Determine why the chain saw pulls the cylinder

by:Jiali     2020-06-25
There are many reasons that cause the chain saw to pull the cylinder. For ordinary users and even many technicians engaged in related work, it is not an easy task to quickly and accurately analyze and determine the cause of the failure. Today, afterburner analyzes several main reasons for pulling cylinders, hoping to help the majority of users and maintenance personnel. When the engine is working, the piston and the piston ring reciprocate in the cylinder at high speed. Due to the huge expansion thrust generated by the combustion of the mixed gas, the oil film formed by the piston and the piston ring is in close contact with the inner wall of the cylinder. Under normal circumstances, due to the isolation and cushioning effect of the oil film, the piston and the piston ring are not in direct contact with the cylinder. However, under certain circumstances, the two will directly contact, sliding friction occurs, and a large amount of heat is generated. If the heat dissipation conditions are not effectively improved, the metal surface of the piston, piston ring or cylinder inner wall may be melted, causing the sliding friction surfaces to fuse together, resulting in longitudinal pull marks during the high-speed movement of the piston. The so-called pulling cylinder phenomenon. The main reasons are as follows: 1. Insufficient initial running-in of the engine, pulling cylinder caused by high-speed operation For the new engine that has just been assembled, the seemingly smooth inner surface of the cylinder and the outer peripheral surface of the piston and piston ring are actually composed of countless micron-level uneven protrusions. Before the engine can really run freely, it must be matched. The surface is initially run-in, and the protruding peaks are cut off to make the surface smooth and the force is even. When sliding, it does not bite each other, and the operation can be smooth. Therefore, the new engine must be run in to a certain extent after assembly. Otherwise, the higher projections on the mating surface of the piston, piston ring and cylinder will be subjected to concentrated loads. The pressure per unit area is very large. After friction, the temperature rises and the projections soften and melt, causing the cylinder to pull. 2. Pull cylinder caused by overheating of piston When the piston overheats, local abnormal expansion occurs. Due to the deformation of the piston, the piston ring groove is twisted into a wave shape, and the piston ring and the piston ring groove bite together for a short time or even a long time, causing the piston ring to fail to work properly. At this time, the edge of the piston ring is in direct contact with the inner wall of the cylinder at a very high pressure, so that the piston ring not only loses the function of sealing the combustion gas, but also destroys the oil film on the inner wall of the cylinder, thereby causing cylinder pulling. The main causes of piston overheating are: ① The poor oil quality of gasoline causes abnormal combustion of the mixture and generates high temperature; ② Poor fuel supply system, ignition setting or adjustment, resulting in abnormal combustion of the mixed gas and high temperature; ③The poor design of the piston body or the poor density of the aluminum parts of the piston makes the piston overheated due to insufficient heat transfer from the piston to the cylinder; ④ Overheating caused by insufficient external cooling of the engine. 3. Pull cylinder caused by improper piston ring clearance When the engine is running, carbon deposits are inevitably generated in the combustion chamber. Although the engine oil and the piston ring have the function of removing carbon deposits, it is very difficult to remove the carbon deposits accumulated in the piston ring groove, especially in the first air ring groove. When the carbon deposit in the piston ring groove gradually increases, it will bring improper backlash and backlash to the piston ring, hinder the movement of the piston ring, apply abnormal pressure to the piston ring, and make the piston ring directly contact with the inner wall of the cylinder , Which causes pulling cylinder. The main causes of carbon deposition in the combustion chamber are as follows: ① The quality of the gasoline used is poor; ② Abnormal combustion of the mixed gas; ③ The gas mixture in the oil supply system is too thick; ④ Ignition system, especially spark plug ignition failure; ⑤ There is too much oil remaining on the inner wall of the cylinder; ⑥ Leakage of engine oil from the cylinder head into the combustion chamber. 4. Pulling cylinder caused by improper gap between piston and cylinder Due to the poor machining accuracy of the inner diameter of the cylinder and the outer diameter of the piston and piston ring, the gap between the piston and the cylinder is improper, causing the chain saw to pull the cylinder. When the gap is too small, the contact pressure between the piston and the cylinder increases or even locks; when the gap is too large, the swing amount of the piston head increases, the piston head and the skirt part contact the cylinder part, or the piston ring edge part contacts the cylinder part , The pressure per unit area is very high, which causes pulling cylinder. 