What is blade pass frequency?

Blade pass frequency (BPF) is the rate at which blades or vanes pass a fixed point. BPF = number of blades × shaft rotational frequency. It is the dominant vibration frequency in fans, pumps, compressors, and turbines.

What is the difference between BPF and GMF?

BPF (blade pass frequency) applies to rotating blades/vanes passing a stationary point. GMF (gear mesh frequency) applies to intermeshing gear teeth. Both equal the number of elements times the shaft frequency, but BPF involves fluid interaction while GMF involves mechanical tooth contact.

What causes elevated blade pass frequency?

Elevated BPF can be caused by: uneven blade spacing, blade-to-housing clearance issues, inlet or outlet flow disturbances, damaged or fouled blades, resonance with structural natural frequencies, or cavitation in pumps.

How does blade count affect vibration?

More blades generally produce a higher BPF at lower amplitude per pulse. Fewer blades produce lower BPF with stronger individual pulses. Odd blade counts distribute forces more evenly than even counts, potentially reducing vibration.

How does BPF differ in pumps vs fans?

In pumps, BPF (often called vane pass frequency) interacts with volute cutwater or diffuser vanes, and elevated BPF may indicate cavitation or recirculation. In fans, BPF interacts with inlet guide vanes or scroll cutoff, and elevated BPF often indicates clearance or flow problems.

Free Engineering Tool #037

Blade Pass Frequency Calculator

Calculate blade pass frequency (BPF) for fans, pumps, and impellers. Includes harmonics 1× through 5× and shaft frequency.

BPF Fans Pumps Impellers

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)

Blade Pass Frequency (BPF)

Blade pass frequency is the rate at which blades or vanes pass a fixed reference point. It applies to fans, pumps, compressors, turbines, and any rotating machinery with blades or vanes.

Z — number of blades or vanes
RPM — shaft rotational speed
f_s — shaft frequency = RPM/60 (Hz)

Shaft Frequency

Why Harmonics Matter

BPF harmonics (2×BPF, 3×BPF, etc.) provide diagnostic information:

1×BPF dominant: Normal operation — pressure pulsations from blade passing
Elevated 1×BPF: Clearance problems, flow disturbance, or resonance
Multiple BPF harmonics: Non-sinusoidal pressure pulses — blade damage, uneven spacing, or cavitation
BPF with shaft-speed sidebands: Individual blade damage or buildup

Practical Example

Example — Centrifugal Pump, 6 Vanes, 2950 RPM

Given: Z = 6, RPM = 2950

f_s = 2950/60 = 49.17 Hz

BPF = 6 × 49.17 = 295 Hz

2×BPF = 590 Hz, 3×BPF = 885 Hz

Period = 1/295 = 3.39 ms

Tip: In centrifugal pumps, the interaction between impeller vanes and volute cutwater is the primary source of BPF vibration. Optimizing the gap between impeller OD and cutwater (typically 5–10% of impeller diameter) can significantly reduce BPF amplitude.

Blade Pass Frequency Calculator | BPF for Fans & Pumps

Published by admin on February 15, 2026 February 15, 2026

Results

BPF Harmonics

Blade Pass Frequency (BPF)

Shaft Frequency

Why Harmonics Matter

Practical Example