Class 2 (2nd Engineer) MEP 📅 Aug 2024

Exam Question

(a) Describe how it is determined whether a crankshaft was twisted during a major "smash up" in a main engine. (4)

(b) Explain where twisting is most likely to occur. (4)

(d) Explain briefly what adjustments and precautions should be instituted when putting an engine with a twisted crankshaft back into service (4)

Reference Answer

(a) Determining Crankshaft Twist After a "Smash Up":
A crankshaft's twist is assessed by examining witness marks. Before installation, these marks are etched onto both the crankshaft journal and the mating crank web. The interference fit between these parts (approximately 1/570 to 1/600) creates a compressive load of about 77 MN/m². If the crankshaft experiences an extreme load (e.g., sudden engine stall, starting with a liquid-filled cylinder, or bottom-end bearing failure), the journal might slip on the web. Misalignment of the witness marks indicates the degree of twist.
(b) Likely Locations for Twisting
Twisting typically occurs at the journal-to-web interface, especially under extreme loads or impacts.

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