Exam Question

(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.