Created on:2021-12-28 10:13

Fault diagnosis and searching method of hydraulic system

Fault diagnosis and searching method of hydraulic system

1. General principles of hydraulic system fault diagnosis

Correct analysis of faults is the premise of troubleshooting. Most system faults do not occur suddenly. There is always an omen before they occur. When the omen develops to a certain extent, faults will occur. The causes of failure are various, and there is no fixed law to follow. Statistics show that about 90% of the faults of the 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, determine whether the working conditions and peripheral environment of the hydraulic system are normal. Firstly, it is necessary to find out whether it is the fault of the mechanical part or electrical control part of the equipment or the fault of the hydraulic system itself, and find out whether various conditions of the hydraulic system meet the requirements of normal operation.

(2) Regional 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 types and conduct comprehensive analysis. According to the final phenomenon of the fault, gradually find m various direct or indirect possible causes. In order to avoid blindness, comprehensive analysis and logical judgment must be carried out according to the basic principles of the system to 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 judge the fault phenomenon quickly; It has certain detection means and can make accurate quantitative analysis of faults.

(5) When verifying the possible fault causes, generally start from the most likely fault causes or the most easily inspected places, which can reduce the assembly and disassembly workload and improve the diagnosis speed.

2. Fault finding method of hydraulic equipment

The following introduces several simple and practical fault diagnosis methods of hydraulic system.

1. Replacement method

Replace (exchange) the same components on the hydraulic equipment of the same type, structure and principle at the same position to prove whether the replaced components work reliably. For example, replace the components of machine a to machine B and start the machine for observation to prove the quality of this component.

The advantage of replacement method is that it can be used to accurately diagnose the faults of hydraulic equipment even if the technical level of repairmen is low. However, the application of this method must be based on the hydraulic equipment with the same type, structure, hydraulic principle and hydraulic components. Therefore, this method has great limitations and blindness.

2. Auxiliary method

With the help of simple auxiliary parts, it can diagnose whether the hydraulic components of hydraulic equipment have faults.

(1) Oil plugging method. If the oil ports of valve components and oil cylinders are blocked, these hydraulic components can be diagnosed for leakage and failure.

(2) Artificial commutation method. By reversing the valve components with the ejector rod, it can diagnose whether the reversing valve is stuck, the valve core is not in place and other faults.

The auxiliary method can diagnose whether there is a fault of hydraulic components without disassembling parts, which avoids too much disassembly workload, reduces the fault diagnosis time, and is convenient for rapid diagnosis, especially for large oil cylinder seals. This kind of fault diagnosis has good practicability.

3. Empirical method

By mastering the hydraulic system of hydraulic equipment, the repairman is familiar with the structure and working principle of each hydraulic element, and makes a comprehensive analysis and comparison of the faults of hydraulic equipment on the basis of accumulating rich experience in the repair of hydraulic equipment, so as to quickly and accurately diagnose the faults of hydraulic equipment. To sum up, the general fault parts and causes of hydraulic equipment can be roughly divided into the following five aspects.

(1) Improper adjustment of hydraulic components, such as incorrect adjustment of pressure and flow of oil pump, motor, sequence valve, direction valve, overflow valve, unloading valve and balance valve.

(2) The sealing element is damaged or impurities make the hydraulic element unable to work normally.

(3) Hydraulic components are worn or damaged, such as sealing failure of valve components, spring failure, excessive clearance, etc.

(4) Failure of control mechanism (electrical appliance), such as relay failure, poor contact or damage of button, incorrect installation of electromagnet, wrong connection of motor phase line and other reasons, resulting in misoperation of hydraulic components or program error.

(5) Failure of auxiliary mechanism, such as improper adjustment or damage of limit switch position, damage of pressure gauge, damage or false signal of pressure relay, filtering problem of oil tank, etc.

4. Sensory method

(1) Look. Observe the working state of the hydraulic system. Generally, there are six Views: one is to see the speed, that is, to see whether the motion speed of the actuator changes; Second, check the pressure, that is, check whether the pressure at each measuring point of the hydraulic system fluctuates; Three look at the oil, that is, to observe whether the oil is clean, whether it is deteriorated, whether the oil quantity meets the requirements, whether the viscosity of the oil meets the requirements and whether the surface has foam or not. Fourth, check the leakage, that is, check whether each joint of the hydraulic system leaks, drips and oil dirt; V. vibration, i.e. whether there are abnormal phenomena such as runout and impact when moving parts such as piston rod or worktable are running; Sixth, look at the products, that is, judge the working state of the moving mechanism from the processed products, and observe the stability of the system pressure and flow.

