Shaft Alignment


There is always an interaction between the ship steelwork and the propulsion plant, whateverthe situation, at quay (static interaction), or during operation (dynamic interaction). The bearing offsets and the machining parameters of stern tube bushes are directly governing this interaction, so they must be carefully determined for ensuring a good compatibility between hull and propulsion plant in every relevant loading and operation condition. This can be achieved accurately by using elastic alignment calculations methodology which takes into account flexibility and deformations of steelwork, propeller forces and moments and accurate oil film characteristics in running conditions.

It is necessary to check that:

  • in static conditions, the maximum value of static pressure is below acceptable limit,
  • in running conditions, the thickness of the lubricant film is above acceptable limit to avoid temperature increase and bearing damage.

The stiffness of the lubricant film is an important parameter regarding the possible high vibration response of line shafting in lateral vibrations which must be avoided.

The methodology of analysis is based on a simultaneous treatment of the static and dynamic interaction between hull and propulsive plant by performing:

  • Elastic shaft alignment calculations for an accurate determination of optimum alignment conditions (shaft bearing offsets),
  • Vibration analysis for avoiding high vibrations levels.

Elastic Alignment methodology can be applied to any type of vessel (container vessel, LNG carrier, bulk carrier, tanker, Ropax…) and to any type of propulsion plant (low rpm diesel engine, diesel electric, turbine with main gear arrangement…).

Elastic alignment calculations for determination of theoretical optimum alignment conditions are performed using LILAS software. The implementation of these conditions on board (practical alignment operations) requires to perform specific shaft alignment measurements.

There is a risk in using classical shaft alignment methodology in which stiffness of bearing supports is assumed to be infinite and in which only static conditions can be accurately investigated. This risk must be assessed and elastic alignment methodology must be used if the risk is found to be high. Elastic alignment calculations are particularly relevant for big ships and for twin screw vessels.