Created on:2021-09-28 14:21

Lifting and transportation equipment_ Failure of hydraulic branch of outrigger retraction and release of truck crane

Lifting and transportation equipment_ Failure of hydraulic branch of outrigger retraction and release of truck crane

(1) Working principle and fault phenomenon

Figure C shows the hydraulic system diagram of outrigger retraction and release branch of q2-8 truck crane. Because the supporting force of automobile tires is limited and elastic, the front and rear legs must be put down before lifting operation to make the automobile tires overhead and bear the load with the legs, so as to ensure the safety of operation; When the car is driving, the outrigger must be retracted.

When the manual directional valve a works in the left position, the pressure oil output by the hydraulic pump enters the rodless cavity of the front outrigger hydraulic cylinder through valve A and the hydraulic control check valve, pushing the piston down to put down the front outrigger; When valve a works in the right position, the pressure oil enters the rod cavity of the front outrigger hydraulic cylinder through valve A and, pushing the piston upward to retract the front outrigger. In order to ensure that the outrigger can be reliably locked when parked at any position, a double-sided hydraulic lock composed of hydraulic control check valve is set in the oil circuit. When valve A is in the neutral position, the hydraulic pump unloads.

B is used to control the retraction and retraction of the rear outrigger hydraulic cylinder, and its working principle is the same as that of the front outrigger hydraulic cylinder.

During the operation of the system, although the front and rear outriggers are put down, the vehicle body cannot be lifted to make the tires overhead.

(2) Reasoning analysis

From the symptom of the fault to finding out the real cause of the fault, it can be carried out in three steps; ① Infer the essential cause of the fault from the symptoms of the fault; ② Infer the common causes that may lead to the fault from the essential causes of the fault; ③ Infer the real cause of the fault from the common causes.

The essential reason for the failure of the hydraulic cylinder is that the oil pressure in the cylinder is insufficient or the movement resistance is too large, so that the hydraulic cylinder can not promote the load movement. Hydraulic cylinder, overflow valve, reversing valve, pipeline system and may all fail, resulting in insufficient pressure; The fault in one aspect may be caused by different reasons. Serious oil leakage of hydraulic cylinder, excessive load or friction resistance, and blocked oil inlet will cause insufficient pressure; The adjustment pressure of overflow valve is too low, the spring of main (pilot) valve is invalid or too soft, the seal between pilot valve and valve seat is poor, the damping hole of main valve core is blocked, etc; The pressure can still be insufficient due to the failure of the reversing valve, insufficient oil in the oil tank, blocked oil circuit and other reasons; Hydraulic pump is a power component. Insufficient flow or low volumetric efficiency of the pump will also cause insufficient pressure, so that it can not push the load. Insufficient rotating speed of the hydraulic pump, mixed air in the pump, too small displacement or air leakage at the oil suction port will make the pump flow insufficient, the seal failure of the pump and serious wear of the friction pair in the pump will make the volumetric efficiency too low. After having a certain understanding of the occurrence law of the above faults, the fault analysis block diagram can be drawn. Its purpose is to determine the process of on-site fault analysis and troubleshooting according to reasonable ways and steps, so as to find the best way and find out the real cause of one or more faults from the possible common fault causes by reasoning. The fault analysis block diagram of truck crane outrigger retraction and retraction hydraulic system is shown in Figure D. Inferring the common causes of faults from the essential causes of faults level by level depends on the learning of theoretical knowledge and the accumulation of work experience.

D.jpg

(3) Field inspection

There are several methods for on-site detection.

① Instrumental analysis. When the hydraulic system fails, the performance of the hydraulic device will inevitably change, which will be characterized by characteristic parameters. The characteristic parameters include pressure, flow, temperature, movement speed of actuator, noise, oil state (pollution degree, viscosity, etc.) and leakage. Among them, pressure is the most important parameter, which is very sensitive to the variation of hydraulic device state. Using pressure gauge to test the pressure of relevant parts of the system to judge its state is the most common fault detection method. When measuring different parameters, different test instruments shall be selected according to the requirements of field conditions and control accuracy.

② Partial stop method, that is, temporarily stop the work of a part of the hydraulic system and observe the impact on fault symptoms.

③ Exploratory counter evidence method, that is, tentatively change some working conditions in the hydraulic system and observe the impact on fault symptoms. If the hydraulic cylinder does not act, remove the external load of the hydraulic cylinder and check whether the hydraulic cylinder can act normally, which can prove whether the hydraulic cylinder does not act due to excessive load.

④ Comparison method, that is, replace the same components in the system with standard or qualified components, and judge whether the replaced components fail through the comparison of working conditions.

(4) Diagnostic route

① According to the principle from simple to complex and from easy to difficult, first check the oil volume of the oil tank, the external leakage of the hydraulic cylinder, and whether the filter and pipeline are blocked. After inspection, it can be seen that the oil tank has sufficient oil, the hydraulic cylinder has no external leakage, and the filter and pipeline are not blocked.

② Check the hydraulic cylinder for failure. Since the outrigger hydraulic cylinder can be put down and stowed, it indicates that the reversing valve can be reversed and the hydraulic cylinder is not stuck. Using the "trial and counter evidence method", put down the outrigger hydraulic cylinder but do not touch the ground. When it is stationary for a period of time, the piston of the hydraulic cylinder still stops in place, indicating that the hydraulic cylinder has no internal leakage and no fault.

③ Check the overflow valve for failure. First check whether the adjustment pressure is too low. Using the "instrument analysis method", fully open the overflow valve, start the hydraulic pump, make the directional valve a work in the left position, gradually tighten the pressure regulating hand wheel of the overflow valve, and observe the change of the pressure gauge: no matter how to tighten the pressure regulating hand wheel, the maximum pressure indicated by the pressure gauge is only 6Mpa, which can not reach the working pressure of the pump (21MPa), indicating that the pressure can not go up. If the pressure of the overflow valve is not adjusted high, it may be the failure of the overflow valve or other reasons. In order to avoid disassembling the qualified overflow valve for inspection by mistake, the "comparison method" is adopted, that is, the overflow valve is removed and replaced with a standby overflow valve of the same model. The above inspection process is repeated, and it is found that the maximum adjustment pressure is still only 6Mpa. It can be seen that the overflow valve has no fault.

④ Check the failure of the hydraulic pump. After eliminating the above possible causes, it can be concluded that the real cause of the fault is the hydraulic pump. Remove the hydraulic pump and disassemble it for inspection. It can be seen that the cylinder block and oil distribution plate, plunger and cylinder block are worn to varying degrees. Wear causes serious internal leakage of the hydraulic pump, so that the pressure of the hydraulic system can not go up, resulting in the outrigger hydraulic cylinder can not lift the body. Change a hydraulic pump of the same model, and the fault can be eliminated.

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