2 interference fit. Proper interference should be maintained between the parts that are tightly fitted. If the interference is too small, the parts cannot be tightened and the parts are loose or displaced during operation, which may cause the parts to wear or the parts to malfunction. If the interference is too large, the parts may be damaged or even broken during the assembly process. This is especially the case with cast iron parts. The amount of interference depends on the type of fit and the size of the part. For parts that are assembled after heating, the interference can be slightly larger; for parts that are assembled at room temperature or smaller, the interference should be smaller.
Secondly, ensure that the relative position between the parts is correct. There should be a correct relative position between the parts or fittings. Failure to do so will cause the parts to work abnormally, accelerate the wear and tear of the parts, and sometimes even make the assembly work difficult. Such relationships include verticality, parallelism, and concentricity.
1 verticality. For example, in the case of boring, the cylinder center line should be perpendicular to the crankshaft center line, otherwise the position of the piston in the cylinder will be skewed to form unilateral wear. Another example is that the plane of the flywheel is not perpendicular to the centerline of the crankshaft, and the flywheel plane is yawed during operation, which causes the clutch to tremble.
2 parallel degrees. For example, when the shafts of the mounting gears are not parallel to each other beyond the allowable limit, the gears may not mesh properly and may sound when working.
3 coaxiality. When the flywheel is not concentric with the crankshaft, the engine will tremble due to an imbalance during rotation. When the flywheel housing hole is not concentric with the crankshaft, the first shaft of the transmission is deflected, except for the sound that occurs during operation, and jumps in the direct gear.
Third, to maintain the coordination relationship of machine processing. Some parts are assembled together for processing. In the repair assembly, in order to ensure the correct relationship or accurate geometry of the machining, it is not interchangeable. For example, the crankshaft bearing hole after the upper and lower cylinders are combined with the cylinder, and the cylinder head, the connecting rod and the connecting rod bearing cap are arranged in the precision machining of the cylinder bore to be processed and installed.
Fourth, the sealing device of each part of the machine is guaranteed to be good. The function of the sealing device is mainly to prevent air leakage, water leakage, oil leakage, and to prevent dust and moisture from entering the inside of the machine. Materials used as seals are available in a variety of gaskets and oil seals. If the seal is poor, the work of the assembly will be deteriorated. For example, when the cylinder head gasket leaks, the sealing performance of the cylinder is destroyed, thereby degrading the power of the engine. Therefore, the gasket should be properly fitted and the oil seal should be properly installed to ensure reliable sealing.
(3) Assembly torque and bolt connection The assembly of each stud and nut should pay attention to the torque and sequence of tightening. If the torque is too large, the stud will be broken; the torque will be too small to meet the tightness requirements during assembly. Therefore, the bolts of important parts have the specified torque data, as shown in Table 1. Parts with multiple screws should be tightened in a certain order to avoid uneven force, causing warpage and even cracking of the parts. A reasonable tightening sequence is: starting from the center and then tightening left and right symmetrically. For four, six or eight screw-connected parts, they are generally tightened diagonally.
In terms of connecting bolts, the effective length of the threaded hole of the cylinder is 2 to 3 mm larger than the working length of the thread, and the elongation of the connecting bolt must be measured after disassembly. In terms of tightening torque of several key connecting bolts such as cylinder connecting bolts, cylinder head connecting bolts, connecting rod tightening bolts, and magneto-electric tightening bolts, the requirements are generally three times tightened, and the torque distribution values ​​are generally 30%, 80%, and 100%. The manufacturing precision requirements of the five key connecting bolts are all 10.9. Test each time the bolt is tightened and remove it, and measure the elongation; the key bolt is used no more than 3 times.
The key components used in the wrench are assembled with a fixed-force wrench or an electric wrench. If an air-pneumatic wrench is used, the lubrication system should be added at the same time. Regular sampling is required. The crankshaft requires a rotational torque test and an idle torque test to ensure the basic performance of each engine. data.
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