Maintenance and Repairs

Crankshaft Renewal on a MAN B&W 4 Stroke Engine

Lifting engine frame to replace crankshaft on a medium speed engine after wiping the bearings
 

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 700C) 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 MAN B&W medium speed engine. Thanks to Shaun Towers for the pics.

The engine is isolated and drained; Turbocharger, air cooler, heads, running gear and liners removed. Camshaft (which is in sections) is removed along with fuel pumps.

Flywheel disconnected and suspended. End cover removed and timing gears removed

Detail of lifting arrangement mounted on entablature.

The frame is lifted using chain blocks....

.....and landed on supports At this point the crankshaft is still in position, with some of the bearing caps removed.

Strops are attached to the crankshaft, passing up the cylinder bores to chain blocks. The remaining bearing caps are removed, and the crankshaft lowered.

The crankshaft is lowered onto wooden blocks as shown.

The weight of the frame is taken on the chain blocks, the  supports removed from one side, and using chain blocks, the crankshaft is removed sideways....

...and out of the engine.

This shows the damaged journal caused by an overheated run bearing. 

The old crankshaft is lifted clear, and the new crankshaft is lowered down to the side of the engine.

Using chain blocks and strops, the new crankshaft is moved into position under the engine frame.

Care is taken not to damage the journals as the crankshaft is manoeuvred into position.......

....and lifted up into the engine frame..

 

After Smoko.......

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