FFT call-out cheat sheet (Pk velocity in/s, CPM)
Keep with the analyzer. Lessons and drill at the course site. Database band alarms and OEM limits override everything here.
The 60-second routine
- Header: machine, point, RPM, overall Pk. Above ISO zone? Above 25% of last route?
- 1X: harmonic cursor on running speed.
- Harmonics: how many, which tallest. 1 = unbalance, 2X big = misalignment, 4+ ladder = looseness.
- Between them: non-integer orders = bearing (above 1X) or whirl/belt (below 1X). Exact 0.5X = looseness/rub.
- Sidebands: spacing 1X = inner race / rotor bar / gear; spacing pole-pass = electrical.
- Floor: haystack = late bearing or lube; rising grass on pump = cavitation.
- Compare: last route, sister machine, H vs V vs A. Decide: Normal / Watch / Call out / Urgent.
Signatures
| Fault | Spectrum | Direction / where | Confirm |
|---|---|---|---|
| Unbalance | 1X dominant, harmonics <15% of 1X | Radial, H about = V | H-V phase 90 deg, steady |
| Angular misalignment | 1X, 2X, 3X | Axial, coupling end | Axial phase 180 across coupling |
| Parallel misalignment | 2X > 1X (2X > 50% suspect, >150% severe) | Radial, coupling end | Radial phase 180 across coupling; zoom to rule out 2xLF |
| Looseness | 4-10 harmonics, 0.5X / 1.5X, raised floor | Vertical, one bearing or foot | Erratic phase, clipped waveform, hand on feet |
| Bearing outer race | Non-integer peak about 0.4 x Nb x RPM + harmonics | Radial, that bearing | PeakVue same freq; data sheet BPFO |
| Bearing inner race | About 0.6 x Nb x RPM + harmonics, 1X sidebands | Radial | PeakVue with 1X sidebands; modulated waveform |
| Bearing late (stage 4) | Haystacks, discrete peaks smeared, 1X rising | Radial | PeakVue may drop; temperature up |
| Electrical rotor | 1X with pole-pass sidebands (slip x poles); RBPF with 2xLF sidebands | Motor only | Vanishes at power-off; MCSA; needs zoom |
| Electrical stator | 2xLF (7,200 CPM) high, no sidebands | Motor only | Zoom separates from 2X; thermal |
| Vane pass | Vanes x 1X, 1X sidebands, 2xVPF | Pump H/V | Ask flow vs BEP; cutwater gap; sister pump |
| Cavitation | Random broadband floor rising to the right | Pump end | Gravel sound; suction pressure, strainer |
| Resonance | Broad hump, not an integer order, near 1X/2X/VPF | One direction much higher | Bump test; 90 deg phase shift |
| Oil whirl (sleeve) | 0.42-0.48X, wanders, may exceed 1X | Large sleeve-bearing motors | Oil temp/viscosity, clearance, load |
| Rub | Exact 1/2X, 1/3X, many harmonics, broad floor | Sleeve bearings, seals | 1X may drop; waveform truncated |
How big: ISO 20816-3 zones in Pk in/s (overall)
| Machine | A/B | B/C | C/D |
|---|---|---|---|
| Group 1 (>400 HP) rigid | 0.13 | 0.25 | 0.40 |
| Group 1 flexible | 0.20 | 0.40 | 0.61 |
| Group 2 (20-400 HP) rigid | 0.08 | 0.16 | 0.25 |
| Group 2 flexible | 0.13 | 0.25 | 0.40 |
A/B = fine. C = short-term only, call out with a date. D = urgent. Pk = rms x 1.414. 1,250 HP 3,600 RPM units = Group 1.
How fast: trend rule
- Within 25% of last 3 routes: normal scatter.
- +25 to +100%: Watch, shorten interval.
- Doubled (6 dB): Call out even in Zone B.
- Tripled or climbing every route: Urgent.
- PeakVue rising, velocity flat: stage 1-2 bearing. Watch, plan spare.
PeakVue rule of thumb (g Pk overall)
| Speed | Alert | Fault |
|---|---|---|
| 3,600 RPM | 10 | 20 |
| 1,800 RPM | 6 | 12 |
| 1,200 RPM | 4 | 8 |
PeakVue with a frequency = defect. PeakVue with no frequency = lubrication or cavitation.
Called out at any level
- Non-synchronous harmonic family (bearing) with sidebands or hump
- Sub-synchronous peak on a sleeve bearing
- Half-order subharmonics
- Pole-pass or 1/3-LF sidebands
- Cavitation floor on a pump
- 2X over 150% of 1X
- Broad peak within 20% of a forcing frequency (block balancing until bump-tested)
Write it in four sentences
1 Observation (point, direction, numbers) 2 Interpretation ("pattern consistent with ...") 3 Severity and trend (ISO zone, change since last route) 4 Recommendation with a date ("recommend ... not later than ...; re-survey after correction").
Example: M2A axial: 1X-6X harmonics, 4X dominant 0.49 in/s Pk, overall 0.675. Consistent with angular misalignment and coupling distress. ISO Zone D (Group 1 rigid), up 3x. Recommend laser alignment and coupling inspection within 2 weeks; confirm with axial phase across coupling.
Resolution trap: at 75,000 CPM / 1,600 lines each line is 47 CPM; 2X (7,160) and 2xLF (7,200) on a 2-pole motor are one line apart. Zoom (Fmax 6,000, 3,200 lines) before calling either. Severity words: Normal (log) / Watch (note, shorter interval) / Call out (finding, recommendation, date) / Urgent (finding plus same-day call to the station).