusted from the shutdown state to the standby state before operation. At this time, the oil pump motor 1 is started, and the oil pump motor 1 drives the acting force double gear oil pump 2 and the reaction force double gear oil pump 3 to operate. The opening of the acting force proportional overflow valve 8 is zero, and the opening of the reaction force proportional overflow valve 7 is 50%. At this time, the acting force variable loading local pressure gauge 11 and the acting force fixed loading pressure transmitter 12 show zero, and the reaction force variable loading local pressure gauge 9 and the reaction force variable loading pressure transmitter 10 feedback greater than or equal to 2.5 MPa. The hydraulic oil in the acting force circuit returns to the oil tank 18 via the acting force electromagnetic directional valve 13, and the hydraulic oil in the reaction force circuit enters the rodless cavities of the first hydraulic cylinder 171, the second hydraulic cylinder 172, and the third hydraulic cylinder 173 respectively through a three-way pipeline.

[0035] After the rodless chamber strokes of the first hydraulic cylinder 171, the second hydraulic cylinder 172, and the third hydraulic cylinder 173 are in place, energize the acting force electromagnetic directional control valve 13 and the reaction force electromagnetic directional control valve 14 to lock the pressure in the oil pipes of the cylinders, achieve fixed load online switching, maintain the system in the fixed load mode, keep the internal pressure of the system self-maintained, and the hydraulic oil pumped out by the acting force double gear oil pump 2 and the reaction force double gear oil pump 3 directly returns to the oil tank 18 via the acting force electromagnetic directional control valve 13 and the reaction force electromagnetic directional control valve 14. Stop the oil pump motor 1, and thus the process of lifting the grinding roller is completed.

[0036] II. Process of lowering the grinding roller: The process of lowering the grinding roller is from the standby state of the coal mill to the shutdown state. At this time, start the oil pump motor 1, and the oil pump motor 1 drives the acting force double gear oil pump 2 and the reaction force double gear oil pump 3 to operate. Since the acting force electromagnetic directional control valve 13 and the reaction force electromagnetic directional control valve 14 are energized, the hydraulic oil pumped out by the acting force double gear oil pump 2 and the reaction force double gear oil pump 3 directly returns to the oil tank 18 via the electromagnetic directional control valve. De-energize the acting force electromagnetic directional control valve 13 and the reaction force electromagnetic directional control valve 14, and switch the oil circuit of the electromagnetic directional control valve to the proportional overflow valve circuit, and set the opening of the reaction force proportional overflow valve 7 to zero. At this time, the rodless chambers of the first hydraulic cylinder 171, the second hydraulic cylinder 172, and the third hydraulic cylinder 173 start to empty due to the self-weight of the grinding roller and the loading frame, and the hydraulic oil in the rodless chambers returns to the oil tank via the double-barrel filter 6.

[0037] After the rodless chamber stroke is in place, stop the hydraulic station oil pump motor 1, and thus the process of lowering the grinding roller ends.

[0038] III. Process of loading the grinding roller: The process of loading the grinding roller is from the standby position of the coal mill to the normal working state. Start the oil pump motor 1. At this time, since the acting force electromagnetic directional control valve 13 and the reaction force electromagnetic directional control valve 14 are in the energized position, the hydraulic oil directly returns to the oil tank 18 after passing through the electromagnetic directional control valve. De-energize the acting force electromagnetic directional control valve 13 and the reaction force electromagnetic directional control valve 14, and switch the oil circuit of the hydraulic station back to be controlled by the reaction force proportional overflow valve 7 and the acting force proportional overflow valve 8. According to the coal feeding amount of the coal mill, the opening of the acting force proportional overflow valve 8 increases from 0% to 30%, and the rodless chambers of the first hydraulic cylinder 171, the second hydraulic cylinder 172, and the third hydraulic cylinder 173 start to be filled with hydraulic oil. Since the power of the acting force oil pump is much higher than that of the reaction force oil pump, the pressure in the rodless chamber is higher than that in the rodless chamber, the piston of the cylinder moves downward, and the pull rod, the loading frame, and the grinding roller connected to the piston move downward. Adjust the opening of the proportional overflow valve to set the acting force proportional overflow valve 8 to 413161 and the reaction force proportional overflow valve 7 to 313161. At this time, the grinding roller operates under the variable load condition.

