In recent years, free forging hydraulic presses of different specifications have been widely used in various metallurgical industries and other fields in China, and the related operation and maintenance management of equipment has also been upgraded accordingly. However, based on actual research, it can be found that some workers still lack in the operation and management of the hydraulic system of the hydraulic press, resulting in frequent failures and hidden dangers in the hydraulic system, which seriously affects equipment and production. To change this situation, this article focuses on the specific research of fault diagnosis and maintenance of the hydraulic system of the free forging hydraulic press.

1. Composition of hydraulic system for free forging hydraulic press

1.1 Inlet and return control valve block

In the hydraulic system of the free forging hydraulic press, the main cylinder inlet valve block is the main control valve block component, consisting of an integrated proportional flow control valve. When the hydraulic press is running, the hydraulic system can be pressurized through this flow control valve to fully ensure the safe and stable operation of the press master cylinder, and to improve the pressure relief speed of the master cylinder. At the same time, the hydraulic system includes a return cylinder control valve block, which can accurately control the inlet and outlet of the return cylinder and provide necessary back pressure for its moving parts during the operation of the press.

1.2 Liquid filling system

The filling system in the hydraulic system mainly consists of a buffer tank, a filling tank, a filling valve block, etc. The filling tank can replenish oil to the main cylinder through the filling valve when the press rapidly descends, preventing the main cylinder from experiencing poor phenomena such as suction during the rapid descent process. The buffer tank is usually located between the filling tank and the filling valve block, which can buffer the flow of filling and discharging liquid. The buffer tank has the application advantage of controlling the amount of filling and discharging liquid. If there is a large amount of filling and discharging liquid, the buffer tank can achieve smooth filling and discharging of large flow.

1.3 Main control valve station

In the hydraulic system of a free forging hydraulic press, the main control valve station is mainly composed of different control valve blocks, which are usually installed near the execution cylinder, press, and basement next to the press. These valve blocks can effectively reduce the impact vibration generated during the operation of the press. And it can also scientifically and reasonably control parameters such as the compression volume of high-pressure liquid in the compressor and the length of high-pressure pipelines, making the operation of the compressor smoother and preventing potential safety hazards during operation.

2. Common examples and analysis of hydraulic action abnormal faults

In a certain case, a hydraulic system experienced frequent pressure fluctuations during actual operation, resulting in abnormal hydraulic action. Specifically, the system pressure fluctuates up and down around the set value, with a fluctuation amplitude of ± 13%, and the fluctuation frequency is relatively high, 8-10 times per minute. This unstable pressure condition makes it difficult to control the size and shape of forging blanks, seriously affecting production efficiency and product quality.

In the handling of the fault, the staff immediately checked whether there were any abnormal conditions in the hydraulic system, and observed the fluctuation of the pressure gauge pointer, recording the amplitude and frequency of the fluctuation. Afterwards, the oil level in the fuel tank was checked and it was found to be normal without any abnormal decrease. Finally, carefully check the pressure at key positions such as the outlet of the hydraulic pump and the inlet and outlet of the hydraulic cylinder, and measure specific parameter values.

In order to further determine whether the hydraulic system malfunction is caused by the relief valve, the staff designed an experiment to test the working status of the relief valve. Remove the overflow valve from the system, connect it to the testing circuit, apply the set pressure, and observe the action of the valve core. The experimental results showed that the valve core was stuck and unable to adjust the pressure normally, indicating that the fault was caused by the overflow valve. Further disassembly and analysis revealed that there was wear and solid particle contamination on the surface of the overflow valve core. Afterwards, the cleanliness of the hydraulic oil was checked, and it was found that there were a large number of metal particles and impurities in the hydraulic oil, which caused severe oil pollution and resulted in wear and jamming of the valve core, leading to malfunctions.

After determining the cause of the malfunction, the maintenance personnel replaced the overflow valve as soon as possible. The new overflow valve was replaced to ensure that the valve core acted flexibly without any jamming. Install the overflow valve into the system and conduct a functional test to ensure stable pressure. At the same time, discharge the old oil from the hydraulic system, thoroughly clean the oil tank, replace it with new and clean hydraulic oil, and ensure that the hydraulic oil meets the system requirements. Not only that, hydraulic oil filters have also been installed to improve the cleanliness of the hydraulic oil and prevent further contamination. After the maintenance is completed, a comprehensive debugging of the hydraulic system is carried out to ensure that all components work properly.

By comparing the data before and after maintenance, it can be seen that the hydraulic system pressure is more stable after maintenance, and the deviation between the actual pressure values and the set values at each monitoring point is significantly reduced. The stability of the hydraulic system has been significantly improved, effectively reducing the probability of hydraulic system failures and abnormal hydraulic actions.

