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A noise analysis based on coherence method has been developed and utilized as a diag - nostic tool for defect detection of a gear box. A case study has been carried out on a multiple reduction gear box and the emitted noise is measured during normal working environment thus the signals are subjected to external disturbing noise. Using the coherence method, the original signals are identified and further analysis is carried out using enhanced Campbell as well as Order-ratio diagrams. The results show that the coherence method is a powerful tool in noise analysis.

An experimental study has been carried out on a transmission system which consists of a pair of spur gears mounted using ball bearings on a steel frame and rotated by a frequency controlled A-C electromotor. The vibration sources among the rotating components of the system have been investigated using Campbell diagram technique and its influence on the frequency spectrum at the measuring points located on the frame has been identified. Harmonic as well as random excitation techniques have been conducted to reveal the transfer-function and the frequency spectrum at those measuring points. It is observed, for low frequency range (up to 125 Hz) similar transfer functions are obtained through out the measuring points. On the other hand for high frequency (above 125 Hz) the transfer functions are significantly influenced by the local propagation path.