Precautions for disassembling and assembling hydraulic cylinders
Thread Content
As hydraulic cylinders become increasingly common in China, Engineer Li shares some precautions for disassembling and assembling them: 1. Before disassembling a hydraulic cylinder, the pressure in the hydraulic circuit should be released first. Otherwise, when the oil pipe fitting connected to the cylinder is loosened, the high-pressure oil in the circuit will spray out rapidly. When releasing pressure from the hydraulic circuit, first loosen the handwheels or pressure-regulating screws located on the relief valve and similar components to allow the pressure oil to be released, and then cut off the power supply or the power source to stop the operation of the hydraulic system. 2. During disassembly, care must be taken to avoid damaging the threads at the top of the piston rod, the oil port threads, as well as the surface of the piston rod and the inner wall of the cylinder liner. To prevent slender components such as piston rods from bending or deforming, they should be supported evenly using shims when placed. 3. Disassembly should be carried out in order. Due to the varying structures and sizes of hydraulic cylinders, the disassembly sequence also varies slightly. Generally, the oil in both chambers of the cylinder should be drained, followed by the removal of the cylinder head, and finally the removal of the piston and piston rod. When removing the cylinder head of a hydraulic cylinder, special tools must be used for the key or snap ring in an internal key-type connection; flat shovels are prohibited from being used ； For flanged end caps, they must be pushed out using screws; hammering or forceful prying is not allowed. When it is difficult to pull out the piston and piston rod, do not force them out; instead, identify the cause first before attempting to disassemble them. 4. Before and after unloading, efforts should be made to create conditions that prevent the components of the hydraulic cylinder from being contaminated by dust and impurities in the surrounding environment. For example, disassembly should be carried out in a clean environment as much as possible ； After disassembly, all parts must be covered with plastic sheeting; do not use cotton cloth or other types of work cloth for covering them. 5. After disassembling the cylinder, it is necessary to conduct a thorough inspection to determine which parts can be used again, which parts can be repaired and then reused, and which parts must be replaced. 6. All parts must be carefully cleaned before assembly. 7. The sealing devices in various locations must be installed correctly. 8. Special wrenches should be used to tighten threaded connectors, and the torque value must meet the standard requirements. 9. After the piston and piston rod are assembled, it is necessary to determine whether their coaxiality and straightness along the entire length are within acceptable limits. 10 After assembly, the piston assembly should move without any sense of obstruction or uneven resistance. 11. When installing the hydraulic cylinder on the main unit, a seal ring must be placed between the inlet and outlet port connections and tightened properly to prevent oil leakage. 12. After assembly as required, several back-and-forth movements should be made under low pressure to expel the gas from the cylinder.
When the hydraulic cylinder moves slowly or does not move at all, first check its appearance, and then check the return oil filter. Due to factors such as wear of the hydraulic cylinder, particles are generated; as the piston of the hydraulic cylinder operates frequently, a considerable portion of these particles are trapped by the oil return filter on their way back to the oil tank along with the working fluid. If large black rubber pieces, copper particles of various sizes, and gray or pale yellow translucent nylon material are found in the oil return filter element, it indicates that the piston seal of the hydraulic cylinder is damaged ； The black rubber pieces come from the piston seals, the copper particles come from the copper support rings, while the gray or light yellow nylon material comes from the wear-resistant rings. As an example, one of our bureau’s Hitachi UH171 backhoe excavators experienced a decrease in the speed of its bucket during operation; it was still able to meet the work requirements at that time. However, soon after, the bucket’s movement became extremely slow, to the point where it could no longer function properly. During the shutdown inspection, it was found that the operation of other devices controlled by the main pump that powers the bucket was normal. It indicates that both the main pump and the main relief valve are in good condition ； The return oil filter was inspected, and a large amount of black rubber pieces, small copper particles, and brown nylon material were found. Analysis suggests that such a large number of particles cannot originate from the control valve; they must come from the bucket hydraulic cylinder. After disassembling the hydraulic cylinder, it was found that the piston seal rings and wear-resistant rings of the cylinder were completely damaged, the support rings had broken into several pieces, and the cylinder wall was severely scratched. The main reason for this analysis is the fracture of the copper support ring due to factors such as fatigue; as the piston rod of the hydraulic cylinder moves in and out frequently, the fracture edges continuously scrape against the inner wall of the cylinder, causing the wall to become rough and damaged, which leads to internal leakage and a decrease in the cylinder’s speed ； As operating time progresses, the wear on the cylinder walls, as well as damage to the sealing rings and wear-resistant rings, increases continuously. This leads to an increase in internal leakage, ultimately resulting in a significant reduction in the speed of the hydraulic cylinder.
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