Introduction to the basic circuit of hydraulic lifting system The hydraulic lifting system of the dump truck is composed of hydraulic pump, check valve, rotary valve or pneumatically controlled distribution valve and hydraulic cylinder. The basic circuit has two forms, due to the inner wall of the hydraulic cylinder. The clearance between pistons is relative to each other and there is inevitable leakage. China's dump truck technical standards stipulate that when the carriage is lifted to any position between 10° and 20° under a state of overload of 10%, it will stay for 5 minutes and the self-descent capacity of the lifting mechanism of the cargo box shall not exceed 3°. Qualified, otherwise, it will be seen as falling too fast and requiring troubleshooting. Failure analysis and diagnosis method In the absence of leakage, it can be judged that the "cylinder slipping" is caused by internal leakage. How to determine which element caused the internal leakage? The method is as follows: first lift the cargo box to the highest or a certain position, and the rotary valve or the pneumatic control valve is placed at the stop position, and the power take-off stops. For the EQ3208 Dump Truck, the pneumatic control valve should be checked. The combination of a one-way valve and a three-position, four-way valve that controls the flow of fluid is called an air-controlled distribution valve. First remove the air control valve from the car, remove the shunt block, check the cause and diagnosis method of the “Cycling Cylinder†of the pneumatic control dump truck Li Biao, Sun Tieying, Senior Technician, Shiyan City, Hubei Province, China In the mouth, pour about 10ml of kerosene into the interface A with a one-way valve, and observe whether there is oil leaking from the tubing below. If there is oil leakage, there is internal leakage and a “cylinder slipping†phenomenon will occur. If the EQ3092 dump truck, first check the rotary valve. Remove the low-pressure oil pipe of the rotary valve, and after 2 minutes, observe whether there is oil leaking from the low pressure pipe. If there is no oil dripping, the poppet valve needle and the valve seat are closed well and there is no internal leakage phenomenon. Conversely, there is a foreign object that will trap the valve needle, valve seat, or the poppet valve will often open due to overload, damaged by the impact. Need to be cleaned or repaired and replaced. After checking the check valve and confirming that there are no problems, check the check valve installed at the outlet of the gear pump to prevent the hydraulic oil from flowing back to the oil pump. By observing whether the pump shaft of the gear pump has impact and rotation, it can be confirmed whether there is internal leakage. Because the valve needle and the valve seat are stuck when the material is loosely closed, internal leakage will occur, and the valve needle and the valve seat will also be damaged due to damage, so that the hydraulic oil will flow back and the gear pump pump shaft will have impact and rotation phenomenon. You can also remove the oil outlet from the oil pump and pour a small amount of kerosene from the P2 direction to observe if there is oil leaking from P1. If the oil is exposed, there is evidence of internal leakage. Should be disassembled and cleaned. If the valve seat is damaged or damaged, it must be repaired or replaced. Prior to loading, the opening pressure of the rotary valve has been adjusted on a dedicated hydraulic test bench and no adjustment is required during use. During the trial production, the rotary valve was tested on the bench for 10,000 times. The valve core wear was only 0.0018mm and the performance was reliable. However, from the service of recent years, it has been found that users have added spacers between the spring and the plug, and replaced the original spring with a spring with a greater rigidity, and some even blocked the overflow hole between the valve seat and the valve needle. Die to meet the goal of increasing system pressure and meet expectations for overload. However, the springs will be plastically fractured, and the valve needles and valve seats will have more wear and tear. This will not only cause a “sliding cylinder†phenomenon in the rotary valve, but also pose potential safety hazards to the entire lifting mechanism and reduce the life of other components. Check the hydraulic cylinder. Remove the hydraulic oil return pipe (that is, the low pressure oil pipe) and wait for 2 minutes to see if any needle oil forms a linear flow (flow rate ≥25ml/min) from the pipe. If it leaks, it will cause " Cylinder phenomenon. According to different structures, there are two kinds of sealing methods for the piston and the inner wall of the cylinder of the hydraulic cylinder: one is sealed with a piston ring, and the other is sealed with a sealing ring. If the hydraulic cylinder fails, there may be the following reasons: (1) The piston ring seal fails. 1 When the piston ring moves, internal leakage due to oil leakage from the interface (the piston ring should be installed in a wrong position). 2 In the uneven construction site of the vehicle, uneven loading of the cargo causes the piston rod to bear a large radial impact load, causing the piston rod to bend, and the coaxiality of the piston and the piston rod to decrease. A "bias cylinder" appears on one side of the piston. The gap between the side and the cylinder is reduced, causing the inner wall of the cylinder to produce uneven wear and internal leakage. In severe cases, the piston ring is stuck in the ring groove, causing the cylinder to pull. The straightness and roundness of the 3-cylinder tube are required to be high, and the clearance with the piston is only 0.08-0.20 mm. In use, due to the high temperature deformation, the piston ring is stuck in the ring groove, which also produces internal leakage, even Pull the cylinder. After-sales service statistics show that 93% of the cylinder's internal leakage occurred on the sealing failure of this type of structure. (2) Failure of the seal ring. 1 The material of the sealing ring is too soft, and the pressure rating and deformation rate do not conform to the standards. When the piston and cylinder cooperate, the clearance is too large. During the movement, the sealing ring can easily penetrate into the sealing gap and damage, resulting in seal failure. 2 There is a burr or hard spot on the oil return cover or oil return hole of the cylinder wall. When the sealing ring passes through, abnormal damage occurs locally, causing internal leakage. 3 The expiration of the wear life (the occurrence of aging failure, wear, damage, etc.) has not been replaced in time. (3) Hard foreign materials such as iron chips and sand enter the hydraulic cylinder. Iron chips or hard foreign matter are embedded in it, causing the cylinder to leak. (4) The piston limit valve does not close tightly. Valve needle, valve seat is caught by dirt or impurities, spring damage, etc. (5) Cracks in the piston can also cause internal leakage. Inspection and Prevention In order to prevent the above failures, the following inspection items should be done: (1) Mainly check the inner cylinder surface dimensional tolerances and geometrical position tolerances, and whether there is longitudinal pull marks, if there is trace deformation and shallow pull marks, The cylinder is repaired with a powerful honing process; the inner surface of the cylinder barrel is heavily worn, deep drawing marks are present, the hydraulic cylinder can be replaced, and the method of bonding can also be used for repair. (2) Check whether there is any burr on the oil return cover or oil return hole, if any. (3) Check the piston, limit valve, valve seat, valve needle, spring, ring groove, etc. If there is any crack, ring groove damage, spring damage, and poor sealing at the valve needle and valve seat, replace it. From the above analysis, it can be seen that the solid particles will not only aggravate the wear of the hydraulic components, but also may block the gaps and orifices of the components, causing malfunction of the control components and causing internal leakage and other system failures. According to statistics, 70% to 85% of hydraulic element failures are attributed to oil contamination. Therefore, maintenance personnel should pay great attention to the cleanliness of oil during manufacture, use, and maintenance to reduce the occurrence of failures.
Tube cutting machines are used for cutting long metal stock or scrap, generally where accuracy is not an important consideration, and the size or shape of the material makes other cutting or shearing options (such as a cutting torch) impractical. Alligator shears are often used in conjunction with large metal shredders in the metal recycling industry, to [clean" or prepare scrap for shredding by removing unwanted fittings and other parts the Shredder will not accept.
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