What Is Machinery Vibration Analysis and How to Start

Machinery vibration analysis is condition monitoring of rotating machines based on the vibration of their bearing housings. A sensor measures the overall vibration level, which is compared with the ISO 10816-3 limits and with the machine's own baseline; when the level rises, the spectrum shows the cause: unbalance, misalignment, looseness or a bearing defect.

This article is the entry point to our vibration analysis section. Each part links to a more detailed article.

What is measured

Vibration is described by three quantities. They are linked through frequency, but they behave differently across the frequency range: at the same velocity, acceleration amplitude rises with frequency and displacement amplitude falls. Velocity depends least on machine running speed, which is why it is the usual choice for overall assessment.

Quantity	Units	Where it is useful
Velocity	mm/s, RMS	Overall machine condition. ISO 10816-3 uses RMS in the 10–1000 Hz band
Acceleration	g, m/s²	High frequencies: bearing impacts, gear mesh
Displacement	µm	Low frequencies and slow machines; shaft motion relative to the housing

There are two types of sensors. Accelerometers and velocity sensors are mounted on the bearing housing and measure absolute housing vibration. Non-contact proximity probes measure shaft displacement relative to the bearing housing. This matters because there is a finite clearance between the rotor and the bearings, seals and casing, and housing vibration is not the same as shaft motion.

Velocity limit tables for motors, pumps and fans above 15 kW are in the article "Electric motor vibration limits per ISO 10816-3".

Overall level, spectrum and time waveform

Vibration analysis works at three levels of detail:

1. Overall level (one number, such as 3.2 mm/s) answers "is there a problem". It is compared with the standard's zones and with the trend.
2. Spectrum is calculated from the signal with the Fast Fourier Transform (FFT). It shows at which frequencies the vibration is concentrated and answers "what exactly is wrong". The reference frequency is shaft running speed (1x).
3. Time waveform and envelope. A non-sinusoidal waveform points to looseness. Envelope analysis isolates modulation and helps find early bearing and gear defects that are hard to see in a normal spectrum.

A step-by-step guide to reading spectra is in "How to read an FFT spectrum".

Which faults vibration analysis detects

Fault	Sign in the spectrum or waveform	More
Unbalance	Dominant peak at running speed, 1x	Field balancing
Shaft misalignment	1x and 2x peaks, often elevated axial vibration	Shaft misalignment
Mechanical looseness	Non-sinusoidal waveform, 3x, 4x, 5x harmonics
Rolling bearing defects	High-frequency content at BPFO, BPFI, BSF, FTF	Bearing failure stages
Gear defects	Sidebands around gear mesh frequency	Gearbox diagnostics
Resonance	Amplified vibration at a natural frequency of a component or structure
Broken rotor bars in induction motors	Sidebands at pole pass frequency	Causes of motor vibration

1x and 2x peaks alone are not enough for a diagnosis: unbalance, misalignment and rubs can all produce them. Conclusions are drawn from several signs together: measurement direction (radial or axial), phase, waveform shape and trend.

Route-based and online monitoring

	Route-based (periodic) monitoring	Online monitoring
How it works	An engineer walks a set route with a portable data collector	Wired or wireless sensors stay on the machine
Data frequency	Per the route schedule	Automatic, no one on site needed
Which machines	Where periodic checks are enough	Critical, remote, hard-to-reach, high-speed
What it takes	Trained staff and route planning	Sensor installation and a link to the platform

In practice both are combined. Routes cover most auxiliary machines. Online sensors go where a stop is expensive or where access is difficult or unsafe. Wireless sensors remove the need for cabling: see "Wi-care wireless vibration monitoring".

How to start: first steps of a programme

1. Machine list and criticality. List pumps, fans, motors, gearboxes and compressors. Mark the ones whose stop halts production.
2. Monitoring method. For each machine choose a route or an online sensor.
3. Measurement points. On the bearing housings in two radial directions; axial during periodic rounds. Mark the points so that each reading is taken at the same spot.
4. Baseline. The first reading in normal operation: rated speed, load, bearings at operating temperature. Later readings are compared at the same speed, load and temperature.
5. Alarm levels. For the overall level use the ISO 10816-3 zones and the rule that a change of 25% of the zone B upper limit is significant (see the vibration limits article).
6. Analysis and action. When vibration rises, take a spectrum, find the cause and plan the repair: balancing, alignment, tightening, bearing replacement or lubrication.
7. Training. In-depth spectrum analysis needs a trained analyst. According to I-care, it usually requires ISO 18436 Category II or III certification.

Which instrument to start with

• A portable data collector and analyser for routes: the Bently Nevada SCOUT220-IS, with 2 simultaneous channels, up to 12,800 lines of spectral resolution, Fmax up to 80 kHz and intrinsically safe ratings. Routes are prepared and data analysed in System 1 software. How a simple meter differs from an analyser: "Vibration meter or vibration analyser".
• Online monitoring without cables: the I-care Wi-care 130 G23 wireless triaxial sensor.

KEG TRK supplies Bently Nevada, I-care and SDT equipment in Kazakhstan and helps set up vibration monitoring programmes. Price on request.

Sources

• I-care. What is Vibration Analysis in Predictive Maintenance? — https://www.icareweb.com/knowledge/predictive-maintenance/what-is-vibration-analysis-predictive-maintenance/
• Bently Nevada (Baker Hughes), Orbit. Accelerometers: What do I use? — https://www.bakerhughes.com/bently-nevada/orbit-home/orbit-article/accelerometers-what-do-i-use
• Bently Nevada. SCOUT220-IS and SCOUT240-IS Portable Data Collectors Datasheet 109M0226 — https://dam.bakerhughes.com/m/54b98a9e1e80eaf6/original/SCOUT220-IS-and-SCOUT240-IS-Portable-Data-Collectors-Datasheet-109M0226-pdf.pdf
• ISO 10816-3:1998 — via the article "Electric motor vibration limits".