Early failure warning from vibration data: the ISO 20816 logic and a general approach

On a rotating machine, vibration is one of the earliest visible signs of failure: imbalance, misalignment, looseness and bearing damage all change it. The question is how to tell when the change reaches a level that "asks for maintenance". This note explains the logic of the approach without giving numbers; the values that apply to your machine are in the standard itself and in the manufacturer's documents.

What do the standards do?

The ISO 10816 series has been replaced by the ISO 20816 series. For rotating machines, it divides the broadband vibration severity measured on structures (mostly velocity, RMS) into four zones according to machine type, power and foundation stiffness: A is the level of a newly commissioned machine, B is suitable for long-term operation, C is limited-duration operation, D is risk of damage. The zone limits change with the machine group, so there is no single "safe mm/s" figure. Read the table from the standard; do not guess.

The standard is not enough: baseline and trend

The zone assessment looks at the absolute level. For early warning, relative change is often more valuable: the level measured on the same machine, at the same point, under the same operating condition while it is healthy is the baseline. A clear rise against the baseline shows that something has changed even while the machine is still in zone B. Set alarm thresholds using both yardsticks: the standard zone (absolute limit) and a multiple of the baseline or a slope (relative limit). Choose the multiples for your machine and experience; an "example" multiplier is not a standard.

Units: acceleration, velocity, displacement

The same vibration is expressed in three units, and they convert into each other by frequency. For a sine, peak velocity = peak acceleration / (2πf). Example: for a peak acceleration of 0.5 g (0.5 × 9.81 = 4.905 m/s²) at 100 Hz, peak velocity is 4.905 / 628.3 = 7.81 mm/s, and RMS velocity, dividing that by √2, is 5.52 mm/s. Because a real signal is not single-frequency, this conversion is valid only for one component; for broadband, velocity comes from measuring velocity directly or integrating the signal.

Common mistakes when setting alarms

• **Changing the measurement point and direction.** Mounting the sensor somewhere else changes the level. Mark the points and keep the same direction and mounting method.
• **Ignoring the operating condition.** When load and speed change, vibration changes; compare data under similar conditions.
• **Alarming on a single measurement.** A momentary spike (impact, measurement error) produces a false alarm. Require it to persist over several consecutive measurements.
• **Looking only at the overall level.** Bearing damage barely moves the overall level in its early stage; you need a high-frequency band or spectrum detail. For spectra, pay attention to the measurement settings (sampling, window, resolution).
• **Not defining the action after an alarm.** What does the "alarm" trigger? Inspection, lubrication, re-measurement, a planned stop: write the action down in advance for each threshold.

Try it with the tools

You can place a velocity value you measured into a zone for your machine group in the vibration severity evaluator and compare it with your baseline; the limits in the tool are for illustration, and what binds you is the standard and the manufacturer's document. For unit conversion there is the vibration unit converter, for spectrum settings the FFT measurement setup. How to proceed without labelled failure data is in this note. For setting up vibration monitoring, get in touch.