Crankshaft deflections are measured to detect the misalignment of main bearings. The misalignment occurs due to bearing wear or deflection of crankshaft. The horizontal and the vertical misalignment are checked.
- Bearing temperature increases
- Damaged bearings
Conditions under which crankshaft deflection to be taken:
- Ship should be afloat (i.e. not in dry dock)
- Calm weather
- Comparison should be made under same temperature and loaded condition
The crankcase alignment is checked using a dial gauge. The gauge is fitted between the adjacent webs, opposite the crankpin at half of the diameter from the shaft centre. The dial gauge measures the crank web spread at different angular position of the shaft.
- Make an enclosed space permit
- Stop engine and allow to cool
- Block the starting mechanism
- Shut off starting air supply
- Engage the turning gear
- Open indicator cock ( because air will compress and give wrong reading on turning the engine)
- Stop lubrication oil pump
- Put tag “ Men at work”
- Open crankcase and allow ventilation before entry
- Closing of the crank throw (compression of the gauge) is regarded as negative and opening of the crank throw is regarded as positive.
- The procedure is repeated for each unit and measurements are recorded.
Reasons of misalignment:
- 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