Study On Working Characteristics Of A Ship Propulsion System And Analysis Of Bearing Lubrication

A complete multi-body dynamic model of a large transport ship propulsion system,including marine diesel engine, shaft and propeller connected by the non-liner joints, isestablished based on the finite element method and multi-body dynamics method. And asthe external load, the engine combustion pressure and exciting force of propeller are loadedon the propulsion system. In this thesis, the calculation and analysis of workingcharacteristics are performed by means of the integration of simulation and experimentalstudy，respectively under rated working condition, one cylinder misfire working conditionand different load working condition for marine propulsion system. Besides, based on theelastohydrodynamic lubrication theory, the lubrication characteristics of main bearing areinvestigated. That provides a significant reference for optimization design of marine powerplant performance and system, and for study of bearing lubrication characteristics.The mainwork is as follows:(1) Under the rated working condition, the working characteristics of global vibrationcharacteristic for propulsion system, vibration characteristic at fore end of crankshaft,orbital path and load of all main bearings, etc, are analyzed. The results show that, theglobal vibration displacements of crankshaft at the X, Y, Z direction are all relatively small,and the displacement at Z direction has the typical ‘piston’ vibration characteristic. Themaximum torsional displacement at fore end of crankshaft is2.62mrad, and the maximumamplitude of axial vibration is3.4mm. The vertical load of No.6main bearing is maximumin all main bearings. The orbital paths of all main bearings are in an allowable range, andbecause of journal tilting, the axis offset of both sides cross section for intermediate shaftare obviously greater than that of the middle section. The comparison made with the shiptrial test data shows that the difference between measured result and calculated result forangular displacement at fore end of crankshaft is3.15%, and the correctness of calculatedresult is verified.(2) With regard to the one cylinder misfire working condition in sailing, the thesis alsomakes some corresponding analyses. Under1#~6#cylinder misfiring condition respectively,the working characteristics of global vibration characteristic, vibration characteristic at foreend of crankshaft, and load of all main bearings, etc, are investigated. The results show that,under the working conditions of2#,4#,6#cylinder misfiring respectively, the vibration displacement of crankshaft has a similar trend at the first half cycle, and a bigger differencehappens under the influence of different cylinder misfire at the later half cycle. Under theinfluence of cylinder firing interval and distance to the fore end, the vibration displacementat fore end of crankshaft has a significant difference. The vertical load of3#and5#mainbearing is maximum,1#and7#main bearing is relatively small under unilateral loaded, and8#main bearing is minimum because it’s far from each cylinder.(3) Based on the above calculation, under4#cylinder misfiring and rated conditions,the working characteristics of vibration characteristic at fore end of crankshaft, and at flangeend for propulsion shafting, load of all main bearings, etc, are compared and analyzed. Theresults show that, compared with the rated condition, the maximum torsional angulardisplacement at fore end of crankshaft is increased by13%, and by70%for maximum axialdisplacement. At the normal time of firing, the maximum angular displacement of flangeend for propulsion shafting is increased by766%, at the same time the vertical load of4#and5#main journal is reduced obviously, the direction of load also changes. The maximumvertical load of journal at foreword stern bearing location is increased by34.5%, and by26.9%at rear stern bearing location, and the fluctuation is more obvious.(4) In the light of low load and overload traveling conditions in the sailing, thevibration characteristic and torsional stress for propulsion system under different loadcondition are compared and investigated. The results show that the vibration displacementand torsional stress are much larger under working condition near the critical resonancespeed and speed restricted range than that under other working conditions. And in otherconditions, as the load increases, the displacement and stress correspondingly increase. Thecomparison made with the ship trial test data shows that, the calculated result and measuredresult of torsional angular displacement at fore end, stress of crankshaft, intermediate shaftand stern shaft are in good agreement, and the correctness of calculated result is verified.(5) The lubrication characteristics of minimum film thickness, maximum film pressure,friction loss power, and oil fill rate, etc, for3#main bearing under the rated condition and85%load condition are calculated and analyzed. The results show that, the minimum filmthickness of3#main bearing is4.65μm and6.47μm respectively under the two differentconditions, and high oil pressure zone is concentrated at the left edge of lower half bearing.The maximum friction loss power and average friction loss power under rated condition arelarger by29.37%and15.81%respectively than that under85%load condition. Oil inflowsand outflows are held balance, and the oil fill rate under85%load condition is relativelyhigher. The maximum misalignment degree of No.3main journal around X axis is0.018deg and0.016deg, and-0.03deg around Y axis.