(4) Assembly clearance control
In addition to the selection of the connecting rod and the main bearing of the key components, the other assembly clearances are guaranteed by the quality of the parts after the coordination of the drawings. During installation, under the premise of ensuring the coaxiality of the main bearing holes on the body, pay attention to the clearance between the bearing bush and the journal. The clearance between the main journal and the bamboo bush (large tile) is 0.035~0.080mm, and the repair wear limit is 0.10 mm. The gap between the connecting rod neck and the bearing bush (small tile) is 0.030~0.07 mm, and the repair wear limit is 0.10 mm.
In order to meet the needs of normal engine operation, the crankshaft must have a proper axial clearance of 0.10mm~0.20mm. If the clearance is too small, the machine will be stuck due to thermal expansion. If the clearance is too large, the piston connecting rod set will be given. The parts are subject to abnormal wear. When the axial clearance is too small or too large, the thrust washer should be replaced for adjustment. The radial clearance of the crankshaft must comply with the regulations. If it is too small, the resistance will increase and the wear will be aggravated. The radial clearance of the crankshaft can be checked with a plastic plug gauge, with a dedicated gauge gauge and the width of the gap strip. After the crankshaft connecting rod piston and cylinder are assembled and installed, the rotation torque is guaranteed to be between 9 and 12 NM, which is disadvantageous if it is too small or too large.
Inspection after crankshaft installation. When installing the crankshaft, check whether the oil hole of the bearing is aligned with the oil hole of the bearing housing, and whether the clearance and end clearance meet the technical requirements. The method of checking the end gap is to use a pry bar to pry the crankshaft to one end, and then measure between the thrust washer and the thrust surface of the crankshaft with a thickness gauge of appropriate thickness. Or place the dial indicator at the front end of the crankshaft, use a large screwdriver to swing the crankshaft back and forth, and observe the needle indication value. Inspection after installation of the camshaft. After the camshaft is installed, in addition to checking its valve clearance and end play, check whether the timing gear marks are aligned. When the timing mark is not correct, it will change the engine's valve timing, reduce engine power, and even fail to start the engine. Among them, the valve clearance, the connecting rod bearing clearance, and the cylinder clearance play a crucial and decisive role in the reliability and durability of the engine.
1 Valve clearance: The intake valve clearance is 0.15~0.22mm, and the exhaust valve clearance is guaranteed to be 0.11~0.20mm. It is necessary to ensure that the gap between each group of valves and the four groups of inlet and exhaust valves is uniform, and at the same time, after the running-in period is appropriate Adjustment to ensure uniformity of intake and exhaust and reduce engine noise. After the valve lock clip is installed, it is required to use a hammer to strike, to detect whether the lock clip is loose or the installation is not in place. Apply the abrasive paste on the valve slope and move the valve into the cylinder head for several times. After installing the valve, inject kerosene into the intake and exhaust passages to check the leakage. Considering the tightness requirements for long-term valve operation, the valve angle is larger than the angle of the cylinder head race.
2 Linkage bushing clearance: The coordination of the connecting rod, connecting rod and crankshaft connecting rod neck is the most important. The engine assembly clearance is matched by the optional connecting rod to ensure the oil film gap is 0.023~0.048, and the bearing thickness tolerance is +-0.003, as shown in Table 2 below. The crankshaft, cylinder block and main shaft are similar principles. The main bearing is matched to ensure the oil film gap. The diameter of the connecting rod big hole is 48.712~48.731, the diameter of the connecting rod neck is 44.962~44.981, the thickness of the connecting rod is 1.844+-0.003, the theoretical gap between the hole and the journal is 1.873~1.911, and the clearance is 0.023~0.048.
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