Every rotating machine vibrates. The question is whether the vibration is normal or a warning sign. A 50 HP motor on a rigid foundation running at 3,600 RPM will always have some vibration from residual imbalance, bearing clearance, and coupling misalignment. The difference between a healthy machine and a failing one often comes down to a few thousandths of an inch of displacement or a few millimeters per second of velocity.
ISO 10816 is the international standard that defines vibration severity zones for rotating machines based on broadband velocity measurements. It divides machines into groups based on size and foundation type, then assigns four severity zones: A (good), B (acceptable), C (alarm), and D (danger/shutdown). Understanding which group your machine falls into and what the zone boundaries mean is the foundation of any vibration-based predictive maintenance program. This guide covers the measurement basics, the ISO classification system, and the practical decisions that follow a vibration reading.
Displacement, Velocity, and Acceleration: Which One to Measure
Machine vibration can be measured in three units: displacement (mils peak-to-peak or microns), velocity (inches per second or mm/s), and acceleration (g's). Each unit emphasizes a different frequency range and tells you something different about the machine's condition.
Displacement measures how far the shaft or housing moves. It emphasizes low-frequency vibration (below about 10 Hz or 600 CPM). Displacement is most useful for slow-speed machines (below 600 RPM) where the energy is concentrated at low frequencies. Proximity probes mounted in bearing housings measure shaft displacement directly and are standard on large turbomachinery.
Velocity measures how fast the surface is moving. It is proportional to the vibration energy and provides a relatively flat response across the frequency range from about 10 Hz to 1,000 Hz (600 to 60,000 CPM). This makes velocity the best general-purpose measurement for most industrial machinery running between 600 and 12,000 RPM. ISO 10816 is based on broadband velocity RMS (root mean square) for this reason.
Acceleration measures the rate of change of velocity. It emphasizes high-frequency vibration (above 1,000 Hz) and is most sensitive to impacts, bearing defect frequencies, and gear mesh problems. Acceleration is the primary measurement for rolling element bearing condition monitoring using envelope analysis or high-frequency demodulation techniques. Most modern accelerometers output acceleration, and the data collector integrates mathematically to produce velocity and displacement readings.
ISO 10816 Machine Groups
ISO 10816-3 (revised as ISO 20816-3) classifies in-situ industrial machines into two groups based on power rating and shaft height, and then evaluates each group against two support classes — rigid or flexible. The severity zone boundaries differ by group and support class because a large machine on a rigid foundation behaves differently from a medium machine on a flexible frame.
The two machine groups:
- Group 1: Large machines with rated power above 300 kW (up to 50 MW); electrical machines with shaft height of 315 mm and above. Includes large motors, generators, and compressors in power plants, large process facilities, and heavy industry. These machines commonly have sleeve bearings.
- Group 2: Medium-sized machines with rated power above 15 kW up to 300 kW; electrical machines with shaft height from 160 mm up to 315 mm. This covers the majority of industrial motors, fans, and compressors in typical plants, usually on rolling-element bearings.
Each group is then judged as rigid or flexible support: the foundation classification matters because a flexible foundation amplifies certain frequencies and allows more motion at the bearing housing even when the shaft vibration is identical. A machine on a rigid concrete pad might read 0.15 in/s velocity while the same machine on a flexible skid reads 0.25 in/s, with no difference in actual shaft condition. The ISO zones account for this by setting wider boundaries for flexible-mounted machines.
Note that the 2009 revision of ISO 10816-3 removed pumps from its scope: rotodynamic pumps (including close-coupled pumps with integrated drivers) are evaluated under ISO 10816-7, which has its own category and zone structure. The motor driving a pump is still evaluated under 10816-3.
Severity Zone Boundaries and What They Mean
ISO 10816-3 defines four severity zones using broadband velocity RMS measured on the bearing housing in three directions (horizontal, vertical, and axial). The zone boundaries in mm/s RMS (Tables A.1 and A.2 of the standard) are:
Group 1, rigid support (large machines): Zone A: 0 to 2.3 mm/s; Zone B: 2.3 to 4.5 mm/s; Zone C: 4.5 to 7.1 mm/s; Zone D: above 7.1 mm/s.
Group 1, flexible support: Zone A: 0 to 3.5 mm/s; Zone B: 3.5 to 7.1 mm/s; Zone C: 7.1 to 11.0 mm/s; Zone D: above 11.0 mm/s.
