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The order tracking method is used to extract the harmonic components related to rotational frequency of the machine. The machine vibration pattern is a mixture of excitation frequencies, usually related to rotational speed, such as unbalance, eccentricity, bearing faults and others and machine response function, which relates to machine natural frequencies based on the structure and mounting of that machine.

With order extraction, we can see a specific harmonic component which relates to a certain machine fault. That is - the first order (harmonic) usually relates to unbalance of the machine, the second harmonic often relates to eccentricity, such as if we have for example 9 rotor blades, the 9th harmonic relates to errors on the blades. Or, if we have for example 31 teeth on a gear, then the 31st harmonic will show the gear mesh frequency.

These are excitations forces which produce vibration accelerations. The ratio between excitation and system response is defined by the system transfer curve. Thus, the final measured vibration of the system is a product of the excitation force and the system transfer curve. Since the transfer curve is fixed, we get different responses for excitations at different rotation speeds.

When the excitation passes natural frequency, we get so-called resonance with increased vibration amplitudes, which could be fatal to the machine.