MARINE TECHNICAL ASSISTANCE — SHAFT ALIGNMENT STUDIES AND ANALYSIS

Ensuring Optimum Propeller Shaft Performance

Correct shaft alignment is a key issue for ships, especially on large vessels such as containerships, bulks, tankers and gas-carriers. Any incorrect alignment of the shaft can result in undesirable vibrations and higher operating costs. Furthermore, damage to the stern-tube bearings or other key parts of the vessel’s propulsion equipment could result in the ship being immobilised.

What are shaft alignment studies and analyses?
Bureau Veritas offer shipbuilders and ship owners a complete set of services around rational shaft alignment. The shaft alignment studies and related analyses must take into account the type of ship, the shaft arrangement and the required degree of accuracy. They include theoretical studies and practical operation.

What are the key benefits?
Avoid undesirable vibration and resulting negative effects.
Prevent costly repairs and immobilisation of the vessel.
Ensure operating the line shafting in the best running conditions for its optimal behaviour, particularly in the stern tube area.
Improve efficiency of the whole propulsion system, including the main engine and eventual gear box.

OUR APPROACH
Theoretical studies
Rational Alignment Analysis. The conventional study which considers the bearings as infinitely rigid includes the following items:
- Basic calculations with influence coefficients of bearings.
- Rational distribution of reactions in way of bearings, including gear shaft bearings.
- Boring out of stern tube and bushes.
- Details or practical operations of alignment, allowing realisation of calculated rational alignment.
Elastic Alignment Analysis. This complementary study may be carried out according to the results obtained previously. This analysis, which takes into account the elasticity of bearing material and hull structure
deformation, provides the following information:
- Distribution of reactions
- Deflection of material
- Deflection of hull structure
Influence of oil film. This approach determines the following:
- Distribution of pressure
- Distribution of reaction
- Oil film thickness
in relation to rpm, shaft alignment, structure elasticity, and propeller forces & moments.
Practical operation
Survey during mounting. This survey involves mainly of checking what enables us to achieve the theoretical rational alignment study.
- Checking of relative position of bores and their predetermined slopes.
- Checking of coupling conditions to fit the intermediate bearing.
- Checking of coupling conditions for positioning the reduction gear or main engine.
Jack up test. This test enables us to determine the real reaction in terms of accessible bearings and
verify their influence coefficients.
- Intermediate bearings.
- Fwd bush or aft bearing of shaft reduction gear (in relation to the shaft arrangement).
Alignment checking. The results of alignment checking by optical check or laser methods are analysed and compared with the boring conditions calculated for the involved line shafting (ship newly built or in repair).
The laser method also enables you to verify the line shafting alignment conditions without disconnecting the shafts, the main engine or the gear shaft.