At the shipyard when the engine is installed, the engine height and athwartships position is adjusted in the proper way that the crankshaft center line is aligned with the output shaft center line. Any small misalignment due to the flexing of the hull and the weight of the flywheel is shown up in the crankshaft deflection readings which are recorded when the vessel is new and should be kept within the limits stated by the maker.
During the operation, the crankshaft main bearings and shaft bearings wear. If the wear-down equal on all bearings, then the alignment would not alter. However, the crankshaft bearings will not wear at the same rate. This rate can be influenced by overloading of individual cylinders or by the hogging and sagging of the hull, as well as factors such as corrosion and erosion within the bearings. Wear-down in the bearings will lead to excessive bending of the crankshaft as the firing loads are transmitted into the crankshaft, which as it rotates is subject to cyclic stressing. If mentioned wear factors allowed to become excessive, crankshaft failure could result.
As the main bearings wear down, the crankshaft center line may not now be in line with the output shaft center line. This will cause excessive load on the bearings adjacent to the coupling flange which will increase the rate of wear, resulting in possible bearing failure.
Wear-down should be kept within prescribed limits. Crankshaft deflections should be regularly checked and loading on output shaft bearings can be ascertained if doubt exists by jacking the shaft clear of the bearing and noting the jacking load.
