KEYSTONEIndustrial Services
Interactive trainer

Reading a vibration spectrum

A motor and pump with a vibration you can feel but not name. Six faults, one spectrum, and the three axes that tell them apart: where the peaks land — 1×, 2×, or high in the bearing band — and whether they jump when you turn the probe axial.

By Francisco Padilla Interactive Technician level

Before you start — job brief

Vibration analysis is done on running equipment, so the hazards are the rotating and hot parts, not the meter. Keep the accelerometer cable and your hands clear of couplings, sheaves and fan blades, and mind pinch points and hot surfaces while you place the probe.

  • Keep cables, tools and hands clear of rotating couplings, belts and fans
  • Mind hot bearing housings and casings
  • Take readings at the bearings, in three directions: horizontal, vertical, axial
  • The pattern of the peaks — and which axis they favour — names the fault

The spectrum is a fingerprint

Overall vibration tells you a machine is sick; the spectrum tells you what it has. Splitting the vibration into its frequencies — the FFT — turns a shake you can feel into a chart you can read, because each fault vibrates the machine at a frequency tied to what is actually happening inside it. Imbalance throws the rotor once per revolution, so it lives at 1× running speed. Misalignment flexes a coupling twice per turn, so it lives at 2×. A cracked bearing race rings far above running speed. Read where the peaks land, and the machine has told you its diagnosis.

Two habits do most of the work: read where the tallest peak sits relative to running speed, and compare the three directions — horizontal, vertical, and axial. The frequency sorts the fault into a family; the axis often settles which member it is.

Where the peak lands

  • 1× running speed, dominant. Imbalance — or a bent shaft. One clean peak, everything else small.
  • 2× running speed, dominant. Misalignment. The most common finding on any coupled machine, and the most common thing done wrong.
  • A run of harmonics — 1×, 2×, 3×, 4× and up. Mechanical looseness. The machine is pounding against its own slack and smearing energy across many orders.
  • High up in the frequency range, non-synchronous. A rolling-element bearing defect. The energy is nowhere near running speed; it is a haystack up high that grows as the bearing fails.
  • At twice line frequency (120 Hz on 60 Hz power). Electrical. Not tied to shaft speed at all, and load-sensitive.

Field note — the axial reading and the coast-down

Two quick checks resolve the look-alikes. First, the axial direction: imbalance is quiet axially, so a strong 1× that is biggest in the axial direction is a bent shaft, not imbalance — and misalignment shows itself by pushing axially at 2×. Second, the coast-down: kill the power and watch the peak. A mechanical fault rides the shaft down as it slows; an electrical fault at twice line frequency vanishes the instant the field collapses. Those two moves separate half the faults in the book.

Take the readings where it matters

A spectrum is only as good as where you put the probe. Read at the bearing housings, not the guard or the frame, and take all three directions at each bearing — the pattern across locations tells you which end of the machine, or which side of the coupling, owns the fault. A bearing defect shows up loudest at the bad bearing; misalignment straddles the coupling; a resonance can make one direction wildly higher than the physics of the fault would suggest. One reading is a data point; the set is the diagnosis.

What normally goes wrong, in order

  1. Misalignment — the everyday coupled-machine fault, and the one alignment tools pay for
  2. Imbalance — from build-up, wear, or a thrown weight, easy to confirm and fix
  3. Looseness — soft feet and worn fits that amplify everything else
  4. Bearing wear — caught early by the high-frequency band before it seizes
  5. Bent shafts and electrical faults — less common, sorted by the axial reading and the coast-down

Vibration analysis is not magic, and it is not only for specialists with six-figure analyzers. The core of it is this handful of patterns and two comparisons. Read where the peak lands, check the axial direction, and cut the power to watch what disappears — and a nameless shake becomes a work order with a part number on it.