From symptoms to solutions: how we diagnose vibration in a propulsion shaft line

In our previous article, we explored why vibrations can occur in a vessel’s propulsion shaft line, the different types of vibration that may arise, and the consequences that abnormal operation can have on bearings, seals, shafts, couplings and gearboxes.

However, detecting a vibration is only the beginning. The real challenge starts when we need to identify what is causing it.

1. How can vibration be detected and diagnosed in time?

The first step in resolving a vibration issue is to determine what is vibrating, at what frequency, and under which operating conditions.

Diagnosis must be carried out systematically, avoiding the replacement of components before the underlying cause has been identified.

The following tools and inspection procedures can be used:

Vibration analysis

Accelerometers and other sensors can be used to measure vibrations at strategic points throughout the installation, such as the engine, gearbox, supports and bearings.

Analysing frequencies, amplitudes, rotational orders and phase data helps identify the probable source of the vibration.

Alignment checks

The alignment of the propulsion shaft line is a critical factor.

Incorrect alignment can lead to uneven load distribution across the bearings and increase the loads acting on the shaft, couplings and other components.

Where necessary, laser alignment systems can be used to check the relative positions of the different components.

Inspection of the shaft and its components

When there are signs of damage, the shaft must be inspected, particularly in areas subject to higher stress concentrations.

Depending on the material, geometry, condition and nature of the problem, inspection methods may include visual examination, dye penetrant testing, magnetic particle testing or ultrasonic testing, among others.

These techniques do not measure vibration. Instead, they help determine whether the stresses to which the component has been subjected have caused damage.

Propeller inspection

The propeller must be inspected to rule out damage, deformation, imbalance or changes to its geometry.

A damaged or improperly balanced propeller can generate excitation forces that are transmitted throughout the propulsion shaft line.

Torsional vibration analysis

In certain installations, torsional vibration analysis may be required to study the dynamic behaviour of the propulsion shaft line and identify potential resonance conditions.

This analysis is particularly relevant during the design of certain propulsion systems and whenever major modifications are made, such as changes to the engine, gearbox, coupling or propeller.

2. Vibration is a symptom, not a diagnosis

This is one of the most important points to keep in mind.

Detecting vibration in a shaft does not necessarily mean that the shaft needs to be replaced.

Before making any decisions, the source of the vibration must be identified.

For example, vibration detected in the aft section of a vessel could be associated with:

• A damaged or unbalanced propeller.
• Misalignment of the propulsion shaft line.
• A bent shaft.
• Bearing wear or excessive clearance.
• Coupling problems.
• Excitation forces generated by the propeller.
• Problems with the gearbox or engine.
• Operating conditions close to a natural frequency.

For this reason, an accurate diagnosis must consider the propulsion shaft line as a complete system.

3. Solutions and best practices: the Metalnox approach

The best strategy for managing vibration is to take action before it develops into a failure.

At Metalnox, we believe that high-quality component manufacturing and repair must go hand in hand with proper diagnosis of the installation.

Precise alignment

Correct alignment allows the engine, gearbox, bearings and shaft to operate within their intended conditions.

Alignment must take into account not only the vessel's static position but also actual operating conditions and potential deformation of the structure and supports.

Precision-manufactured and repaired shafts

Shaft straightness, concentricity, geometry and surface finish are important factors in ensuring proper operation.

Material selection must also reflect the characteristics of the installation and the loads to which the shaft will be subjected, taking into account factors such as strength, corrosion resistance, fatigue and service conditions.

Propeller quality control

A properly manufactured, repaired and inspected propeller helps reduce unnecessary excitation forces throughout the propulsion shaft line.

Blade repairs should not be limited to restoring the propeller's visual appearance. The blade's geometry and balance must also be restored within the applicable tolerances.

Bearing and stern tube maintenance

The condition of the bearings and sealing systems must be an integral part of propulsion shaft line maintenance.

Wear, excessive clearance or incorrect loading can alter the dynamic behaviour of the assembly and contribute to the development of vibration.

Diagnose before replacing components

When vibration occurs, replacing components without identifying the cause may only resolve the problem temporarily or may fail to resolve it altogether.

Diagnosis must therefore be the starting point for determining which intervention the installation actually requires.

4. From symptoms to solutions

Vibration is part of the dynamic behaviour of any propulsion system, but abnormal vibration should never be ignored.

The key is to distinguish between the normal operating behaviour of the installation and vibration that may indicate an underlying problem.

A correctly aligned propulsion shaft line, a propeller in good condition, properly adjusted bearings, and a precisely manufactured or repaired shaft are all essential to achieving reliable, long-lasting power transmission.

Above all, remember that vibration is a symptom, not a diagnosis. Correctly identifying its source makes it possible to address the root cause and avoid unnecessary repairs or more extensive damage.

Have you noticed vibration, unusual noise or abnormal movement in your vessel's propulsion shaft line?

At Metalnox, we have experience in the manufacture, repair and maintenance of propulsion shaft line components, helping you identify the problem and find the right solution.