Main Bearing Failures

The vessel which you are about to join as Chief Engineer has recently suffered the main bearing failures. State, with reasons for your answer, each of the following.

The information and documents you would require in order to enable you to assess the cause of such failures.

Ans)

The most recent bearing clearance readings and when they were taken with details of shims fitted or removed. Bearing clearances on thin wall bearings are non adjustable: thick wall bearing clearances are adjusted with shims . The wrong clearance will lead to breakdown of hydrodynamic film as will uneven shim thickness either side of bearing

Crankshaft deflection readings for ballast and loaded condition: This will give an indication of the alignment of the crankshaft in the bedplate. If outside of manufactures recommendations, it may cause overloading of bearing s which are high.

Sets of recent power and draw cards: This will give an indication as to the cylinder power balance of the engine. Excessive loading in a particular unit, or overload of the engine in general will lead to higher bearing loads.

Details of recent Lo tests on the crankcase oil shipboard or from spectrograph analysis ashore. On board tests will give a guide to water content which can cause corrosion especially sea water corrosion, viscosity, too high indicates contamination by cylinder oil/products of combusting causing poor heat transfer and damage to bearings by calcium deposits; too low indicates contamination by fuel oil, leading to breakdown of hydrodynamic film. Spectrograph tests will give an indication to particles of metals present which may indicate if the filtration/purification system is efficient as well as a clue to which engine parts are wearing.

The last survey reports for the main bearings prior to failure together with dates, clearances, journal condition and diameter, (ovality can lead to failure) and any other information, which may be available e.g., photos. If the failed bearings were surveyed at the same time, there could have been mistakes made when reassembling (tightening sequence, tightening pressures)

Reports as to the events leading up to bearing failure; indications of failure, damage reports.

Ovality of journals is due to the varying load on the crankshaft due to the varying cylinder pressure if the joviality approaches 25% of the bearing clearance then the hydrodynamic lubrication will be affected, This leads to failure of the bearing through wiping of the white metal, regular measurement will allow the problem to be rectified before damage occurs. Avoid overloading.

Increased surface roughness of journal: This will cause metal contact with the bearing material, and therefore wiping, because the oil film is no longer thick enough to separate the two surfaces. This increased roughness can be caused by abrasive damage to contamination, corrosive damage due to sea water contamination (tin oxide encrustation) or acidic oil. Avoided by correct filtration, purification and replenishment of lub oil when required.

Spark erosion of the bearing material due to potential difference between hull and prop shaft due to the bronze propeller being immersed in sea water (an electrolyte ) This earths down through the main bearing shells.

Cracking of bearing material: Can be caused by incorrect bonding between white metal and backing, but also caused by fretted bearing housings (inadequate for steel shell backing), misalignment between bearing cap and housing ( incorrect tightening of nuts )and overloading. Avoided by following manufacturers instructions when refitting bearings; use only engine builders spares, tighten bearing cap nuts equally and opposite, not diagonally, if 4 are fitted.

Regular checking and recording of main bearing clearances when carrying out a crankcase inspection. At the same time check for squeezed out white metal and white metal in bottom of crankcase. Any misalignment of the crankshaft investigated and rectified. As well as leading to bearing failure, this can lead to crankshaft failure due to excessive bending(cyclic stressing). Misalignment may be due to uneven bearing wear down indicating the likelihood of another imminent failure, fretting of chocks or hull distortion .

If the engine is a two stroke crosshead engine, and there is evidence of contamination of the crankcase lub oil with cylinder oil then it may be prudent to discontinue the practice of purifying the drains from the piston rod stuffing boxes to the main engine sump.

Regular L C analysis can help prevent damage to bearings by water/acid attack. Early warning of presence of water will allow investigation into cause, Efficient purification will remove water and solid particles. Efficient filtration will prevent scoring of shaft and bearing

Ensure the shaft is earthed via slip rings to the hull to prevent spark erosion. Correct fuel injection can be verified by regular power/draw cards, which will prevent overloading or uneven loading of the engine. On a medium speed engine a power balance can be obtained from compression/peak pressures.

If the crankshaft main journals are damaged by scoring or are oval, then this can be corrected by grinding and fitting undersize bearings.