1. Judgment of the cause of the automatic descent failure of the excavating mobile arm
After using the hydraulic excavator for a period of time, some of the booms will automatically fall, resulting in a slower boom lifting speed, or even "lifting", which seriously affects work efficiency. The main reasons for the above failures are as follows: ① The hydraulic cylinder oil seal is damaged. ②The control valve stem and valve hole are scratched or worn. ③ The overload valve controlling this action is damaged. ④ The pilot control valve is damaged.
There are many judgment and elimination methods for the above four aspects. Based on the experience of on-site diagnosis, a quicker and simpler judgment and elimination method is introduced.
Hydraulic excavator hydraulic system principle is shown in Figure 1-7.
1. Judgment of pilot control valve
In the pilot oil circuit of modern excavators, there are working devices to control the oil circuit master switch for safety protection. When it is locked, the pilot oil path to the cab control valve is cut off, so the quality of the pilot control valve can be judged by the opening and closing of the locking valve. The specific method is: put the excavator in the position shown in Figure 1-8, and place the safety lock valve in the locked and free positions, if the boom lowering speed changes, it proves that the pilot control valve is defective and the pilot control valve is faulty The main reasons are: ① There is no gap between the rotating disk A and the piston B, which causes the boom to drop when the main control lever is in the neutral position, and the normal adjustment gap should be in the range of 0.3~0.5mm (see Figure 1-9). ②The spring is damaged. ③ Worn spool valve.
2. Judgment of overload valve
Using the "replacement method" will control the overload valve and control action of the bucket or stick cylinder.
The overload valve of the arm is interchanged to judge its quality. If the service life of the excavator is long, and the site of the rated load Shi Shilu can not judge the quality of other overload valves, it can process a 6mm thick steel plate with a surface roughness Ra=0.8~1.6um to replace the overload valve.
Then cut off the overload oil circuit and the return oil circuit. If the boom is still down, it indicates that the overload valve is normal. The reasons for the failure of the overload valve are: ① The pressure regulating spring is broken or the spring elasticity is reduced. ② The sealing between the valve and the valve seat is not good. ③ The valve core is worn. The treatment methods are as follows: replacement of the gasket, reassembly and replacement of the pilot valve pressure spring. Repair the sealing cone surface of the pilot valve and clean the repair cone valve, etc.
3. Judgement of hydraulic cylinder oil seal and hydraulic cylinder control valve
When the above two valves work normally, operate the excavator as shown in Figure 1-8, stop the engine, and operate the boom pilot control valve to the "lift" position. If the falling speed increases, it proves that the hydraulic cylinder oil seal is damaged. . The reason is that when the working device is in the state shown in Figure 1-8 (the rodless cavity at the bottom of the hydraulic cylinder is pressurized and the oil pressure increases), the hydraulic oil leaks to the side with the rod cavity, and when the tip has the rod cavity, the pressure increases At the time, maintain its equilibrium pressure according to its volume ratio (different depending on the amount of oil leaked). At this time, the rate of decline is slow. If the pilot control valve is subsequently operated to "lift", there is rod cavity hydraulic oil and oil return at the top of the cylinder When it is connected, the oil in the rod cavity flows into the drain circuit, the balance is destroyed, and the descent speed becomes faster. If the descent speed does not change, it indicates that the control valve is damaged.
Attention should be paid to several points when using the above elimination methods: ① Safety lock valve should be installed in the pilot oil circuit. ②The hydraulic system needs to be equipped with an accumulator and its function is normal.
2. Failure phenomenon and reason analysis of excavation manipulator boom
When the operating handle of the excavator is moved to the "boom down" position, the boom cannot be lowered smoothly, but there is an intermittent drop of one drop and one stop, which causes the whole machine to shake violently back and forth in the direction of the boom, so that it cannot be normal jobs.
Fault tree analysis is shown in Figure 1-10.
1. Detection of possible causes
The pressure of the main oil circuit system detected by the pressure gauge is 27.8MPa, which meets the regulations. The other operating speeds and forces are normal, which can eliminate the cause of the insufficient pressure and flow of the main oil circuit. The pressure of the pilot control system is 2.94MPa, which is in compliance with the regulations. The other operations are operated normally, which can eliminate the cause of the lack of pilot control oil pressure: after dismantling and inspecting the boom stem in the right multi-way valve, it is found that the stem is somewhat Stuck, but no foreign objects, and no scratches on the surface of the stem: After the disassembly and inspection of the boom lower speed limit valve, the one-way valve is found to be well closed and there is no spring breakage.
Preliminary judgment, the failure is caused by the out-of-round deformation of the boom stem.
After reviewing the data, it was found that the structure and dimensions of the boom valve stem and the stick valve stem in the multi-way valve are completely the same, and they can be interchanged. Then the two machines were installed in reverse, and the test machine found that the shaking phenomenon of the whole machine disappeared before and after, and the boom The descent is stable, and the action during the stick operation is not stable. Prove that the above judgment is correct. Take out the boom stem from an old multi-way valve in stock and replace it, you can immediately resume work.
2. Theoretical explanation of fault phenomena
Due to the out-of-round deformation of the boom valve stem, the stroke is not in place during the descent operation, the throttling phenomenon occurs in the oil inlet of the boom cylinder with the rod cavity, and the oil intake is insufficient; and the boom is under the action of the gravity moment of the entire working device Forcible descent makes the volume of the rod cavity of the boom cylinder increase faster than the oil inlet speed, which causes a vacuum phenomenon (ie, negative pressure) in the rod cavity of the boom cylinder. Under the action of negative pressure, a reaction force directed to the rod cavity from the rodless cavity is generated to prevent the boom from descending. The greater the vacuum, the greater the reaction force. When the reaction torque and the gravity moment of the working device reach equilibrium, the boom lowering is forced to stop.
After the boom is stopped, the volume of the rod cavity of the boom cylinder no longer increases, and the oil intake continues. At this time, the vacuum degree is reduced, the reaction force is reduced, and its torque can no longer support the gravity moment of the working device. So the boom continues to descend, and then there is a vacuum in the rod cavity, and the boom descends and stops... In this way, the boom lowering process is an alternating process of "descent stops and descends and stops again and again...". During the boom descent, due to the periodic speed change of the boom, a periodic forward and backward tilting moment in the direction of the boom will be generated for the whole machine. Under the action of this tilting moment, the entire excavator During the boom lowering process, it rocked back and forth along the boom direction, and it became more and more intensified, so that it could not continue to work.
