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1. What is hydraulic cylinder creeping?
When the piston rod of the hydraulic cylinder extends or retracts under the action of oil pressure, the speed is often uneven, and sometimes accompanied by vibration and abnormal noise, which causes the vibration of the entire hydraulic system and drives the vibration of other parts of the main engine. It often occurs during the main engine debugging process. Sometimes the speed is fast, this phenomenon will be alleviated. In addition to this phenomenon caused by the hydraulic system pipeline, the vibration generated by the hydraulic cylinder itself also often causes this phenomenon.
2. Causes of hydraulic cylinder creeping
1) Low-speed creeping caused by gas in the rod chamber and rodless chamber of the hydraulic cylinder. Because the gas is mixed in the hydraulic oil, the volume changes under the action of pressure, and even the gas explodes instantly under high pressure, causing the speed of the hydraulic cylinder to be unstable.
2) Low-speed creep caused by improper hydraulic cylinder design clearance. The sliding clearance between the piston and cylinder body, piston rod and guide sleeve inside the hydraulic cylinder is too large, causing uneven pressure on the sliding surface, uneven friction, and low-speed creep of the hydraulic cylinder. If the sliding clearance is too small, coupled with the tolerance of component manufacturing, it will also cause uneven pressure on the sliding surface, uneven friction, and low-speed creep of the hydraulic cylinder.
3)Low-speed creep caused by uneven friction of the guide element in the hydraulic cylinder. The commonly used guide materials and non-metallic support rings of the hydraulic cylinder, especially the non-metallic support rings, are uneven in size. Some non-metallic support rings increase or decrease in size with the change of oil temperature, that is, the poor dimensional stability in the oil directly causes the change of the matching clearance, which can easily cause the speed of the hydraulic cylinder to be unstable.
4)Hydraulic cylinder creeping at low speed caused by sealing material problems. Common sealing materials for hydraulic cylinders include butadiene rubber, polyurethane rubber, polytetrafluoroethylene, etc. The hardness, strength, and followability of the materials directly affect the friction between the material and the sliding surface. In addition, for lip seals, fluctuations in oil pressure cause changes in the contact pressure between the sealing area and the contact surface, thereby causing changes in the speed of the hydraulic cylinder.
5)Influence of parts processing accuracy. The processing accuracy of the inner wall of the hydraulic cylinder and the piston rod surface has a great influence on the low-speed stability of the hydraulic cylinder. In particular, the geometric accuracy has a greater impact, among which straightness is the key. It is the most difficult to ensure straightness during the processing. For hydraulic cylinders with long strokes, the straightness of the inner wall of the hydraulic cylinder and the piston rod surface is the main factor affecting the low-speed stability of the hydraulic cylinder.
3. Solutions
1)The low-speed creep caused by the gas in the rod chamber and rodless chamber of the hydraulic cylinder can be exhausted by repeatedly running the hydraulic cylinder. If necessary, an exhaust device can be installed in the pipeline or the two chambers of the hydraulic cylinder to exhaust the gas when the hydraulic system is working.
2) For low-speed creep caused by improper clearance design of hydraulic cylinder, the sliding clearance between the piston and cylinder body, piston rod and guide sleeve inside the hydraulic cylinder can be correctly designed.
3) For low-speed creep caused by uneven friction of the guide element in the hydraulic cylinder, it is recommended to use metal as the guide support, such as QT500-7, ZQAL9-4, etc. If non-metallic support rings are used, it is recommended to use non-metallic support rings with good dimensional stability in oil, especially the thermal expansion coefficient should be small. In addition, the thickness of the support ring must strictly control the dimensional tolerance and thickness uniformity.
4)For low-speed creep of hydraulic cylinder caused by sealing material problems, it is recommended to use a combination seal ring with polytetrafluoroethylene as the seal, such as the commonly used Gly ring, Step seal, etc., under conditions where the working conditions allow; if a lip seal is selected, it is recommended that the material is preferably a seal made of butyl rubber or similar materials, which has better followability.
5) The impact of parts processing accuracy. In the manufacturing process of hydraulic cylinders, the processing accuracy of the cylinder inner wall and piston rod surface should be strictly controlled, especially the geometric accuracy, especially straightness is the key. In domestic processing technology, the processing of the piston rod surface is basically grinding after turning, and there is no big problem in ensuring straightness. However, there are many processing methods for the processing of the cylinder inner wall, including boring-rolling, boring-honing, direct honing, etc. However, due to the gap between the basic level of domestic materials and that of foreign countries, the straightness of the pipe blank is poor, the wall thickness is uneven, the hardness is uneven, and other factors often directly affect the straightness of the cylinder inner wall after processing. Therefore, it is recommended to use boring-rolling and boring-honing processes. If direct honing is used, the straightness of the pipe blank must be improved first.
