OWING to vibration troubles which occasionally arise in engine crankshaft propeller systems, it has become necessary to investigate such possibilities in the design stage. In this paper we are concerned with the torsional vibration of an engine crankshaft coupled with flexural vibration of the propeller. The engine crankshaft will have a number of throws, which it has become customary to replace by “equivalent” pulleys which have the same moments of inertia as the complete crankthrows and are usually designated as engine “masses”. Assuming that there is a node in the propeller shaft, we may represent the torsional stiffness of the propeller shaft between the gears and the node by ca / (l−γ) where ca is the torsional stiffness of the propeller shaft; the torsional stiffness of the other portion of the propeller shaft will then be ca/γ. The frequencies of torsional vibrations of the engine crankshaft system about the assumed node are found for various values of γ. The corresponding frequencies of flexural vibrations of the propeller arc also found for the same values of γ. The two sets of frequencies may then be plotted against γ, giving engine crankshaft and propeller frequency curves respectively.
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