Marine Machinery Vibration Analysis: Seven Ship Vibration Patterns to Investigate

Imbalance

Rotor imbalance means the rotating mass is not distributed evenly around the shaft centreline. On a ship this may follow deposits on a fan or impeller, a damaged component, an incorrect balance correction, coupling eccentricity or a change in the rotating assembly.

A strong component at rotational speed is often investigated first, particularly in the radial directions. The pattern becomes more persuasive when amplitude follows speed, phase is stable and the same response appears at technically related measurement points. A one-times-running-speed peak is not proof by itself because misalignment, bent shafts, eccentricity and structural response can produce similar evidence.

• Record rotational speed and machine load.
• Compare horizontal, vertical and axial directions.
• Check phase, run-up or coast-down behaviour where practicable.
• Inspect for deposits, damage, eccentricity and recent maintenance changes.

Misalignment

Misalignment can exist between coupled shafts, across a flexible coupling or between a machine and its driven equipment. Thermal growth, soft foot, pipe strain, foundation movement and installation error can all change alignment after commissioning.

Investigators often review running-speed harmonics, axial vibration, coupling-end measurements, phase relationships, temperature and coupling condition together. Elevated axial response or a noticeable two-times-running-speed component may support the diagnosis, but neither is unique to misalignment.

• Measure both sides of the coupling in consistent directions.
• Document hot or cold operating condition and recent alignment work.
• Check soft foot, hold-down condition and connected-pipe loading.
• Compare vibration with coupling wear, temperature and alignment records.

Mechanical looseness

Mechanical looseness may involve hold-down bolts, bearing fits, foundation interfaces, cracked supports, soft foot, worn clearances or movement between assembled parts. Onboard structures can transmit this response well beyond the original source.

Possible indicators include multiple harmonics, impacts, waveform distortion, directional differences and a response that changes abruptly with load. The investigation should separate true machine looseness from a flexible deck, foundation resonance or movement in connected piping.

Good diagnostic evidence connects every reading to the exact measurement point, direction, speed, load and vessel condition in which it was captured.

• Inspect fasteners, feet, chocks, bearing housings and support welds.
• Compare machine casing movement with foundation or deck movement.
• Review waveform impacts as well as spectrum peaks.
• Repeat measurements after any tightening or support correction.

Resonance

Resonance occurs when an excitation frequency approaches a natural frequency of the machine, foundation, deck, pipework or supporting structure. A comparatively small forcing input can then produce a large measured response.

A sharp amplitude change over a narrow speed range, accompanied by a phase change, is stronger evidence than a single steady-speed reading. Run-up, coast-down, impact testing or operating-deflection observations may help distinguish resonance from a fault that simply increases with load.

• Record amplitude and phase through the available speed range.
• Compare machine, foundation, deck and connected-system points.
• Identify blade-pass, vane-pass, gear-mesh and shaft-order excitation.
• Confirm whether the response disappears away from the critical condition.

Rolling-element bearing indicators

Developing rolling-element bearing defects may first appear in higher-frequency acceleration or enveloped data before overall velocity becomes severe. Lubrication condition, mounting, load zone, contamination and shaft speed all influence the pattern.

Bearing characteristic frequencies and sidebands can support a diagnosis when the correct bearing geometry and speed are known. The conclusion is stronger when vibration agrees with temperature, lubrication findings, noise, inspection history or repeat trends.

• Use repeatable bearing-housing measurement points.
• Capture suitable high-frequency data in addition to overall values.
• Confirm bearing designation, speed and load where available.
• Avoid declaring a failed bearing from one isolated spectrum.

Gear-related response

Gearboxes can generate response at shaft orders, gear-mesh frequency and sidebands related to modulation. Tooth wear, local damage, eccentricity, backlash, alignment, load variation and lubrication condition can alter the pattern.

Gear-mesh energy alone does not prove damage. The analyst should relate frequency components to actual shaft speeds and tooth counts, compare load states and review whether sidebands, impacts or waveform changes are developing over time.

• Confirm input, intermediate and output shaft speeds.
• Use tooth counts to identify expected gear-mesh components.
• Compare casing locations near relevant bearings and gear stages.
• Review oil debris, temperature, lubrication and inspection evidence.

Hydraulic or aerodynamic excitation and cavitation

Pumps, fans, compressors and flow systems can vibrate because of cavitation, recirculation, turbulence, vane-pass excitation, flow restriction or operation away from the intended duty point. The vibration source may therefore be hydraulic or aerodynamic rather than a mechanical defect.

Broadband high-frequency energy, changing noise, unstable pressure or flow and a response linked to valve position or operating demand can guide the investigation. Measurements should be reviewed alongside process data, suction conditions, filters, dampers and system configuration.

• Record pressure, flow, valve or damper position and operating demand.
• Compare suction and discharge conditions.
• Check whether vibration changes when the process condition changes.
• Separate flow excitation from bearing, alignment and foundation effects.

Why no pattern should be diagnosed from one plot alone

A defensible ship vibration diagnosis combines machine knowledge with repeatable measurements. The analyst should know where the sensor was mounted, which direction was measured, the instrument settings, rotational speed, vessel condition, machinery load and whether nearby systems were operating.

Useful evidence may include overall velocity, acceleration, displacement where appropriate, time waveform, spectrum, phase, rotational order, temperature, lubrication information, photographs, maintenance history and comparison with a sister machine or baseline. Applicable manufacturer limits, class or contract criteria and relevant ISO guidance should be confirmed for the actual equipment and decision.

When the symptom is mainly felt in cabins, decks or occupied spaces, the correct scope may instead be a whole-body and habitability vibration survey or a structural and hull vibration survey. Use the torsional, machinery and structural vibration decision guide to separate the measurement quantity first. Cyclic shaftline twisting requires torsional vibration measurement, while an uncertain multi-source problem may need noise and vibration troubleshooting.