Setting Vibration Alarm Limits That Work on Real Machines

Alarm limits that are set too low cause nuisance trips, and limits set too high let real faults develop into failures. Most plants copy default values into the monitoring system and never check them against the machine or the standard. After reading this article you can set alarm and trip limits from ISO 10816-3 zones, your own baseline data and the dominant fault type, and document the decision so the next engineer can audit it.

Start from the machine, not from a default table

A vibration limit is only meaningful when you define the measurement point, the direction, the quantity and the frequency band. The same motor can read 2 mm/s RMS on its foot and 6 mm/s RMS on the bearing cap in the axial direction; both values can be normal. Before you write any number into the monitoring system, record:

• Measurement point (bearing housing, foot, shaft probe) and direction (H, V, A)
• Quantity: velocity RMS in mm/s for general machine condition, acceleration in g for bearings, displacement in µm for shaft proximity probes
• Frequency band: broadband 10 Hz to 1 kHz is the usual default, but rolling bearing faults live above 1 kHz
• Machine group and mounting per ISO 10816-3: rigid or flexible foundation, and power rating above 15 kW up to 50 kW or above 50 kW

ISO 17359 describes this as the first step of condition monitoring strategy: know what you measure and why, before you judge the values. ISO 13373 gives the measurement procedures for collecting comparable data.

Use ISO 10816-3 zones as the starting point

ISO 10816-3 (now carried forward as ISO 20816-3) divides vibration severity into four zones:

• Zone A: new commissioned machine
• Zone B: unrestricted long-term operation
• Zone C: restricted operation, plan corrective action
• Zone D: vibration severe enough to cause damage

The standard gives the zone boundary values in mm/s RMS for each machine group and foundation type. For a typical medium-size machine on a rigid foundation, the boundary between B and C sits around 4.5 mm/s RMS and between C and D around 7.1 mm/s RMS; for flexible foundations the boundaries are higher. The exact values depend on the machine group, so look them up rather than memorise them. Our ISO 10816-3 Vibration Severity Evaluator returns the correct zone for your measured value and machine group.

A common and defensible starting rule:

• Alarm level: upper edge of Zone B
• Trip level: lower edge of Zone D, or the manufacturer's trip value if the OEM specifies one

This gives you Zone C as a working buffer, where you investigate but keep running.

Refine the limits with your own baseline

Standards describe machine classes, not your specific machine. A fan that has run for years at 2.8 mm/s RMS has a different normal state than a nominally identical fan at 1.2 mm/s. ISO 17359 recommends trending against baseline data. Use this procedure on site:

1. Measure velocity RMS at every defined point and direction, in H, V and A, at a stable operating condition. Take at least three readings and average them.
2. Record the baseline in the monitoring system with the date, machine load, speed and temperature.
3. Set the alarm at the higher of the two values: the Zone B boundary, or 2.5 times the baseline RMS. The factor 2.5 is a widely used engineering rule of thumb, not a standard requirement; adjust it if the machine shows high scatter.
4. Set the trip at the higher of the Zone D boundary or the OEM trip value.
5. Re-check the baseline after any overhaul, alignment or rebalancing, and update the limits.

If you rebalance the machine, ISO 21940-11 gives the permissible residual unbalance from the rotor mass and service speed, and ISO 21940-14 covers the balancing of flexible rotors. After field balancing with our Single-Plane Balancing Calculator or Two-Plane Balancing Calculator, measure a new baseline; a well-balanced machine should sit clearly inside Zone A or B.

Set separate limits for different fault types

Broadband velocity RMS detects unbalance, misalignment and looseness well, but it can miss early rolling bearing damage. A spalled bearing raises high-frequency acceleration long before the 1x RPM component moves. Practical split:

• Unbalance, misalignment, looseness: alarm on velocity RMS in the 10 Hz to 1 kHz band
• Rolling bearings: alarm on acceleration or on envelope/demodulated high-frequency vibration, with limits from the bearing manufacturer or from baseline trend
• Journal bearings and sleeve-bearing machines: watch shaft displacement in µm on proximity probes; ISO 21940-21 addresses shaft vibration of typical industrial machines

When an alarm fires, the spectrum tells you which limit mattered. If you need to identify a pattern, compare the measured spectrum against the Vibration Fault Spectrum Simulator or run the Vibration Diagnosis Wizard before you change any setpoint.

Watch for resonances before you tighten limits

If a machine sits near a structural or rotor resonance, its broadband level depends strongly on speed and temperature, and a fixed alarm limit produces false calls. Check the phase and the frequency of the dominant peak: a resonance shows a peak at a natural frequency, not at 1x RPM, and the phase shifts sharply through the resonance. The Resonance & Critical Speed Simulator shows this behaviour clearly.

For a resonant machine, either move the alarm band to exclude the resonance region and monitor it separately, or fix the resonance itself by stiffening, damping or rebalancing. Do not simply raise the limit; a resonance that grows with time will still damage the machine inside the alarm threshold.

Document the limits so the next engineer can audit them

Every alarm setpoint in the monitoring system should have a record with five entries:

1. Point, direction and quantity, with units
2. Alarm and trip values, and the frequency band they apply to
3. The rule used: ISO 10816-3 zone, baseline factor, or OEM value
4. The baseline measurement date and operating condition
5. The date of the last limit review

Review the limits at least annually, and after any modification to the machine, its foundation or its operating speed. A limit without a documented origin is a guess, and guesses drift upward after every nuisance trip.

When to call for an on-site measurement

Call for an on-site measurement when the broadband level sits in Zone C and you cannot separate the fault sources, when a resonance or critical speed limits your setpoints, or when a machine trips repeatedly with no clear spectral signature. Field balancing with phase measurement, orbit analysis and a 24-bit DAQ chain resolves most of these cases in one visit. REI Balance offers field balancing and vibration analysis on site; contact us via /en/contact to schedule a measurement.