On-site detection and diagnosis of engineering machinery hydraulic system failure
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1 characteristics of engineering machinery hydraulic system failure
 
The hydraulic mechanical transmission system is mainly composed of a hydraulic pump, a control valve, a torque converter, a transmission, and a power shifting shift valve, and the fault is usually manifested as a walking force or a poor hydraulic clutch engagement.
 
The hydraulic system of the working device is mainly composed of a hydraulic pump, a control valve, a hydraulic motor and a hydraulic cylinder. The failure is mainly manifested by the running or turning of the motor, the extension and retraction of the piston of the hydraulic cylinder.
 
The common feature of these two system failures is that the system pressure is insufficient.
 
2 fault detection and diagnosis
 
(1) Initial inspection and diagnosis on site
 
Corresponding to the hydraulic system diagram to analyze the location and initial cause of the fault, we must not neglect the reason that looks very simple, and must not be blindly dismantled to avoid unnecessary losses. The following steps should be carried out during the specific inspection process.
 
1 Know the situation to the driver, and do a detailed understanding of the state and sound of the machine when the fault occurs, avoiding the fuss and making it difficult. For example, a 966D loader found that the machine was running weak after changing the oil to the transmission. The torque converter oil temperature is too high. After inspection, it was found that the added transmission oil number was wrong, and the fault was quickly eliminated after the cause of the failure was clarified.
 
2 Perform the necessary specific operations. Sometimes, the driver's causal relationship to the machine failure is unclear, making the diagnosis difficult, and the necessary on-site operations will benefit a lot.
 
3 oil quality, oil quantity inspection. This content seems to be simple, but it is often overlooked. For example, a 966D loader (the running mechanism is a hydraulic pressure transmission system), the transmission oil has been released during the driver's vacation. When the assistant moved to the site after the relocation, he found that the machine could not walk. He thought it was a big fault, but the maintenance personnel only solved the problem by listening to the sound and checking the dipstick at the scene, and avoided the occurrence of a big accident. Another example is that a Hitachi EX220-2 excavator found that the hydraulic oil was insufficient after repairing the hydraulic cylinder, and the hydraulic oil purchased on site was the reclaimed oil refined by the earth method, which was added to the fuel tank and caused oil pollution. Deteriorating foaming, causing the machine to move weakly, and the fault is eliminated after replacing the hydraulic oil. Therefore, the inspection of oil quality and oil quantity must be paid enough attention; otherwise, the hydraulic pump will be burned out and the transmission system will be damaged.
 
4 Check the various filter elements. The oil filter is a cleaning tool for the hydraulic system. In the fault diagnosis, checking the oil filter (the degree of dirt on the oil filter, the properties of various impurities on the filter element, etc.) can provide a basis for further analysis of the fault. For example, a Kato HD820 excavator found that the whole machine was weak after running for about 4000h; when the hydraulic system oil filter was dismantled, it was found that the filter element was damaged, the oil return port was blocked, and the fault was eliminated after replacing the filter element.
 
If the fault cannot be eliminated after the initial inspection, the instrument should be used for more detailed testing.
 
(2) Instrument diagnosis of hydraulic system
 
In the general on-site inspection, because the flow detection is difficult, and the failure of the hydraulic system often manifests as insufficient pressure, in the on-site inspection, the method of detecting the system pressure is more used. For example, a 966D loader, after running for 6000h, found that its walking force is weak, detecting the pressure value of the torque converter inlet and outlet, the result is normal; operating the power shifting shift valve, when measuring the direction clutch pressure, the pressure is only 0.5MPa , that is, can not establish normal pressure. After solving the full transmission, it was found that the oil seal in the direction clutch oil passage was damaged, causing leakage of hydraulic oil, and the fault was eliminated after replacing the oil seal. Another example is an EX220-5 excavator. After running for 3000 hours, it was found that the walking deviation was detected. The pressure of the walking system was found to be 32 MPa on the left and 26 MPa on the right. After adjusting the pressure of the right travel safety valve, the fault was eliminated.
 
(3) Computer diagnosis
 
With the wide application of electromechanical and hydraulic integration in construction machinery, a single stress test can no longer meet the needs of on-site inspection. Now more and more imported construction machinery, its fault diagnosis must be completed by means of a special inspection computer. The measured data is rich, small in size and easy to carry. For example, a Hitachi EX220-2 excavator, the hydraulic system of the working device is weak, and when the handle of the excavator is operated, the sound is changed and smoke is emitted. When using the detection computer, it was found that the flow rate of the hydraulic pump did not change significantly. When the pressure increased, the engine changed sound. According to the analysis, the flow rate of the hydraulic pump was too large, and the swash plate could not adjust the flow. Disassemble the hydraulic pump servo valve and find that the connecting pin of the servo valve and the hydraulic pump flow adjustment swash plate is broken, and the fault is eliminated after replacing the pin.
 
(4) Other diagnostic methods
 
In-situ maintenance often uses a non-instrumental replacement diagnostic method, which is often used in the overall testing of different types of machines, that is, if there is no testing instrument on site or the components to be inspected are not suitable for disassembly, they can be replaced with other When the components on the model machine are inspected, it is possible to quickly diagnose whether there is a malfunction. For example, if a CAT320L excavator is working less than 500h, the hydraulic system of the working device is weak. At that time, there is no testing instrument on site. According to the experience, the main safety valve is faulty. However, the main safety valve is disassembled on the spot and the cone of the pilot needle valve is not obvious. The wear and tears will be replaced by the main safety valve on the same 320L excavator of the same model and the safety valve will be replaced after the test. This type of conversion diagnosis is simple and easy, but it must be judged accurately.

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