A complete guide to crankshaft machining process
The crankshaft is an important part of the engine. It is mostly made of carbon structural steel or ductile iron. It has two important parts: the main journal and the connecting rod journal (and others). Rod machining is the processing of the rod that transmits motion between the reciprocating parts (such as pistons) and rotating parts (such as crankshafts) of the machine.
The main journal is installed on the cylinder block, the connecting rod journal is connected to the connecting rod big end hole, and the connecting rod small end hole is connected to the cylinder piston. It is a typical crank slider mechanism. The lubrication of the crankshaft mainly refers to the lubrication of the connecting rod big end bearing and the crankshaft connecting rod journal and the lubrication of the fixed points at both ends. The rotation of the crankshaft is the power source of the engine and the source power of the entire mechanical system.
1.How a crankshaft works
The crankshaft is one of the most typical and important parts in the engine. Its function is to convert the gas pressure transmitted by the piston connecting rod into torque, output work as power, drive other working mechanisms, and drive the auxiliary equipment of the internal combustion engine to work.
2.Crankshaft machining process
①External milling of crankshaft main journal and connecting rod journal
When processing crankshaft parts, due to the influence of the structure of the disc milling cutter itself, the cutting edge and the workpiece are always in intermittent contact and there is an impact. Therefore, the clearance link is controlled in the entire cutting system of the machine tool, which reduces the vibration caused by the movement clearance during the machining process, thereby improving the machining accuracy and the service life of the tool.
②Grinding of crankshaft main journal and connecting rod journal
The tracking grinding method takes the center line of the main journal as the center of rotation, and completes the grinding of the crankshaft connecting rod journal in one clamping (it can also be used for main journal grinding). The grinding method of the connecting rod journal is controlled by CNC The feed of the grinding wheel and the rotary motion of the workpiece are linked by two axes to complete the feed of the crankshaft. The tracking grinding method adopts one clamping and completes the grinding process of crankshaft main journal and connecting rod journal sequentially on a CNC grinding machine, which can effectively reduce equipment costs, reduce processing costs, and improve processing accuracy and production efficiency.
③Crankshaft main journal and connecting rod neck fillet rolling machine tool
The rolling machine is used to improve the fatigue strength of the crankshaft. According to statistics, the crankshaft life of the ductile iron crankshaft after fillet rolling can be increased by 120% to 230%; the life of the forged steel crankshaft can be increased by 70% to 130% after fillet rolling. The rotational power of rolling comes from the rotation of the crankshaft, which drives the rollers in the rolling head to rotate, and the pressure of the rollers is implemented by the oil cylinder.
The most common fatigue damage of the crankshaft, the main stressed part of the engine, is metal fatigue damage, that is, bending fatigue damage and torsional fatigue damage, and the occurrence probability of the former is greater than that of the latter. Bending fatigue cracks first develop in the connecting rod journal (crank pin) or main journal fillet, and then develop towards the crank arm. Torsional fatigue cracks occur in poorly processed oil holes or fillets, and then develop in the direction of the axis. Metal fatigue failure is the result of variable stress that changes periodically with time. Statistical analysis of crankshaft damage shows that about 80% is caused by bending fatigue.
3.The main cause of crankshaft fracture
1.Long-term use of engine oil deteriorates; severe overloading and overhanging cause long-term overloading of the engine and cause tile burning accidents. The crankshaft was severely worn due to engine burnout.
2.After the engine is repaired, the vehicle has not passed the running-in period, that is, it is overloaded and overslung, and the engine is overloaded for a long time, so that the crankshaft load exceeds the allowable limit.
3.Overlay welding is used in the repair of the crankshaft, which destroys the dynamic balance of the crankshaft, and the balance check is not done. The unbalance exceeds the standard, which causes a large vibration of the engine and leads to the fracture of the crankshaft.
4.Due to poor road conditions and severe overloading of the vehicle, the engine often runs within the critical speed of torsional vibration, and the shock absorber fails, which will also cause the crankshaft to break due to torsional vibration fatigue damage.
4.Crankshaft maintenance precautions
1.During the crankshaft repair process, the crankshaft should be carefully inspected for cracks, bending, twisting and other defects, as well as the wear of the main bearing bush and the connecting rod bearing bush, to ensure that the main journal and the main bearing bush, the connecting rod journal and the connecting rod bearing bush The clearance between them is within the allowable range.
2.Crankshaft cracks mostly occur at the transition fillet between the crank arm and the journal, and at the oil hole in the journal.
3.When repairing and reinstalling the crankshaft, ensure the running balance of the flywheel.
4.After major accidents such as tile burning and cylinder smashing have occurred in the internal combustion engine, the crankshaft should be comprehensively overhauled.
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