[0039] Energize the acting force electromagnetic directional control valve 13 and the reaction force electromagnetic directional control valve 14, switch the oil circuit to operate under the fixed load condition, and stop the oil pump motor 1. Thus, the process of loading the grinding roller ends.

[0040] 4. Roller unloading process: Roller unloading process is the process of coal mill from working position to standby position. Start the oil pump motor 1, at this time, the oil pump motor 1 drives the action force double gear oil pump 2 and the reaction force double gear oil pump 3 to run, but because the action force electromagnetic reversing valve 13 and the reaction force electromagnetic reversing valve 14 are in the energized state, the hydraulic oil directly returns to the oil tank 18 through the electromagnetic reversing valve. The electromagnetic reversing valve is powered off, and the hydraulic station is switched to the variable loading mode controlled by the proportional relief valve online, and the action force proportional relief valve 8 is opened to 0%, and the reaction force proportional relief valve 7 is opened to 50%. The pressure in the rodless cavity of the first hydraulic cylinder 171, the second hydraulic cylinder 172 and the third hydraulic cylinder 173 increases, and the hydraulic oil in the rod cavity returns to the oil tank after passing through the action force proportional relief valve 8 and the double cylinder filter 6. After the piston in the rodless cavity is lifted to the right position, the action electromagnetic reversing valve 13 and the reaction electromagnetic reversing valve 14 are energized again, the oil pipeline behind the electromagnetic reversing valve is kept under pressure, and the constant loading operation is performed. The hydraulic station loading oil pump motor 1 is stopped, and the coal mill roller unloading process is completed.

[0041] 5. An action force loading pressure transmitter 16 and a reaction force loading pressure transmitter 15 are arranged behind the action force electromagnetic reversing valve 13 and the reaction force electromagnetic reversing valve 14 to ensure that the pressure deviation corresponding to the pressure and the coal feed rate curve during fixed loading operation is ≤3MPa. When the pressure deviation is too large, the motor starts automatically and the electromagnetic reversing valve automatically loses power, and the fixed loading is switched online to the variable loading condition to ensure the safe and stable operation of the equipment.

