On Site Detection And Diagnosis Of Hydraulic System Failure Of Construction Machinery

On-site detection and diagnosis of hydraulic system failure of construction machinery

1. The characteristics of hydraulic system failure of construction machinery

The hydraulic mechanical transmission system is mainly composed of hydraulic pumps, control valves, torque converters, transmissions and power shift transmission valves, etc. Its faults are usually manifested as weak walking or 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. Its faults are mainly manifested in the weakness of the motor's walking or rotation, and the slow extension and retraction of the hydraulic cylinder piston.

The common feature of these two system failures is: insufficient system pressure.

2. On-site fault detection and diagnosis

(1) Preliminary inspection and diagnosis on site

Analyze the location of the failure and the initial cause according to the hydraulic system diagram. The seemingly simple cause should not be ignored, and the dismantling should not be blind to avoid unnecessary losses. The following steps should be scanned during the specific inspection process.

① Learn about the situation from the driver, and have a detailed understanding of the state of the machine when the fault occurs, the sound, etc., to avoid making a fuss and making it difficult. For example, a 966D loader found that the machine was unable to walk after changing the oil in the transmission. Torque converter oil temperature is too high. After inspection, it was found that the added transmission oil number was wrong. After the cause of the fault was found out, the fault was quickly eliminated.

② Carry out the necessary specific operations. Sometimes, the driver is not clear about the cause and effect of the machine failure, which makes the diagnosis of the fault difficult. At this time, the necessary on-site operations will benefit a lot.

③Inspection of oil quality and quantity. This content seems simple, but it is often overlooked in implementation. For example, a 966D loader (its running mechanism is a hydraulic drive system), the driver has already drained the transmission oil during vacation. When the assistant came to drive after the relocation of the construction site, he found that the machine could not run. He thought it was a major malfunction. However, the maintenance personnel only listened to the sound and checked the oil dipstick to solve the problem and avoid major accidents. Another example is a Hitachi EX220-2 excavator. After repairing the hydraulic cylinder, it was found that the hydraulic oil was insufficient, and the hydraulic oil purchased on site was regenerated oil refined by the local method. After being added to the oil tank, it caused oil pollution. Deterioration and foaming caused the machine to be weak, and the fault was eliminated after replacing the hydraulic oil. Therefore, sufficient attention must be paid to the inspection of oil quality and quantity; otherwise, the hydraulic pump will be burned out and the transmission system will be damaged.

④Check various filter elements. The oil filter is a cleaning tool for the hydraulic system. During 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 Jiateng HD820 excavator was found to be weak after running for about 4000 hours; when the oil filter of its hydraulic system was disassembled and inspected, it was found that the filter element was damaged and the oil return port was blocked. After replacing the filter element, the fault was eliminated.

If the failure cannot be eliminated after passing the above preliminary inspection, a more detailed inspection should be done with the aid of the instrument.

(2) Instrument diagnosis of hydraulic system

In general on-site inspections, because it is difficult to detect the flow rate, and the failure of the hydraulic system is often manifested as insufficient pressure, in the on-site inspection, more methods are used to detect system pressure. For example, a 966D loader was found to be weak after operating for 6000 hours, and the pressure values ​​at the inlet and outlet of the torque converter were tested, and the results were all normal; when the power shift valve was operated and the directional clutch pressure was measured, the pressure was only 0.5MPa , That is, normal pressure cannot be built. After unpacking the transmission, it was found that the oil seal in the directional clutch oil passage was damaged, causing hydraulic oil leakage. The fault was eliminated after replacing the oil seal. For another example, an EX220-5 excavator was found to run off track after 3000 hours of operation. The pressure of the walking system was found to be 32MPa on the left and 26MPa on the right. After adjusting the pressure of the right walking safety valve, the fault was eliminated.

(3) Computer diagnosis

With the widespread application of electromechanical-hydraulic integration in construction machinery, a single pressure test can no longer meet the needs of on-site inspection. Nowadays, more and more imported construction machinery require the help of special inspection computers to complete the fault diagnosis. The measured data is rich, small in size and easy to carry. For example, on a Hitachi EX220-2 excavator, the hydraulic system of the working device is weak. When the handle of the excavator is operated, the sound of the engine is changed and thick smoke is emitted. It was found by the detection computer that there was no significant change in the flow of the hydraulic pump, and the engine became sound when the pressure increased. After analysis, it was found that the flow of the hydraulic pump was too large and the swash plate could not adjust the flow. The hydraulic pump servo valve was disassembled, and it was found that the connecting pin between the servo valve and the flow adjustment swash plate of the hydraulic pump was broken. After the pin was replaced, the fault was eliminated.

(4) Other diagnostic methods

In the field maintenance, the exchange diagnosis method without the instrument is often used. This method is often used in the overall test of different types of machines. That is, if there is no inspection instrument on site or the inspected component is more precise and it is not appropriate to disassemble, it can be replaced with other same The components on the model machine are being inspected, which can quickly diagnose whether there is a fault. For example, when a CAT320L excavator works for less than 500 hours, the hydraulic system of the working device is weak, and there is no detection instrument on site at that time. Based on experience, it is preliminary judged that the main safety valve is faulty; but the main safety valve is disassembled on site, and the pilot needle valve cone surface is not obvious The main safety valve on another 320L excavator of the same model on the same site was exchanged with this safety valve. The fault was eliminated after the test machine. This exchange diagnosis method is simple and easy to implement, but it must be judged accurately.

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