Chainsaw knowledge! Determine why the chain saw pulls the cylinder
2019-04-02

There are many reasons for the engine to pull the cylinder. For the average user and even many technicians engaged in related work, it is not easy to quickly and accurately analyze and judge the cause of the fault. Today, Xiaobian analyzes several main reasons for pulling the cylinder, hoping to help the users and maintenance personnel.


When the engine is in operation, the piston and piston ring reciprocate at high speed in the cylinder. Due to the huge expansion thrust generated by the combustion of the mixture, the oil film formed by the piston and the piston ring through the oil is in close contact with the inner wall of the cylinder. Under normal circumstances, due to the isolation and cushioning of the oil film, the piston and the piston ring are not in direct contact with the cylinder, but in some specific cases, the two will directly contact, generate sliding friction, and generate a large amount of heat. If the heat dissipation conditions are not effectively improved, the metal surfaces of the piston, piston ring or cylinder inner wall may be melted, so that the sliding friction surfaces are fused together, and longitudinal tensile marks are generated in the high-speed movement of the piston. The so-called pull cylinder phenomenon.


The main reasons are as follows:


1. The initial running-in of the engine is insufficient, and the pulling cylinder caused by high-speed operation


For the new engine just assembled, the seemingly smooth inner surface of the cylinder and the outer peripheral surface of the piston and piston ring are actually composed of innumerable micron-level bumps. Before the engine can really work as desired, it must be matched. The surface is initially run-in, the sharp peaks are removed, the surface is smooth, the force is even, and the teeth do not bite each other when sliding, and the operation can be smooth. Therefore, the new engine must be grounded to some extent after assembly. Otherwise, on the higher protrusions on the mating surface of the piston, piston ring and cylinder, it will be subjected to concentrated load, the pressure per unit area is very large, the temperature rises after friction, and the protrusion softens and melts, so that the cylinder is pulled.


2. Pull cylinder caused by overheating of the piston


Local abnormal expansion occurs when the piston overheats. 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 are temporarily and evenly engaged for a long time, so that the piston ring cannot work normally. At this time, the rim portion of the piston ring directly contacts the inner wall of the cylinder with a very high pressure per unit area, 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 the cylinder to be pulled.


The main causes of overheating of the piston are:


① The quality of the gasoline oil is poor, causing the mixture to burn abnormally and generate high temperature;


② The oil supply system, ignition setting or poor adjustment causes the mixture to burn abnormally and generate high temperature;


③ poor design of the piston body or poor density of the piston aluminum parts, so that the piston is 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 oil and the piston ring have the function of removing carbon deposits, it is very difficult to remove carbon deposits accumulated in the piston ring groove, particularly in the first gas ring groove. When the carbon deposit in the piston ring groove is gradually increased, it will bring improper backlash and backlash to the piston ring, hinder the movement of the piston ring, and apply abnormal pressure to the piston ring, so that the piston ring directly contacts the inner wall of the cylinder and rubs. , causing the pull cylinder.


There are several reasons for the carbon deposit in the combustion chamber:


① The gasoline used is of poor quality;


② mixed gas abnormal combustion;


③ The mixture of the oil supply system is too rich;


④ ignition system, especially spark plug ignition failure;


⑤ Excessive oil residue on the inner wall of the cylinder;


⑥ Leaking oil from the cylinder head into the combustion chamber.


4. Pulling cylinder caused by improper clearance between piston and cylinder


Due to the cylinder inner diameter and the poor machining accuracy of the outer diameter of the piston and the piston ring, the gap between the piston and the cylinder is improperly caused to pull the cylinder. When the clearance is too small, the contact pressure between the piston and the cylinder increases or even locks; when the clearance is too large, the swinging amount of the piston head is increased, the piston head and the skirt are in contact with the cylinder portion, or the rim portion of the piston ring is in contact with the cylinder portion. The pressure per unit area is very high, causing the cylinder to be pulled.