3. Key points for hydraulic system fault diagnosis and maintenance

3.1 Maintenance of oil tank and filling system faults in hydraulic system

In the hydraulic system of a free forging hydraulic press, the oil tank and filling system are important components of the hydraulic system. If a fault occurs, it will directly affect the normal operation of the hydraulic pump, control valve, and even the entire hydraulic system of the hydraulic press. Therefore, it is necessary to strengthen its fault diagnosis and maintenance.

Firstly, measures should be taken to prevent faults such as fuel tank damage and pipeline oil leakage. Carefully inspect the oil tank body, air filter, and other components to prevent contamination of the hydraulic oil in the tank from affecting the entire hydraulic system. At the same time, pipeline connections and other parts are also high-risk areas for faults. Timely inspection and analysis should be conducted to determine whether there are any adverse conditions such as loose fasteners, aging or damaged seals, or damaged welds in the pipeline. If a malfunction is found, it is necessary to repair or replace the oil tank in a timely manner, regularly replace the air filter element of the oil tank, replace damaged seals and pipelines, and re tighten them. Close attention should be paid to the cleanliness of the hydraulic oil in the oil tank, thereby reducing adverse effects on the normal operation of the system. When checking the cleanliness of hydraulic oil in the oil tank, a hydraulic oil online monitoring system FCU pollution monitor can be installed. If the impurities such as moisture and solid particles in the hydraulic oil exceed the standard, measures must be taken in a timely manner, such as replacing the filter element at the suction port and using an oil filter to remove the pollutants in the hydraulic oil tank as soon as possible. If air or gas is mixed into the oil, it is easy to cause bubbles in the hydraulic system, which should be eliminated in a timely manner. In addition, if the hydraulic oil has undergone chemical changes, it should be prohibited from use. Filter the clean oil and inject it into the oil tank for 15-20 minutes until the clean hydraulic oil is discharged. Then add the hydraulic oil in the main oil tank to the required level.

Secondly, in addition to the fuel tank, the filling system is also an important component of the hydraulic system and requires diagnosis and maintenance of corresponding faults. Firstly, if the filling pipeline is too long and has a large diameter, it should be adjusted in a timely manner, otherwise the filling system will generate significant impact and vibration during drainage or filling, which will cause damage to the pipeline to a certain extent. At this point, the fault repair personnel can readjust the placement of the buffer tank to reduce vibration and damage to the filling pipeline. Secondly, due to factors such as improper pressure and liquid level control, the filling system is prone to problems such as high flow and pressure relief. Maintenance technicians can install sensors at locations such as low-pressure compensators and filling tanks in the hydraulic system to implement dual control and safety protection for the system's liquid level and pressure. At the same time, specialized compensation systems and intelligent oil discharge systems can be set up to intelligently and automatically adjust the liquid and air pressure in the filling tank, as well as the ratio between the two, ensuring that the hydraulic oil can be maintained within the standard range of liquid level and pressure values, so that the filling system and hydraulic system can work normally and safely. In addition, it is necessary to prevent overloading and overflow of the filling system. During the operation of the free forging hydraulic press, the deformation resistance of the workpiece should be controlled to avoid the deformation resistance value of the billet exceeding the equipment load capacity, and to prevent overloading from causing adverse effects or even damage to the hydraulic system such as the return and pressure relief of the hydraulic press.

3.2 Diagnosis of Abnormal Pressure between Main Pump and Cylinder in Hydraulic System

The main pump and oil cylinder in the hydraulic system of the free forging hydraulic press are prone to malfunction. Firstly, in hydraulic systems, hydraulic cylinders may experience low oil pressure, leading to abnormal cylinder pressure and movement. This type of malfunction is mainly caused by abnormal pressure in the main pump and abnormal pipelines and pressure control valves in the oil cylinder. For example, if the working pressure of the main pump is much lower than the rated pressure, there may be excessive throttling resistance in the control pipeline, or if the pressure control valve fails or the control pressure cannot be adjusted properly. At this point, the troubleshooting personnel need to carefully test the control pressure of each part and readjust the pressure to a reasonable range. At the same time, hydraulic cylinders may experience abnormal faults with slow initial movement, mainly caused by temperature factors. When the external temperature is low, the viscosity of hydraulic oil will increase, and the flow of hydraulic oil will slow down, resulting in slow action of the hydraulic cylinder. During maintenance and diagnosis, hydraulic oil with stronger viscosity and temperature performance can be used to ensure that it can increase the starting oil temperature under low temperature conditions with the help of corresponding heating equipment, ultimately improving the starting speed. Secondly, in terms of various main hydraulic pumps in the hydraulic system, it is possible to diagnose and repair the inlet and outlet pressures of hydraulic pumps, as well as faults in pump components. Carefully check whether the pressure at the inlet and outlet of the oil is normal, whether there is air entering, and if there is air entering, exhaust it in a timely manner. After confirming that there is no air entering, check whether there is any abnormal noise, such as adding lubricating grease to the bearing area, and then carefully observe the size of the pump noise. For the parts inside the pump, the clearance between the parts should be adjusted reasonably to prevent faults such as the inability to increase the operating pressure of the pump. At the same time, quality management of component assembly should be strengthened to avoid hydraulic pump failures.

