Failure Analysis of Gearbox and Clutch Shaft from a Marine Engine
Two shafts that transmit power from the engine to the propeller of a container ship failed after a short time in service. The shafts usually have a 25 year lifetime, but the two in question failed after only a few years. One of the shafts, which carries power from a gearbox to the propeller, is made of low alloy steel. The other shaft, part of a clutch mechanism that regulates the transmission of power from the engine to the gears, is made of carbon steel. Fracture surface examination of the gear shaft revealed circumferential ratchet marks with the presence of inward progressive beach marks, suggesting rotary-bending fatigue. The fracture surfaces on the clutch shaft exhibited a star-shaped pattern, suggesting that the failure was due to torsional overload which may have initiated at corrosion pits discovered during the examination. Based on the observations, it was concluded that rotational bending stresses caused the gear shaft to fail due to insufficient fatigue strength. This led to the torsional failure of the corroded clutch shaft, which was subjected to a sudden, high level load when the shaft connecting the gearbox to the propeller failed.
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