Machinery rarely fails without warning; it fails without anyone listening. A bearing that will seize in March is usually ringing at its own defect frequency by December, and a misaligned pump shows its problem at twice running speed long before the coupling goes. This guide covers severity zones, baselines, fault frequencies and sensor placement for ship machinery. Trend vibration readings free in Marine Inspection.
Marine vibration analysis that hears a failure months before it happens
Every rotating fault has a signature in the spectrum. Learn the few that matter, take readings the same way every time, and the trend tells you what to open up next.
Marine vibration analysis: what condition monitoring can and cannot see
Vibration analysis is strongest on rotating machinery and weakest where combustion forces drown the signal. Knowing where it works is what keeps a monitoring programme credible. See a machinery route set up in a demo.
Rotating machinery
Pumps, fans, motors, compressors, purifiers, turbochargers and gearboxes: unbalance, misalignment, looseness, rolling bearing and gear defects.
Reciprocating engines
Diesel engines are judged against ISO 10816-6 and the maker's limits; combustion forces dominate, so trends and comparisons between units matter more than absolute values.
Other methods
Class condition monitoring schemes combine vibration with oil analysis, performance monitoring, thermography and motor current analysis.
Vibration severity zones: ISO 20816-3 limits for ship machinery
ISO 20816-3 (which replaced ISO 10816-3) grades broadband velocity, in mm/s RMS measured on the bearing housing, into four zones. Rotodynamic pumps are assessed under ISO 10816-7 and reciprocating engines under ISO 10816-6, so check which part covers each machine before setting alarms.
Group 1 machine on a rigid foundation (above 300 kW)
| Machine group | Foundation | A / B | B / C | C / D |
|---|---|---|---|---|
| Group 1, above 300 kW | Rigid | 2.3 | 4.5 | 7.1 |
| Group 1, above 300 kW | Flexible | 3.5 | 7.1 | 11.0 |
| Group 2, 15 to 300 kW | Rigid | 1.4 | 2.8 | 4.5 |
| Group 2, 15 to 300 kW | Flexible | 2.3 | 4.5 | 7.1 |
Values in mm/s RMS, 10 to 1,000 Hz (2 to 1,000 Hz below 600 r/min), as commonly published from ISO 20816-3. Use them as a starting point; alarms set relative to each machine's own baseline catch change much earlier than absolute zones.
Building a baseline vibration signature
A reading without a baseline is an opinion. Class rules for condition monitoring take the same view: baseline data are taken under documented reference conditions when the machine is in its initial condition, and every later reading is compared against it. Our guide to integrating condition monitoring sensors covers units and thresholds for permanent installations.
Pick the moment
At commissioning, after overhaul or after a bearing change, once the machine has run in.
Fix the conditions
Record load, speed, temperature and valve positions. Repeat them for every later reading.
Measure three ways
Horizontal, vertical and axial at each bearing, at points marked so they are found again.
Keep the spectrum
Store the full spectrum and envelope, not only the overall value; the shape is what diagnoses.
Set relative alarms
Alert on change from baseline as well as on the ISO zone, and tighten intervals when a reading moves.
Fault frequencies: reading a vibration spectrum
Most shipboard diagnoses come from a handful of patterns. Where the peak sits, and in which direction it is strongest, narrows the cause before anything is opened. Log spectrum findings against each machine free.
| Fault | Spectrum sign | Strongest direction | Confirm with |
|---|---|---|---|
| Unbalance | Dominant peak at 1x running speed | Radial | Phase steady, amplitude rises with speed |
| Misalignment | High 2x, often with 1x | Axial | Phase across the coupling; laser alignment check |
| Mechanical looseness | Series of running-speed harmonics | Radial, often vertical | Feet, bolts and soft foot inspection |
| Oil whirl (sleeve bearings) | Sub-synchronous peak near 0.42 to 0.48x | Radial | Oil temperature, pressure and clearance |
| Rolling bearing defect | BPFO, BPFI, BSF or FTF and harmonics | Load zone, radial | Envelope or acceleration spectrum; oil and temperature |
| Gear wear | Gear mesh (teeth x speed) with sidebands | Radial and axial | Oil debris analysis; tooth inspection |
Rolling bearing defect frequencies
fr = shaft speed (Hz), n = number of rolling elements, d = element diameter, D = pitch diameter, θ = contact angle
(n / 2) × fr × (1 − d/D × cos θ)outer race(n / 2) × fr × (1 + d/D × cos θ)inner race(D / 2d) × fr × (1 − (d/D × cos θ)²)rolling element(fr / 2) × (1 − d/D × cos θ)cageBearing makers publish these as multiples of running speed for each part number, so most ships look them up rather than calculate them.
Vibration sensor placement on marine machinery
The same machine can read two zones apart depending on where and how the sensor is held. Placement and mounting are what make one month's reading comparable with the next. See marked measurement points in a demo.
