Shafting lateral vibrations|
Shafting lateral vibrations are characterized by shafting segments oscillating in a plane passing through the shaft's neutral position. For simplified description purposes only, the shaft axis may be considered the shaft neutral position.
Lateral vibrations may be considered as a special case of the more general whirling vibrations, which represent the resultant motion of two concurrent motions, each in perpendicular planes passing through the shaft neutral position.
Lateral vibrations are mainly excited by the propeller weight, propeller induced variable forces and shafting segments' weight and imbalance. The amplitudes of lateral vibrations are generally enlarged by the increased span between the line shaft bearings. However, it must be clearly understood that a small inter-bearing distance could also provoke enlarged lateral vibrations. It is especially the case with the stern tube bearings, if the forward stern tube bearing becomes unloaded.
In general, the basic design countermeasure against the unacceptable shafting lateral vibrations is to ensure that the lateral natural frequencies are positioned sufficiently far away with respect to the propeller rotation speed.