Domain Fundamentals

Some fundamental signal processing techniques and diagnostic features for gearbox components are provided below. The information provided is only intended as a brief introduction to existing domain knowledge and is NOT meant to guide the challenge participants in any way.

Time Synchronous Averaging

The common use of the synchronous averaging technique is the attenuation of both non-coherent components and the non-synchronous components, like noise in an accelerometer signal collected from rotating machinery, to negligible levels. Synchronous averaging is a fundamental process to many shaft, bearing, and gear diagnostics algorithms. A common rule of thumb is that the amount of attenuation is inversely proportional the square root of the number of averages. This rule is highly representative for the non-coherent components but is not representative for the non-synchronous terms. More information about this technique may be found in the paper:

  • Hochmann, D.; Sadok, M., “Theory of synchronous averaging,” Proceedings of 2004 IEEE Aerospace Conference, 2004. Vol.6, no., pp. 3636-3653 Vol.6, 6-13 March 2004 (PDF).

Accelerometer Data Analysis

Accelerometer data can be analyzed in a variety of domains like time, frequency, wavelet, etc. The most common ones are time and frequency. Some popular time domain techniques are decimating, digital filtering, averaging, and RMS. Frequency domain analysis typically comprises Fourier transform, windowing, and spectral analysis of power and amplitude. More information about processing accelerometer data can be found here.

Gearbox Component Fault Signatures

The main components of gearboxes are gears, bearings and shafts. Their common fault modes have been extensively studied in literature. The following table lists the generic locations of frequency domain features that accompany the common faults. More information can be found in the following books:

  • J. M. Vance, “Rotordynamics of Turbomachinery“, John Wiley & Sons, Inc., New York, USA, ISBN 0-471-80258-1, 1988.
  • Keith Mobley, “Vibration Fundamentals“, Elsevier, ISBN 978-0-7506-7150-7, 2001.
Common fault modes and features of a gearbox



Fault Modes


Gear Change rotation speed Crack in gear Gear natural frequency
Sidebands around the gear natural frequency
Cracked/broken tooth Sidebands around gear mesh frequency
Excessive wear or clearance Sideband spacing
Bearing Support rotating shaft Bearing race defect Approximately, (n/2) x shaft speed, n is the number of balls
Excessive bearing clearance Sub-synchronous whirl
Shaft Transmit torque Rotor imbalance 1 x shaft speed
Shaft misalignment 2 x shaft speed
high axial vibration
Mechanical looseness higher harmonics of shaft speed