1.2 Introduction

Normal rolling-element bearings generate an easily identifiable ball or roller pass frequency when operating. Bearing defects amplify the amplitude of these frequencies. By using the high-frequency resonance technique (or envelope detection), these frequencies can be isolated and demodulated to give an indication of bearing condition.1

Defects in a bearing generally will produce impacts when in contact with mating parts. The secondary effect of this impact is to excite resonance in the races, rolling-elements or other structural elements. These high-frequency resonances decay exponentially and are modified (modulated) at ball/roller pass frequency in a manner that can be easily detected. The basic signal is obtained using a high-frequency accelerometer located as near the bearing as possible. The resulting spectrum is analysed to detect possible resonant frequencies. Once the frequency of interest has been found, the signal is narrow band filtered and the filtered output is envelope-detected. The spectrum of the envelope is obtained and analysed in the range of defect frequencies. The frequency of interest is generally some structural resonance, which can be found analytically or experimentally. Quite a few investigations have been reported using this technique. ^{2 5}