Vibration Monitoring System

What sets this type apart

A portable vibration meter tells the engineer the condition of a machine at the moment someone walks up with the instrument; a permanently installed vibration monitoring system tells the machine's condition continuously, catching a developing fault between rounds and, where wired to a trip relay, stopping the machine automatically before a bearing failure or blade loss becomes a major repair. The distinction matters most on machines that run unattended or where sudden failure has serious consequences, such as main engine turbochargers, reduction gears, and large electric propulsion motors.

Main components

Sensors

Accelerometers, bolted or stud-mounted close to the bearing housing, measure vibration on rolling-element bearing machines; eddy-current proximity probes, reading directly off the shaft through the oil film, are used on fluid-film (journal) bearing machines such as turbochargers and large turbines, since accelerometers on the casing alone miss shaft-relative movement that matters most on those machines.

Monitoring rack and signal conditioning

A rack converts raw sensor signal into overall vibration amplitude and, on more capable systems, spectral data that separates unbalance, misalignment and bearing defect frequencies from each other rather than lumping them into one number.

Alarm and trip relays

Two-stage setpoints are typical: an alert level that raises an alarm for the watchkeeper to investigate, and a danger level that can be wired to trip the machine automatically. Trip setpoints are normally set with a delay or voting logic across more than one sensor to avoid a nuisance trip from a single spurious reading shutting down running machinery.

Data logging and trending

Continuous logging lets an engineer trend vibration amplitude over weeks or months, which is often more useful than the absolute value at any one moment, since a slow upward trend toward the alert level is the earlier and more reliable warning.

Selection and sizing

Sensor type follows bearing type: proximity probes for fluid-film bearings, accelerometers for rolling-element bearings, and both together on machines that combine bearing types, such as a turbocharger with a shaft in fluid-film bearings driven by a rotor that also needs axial position monitoring. API 670 is the reference standard most monitoring systems on larger machinery are built against, defining sensor mounting, signal processing and protection logic even where the ship itself is not contractually bound to the full standard.

Regulations and class

Class rules for main propulsion machinery increasingly recognise continuous condition monitoring, including vibration, as a basis for extending or adjusting some survey intervals under condition-based or reliability-centred maintenance schemes, provided the monitoring system, its calibration and its data are demonstrably maintained. Where fitted as a safety trip on machinery covered by class rules, the trip function itself is surveyed as part of the machinery protection system, separate from any maintenance-interval benefit.

Typical faults

• Sensor cable damage from heat or vibration chafe - intermittent or lost signal misread as a genuine machine problem.
• Proximity probe gap drift - false trend readings on fluid-film bearing machines if the probe-to-shaft gap changes from the calibrated setting.
• Single-sensor trip logic left in service after a sensor fault - nuisance trips, or worse, a masked real fault if the faulty sensor reads artificially low.
• Alarm setpoints left at generic factory defaults - alerts too late or too often for the specific machine's actual baseline vibration signature.
• No baseline recorded after overhaul - trend comparison against the wrong reference point, hiding a real change introduced during the work.

What to look for in a supplier

• Sensor and rack compatibility with the ship's existing alarm and monitoring system rather than a standalone display no one checks.
• Calibration service for proximity probes specifically, since gap calibration drift is a common, hard-to-spot source of bad data.
• Spectral analysis capability, not just an overall amplitude reading, if the system is meant to support condition-based maintenance decisions.

Record a vibration baseline immediately after every bearing overhaul or alignment job, before returning the machine to normal service - without it, later trend readings have nothing reliable to compare against.