5. Pulling cylinder caused by poor surface shape of piston and cylinder When the engine is working, the ideal shape of the contact surface of both the piston and the cylinder should be cylindrical, so that the two can evenly contact everywhere to prevent the generation of large surface pressure. Due to the effects of thermal expansion and contraction, at room temperature, the shape of the piston skirt is barrel-shaped in the longitudinal direction and elliptical in the horizontal direction. This complex geometry has high precision requirements. However, due to reasons such as design, manufacturing, processing, and materials, it may cause unsatisfactory shape and cause pulling cylinders. The main reasons are: ① Insufficient machining accuracy of the piston skirt causes shape deformation; ②Poor machining of the inner surface of the cylinder (out-of-roundness and cylindricity); ③ The machining accuracy of the outer diameter of the piston pin or the inner diameter of the piston pin hole is insufficient, resulting in a gap between the two that is too small, or a foreign object is sandwiched between the two, which causes the piston pin to expand after the engine runs and locks with the pin hole, preventing the piston skirt The expansion of the part in the direction of the pin hole, so that the skirt cannot reach the ideal shape; ④When the poor design of the piston skirt structure results in insufficient rigidity, the front and rear bearing surfaces of the skirt are compressed and deformed, causing the skirt side to increase the local force in the direction of 45° and pull the cylinder; ⑤ Poor cylinder cylinder liner structure design or poor material causes deformation of the cylinder, which causes the piston and cylinder to cooperate too tightly near the top dead center, and the frictional force increases, thereby causing cylinder pulling. 6. Pull cylinder caused by poor surface treatment From a material point of view, the piston and piston ring move at high speed in the cylinder, friction generates heat, and in serious cases, the metal surface melts and adheres together. Therefore, it is generally necessary to perform various surfaces on the piston, piston ring and even the inner surface of the cylinder Treatment, such as chromium plating, nickel plating, molybdenum spraying, nitriding, phosphating, PVD (physical vapor deposition), etc., to cover the surface with a layer of high melting point material, so as to achieve the purpose of not easy to produce high temperature fusion. However, if the surface treatment is poor, such as insufficient plating thickness and poor adhesion, the high temperature fusion of the piston and ring and the cylinder is inevitable. 7. Pull cylinder caused by oil lubrication failure When the engine is working, the oil forms a layer of oil film between the cylinder, the piston and the piston ring, which can effectively avoid the direct contact friction between the two. However, the following conditions will prevent the engine oil from fully exerting its lubricating effect, resulting in cylinder pulling: ① The amount of oil is insufficient. Obviously, when the amount of oil added is insufficient, and the engine continues to consume oil during operation and cannot be replenished in time, it cannot establish an oil film of sufficient thickness to pull the cylinder. ② Engine oil emulsification. Especially in winter, when the temperature of the engine is reduced after the engine is turned off, the moisture in the air in the lower part of the cylinder and the crankcase (inhaled from the vent pipe) condenses into water and mixes into the engine oil, which will gradually cause the emulsification of the engine oil. At the same time, the mixed gas leaking from the combustion chamber is also continuously integrated into the engine oil, which promotes the emulsification of the engine oil. After the emulsification, the oil concentration becomes thinner and the oil is deteriorated. It is not easy to establish an effective oil film between the cylinder and the piston and piston ring. ③ Improper selection of oil grade. Depending on the region and season, the appropriate grade of oil should be selected. If not selected properly, the oil will be too viscous and poor in fluidity in the winter, and too thin in the summer will result in poor lubrication. 8. Pulling cylinder caused by foreign matter and impurities When there are hard foreign objects between the cylinder, the piston, and the piston ring, the foreign objects play the role of grinding material, which accelerates the wear of the surfaces of the two, and pulls the cylinder in severe cases. The sources of foreign objects mainly include the following aspects: ① Dust and other foreign objects brought in by the cleaning of engine parts; ②The remaining burrs on engine parts; ③ Iron and aluminum scraps ground during the operation of the engine; ④ Dust that enters the air due to poor filtration of the air filter; ⑤ Carbon deposits caused by insufficient combustion of engine oil and mixed gas; ⑥ The iron sand remaining in the muffler during the sandblasting process of the muffler is sucked into the combustion chamber.
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