| The crankshaft is one of important parts of diesel engine,whose torsional vibration is relevant to the safety and reliability of diesel engines.Due to the crankshaft’s own defects and its stress characteristics under different working conditions,crack failures often occur on the surface of crankshaft,and the continuous development of cracks will eventually cause the crankshaft to failure.Study on torsional vibration of cracked crankshaft of diesel engine has certain practical value.Therefore,this thesis makes the following study.The torsional vibration characteristics of crankshaft is analyzed.The crankshaft equivalent model is simplified according to the rules,and the inherent properties of the crankshaft are calculated using the Holzer method,transfer matrix method,and finite element method.After ensuring the correctness of the free torsional vibration algorithm,the forced torsional vibration of the diesel engine crankshaft is analyzed.Synthesize the characteristics of the model mechanism,the excitation torque applied to the crankshaft by gas pressure and inertial force of the reciprocating components are calculated,and the excitation characteristics of the multicylinder engine is considered.In this way,the critical speed and the relative amplitude vector sum,predict the crankshaft torsional vibration are calculated.The forced torsional vibration of crankshaft is calculated and analyzed.The torsional vibration characteristics of diesel engine crankshaft is further explored,and the torsional vibration characteristics of diesel engine under various special operating conditions is analyzed.These include: uneven firing conditions for diesel cylinders,single-cylinder flameout conditions,clockwise firing conditions,and changing firing order conditions.For these types of operating conditions,single-cylinder flameout will have a greater impact on crankshaft torsional vibration.When the diesel engine is running,it is necessary to regularly monitor the pressure and working conditions in the cylinder to avoid the diesel engine working under the above-mentioned fault conditions for a long time,and to ensure that the crankshaft is not damaged.The crankshaft crack fault due to its own defects caused by the manufacturing process and long-term exposure to alternating stress conditions is analyzed.The three-dimensional crankshaft model and the cracked crankshaft are established,and the finite element analysis of the crankshaft and cracked crankshaft is carried out to obtain the variation trend of its natural frequency.Based on the theory of fracture mechanics,stiffness variation of the shaft segment with cracks is calculated,the specific impact of the crack on shaft segment stiffness is analyzed.Dynamic characteristics of the crack is considered,a suitable model is selected and then brought into the crankshaft dynamic equation to change the parameters,torsional vibration response of the crankshaft segment during a firing cycle is calculated. |
