Abstract: A mechanical fault detection method and system computes a similarity index to quantify the relationship of a measured spectral signature to a theoretical fault signature. A vibration sensor detects vibrations on machinery with rotating components. The vibration sensor generates a representation of the vibration and provides it to a vibration analyzer. The vibration analyzer is configured to identify peak amplitudes in the generated representation, determine a corresponding frequency for each of the peak amplitudes, and match the determined corresponding frequencies to a theoretical set of frequencies. The analyzer determines a number of matching frequencies and identifies a detection condition when the number of matching frequencies meets a predetermined criterion. The system and method combines vibration amplitude threshold detection with similarity index threshold detection to significantly reduce false fault alarms and false pass errors.