Analysis of propeller shaft transverse vibrations

Warikoo, Raman (1989) Analysis of propeller shaft transverse vibrations. Masters thesis, Memorial University of Newfoundland.

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Abstract

The aim of this project was to find a simple, practical and sufficiently accurate method for finding the natural frequencies of a propeller shaft assembly. The need for such methods is felt at the early design stages when sufficient data about the system is not available and the need to restrict the cost of analysis is of importance. Due to these facts it is clear that the methods based on the discretization of the continuum, which can estimate the natural frequencies accurately may not be advisable to go in for in the initial stages. -- In the class of approximate methods, to which this work belongs, investigations done so far have crudely modelled the propeller, which was avoided in the present work. The propeller was considered to be a flexible rotor mounted with blades. -- To solve the problem analysis was carried out in three parts. First the blade was studied for the natural frequencies, mode shapes, static deflections and the steady state stresses. The blade was assumed to be a cantilever and the results were cross-checked by a preliminary finite element analysis using beam elements. The assumption was found to be quite valid after doing the finite element analysis using 3-D isoparametric 20 noded elements. For the airfoil blade effects of rotation, shear deflection, rotary inertia, pitch setting angle were taken into consideration. Second the shaft-rotor system was studied and the equation for the transverse vibrations was derived in the complex plane which was solved for the natural frequencies. 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. Lastly the natural frequencies of the blades and the shaft-rotor system were coupled to give the resultant natural frequency of the propeller shaft assembly. The hydrodynamic effect has been taken into consideration by incorporating the added mass effects for the propeller. Two approximations for the natural frequency of the assembly were arrived at, the zeroth order and the first order. The true value of the frequency is supposed to lie in between the two limits. Effects due to change in the number of blades, blade geometry and the shaft parameters on the propeller shaft assembly were checked for. Agreement between the results obtained from the present work and with those available in the literature was found to be excellent. Finite element analysis comparisons also were very satisfactory.

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