Vibration Analysis for Beginners: What the Spectrum Is Telling You

A single overall vibration number tells you how much a machine shakes. It says nothing about why. Two machines can read the same overall level for completely different reasons — one out of balance, the other with a bearing on its way out — and they need completely different fixes.

The spectrum exists to answer the why. It splits the shaking into its component frequencies, so you can see which part of the machine is doing the shaking.

From a waveform to a spectrum

A sensor on a bearing housing picks up everything at once: shaft, coupling, gears, bearings, a loose bolted joint. Plotted against time, that signal is a tangled waveform.

The Fast Fourier Transform, FFT for short, sorts it out. It breaks the waveform into sine waves, each with its own frequency and amplitude, plotted as peaks. Frequency runs along the bottom axis, usually in hertz; amplitude up the side, usually as velocity in millimetres per second RMS.

Reading starts with one reference: running speed. A shaft at 1,500 rpm turns 25 times a second, so its running-speed frequency — written 1× — is 25 Hz, and 2× is 50 Hz. Every peak gets read against that reference: 1× means once per revolution, 2× twice per revolution, and a peak that lines up with neither is telling you something else entirely.

Measure in the right place, the same way every time

Sensors go on the bearing housing, on solid structure close to the bearing — not on a cover, a guard, or a cooling fin. Take three directions at each bearing: horizontal, vertical, and axial, parallel to the shaft. Some faults show up radially and some axially, and the comparison between directions is often what separates one fault from another.

Take readings at the same marked points, in the same directions, under comparable load and speed, every time. Vibration analysis lives on comparison: against the machine's own baseline, against the last reading, against the same bearing on a sister machine. A single reading with no history behind it is hard to judge.

Velocity in mm/s RMS is the usual default for general machinery monitoring, over a band of roughly 10 to 1,000 Hz, and it suits the common low-frequency faults. Acceleration suits high-frequency work, like early bearing defects, and displacement suits slow machines. Picking the wrong parameter makes the fault you are looking for hard to see.

The four patterns you will meet first

Most routine spectra come down to four patterns.

Unbalance: a tall peak at 1×. A heavy spot on a rotor pulls it once per revolution, so the spectrum shows one dominant peak at running speed, mostly radial, with little else around it. The force grows with the square of speed. The fix is balancing, and nothing else substitutes for it.

Misalignment: 1× plus a strong 2×, with high axial readings. Two shafts that are not lined up push and pull on each other twice per revolution through the coupling, which is why 2× shows up alongside 1×. Axial vibration is the giveaway: if the axial reading at the coupling is high compared with the radial readings, suspect alignment before anything else. Balancing a misaligned machine wastes the one outage you had.

Looseness: a forest of harmonics. A loose foot, a cracked base, or a worn housing lets the machine rattle, and rattling is not a clean once-per-revolution event. The spectrum fills up with peaks at 1×, 2×, 3×, 4×, and beyond, sometimes with half-speed (0.5×) peaks mixed in. When you see that forest, check bolts, feet, and fits first.

Bearing defects: peaks that do not line up with running speed. A damaged rolling-element bearing vibrates at frequencies set by its own geometry — the ball-pass frequency of the outer race (BPFO), of the inner race (BPFI), the ball spin frequency, and the cage frequency — none of which are whole multiples of shaft speed. That is the clue: peaks sitting off the 1× grid, often with sidebands spaced 1× apart, and a rising noise floor as damage spreads. These frequencies need the bearing's actual dimensions from its datasheet; guessing them from a generic table is how a real defect gets misread.

Gears confuse beginners for a similar reason. A gear pair vibrates at the mesh frequency — teeth multiplied by shaft speed — with sidebands at shaft speed around it. If the mesh peak and its sidebands grow, the gear set, not the rotor, is where to look.

Phase is the tie-breaker

Frequency tells you what is vibrating. Phase tells you how it is moving, and it settles arguments frequency alone cannot. Unbalance and misalignment can both produce a strong 1× peak. Phase separates them: bearings moving together point to static unbalance, a 180-degree split across the coupling points to misalignment, and erratic phase points to looseness.

Phase needs a reference — a tachometer mark on the shaft — and the same setup at each point, or the comparison is meaningless. It is one extra step on a route, and it is the step that stops a wrong diagnosis from becoming a wrong repair.

How bad is it? Use the zones, then the trend

A pattern tells you the fault. A number tells you how urgent it is. ISO 20816, which has replaced the older ISO 10816 series part by part, sorts broadband RMS velocity into four zones: A for newly commissioned machines, B for unrestricted long-term operation, C for unsatisfactory continuous operation where remedial action should be planned, and D for levels likely to cause damage.

The boundaries depend on the machine. Under ISO 20816-3, for rigidly mounted machines, Group 1 (above 300 kW) uses 2.3, 4.5, and 7.1 mm/s RMS as the A/B, B/C, and C/D boundaries; Group 2 (15 to 300 kW) uses 1.4, 2.8, and 4.5 mm/s. Flexibly mounted machines carry higher boundaries, so getting the machine group and support type right matters more than memorising one table.

Even so, the trend usually beats the zone. A machine that has sat at 1.8 mm/s for a year and suddenly reads 2.6 mm/s has a story to tell, even though it is still in Zone B. Watch the change, not only the label.

What a spectrum cannot do

A spectrum suggests a fault; it does not prove one. It cannot tell you how long a bearing has left. Confirm the reading before acting on it: repeat it, check phase, look at temperature and oil condition, and inspect when the opportunity comes. The spectrum narrows the search. The inspection closes it.

The Takeaway

Read a spectrum in this order: running speed first, so every peak has a reference; the pattern second — 1× for unbalance, 1× plus 2× with high axial for misalignment, a forest of harmonics for looseness, off-grid peaks for bearing defects; phase third, to settle the cases where two faults look alike; and the trend against the machine's own baseline, judged against the right ISO 20816 zone boundaries, to decide how urgent it is. Four steps, and a wiggly line becomes a decision: what is wrong, and what to do about it first.

Further Reading

Preventive, Predictive, Condition-Based — Choosing the Right Mix — Where vibration monitoring sits among the maintenance philosophies, and how to decide what each asset actually gets.

Plant maintenance — pillar guide — The full guide to maintenance planning, where routine vibration routes sit alongside overhauls, inspections and budgeting.

Vibration Analysis & Machinery Diagnostics — A full spectrum-diagnostics reference, with the fault-pattern table behind the unbalance, misalignment, looseness, bearing and gear signatures used here.

ISO 20816 Vibration Limits: Zones A to D Chart in mm/s — Source for the ISO 20816-3 zone boundaries for Group 1 and Group 2 rigidly mounted machines quoted above.

BS ISO 20816-2:2017 — The standard itself, covering land-based gas turbines, steam turbines and generators above 40 MW with fluid-film bearings.