Class 2 (2nd Engineer) MEKM 📅 Mar 2025

Exam Question

(a) Define the term Torsional Vibration with respect to an engine crankshaft, stating the effect that high levels can have on an engine crankshaft (6)

(b) Explain how engine deterioration influences the risk of Torsional Vibration, stating what can be done to minimise that risk (5)

(c) Explain TWO possible reasons for the activation of a Torsional Vibration alarm after an engine has been started if there had been no previous history of such an alarm and if no maintenance had been undertaken on the engine whilst it was stopped (5)

Reference Answer

(a) Torsional vibration is caused by forces applied to the crankpin by the connecting rod, which vary according to the angle of thrust exerted by the connecting rod and the cylinder firing pressure. It occurs during the firing and compression strokes.
This stress is cyclic, meaning the crankshaft twists and untwists along its length. In direct-drive engines, torsional vibration can be exacerbated by an unbalanced engine cylinder or propeller shaft, potentially caused by a damaged propeller.
An increase in torsional vibrations results in higher torsional stress, which adds to the existing stress levels. This increase in stress can lead to the generation and growth of cracks in high-stress areas of the crankshaft. If left unaddressed for an extended period, this condition can lead to the crankshaft breaking.
(b) As the engine deteriorates over time, the materials weaken due to fatigue. Fatigue occurs when a material becomes "tired" and fails at a stress level below its nominal strength. Torsional vibration is a cyclic stress that causes the crankshaft to twist and untwist repeatedly.

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