Crankshaft is the intermediate part of a marine engine, which transfers the power of a firing cylinder from the reciprocating piston to the rotating propeller (or alternator in case of a generator).
The working of other components of the engine depends upon the correct rotation of the crank shaft such as camshaft for fuel timing, firing order of units etc. Failure of a single part of the crankshaft can stall the engine as well as the ship.
As a marine engineer working on a ship, one should know various reasons which can lead to failure of this important component.
Reasons for failure of crankshaft
Fatigue Failure: Majority of steel crankshaft failure occurs because of fatigue failure, which may originate at the change of cross-section such as at the lip of oil hole bored in the crankpin.
Failure due to Vibration: If the engine is running with heavy vibration especially torsional vibration, it may lead to crack in the crankpin and journal
Insufficient lubrication: If the lubrication of bearing in the crankshaft is starved, it may lead to wipe out of the bearing and failure of the crankshaft
Over Pressurised Cylinder: It may happen that there is hydraulic lock (water leakage) inside the liner and due to extreme pressure the crankshaft may slip or even bent (if safety valve of that unit is not working).
Cracks: Cracks can develop at the fillet between the journal and the web, particularly between the position corresponding to 10 o’clock and 2 o’clock when the piston is at T.D.C.
Reasons for Crankshaft Misalignment
Crankshaft of a marine engine is a massive component when fully put together in the engine. Initially the complete crankshaft is aligned in a straight line (connection drawn from the centre of the crankshaft makes a straight line) before setting it on the top of main bearings.
But with time due to various factors, the straight line may deviate and misalign. A degree of misalignment is acceptable within limits but if the value goes beyond that rated by the manufacturer; it may lead to damage or even breakage of the crankshaft.
Following are the reasons for misalignment of crankshaft-
- Damage or wipe-out of the main bearing
- Loose engine foundation bolt leading to vibration
- Deformation of ship’s hull
- Crack in the bearing saddle
- Loose main bearing bolt leading to damage of main bearing
- Very high bending moment on the crankshaft due to excessive force from piston assembly
- Grounding of the ship
- Crankcase explosion or fire
- A defective or worn out stern tube or intermediate shaft bearings
- Loose or broken chokes in the foundation
- Bearing pockets cracked
- Bedplate deformed – transverse girder damaged
- Tie bolts slack or broken
- Weakening of structure due to corrosion
It is thus advisable to regularly have a crankcase inspection and crankshaft deflection (to check the misalignment).