ArticleComments on Rolling Element Bearing Analysis
by Ronald L. Eshleman
This paper serves as a summary of the vibration analysis techniques available at the time to detect and track the severity of rolling element bearing defects. Both low frequency, velocity spectra detection and high frequency detection techniques are discussed with examples given. The author relates the strengths and weaknesses of each technique. A helpful restatement of the four stages of bearing degradation first classified by J.C. Berggren in 1988 is provided with helpful comments by the author.
“It has been found that defects in rolling element bearings can be identified and quantified with vibration analysis. The many rolling element bearings on large and small equipment have motivated the development of methods for bearing analysis. Defects on the bearing elements, excessive wear, and lack of lubrication produce, identifiable vibration signals. Bearing defects cause bearing impacts at frequencies governed by the operating speed of the unit and the geometry of the bearings. In later stages of wear these frequencies can sometimes be identified with the human ear and analog meters. Excessive wear and defects cause the bearing to ring at its natural frequencies, a phenomenon utilized in high-frequency demodulation techniques. Lack of lubrication inhibits rolling and leads to skidding and slipping that cause surface frequencies generated by friction.
Scientific vibration analysis of bearings is a relatively re-cent development because the sophisticated electronics now available are required to implement them. It has been noted that bearings produce discrete frequencies in the stage of initial failure and broad-band vibration in the final stages of wear . Vibration techniques have been developed to identify faults in rolling element bearings . Spectrum shape was first identified as a symptom of a defective bearing. The technique was perfected when it was found that sum and difference frequencies are generated by the defective bearing. The FFT analyzer with high-frequency resolution is required for these techniques. Other techniques are concerned with acoustic emission and high-frequency vibrations [3, 4, 5]. Some of these techniques involve a single index or number read from a meter rather than an analysis that may indicate condition of the bearing. The demodulation, or enveloping, technique, which provides a spectral acceleration record of bearing frequencies demodulated from the natural frequency response of the system, is an analytical technique.”
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