Condition-Monitoring Based on Structure-borne Ultrasound Analysis

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 analysed 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 analysed signals: Harmonic waves of audible sounds or percussion-type stimulations are not evaluated but the portion of friction sounds emerging in the ultrasonic range beyond audible frequencies. These friction sounds are widely unaffected by ambient noise and other sources of interference. ... mehrAmong the variety of the available monitoring methods for machinery the acoustic ones have to be emphasised as all-purpose methods and easily to be applied. An advantage of this principle is the fact that the sensors do not have to be positioned in direct proximity to the component parts in question and also that one sensor often suffices to observe a multitude of machine elements. In addition to mere sound analyses, which consider the audible frequency range, methods have been established which include non-audible high-frequency waves. A basic distinction has to be made between methods which analyse the waves occurring in the monitored process itself and those which profit from artificially produced ultrasonic waves, as for instance non-destructive material testings by means of echo techniques, with which also an emitter (sound generator) is required apart from the ultrasonic receiver. The latter are not subject of this paper. Natural causes for structure-borne sound waves can be varying processes during which energy is transformed and partly released as sound waves. Examples are plastic deformations, crack growth, corrosion effects, or also turbulent flows of fluids and cavitation. Moreover, material vibrations by friction also occur up to high-frequency ranges. These show themselves by continuous signals in the ultrasonic range and are used as analysing basis in the approach subject of this paper. A highpass filtered time signal is captured which is subsequently transformed into the frequency range by means of Fast-Fourier-Transformation (FFT).