(2) Listen. Use hearing to judge whether the hydraulic system works normally. Generally, there are four listeners: first, listen to the noise, that is, whether the noise of the hydraulic pump and system is too large, and whether the hydraulic valve and other components scream; Second, listen to the impact sound, that is, listen to whether the impact sound is too loud when the actuator is reversing; Third, listen to the leakage sound, that is, listen to whether there is a slight and continuous sound inside the oil circuit board; Fourth, listen to the knocking sound, that is, listen to whether there is a knocking sound in the hydraulic pump and pipeline.

(3) Touch. Touch the temperature rise and working condition of moving parts with your hand. Generally, there are four touch: first, touch the temperature rise, that is, touch whether the temperature of pump, oil tank and valve body is too high; Second, touch the vibration, that is, touch the moving parts and pipes with your hand for vibration; Three touch crawling, that is, when the workbench runs at a slow speed, touch it with your hand to see if there is crawling; Four touch the tightness, that is, the tightness of the stop iron and microswitch by hand.

(4) Smell. The smell is mainly to smell whether the oil has bad smell.

(5) Check. Inspection refers to consulting technical data and relevant fault analysis and repair records and maintenance records.

(6) Ask. Ask is to ask the equipment operator to understand the normal working conditions of the equipment. There are generally six questions: first, whether the hydraulic system works normally; 2. Ask about the latest replacement date of hydraulic oil, cleaning or replacement of filter screen, etc; Three asked whether the pressure regulating valve or speed regulating valve before the accident was adjusted, there was no abnormal phenomenon. IV. ask whether the hydraulic parts or seals have been replaced before the accident; 5. Ask about the working difference of the hydraulic system before and after the accident; 6. Ask what kind of accidents often occurred in the past and how they were eliminated.

Sensory detection is only a qualitative analysis. If necessary, relevant components should be quantitatively analyzed and tested on the experimental bench.

5. Analytical method

Analysis method is a fault diagnosis method based on the working mechanism of hydraulic system. Many hydraulic system faults can be solved by analysis method, but the requirements for hydraulic system fault analysts are high. They must fully understand and be familiar with the working principle of hydraulic components and circuits.

In the fault analysis, the following problems deserve attention.

(1) Deeply analyze the hydraulic system diagram, combined with the relevant electromagnet action table and relevant circuit diagram, sort out the complete working mechanism of the circuit; At the same time, correctly understand the design intention and idea of the circuit, the technical measures taken and the relevant background.

(2) The working schematic diagram is corresponding to the real object to form a specific impression. The pipeline schematic diagram in the hydraulic circuit is often very different from the real object. If possible, clarify the collusion and barrier relationship between the valve holes on the valve plate. These factors are closely related to the future circuit inspection.

(3) Refer to relevant books and materials, find out the judgment basis for judging the characteristics of hydraulic devices, and then judge them.

(4) Refer to relevant books and equipment instructions to explore failure mechanism and relevant analysis and test methods.

6. Application of Ferrography Technology

Ferrography technology is used to diagnose and monitor the fault of hydraulic system. Ferrography technology takes the wear of mechanical friction pair as the basic starting point. With the help of Ferrograph, the wear particles and other pollution particles in hydraulic oil are separated and made into ferrography sheets, which are then observed under ferrography microscope or scanning electron microscope, or deposited in glass tube according to the size, and quantitatively detected by optical method. Through the above analysis, we can accurately obtain important information about wear in the system. Therefore, further study the wear phenomenon, monitor the wear state, diagnose the fault precursor, and finally make the system failure prediction. Ferrography technology can be effectively applied to the detection and monitoring of oil pollution degree, wear process analysis and fault diagnosis of hydraulic system of construction machinery, and has the advantages of intuition, accuracy and more information. Therefore, ferrography technology has become a powerful tool for fault diagnosis and analysis of mechanical engineering hydraulic system.

7. Special instrument detection method

The special instrument detection method uses a special hydraulic system fault detection instrument to diagnose the system fault, which can detect the hydraulic system fault quantitatively. There are many special portable hydraulic system fault detectors at home and abroad to measure flow, pressure and temperature, as well as the speed of pumps and hydraulic motors.

8. Condition monitoring method

There are many kinds of instruments used in condition monitoring method, usually including pressure sensor, flow sensor, displacement sensor and oil temperature monitor. Input the tested data into the computer system, and the computer provides various information and technical parameters according to the input data, so as to distinguish the working condition of a certain hydraulic element and a certain part of the hydraulic system, and send out signals such as alarm or automatic shutdown. Therefore, condition monitoring technology can solve the diagnosis of difficult faults that can not be solved by human feeling alone, and provide information for predictive maintenance.

The condition monitoring method is generally applicable to the following hydraulic equipment:

(1) Hydraulic equipment and automatic lines that have a great impact on the whole production after failure;

(2) Hydraulic equipment and control system that must ensure its safety performance;

(3) Expensive precision, large, rare and key hydraulic system;

(4) Hydraulic equipment and hydraulic control system with too high repair cost or too long repair time and too much loss.

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