Study On The Torsional Vibration Characteristic And Stress Engineering Prediction Of Ship Shafting

Ship propulsion shafting is an important part of marine power plant, including the transmission shaft which is from the Main Engine output thrust bearing to the propeller and the shafting attachment. The main function of ship shafting is to transfer power to propeller making it rotating; meanwhile, thrust force is transferred to the hull, and push ship going forward. In the navigation, complex loads are acting on the propulsion shafting, thus the shafting torsional vibration is an important factors affecting the reliability of ship power plant. If the shafting is always working in the dangerous critical speed region, it may lead to shafting dynamic performance declining, shafting distortion and fatigue invalidation, or even shafting broken. These all affect the ship navigation performance and security, so the study of shafting torsional vibration has great significance.The torsional vibration is one type of mechanical vibrations, its calculation probably can date back to Newton era, but as a practical production problem, it appears much later. The ship propulsion shafting torsional vibration calculation could provide prediction, testing and guiding functions for the design, construction and installation of shafting; it is also one of the essential elements during the design, manufacture, installation and testing of shafting; it provides strong guarantee to propulsion device's reliable and safe running; it has important guidance significance to the further study on the reliability of propulsion shafting, power plant fault diagnosis and power plant simulation calculation.There are a lot of torsional vibration calculation methods, however, no matter which method it uses, the formula involved is basically similar. Many experience formulas are used in the torsional vibration stress calculation, especially in the shipyards, where the experience formulas are widely used, but some experience formulas are not accurate, there are some errors between calculation stress and measured stress. To improve the accuracy of the experience formulas'calculation and the stress engineering prediction, some experience formulas need to be modified based on the ship torsional vibration test.This thesis uses one kind of traditional experience formula to calculate two ships'( CSAZALEA and SHENGFAHAI ) shafting torsional vibration stress respectively, which are 57,000t bulk carriers built in Zhejiang Zhenghe Shipyard and Jiangsu Jiangdu Shipyard. By testing the two Bulk carriers'torsional vibration stress, the measured stress is compared to the calculation value, the result shows that torsional vibration stress errors from experience formula are all more than 8% at the intermediate shaft, propeller shaft and crankshaft, the largest error exceeds 15%. In this case, the large error will result in accuracy reducing of torsional vibration stress prediction and have great impact on the whole design, construction and installation of ship shafting.As the large error exists, it provides three modification methods including linear modification, quadratic polynomial modification and power modification according to the different circumstances. Base on the result of shafting torsional vibration test to the SUNRAY bulk carrier which is built in Jiangsu Hantong Shipyard, the calculation stress got from modified formula is compared with the measured stress respectively at the part of intermediate shaft, propeller shaft and crankshaft, and the result proves that the modified stress formula can increases the accuracy and different shafting parts using different modified formulas makes the highest accuracy. And suggestion that linear modified formula be used in the intermediate shaft stress calculation, power modified formula be used in the propeller shaft stress calculation and quadratic polynomial modified formula be used in the crankshaft stress calculation is given to obtain a higher accuracy of torsional vibration calculation.For the main factors affecting experience formula accuracy are physical parameters and balance torque at the intermediate shaft, the linear modified formula has the minimal error. The torsion angle of propeller shaft is biggest in the whole ship shafting and can be seen as a large deformation case, thus the power modified formula has the minimal error. The stress quadratic coefficient has a greater impact at the crankshaft, so quadratic polynomial modified formula has the minimal error.The modified formula can be used to calculate ship shafting torsional vibration stress, it has important practical significance to improve the accuracy of calculation and engineering prediction of torsional vibration stress; it also has great guiding significance to the design, construction and installation of ship shafting.