Practical methods for fault diagnosis and troubles

2022-09-22
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Practical methods for fault diagnosis and troubleshooting of hydraulic system of construction machinery

1. General principles of fault diagnosis of hydraulic system

correct analysis of faults is the premise for troubleshooting. Most system faults do not occur suddenly, and there are always omens before they occur. When the omens develop to a certain extent, faults will occur. The causes of the failure are various, and there is no fixed law to find. Statistics show that about 90% of the faults of hydraulic system are caused by poor use and management. In order to diagnose faults quickly, accurately and conveniently, we must fully understand the characteristics and laws of hydraulic faults, which is the basis of fault diagnosis

the following principles should be followed in fault diagnosis:

(1) first of all, judge him to make a rough statistics to show whether the working conditions and peripheral environment of the hydraulic system are normal. First, find out whether it is the fault of the mechanical part of the equipment or the electrical control part, or the fault of the hydraulic system itself. At the same time, find out whether the various conditions of the hydraulic system meet the requirements of normal operation

(2) area judgment determine the area related to the fault according to the fault phenomenon and characteristics, gradually reduce the scope of the fault, detect the components in this area, analyze the causes, and finally find out the specific location of the fault

(3) master the fault type and conduct comprehensive analysis. According to the final phenomenon of the fault, gradually find out a variety of direct or indirect possible causes. In order to avoid blindness, we must conduct comprehensive analysis and logical judgment according to the basic principles of the system, reduce the gradual approach of the suspected object, and finally find out the fault location

(4) fault diagnosis is based on operation records and some system parameters. Establish system operation records, which is the scientific basis for preventing, discovering and dealing with faults; Establish the equipment operation fault analysis table, which is a high summary of the use experience, and is helpful to quickly judge the fault phenomenon; It has certain detection means and can make accurate quantitative analysis of faults

(5) when verifying possible fault causes, generally start from the most likely fault cause or the place that is most easy to check, which can reduce the workload of assembly and disassembly and improve the diagnosis speed

2. Fault diagnosis method

at present, the traditional method to find the fault of hydraulic system is logical analysis to approach the fault step by step. The basic idea of this method is comprehensive analysis and conditional judgment. That is, the maintenance personnel can judge the cause of the fault by experience through observation, listening, touching, simple testing and understanding of the hydraulic system. When the hydraulic system fails, there are many possible causes of the failure. Using the method of logic algebra, the possible fault causes are listed, and then the logical judgment is carried out one by one according to the principle of "easy before difficult", and the fault causes and specific conditions causing the fault are finally found out

this method requires maintenance personnel to have basic knowledge of hydraulic system and strong analysis ability in the process of fault diagnosis, so as to ensure the efficiency and accuracy of diagnosis. However, the diagnosis process is cumbersome, which requires a lot of inspection and verification, and can only be analyzed qualitatively. The fault cause of diagnosis is not accurate enough. In order to reduce the blindness and experience of system fault detection and the workload of disassembly and assembly, the traditional fault diagnosis method is far from meeting the requirements of modern hydraulic system

in recent years, with the development of hydraulic system towards large-scale, continuous production and automatic control, a variety of modern fault diagnosis methods have emerged. For example, ferrography technology can judge the quantity, shape, size, composition and distribution law of various abrasive particles separated from the oil, and timely and accurately judge the wear position, form and degree of components in the system. Moreover, quantitative pollution analysis and evaluation of hydraulic oil can be carried out to achieve detection and fault prevention. Another example is the expert diagnosis system based on artificial intelligence, which simulates the problem-solving methods of experienced experts in a certain field through computers. The fault phenomenon is input into the computer through the man-machine interface. The computer can calculate the cause of the fault according to the input phenomenon and the knowledge in the knowledge base, and then output the cause through the man-machine interface, and put forward the maintenance plan or preventive measures. These methods bring broad prospects for hydraulic system fault diagnosis and lay a foundation for hydraulic system fault diagnosis automation. However, most of these methods require expensive detection equipment and complex sensor control system and computer processing system, and some methods are difficult to study. At present, it is not suitable for on-site promotion. A simple and practical fault diagnosis method of hydraulic system is introduced below

2.1 fault diagnosis system based on parameter measurement

whether a hydraulic system works normally depends on whether the two main working parameters, namely, pressure and flow, are in normal working state, and whether the parameters such as system temperature and actuator speed are normal at the same time. The fault phenomenon of hydraulic system is various, and the fault reason is also the synthesis of many factors. The same factor may cause different fault phenomena, and the same fault may correspond to many different causes. For example, oil pollution may cause hydraulic system pressure, flow or direction faults, which brings great difficulties to hydraulic system fault diagnosis

the idea of fault diagnosis with parameter measurement method is as follows: when any hydraulic system works normally, the system parameters work near the design and set values. If these parameters deviate from the predetermined values during work, the system will fail or may fail. That is, the essence of the failure of the hydraulic system is the abnormal change of the working parameters of the system. Therefore, when the hydraulic system fails, it must be a certain component or some components in the system that have failed. Further, it can be concluded that the parameters of a certain point or points in the circuit have deviated from the predetermined value. This indicates that if the working parameters of a certain point in the hydraulic circuit are abnormal, the system has failed or may have failed, and the maintenance personnel need to deal with it immediately. In this way, on the basis of parameter measurement, combined with logical analysis method, the fault can be found quickly and accurately. Parameter measurement method can not only diagnose system faults, but also predict possible faults, and this prediction and diagnosis are quantitative, which greatly improves the speed and accuracy of diagnosis. This kind of detection is direct measurement, with fast detection speed, small error and simple detection equipment, which is easy to be popularized and used in the production site. It is suitable for the detection of any hydraulic system. During measurement, there is no need to shut down and damage the hydraulic system. Almost any part of the system can be detected. It can not only diagnose existing faults, but also monitor and predict potential faults

