Replacing a crankshaft on a medium speed engine can become necessary after a bearing failure if damage to the crankshaft journals has occurred, and regrinding of the crankshaft is not feasible. On older engines a 0.4% carbon steel (EN8 or BS970 080M40) was used for the manufacture of crankshafts. This material could withstand overheating, and often could be reground.
Modern crankshafts for medium speed engines are manufactured from high tensile steel; for instance a 3% chromium molybdenum nitriding steel (EN40B or BS 970 722M24). Whether these shafts are surface hardened or otherwise, the severe overheating that can occur (above 700°C) when a bearing fails may render the crankshaft beyond repair. On a none hardened crankshaft, bending and cracking can occur, together with localised hardening. Where the crankshaft has been surface hardened, then annealing can occur, together with cracking.
The following photographs were taken during a crankshaft replacement on a Wartsila 32 engine. Thanks to John Koufopoulos for the photos.
In the case where an underslung crankshaft has to be replaced, the following gives a guide to the procedure.
Above and Below: The new crankshaft is being lifted and pulled into position. Note the Wooden blocks (below) supporting the frame.
Once the crankshaft has been lifted into position and secured with bearing caps the engine frame can be lowered back into position, and the holding down bolts replaced. reassembly is basically the reverse of disassembly, with checks for crankshaft alignment. Rechocking may be required if the alignment has been disturbed, with the engine originally mounted on resin chocks.