A new method for strength evaluation of propulsion shafting coupling bolts based on the actual failure mode
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概要
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Damages to coupling bolts connecting propeller shaft, intermediate shaft and crankshaft of the main engine while infrequent also occur repeatedly. This paper identifies the exact causes of such damage through an in-depth examination of several bolt failure cases. It is found that contrary to their design criterion to avoid the shear failure at the face of the couplings, the bolts break off within the coupling flanges in bending. Examinations of the fracture surfaces reveal that the cracks initiate on the bolt surface of the reamer portion due to fretting fatigue that occurs in between bolt and flange hole surfaces. After initiation, the cracks are driven by torque-induced bending stress. The longitudinal location of maximum bending stress is determined by both experimental and theoretical analysis. Experimental and theoretical analysis results show that the maximum bending stress is mainly controlled by the bolt pretension force, the diametral interference between the reamer and the flange holes, and the contact length of the bolt reamer portion, in addition to the transmitted torque itself. Adequate bolt pretension forces reduce the bending stress in bolts by transmitting more torque through the friction between the flange coupling faces. The extent of shrinkage between reamer and hole is believed to change the supporting stiffness from flanges to bolts; the stiffer the supports, the smaller the bending stress. In addition, a shorter contact portion between reamer and hole shows a favorable effect on the bolt bending. As for the amplitude of the transmitted torque, the damage experienced cases are all estimated to be subjected to relatively large torsional vibratory torque. As the result of clearly identifying the affecting factors to the bolt failures, the Society's rule requirement for determining coupling bolt diameter has been changed to take into account the effect of vibratory torque. In addition to the change in rule requirement, the authors recommend preventive measures addressing the effects of the reamer shrinkage extent and the length of the reamer contact portion as well as the bolt pretension.
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