What a vibration tells you, and what it doesn't

In brief

• A machine repeats itself every revolution: defect signatures are read in multiples of rotation (1x, 2x…).
• The signature table is established guidance, not a verdict and not a normative requirement.
• Without a phase reference, unbalance, misalignment, resonance and rubs cannot be told apart.
• A continuous measurement does not make the diagnosis: it notices that something has changed, and tells you where and when to look.

The machine repeats itself every revolution

A rotating machine repeats itself revolution after revolution, defects included. That is why one reasons in multiples of the rotation frequency, called orders. A motor at 1,480 rpm makes about 24.7 revolutions per second: its 1x sits around 24.7 Hz, its 2x around 49.3 Hz. If the speed changes, the rotation-related lines all move together; the ones that stay put do not come from rotation, like the electrical component at 100 Hz discussed further down.

Without a speed signal, the 1x is an estimate. Without a tachometer or encoder the speed has to be derived from the signal itself: it works with a clean 1x and near-constant speed, but it remains an inference. If the 1x falls, so does the judgment on what is 2x and 3x.

The typical signatures

This is the table hanging in half the world’s workshops. It serves as guidance, not as a verdict.

Defect	In the signal	On the shop floor
Unbalance	Stable 1x at constant speed	Grows with speed, uniform hum on the support
Shaft misalignment	1x, 2x or higher harmonics, sometimes axial	Hot coupling, worn inserts, leaking seals
Mechanical looseness	1x and harmonics, sometimes subharmonics, erratic values between start-ups	Loose anchors, shims that move, changing noise
Rolling bearing degradation	Broadband high-frequency energy, unstable and rising readings	Whistle or metallic rasp, housing getting hot
Pump cavitation	Noise floor rising above 1-2 kHz, no sharp lines	Gravel-like noise in the impeller, fluctuating flow
Resonance	Amplification near a natural frequency, strong phase changes	Explodes at a certain speed, calms down above and below

These rows are established engineering practice, not requirements. A similar table sits in an informative annex of ISO 13373-1:2002, and ISO 13373-3:2015 uses fault tables as good practice among experts, allowing other approaches.

Three levels not to confuse

• The standard says so: broadband severity with zones and baseline (ISO 20816 series) and the structured diagnostic path (ISO 13373-3 and ISO 13379-1), which calls for amplitude and phase of 1x and 2x, multiple measurement points, machine data and a declared confidence level.
• It is engineering practice: the signature-defect associations at the level of detail taught in courses, the half-harmonics in looseness, cavitation as a rising noise floor, which appears in no ISO table.
• It is your own reasoning: the hypothesis you build knowing that machine, that process, that failure history. Legitimate, as long as it is declared for what it is.

How slippery the second level is was shown by a 1999 test rig (IMAC): controlled misalignments, varying coupling, magnitude and speed, produced no guaranteed characteristic spectrum. The dominant harmonic changed from test to test. So no: a dominant 2x does not mean misalignment.

Without phase, many diagnoses cannot be separated

Phase is the angular position of the vibration relative to a reference turning with the shaft: it takes a notch, an encoder or a probe. Without it, phase is not less precise: it simply is not there.

ISO 13373-3 names it as the tool for distinguishing misalignment, resonance, rubs and unbalance: the four causes that, without a phase reference, collapse into one, because they all produce energy at 1x or its multiples.

The electrical trap. On the 50 Hz grid the electromagnetic force generates an exact component at 100 Hz. A two-pole motor at full load runs between 2,900 and 2,950 rpm: its mechanical 2x falls between 96.7 and 98.3 Hz, a few hertz from the electrical 100 Hz. Without a speed reference and sufficient resolution the two get confused.

It is not a limitation of cheap instruments, it is a codified fact: ISO 13379-1 asks for detectability and confidence scores for each failure mode, and low scores mark precisely the symptoms that cannot be distinguished from those of other modes.

Bearings: what can be calculated and what can only be seen

The characteristic frequencies of a bearing (outer race, inner race, rolling element, cage) are pure kinematics: they depend on the geometry and scale with shaft speed. Without geometry and without speed they cannot be calculated. Whoever declares an outer-race defect without those two pieces of data is skipping a step.

Even when calculated they are not exact: the contact angle changes with load and the rolling elements slip, so the real values drift by a few percent. That is what distinguishes them from mechanical harmonics: they are not integer multiples of rotation.

On envelope analysis: the classic technique demodulates a high-frequency resonance band, often around 8-10 kHz. With a usable band of a few kilohertz you work on lower resonances, of the machine and the support: a different technique, a different sensitivity. Always ask which band it is done on.

What remains visible even without the bearing data:

• the level and trend over time of the high-frequency energy;
• the impulsiveness indicators, such as crest factor and kurtosis;
• a non-integer periodicity in the envelope spectrum, to be named only after entering the bearing data.

What it takes to close a diagnosis

Severity scale and diagnostics are two different worlds. ISO 20816-3 evaluates broadband vibration, with no frequency components or phase, and excludes the evaluation of bearings and gears: a zone A, B, C or D is not a bearing diagnosis.

The honest point, and it is not discouraging: a continuous measurement does not make the diagnosis. It notices that something has changed, when and how fast, and tells you where to look and when it is worth going. The diagnosis comes afterwards, with:

1. more measurement points, on every bearing;
2. the three directions: horizontal radial, vertical radial, axial;
3. a phase reference and a shaft speed signal;
4. the machine data: bearing geometry, gear ratios, number of blades or rotor bars;
5. the operating conditions of the moment: a measurement in a transient is not comparable;
6. someone who knows that machine and, for the fine reading, certified expertise.

And no measurement predicts the date of a failure: whoever promises you one is selling, not measuring.