Abstract
The problem of the accurate estimation of resonant amplitudes of torsional vibration in small marine Diesel-engined propulsion sets is considered from the viewpoint of the design engineer. A short discussion on torsiographs is followed by the basic theory of the energy method of calculation, with particular reference to the calculation of vibration amplitudes assuming linear damping proportional to the phase velocity of the vibration. Harmonic input torque diagrams are given for a small marine two-stroke engine.
The methods put forward by different investigators are considered by comparing the calculated results with those obtained by torsiograph from a number of marine Diesel units investigated by the author. Both the one- and the two-noded forms are considered. In the former, propeller damping is shown to be the main influence and a method of computation put forward by Dorey is shown to be satisfactory after slight modification. For the two-noded form the position is not so satisfactory. Hysteresis damping in the crankshaft, viscous damping at the bearings, overall velocity damping, etc., are considered in turn and rejected as unsatisfactory. The importance of apparent damping effects—particularly cyclic fluctuations of speed or cyclic irregularity—is stressed, and a method is evolved in which this consideration is utilized in conjunction with results published by Major Carter which the author has found fairly satisfactory to date. It is concluded that much has still to be learnt on the subject before empirical methods can be dispensed with.
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