MAIN SHAFT BEARINGS - EXCESSIVELY LOADED AND VERY LIGHTLY LOADED

Excessively loaded bearing:

• High bearing temperature (overheating).
• Excessive wear of the bearing white metal (rapid wear).
• Wiping/ scuffing of the bearing surface.
• Increased vibration/ noise from the bearing.
• Oil film breakdown, possible seizure.
• The bearing may show fretting/ hammering.

Very lightly loaded bearing:

• The bearing may "float"/ not carry its share of the load, causing the shaft to lift off the bearing.
• Oil film may not be maintained properly (the bearing may run with a thin/ interrupted film).
• Possible knocking/ hammering as the shaft lifts and drops.
• Uneven wear/ the bearing may not wear evenly.
• The bearing may run cooler than normal.

• Excessively loaded bearing: The overload on one bearing increases the load on the adjacent bearings (the load is redistributed), causing them to also overheat/ wear. The shaft deflection increases, and the adjacent bearings may become overloaded or misaligned.
• Very lightly loaded bearing: If one bearing is lightly loaded, the adjacent bearings carry more load (they become overloaded), causing them to overheat/ wear. The shaft may also be unsupported at the lightly loaded bearing, increasing deflection and stress.

• Excessively loaded bearing:
• Check and correct the bearing alignment (crankshaft alignment/ deflection).
• Check the bearing clearance and adjust/ renew the bearing.
• Check the oil supply (pressure, flow, condition) to the bearing.
• Check the shaft/ journal condition (out-of-round, taper).
• Correct the cause of the overload (e.g. misalignment, foundation/ chock problem, or a bent shaft).
• Very lightly loaded bearing:
• Check and correct the bearing alignment so the load is distributed evenly.
• Adjust the bearing clearance/ shims so the bearing carries its share of the load.
• Check the oil supply and ensure a proper oil film is maintained.
• Correct the cause (e.g. misalignment, or a change in the shaft/ engine condition).

Why load distribution on main shaft bearings changes in service:

• Wear of the bearings and journals changes the clearances and the shaft position, redistributing the load.
• Thermal expansion/ distortion of the engine frame, bedplate and foundation changes the alignment.
• Settling/ deterioration of the chocks and foundation.
• Changes in the crankshaft deflection due to wear or distortion.
• Changes in the engine load/ operating conditions.
• A bent or distorted crankshaft.
• Wear of the main bearing shells (uneven wear).