2.1.1 principle of parameter measurement method

as long as the working parameters at any point required in the hydraulic system circuit are measured and compared with the normal value of the system operation, we can judge whether the system working parameters are normal, whether there is a fault and where the fault is located

the working parameters in the hydraulic system, such as pressure, flow, temperature, etc., are non electrical physical quantities. When using general instruments to measure indirectly, these non electrical quantities need to be converted into electrical quantities by using physical effects first, and then processed by amplification, conversion and display. The measured parameters can be represented and displayed by the converted electrical signals. From this, we can judge whether there is a fault in the hydraulic system. However, this indirect measurement method requires various sensors, the detection device is more complex, and the measurement result error is large and not intuitive, which is not convenient for field promotion and use

through years of teaching and production practice, I designed a simple and practical hydraulic system fault detection circuit. The detection circuit is usually connected in parallel with the tested system. This connection requires a double ball valve tee joint set at the tested point, which is mainly used for non disassembly detection of the system. It can directly and quickly detect various parameters of the required points of the hydraulic system without any sensors. It can simultaneously detect the three parameters of pressure, flow and temperature in the system, and the speed and speed of the actuator can be calculated by measuring the outlet flow. For example, as long as the double ball valve tee is installed at the pump outlet and the inlet and outlet of the actuator, the approximate position of the fault (pump source, control transmission part or actuator part) can be immediately diagnosed by measuring the pressure, flow and temperature values at points 1, 2 and 3. By adding parameter detection points, the fault area can be reduced

when the system works normally, valve 1 is opened and valve 2 is closed, and the dust cover is installed on the detection mask to prevent pollution. During detection, as long as the detection circuit is connected with the detection port, tighten the union thread and open valve 2. By adjusting valve 1 and overflow valve 7, parameters such as pressure, flow, temperature and speed can be easily measured. However, when the system piping is required, the double ball valve tee shall be configured as a nozzle or elbow joint at the position where the system parameters need to be detected

1,2. Globe valve 3, 8 Hose 4 Pressure gauge 5 Flowmeter 6 Thermometer 7 Overflow valve 9 Filter

2.1.2 parameter measurement method

step 1: to measure the pressure, first screw the hose connector of the detection circuit and the threaded joint of the double ball valve tee. Open the ball valve 2, close the overflow valve 7, and cut off the oil return channel. At this time, the pressure value of the measured point (the actual working pressure of the system) can be directly read out from the pressure gauge 4

but it is scattered intensively

step 2: measure the flow and temperature, slowly loosen the handle of overflow valve 7, and then close ball valve 1. Readjust the overflow valve 7 so that the reading of pressure gauge 4 is the measured pressure value. At this time, the reading of flowmeter 5 is the actual flow value of the measured point. At the same time, the oil temperature value can be displayed on thermometer 6

step 3: measure the speed (speed) no matter the pump, motor or cylinder, its speed or speed only depends on two factors, namely, the flow and its own geometric size (displacement or area), so as long as the output flow of the motor or cylinder (for the pump, it is the input flow), divide it by its displacement or area to get the speed or speed value

2.2 example of parameter measurement method

the following phenomena occurred in the commissioning of this system: the pump can work, but the pressure of the high-pressure pump supplying the mold closing cylinder and injection cylinder does not go up (the pressure is adjusted to about 8.0MPa, and can no longer be adjusted up), the pump has slight abnormal mechanical noise, the water cooling system works, the oil temperature and oil level are normal, and there is oil return

from the circuit analysis, there are the following possible causes of the fault:

(1) overflow valve fault. Possible causes: incorrect adjustment, spring yielding, blocked damping hole, stuck spool valve

(2) fault of electrohydraulic directional valve or electrohydraulic proportional valve. Possible causes: the return spring is broken, the control pressure is not enough, the slide valve is stuck, and the control part of the proportional valve is faulty

(3) hydraulic pump failure. Possible causes: the pump speed is too low, the vane pump stator is abnormally worn, the seal is damaged, a large amount of air enters the pump suction, and the filter is seriously blocked

fault diagnosis method:

(1) apply the traditional logical analysis step-by-step approximation method. It is necessary to analyze, judge and test all the possible causes one by one, and finally find out the cause of the fault and the specific components causing the fault. The diagnosis process of this method is cumbersome and requires a lot of assembly, disassembly and verification work. It has low efficiency and long construction period. It can only be qualitative analysis, and the diagnosis is not accurate enough

(2) apply the fault diagnosis system based on parameter measurement. When piping the system, set double ball valves and tees at the outlet a of the pump, the front B of the reversing valve and the inlet C of the cylinder, and then use the fault diagnosis and detection circuit to limit the system fault to a certain area and root in a few seconds

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