3.3 Reasonable selection of hydraulic system fault diagnosis and maintenance methods

There are many types of faults in the hydraulic system of the free forging hydraulic press, and targeted fault diagnosis and maintenance methods should be adopted.

Firstly, common faults in hydraulic systems can be classified into random faults and deterministic faults based on their specific nature and characteristics. According to the indication form, it can be divided into alarm and no alarm display, etc. The hydraulic system of the free forging hydraulic press is complex, and the faults in the hydraulic system are characterized by strong concealment, sporadic occurrence, and complexity. When diagnosing, analyzing, and repairing faults, it is necessary to fully understand and master the functions of each component in the hydraulic system and the entire hydraulic system. For example, the hydraulic schematic diagram of the hydraulic system, the working principle and parameters of hydraulic components, and the correlation between component parameters and hydraulic actions in the actual hydraulic system can be compared and analyzed. Then, combined with professional fault diagnosis capabilities, complex and hidden fault causes can be identified.

Secondly, targeted diagnosis and maintenance methods can be selected based on the possible phenomena that may occur in the hydraulic system when component failures occur. Under normal circumstances, common faults in hydraulic systems mainly include pressure imbalance and vibration noise. At this time, hydraulic system faults require the coordination of mechanical and electrical systems for inspection. The same fault cause can lead to multiple different fault phenomena, and the same fault cause is also more complex. The fault phenomena of unstable pressure and vibration noise may occur simultaneously. When diagnosing and repairing faults, multiple symptoms can be combined to locate the source of the fault. For example, the fault source can be located by setting a reasonable detection order, using a sequence from simple to complex or following a primary to secondary order, which can improve the timeliness of hydraulic system fault diagnosis, maintenance, and handling [2]. For example, in cases where the cause of the malfunction is unknown, simple and easy to disassemble components in the hydraulic system can be inspected first, followed by complex components such as motors and hydraulic cylinders. Finally, suspicious fault points can be further examined to identify the cause of the malfunction.

In addition, for more complex hydraulic system faults, multiple algorithms and physical sensory diagnosis and maintenance can be combined. For example, in terms of physical senses, auscultation and palpation can be used to analyze and determine hydraulic component faults through auditory and tactile senses. Analyze the temperature and vibration of hydraulic control components through sensory analysis to determine if the valve core is disturbed by foreign objects, and then promptly disassemble, clean, or replace the components according to the actual situation. In terms of algorithms, inference verification method and comprehensive judgment method of turning parts into whole can be used. The reasoning verification method mainly refers to actively assuming the cause of the fault, then selecting a reasonable algorithm, continuously verifying and analyzing the conjecture until the verification is passed, and identifying the cause of the fault. In the process of fault diagnosis, different technical methods can be used to examine the same fault problem, and then the final fault diagnosis result can be obtained by comprehensive comparison.

3.4 Strengthen the informationization construction of hydraulic system fault diagnosis and maintenance

In the context of informatization, in the diagnosis and maintenance of hydraulic system faults in free forging hydraulic presses, information devices such as visualized monitoring hardware for forging site production and operation data can be used to achieve fault diagnosis and maintenance of the hydraulic system. Firstly, with the support of information technology software, hydraulic system fault diagnosis and maintenance personnel can more accurately select fault diagnosis and maintenance methods. In order to further improve the application effect, the software's functions and modules can be optimized and designed. For example, a data processing module can be designed according to the actual situation, adding a series of functions such as software database queries, and setting up a query and editing data module based on the C/S structure to achieve clear division of hydraulic system fault data access, which can accurately and effectively improve the efficiency of fault diagnosis for maintenance operators. In hardware devices, abnormal information feedback function can be used, with the help of intermediate conversion devices such as network relays, and data mining, data filtering, data cleaning and other methods can be used to analyze and determine possible abnormal data information in the production site, forging hydraulic press and hydraulic system. The identified abnormal information signals and alarm lights can be connected to each other, and the abnormal fault data information of the hydraulic system can be transmitted to the forging production site to remind on-site personnel to handle it in a timely manner. Abnormal fault data can also be transmitted to different mobile terminal devices, and abnormal warning information can be digitized and visualized, providing convenience for the diagnosis and maintenance of hydraulic system abnormal faults.

4. Conclusion

The operation failure of the free forging hydraulic press will directly affect the quality of the hydraulic system and forging operation. It is necessary to adopt relevant technical points and methods for the main components of the hydraulic system of the free forging hydraulic press, such as the oil tank and filling system, hydraulic pump and hydraulic cylinder, to diagnose and repair the faults of the hydraulic system, and strengthen the preventive and information-based fault diagnosis and maintenance efforts of the hydraulic system to ensure the safe and stable operation of the hydraulic system and hydraulic press.

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