scope:

General

Propulsion shafting alignment is a process that consists of:

• The design and analysis

• The alignment procedure

• Measurements and verification

The terminology and requirements for shafting alignment vary depending on the type of the system analyzed (propulsion, gen-set, etc.), the powertrain size, and the alignment process itself.

The following terminology is used in these Guidance Notes.

Propulsion Shafting: A system of revolving rods that transmit torque and motion from the prime mover to the propeller. The shafting is supported by bearings, whose number and position is determined based on allowable bearing loads and lateral vibration (whirling) requirements. Static shafting alignment analysis criteria defines the acceptable load distribution and contact condition between shafts and bearings.

These Guidance Notes distinguish between "alignment process" and "alignment procedure". The alignment process encompasses all shafting alignment activity starting with analyses and reviews, to installation procedures, condition verification and trials, while the alignment procedure in itself refers only to shafting alignment production work activities on site.

Propulsion Shafting Alignment: A static condition observed at the bearings supporting the propulsion shafts. To suitably define propulsion shafting alignment, a minimum set of parameters are defined in the design stage and subsequently confirmed acceptable on board:

• Bearing vertical offsets

• Bearing reactions

• Bearing to shaft contact condition (i.e. misalignment angles)

• Crankshaft web deflections

• Gear mesh misalignment.

Alignment is considered satisfactory when these parameters can be controlled in the static and dynamic condition and maintained within the required limits under all operating conditions of the vessel for all service drafts and service temperatures.

A change in the vessel's draft due to altered loading conditions affects hull girder deflections. This in turn influences bearing offsets, causing a redistribution of the load among the bearings. Achieving a satisfactory alignment design for the loading conditions included in the vessel's loading manual requires several repeated analyses.

A temperature rise or drop also affects bearing offsets. However, unlike the hull deflections which affect all bearings in the system simultaneously, effect of temperature changes tend to be local to a particular bearing, or set of bearings, such as in the main engine and gear-box.

Although shafting alignment analysis and procedure are conducted for a static condition, the industry often uses the term dynamic alignment to describe the running condition of the propulsion shafting. This terminology raises some controversy when the dynamic or "running" condition is defined by utilizing dynamic propeller loads in an essentially static analysis. This "quasi-static" approach potentially draws misleading conclusions about aft stern tube bearing loading, slope boring and overall performance.

The propeller loads accounted for in a quasi-static analysis must be defined for a service condition that is deemed critical for propulsion system performance, namely maneuvering at high speeds with large rudder angles. Merely accounting for dynamic propeller loads for steady straight-ahead runs does not contribute to a safer or better alignment design and may result in misleading conclusions and a substandard aft stern tube bearing slope boring design.

The quasi-static approach can only be considered appropriate when all relevant loads are accounted for, including also the most critical service load condition for those components evaluated in the design. Moreover, the quasi-static analysis of the aft stern tube bearing contact entirely neglects the hydrodynamic loads components acting on the bearing.

Descriptions, processes, procedures and proposed solutions for shafting alignment issues outlined in these Guidance Notes are based upon industry-wide accepted best practices and may not necessarily fully reflect ABS Rule requirements.

ABS propulsion shafting alignment requirements and limits are defined in the relevant part of the applicable set of ABS Rules and must be referred to for Classification purposes.