Vibration analysis detects the characteristic frequency signatures of developing mechanical faults in rotating machinery (bearing defects, shaft imbalance, misalignment, looseness, and gear wear), typically weeks to months before they progress to catastrophic failure. It is the most widely implemented predictive maintenance technology globally, backed by decades of validated case studies and formalised in international standards including ISO 10816, ISO 20816, and ISO 13373. For Singapore facilities operating motors, pumps, fans, compressors, and gearboxes in demanding tropical conditions, a structured vibration programme is both a reliability and a cost-management investment. This guide explains the physics of machine vibration, the main fault signatures and their frequencies, how to collect and analyse vibration data, what instruments are appropriate for different applications, and how to interpret results against recognised severity criteria.
The Physics of Machine Vibration
Every rotating machine vibrates: vibration is generated by forces acting on the machine structure, imbalance in the rotating mass, misalignment between coupled shafts, rolling element contact in bearings, meshing of gear teeth, or hydraulic forces in pumps and fans. In a healthy machine these forces are small, symmetric, and consistent, so the resulting vibration is low in amplitude and stable over time. As a machine develops a fault, additional forces appear; a bearing with a developing surface defect on the outer race generates a repetitive impact each time a rolling element passes over the defect, at a calculable frequency (the Ball Pass Frequency Outer race, BPFO) derived from bearing geometry and shaft speed, which spectral analysis reveals as a peak in the spectrum, often with sidebands at shaft speed harmonics. This is the power of vibration analysis: each fault type has a characteristic frequency signature determined by machine geometry, so an analyst who identifies these signatures can not only confirm a fault is present but often pinpoint which bearing, gear pair, or rotor component is affected.
Common Fault Types and Their Frequency Signatures
Imbalance, the most common rotating machine fault, occurs when the mass centre of the rotating assembly does not coincide with its geometric centre of rotation, appearing as a strong peak at 1× running speed in the radial direction and corrected through precision balancing, in-place or on a workshop balancing machine. Misalignment between a motor and its driven load generates forces at 1× and 2× running speed, angular misalignment predominantly producing axial vibration and parallel (offset) misalignment producing radial vibration at 2X; precision laser alignment tools, available through Unitest Instruments, correct misalignment to within OEM tolerances. Bearing defects generate vibration at the four bearing defect frequencies, BPFO, BPFI, BSF and FTF, calculated from bearing geometry and shaft speed; early stages are best detected above 5,000 Hz using enveloping or demodulation, and as the defect progresses sidebands proliferate, the overall level rises, and late-stage failure brings broadband noise and structural resonance excitation. Looseness, whether loose foundation bolts, loose rotor fit, or bearing clearance, produces a rich harmonic series at shaft speed and sometimes half-order subharmonics; combining vibration analysis with physical inspection of fasteners and bearing fits usually resolves ambiguity with other fault patterns. Gear faults produce vibration at the gear mesh frequency (teeth count × shaft speed) and its harmonics; tooth wear or breakage modulates this frequency with sidebands at the affected gear's shaft speed, distributed wear producing broadly elevated sidebands and a cracked tooth producing modulation at 1× shaft speed of the defective gear.
Vibration Measurement Parameters
Vibration is measured in three parameters, each sensitive to different fault types and frequency ranges:
| Parameter | Unit | Best For | Frequency Range |
|---|---|---|---|
| Displacement | mm or µm (peak-to-peak) | Low-speed machinery, shaft orbit analysis | <10 Hz |
| Velocity | mm/s (RMS) | General machinery condition, ISO 20816 compliance | 10 Hz – 1,000 Hz |
| Acceleration | g or m/s² (peak or RMS) | High-frequency bearing defects, gearboxes | 1,000 Hz – 20,000 Hz |
ISO 20816 (the successor to ISO 10816) specifies vibration severity criteria for a wide range of machine types, including induction motors, steam and gas turbines, pumps, fans, compressors, and generators, in terms of velocity (mm/s RMS); compliance with these thresholds is often required for insurance, warranty, or regulatory purposes.
Instruments, Sensor Selection and Mounting
Vibration data collection instruments range from simple single-axis pen-type meters to sophisticated route-based multi-channel analysers with built-in spectrum analysis and fault frequency databases. For a route-based PdM programme, a handheld analyser should offer FFT spectrum analysis to at least 20,000 Hz, bearing defect frequency calculation from bearing geometry or a built-in database, overall vibration level (velocity and acceleration) trending, route storage to pre-load the machine list and measurement point sequence, Bluetooth or USB data transfer to PdM software, and a robust industrial enclosure. Fluke vibration analysers distributed by Unitest Instruments meet these requirements and integrate with Fluke Connect condition monitoring software; for basic overall level monitoring at a budget, the Fluke 805 vibration meter provides a quick-check reading and bearing condition number for routine walkdown checks.
The accelerometer is the most critical component in the measurement chain; for bearing fault detection, a sensor with a flat frequency response to at least 15 kHz is required, as lower-frequency sensors miss the high-frequency content characteristic of early-stage bearing defects. Mounting method significantly affects the usable frequency range: stud mounting gives direct metal-to-metal contact and the highest frequency response for permanent monitoring points; magnetic mounts are quick-attach and usable to roughly 5–7 kHz for route-based monitoring at standard bearing points; probe or handheld contact is only suitable for low-frequency overall level checks, not bearing defect detection. For repeatable trending, measurement points should be marked on the machine housing and the same mount method used at every visit, since a 10% change in mounting position can produce more signal variation than a genuine 10% change in machine condition.
Calibration, Alert Thresholds and Trending
Vibration analysers and their accelerometers must be calibrated at defined intervals, with calibration traceable to national standards. Unitest Instruments provides SAC-SINGLAS accredited calibration across eight measurement disciplines relevant to vibration instruments; for ISO 20816 compliance, calibration traceability is particularly important since measurements are compared to defined numerical thresholds. Accelerometers are sensitive devices, and a dropped sensor can permanently alter its sensitivity, producing systematically incorrect amplitude readings that corrupt the trend data maintenance decisions rely on, so any accelerometer that has been dropped or shocked should be returned for calibration before further use.
ISO 20816 provides machinery-type-specific velocity severity bands, but these are absolute thresholds for generic machine classes; for an optimised programme, statistical thresholds based on your specific machine's historical baseline are more sensitive to genuine degradation and produce fewer false alarms. A practical approach uses the machine's own baseline data plus two-sigma and three-sigma control limits as advisory and alarm thresholds respectively, analogous to statistical process control applied to machine condition. Bearing-specific fault frequency amplitude trending, tracking BPFO, BPFI, BSF, and FTF amplitudes over successive cycles, is more sensitive to bearing degradation than overall level trending alone, often detecting a developing defect significantly earlier. Read our practical guide to setting up a full PdM programme.
Integrating Vibration with Other PdM Technologies
Vibration analysis is most powerful in combination with other condition monitoring technologies. A bearing running hot with elevated high-frequency vibration is a more compelling repair candidate than one showing high vibration alone, since the thermal evidence confirms the vibration is not a measurement artefact; combining vibration with oil analysis for gearboxes and large oil-lubricated machines provides both a mechanical fault signature and chemical evidence of wear debris in the lubricant. For Singapore facilities with motors driving critical pumps and fans, integrating vibration monitoring with motor current analysis covers both the mechanical and electrical failure modes. Contact Unitest Instruments to discuss a complete condition monitoring instrument package for your facility.