Applied

Engine Repairs

Main Engine Crankshaft

The repairs described below are to aid maintenance planning and not a forum for denigrating machinery, companies or personnel

History

The engine is a 280MM bore 9 cylinder medium/high speed engine turning at 1000rpm. The oil mist detection alarm was sounded and the engineer on duty went to investigate. Approaching the engine a heavy mechanical noise was noted and the engine ws stopped by local control of the fuel rack

Investigation

A crankcase inspection revealed than both upper and lower bearings were absent from one con rod which had thus been running metal to metal.



Upper shell


damage indicated that the shell had been trapped before exiting. All white metal was absent



lower shell


The impact on the lower shell had been sufficient to fold it completely



The increased clearance allowed slight contact between piston skirt and balance weights. Rub had led to blueing of the con rod


The oil supply was via a main bearing. This was opened and found to be in good condition without damage. Lube oil analysis prior to this indicated no fault. Oil sample taken after the event was satisfactory with no indication of fuel dilution

During repairs all con rod bearings and main bearing were inspected and no damage was note other than scoring discussed in the conclusions. The engine had 800 hours since all units had been overhauled.

hardness testing of the journal indicated results far in excess of maximums. Spot grinding using a hand grinder to 1.6mm indicated that hardness was still unacceptably high. As this was approaching maximum allowable decision taken to replace crankshaft

Repair

All units were removed. All ancillaries such as lube oil pumps & filters, jacket cooling water pump, air cooler etc were removed. The block was detached from the bedframe and then lifted using chainblocks


And placed on stands

The sump pan was then dropped. The crankshaft was supported using chainblock and strops passing though the liners and all main bearing released The new crankshaft was support using extending bolts between the webs to prevent bending and then placed on a slide under the frame and brought into position.


Concluding

The level of destruction meant that determination of cause of failure was difficult and was one of removal of possibilities. The vibration damper had been renewed 5 years before therefore torsional vibration problem were unlikely although question marks had existed over it alignment to the gearbox.
No evidence of lubrication fault could be found and it was felt unlikely that incorrect assembly would have allowed the unit to operate successfully for 800hours.


Scratch marks were noted on several bearings and was probably caused by poor technique when changing lube oil filters. Specifically the filter housing ws not being drained and cleaned before inserion of new filters. These marks lead to a disproportionate loss of bearing loading surface due to the gradual rather than stepped collapse of oil film in the vicinity. Damage to bearing shells means this could not be proved or disproved.
Big end ovality was on manufacturers limits. It was felt that this was the most likely cause of failure.