What is a Campbell diagram?

A Campbell diagram (also called interference diagram) plots excitation frequencies (harmonics of running speed) against natural frequencies. Where harmonic lines cross natural frequency lines, a potential resonance or critical speed exists.

What are harmonic orders?

Harmonic orders represent multiples of the rotational speed. 1× is the fundamental (1 per revolution), 2× is twice per revolution (common for misalignment), and higher orders relate to blade pass, gear mesh, etc.

How do I find critical speeds from the diagram?

Critical speeds occur where harmonic order lines intersect natural frequency lines. The crossing RPM = fn × 60 / order, where fn is the natural frequency in Hz and order is the harmonic number.

Why are critical speeds dangerous?

At critical speeds, a harmonic excitation coincides with a natural frequency, causing resonance. This leads to amplified vibration, potential structural damage, and accelerated bearing/component wear.

How do I avoid critical speeds?

Ensure operating speed is at least 20% away from any critical speed. Options include changing natural frequencies (stiffening/mass changes), adding damping, or adjusting operating speed range.

Free Engineering Tool — #015

Campbell Diagram Calculator

Plot interference between harmonic excitation orders and natural frequencies. Identify critical speed crossing points for rotating machinery.

Up to 5 Natural Frequencies Harmonics 1×–10× Critical Speeds

Critical Speed Crossings

Total Crossings in Speed Range

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Speed Range

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Natural Frequencies

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Campbell Diagram

Campbell Diagram Principle

The Campbell diagram plots excitation frequency vs. rotational speed. For each harmonic order k, the excitation frequency at speed n is:

A crossing (critical speed) occurs when a harmonic excitation matches a natural frequency:

Common Harmonic Orders

Order	Source
1×	Unbalance, bow, eccentricity
2×	Misalignment, elliptical bearing, looseness
3×	Misalignment (severe), coupling defects
k× (blade pass)	Number of blades × RPM
k× (gear mesh)	Number of teeth × RPM

Practical Example

Example — Centrifugal Pump

Given: f_n1 = 25 Hz, f_n2 = 48 Hz, Harmonics: 1×, 2×, 3×

1× crossing at f_n1: RPM = 25 × 60 / 1 = 1500 RPM

2× crossing at f_n1: RPM = 25 × 60 / 2 = 750 RPM

1× crossing at f_n2: RPM = 48 × 60 / 1 = 2880 RPM

⚠️ Note: Keep operating speed at least 20% away from any critical speed. If a critical speed must be passed during run-up, accelerate quickly through the resonance zone.

The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.