Knowledge / Ship systems

Shaft alignment: from centreline geometry to bearing load and oil film

Explore shaft-bearing load sharing, thermal and hull changes, influence coefficients and the limits of alignment measurements.

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Aligning a propeller shaft involves more than placing every centre on a straight line. The shaft bends under its own weight and propeller loads, while bearing positions and stiffness determine how those loads are shared. A suitable alignment establishes acceptable bearing reactions, contact conditions and lubrication behaviour across the relevant operating conditions.

Bearings perform different functions

Radial supports, the thrust bearing, couplings and machinery bearings carry different load components. Propeller mass is not the same as hydrodynamic propeller loading. The former exists when stationary; the latter changes with rotation, advance and manoeuvring. A coupling that can be closed geometrically does not establish that every bearing has the intended load.

A shaft on multiple supports can behave as a statically indeterminate beam. Equilibrium equations alone do not determine every reaction: shaft bending stiffness, support compliance and offsets are also needed. A small change in one bearing's elevation can redistribute load to neighbouring bearings while the total external load remains unchanged. Unexpectedly low loading deserves attention as well as overload.

Cold harbour conditions do not represent all operation

Thermal growth of machinery foundations, hull deformation, draught and tank distribution can change relative bearing positions. A ship measured in dry dock does not have the same support conditions as a loaded ship afloat. Temperature, draughts, tank state and machinery condition should accompany a comparison between calculation and measurement.

DNV's 2018 technical note explains how high-speed turning can introduce additional propeller bending moments, particularly for some stern-tube arrangements. That dated note does not establish a speed limit for every ship. The engineering implication is that one straight-ahead condition does not represent the full manoeuvring load envelope.

Why average pressure may be insufficient

In an oil-lubricated journal bearing, rotation and viscosity help establish the film carrying the load. Shaft slope relative to the bearing axis can make load distribution uneven along the bearing. The same total reaction can be spread over a broad region in one condition and concentrated near an edge in another. Dividing reaction by nominal projected area can conceal local contact and film conditions.

DNV's current service explanation describes evaluating a hydrodynamic oil-film criterion alongside bearing load and relative slope. Lubricant type and viscosity enter that assessment. This does not mean all environmentally acceptable lubricants are interchangeable, or that a single viscosity number determines acceptance.

Using influence coefficients to understand a small offset

Consider an illustrative linear model with three bearings at x = 0 m, 3 m and 5 m. Their initial vertical reactions are 80, 100 and 120 kN. Assume calculated influence coefficients for a change in the middle bearing's elevation of −20, +50 and −30 kN/mm respectively. These are hypothetical coefficients, not design data for an actual shaft.

Raising the middle bearing by 0.40 mm changes the reactions by −8, +20 and −12 kN, giving new loads of 72, 120 and 108 kN. Total reaction remains 300 kN, and the moment of the changes about x = 0 is 20 × 3 − 12 × 5 = 0 kN·m. The example demonstrates redistribution without changing external load. It cannot establish bearing acceptance without the relevant manufacturer and class limits.

The linear approximation assumes small changes and an unchanged contact regime. Continuing with the same coefficients can become invalid if a bearing loses contact or its contact region changes substantially. A real model may include stepped shafts, coupling stiffness, distributed bearing contact and operating forces. DNV's calculation-tool overview illustrates considering reactions, deflections and moments together; a software name does not establish model validity.

Different measurements produce different evidence

An optical or laser reference gives position relative to a geometric line. Coupling gap-and-sag measurements describe the relationship between separated components in a particular condition. Jack-up measurements help infer bearing reaction from force-displacement behaviour. None of these methods answers every question addressed by the others.

The jack's distance from the bearing centre, shaft bending, adjacent supports and friction affect interpretation. Treating jack force as bearing reaction without the relevant correction can be wrong. If lifting and lowering curves differ, that difference needs explanation. When measured conditions do not match the model, changing offsets simply to make the numbers agree is not a reliable diagnosis.

Combine temperature, vibration and lubricant evidence

A rise in bearing temperature can reflect concentrated loading, oil supply, cooling or measurement problems. One temperature reading does not diagnose alignment error conclusively. Low vibration likewise does not prove adequate oil film at every bearing. Comparing trends without speed, load and manoeuvring condition leaves the comparison incomplete.

An alignment assessment should connect calculation assumptions, measured support conditions and operating evidence through a consistent physical explanation. Following a lubricant change, propeller work or foundation repair, an earlier acceptance record does not automatically establish the new condition. An adjustment that improves one bearing can move another closer to its limit; assessing that redistribution is central to the task.

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