What does this calculator compute?

It computes the kinematic passage frequency fBPF = Z n/60 for Z equally recurring blades or vanes and speed n in revolutions per minute. It does not predict vibration or sound amplitude.

Does a BPF peak prove a fault?

No. A component at BPF can arise from normal rotor-stator or blade-casing interaction. Its amplitude is installation and operating-point dependent; diagnosis requires comparison with baseline or trend data and other evidence.

Do BPF harmonics identify blade damage or cavitation?

No single harmonic or sideband pattern uniquely identifies a cause. Harmonics and modulation are analysis targets, not automatic fault labels.

Is this an ISO acceptance calculator?

No. The frequency equation is kinematic. This page does not apply ISO vibration severity limits or provide acceptance criteria.

Free Engineering Tool #037

Blade Pass Frequency Calculator

Calculate the kinematic passage frequency for a known integer blade or vane count and measured shaft speed. Frequency alone does not predict amplitude or identify a fault.

BPF / VPFKinematic FrequencyNot an ISO Limit

Results

Blade Pass Frequency (BPF)

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BPF as Order of Shaft Speed

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Shaft Frequency

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BPF Period

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BPF Harmonics

Harmonic	Frequency (Hz)	Order (× shaft)	Period (ms)

Passage frequency

f_BPF = Z × n / 60 = Z × f_s [Hz]

f_s = n / 60 [Hz]

Z is the integer number of recurring rotor elements, n is shaft speed in r/min, and f_s is shaft rotational frequency. For a fixed observation point, Z equally recurring elements produce Z passages per revolution. ANSI/AMCA 301-22 gives this same fan blade-pass equation in its sound-rating method (Eq. 5.7).

Harmonics and sidebands

The table lists mathematical integer harmonics h·f_BPF. Real spectra may contain BPF, harmonics and modulation components, but their presence and amplitude depend on rotor/stator geometry, flow, loading, sensor location, structure and acoustic response. A calculated line is a frequency marker, not a predicted spectral amplitude.

Diagnostic boundary

Do not diagnose from one line. A BPF peak, its harmonics or shaft-spaced sidebands do not uniquely prove clearance error, blade damage, fouling, resonance, recirculation or cavitation. Compare like-for-like operating points with baseline/trend data and use corroborating phase, waveform, pressure, acoustic, process and inspection evidence. Cavitation assessment also requires hydraulic operating and NPSH evidence.

Worked calculation

Six vanes at 2950 r/min

f_s = 2950/60 = 49.1667 Hz

f_BPF = 6 × 49.1667 = 295 Hz

Period = 1/295 = 3.3898 ms; 2×BPF = 590 Hz; 3×BPF = 885 Hz.

Sources and standard scope

ANSI/AMCA Standard 301-22, Eq. 5.7 — fan blade-pass frequency.
NASA technical report 20030064154 — BPF/harmonics in rotor–stator forced response and modulation context.
Grundfos cavitation reference — cavitation and NPSH context.

Scope: steady shaft speed and one integer count of equally recurring elements. Variable speed smears or sweeps the frequency; nonuniform pitch can create additional components. This calculator is not an ISO severity or acceptance assessment.