Analysis of propeller shaft transverse vibrations

Abstract

A simple, practical and sufficiently accurate method for finding the natural frequencies of a propeller shaft assembly is described. The need for such methods is felt at the early design stages when sufficient data about the system are not available and the need to restrict the cost of analysis is important. This need cannot be fulfilled at this stage by the use of methods based on discretization of the continuum, even if they yield much more accurate estimates. The problem is divided into three parts. First, the propeller blade, assumed to be a cantilever, was analysed for the natural frequencies and mode shapes. The validity of the assumption was confirmed by finite element analysis, the effects of rotation, shear deflection, rotary inertia and pitch angle setting were considered. Second, the equation for the transverse vibration of the shaft-rotor system was derived in the complex plane and then solved for the natural frequencies. The effects of forward and reverse whirl, fixed and simply supported forward bearing end conditions, tail shaft length variation, rotary inertia and shear deflection were investigated. Third, the natural frequencies of the blades and the shaft-rotor system were coupled to give the resulting natural frequency of the propeller shaft assembly. The hydrodynamic effect was taken into consideration by incorporating the added mass effects for the propeller. Two approximations for the natural frequency of the assembly were derived, the zeroth order and the first order; the true value of the frequency lies between these two limits.