Maintenance Precautions for Crankshaft Connecting Rod Mechanism of Single-Cylinder Diesel Engine

(1) Installation Precautions for Diesel Engine Bearings. When installing bearings, carefully check the wear condition of the bearings. If the bearing alloy is melted, peeling, scratched, or excessively worn (alloy thickness less than 0.3mm), or if the contact area between the bearing steel back and the seat hole is less than 75%, replace with a new bearing. After the bearing is installed into the seat hole, the bearing end face should be a certain height H above the seat hole plane to ensure that the bearing and seat hole have appropriate interference fit. Generally, H is 0.05～0.10mm. If the H value is too small, there will be gaps at the bearing joint, the bearing will easily loosen, the fit will be poor, the seat hole will wear more intensely, causing bearing fatigue peeling and melting; if the H value is too large, the bearing will easily deform, the normal fit between the bearing and the journal will be damaged, and even lead to bearing burnout. Therefore, the height of the bearing must be considered when assembling bearings. If the height is too large, it can be filed or sanded; if the height is too small, the bearing should be refitted or the seat hole repaired. Note that it is strictly forbidden to add pads to the back of the bearing to raise it, so as to avoid affecting heat dissipation and causing the bearing to loosen and be damaged. The connecting rod bearing is equipped with a "locating lip". During installation, the two locating lips are respectively inserted into the corresponding grooves on the big end of the connecting rod and the connecting rod cap to prevent the bearing from rotating and moving axially during operation. 

(2) Precautions for disassembling and assembling the flywheel. If the method of disassembling and assembling the flywheel is not correct, it will cause the flywheel and crankshaft to loosen, resulting in serious consequences such as damage to the flywheel, crankshaft and balance shaft. The correct method is:

 ① Use special tools to disassemble the flywheel. Do not knock or hit it to avoid damaging the flywheel. If the flywheel cannot be removed with special tools, the flywheel can be removed together with the crankshaft and the crankshaft can be pressed out with a press. 

② After the flywheel or crankshaft is replaced, the contact area between the flywheel taper hole and the crankshaft should be more than 75%. Otherwise, it should be replaced to avoid loosening during operation. 

③ Before assembly, clean the mating surfaces of the flywheel and crankshaft, as well as parts such as the key, to remove any impurities. When installing the key, ensure it is flat; remove any burrs. After tightening the flywheel nut, loosen and remove it, then check if the shoulder at the rear end of the crankshaft screw is lower than the plane of the flywheel bore. If the shoulder is lower than the plane of the flywheel bore, the flywheel is securely fastened to the crankshaft. If the shoulder is flush with the plane of the flywheel bore, the flywheel is not properly tightened. In this case, use a 3-5mm thick shim with an inner diameter larger than the diameter of the crankshaft shoulder, install it, then tighten the nut, and finally tighten the anti-loosening washer to prevent loosening. 

④ After installing the flywheel, check and determine if the flywheel and crankshaft are loose. If the decompression handle is moved to the decompression position and there is no sound when cranking the engine, but a "crunching" sound is heard when the decompression handle is moved back, it indicates that the flywheel is not tightened and should be carefully checked and reinstalled. When the diesel engine is working, if the diesel engine speed is suddenly increased or the clutch is released to unload the diesel engine, the flywheel will make a dull "crunching" sound, and the louder the sound is, the more rapid the speed change, indicating that the flywheel and crankshaft are not tight and are loose. It should be carefully checked and reinstalled.

 (3) The flywheel should not be enlarged at will. The flywheel and crankshaft have undergone a balance test before the diesel engine leaves the factory. The flywheel is marked with timing angle and top and bottom dead centers. If the flywheel is enlarged at will or a flywheel of a different model is used instead, the dynamic balance of the diesel engine will be damaged due to the different flywheel mass, the engine vibration will increase, and the speed regulation performance will deteriorate. At the same time, using an enlarged flywheel will increase the load on the crankshaft main bearing at the flywheel end and aggravate wear. After the flywheel is enlarged, the load on the diesel engine and the load on the tractor's steering and braking mechanism also increase. When it exceeds the limit allowed by the design performance, not only will the power and economy of the diesel engine deteriorate, but accidents such as steering and braking failure will also easily occur. "Nine out of ten accidents are caused by speeding." Arbitrarily enlarging the flywheel is an important cause of unsafe accidents of small tractors. It is necessary for agricultural machinery safety supervision departments in various places to prohibit the enlargement of flywheels. 

(4) Requirements for disassembly and assembly of timing gears. The function of timing gears is to ensure the correct coordination between the valve train mechanism and the crankshaft connecting rod mechanism to realize valve timing. Timing gears include crankshaft gears, camshaft gears, balance shaft gears and governor gears (idler gears). There are marks on the timing gears. Do not install them incorrectly. Under normal circumstances, timing gears are rarely disassembled. If it is necessary to disassemble and repair, the meshing marks on the circumference of the timing gear must be checked so that they can be aligned according to the marks during assembly. If the markings are unclear or missing, a chisel can be used to mark the teeth of the two meshing gears; if the crankshaft gear and camshaft gear are both removed, the positions of the gears and the relevant shafts must be marked; after removing the circlip, the governor gear can be taken out together with the bushing. Before installing the timing gear, the parts should be cleaned and the gears checked for cracks, breaks or other damage. When assembling, the meshing marks must be aligned. Do not install it incorrectly, otherwise, the valve timing and fuel injection timing will be disordered, resulting in the diesel engine failing to start or increased vibration. The assembly of the camshaft gear and the crankshaft gear and the relevant shafts should be aligned with the marks, and the installation should be firm and not loose. After the timing gear is assembled, check whether the marks are correct and whether the meshing is normal. When rotating the gear, there should be no biting or wobbling, and no large axial movement. 

(5) Adjustment of the axial clearance of the camshaft of the 195 diesel engine. The camshaft needs to have a certain axial clearance due to thermal expansion during operation. However, if the clearance is too large, exceeding 2.5mm, it will cause axial movement of the camshaft, aggravate the wear of parts, and the change in the position of the camshaft will cause changes in the valve timing, affecting the normal operation of the diesel engine. Therefore, the camshaft should be axially positioned. When the diesel engine is working, due to the wear and loosening of parts, or after the timing gear cover has been disassembled and reassembled, the camshaft clearance needs to be adjusted to ensure the normal operation of the camshaft. The axial positioning of the camshaft of the 195 diesel engine is achieved by the thrust screw on the gear cover. When adjusting, the thrust screw needs to be tightened to the bottom to eliminate the axial clearance, and then backed out about 1/4 turn. 

(6) The vent hole of the diesel engine crankcase should not be blocked. During the operation of the diesel engine, whenever the piston moves downward, the gas in the crankcase (including the leaked combustion gas) is compressed, which increases the gas pressure in the case, causing the oil to leak out at the oil pan mating surface and creating resistance to the piston movement. Therefore, the diesel engine crankcase should have a vent hole for venting to the atmosphere. Different diesel engine models have different locations for the vent holes on the crankcase, but their working principle is the same: a one-way valve. When the piston moves from bottom dead center to bottom dead center, the crankcase pressure increases, opening the valve and allowing gas to escape. When the piston moves from bottom dead center to top dead center, the crankcase pressure drops to negative pressure as the space expands. This negative pressure blocks the passage between the crankcase and the outside, preventing dust from entering the crankcase, keeping the lubricating oil clean, and reducing the chance of lubricating oil leakage during diesel engine operation. Therefore, the vent holes of the diesel engine crankcase should be kept unobstructed to ensure that the crankcase can breathe.