Group 2, rigid support (medium machines): Zone A: 0 to 1.4 mm/s; Zone B: 1.4 to 2.8 mm/s; Zone C: 2.8 to 4.5 mm/s; Zone D: above 4.5 mm/s.
Group 2, flexible support: Zone A: 0 to 2.3 mm/s; Zone B: 2.3 to 4.5 mm/s; Zone C: 4.5 to 7.1 mm/s; Zone D: above 7.1 mm/s.
In imperial units (inches per second peak), multiply mm/s RMS by 0.0557 to get in/s peak (approximately). A common US rule of thumb is: below 0.1 in/s peak is good, 0.1 to 0.3 is acceptable, 0.3 to 0.5 is alert, above 0.5 is danger. These rough thresholds correspond loosely to the Group 2 zones but should not replace the actual ISO classification for your specific machine.
Zone A means newly commissioned machines should fall here. Zone B is acceptable for long-term operation. Zone C means the machine can run but should be monitored closely and maintenance should be planned. Zone D means vibration levels are damaging and the machine should be shut down or reduced to minimum duty until the cause is identified and corrected.
What the Most Common Vibration Frequencies Mean
Broadband vibration severity tells you whether a problem exists, but it does not tell you what the problem is. Frequency analysis (FFT spectrum) identifies the source. The most common vibration patterns in industrial machines:
- 1× RPM (running speed): Imbalance. The dominant peak is at shaft speed. Fix by balancing the rotor. The most common single cause of elevated vibration in rotating machinery.
- 2× RPM: Misalignment (angular) or looseness. A strong 2× component with axial vibration usually points to angular misalignment at the coupling. Mechanical looseness also generates 2× and higher harmonics.
- 1× and 2× RPM in axial direction: Combined offset and angular misalignment. Both coupling alignment components are out of tolerance.
- Sub-synchronous (<1× RPM): Oil whirl in sleeve bearings (typically at 0.42 to 0.48× RPM) or rubbing. Sub-synchronous vibration is always a concern and should be investigated immediately.
- Bearing defect frequencies (BPFO, BPFI, BSF, FTF): Rolling element bearing damage. Each defect location produces a characteristic frequency that can be calculated from the bearing geometry. These typically show up in the acceleration spectrum or in envelope (demodulated) analysis.
- Gear mesh frequency (number of teeth × RPM): Gear problems. Normal gears produce a gear mesh frequency peak. Sidebands around the mesh frequency indicate gear wear, eccentricity, or tooth damage.
A reliable vibration program starts with baseline readings on healthy machines. Without a baseline, you cannot distinguish between normal vibration for that specific machine and a developing problem. Take three-axis readings at each bearing location immediately after installation or overhaul, and store them as the reference for all future comparisons.
Practical Measurement: Where and How to Take Readings
Measurement location and mounting method have a bigger effect on data quality than the quality of the sensor. A $5,000 accelerometer loosely held against a bearing housing with a stick gives worse data than a $200 sensor properly stud-mounted. Mounting method affects the usable frequency range:
- Stud mount (threaded into housing): Best response, usable to 10,000 Hz or higher. Required for bearing condition monitoring.
- Adhesive mount (flat magnet or glue pad): Good response to about 5,000 Hz. Adequate for most routine monitoring.
- Magnetic mount (two-pole magnet): Response rolls off above 2,000 Hz. Acceptable for overall velocity readings and trending but misses high-frequency bearing defects.
- Hand-held probe: Inconsistent, operator-dependent, usable only for gross screening below 1,000 Hz. Not suitable for trending or bearing analysis.
Take readings at each bearing location in three directions: horizontal (perpendicular to the shaft), vertical, and axial (parallel to the shaft). Mark the measurement points with a paint dot or engraved mark so every reading is taken at exactly the same spot. Consistency is more important than absolute accuracy for trending.
Machine operating condition must be consistent between readings. Measure at the same load, speed, and temperature. A centrifugal pump at half flow vibrates differently than the same pump at design flow because of hydraulic forces. Record the process conditions alongside the vibration data. A vibration increase that coincides with a process change is not a machine problem; it is an operating condition change.
The reading interval depends on the criticality of the machine and the rate of change expected. Monthly readings are typical for general plant equipment. Weekly or continuous monitoring is appropriate for critical machines where a failure would cause a plant shutdown. If a machine transitions from Zone B to Zone C, increase the monitoring frequency regardless of the normal schedule.
Vibration Severity Checker
Assess machine vibration severity per ISO 10816. Enter velocity readings and get zone classification with maintenance recommendations.