LINE SHAFT SYSTEM :

The line shaft system of a ship comprises of Tail-end shaft, Intermediate shaft. Thrust Shaft and Engine Crankshaft. The shafting supports are arranged so that there will be no overload'of the bearings due to changes in ship attitude between ballast and loaded conditions.

In older ships with midship engine and smaller length, shaft system tended to be long and diameters small, these flexible shafts were installed in very stiff ships, anc alignment and method of support was not a problem.

The increase in ship size has resulted in shafts of large diameter (power increase at lower revolutions). This has been accompanied by a trend towards "Aft End" engine installations, with short length of shaft giving a very stiff shaft system. With the improved ship design, there is considerable reduction in ship scantlings which made larger ships fairly flexible. Flexible ship and stiff shaft has led to a greater risk of damage to the shaft bearings. The problem is further aggravated by the large, slow rotation propeller with its overhang mass. The overhang mass of the propeller has a significant effect on the stern tube bearing. In an effort to protect this bearing, it is usually fitted in a stern frame that is bored on a "SLOPE".

In an effort to reduce"the effect of ship flexing, the "Fair Curve" alignment method is widely used, particularly with aft end installation.

In the shaft alignment procedure the follwoing criteria should be met:

a)                 Similar and known positive loads on all bearings,

b)                 Acceptable levels of vibration.

c)                  Maximum distance between bearings (giving flexibility).

d)                 Bending stresses in each shaft section to be at acceptable levels.

e)                 Minimum transfer of bending moment from shaft to crankshaft.     

f)                      Adequate allowance for thermal lift of engine, cold to operating condition.

g)                 Maintenance of the above criteria in both ballast and loaded condition.

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