[0042] The preferred embodiments of the present invention disclosed above are only used to help explain the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific implementation methods described. Obviously, many modifications and changes can be made according to the content of this specification. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that people in the relevant technical field can well understand and use the present invention. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A hydraulic fixed and variable loading on-line switching system for a wheel type medium-speed coal mill, comprising a hydraulic oil tank (18), characterized in that: The hydraulic oil tank (18) is provided with two oil outlets and seven oil return ports. The two oil outlets are respectively connected to the acting force oil circuit and the reaction force oil circuit. An acting force double gear oil pump (2) and a reaction force double gear oil pump (3) are arranged in the hydraulic oil tank (18). The acting force oil circuit includes a pump motor (1). The pump motor (1) is sequentially connected to an acting force electromagnetic directional valve (13) through an acting force double gear oil pump (2), an acting force fixed load safety overflow valve (4) and an acting force proportional overflow valve (8). The acting force electromagnetic directional valve (13) is respectively connected to the oil return ports of a first hydraulic cylinder (171), a second hydraulic cylinder (172), a third hydraulic cylinder (173) and the hydraulic oil tank (18). The reaction force circuit includes a pump motor (1). The pump motor (1) is sequentially connected to a reaction force electromagnetic directional valve (14) through a reaction force double gear oil pump (3), a reaction force fixed load safety overflow valve (5) and a reaction force proportional overflow valve (7). The reaction force electromagnetic directional valve (14) is respectively connected to the oil return ports of a first hydraulic cylinder (171), a second hydraulic cylinder (172), a third hydraulic cylinder (173) and the hydraulic oil tank (18). An acting force loading pressure transmitter (16) and a reaction force loading pressure transmitter (15) are respectively arranged at the outlets of the acting force electromagnetic directional valve (13) and the reaction force electromagnetic directional valve (14). An acting force variable loading local pressure gauge (11) and an acting force fixed loading pressure transmitter (12) are arranged at the inlet of the acting force electromagnetic directional valve (13). A reaction force variable loading local pressure gauge (9) and a reaction force variable loading pressure transmitter (10) are arranged at the inlet of the reaction force electromagnetic directional valve (14). The acting force electromagnetic directional valve (13) is also connected to the reaction force electromagnetic directional valve (14) and is connected to the oil tank (18) through a double - barrel filter (6). Roller lifting process: The roller lifting process is when the coal mill is adjusted from the shutdown state to the standby state before operation. At this time, the oil pump motor (1) is started, and the oil pump motor (1) drives the action force double gear oil pump (2) and the reaction force double gear oil pump (3) to operate. The opening of the action force proportional relief valve (8) returns to zero, and the opening of the reaction force proportional relief valve (7) is 50%. At this time, the action force variable load on-site pressure gauge (11) and the action force fixed load pressure transmitter (12) display zero, and the reaction force variable load on-site pressure gauge (9) and the reaction force variable load pressure transmitter (10) feedback is greater than or equal to 2.5MPa. The hydraulic oil of the action force circuit returns to the oil tank (18) through the action force electromagnetic reversing valve (13), and the hydraulic oil of the reaction force circuit enters the oil tank (18) through the three-point pipeline. The rodless chambers of the first hydraulic cylinder (171), the second hydraulic cylinder (172) and the third hydraulic cylinder (173) are in place; after the rodless chambers in the first hydraulic cylinder (171), the second hydraulic cylinder (172) and the third hydraulic cylinder (173) have reached their strokes, the action electromagnetic reversing valve (13) and the reaction electromagnetic reversing valve (14) are powered on, the pressure in the cylinder pipeline is locked, and the fixed loading online switching is realized, the system is maintained in the fixed loading mode, the internal pressure of the system is self-maintained, and the hydraulic oil pumped out by the action force double gear oil pump (2) and the reaction force double gear oil pump (3) is directly returned to the oil tank (18) via the action force electromagnetic reversing valve (13) and the reaction force electromagnetic reversing valve (14); the oil pump motor (1) is stopped, and the roller lifting process is completed; Roller lowering process: Roller lowering process is when the coal mill is released from standby state to shutdown state. At this time, the oil pump motor (1) is started, and the oil pump motor (1) drives the action force double gear oil pump (2) and the reaction force double gear oil pump (3) to operate. Since the action force electromagnetic reversing valve (13) and the reaction force electromagnetic reversing valve (14) are energized, the hydraulic oil pumped out by the action force double gear oil pump (2) and the reaction force double gear oil pump (3) directly returns to the oil tank (18) through the electromagnetic reversing valve, and the action force electromagnetic reversing valve (13) and the reaction force electromagnetic reversing valve (14) are energized. (13) The reaction force