Class 2 (2nd Engineer) MEKG 📅 Dec 2023

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. (4)

(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. (6)

Reference Answer

### (a) Torsional Vibration: Definition and Effects
**Definition:**
Torsional vibration is the periodic angular oscillation or twisting and untwisting of a crankshaft along its longitudinal axis. The crankshaft, flywheel, intermediate shafting, and propeller can be modeled as a multi-mass elastic system. The crank throws, connecting rods, and pistons represent the masses (moments of inertia), while the journal sections of the crankshaft act as torsional springs connecting them.
This system possesses one or more natural frequencies of vibration. Torsional vibration occurs when the frequency of the periodic turning moment (torque) applied to the crankshaft, or one of its harmonics, coincides with one of these natural frequencies. This condition is known as resonance, which leads to a significant amplification of the vibrational amplitude and associated stresses.

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