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Horror Stories

Failed Bottom End

 

 

This incident happened when the bottom end of a V12 Pielstick 2.5 engine failed. With no cap to hold it in place, the con-rod unseated from the crank pin and struck the counterweight. The impact sheared the pin securing the counterweight in its notches. As the crankshaft continued to rotate, it struck the counterweight and con-rod, jamming them against the lower edge of the No 3 cylinder liner. The force transmitted by the con-rod top end to its gudgeon pin caused the piston skirt to break into several pieces. The impact of the con-rod bottom end striking the starboard casing wall punctured the casing, damaging the oil and fresh water lines.

 

Inspection of the main engine revealed that the connecting rod (con-rod) had separated from piston No 3. The piston crown had separated from the skirt, which was in pieces. The crankshaft was bent at the No 4 main bearing. The exhaust valve and push rods for the exhaust and inlet valves in the No 3 cylinder head were also bent. The No 6 counterweight and the piston end of the No 3 con-rod were jammed between the crank pin and the bottom of the liner and the casing.

 

The Pielstick engine is fitted with fixed centre con rods where the split between the connecting rod and bottom end is at 45 to the con rod centre line. The location of the two parts is by serrations. Although this design prevents the typical cyclic stressing experienced by the  bottom end bolts in a conventional bottom end arrangement, the design can lend itself to fretting of the serrations especially if the bottom end is incorrectly tightened. Manufacturers recommend that the serrations on the bottom end are closely examined and are crack detected every 24000 hours, especially in the upper portion of the con-rod, where the stresses are normally concentrated.

 

In this case  the con-rod cap fractured near its centre which is unusual. It originated in a corrosion pit on the contact surface with the bearing shell, then propagated to over 95 percent of the cap surface through high cycle fatigue. The cap subsequently failed in overload in the smallest section. The break-up of the cap caused the four bolts to fracture.

 

 

Surface of crack in con-rod cap. Note the fatigue striations covering over 95 percent of the surface. The letter "O" marks the point of origin of the crack, and "D" indicates the bearing shell contact surface.

 

A dye penetrant test on the bottom end cap shows that cracking was present, although this was not the cause of the failure.

Information of this event was taken from the Transportation Safety Board of Canada website. The full report can be downloaded here in pdf format

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