electromagnetic reversing valve (14) is powered off, and the electromagnetic reversing valve oil circuit is switched to the proportional relief valve circuit, and the opening of the reaction proportional relief valve (7) is reset to zero. At this time, the rodless chambers of the first hydraulic cylinder (171), the second hydraulic cylinder (172) and the third hydraulic cylinder (173) begin to be emptied due to the dead weight of the grinding roller and the loading frame, and the hydraulic oil in the rodless chamber returns to the oil tank through the double-tube filter (6); after the rodless chamber stroke is in place, the hydraulic station oil pump motor (1) is stopped, and the grinding roller lowering process is completed; Roller loading process: The roller loading process is the process of the coal mill from the standby position to the normal working state. The oil pump motor (1) is started. At this time, because the action force electromagnetic reversing valve (13) and the reaction force electromagnetic reversing valve (14) are in the energized position, the hydraulic oil returns directly to the oil tank (18) after passing through the electromagnetic reversing valve; the action force electromagnetic reversing valve (13) and the reaction force electromagnetic reversing valve (14) are powered off, and the hydraulic station oil circuit is switched back to the reaction proportional relief valve (7) and the action proportional relief valve (8) for control. According to the coal feed amount of the coal mill, the opening of the action force proportional relief valve (8) increases from 0% to 3%. 0%, hydraulic oil begins to be injected into the rod chambers of the first hydraulic cylinder (171), the second hydraulic cylinder (172) and the third hydraulic cylinder (173). Because the power of the action force oil pump is much higher than that of the reaction force oil pump, the pressure in the rod chamber is higher than that in the rodless chamber. The cylinder piston moves downward, and the pull rod connected to the piston, the loading frame and the grinding roller move downward. The opening of the proportional relief valve is adjusted. At this time, the grinding roller is in a variable loading condition. The action force electromagnetic reversing valve (13) and the reaction force electromagnetic reversing valve (14) are powered on, the oil circuit is switched to a fixed loading condition, and the oil pump motor (1) is stopped. At this point, the grinding roller loading process ends. Roller unloading process: Roller unloading process is the process of the coal mill from the working position to the standby position. The oil pump motor (1) is started. At this time, the oil pump motor (1) drives the action force double gear oil pump (2) and the reaction force double gear oil pump (3) to operate. The action force electromagnetic reversing valve (13) and the reaction force electromagnetic reversing valve (14) are in a powered state. The hydraulic oil directly returns to the oil tank (18) through the electromagnetic reversing valve. The electromagnetic reversing valve is powered off, and the hydraulic station is switched online to the variable loading mode controlled by the proportional relief valve. The action force proportional relief valve (8) is opened to 0% and the reaction force is turned off. The proportional relief valve (7) is opened to 50%, the pressure in the rodless chambers of the first hydraulic cylinder (171), the second hydraulic cylinder (172) and the third hydraulic cylinder (173) increases, and the hydraulic oil in the rod chamber returns to the oil tank after passing through the force proportional relief valve (8) and the double-tube filter (6); after the piston in the rodless chamber is lifted to the right position, the force electromagnetic reversing valve (13) and the reaction force electromagnetic reversing valve (14) are re-energized, the oil pipeline behind the electromagnetic reversing valve maintains pressure, and the constant loading operation is carried out; the hydraulic station loading oil pump motor (1) is stopped, and the coal mill roller unloading process is completed; An action force loading pressure transmitter (16) and a reaction force loading pressure transmitter (15) are arranged behind the action force electromagnetic reversing valve (13) and the reaction force electromagnetic reversing valve (14) to ensure that the pressure deviation corresponding to the pressure and the coal feeding amount curve during fixed loading operation is ≤3MPa. When the pressure deviation is too large, the motor starts automatically and the electromagnetic reversing valve automatically loses power, and the fixed loading is switched to the variable loading condition online to ensure the safe and stable operation of the equipment. The first hydraulic cylinder (171), the second hydraulic cylinder (172), and the third hydraulic cylinder (173) are respectively connected to the first accumulator (191), the second accumulator (192), and the third accumulator (193). The first hydraulic cylinder (171), the second hydraulic cylinder (172), and the third hydraulic cylinder (173) are respectively connected to the first pressure relief safety valve (201), the second pressure relief safety valve (202), and the third pressure relief safety valve (203). The hydraulic fixed and variable loading on-line switching system of the wheel type medium-speed coal mill further includes a standby motor (21), a standby working oil pump (22), and a standby reaction oil pump (23). The standby motor (21) is connected to the outlet of the working double gear oil pump (2) through the standby working oil pump (22), and the standby motor (21) is connected to the outlet of the reaction double gear oil pump (3) through the standby reaction oil pump (23).

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