Research On Bending/Torsion Composite Strength And Fatigue Life Of 494Q Diesel Engine Crankshaft

Crankshaft is an important moving part of diesel engine.During the operation of diesel engine,the crankshaft is subjected to alternating loads such as tension,compression,bending and torsion.The crankshaft structural parameters,materials and other factors affect the strength of crankshaft and the reliability of diesel engine.In order to meet the increasingly stringent emission regulations and the requirements of power and economy,diesel engines usually adopt technologies such as turbocharger,inter-cooling and electronic high-pressure injection,so that the maximum explosion pressure in the cylinder of diesel engine is greatly improved and the alternating load of crankshaft increases accordingly,which intensifies the stress concentration in the main journal and crank pin transition fillet areas.Research on the strength and fatigue life of crankshaft bending,torsion,bending/torsion composite loads are of great significance for improving the strength and fatigue life of crankshafts.Taking 494Q diesel engine crankshaft as the research object,the crankshaft model of bending/torsion composite was established.The load and constraint boundary conditions of crankshaft were determined.The variation law of crankshaft transition fillet stress and deformation with crank angle was discussed during the actual working process of diesel engine.The sensitivity of crankshaft structural parameters was analyzed by means of finite element simulation,and an optimization scheme was proposed.Based on fatigue cumulative damage rule,fatigue life analysis method and average stress correction method,the load spectrum was drawn and compared the optimized fatigue life and safety coefficient of crankshaft with the original.A fatigue test bench was built.According to the principle of paired lifting method,bending and torsion fatigue tests of crankshaft were carried out,and the relationship between fatigue ultimate load and reliability was discussed.The main research work were as follows:The indicator diagram of 494Q diesel engine was actually measured,and the boundary conditions of crankshaft bending,torsion and bending/torsion composite loads were analyzed.The ANSYS WORKBENCH was used to study the variation of fillet stress and deformation with the crank angle under bending load,maximum torsion load and bending/torsion composite load.The results showed that the maximum stress of crankshaft mainly occurred at the transition fillet.When the fourth cylinder of diesel engine worked,the maximum stress occurred at the right transition fillet of the fourth crank pin,the maximum stress was 220.76 MPa.When crankshaft was subjected to the maximum torsion load,the maximum torsion stress was 153.2 MPa,which occurred at 25°CA after TDC.When the crankshaft was subjected to the bending/torsion composite load,the maximum bending/torsion composite stress was 278.7 MPa,which was 26%larger than the maximum stress when the fourth cylinder of diesel engine worked,and occurred at 15°CA after TDC.The effects of structural parameters,materials on crankshaft strength were studied.Based on the sensitivity analysis theory,Crankshaft structural parameters such as main journal radius,crank pin radius,and transition fillet radius were selected for stress and deformation sensitivity analysis.According to the sensitivity analysis results,apply response surface method,the optimization scheme of crankshaft structure parameters was proposed.The results showed that the transition fillet radius had the greatest influence on crankshaft stress and deformation,and the corresponding stress and deformation sensitivity were-0.426 and-0.563 respectively.The finite element analysis of the optimization scheme reduces the maximum stress of crankshaft by 11.8%and the maximum deformation by 12.5%,the optimization purpose was achieved.The crankshaft fatigue life was predicted by the fatigue cumulative damage rule,fatigue life analysis method and the average stress correction method.The fatigue damage and safety factor of crankshaft were analyzed and compared with the optimization scheme's fatigue life and safety factor.The results showed that the crankshaft fatigue life before optimization was 2.452×10~9cycles,and the safety factor was 1.75.After optimization,the fatigue life of the crankshaft was increased by 17.2%and the safety factor was increased by 8.5%.According to the fatigue strength theory,the static strength safety factor of crankshaft was 2.9 and the bending/torsion composite fatigue strength safety factor of crankshaft was 1.77.Using the fatigue test method,the crankshaft fatigue test bench was built,and the bending and torsion fatigue tests of the crankshaft were carried out.Based on the principle of paired lifting method,the relationship between bending and torsion fatigue ultimate load and reliability under different survival rates were discussed.The results showed that at 99.99%survival rate,the crankshaft bending fatigue limit was 2036 N·m and the safety factor was 1.93;The crankshaft torsion fatigue limit was 4502 N·m and the safety factor was 3.91.The safety factor of crankshaft bending/torsion composite fatigue strength was 1.73.The test and simulation results had good consistency.