How do I determine the correct Fmax for vibration measurement?

Fmax should be at least 3.5× the highest bearing defect frequency. For rolling element bearings, BPFO is typically 0.4×N×RPM/60 where N is the number of rolling elements. A common rule: Fmax ≈ 40× shaft speed for standard bearings, 80× for high-speed machinery.

How many FFT lines should I use?

Use 800 lines for routine monitoring, 1600 lines for detailed analysis, 3200 or 6400 lines for high-resolution bearing diagnostics. More lines = better frequency resolution but longer acquisition time.

What measurement parameters should I collect?

Velocity (mm/s RMS, 10-1000 Hz) for overall machine condition per ISO 10816. Acceleration (g) for bearing defects and high-frequency issues. Displacement (μm pk-pk) for low-speed machines (<600 RPM). Envelope/demodulation for early bearing defect detection.

How many measurement points per bearing?

Minimum 2 per bearing: horizontal (H) and vertical (V). Add axial (A) for thrust bearings and to detect misalignment. For critical machines, measure H+V+A at each bearing = 3 points per bearing position.

How is data storage calculated for vibration measurements?

Each spectrum requires FFT_lines × 4 bytes (float32). Total storage = number_of_points × parameters_per_point × FFT_lines × 4 bytes. A typical route with 50 machines × 4 points × 3 parameters × 800 lines = ~1.9 MB per route.

Free Engineering Tool

Vibration Measurement Plan Generator

Calculate recommended frequency range, measurement points, acquisition time, and data storage. Generate a downloadable measurement plan.

ISO 10816 ISO 20816 Bearing Defects

Machine Type

Shaft Speed (RPM)

Bearing Positions (2–8)

FFT Lines

Rolling Elements per Bearing (N, for BPFO calc)

Machine Tag / ID

Measurement Parameters

Velocity (mm/s RMS) Acceleration (g peak) Displacement (μm pk-pk) Envelope / Demodulation

Quick presets

Measurement Plan

Recommended Fmax

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Total Measurement Points

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Frequency Resolution (Δf)

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BPFO Estimate

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Estimated Acq. Time per Point

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Total Measurement Time

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Data Storage per Route

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Parameters per Point

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Maximum Frequency (Fmax) Recommendation

Fmax must capture the highest-frequency defect of interest. For rolling element bearings, the outer race defect frequency (BPFO) is approximately:

To capture 3rd harmonic of BPFO with sidebands, the recommended Fmax is:

Frequency Resolution & Acquisition Time

The frequency resolution determines the minimum resolvable frequency difference:

The time record length T (acquisition time for one average) equals 1/Δf. With 4 averages (75% overlap), total acquisition time ≈ 1.75 × T.

Data Storage Calculation

Each spectrum stores amplitude values for every FFT line. Storage per measurement:

Measurement Directions

Direction	Code	Detects
Horizontal	H	Unbalance, looseness, misalignment
Vertical	V	Unbalance, bearing defects
Axial	A	Misalignment, thrust bearing, blade pass

Practical Example

Example — Centrifugal Pump at 2960 RPM, 4 bearings

Shaft frequency: f = 2960/60 = 49.3 Hz

BPFO estimate (12 rollers): 0.4 × 12 × 49.3 = 236.8 Hz

Fmax: 3.5 × 236.8 = 828 Hz → round to 1000 Hz

Points: 4 bearings × 3 directions = 12 points

Δf (800 lines): 1000/800 = 1.25 Hz → can resolve 1× at 49.3 Hz easily

Storage: 12 × 3 params × 800 × 4 = 115,200 bytes ≈ 113 KB

⚠️ Note: For gearbox vibration analysis, Fmax should cover gear mesh frequency and its harmonics: GMF = N_teeth × shaft_speed. This calculator focuses on rolling element bearing machines.