Usable frequency by mounting method
Bearing defect detection needs the high-frequency range, which is why stud mounting is the norm for bearing monitoring. Source: ToolGrit vibration analysis guide.
- At the bearing, on the housing, as close to the load zone as the guard allows
- Three axes at each bearing: horizontal, vertical and axial
- Same spot every time, painted or fitted with a mounting pad
- Solid metal path, never on covers, guards or thin sheet
- Same conditions, load, speed and temperature noted with each reading
Every reading on the same point, every trend on one screen
Routes, marked points, baselines and alarm levels per machine, with readings logged offline in the engine room and trended for the chief engineer and the office.
Ship machinery to put on a vibration monitoring route
Start with what would stop the ship or fail a survey, then add the rest. Intervals tighten as soon as a machine moves from zone B toward C. Pair the route with our lube oil analysis guide, since wear metals and vibration confirm each other.
| Machine | Faults vibration finds early | Typical route interval |
|---|---|---|
| Seawater and cooling pumps | Misalignment, bearing wear, impeller unbalance, cavitation | Monthly; quarterly is a common class minimum for centrifugal pumps |
| Diesel generators | Generator bearing wear, coupling misalignment, foundation looseness | Monthly, plus after every overhaul |
| Turbochargers | Rotor unbalance from fouling, bearing deterioration | Trend with each performance check |
| Separators (purifiers) | Bowl unbalance, spindle bearing wear | Monthly and after bowl cleaning |
| Air compressors | Bearing wear, looseness, valve problems | Monthly |
| Fans and blowers | Unbalance from fouling, belt and bearing faults | Monthly or quarterly |
| Cargo and ballast pumps | Bearing and seal wear, misalignment | Before and after cargo campaigns |
| Shafting and gearboxes | Gear mesh wear, shaft line misalignment | Per class CM scheme or maker |
Weekly or continuous monitoring is usual for the most critical machines. Your SMS, the maker and any class condition monitoring arrangement set the intervals that bind your ship.
Condition monitoring and class: earning survey credit
A class-approved condition monitoring arrangement can let the surveyor accept good trend data instead of opening a machine. The bar is evidence, not equipment. See our PMS class survey arrangement guide for how the wider arrangement is approved.
Examples from Turk Loydu Chapter 25, Machinery Condition Monitoring. Each society words this differently; your class rules decide.
From vibration alarm to work order
A rising trend is only useful if it changes what the crew does next. An escalation ladder agreed in advance stops alarms being acknowledged and forgotten, and stops healthy machines being opened on a hunch. Our predictive maintenance checklist covers the wider programme.
Normal route interval; compare with baseline
Shorten the interval; take a spectrum and envelope
Confirm with a second method; raise a planned work order
Stop or change over to standby; repair before restart
How Marine Inspection turns vibration readings into decisions
Most ships already take readings. What goes missing is the link between the reading, the trend and the job that follows. Marine Inspection keeps every machine's readings, baseline and work history in one place. Start your machine health log free.
Illustrative example. Values and limits come from your own baseline and machine group.
Routes with marked points
Each machine carries its measurement points, axes and reference conditions, so the third engineer takes the reading the way the chief did.
Trends against baseline
Readings are logged as numbers and trended, with alerts when a value moves from baseline or crosses a zone limit.
Readings that become work
A zone C reading raises a work order with the spectrum photo attached, tracked through repair and the after-repair baseline.
Evidence for class
Baselines, readings and repairs stay with the equipment record, ready for the annual condition monitoring report.
Marine vibration analysis and condition monitoring: FAQs
What vibration level is too high for a ship's pump or motor?
Under ISO 20816-3, a group 1 machine on a rigid foundation enters zone C above 4.5 mm/s RMS and zone D above 7.1 mm/s. Rotodynamic pumps have their own part, ISO 10816-7, and a rising trend from baseline matters as much as the absolute number.
What does a high 2x running speed peak mean?
Most often misalignment, especially when it is strongest in the axial direction. Looseness can also show 2x, usually with more harmonics. Confirm with phase readings and an alignment check before opening anything.
How do you detect a failing rolling bearing?
Look for the bearing defect frequencies, BPFO, BPFI, BSF or FTF, in an envelope or acceleration spectrum. They appear long before overall velocity rises, which is why stud-mounted sensors are used for bearing monitoring.
Where should vibration sensors be placed?
On the bearing housing, close to the load zone, in horizontal, vertical and axial directions, at marked points on a solid metal path. Never on covers or guards.
Can vibration analysis be used on diesel engines?
Yes, with care. Reciprocating engines are assessed under ISO 10816-6 and the maker's limits, and the most useful approach is trending each unit against itself and its sister engines.
Can condition monitoring replace opening up machinery for class?
Under an approved class condition monitoring arrangement, good trend data can be accepted instead of dismantling, after a probation period and with annual reporting. See class-ready CM records in a demo.
Hear the failure early, and have the trend to prove it
Vibration routes, baselines, fault findings and the work orders that follow, kept against every machine and ready for the chief, the office and class.
