Vibration Reference Panel
Pick an equipment type — or filter by bearing or sensor first — to read its typical construction, monitoring approach, and the governing acceptance criteria, alongside the frequency-diagnosis and condition-rating references below.
Figures throughout this page are compiled for orientation and spec-review discussion. Verification depth varies by row — some criteria were checked directly against primary-source standard text, others come from secondary references or community approximations that are flagged in their own notes — but none of it should be relied on for a contractual, safety, or compliance decision without confirming exact values against the current edition of the applicable standard.
Equipment lookup
Filter by bearing or sensor type to narrow the list, then choose the equipment class.
| Standard | Acceptance criteria |
|---|
Speed & frequency correlation
Every diagnosis starts from expressing a measured frequency as a multiple ("order") of running speed.
| Order / frequency | Common name | Typically indicates |
|---|---|---|
| ~0.38–0.48X | Oil whirl / whip | Fluid-film bearing instability; "whip" locks in when whirl frequency crosses a rotor critical speed |
| 0.5X | Half-speed / stall | Loose bearing fit, or aerodynamic rotating stall in fans and compressors |
| 1X | Running speed | Unbalance (by far the most common cause); also bow, eccentricity, thermal rub |
| 2X | Twice running speed | Misalignment (angular or parallel), mechanical looseness, bent shaft |
| 3X–5X | Low harmonics | Severe coupling misalignment, coupling wear |
| nX (many) | Harmonic series | Mechanical looseness, rubbing, impacting — the more harmonics, the more impact-like the fault |
| Blades × 1X | Blade / vane pass | Flow disturbance, vane-to-casing gap, recirculation at off-BEP operation |
| Teeth × 1X ± sidebands | Gear mesh | Tooth wear or pitting; sideband spacing identifies which gear/shaft is modulating |
| BPFO / BPFI / BSF / FTF | Bearing defect frequencies | Rolling-element race, ball, or cage wear — non-integer multiples calculated from bearing geometry |
| Line freq. / 2× | Electrical (50/60, 100/120 Hz) | Broken rotor bars, static/dynamic eccentricity, electrical unbalance in motors and generators |
| High-frequency / broadband kHz | Ultrasonic energy | Early-stage rolling-element defects, cavitation, rubs, lubrication starvation |
ISO 10816 / 20816 classification by equipment type
Every equipment class in the lookup above maps to exactly one ISO 10816/20816 part — never both a rotodynamic and a reciprocating scale at once. Five shared legends (below) cover most of the mapping; the table underneath says which legend, if any, applies to each equipment type, and whether that equipment's own API/HI/AMCA/CTI card above already supersedes it as the operative acceptance basis.
Group 1 (>300 kW) reads higher: rigid 2.3/4.5/7.1, flexible 3.5/7.1/11.0 mm/s RMS — see the full industrial-machinery row in the table below (general-industrial fallback).
| Machine / speed | Zone A/B | Zone B/C | Zone C/D |
|---|---|---|---|
| Steam turbine/generator, 1,500–1,800 rpm | 2.8 | 5.3 | 8.5 |
| Steam turbine/generator, 3,000–3,600 rpm | 3.8 | 7.5 | 11.8 |
| Gas turbine, all sizes > 3 MW | 4.5 | 9.3 | 14.7 |
Shaft relative displacement (peak-to-peak, µm) is speed-dependent rather than a fixed table: Zone A/B = 4,800/√n, B/C = 9,000/√n, C/D = 13,200/√n, where n = rated speed in rpm. Part 2 targets steam turbine/generator sets in the >40 MW class; Part 4 covers gas turbines >3 MW using the same housing-velocity numbers as the gas-turbine row above.
| Group | Zone A | Zone B | Zone C | Zone D |
|---|---|---|---|---|
| 1 — Horizontal generator sets, fluid-film, 60–1,800 rpm | ≤ 1.6 | 1.6–2.8 | 2.8–4.5 | > 4.5 |
| 2 — Vertical generator sets, fluid-film, 60–1,800 rpm | ≤ 2.3 | 2.3–4.0 | 4.0–6.4 | > 6.4 |
| 3 — Pump-turbines / storage pumps, fluid-film | ≤ 2.8 | 2.8–4.5 | 4.5–7.1 | > 7.1 |
| 4 — Radial / axial / mixed-flow hydro pumps, fluid-film | ≤ 3.5 | 3.5–5.6 | 5.6–9.0 | > 9.0 |
Select the group matching the unit's configuration — horizontal vs. vertical generator set, pump-turbine, or straight hydro pump — not head or power, which the standard treats as secondary within a group.