5. Pull cylinder caused by poor surface shape of piston and cylinder


When the engine is in operation, the ideal shape of the contact surfaces of the piston and the cylinder should be cylindrical, so that the two are evenly contacted everywhere to prevent the generation of large-face pressure. Due to the effect of thermal expansion and contraction, at normal temperature, the shape of the piston skirt is in the shape of a barrel in the longitudinal direction and an elliptical shape in the lateral direction. This complex geometry has high precision requirements. However, due to design, manufacturing, or materials, it is possible to cause the shape to be unsatisfactory and cause the cylinder to be pulled.


The main reasons are:


① The piston machining accuracy is insufficient to cause shape deformation;


② cylinder inner surface shape processing is poor (roundness, cylindricity is out of tolerance);


③ 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 is too small, or there is a foreign object between the two, so that the piston pin expands and locks with the pin hole after the engine is running, hindering the piston skirt The expansion of the part in the direction of the pin hole, so that the skirt cannot reach the desired shape;


④ When the design of the piston skirt is poor, the rigidity of the skirt is insufficient, and the front and rear force surfaces are pressed and deformed, so that the lateral force of the skirt is increased in the direction of 45° to pull the cylinder;


The cylinder design of the 5-cylinder cylinder liner is poorly designed or the material is poor, causing the cylinder to deform, so that the piston and the cylinder are tightly tightened near the top dead center, and the frictional force is increased, thereby causing the cylinder to be pulled.


6. Pull cylinder caused by poor surface treatment


From the material point of view, the piston and the piston ring move at high speed in the cylinder, and the friction generates heat. In severe cases, the metal surface melts and bonds together. Therefore, various surfaces of the piston, the piston ring and even the inner surface of the cylinder are generally required. Treatment, such as chrome plating, nickel plating, molybdenum spraying, nitriding, phosphating, PVD (physical vapor deposition), etc., the surface is covered with a layer of high melting point material, so as to achieve the purpose of high temperature fusion. However, if the surface treatment is poor, such as insufficient plating thickness and poor adhesion, high temperature fusion between the piston and the ring and the cylinder is unavoidable.


7. Pull cylinder caused by engine lubrication failure


When the engine is working, the oil forms an oil film between the cylinder and the piston and the piston ring, which can effectively avoid the direct contact friction between the two. However, the following conditions may cause the oil to not fully exert its lubricating effect, resulting in the pulling cylinder:


① The amount of oil is insufficient. Obviously, when the amount of oil is insufficiently added, the engine can not be replenished in time during operation, and it is impossible to build a film of sufficient thickness to pull the cylinder.


② oil emulsified. Especially in the winter, when the engine is turned off, the temperature is lowered, and the moisture in the air in the lower part of the cylinder barrel and in the crankcase (inhaled from the vent pipe) is condensed into water and mixed into the oil, which gradually leads to emulsification of the oil. At the same time, the mixture leaking from the combustion chamber is constantly integrated into the oil to promote the emulsification of the oil. The concentration of the emulsified oil becomes thinner, which deteriorates the oil and makes it difficult to establish an effective oil film between the cylinder and the piston and the piston ring.


③ oil brand selection is not appropriate. According to the region and season, the oil of the appropriate grade should be selected. If it is not properly selected, the oil in the winter will be too sticky and the fluidity will be poor. In the summer, the oil will be too thin, which will lead to poor lubrication.


8. Pulling cylinder caused by foreign matter and impurities


When there is a hard foreign matter between the cylinder and the piston and the piston ring, the foreign matter acts as a grinding material, so that the surfaces of both surfaces are accelerated to wear, and the cylinder is pulled when it is severe.


The source of foreign matter mainly has the following aspects:


① foreign matter such as dust that is not cleaned by the engine parts;


② burrs remaining on the engine parts;


③ iron scraps and aluminum scraps that are ground during engine operation;


④ air filter bad filter and dust entering with air;


⑤ carbon deposits caused by insufficient combustion of engine oil and mixed gas;


⑥ The iron sand remaining in the muffler during the blasting treatment of the muffler is sucked into the combustion chamber.


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