Recent results in condition monitoring of machine elements by acoustic emission analysis are presented. A special method based on the evaluation of structure-borne noise emissions in the ultrasonic range is described. The ultrasound-signals caused by friction processes are captured by a broadband piezoelectric sensor and analyzed subsequently. The method has proven to be suitable for detecting the occurrence of friction between solid objects in a very reliable way. This leads to a variety of possible applications wherever occurrence of solid body friction has to be considered as an indication of failure or wear. In addition to tribometer tests, experiments with sliding bearings and slide ring seals are presented exemplarily. In both cases promising results were achieved. The significant difference of the presented method compared to other sound-based methods is in the nature of the analyzed signals: Harmonic waves of audible sounds or percussion-type stimulations are not evaluated but the portion of friction sounds emerging in the ultra-sonic range beyond audible frequencies. These friction sounds are widely unaffected by ambient noise and other sources of interference.

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