Study On Bearings Lubrication For Marine Diesel Propulsion Shafting Based On Multi-body Dynamics

Marine propulsion shaftings, composed of crankshaft, intermediate shaftings, stern shaftings and propeller, transmit power generated by marine diesel engine to propel vessel forward. Marine propulsion shaftings are supported by sliding bearings such as main bearings, intermediate bearings, fore and aft sterntube bearings built-in stern tube. Marine propulsion system has particular structure and complicated loads. In-depth analysis of the lubrication characteristics for friction pairs of marine propulsion shaftings and its influencing factors has important practical value for improving the economy and reliability of marine power plant and optimizing the shafting design. Currently, it is difficult to develop effective experimental study for the analysis and prediction of bearing lubrication characteristics for large low-speed two-stroke marine diesel engine propulsion shafting. In this paper, the multi-body dynamics model of marine diesel engine propulsion shafting was established, based on elastohydrodynamic lubrication theory, the lubrication characteristics of all bearings and its influencing factors were analyzed thoroughly.The propulsion system of a39000deadweight tonnage oil tanker propelled by a MAN B&W6S50MC-C diesel engine was taken as study objective, and the three dimensional solid finite element models of flexible crankshaft, intermediate shaft, and stern shaft, main bearings of diesel engine, intermediate bearing and sterntube bearings were established. Considering dynamic interaction behaviors of all flexible bodies and the fluid-solid interaction behavior between flexible bodies and elastohydrodynamic lubrication oil film, Modal Reduced-Substructuring method was employed for dynamic reduction of finite element models, the resulting files generated from reduced FE models were employed to create multi-body dynamics model of propulsion system.Considering purely the engine pistons, connecting rods, crankshaft and main bearings, but ignoring intermediate shafting, stern shaft and propeller, the simulation study on lubrication characteristics of MAN B&W6S50MC-C main bearings was carried out at rated working conditions. Simulation results show that:the loads and lubrication conditions of main bearings were quite different to each other, and the lubrication conditions of No.2main bearing was poor. The influences of bearings structure parameters and operating conditions to the lubrication characteristics of main bearings were quite different. Once one cylinder misfire, engine load should decrease, and the load on each bearing varied differently due to the influence of inertia forces. Supposing the bearing offset owing to mis-assembly probably, the bearing lubrication changed. In the case of bearing tilting, the bearing load changed very small, but the bearing lubrication deteriorated sharply, and high oil pressure zone deviated to bearing edges.Based on the above multi-body dynamics model, considering the influence of driving shafting and propeller, full shafting model for lubrication simulation of large low-speed two-stroke marine diesel engine propulsion system was created. The boundary conditions exerting on this model were closer to actual situation, and the lubrication characteristics of main bearings, intermediate bearing and sterntube bearings were more reasonable and more accurate. Simulation results show that:The lubrication characteristics of the main bearings close to engine output end changed significantly, but small change appeared on the main bearings close to engine free end. The load distributed uniformly on the intermediate bearing in axial direction, and the intermediate bearing lubricated well and worked smoothly. As the impact of the propeller gravity, the edge effect of the aft sterntube bearing load was very obvious, and high oil pressure zone located at the aft edge of aft sterntube bearing. With the increasing running speed of propeller shaft, the lateral vibration of shafting strengthened, and the bearing loads increased accordingly. As the impact of gyroscopic effects, the tilting angle of stern shaft decreased, and the sterntube bearing lubrication improved accordingly.In summary, the multi-body dynamics model of marine diesel engine propulsion shafting was established and the lubrication characteristics of sliding bearings were simulated in this paper. The multi-body dynamics model and interaction simulation method provides an effective research method to the analysis and prediction of bearing lubrication characteristics for marine propulsion shafting, and fast and effective optimization design of sliding bearings were achieved.