Part 9 does not itself publish a flat lookup table — it defines Grades 1–4 via frequency-dependent Annex A rating curves. The values below are the engineering community's backfilled baseline, carried over from the withdrawn predecessor standard ISO 8579-2:1993, not a verified reproduction of Part 9's own curves. Treat as a working approximation, not a citable clause.
| Housing velocity by power class (mm/s RMS, 10 Hz–2,000 Hz) | Grade 1 | Grade 2 | Grade 3 | Grade 4 |
|---|---|---|---|---|
| 10–200 kW | 1.1 | 1.8 | 2.8 | 4.5 |
| 200–1,000 kW | 1.8 | 2.8 | 4.5 | 7.1 |
| 1,000–10,000 kW | 2.8 | 4.5 | 7.1 | 11.2 |
| > 10,000 kW | 4.5 | 7.1 | 11.2 | 18.0 |
| Shaft displacement by speed (µm p-p, 2–500 Hz) | Grade 1 | Grade 2 | Grade 3 | Grade 4 |
|---|---|---|---|---|
| ≤ 500 rpm | 45 | 70 | 110 | 150 |
| 500–1,800 rpm | 35 | 55 | 85 | 120 |
| 1,800–4,500 rpm | 25 | 40 | 65 | 90 |
| > 4,500 rpm | 15 | 25 | 40 | 60 |
Grade 1/2 typically govern factory acceptance testing; Grade 3 ≈ Zone B (unrestricted continuous operation), Grade 4 ≈ Zone C boundary (restricted run time). Casing acceleration has no flat table — Annex A scales it from the gear-mesh frequency and the grade's velocity limit (a ≈ 2π·f·v×10⁻³ m/s², true peak); ~10 m/s² is a typical Grade 2 baseline and readings above ~25 m/s² trigger an unsatisfactory (Zone C/D) call regardless of power.
| Class | Zone A | Zone B | Zone C | Zone D |
|---|---|---|---|---|
| 1 — Small engines, < 15 kW components | ≤ 1.12 | 1.12–1.8 | 1.8–2.8 | > 2.8 |
| 2 | ≤ 1.8 | 1.8–2.8 | 2.8–4.5 | > 4.5 |
| 3 — Medium engines / compressors | ≤ 2.8 | 2.8–4.5 | 4.5–7.1 | > 7.1 |
| 4 | ≤ 4.5 | 4.5–7.1 | 7.1–11.2 | > 11.2 |
| 5 — Large, rigidly mounted engines | ≤ 7.1 | 7.1–11.2 | 11.2–18.0 | > 18.0 |
| 6 — Large, resiliently mounted engines | ≤ 11.2 | 11.2–18.0 | 18.0–28.0 | > 28.0 |
| 7 — Heavy industrial reciprocating pumps | ≤ 18.0 | 18.0–28.0 | 28.0–45.0 | > 45.0 |
Displacement (µm RMS) and acceleration (m/s² RMS) are tracked in parallel per the standard's three-parameter approach — velocity above is the figure most commonly quoted. This seven-class table condenses the 1995 edition's original twelve-tier severity-grade ladder (1.12/1.8/2.8/4.5/7.1/11.2/18/28/45/71/112/180 mm/s, each an upper boundary) into the classes practitioners actually cite; confirm which class applies against the OEM's stated classification or the purchased standard text. Bearing type here is a package-level convenience, not an engine-level fact: a reciprocating engine's own main/rod bearings are always fluid-film — a "rolling" tag on an equipment card in this reference (e.g. diesel gensets) only reflects the generator end or auxiliary components being counted as part of "the equipment."
None of the five legends apply to this equipment — its ISO part (right, in the table below) doesn't reduce to a shared zone/grade scale. Read the Applicable part column and its governing-basis note instead.
| Equipment | Applicable part | Legend / governing basis |
|---|
Baseline measurements
Why a baseline matters more than any single absolute reading: two nominally identical new machines can sit at meaningfully different absolute vibration levels — manufacturing tolerance, foundation stiffness, and installation quality all shift the honest "healthy" number. Acceptance-standard limits (the table above) tell you whether a machine is broadly acceptable; a baseline tells you whether this specific machine has changed.
A baseline is the full signature — overall amplitude and spectrum, at multiple load points across the operating range — captured while the machine is known-good: at commissioning, immediately after a mechanical run test, or right after an overhaul/rebuild. Every later reading gets trended against it, not just checked against the standard's absolute limit.
- Capture at commissioning — overall + full FFT spectrum, all measurement points/directions, before the machine accumulates any operating hours.
- Re-baseline after major work — any overhaul, rotor rebalance, bearing replacement, or realignment invalidates the old baseline.
- Record load conditions — flow/load-dependent machines (pumps off-BEP, partially loaded generators) need a baseline at more than one operating point.
- Trend the deviation, not just the absolute value — a doubling from baseline is a stronger signal than a reading still inside Zone B.
Vibration unit & amplitude conversion calculator
Single-frequency (sinusoidal) motion only — velocity = 2πf·displacement, acceleration = 2πf·velocity (f in Hz). Enter one known reading with its frequency/speed and amplitude convention; every other quantity, convention, and unit system updates live.
| Quantity | Peak | Peak-to-peak | RMS |
|---|
Each cell shows metric · US customary. Peak-to-peak is conventionally used for displacement, RMS for velocity, and peak (often true-peak) for acceleration — all three conventions are shown for every quantity so you can read off whichever your governing standard calls for. This calculator applies to a single tone; it does not recover true broadband RMS from a peak reading on a real multi-frequency signal — measure that directly.